Patent Publication Number: US-6907048-B1

Title: Method and apparatus for transporting ethernet data packets via radio frames in a wireless metropolitan area network

Description:
This is a Continuation-in-Part of application Ser. No. 08/950,028, filed Oct. 14, 1997 now abandoned, the contents of which are hereby incorporated by reference. This application claims the benefit of U.S. Provisional Application Ser. No. 60/086,459, entitled, “Method and Apparatus for Wireless Communication of Fast Ethernet Data Packets,” filed May 22, 1998. 
    
    
     FIELD OF THE INVENTION 
     The invention relates to a terminal for a wireless network for a metropolitan area. More particularly, the invention relates to a terminal for transporting Ethernet data packets via radio frames in a wireless metropolitan area network. 
     BACKGROUND OF THE INVENTION 
     Computers utilized in modern office environments are typically coupled to a local area network (LAN). The LAN allow users of the computers to share common resources, such as a common printer included in the network, and allows the users to share information files, such as by including one or more file servers in the network. In addition, the users are typically able to communicate information with each other through electronic messaging. A commonly utilized type of LAN is Ethernet. Currently, a variety of products which support Ethernet are commercially available from a variety of sources. Other types of LANs are also utilized, such as token ring, fiber distributed data interface (FDDI) or asynchronous transfer mode (ATM). 
     LANs are often connected to a wide area network (WAN) via a telephone modem. Thus, information is communicated over the WAN via a communication link provided by a telephone service provider. These telephone links, however, are generally designed to have a bandwidth that is sufficient for voice communication. As such, the rate at which information can be communicated over these telephone links is limited. As computers and computer applications become more sophisticated, however, they tend to generate and process increasingly large amounts of data to be communicated. For example, the communication of computer graphics generally requires a large amount of bandwidth relative to voice communication. Thus, the telephone link can become a data communication bottleneck. 
     Business organizations and their affiliates are often spread over several sites in a metropolitan or geographical area. For example, a business organization can have a headquarters, one or more branch offices, and various other facilities. For such business organizations, LANs located at the various sites will generally need to communicate information with each other. Wireless communication links for connecting local area networks are known. For example, U.S. Pat. No. 4,876,742, entitled “Apparatus and Method for Providing a Wireless Link Between Two Area Network Systems,” and U.S. Pat. No. 5,436,902, entitled “Ethernet Extender,” each disclose a wireless communication link for connecting LANs. 
     Availability is a measure of the average number of errors which occur in digitally transmitted data. An availability of 99.99 percent is commonly required for radio communications. For an availability of 99.99 percent, the average error rate for digitally communicated data must be maintained below 1×10 −6  errors per bit, 99.99 percent of the time. The integrity of a wireless communication link, however, is largely dependent upon transient environmental conditions, such as precipitation. Environmental precipitation causes a severe attenuation of the transmitted signal. For example, to maintain an availability of 99.99 in the presence of environmental precipitation, the signal must be transmitted at a level that is 24 dB/km higher than otherwise. Therefore, to ensure an acceptable data error rate under all expected conditions, data is typically communicated over a wireless communication link at a relatively high power and at a relatively low rate. The amount of data required to be communicated over the wireless link, however, can vary widely over time and can vary independently of environmental conditions. In addition, wireless links, especially those operated at high power levels, can cause interference with other wireless links operating in the same geographical area. Thus, the wireless link can become a data communication bottleneck. 
     Therefore, a technique is needed for efficiently and cost effectively communicating data over a wireless link between Ethernet local area networks. 
     Known wireless transmission systems for LAN have a disadvantage in that they require conversion from the LAN protocol to an intermediate protocol prior to wireless transmission. Such known systems perform conversion to a telephony protocol or to an asynchronous transfer mode (ATM) protocol. 
     Therefore, what is needed is a technique for communicating data over a wireless link between local area networks which does not suffer from these drawbacks. 
     SUMMARY OF THE INVENTION 
     The invention is a method and apparatus for transporting Ethernet data packets via radio frames in a wireless metropolitan area network. A terminal for transporting data packets via radio frames includes a data packet receiver for receiving data packets for communication over a wireless link wherein not every data packet has a same length, a data packet formatting apparatus coupled to the data packet receiver, the data packet formatting apparatus for formatting the data packets according to radio frames wherein the radio frames each have a same length and wherein the data packets are formatted into the radio frames such that boundaries for the data packets are not necessarily aligned with boundaries for the radio frames, and a wireless transceiver coupled to the packet formatting apparatus, the wireless transceiver for communicating the radio frames over the wireless link. The terminal need not convert the Ethernet data packets into a telephony communication protocol or into an asynchronous transfer mode (ATM) protocol prior to communication of the radio frames over the wireless link. The terminal can include a data packet synchronizer coupled to the data packet receiver and to the data packet formatting apparatus for synchronizing the data packets to a clock signal associated with the radio frames. The data packets can be Fast Ethernet data packets. The data packets can be time-division multiplexed into the radio frames. Each radio frame can include a data field having a predetermined length for receiving the data packets. The data packets can be received by the data packet receiver separated by an inter-packet gap and wherein a code representative of the inter-packet gap is stored in the data field between the data packets. The data packet receiver can receive the data packets from a local area network coupled to the data packet receiver via a twisted pair of wires. The data packet receiver can be an Ethernet switch. The Ethernet switch can include a 100BASE-T port for receiving the data packets from the local area network. The Ethernet switch can include an MII (media independent interface) for providing the data packets to the packet formatting apparatus. The data packet receiver can receive the data packets from a local area network coupled to the data packet receiver via a fiber optic cable. The radio frames can be communicated over the wireless link according to full-duplex communication. 
     According to another aspect of the present invention, a method of transporting data packets via radio frames includes steps of receiving data packets wherein not every data packet has a same length, formatting the data packets according to radio frames wherein the radio frames each have a same length and wherein the data packets are formatted into the radio frames such that boundaries for the data packets are not necessarily aligned with boundaries for the radio frames, and communicating the radio frames over the wireless link. Each radio frame can include a data field having a predetermined length and wherein the step of formatting can include a step of placing the data packets into the data field. The step of receiving data packets can include a step of receiving an inter-packet gap between each packet. The method can include a step of synchronizing the data packets to a clock signal associated with the radio frames. The step of formatting the data packets can include a step of inserting a code representative of an inter-packet gap into radio frames between each data packet. The step of receiving the data packets can include a step of receiving the data packets from a local area network via a twisted pair of wires. The method can comprise a step of mapping portions of the radio frame to quadrature amplitude modulation symbols. The step of formatting the data packets according to radio frames can include a step of time-division multiplexing the data packets into the radio frames. The data packets can be Fast Ethernet data packets. The method does not require a step of converting the data packets into a telephony communication protocol or into an asynchronous transfer mode (ATM) protocol prior to communication of the radio frame over the wireless link. The method can include steps of receiving the radio frames from the wireless link, and reconstructing the data packets from received radio frames. 
     According to a further aspect of the present invention, a method of transporting data packets via radio frames includes steps of receiving data packets wherein not every data packet has a same length and wherein each data packet is separated by an inter-packet gap, formatting the data packets according to radio frames wherein each radio frame includes a data field having a predetermined length and wherein the data packets are formatted into each data field one after the other wherein each data packet is separated from its neighbors by a code representative of an inter-packet and wherein boundaries for the data packets are not necessarily aligned with boundaries for the radio frames such that the data packets are time-division multiplexed into the radio frames, and communicating the radio frames over the wireless link. The data packets can be Fast Ethernet packets. The method can include a step of synchronizing the data packets to a clock signal associated with the radio frames. The method need not include a step of converting the data packets into a telephony communication protocol or into an asynchronous transfer mode (ATM) protocol prior to communication of the radio frames over the wireless link. The code representative of an inter-packet gap can be representative of an inter-packet gap of approximately of 0.96 μs. 
     According to yet another aspect of the invention, a method of transporting Fast Ethernet data packets via radio frames over a wireless link includes steps of receiving Fast Ethernet data packets into a receiver from a first local area network via a twisted pair of wires, formatting the data packets according to radio frames wherein the radio frames each have a same length and wherein the Fast Ethernet data packets are formatted into radio frames such that boundaries for the Fast Ethernet packets are not necessarily aligned with boundaries for the radio frames, communicating the radio frames over the wireless link, receiving the radio frames from the wireless link, reconstructing the Fast Ethernet data packets from received radio frames, and communicating reconstructed Fast Ethernet data packets to a second local area network. Each radio frame can include a data field having a predetermined length and wherein the step of formatting can include a step of placing the data packets into the data field. The step of formatting the data packets according to radio frames can include a step of time-division multiplexing the data packets into the radio frames. The method can include a step of synchronizing the data packets to a clock signal associated with the radio frames. The method need not include a step of converting the data packets into a telephony communication protocol or into an asynchronous transfer mode (ATM) protocol prior to communication of the radio frames over the wireless link. 
     According to a still further aspect of the present invention, a terminal for transporting data packets via radio frames includes a data packet receiver for receiving data packets for communication over a wireless link wherein not every data packet has a same length, a packet formatting apparatus coupled to the data packet receiver, the packet formatting apparatus for formatting the data packets according to radio frames. The packet formatting apparatus includes means for performing forward error correction on data from the data packets thereby forming error corrected data, means for inserting the error corrected data into radio frames, and means for randomizing data within the radio frames. The terminal also includes a wireless transceiver coupled to the packet formatting apparatus, the wireless transceiver for communicating the radio frames over the wireless link. The data packets can be Fast Ethernet data packets. The radio frames can each have a same length and wherein the packet formatting apparatus formats the data packets into the radio frames such that boundaries for the data packets are not necessarily aligned with boundaries for the radio frames. The terminal need not convert the Ethernet data packets into a telephony communication protocol or into an asynchronous transfer mode (ATM) protocol prior to communication of the radio frames over the wireless link. The data packets can be time-division multiplexed into the radio frames. The terminal can also include a data packet synchronizer coupled to the data packet receiver and to the data packet formatting apparatus for synchronizing the data packets to a clock signal associated with the radio frames. The packet formatting apparatus can also include means for mapping portions of the radio frame to quadrature amplitude modulation symbols. The radio frames can be communicated over the wireless link according to full-duplex communication. 
     According to another aspect of the present invention, a method of transporting Ethernet data packets via radio frames includes steps of receiving Ethernet data packets wherein each data packet includes a data valid bit for each portion of packet data, and removing each data valid bit. The method can also include steps of formatting the Ethernet data packets according to radio frames after performing the step of removing each data valid bit, and communicating the radio frames over a wireless link. Each portion of packet data can be four bits long. The radio frames can each have a same length and wherein the step of formatting the data packets according to radio frames is performed such that boundaries for the data packets are not necessarily aligned with boundaries for the radio frames. The method can include a step of synchronizing the data packets to a clock signal associated with the radio frames. The step of formatting the data packets according to radio frames can include a step of time-division multiplexing the data packets into the radio frames. 
     According to yet another aspect of the present invention, a method of transporting Ethernet data packets via radio frames includes steps of receiving Ethernet data packets wherein each data packet includes a preamble and a start-of-frame delimiter, stripping off the preamble and start-of-frame delimiter, formatting the packet data according to radio frames. The step of formatting includes steps of appending a synch field to the packet data, and appending a length field to the packet data. The method can include a step of inserting a synch value which is in accordance with a Willard code into the synch field. The method can include a step of inserting a length value and an error correcting code for correcting errors in the length value into the length field. The error correcting code can be a Golay error correcting code. The radio frames can each have a same length and wherein the step of formatting the data packets according to radio frames is performed such that boundaries for the data packets are not necessarily aligned with boundaries for the radio frames. The method can include a step of synchronizing the data packets to a clock signal associated with the radio frames. The step of formatting the data packets according to radio frames can include a step of time-division multiplexing the data packets into the radio frames. The step of synchronizing can include steps of storing each portion of packet data in successive locations of a buffer, and removing the packet data from the packet buffer prior to performing the step of formatting. The radio frames can each have a same length and wherein the step of formatting the data packets according to radio frames is performed such that boundaries for the data packets are not necessarily aligned with boundaries for the radio frames. Each data packet can include a data valid bit for each portion of packet data and wherein the method further comprises a step of removing the data valid bit for each portion of packet data. Each portion of packet data can be four bits long. The method can include steps of storing the each portion of packet data in successive locations of a buffer, and removing the packet data from the packet buffer prior to performing the step of formatting. The method can include a step of storing the data valid bit for each portion of packet data in association with the four bit portion of packet data. The radio frames can each have a same length and wherein the step of formatting the data packets according to radio frames is performed such that boundaries for the data packets are not necessarily aligned with boundaries for the radio frames. The method can include a step of communicating the radio frames over the wireless link according to full-duplex communication. 
     The present invention provides an improvement in that conversion from the LAN protocol to an intermediate protocol is not required prior to wireless transmission. Rather, the present invention communicates data packets over a wireless link in a highly efficient manner. Thus, according to the present invention, conversion is not required to convert the LAN protocol into a telephony communication protocol, such as PDH (e.g. DS1, DS3, E1 and E3) or SDH (e.g. OC-1, OC-3), or to an asynchronous transfer mode (ATM) protocol prior to communication over the wireless link. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  illustrates a schematic block diagram of a pair of wireless terminals which communicate with each other via a wireless communication link in accordance with the present invention. 
         FIGS. 2A-F  illustrate representative metropolitan area network (MAN) topologies according to the present invention. 
         FIG. 3  illustrates a schematic block diagram of a single wireless terminal  100  in accordance with the present invention. 
         FIG. 4  illustrates a schematic block diagram of the digital signal processing MAC and radio framer included in the CODEC illustrated in FIG.  2 . 
         FIG. 5  illustrates a frame structure for reformed 100BASE-T Ethernet data packets according to the present invention. 
         FIG. 6  illustrates a radio frame according to the present invention. 
         FIG. 7  illustrates a radio super frame according to the present invention. 
         FIG. 8  illustrates a schematic block diagram of a symbol scrambler according to the present invention. 
         FIG. 9  illustrates a schematic block diagram of a differential encoder and characteristic equations according to the present invention. 
         FIG. 10  illustrates a schematic block diagram of a differential decoder and characteristic equations according to the present invention. 
         FIG. 11  illustrates a mapping constellation for a constellation mapper according to the present invention. 
         FIG. 12  illustrates a schematic block diagram of an Ethernet-to-radio frame synchronizing portion of the rate control logic according to the present invention. 
         FIG. 13  illustrates a schematic block diagram of a radio frame-to-Ethernet synchronizing portion of the rate control logic according to the present invention. 
         FIG. 14  illustrates a schematic block diagram of a microwave module and microwave antenna according to the present invention. 
         FIG. 15  illustrates a perspective view of the microwave antenna and a housing for the outdoor unit according to the present invention. 
         FIG. 16  illustrates a schematic block diagram of an alternate embodiment of the digital signal processing MAC and radio framer according to the present invention. 
         FIG. 17  illustrates a frame structure for reformed 100BASE-T Ethernet data packets formed by the MAC and radio framer illustrated in FIG.  14 . 
         FIG. 18  illustrates a schematic block diagram of an adaptive countermeasures block according to the present invention. 
         FIG. 19  illustrates a chart of received signal level vs. time as a result of rain fade. 
         FIG. 20  illustrates a flow diagram for implementing counter-measures according to the present invention. 
         FIG. 21  illustrates a point-to-multipoint metropolitan area network divided into sectors having inner and outer radii according to the present invention. 
         FIG. 22  illustrates a wireless link between two terminals wherein an unauthorized terminal is attempting to eavesdrop on communication between the two terminals. 
         FIG. 23  illustrates an embodiment according to the present invention having multiple digital processing MACs multiplexed to a single radio framer. 
     
    
    
     DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT 
       FIG. 1  illustrates a schematic block diagram of a pair of wireless terminals  100 ,  100 ′ which communicate with each other via a bi-directional wireless communication link  102  in accordance with the present invention. Though a single wireless communication link  102  is illustrated, it will be apparent that a network of wireless communication links can interconnect a plurality of wireless terminals, thereby forming a wireless metropolitan area network (MAN) in accordance with the present invention.  FIGS. 2A-F  illustrate representative MAN topologies which interconnect wireless nodes A-E with wireless links according to the present invention. Each of the nodes A-E can include a wireless terminal identical to the terminal  100  or  100 ′ illustrated in  FIG. 1  for terminating each wireless link. It will be apparent that other MAN topologies can be implemented and that one or more of the nodes A-E can be coupled to one or more other types of networks. 
     Due to availability of portions of the radio spectrum in the 38 GHz frequency band, the wireless link  102  illustrated in  FIG. 1  preferably operates within this frequency band, though another frequency band can be selected. Different channels within the selected band are assigned to nearby wireless links so as to reduce interference between them. The channels are preferably stepped at intervals of 25-50 MHz. Because the 38 GHz radio frequency band is susceptible to rain fade, the manner and path of transmissions via the wireless link  102  are adaptively modified for maintaining a predefined transmission quality in the network in accordance with the teachings of the parent application Ser. No. 08/950,028, filed Oct. 14, 1997, the contents of which are hereby incorporated by reference. 
     Referring to  FIG. 1 , the wireless link  102  preferably includes a primary radio channel  102 A which carries full duplex 100 mega-bits-per-second (Mbps) data traffic, including payload data, and an auxiliary radio channel  102 B which carries full-duplex control data for network management and control over the manner of transmission over the link  102  (link control). For example, changes to the manner of transmission initiated through link control can include changing transmission power, data bit rate, amplitude modulation scheme, spectrum spreading and transmission path. 
     The terminal  100  includes a broadcast device, also referred to herein as an outdoor unit (ODU)  104 , which terminates one end of the wireless link  102 . In the preferred embodiment, the ODU  104  includes a bi-directional radio antenna and is mounted outdoors on a roof-top mast of a building. Also included in the terminal  100  is an extender device, also referred to herein as a top floor unit (TFU)  106 , which is coupled to the ODU via bi-directional communication cables  108 ,  110  and  112  and by power leads  114 . The TFU  106  is preferably located indoors of the building having the ODU  104  located on its roof and as close as practical to the ODU  104 . In preferred embodiment, the TFU  106  is located indoors, ideally in a wiring closet, on the top floor of the building. It will be apparent that the term “top floor unit”, as used herein, refers to the extender unit  106  and its equivalents regardless of its location relative a building. For example, the “top floor unit” is preferably, though not necessarily, located on the top floor of a building. 
     The cable  108  carries full-duplex data traffic between the ODU  104  and the TFU  106  which is received from, or transmitted to, the primary radio channel  102 A. The data traffic communicated via the cable  108  includes payload data for communication over the link  102  and can also include network management and control data. Preferably, data communicated via the cable  108  is in accordance with a Fast Ethernet standard, 802.3u, adopted by the Institute of Electrical and Electronics Engineers (IEEE), such as 100BASE-TX or 100BASE-T4, which operates at a data rate of 100 Mbps. The cable  110  carries half-duplex network management and control data between the ODU  104  and TFU  106 . Preferably, data communicated via the cable  110  is in accordance with an Ethernet standard, such as 10BASE-T, which operates at 10 Mbps. The cable  112  carries serial data for set-up and maintenance purposes between the ODU  104  and the TFU  106 . Preferably, the data communicated via the cable  112  is in accordance with conventional RS423 serial port communication protocol. The cable  114  provides supply power to the ODU  104 . 
     Thus, in the preferred embodiment of the present invention, data is communicated between the TFU  106  and the ODU  104  via each of the cables  108 ,  110  and  112  according to baseband communication frequencies. This is in contrast to systems which communicate data between an indoor unit and an outdoor unit by modulating such data to intermediate frequencies (IF). The baseband communication aspect of the present invention has an advantage over such an IF modulation scheme in that implementation of the TFU  106  is simplified by the present invention. In addition, the cables  108 ,  110  and  112  can be of less expensive construction than would be required for IF communication. 
     A router or switch  116  is coupled to the TFU  106 , and hence, to the terminal  100 , via cables  118  and  120 . The cable  118  preferably communicates data in accordance with the 100BASE-TX or T4 Fast Ethernet standard, while the cable  120  preferably communicates data in accordance with the 10BASE-T Ethernet standard. Alternately, the cable  118  can be a fiber-optic cable, in which case, it preferably communicates data in accordance with 100BASE-FX Fast Ethernet standard. 
     A cable  122  is coupled to a serial port of the TFU  106 . Preferably, data communicated via the cable  122  is in accordance with the RS232 serial port communication protocol. A diagnostic station  124  can be coupled to the cable  122  for performing diagnostics, set-up, and maintenance of the terminal  100 . Because certain aspects of the TFU  106  and ODU  104  can only be accessed from the diagnostic station  124  security over such aspects is enhanced by the requirement that the diagnostic station  124  be directly connected to the TFU  106  via the cable  122 . AC power is supplied to the TFU  106  via a power supply cable  126 . 
     A wired local area network (LAN)  128 , such as an Ethernet LAN located within the building having the terminal  100 , can be coupled to the router or switch  116 . In addition, a wide area network (WAN)  130 , such as a telephone service network which provides access to the world wide web, can be coupled to the LAN  128 . Thus, the wireless link  102  can be accessed from one or more personal computers (PCs), data terminals, workstations or other conventional digital devices included in the LAN  128  or WAN  130 . A network management system (NMS)  132  is coupled to any one or more of the router or switch  116 , the LAN  128  or the WAN  130 . The NMS  132  accesses the wireless link  102  and the terminals  100 ,  100 ′ for performing network management and link control functions (e.g. collecting data regarding operation of the MAN or changing the manner of data transmission over a particular link or links). If the NMS  132  is coupled to the LAN  128 , this access is through the LAN  128 . If the NMS  132  is coupled to the WAN  130 , however, this access is remote via direct dial-up through a telephone service provider or via access through the world wide web. When network management and link control functions are accessed via the world wide web, a web browser is provided in the NMS  132 , while a web server  236  ( FIG. 3 ) is provided in the terminal  100 . In the preferred embodiment, the DS  124  and the NMS  132  are each a personal computer, but can be another type of conventional digital device. 
     The terminal  100 ′ terminates the opposite end of the link  102 , remote from the terminal  100 . In the preferred embodiment, the link  102  can be up to 4 kilometers or more in dry climates (e.g. Wyoming) while maintaining 99.99% link availability and can be up to 1.2 kilometers or more in wetter climates (e.g. Florida) while maintaining 99.99% link availability. Elements illustrated in  FIG. 1  having a one-to-one functional correspondence are given the same reference numeral, but are distinguished by the reference numeral being primed or not primed. Note, however, that because any NMS  132 ,  132 ′ can access the wireless communication link  102  and both terminals  100 ,  100 ′, an NMS  132  or  132 ′ need not be located at each end of the link  102 . 
       FIG. 3  illustrates a schematic block diagram of a single wireless terminal  100 , including a TFU  106  and an ODU  104 , in accordance with the present invention. The TFU  106  includes a 100BASE-T regenerator  200  which is coupled to the cable  118  ( FIG. 1 ) and to the cable  108  (FIG.  1 ). In addition, assuming the cable  118  is a fiber-optic cable, the TFU  106  includes a converter  202  for converting between fiber-optic cable and Category 5 twisted pair cable. The converter  202  is coupled to the fiber-optic cable  118  and to the regenerator  200 . The TFU  106  also includes a 10BASE-T repeater  204  coupled to the cable  120  ( FIG. 1 ) and to the cable  110  (FIG.  1 ). A converter  206  included in the TFU  106  converts between signals in accordance with the RS232 standard and signals in accordance with the RS423 standard. The converter  206  is coupled to the cable  122  ( FIG. 1 ) and to the cable  112  (FIG.  1 ). 
     The TFU  106  also includes an alternating-current to direct-current (AC/DC) power converter  208  coupled to the cable  126  ( FIG. 1 ) and to the cable  114  (FIG.  2 ). The power converter  208  provides power to the TFU  106  and to the ODU  104 . A status indicator  210  included in the TFU  106  displays status of the TFU  106  via light emitting diodes for diagnostic, set-up and maintenance purposes. 
     The TFU  106  provides three interfaces to customer equipment, including the router or switch  116  ( FIG. 1 ) and the DS  124  (FIG.  1 ). These include a full-duplex 100 Mbps interface via the regenerator  200 , a half-duplex 10 Mbps interface via the repeater  204  and an RS232 serial port via the converter  206 . Though the payload data traffic is generally directed through the 100 Mbps interface while network management and link control traffic is generally directed through the 10 Mbps interface, a user of the terminal  100  can combine network management and link control signals with the payload data traffic in the 100 Mbps interface depending upon the particular capabilities of the router or switch  116  (FIG.  1 ). 
     The TFU  106  provides an interface from multiple indoor cables  118 ,  120 ,  122 ,  126 , to a multiple outdoor cables  108 ,  110 ,  112  and  114 . TFU  106  also regenerates/repeats the Ethernet signals in the form of Ethernet data packets, between the cables  108 ,  118  and between the cables  110 ,  120 . Thus, the TFU  104  serves to extend the maximum distance possible between the customer equipment, such as the router or switch  116  (FIG.  1 ), and the ODU  104 . In the preferred embodiment, a distance between the customer equipment and the TFU  106  can be up to 100 meters while a distance between the TFU  106  and the ODU  104  can also be up to 100 meters. Accordingly, in the preferred embodiment, a distance between the customer equipment and the ODU  104  can be up to 200 meters. Because data is communicated between the TFU  106  and ODU  104  at baseband frequencies, however, apparatus for performing IF modulation is not required in the TFU  106 . 
     The ODU  104  includes a 100BASE-T transceiver  212  coupled to the cable  108 , a 10BASE-T transceiver  214  coupled to the cable  110 , an RS423 driver  216  coupled to the cable  112  and a DC-to-DC power converter  218  coupled to the cable  114 . The 100BASE-T transceiver  212 , the 10BASE-T transceiver  214 , and the RS423 driver  216  are each coupled to a coder/decoder (CODEC)  220  included in the ODU  104 . The power converter  218  provides power to the ODU  104 . 
     The CODEC  220  includes a media access control unit (MAC)  222 , having a transmitting portion  224  and a receiving portion  226 , a radio framer  228  and a micro-processor  230  for controlling operation of the ODU  104 . The transmitting portion  224  and the receiving portion  226  of the MAC  222  are coupled to the 100BASE-T transceiver  212  for communicating Ethernet data packets with the 100BASE-T transceiver  212 . The radio framer  228  is coupled to the MAC  222  for translating data from the Ethernet data packets received by the MAC  222  into a radio frames  350  ( FIG. 6 ) suitable for radio frequency modulation and transmission. The radio framer  228  also translates received radio frames  350  ( FIG. 6 ) into packets which it provides to the MAC  222 . 
     The micro-processor  230  is programmed by software so as to implement a TCP/IP stack  232 , a link management (LM) task  234 , a HyperText Transfer Protocol (HTTP) server  236  and a simple network management protocol (SNMP) agent  238 . The micro-processor  230  manages each wireless link of a network of such wireless links (e.g., a MAN), including a local link  102  ( FIG. 1 ) which is coupled directly to the terminal  100 . The micro-processor  230  is accessible via any of the NMS  132  ( FIG. 1 ) and via the DS  124  (FIG.  1 ). Thus, the wireless network of links can be managed locally, such as via an NMS  132  or DS  124  which is wired to the TFU  106 . For this purpose, the microprocessor  230  is assigned an Ethernet (medium access control) MAC address. Alternately, the wireless network of links can be managed remotely, such as via an NMS  132  which is coupled to the WAN ( FIG. 1 ) and which accesses the micro-processor  230  through internet access using TCP/IP (Internet Protocol). The TCP/IP stack  232  provides for this TCP/IP interface through the world wide web. For this purpose, the microprocessor  230  is assigned an internet protocol (IP) address. 
     The LM task  234  provides a function of changing the manner in which data is transmitted over a wireless link, initiated by one of the NMS  132 ,  132 ′. For example, the data rate for the link  102  can be changed via the LM task  132  included in the ODU  104 . This can include sending a link control command over the link  102  to the ODU  104 ′ ( FIG. 1 ) so that both terminals  100 ,  100 ′ communicate data at the same rate. Such commands are received from, and provided to, the microprocessor  230  by a overhead link management (OH/LM) module  240  included in the radio framer  228 . Thus, the radio framer  228  appropriately combines network management and link control traffic provided by the LM task  234  with payload data received from the MAC  222  into radio frames  350  ( FIG. 6 ) for communication over the link  102 . In addition, the radio framer  228  extracts network management and link control traffic from radio frames  350  ( FIG. 6 ) received from the link  102  and provides them to the LM task  234  of the microprocessor  230  via the OH/LM module  240 . While two types of data traffic (payload and link control) are communicated via radio frames  350  (FIG.  6 ), the payload data is considered to be communicated via the primary channel  102 A, while the link control traffic considered to be communicated via the auxiliary channel  102 B. Accordingly, these two channels  102 A and  102 B are time-division multiplexed. 
     A graphical user interface by which the micro-processor  230  can be accessed from an NMS  132 ,  132 ′ ( FIG. 1 ) or DS  124 ,  124 ′ ( FIG. 1 ) for network management and link control purpose, is preferably achieved by the HTTP web server software module  236  which is implemented by the microprocessor  230  located in the ODU  104  and which is assigned a unique IP address. The server software  236  operates in conjunction with the TCP/IP stack  232 . According to this aspect of the invention, the server software  236  is utilized for providing a graphical user interface for through which network management functions are initiated. These functions include retrieving data representative of network conditions in the MAN and changing the manner in which data is transmitted across a wireless link of the MAN. 
     Thus, functions for managing the MAN and its wireless links can be accessed and initiated from network management stations  132 ,  132 ′ (NMS) located in various portions of the MAN, utilizing web browser software resident in the NMS  132 ,  132 ′. This graphical user interface provides a user friendly environment which can operate on, and be accessed by, a variety of different NMS&#39;s obtained from a variety of different manufacturers. For example, an NMS  132 ,  132 ′ can be a workstation manufactured by Sun Microsystems, a PC manufactured by any one of a variety manufacturers or even a set-top box used in conjunction with a television set. Compatibility with the web server is achieved via commercially available web browser software resident in the NMS  132 ,  132 ′. This aspect of the present invention addresses compatibility issues between the NMS  132 ,  132 ′, and the terminal  100 ,  100 ′. 
     The SNMP agent  238  located in the ODU  104  maintains a management information database (MIB statistics) which is a collection of managed objects that correspond to resources of the MAN and of the terminal  100 . The SNMP agent  238  can access the MIB to control certain aspects of the MAN and the terminal  100  and can query the MIB for information relating to the managed objects. The SNMP is accessible through the HTTP server  236 . 
     The ODU  104  also includes a transmit modulator (TX mod)  242 , a receive demodulator (RX demod)  244  and a microwave module (MWM)  246 . The transmit modulator  242  translates from digital baseband output data received from the radio framer  228  to analog waveforms suitable for up-conversion to microwave frequencies and eventual transmission over the wireless link  102 . The analog waveforms formed by the transmit modulator  242  preferably modulate a 490 MHz IF carrier. It will be apparent, however, that a frequency other than 490 MHz can be selected for this purpose. 
     The receive demodulator  244  performs functions which are essentially the opposite of those performed by the transmit modulator  242 . In the preferred embodiment, the receive demodulator  244  receives a 150 MHz IF signal from the microwave module  246 . It will be apparent, however, that a frequency other than 150 MHz can be selected for this purpose. The receive demodulator  244  controls the level of the this signal via automatic gain control (AGC) and, then, down-converts the signal to baseband according to coherent carrier recovery techniques and provides this down-converted signal to the radio framer  228 . 
     The microwave module  246  performs up-conversion to microwave frequency on the 490 MHz IF output signal generated by the transmit modulator  242  and provides this up-converted signal to a microwave antenna  508  ( FIG. 12 ) which transmits the data over the link  102 . In addition, the microwave module  246  receives a microwave frequency signal from the link  102 , down-converts this signal to a 150 MHz IF signal and, then, provides this down-converted signal to the receive demodulator  244 . 
       FIG. 4  illustrates a schematic block diagram of the digital signal processing MAC  222  and radio framer  228  included in the CODEC  220  illustrated in FIG.  2 . The MAC  222  includes rate control logic  250  and rate buffers  252 . The rate control logic  250  receives 100BASE-T Ethernet data packets at 100 Mbps from the 100BASE-T Transceiver  212  ( FIG. 3 ) via a media independent interface (MII) between the MAC  222  and the transceiver  212 . 
     Note that 100BASE-T Ethernet data packets are provided to the transceiver  212  ( FIG. 3 ) as a serial data stream. In accordance with the IEEE 802.3u standard, the serial data stream is encoded utilizing a 4B/5B scheme. According to the 4B/5B scheme, each four-bit portion (nibble) of each 100BASE-T data packet is accompanied by a 1-bit data valid field. Thus, due to the data valid bits, the wire speed for 100BASE-T is actually 125 Mbps, though the serial data communication rate is 100 Mbps assuming the data valid bits are discounted. The transceiver  212  converts this serial data stream into parallel four-bit portions of data (nibbles), a data valid signal (RX_DV) and also recovers a clock signal from the data stream. The nibbles, data valid signal and clock signal are provided to the MAC  222  by the transceiver via the MII interface. 
     The data nibbles, data valid signal and recovered clock signal are then synchronized to a locally generated clock signal. This locally generated clock signal preferably operates at 27.5 Mhz and is derived from a 55 MHz and 10 parts-per-million accuracy crystal oscillator located within the CODEC  220  (FIG.  3 ). The rate control logic  250  detects each 100BASE-T Ethernet data packet received from the transceiver  212 . In the preferred embodiment, the rate control block  250  then checks each such 100BASE-T Ethernet data packet for errors utilizing the frame check sequence (FCS) appended to each 100BASE-T Ethernet packet and strips each 100BASE-T Ethernet data packet of its preamble and start-of-frame delimiter (the frame-check sequence FCS for each 100BASE-T Ethernet packet is preferably retained). The rate control logic  250  also converts each Ethernet data packet from nibbles to bytes. 
     The rate control logic  250  calculates the length of each detected 100BASE-T Ethernet data packet. The rate control logic  250  also determines whether the packet is too long, too short (a runt packet) or is misaligned. 
     The rate control logic  250  then temporarily stores the packets in rate buffers  252 . In the preferred embodiment, the bytes for each packet are clocked into the rate buffers  252  according a clock signal recovered from the data. The rate buffers  252  preferably include two first-in, first-out (FIFO) buffers having 16K entries, one for packets being transmitted and one for packets being received. The FIFO buffers each preferably provides sufficient storage for each entry so that additional information can be stored in the rate buffers  252  along with the byte of data. Such additional information preferably includes the data valid bit for each nibble and an indication of whether the nibble is payload data or overhead for the 100BASE-T Ethernet packets. For example, the overhead can include inter-packet gaps codes (e.g. one byte/octet of all zeros with associated data valid bits de-asserted), and start-of-packet codes. Assuming inter-packet gap codes are stored, preferably only one inter-packet gap code, representative of the minimum required inter-packet gap (e.g. of 0.96 μs), is stored in the rate buffers  252 . 
     The rate control logic  250  then records the previously determined length of the 100BASE-T Ethernet data packet in a length and status FIFO buffer  254 . In addition, the rate control logic  250  stores an indication of the status of the packet (e.g. too long, too short or misaligned) in the length and status buffer  254 . 
     The radio framer  228  is coupled to the MAC  222  and includes the OH/LM block  240  (FIG.  3 ), a packet synch/de-synch block  254 , a Reed-Solomon encoder/decoder (R-S codec)  258 , a framing block  260 , a pseudo-random number (PN) randomizer/de-randomizer block  262 , a differential encoder/decoder  264  and a constellation mapper  266 . 
     The packet synch/de-synch block  256  retrieves the stored 100BASE-T Ethernet data packets from the rate buffers  252  at an appropriate rate which depends, in part, upon the data transmission rate utilized for sending data over the wireless link  102 . In the preferred embodiment, removal of data from the rate buffers  252  for an Ethernet packet is not initiated until the packet has been completely stored. During periods when a complete packet is not available from the rate buffers  252 , then an inter-packet gap code is substituted by the packet synch/de-synch block  254 . 
     In the preferred embodiment of the present invention, the packet synch/de-synch block  256  reforms the 100BASE-T Ethernet data packets according to a reformed frame structure  300  for 100BASE-T Ethernet data packets illustrated in FIG.  5 . The reformed frame structure  300  includes a synch pattern field  302 , a length field  304 , a data field  306  and a frame check sequence (FCS) field  308 . 
     Recall that the rate control logic  250  ( FIG. 4 ) strips each 100BASE-T Ethernet data packet of its preamble and start-of-frame delimiter prior to storing the packet in the rate buffers  252 . Upon retrieving each packet from the rate buffers, the packet synch/de-synch block  256  adds a synch pattern in field  302  and a length value in field  304  to the packet. The length value is retrieved from the length and status buffer  254 . 
     In the preferred embodiment, finite state machines control the synch/de-synch block  256  so as to enable the retrieval of 100BASE-T Ethernet packets from the rate buffers  252  along with the length and status of each, at a appropriate frequency for forming radio frames  350  (FIG.  6 ). A store and forward technique is applied to 100BASE-T Ethernet packets which pass through the transmit portion of the rate buffers  252 . Thus, data packets to be transmitted across the wireless link  102  are completely received into the rate buffers  252  and stored therein prior to being formed into a radio frame  350 . A cut-through technique, however, is preferably applied to 100BASE-T data packets which pass through the receive portion of the rate buffers  252 . Thus, data packets received from the wireless link  102  are forwarded to the transceiver  212  ( FIG. 3 ) as they received without storing the entire data packet in the rate buffers  252 . 
     Table 1 shows the particular bit values for the synch pattern field  302  and for the length value field  304  according to the preferred embodiment of the present invention. 
     
       
         
           
               
               
               
             
               
                 TABLE 1 
               
             
            
               
                   
               
               
                 Synch Field 302 
                 Packet Length Field 304 
                   
               
            
           
           
               
               
               
               
               
               
               
               
               
            
               
                 octet 
                 octet 
                   
                   
                   
                 octet 
                   
                   
                   
               
               
                 1 
                 2 
                 octet 3 
                 octet 4 
                 octet 5 
                 1 
                 octet 2 
                 octet 3 
                 Bit 
               
               
                   
               
               
                 1 
                 1 
                 0 
                 1 
                 0 
                 G[11] 
                 G[7] 
                 G[3] 
                 7 
               
               
                 1 
                 1 
                 0 
                 1 
                 0 
                 G[10] 
                 G[6] 
                 G[2] 
                 6 
               
               
                 0 
                 0 
                 1 
                 0 
                 1 
                 G[9] 
                 G[5] 
                 G[1] 
                 5 
               
               
                 1 
                 1 
                 0 
                 1 
                 0 
                 G[8] 
                 G[4] 
                 G[0] 
                 4 
               
               
                 0 
                 0 
                 1 
                 0 
                 1 
                 0 
                 L[7] 
                 L[3] 
                 3 
               
               
                 1 
                 1 
                 0 
                 1 
                 0 
                 L[10] 
                 L[6] 
                 L[2] 
                 2 
               
               
                 1 
                 1 
                 0 
                 1 
                 0 
                 L[9] 
                 L[5] 
                 L[1] 
                 1 
               
               
                 0 
                 0 
                 1 
                 0 
                 1 
                 L[8] 
                 L[4] 
                 L[0] 
                 0 
               
               
                   
               
            
           
         
       
     
     As shown in Table 1, the synch pattern placed in the synch field  302  is preferably a five-octet (five-byte) pattern defined by a five-bit Willard code [11010]. Essentially, the Willard code is repeated for each octet, but is inverted for two of the five octets. The length value placed in the length field  304  is preferably an eleven-bit value L[10:0] which specifies the number of octets (bytes) of payload data contained in the data field  306 . Thus, the length value L[10:0] can vary for each packet depending upon the length of the data payload included in the 100BASE-T Ethernet packet. In the preferred embodiment, a twelve-bit Golay check sum G[11:0] for the length value is stored along with the length value in the length field  304 , as shown in Table 1. Because the length field  304  is preferably three octets (three bytes) a value of zero (0) is used a place holder between the length value L[10:0] and the Golay check sum G[11:0]. 
     Referring to  FIG. 5 , the data payload from the Ethernet packet is stored in the data field  306 . Note that 100BASE-T Ethernet data packets are conventionally of variable length. In particular, the data payload portion for a conventional 100BASE-T Ethernet packet can vary between 64 and 1518 octets (bytes). Thus, the length of the data field  304  can vary between 64 and 1518 bytes. 
     An important aspect of the reformation of the Ethernet data packets in the reformed frame structure  300  is the omission of the 1-bit data valid field for each nibble of the Ethernet packet. Rather, the nibbles are placed contiguously in the data field  306 . This omission of the data valid bits results in a significant savings in bandwidth required for transmitting the reformed packet frame  300  over the wireless link  102  in comparison to also transmitting the data valid bits over the wireless link  102 . The FCS sequence is retained for each Ethernet packet and placed in the FCS field  308 . 
     The packet synch/de-synch block  256  also receives link control data from the OH/LM  240  and for combining this link control data with the reformed packet frames  300  to be communicated over the link  102 . 
     The R-S codec  258  receives the reformed data packet frames  300  and link control commands from the packet synch/de-synch block  256  and performs Reed-Solomon (R-S) forward error correction coding. The R-S encoded data is then provided to the framing block  260  where the R-S encoded data is formatted according to radio frames  350  (FIG.  6 ). 
       FIG. 6  illustrates a radio frame  350  according to the present invention. The radio frame  350  includes a synch field  352  for synchronizing a receiver to the radio frame  350 , an auxiliary field  354  for network management and link control traffic which is received from the OH/LM  240  to be communicated over the auxiliary channel  102 B of the wireless link  102 , a data field  356 , and an R-S parity field  358 . The value placed in the synch field is preferably 47 hex. 
     In the preferred embodiment, radio frames  350  are continuously formed and transmitted across the wireless link  102  whether or not data from a complete Ethernet packet is queued in the rate buffers  252  ( FIG. 4 ) to be placed in reformed packet frames  400 . During periods when no reformed packet frames are available, the data field  356  of the current radio frame  350  is loaded with idle code (all zeros). Similarly, during periods when no network management commands are queued to be communicated via the auxiliary channel  102 B, then the auxiliary field  354  is loaded with idle code (all zeros). 
     Recall that reformed packet frames  300  have variable length according to the preferred embodiment of the present invention. The data field  356  of each radio frame  350 , however, preferably has a fixed length according to the preferred embodiment of the present invention. Accordingly, the R-S encoded data from the R-S codec  258  is placed contiguously in the data field  356  of each radio frame  350  such that reformed data frame  300  boundaries do not have a predefined relationship to radio frame  350  boundaries. For example, a reformed data frame  300  can span multiple radio frames  350 . Alternately, up to three complete smaller reformed data frames  300  can be included in a single radio frame  350 . Further, during idle periods between communication of reformed packets, an idle code is preferably transmitted as a place holder within the data field  356  of each radio frame  350  to meet the timing requirements needed to synchronize 100BASE-T Ethernet data packets. 
     As radio frames  350  are formed, multiples of the radio frames  350  are combined to form a radio “super frame”  380  (FIG.  7 ).  FIG. 7  illustrates a radio super frame  380  according to the present invention. In the preferred embodiment, each radio super frame  380  includes 16 consecutive radio frames  350  (FIG.  6 ). For the first radio frame  382  of the super frame  380 , the value placed in the synch field  352  is inverted (changed to B8 hex). In the second through sixteenth radio frames  384 , however, the value placed in the synch field  352  remains unchanged. The value placed in the synch field  352  of the first radio frame  386  for a next radio super frame  388 , is also inverted. This inversion of the synch value for the first radio frame  350  of each radio super frame  380  allows the radio super frames  500  to be detected after reception. 
     The radio super frame  380  is provided to the PN randomizer/de-randomizer  262 . The PN randomizer/de-randomizer  262  performs quadrature amplitude modulation (QAM) scrambling on the entire radio super frame  380  except for the inverted synch values placed in the first synch field  352  of each super frame  380 . By disabling the PN randomized de-randomizer  262  for the inverted synch values, the scrambled super frame  380  can be detected upon reception. In preferred embodiment, the scrambling operation maps each octet (byte) of the radio super frame  380  (other than the inverted synch values) to a two successive four-bit symbols utilizing a 13th order polynomial, as shown by the schematic block diagram of the PN randomizer/de-randomizer  262  according to the preferred embodiment of the present invention. 
     Referring to  FIG. 8 , each octet of the radio super frame  380  (other than the inverted synch values) is divided into two successive four-bit portions B[3:0] which are applied to the correspondingly labelled inputs illustrated in FIG.  8 . These inputs correspond to in-phase and quadrature (I&amp;Q) symbol components I 1 ,  10 , Q 1 , Q 0 . A feedback shift register  400  generates the specified 13th order polynomial. Contents of selected memory cells of the feedback shift register  400  are exclusive-OR&#39;d by logical exclusive-OR blocks  402 ,  404 ,  406 , and  408  with each four bit portion b[3:0] of the radio frame. Outputs of the exclusive-OR blocks  402 ,  404 ,  406  and  408  form I&amp;Q symbol components I 1 ′, I 0 ′, Q 1 ′, Q 0 ′. 
     The symbol components I 1 ′, I 0 ′, Q 1 ′, Q 0 ′, are applied to the differential encoder/decoder block  264  (FIG.  4 ).  FIG. 9  illustrates a schematic block diagram of a differential encoder  264 A included in the differential encoder/decoder block  264  ( FIG. 4 ) and characteristic equations according to the present invention. The encoder  264 A forms signal components I 1 ″, I 0 ″, Q 1 ″, Q 0 ″. In the preferred embodiment, the encoder  264 A is implemented by an appropriately preconditioned look-up table. 
     The differential encoder encodes the scrambled symbols from the PN randomizer/de-randomizer  262  such that quantum-phase differencing of the transmitted symbols according to modulo-π/2 recovers the original un-encoded data, independent of which of the four possible quantum-phase alignments is selected in the decoder  264 B illustrated in FIG.  10 . 
       FIG. 10  illustrates a schematic block diagram of the differential decoder  264 B included in the differential encoder/decoder  264  ( FIG. 4 ) and characteristic equations according to the present invention. In the preferred embodiment, the differential decoder  264 B is implemented by an appropriately preconditioned look-up table. 
     The symbol components I 1 ″, I 0 ″, Q 1 ″, Q 0 ′,′ formed by the encoder  264 A are applied to the constellation mapper  266  (FIG.  4 ). The constellation mapper  266  maps four-bit portions of the radio frame  350  to sixteen different symbols, as shown in  FIG. 11 , according to quadrature amplitude modulation techniques (16 QAM). 
       FIG. 11  illustrates a mapping constellation for the constellation mapper  266  ( FIG. 4 ) according to the present invention. In the preferred embodiment, this constellation is defined by a standard adopted by the Digital Audio Visual Counsel (DAVIC). The input symbol components I 1 ″, I 0 ″, Q 1 ″, Q 0 ″, are mapped to the output symbol components Is, Im, Qs, Qm, as shown in Table 2. The mapped symbols are then provided by the constellation mapper  266  ( FIG. 4 ) to the transmit modulator  242  (FIG.  3 ). 
     
       
         
           
               
               
               
             
               
                   
                 TABLE 2 
               
               
                   
                   
               
               
                   
                 I1″, I0″, Q1″, Q0″ 
                 Is, Im, Qs, Qm 
               
               
                   
                 (input) 
                 (output)  
               
               
                   
                   
               
             
            
               
                   
                 0000 
                 1010 
               
               
                   
                 0001 
                 1110 
               
               
                   
                 0010 
                 1001 
               
               
                   
                 0011 
                 1000 
               
               
                   
                 0100 
                 1011 
               
               
                   
                 0101 
                 1111 
               
               
                   
                 0110 
                 1101 
               
               
                   
                 0111 
                 1100 
               
               
                   
                 1000 
                 0110 
               
               
                   
                 1001 
                 0111 
               
               
                   
                 1010 
                 0101 
               
               
                   
                 1011 
                 0001 
               
               
                   
                 1100 
                 0010 
               
               
                   
                 1101 
                 0011 
               
               
                   
                 1110 
                 0100 
               
               
                   
                 1111 
                 0000 
               
               
                   
                   
               
            
           
         
       
     
     Received radio super frames  380  ( FIG. 7 ) are provided to the constellation mapper  266  ( FIG. 4 ) from the receive de-modulator  244  (FIG.  3 ). During radio super frame  380  reception, each radio super frame  380  is converted back from the symbols Is, Im, Qs, Qm, into the symbol components I 1 ″, I 0 ″, Q 1 ″, Q 0 ′, by the constellation mapper  262  performing a reverse of the mapping operation according to the relationships shown in Table 2. 
     In the preferred embodiment of the present invention, the QAM format can be altered dynamically under control of the microprocessor  230  based upon rain fade or interference detected through bit error rates (BER) or upon receiving a link control command. For example the QAM format can be dynamically altered from 16 QAM to 4 QAM. Alternately, the QAM format can be changed from 16 QAM to 4 QAM and with the application of spectrum spreading. As a result, the data transmission bit rate falls, however, the error rate would be expected to fall also. Conversely, the QAM format can be dynamically altered from 16 QAM to 64 QAM which results in a higher data transmission bit rate. 
     Then, the differential decoder  264 B ( FIG. 10 ) decodes the symbol components I 1 ″, I 0 ″, Q 1 ″, Q 0 ″, into the symbol components I 1 ′, I 0 ′, Q 1 ′, Q 0 ′. Next, the radio super frame  380  is detected by the inverted synch values for the first radio frame of each super frame  380 . The symbol components I 1 ′, I 0 ′, Q 1 ′, Q 0 ′, are then provided to the PN randomizer/de-randomizer  262  ( FIG. 4 ) which converts them to the back into the original two successive four-bit portions b[3:0] for each octet of each radio frame  350  ( FIG. 6 ) of the radio super frame  380  (FIG.  7 ). 
     The radio frame  350  is then synchronized to the radio super frame  380  by detecting the non-inverted synch value in the field  352  ( FIG. 6 ) for each radio frame  350 . Forward error correction is performed by the R-S codec  258  (FIG.  4 ). For each radio frame  350  having an error which is uncorrectable by the R-S codec  258 , the R-S codec  258  provides an indication, preferably by setting a flag, which is stored in the rate buffers  252  along with the affected packet data. For each Ethernet packet formed by the rate control logic  250  which is affected by such an uncorrected error as flagged by the R-S codec  258  (FIG.  4 ), the transmit error signal TX ER provided to the transceiver  212  ( FIG. 3 ) via the MII interface, is asserted. A link-layer response can then be applied to cause the packet to be resent. 
     The reformed data frames  300  are then passed from the R-S codec to the packet synch/de-synch block  256 . In the packet synch/de-synch block  256 , the reformed data frames  300  (FIG.  5 ), as well as network management and control data, are detected and extracted from the radio frame  350  structure. For the reformed data frames  300 , this is accomplished by a windowed search technique which utilizes matched filter correlation. The search technique is utilized to locate the five-octet synch value in the synch field  302  (based on the Willard code) for each reformed data frame  300 . When packet synchronization is maintained, the search window preferably encompasses only inter-packet gap periods (when the data field  356  of the radio frame  350  contains the idle code). During periods when packet synchronization is not detected, however, the search window is expanded to encompass the entire packet. Once synchronization is obtained, the window is again reduced. 
     Correlation searching is performed by the packet synch/de-synch block  256  utilizing a matched filter which performs correlation on an octet-by-octet basis. Accumulation by addition is performed on 40 bits of data at a time (5 bytes), as octets slide through the matched filter. The accumulated value is compared to a predetermined threshold for each octet. When the threshold is exceeded, the start of a reformed data frame  300  is indicated. 
     Once a synch value is detected, the length value for the packet and Golay code are read from the length field  304 . The length value is verified utilizing the Golay code. If necessary, the length value is corrected utilizing the Golay code. If the length value is corrupted and uncorrectable, however, the packet is disregarded while searching for a next synch value continues. 
     Assuming the length value is correct or correctable, the reformed data frame  300  is loaded to the rate buffers  252  by the packet synch/de-synch block  256  in eight-bit portions (bytes) for processing into a 100BASE-T Ethernet packet. From the length value, the data valid bit for each byte is also re-generated and stored in the rate buffers  252 . A single inter-packet gap code is stored in the rate buffers  252  to separate each packet. Network management and link control data from the auxiliary field  354  of each received radio frame  350  is provided to the microprocessor  230  ( FIG. 3 ) through time-division de-multiplexing. 
     Then, searching for a next synch value is disabled until the end of the reformed data frame  300 , as indicated by the correct or corrected length value. 
     Reformed data frames  300  are retrieved from the packet buffer  252  under control of the rate control logic  250  and returned to conventional 100BASE-T Ethernet format for the MII interface with the transceiver  212  (FIG.  3 ). This is accomplished by restoring the preamble and start-of-frame delimiter for each 100BASE-T Ethernet packet. Then, the conventional 100BASE-T Ethernet packets are provided to the 100BASE-T transceiver  212  ( FIG. 3 ) at a rate appropriate to the 100BASE-T transceiver  212 . The 100BASE-T transceiver  212  then communicates the packets to the TFU (FIGS.  1  and  3 ). In the preferred embodiment, the rate control logic  250  includes a finite state machine for performing the function of retrieving the Ethernet packets from the rate buffers  252  and providing them to the 100BASE-T transceiver  212 . Thus, the rate control logic  250  synchronizes the packets to a clock signal utilized for communication of the 100BASE-T data packets with the locally generated clock signal which is utilized for forming and communicating radio frames  350  (FIG.  6 ). 
     Referring to  FIGS. 3 and 4 , in the preferred embodiment, the transmit modulator  242  receives four-bit symbols from the constellation mapper  266  of the radio framer  228  in the CODEC  220  at 27.5 Mbaud. Each symbol is converted to a complex in-phase and quadrature (I&amp;Q) voltage and, then, pulse-shaped utilizing a square-root cosine filter in the transmit  11  modulator  242 . Finally, the symbol modulates a 490 MHz intermediate frequency (IF) output signal. The output level of the signal formed by the transmit modulator  242  is selectively adjustable over a continuous range under control of the micro-processor  230 . Adjustments in the output level are preferably made in response to detected rain fade, detected interference or in response to a link control command. The modulated IF signal formed by the transmit modulator  242  is supplied to the microwave module  246 . 
     The receive demodulator  244  preferably includes a 0-dB/20-dB IF attenuator in the receive path which is selectable under control of the micro-processor  230  depending upon the range of the link  102 . Typically, this attenuator is set for 0-dB. For link ranges of less than approximately 50 meters, however, the attenuator is preferably set for 20-dB attenuation. The receive demodulator  244  performs adaptive slope equalization to minimize effects of distortion caused by transmission over the link  102 . Further, the receive demodulator  244  preferably also includes an adaptive time-domain equalizer to perform symbol synchronization, and a matched-filter square-root-raised-cosine process is applied to minimize inter-symbol interference. 
       FIG. 12  illustrates a schematic block diagram of an Ethernet-to-radio frame synchronizing portion  268  of the rate control logic  250  ( FIG. 4 ) and transmit buffer  252 A according to the present invention. The transmit buffer  252 A forms a portion of the rate buffers  252  (FIG.  4 ). 100BASE-T Fast Ethernet packets and a receive data valid signal RXDV are received into the transmit buffer  252 A from the transceiver  212 , as explained above in reference to FIG.  4 . In addition, a clock signal at 25 MHz is derived from the incoming data packet and utilized for clocking the incoming Ethernet data packets into the transmit buffer  252 A. 
     The receive data valid signal RXDV is provided to a first input of an arbitration logic block  270 . In response to a complete Ethernet packet being stored in the transmit buffer  252 A, as indicated by the data valid signal RXDV, the arbitration logic  270  instructs a packet counter  272  to increment a count by one. As Ethernet packets are retrieved from the transmit buffer  252 A, a delayed data valid signal is also retrieved from the transmit buffer  252 A. This delayed data valid signal is applied to a second input to the arbitration logic block  270 . In response to a complete Ethernet data packet being removed from the transmit buffers  252 A as it is supplied to the synch/de-synch logic block  256 , as indicated by the delayed data valid signal, the arbitration logic block  282  instructs the packet counter  272  to decrement the count by one. Thus, the packet counter  272  maintains a current count of complete Ethernet data packets in the transmit buffer  252 A. 
     This count is provided by the packet counter  272  to a threshold compare block  274 . The threshold compare block  274  notifies a read packet state machine  276  when a sufficient number of complete Ethernet packets are stored in the transmit buffer  252 A to initiate retrieval of the packets from the transmit buffer  252 A. In the preferred embodiment, only one complete Ethernet packet need be stored in the transmit buffer  252 A to initiate the read packet state machine  276  to retrieve the packet. Once initiated to retrieve a packet, the read state machine  276  activates a first input to a logic AND gate  278 . A second input to the logic AND gate  278  receives a read frame enable signal from the synch/de-synch logic  256  (FIG.  4 ). This read frame enable signal is activated when the synch/de-synch logic  256  is ready to receive the Ethernet packet data for insertion into a radio frame  350  (FIG.  6 ). 
     An output of the logic AND gate  278  is coupled to a read input of the transmit buffer  252 A for retrieving the packet from the transmit buffer  252 A. As it is being retrieved, the packet is provided to the synch/de-synch logic  256 . 
     An important aspect of the Ethernet-to-radio frame synchronizing portion  268  of the rate control logic  250  ( FIG. 4 ) is that it synchronizes the receiving of Ethernet data packets according to 25 MHz clock signal which is asynchronous with the locally generated clock signal. Note that the 25 MHz clock signal is derived from the incoming Ethernet data packets and is applied to the transmit buffer  252 A for storing the packet data while the locally generated clock signal is applied to the transmit buffer  252 A for retrieving Ethernet packet data from the transmit buffer. Thus, the arbitration logic, packet counter  272  and threshold compare logic  274  operate according to the derived 25 MHz clock, while the read packet state machine  276  and the radio framer  228  ( FIG. 4 ) operate according to the locally generated clock. 
     In the preferred embodiment, the locally generated clock signal is 27.5 MHz. Because the locally generated clock signal is at a higher rate than the clock signal derived from the incoming Ethernet packets, in absence of the synchronizing portion  268  of the rate control logic  250 , it would be possible for the transmit buffer  252 A to become empty while an Ethernet packet is still being received into the transmit buffer  252 A. Thus, the synchronizing portion  268  of the rate control logic  250  avoids this potential problem. 
     Assuming that an adaptive counter measure is employed which reduces the rate at which radio frames  350  ( FIG. 6 ) are formed, this also reduces the rate at which the data from Ethernet packets is retrieved from the transmit buffer  252 A. Assuming this rate is below 25 MHz (e.g. 13.75 MHz), then a complete packet need not be stored in the transmit buffer  252 A prior to initiating retrieval of such a packet. In the preferred embodiment, under such circumstances, cut-through is employed wherein the incoming Ethernet data packet is supplied to the radio framer  228  ( FIG. 4 ) prior to the complete packet being received into the transmit buffer  252 A. 
       FIG. 13  illustrates a schematic block diagram of a radio frame-to-Ethernet synchronizing portion  280  of the rate control logic  250  ( FIG. 4 ) according to the present invention. The receive buffer  252 B forms a portion of the rate buffers  252  (FIG.  4 ). 100BASE-T Fast Ethernet packets recovered from radio frames  350  (FIG.  6 ), and a recovered receive data valid signal RXDV, are received into the receive buffer  252 B from the synch/de-synch block  256 , as explained above in reference to FIG.  4 . The internally generated clock to signal at 27.5 MHz is synchronous with the radio frames  350  ( FIG. 6 ) and utilized for clocking the incoming Ethernet data packets into the receive buffer  252 B. Ethernet data packets stored in the receive buffer  252 B are retrieved and provided to the transceiver  212  ( FIG. 3 ) according to a 25 MHz clock. 
     If no spectrum spreading is employed for data communicated via the link  102 , then the clock signal utilized for clocking data into the receive buffer  252 B preferably operates at 27.5 MHz. Because the clock signal utilized for retrieving data from the receive buffer  252 B preferably operates at 25 MHz, there is no possibility that the receive buffer  252 B will become empty while an Ethernet packet is still being received into the receive buffer  252 B. 
     However, in the event that spectrum spreading is employed for data communicated via the link  102 , however, the clock signal applied to the receive buffer  252 B can operate at a lower frequency (e.g. 13.75 MHz), that is synchronous with the internally generated 27.5 MHz clock signal. In which case, it would be possible for the receive buffer  252 B to become empty while an Ethernet packet is still being received into the receive buffer  252 B. Thus, the synchronizing portion  280  of the rate control logic  250  avoids this potential problem, as explained below. 
     The recovered receive data valid signal is provided by the synch/de-synch block  256  ( FIG. 4 ) to a first input of an arbitration logic block  282  and to a read packet state machine  288 . In response to a complete Ethernet packet being stored in the receive buffer  252 B, as indicated by the recovered data valid signal, the arbitration logic  282  instructs a packet counter  284  to increment a count by one. As Ethernet packets are retrieved from the receive buffer  252 B, a data valid signal RXDV is also retrieved from the receive buffer  252 B. This data valid signal RXDV is utilized by the transceiver  212  ( FIG. 3 ) and applied to a second input to the arbitration logic block  282 . In response to a complete Ethernet data packet being removed from the receive buffer  252 B, and supplied to the transceiver  212  (FIG.  3 ), as indicated by the data valid signal RXDV, the arbitration logic block  282  instructs the packet counter  284  to decrement the count by one. Thus, the packet counter  284  maintains a current count of complete Ethernet data packets in the receive buffer  252 B. 
     This count is provided by the packet counter  284  to a threshold compare block  286 . The threshold compare block  286  notifies a read packet state machine  288  when a sufficient number of complete Ethernet packets are stored in the receive buffer  252 B to initiate retrieval of the packets from the receive buffer  252 B. In the preferred embodiment, only one complete Ethernet packet need be stored in the receive buffer  252 B to initiate the read packet state machine  288  to retrieve the packet. Once initiated to retrieve a packet, the read state machine  288  activates a first input to a logic AND gate  290 . A second input to the logic AND gate  290  receives a LAN read clock enable signal from the transceiver  212  (FIG.  3 ). This LAN read clock enable signal is activated when the transceiver  212  is ready to receive the Ethernet packet data for communication to the TFU  106  (FIG.  1 ). 
     An output of the logic AND gate  290  is coupled to a read input of the receive buffer  252 B for retrieving the packet from the receive buffer  252 B. As it is being retrieved, the packet is provided to the transceiver  212 . Accordingly, this aspect of the present invention prevents the receive buffer  252 B from being emptied while a packet is being provided from the receive buffer  252 B to the transceiver  212  (FIG.  3 ). 
     A first alternate approach for avoiding overflow in the receive buffer  252 B of the terminal  100  during periods when data is being communicated over the wireless link  102  according to maximum transmission rates can be implemented when an Ethernet data source (e.g. a terminal in the LAN  128 ′) is operating at a slightly higher rate than the reference clock utilized for removing data from the receive buffer  252 B. This approach includes monitoring the current depth of the receive buffer  252 B, and as the amount of occupied storage space increases, then the transmission rate of the Ethernet data source is adjusted upward utilizing a voltage controlled oscillator. As the amount of occupied storage space decreases, then the transmission rate of the transceiver  212  is adjusted downward. When the buffer is nearly empty, the transmission rate is set to the nominal level of 25 Mhz. Both the originating and local frequency references must be within 100 parts per million high or low of the IEEE 802.3 Ethernet specified 25 MHz. 
     A second alternate approach for avoiding overflow in the receive buffer  252 B of the terminal  100  during periods when data is being communicated over the wireless link  102  according to maximum transmission rates, involves reducing the minimum inter-packet gap utilized for forwarding packets removed from the receive buffer  252 B. For example, rather than utilizing 12 byte-times to represent the inter-packet gap, the inter-packet can be represented by 11 byte-times. This may result in a violation of the IEEE 802.3 standard for the minimum inter-packet gap, however, this result is expected to be more desirable than the loss of packet data should the receive buffer  252 B overflow. 
     A third alternate approach for avoiding overflow in the receive buffer  252 B of the terminal  100  during periods when data is being communicated over the wireless link  102  according to maximum transmission rates, is for the microprocessor  230  of the terminal  100  to send a link control command to the terminal  100 ′. This link control command provides a pause packet to the layer-two switch  600 ′ (the layer-two switch  600 ′ and associated packet buffers  602 ′ are not shown, however, because the terminal  100 ′ is identical to the terminal  100 , it will be understood that the layer-two switch  600  and packet buffers  602  illustrated in  FIG. 16  have identical counter-parts in the terminal  100 ′, referred to herein as  600 ′ and  602 ′). The pause packet causes the switch  600 ′ to temporarily store packets in its associated packet buffers  602 ′ rather than sending such packets to the receive buffer  252 B. 
       FIG. 14  illustrates a schematic block diagram of the microwave module (MWM)  246  ( FIG. 3 ) and microwave antenna  508  according to the present invention. The MWM module  246  constitutes a wireless transceiver for implementing wireless communication over the link  102  (FIG.  1 ). The MWM  246  includes a transmit up-converter (TX-U/C)  500  coupled to receive signals from the transmit modulator  242 . The TX U/C  500  up-converts 490 MHz IF signals received from the transmit modulator  242  to microwave frequency for transmission over the link  102 . In the preferred embodiment, the frequency of transmission over the link  102  is selectable under control of the micro-processor  230  in 12.5 MHz steps across two adjacent microwave bands (e.g. 38.6-39.2 GHz and 39.3-40.0 GHz). 
     A transmit power amplifier (TX-P/A)  502  coupled to the transmit up-converter  500  amplifies the microwave signals provided by the transmit up-converter  500  to an appropriate level. In the preferred embodiment, the transmit power amplifier  502  has a 1-dB compression point at about 17 dBm. The nominal power is preferably set to 11 dBm, however, the transmit power is selectively controllable by the micro-processor  230  in response to detected rain fade, detected interference or in response to a link control command. 
     A transmit sub-band filter  504  coupled to the output of the transmit power amplifier  502  filters unwanted frequencies from the microwave signal to be transmitted over the link  102 . The microwave module  246  includes a di-plexer  506  coupled to the transmit sub-band filter  504 . The di-plexer  506  couples the microwave module  246  to the microwave antenna  508  for full-duplex communication over the link  102  by the microwave module  246 . The antenna  508  transmits microwave signals over the link  102  and receives microwave signals from the link  102 . 
     A microwave signal received from the link  102  by the antenna  508  is provided to a receive sub-band filter  510  via the di-plexer  506 . The receive sub-band filter  510  filters unwanted frequencies from the received signal and provides a filtered signal to a low noise amplifier (LNA)  512 . Then, the received signal is down-converted, preferably to 150 MHz IF by a receive down-converter (RX D/C)  514 . It will be apparent, however, that a frequency other than 150 MHz can be selected. An intermediate frequency automatic gain control (IF AGC) circuit  516  adjusts the level of the down-converted signal to a predetermined level. An output formed by the IF AGC  516  circuit  514  is provided to the receive demodulator  244 . 
     According to the preferred embodiment of the present invention, a microwave frequency synthesizer  518  included in the microwave module  246  is locked to a precision crystal reference signal and is digitally controlled by the microprocessor  230  ( FIG. 3 ) with a 12.5 Mhz step capability. Two outputs of the frequency synthesizer  516  are each locked to the same crystal reference signal and provided to the transmit up-converter  500  and to the receive down-converter  514  for performing up-conversion and down-conversion, respectively. 
       FIG. 15  illustrates a perspective view of the microwave antenna  508  and a housing  550  for the outdoor unit  104  ( FIGS. 1 and 3 ) according to the present invention. The housing  550  protects the ODU  104  from environmental conditions, such a rain, snow and sunlight, which can be encountered on roof-tops where the ODU  104  is typically positioned. The housing  550  includes a flange  552  for attaching the antenna  508  and cooling fins  554  for dissipating heat generated by the electrical circuits of the ODU  104 . A cable  556  which is preferably weather-resistant and electrically shielded, extends between, and electrically connects, the ODU  104  to the TFU  106  (FIGS.  1  and  3 ). Thus, the cable  556  includes each of the cables  108 ,  110 ,  112  and  114  (FIGS.  1  and  3 ). 
       FIG. 16  illustrates a schematic block diagram of an alternate embodiment of the digital signal processing MAC  222 ′ and radio framer  228 ′ according to the present invention. Elements illustrated in  FIG. 16  having a one-to-one functional correspondence with elements illustrated in  FIG. 4  are given the same reference numeral, but are distinguished by the reference numeral being primed. In one respect, the arrangement illustrated in  FIG. 16  differs from that illustrated in  FIG. 4  in that a layer-two switch  600  and associated packet buffer  602  are added. 
     According to the embodiment of the MAC  222 ′ illustrated in  FIG. 16 , the Ethernet switch  600  is coupled to the transceivers  212 ,  214  ( FIG. 3 ) and to packet buffers  602 . The packet buffers  602  provide a temporary storage for packets while being directed through the switch  600 . The switch  600  is also coupled to the microprocessor  230  via an interface  604  and to the rate control logic  250 ′ via an interface  606 . The switch  600  can be a conventional layer-two Ethernet network switch having a 100BASE-T port coupled to the cable  108  and a 10BASE-T port coupled to the cable  110 . In the preferred embodiment, the switch  600  also includes a 10BASE-T port which is coupled to the microprocessor  230  via the interface  604  and a 100BASE-T MII port which is coupled to the rate control logic  250 ′ via the interface  606 . 
     Network management and link control traffic in the form of Ethernet packets received by the switch  600  from the transceiver  212 , the transceiver  214 , or the interface  606 , and which include the MAC address of the microprocessor  230  as a destination address are directed to the microprocessor  230  via the interface  604  by the switch  600 . Similarly, the microprocessor  230  sends Ethernet packets to the rate control logic  250 ′ via the switch  600  for communication over the link  102  and to the transceivers  212 ,  214  via the switch  600  for communication with the router or switch  116  (FIG.  1 ). 
     In the preferred embodiment, the switch  600  implements a flow control technique in accordance with IEEE 802.3×. According to the present invention, the flow control technique is selectively initiated by the rate control logic  250 ′ sending a pause packet to the switch  600  via the interface  606 . Each pause packet includes an indication of a how long the flow control technique is to remain active. In response to receiving the pause packet, the switch  600  does not provide packets which are received from the transceivers  212 ,  214  or from the interface  604  to the interface  606 . Rather, when the flow control technique is active, the switch  600  temporarily queues such packets by storing them in the packet buffers  602 . The pause signal can preferably be initiated for several hundred milli-seconds while packets are received from the transceivers  212 ,  214  or from the interface  604  without loss of any such packets. When the indicated time expires, the flow control technique is deactivated. Upon deactivation of the flow control technique, the switch  600  retrieves the queued packets from the packet buffers  602  and provides them to the rate control logic  250 ′ via the interface  606 . 
     The rate control logic  250 ′ sends a pause packet with an indicated activation period in response to a halt control signal received from the rate buffers  252 ′ via a signal line  608 . When activated, the halt signal provided via the signal line  608  indicates that the rate buffers  252 ′ are nearly full. The indicated activation period included in the pause packet is appropriate to allow sufficient data to be removed from the rate buffers  252 ′ and communicated over the link  102  via radio frames  350 . 
     As an example of operation of the MAC  222 ′, assume that rain fade or interference is detected in the link  102  by an increase in a measured bit error rate (BER). In response, a link control command is issued by the microprocessor  230  which causes the data rate for the link  102  to be reduced. As a result of this lower data rate for the link  102 , radio frames  350  are formed less quickly and, thus, data is removed from the rate buffers  252 ′ at a lower rate. If the reduced data rate results in the rate buffers  252 ′ becoming nearly full, the rate buffers  252 ′ activate the halt signal via the signal line  608 . In response, the rate control logic  250 ′ sends a pause packet to the switch  600 . Then, while flow control is active, packets received from the transceiver  212 ,  214  or the interface  604  for communication over the link  102  are temporarily queued in the packet buffers  602 . Accordingly, the MAC  222 ′ according to the present invention implements a flow control technique for adapting a current rate of data transmission over the link  102  to a rate at which Ethernet packets are received by the MAC  222 ′ from the TFU  106  (FIGS.  1  and  3 ), without loss of the Ethernet packets. 
     In addition, the embodiment of the MAC  222 ′ illustrated in  FIG. 16  includes an encryption/decryption block  612  coupled between the rate control logic  250 ′ and the rate buffers  252 ′. Accordingly, for packets to be transmitted over the link  102 , the encryption/decryption block  612  encrypts the Ethernet data packets prior to temporarily storing the data packet in the rate buffers  252 ′. Conversely, Ethernet packets received from the link  102  are decrypted by the encryption/decryption block  612  before being provided to the switch  600 . A memory buffer  614  coupled to the encryption/decryption block  612  provides a temporary memory store for use during encryption/decryption of the Ethernet packets. An encryption start control signal line  610  coupled between the encryption/decryption block  612  and the length/status buffer  254 ′ is utilized by the encryption/decryption block  612  to instruct the length/status buffer  254 ′ to provide an encryption tag and sequence number to the packet synch/de-synch block  256 ′. This arrangement which includes the encryption/decryption block  612  provides an advantage over the arrangement illustrated in  FIG. 4  in that data security is enhanced. 
       FIG. 17  illustrates a frame structure  700  for reformed 100BASE-T Ethernet data packets formed by the MAC  222 ′ and radio framer  228 ′ illustrated in FIG.  16 . When the packet is removed from the rate buffers  252 ′ and reformed for insertion to a radio frame  350  (FIG.  6 ), the encryption tag and sequence number provided by the length/status buffer  254 ′ ( FIG. 16 ) are appended to the reformed packet frame  700  in an encryption tag field  702  and a sequence number field  704 , respectively. The encryption tag indicates an appropriate key box utilized to encrypt the data while the sequence number provides synchronization information to the terminal which receives the reformed Ethernet data frame  700  from the wireless link  102 . Fields of the reformed packet frame  700  illustrated in  FIG. 17  which have one-to-one functional correspondence with those illustrated in  FIG. 5  are given the same reference numeral primed. 
     Referring to  FIG. 16 , this arrangement also differs from that illustrated in  FIG. 4  in that the PN randomizer/de-randomizer  262  and the differential encoder/decoder  264  are omitted and, instead, an adaptive countermeasures block  616  takes their place. The adaptive countermeasures block  616  responds to a rate change command issued by the microprocessor  230  by changing the rate at which data is communicated over the wireless link  102 . The rate at which data is communicated can be in response to a detected increase in BER due to rain fade or can be to reduce interference with nearby wireless links, such as to reduce interference between subscriber terminals in a point-to-multipoint network. 
       FIG. 18  illustrates a schematic block diagram of the adaptive countermeasures block  616  according to the present invention. A multiplexer  750  is coupled to the framing block  260 ′ ( FIG. 16 ) for communicating radio super frames  380  ( FIG. 7 ) with the framing block  260 ′. A first PN randomizer/de-randomizer  262 A′, a second PN randomizer/de-randomizer  262 B′ and a first differential encoder/de-coder  264 A′ are each coupled to receive selected radio super frames  380  from the multiplexer  750  depending upon conditioning of the multiplexer  750  by the rate change control signal. 
     In the preferred embodiment, the PN randomizer/de-randomizers  262 A′,  262 B,  262 C′ perform scrambling on the radio super frames  380  in an identical manner to the PN randomizer/de-randomizer  262  illustrated in  FIGS. 4 and 8 . Super frames  380  scrambled by the PN randomizer/de-randomizer  262 A′ are provided to a second differential encoder/decoder  264 B′. The differential encoder/decoders  264 A′,  264 B′ and  264 C′ preferably perform encoding and decoding in an identical mariner to the differential encoder/de-coder  264  illustrated in FIG.  4 . Then, super frames  380  encoded by the second encoder/decoder  264 B′ are provided to a QAM constellation mapper  266 ′. The QAM constellation mapper  266 ′ preferably performs QAM constellation mapping in an identical manner to the QAM constellation mapper  266  illustrated in  FIGS. 4 and 16 . A multiplexer  756  is coupled to the QAM constellation mapper  266 ′ for communicating encoded radio super frames  380  with the Rx demodulator  244  ( FIG. 3 ) and Tx modulator  242  (FIG.  3 ). Thus, when a first path through the PN randomizer/de-randomizer  262 A′, the second differential encoder/decoder  264 B′ and QAM constellation mapper  266 ′ is selected, radio super frames  380  are conditioned identically for transmission and reception as when passing through the PN randomizer/de-randomizer  262 , differential encoder/decoder  264  and QAM constellation mapper illustrated in FIG.  4 . In the preferred embodiment, the first path conditions the radio super frames  380  according to 16 QAM. 
     The third differential encoder/decoder  264 C′ is coupled to the PN randomizer/de-randomizer  262 B′ and to a quadrature phase-shift (QPSK) constellation mapper  752 A. The QPSK constellation mapper  752 A maps portions of the radio frame  350  to QPSK symbols according to quadrature phase-shift keying techniques (QPSK). Super frames  380  are communicated between the QPSK constellation mapper  752 A and the multiplexer  756 . Thus, when a second path through the PN randomizer/de-randomizer  262 B′, the differential encoder/decoder  264 C′ and QPSK constellation mapper  752 A is selected, radio super frames  380  are conditioned for transmission and reception according to QPSK format. 
     A second QPSK constellation mapper  752 B is coupled to the differential encoder/decoder  264 A′ and to a PN randomizer/de-randomizer  262 C′. The QPSK constellation mapper  752 B maps portions of the radio frame  350  to QPSK symbols according to quadrature phase-shift keying techniques (QPSK) identically to the QPSK constellation mapper  752 A. Super frames  380  are communicated between the QPSK constellation mapper  752 B and the multiplexer  756 . Thus, when a third path through the differential encoder/decoder  264 A′, QPSK constellation mapper  752 B and PN randomizer/de-randomizer  262 C′, is selected, radio super frames  380  are conditioned for transmission and reception according to QPSK format with spectrum spreading. Upon reception, super frames  380  routed through this third path are appropriately de-spreaded and decoded for communication with the framing block  260 ′. 
     So that the radio super frames  380  are properly received by a receiving terminal (e.g. the terminal  100  illustrated in FIG.  1 ), it is important the appropriate path is selected through the adaptive countermeasures block  616  for each radio super frame  380 . This can be accomplished by the transmitting terminal  100  notifying the receiving terminal  100 ′ of the manner and rate at which the transmitting terminal  100  is transmitting radio super frames  380 . 
       FIG. 19  illustrates a chart of received signal level vs. time as a result of rain fade. Refer to  FIGS. 1 and 20  and assume that the terminal  100  is receiving data from the terminal  100 ′ via the wireless link  102 . When rain occurs between the terminals  100  and  100 ′, the level of the microwave carrier signal received by the terminal  100 , the received signal level (RSL) falls over time as the rain increases over time. Thus, depending upon the weather conditions, the RSL can eventually fall from a normal level to below threshold levels set at L1-L8. When the RSL is above the threshold level L1, this represents an insubstantial level of rain fade. However, when the RSL is below the threshold level L8, this represents a extreme level of rain fade. The threshold levels L2-L7 represent progressively increasing levels of rain fade between the extremes represented by L1 and L8. The rate at which the RSL falls (the measured slope) can also vary depending upon the weather conditions. Similarly, as the weather conditions improve, the RSL can return the normal level. In response to rain fade, the bit error rate (BER) tends to rise. Thus, the adaptive countermeasures implemented by the present invention can detect the presence of rain fade by measuring the RSL or the BER. 
     In addition, the BER tends to rise in response to interference between nearby wireless links. A significant difference between rain fade and interference, however, is that in the event of interference, the RSL can remain at a normal level while the BER rises. Accordingly, the adaptive countermeasures implemented by the present invention can detect the effects of interference by measuring the BER. 
     Accordingly, in the preferred embodiment, the present invention responds to both the measured RSL and the measured BER. To simplify the following discussion, an example involves a response to rain fade detected by measuring the RSL. It will be apparent, however, that an identical response can be made by measuring the BER. Thus, in the following discussion, the BER, rather than the RSL, is compared to the various thresholds disclosed (in addition, the operators &gt; and &lt; are exchanged with each other). In addition, it will be apparent that a response can be made simultaneously to both the RSL and to the BER with appropriate modifications. 
       FIG. 20  illustrates a flow diagram for implementing counter-measures according to the present invention in response to measured RSL. In the preferred embodiment, the microprocessor  230  ( FIG. 3 ) is appropriately programmed to implement the flow diagram illustrated in FIG.  20 . In a first state  800 , the terminal  100  is configured for communicating data at 16 QAM. Then, program flow moves from the state  800  to a state  802 . In the state  802  a determination is made whether the RSL has fallen below the threshold level L1. If the RSL has not fallen below the threshold level L1, then program flow returns to the state  800 . 
     If, however, the RSL has fallen below the threshold level L2, then program flow moves to a state  804 . In the state  804 , a determination is made whether the rate at which the RSL is changing exceeds a first predefined slope Z 1 . If the rate does not exceed the predefined slope Z 1 , then program flow moves from the state  804  to a state  806 . In the state  806 , a determination is made whether the RSL has fallen below the threshold L4. If the RSL has not fallen below the threshold L4, then program flow returns to the state  800 . 
     If, however, the RSL has fallen below the threshold L4, then program flow moves from the state  806  to a state  808 . If the determination made in the state  804  resulted in a determination that the rate did exceed the predefined slope Z 1 , then the program flow moves from the state  804  to a state  808 . In the state  808 , the terminal is configured to transmit data according to QPSK (without spectrum spreading). Then program flow moves from the state  808  to a state  810 . 
     In the state  810 , a determination is made as to whether the RSL is above the threshold L5. If the RSL is above the level L5, then program flow moves from the state  810  to a state  812 . In the state  812 , a determination is made as to whether the rate at which the RSL is changing exceeds a predefined slope Z 2 . If the rate exceeds the slope Z 2 , then program flow returns to the state  800 . If the rate does not exceed the slope Z 2 , then program flow moves from the state  812  to a state  814 . 
     In the state  814 , a determination is made whether the RSL is above the threshold level L1. If not, then program flow returns to the state  808 . If in the state  814 , the RSL is above the threshold L1, then program flow returns to the state  800 . 
     If, in the state  810 , the RSL is not above the threshold L5, then program flow moves to a state  816 . In the state  816 , a determination is made whether the RSL is below the threshold L6. If the RSL is not below the threshold L6, program flow returns to the state  808 . If, in the state  816 , the RSL is below the threshold  816 , then program flow moves from the state  816  to a state  818 . In the state  816 , a determination is made if the rate of change in the RSL exceeds a predefined slope Z 3 . If the slope Z 3  is not exceeded program flow moves from the state  818  to a state  820 . 
     In the state  820 , a determination is made whether the RSL is below the threshold L8. If not, then program flow returns to the state  808 . If in the state  820  the RSL is not below the threshold L8, the program flow moves to a state  822 . In addition, if, in the state  818 , the slope Z 3  is exceeded, program flow moves to the state  822 . In the state  822  the terminal  100  is configured for communicating data according to QPSK with spectrum spreading. 
     From the state  822 , program flow moves to a state  824 . In the state  824 , a determination is made whether the RSL is below the threshold L7. If the RSL is not below the level L7, then program flow returns to the state  822 . If, in the state  824 , the RSL is above the threshold L7, then program flow moves from the state  824  to a state  826 . In the state  826 , a determination is made whether the rate of change in the RSL exceeds a predefined slope Z 4 . If so, program flow returns to the state  808 . If, in the state  826 , the slope Z 4  is not exceeded, then program flow moves to a state  828 . 
     In the state  828 , a determination is made whether the RSL is above the threshold  828 . If so, program flow returns to the state  808 . If, in the state  828 , the RSL is not above the threshold  828 , then program flow returns to the state  822 . 
     An important aspect of the present invention is that hysteresis is introduced in the flow diagram for changing the manner of data communication in the states  800 ,  808  and  822 , based upon the RSL. Thus, for example, to change from 16 QAM to QPSK, the RSL must fall below L2. However, to change from QPSK to 16 QAM, the RSL must rise above L1 where L1 is higher than L2. This hysteresis reduces the frequency at which the manner of communicating data is changed and prevents oscillations from occurring between any two of the states  800 ,  808  and  822 . 
     In a point-to-multipoint MAN, a single network node communicates radio super frames  380  with a plurality of other nodes.  FIG. 21  illustrates a point-to-multipoint metropolitan area network divided into sectors having inner and outer radii according to the present invention. A single node at a hub  900  communicates with a plurality of subscriber nodes, designated “r” located as various radial distances from the hub  900  and in different directions (sectors). An important advantage of the present invention that changes in manner in which data is communicated over a wireless link can be utilized to reduce interference between nodes in a same sector, but at a different radial distances from the hub  900 . 
     As an example, assume a first subscriber node  902  is located in a sector  904  at a radial distance from the hub  900  that is less than 2 Km. Assume that a second subscriber node  906  is also located in the in the sector  904  but at a radial distance from the hub  900  that is more than 2 Km and less than 4 Km. If both subscriber nodes  902 ,  906  communicate with the hub  900  in the same manner, there is a probability that communications intended for the node  902  will interfere with communications intended for the node  906 . In the preferred embodiment of the present invention, however, the adaptive countermeasures block  616  ( FIGS. 14 and 16 ) of the first subscriber node  902  is conditioned to communicate data in a first manner (e.g. according to 16 QAM), whereas, the adaptive countermeasures block  616  of the second subscriber node  906  is conditioned to communicate data in a second manner (e.g. according to QPSK). The adaptive countermeasures block  616  of hub  900  is conditioned for communication with either of the nodes  902 ,  906 , by changing back and forth between the first and second manner of communicating. This is accomplished by appropriately conditioning the rate control signal applied to the multiplexers  750 ,  756  ( FIG. 18 ) of the hub  900  depending upon which node  902 ,  906  the hub is currently communicating with. 
     In the preferred embodiment of the present invention, a security authentication protocol is implemented for data security purposes against eavesdroppers.  FIG. 22  illustrates a wireless link  102  between two terminals  100  and  100 ′ wherein an unauthorized terminal  950  is attempting to eavesdrop on communication between the two terminals  100 ,  100 ′. Each terminal  100 ,  100 ′ and  950  is preconditioned to periodically authenticate the other terminal opposite the communication link. For this purpose, each terminal is assigned a unique password. 
     Link authentication is accomplished in the following manner: Once communication between the terminals  100  and  100 ′ is established, the terminals  100 ,  100 ′ exchange their passwords. Then, at periodic intervals, the terminal  100  sends a challenge message to the terminal  100 ′. The challenge message includes an identification number and a random number. The terminal  100 ′ receives the random number and calculates a response based upon a mathematical combination of the random number and its unique password. Then the terminal  100 ′ then sends the calculated response to the terminal  100  along with the same identification number it received. 
     The terminal  100  then matches the identification number it receives from the terminal  100 ′ to the challenge message it previously sent and then compares the response it received to an expected response. The terminal  100 ′ determines the expected response based upon its knowledge of the unique password associated with the terminal  100 ′ and upon its knowledge of the random number included in the challenge. If the received response matches the expected response, the terminal  100 ′ sends a success message to the terminal  100 ′. Data communication then resumes. Each terminal  100 ,  100 ′ periodically authenticates the other in a symmetrical manner. 
     If, however, the received response does not match the expected response, an alarm is set in the terminal  100 . In response to the alarm, the terminal  100  maintains the wireless communication link  102  by sending and receiving radio frames  350  ( FIG. 6 ) with the terminal  100 , however, the radio frames  350  sent by the terminal  100  no longer carry 100BASE-T Ethernet data. Instead, the inter-packet gap code is sent. In addition, the terminal  100  is configured to no longer detect and separate 100BASE-T Ethernet packets from received radio frames. Thus, the 100BASE-T traffic in both directions is disabled. The terminals continue attempting to re-authenticate the link, and if successful, communication of 100BASE-T packets resumes. 
     It is important to note that each terminal  100 ,  100 ′,  950 , is configured to successfully receive radio frames at all times, but us configured to successfully receive 100BASE-T packet data only if it receives a response to a challenge message which matches an expected response. The determination of whether a response to a challenge message is appropriate depends upon knowledge of the random number included in the challenge message. 
     Assume that once the link  102  is established, the terminal  950  attempts to eavesdrop. This is an unauthorized intruder who is attempting to receive data from the link. It is expected in such a situation, that the terminal  950  will have its transmitter muted in an attempt to escape detection. Because the transmitter of the terminal  950  is muted, it cannot authenticate with either terminal  100 ,  100 ′. Thus, although the terminal can receive responses to challenge messages sent by the terminals  100 ,  100 ′, it cannot match such a response to an expected response because the terminal  950  will not have knowledge of the random number sent with the response. Thus, an alarm will be set in the terminal  950 . Once this occurs, the terminal  950  can no longer receive 100BASE-T packet data. Accordingly, the attempted eavesdropping is prevented and data security maintained. 
       FIG. 23  illustrates an embodiment according to the present invention having multiple digital processing MACs  222 A″,  222 B″ multiplexed to a single radio framer  228 ″. The MACs  222 A″,  222 B″ can each be identical to the MAC  222 ′ illustrated in  FIG. 16  while the radio framer  228 ″ can be identical to the radio framer  228 ′ illustrated in FIG.  16 . This embodiment enables multiple 100BASE-T Ethernet packets to be received simultaneously, one for each MAC  222 A″,  222 B″. The Ethernet packets are temporarily stored in each MAC  222 A″,  222 ″ and then provided to the radio framer  228 ″ via a multiplexer  980  according to time division multiplexing. The time division multiplexed data is then communicated over the wireless link  102 . According to this embodiment, the wireless link  102  is configured to communicate data at 200 Mbps. It will be apparent that a number, n, of MACs can be coupled to the multiplexer  980  thereby achieving a n×100 Mbps data rate for the wireless link  102 . Such an arrangement is limited by the maximum bandwidth capacity for the wireless link  102 . 
     The present invention has been described in terms of specific embodiments incorporating details to facilitate the understanding of the principles of construction and operation of the invention. Such reference herein to specific embodiments and details thereof is not intended to limit the scope of the claims appended hereto. It will be apparent to those skilled in the art that modifications may be made in the embodiment chosen for illustration without departing from the spirit and scope of the invention. Specifically, it will be apparent to one of ordinary skill in the art that the device of the present invention could be implemented in several different ways and the apparatus disclosed above is only illustrative of the preferred embodiment of the invention and is in no way a limitation.