Abstract:
A LAN adapter device using an interpolative equalizer. The LAN adapter device allows LAN computing devices to connect to the LAN medium. The adapter devices are internal or external to the LAN devices and provide a transceiver and protocol stacks for the LAN devices to communicate with each other. The physical layer of the transceivers includes transmitter having a DMT modulator and demodulator that is dynamically configurable with respect to data rate and spectrum usage. Within the receiver is an equalizer that equalizes the received signal with a simple yet effective equalizer training method, thereby obviating the need for extensive equalizer training and long-term coefficient storage. The equalizer trains on known symbols transmitted on specific carriers that arc spaced apart in the frequency domain and then determines the channel response at those frequencies. The remaining carriers that are modulated with data are equalized based on an estimate of the channel response determined from interpolating between the points previously determined from the known symbols.

Description:
CROSS REFERENCE TO RELATED APPLICATION 
     The present application is a continuation of co-pending U.S. patent application Ser. No. 09/085,960 filed May 27, 1998. 
    
    
     BACKGROUND OF THE INVENTION 
     A. Field of the Invention 
     The present invention relates to Local Area Networks (LANs). More particularly, the invention relates to a LAN architecture specifically designed to be operated within a home environment, but may also be used in a small business setting. Home LANs may be used to connect computers, peripherals, TVs, and audio equipment, as well as less intelligent devices (appliances, thermostats, etc.), and provide connectivity to devices and networks outside the home (e.g., Internet and corporate LANs). 
     B. Description of the Related Art 
     Networks are collections of independent computers that communicate with one another over a shared physical connection, or network medium. Networks are often categorized as Local Area Networks (LAN) and Wide Area Networks (WAN). 
     1. Local Area Networks 
     Local area networks are usually confined to a specific geographic area, such as an office building. LANs, however, are not necessarily simple in design, and may link together hundreds of computers. The development of various standards for networking protocols has made possible the proliferation of LANs in organizations worldwide for business and educational applications. 
     Ethernet is a LAN networking protocol commonly utilized today. Ethernet typically utilizes a “star” or “spoke” topology, where each computer of the LAN is connected to other computers via a central hub. In such a configuration, each computer has its own private connection to the LAN and can be disconnected from the network without interfering with any other computer&#39;s connection. 
     Ethernet LAN technology was standardized by the Institute of Electrical and Electronics Engineers (IEEE) as the 802.3 specification entitled “Carrier Sense Multiple Access with Collision Detection (CSMA/CD) Access Method and Physical Layer Specifications.” Initially, Ethernet technology used coaxial cable in a bus topology, however it has evolved to take into advantage of new technologies such as twisted pair cabling (10 Base-T), fiber optics (10 Base-FL), and 100 Mbps operation (100 Base-X, or Fast Ethernet). The current standard is known as IEEE 802.3u, the contents of which are hereby incorporated by reference. One limitation of 10/100 Base-T is the requirement for “home run” cabling, i.e., each device is connected back to a central hub, as opposed to “daisy chain” connections. 
     The Ethernet system consists of three basic elements: the physical medium; medium access control rules; and a packet format. The physical medium is used to convey Ethernet signals from one computer to another. The medium access control rules are embedded in each Ethernet interface, and allow multiple computers to access the shared Ethernet channel. The Ethernet packet, or frame, consists of a standardized set of fields used to carry data over the system. 
     2. Wide Area Networks 
     Interconnected LAN networks or individual users located in multiple physical locations are known as Wide Area Networks (WAN). The interconnections are performed via services such as dedicated leased phone lines, digital subscriber lines, dial-up phone lines, satellite links, and data packet carrier services. Wide area networking can be as simple as providing modems and a remote access server to allow remote users to dial in; or it can be as complex as linking hundreds of branch offices across the world using special routing protocols. Once type of WAN interconnection mechanism is Asymmetric Digital Subscriber Line. 
     3. Asymmetric Digital Subscriber Lines 
     Asymmetric Digital Subscriber Line (ADSL) is a communication system that operates over existing twisted-pair telephone lines between a central office and a residential or business location. It is generally a point-to-point connection between two dedicated devices, as opposed to multi-point, where numerous devices share the same physical medium. 
     ADSL supports bit transmission rates of up to approximately 6 Mbps in the downstream direction (to a subscriber device at the home), but only 640 Kbps in the upstream direction (to the service provider/central office). ADSL connections actually have three separate information channels: two data channels and a POTS channel. The first data channel is a high-speed downstream channel used to convey information to the subscriber. Its data rate is adaptable and ranges from 1.5 to 6.1 Mbps. The second data channel is a medium speed duplex channel providing bi-directional communication between the subscriber and the service provider/central office. Its rate is also adaptable and the rates range from 16 to 640 kbps. The third information channel is a POTS (Plain Old Telephone Service) channel. The POTS channel is typically not processed directly by the ADSL modems—the POTS channel operates in the standard POTS frequency range and is processed by standard POTS devices after being split from the ADSL signal. 
     The American National Standards Institute (ANSI) Standard T1.413, the contents of which are incorporated herein by reference, specifies an ADSL standard that is widely followed in the telecommunications industry. The ADSL standard specifies a modulation technique known as Discrete Multi-Tone modulation. 
     4. Discrete Multi-Tone Modulation 
     Discrete Multi-Tone (DMT) uses a large number of subcarriers spaced close together. Each subcarrier is modulated using a type of Quadrature Amplitude Modulation (QAM). Alternative types of modulation include Multiple Phase Shift Keying (MPSK), including BPSK and QPSK, and Differential Phase Shift Keying (DPSK). The data bits are mapped to a series of symbols in the I-Q complex plane, and each symbol is used to modulate the amplitude and phase of one of the multiple tones, or carriers. The symbols are used to specify the magnitude and phase of a subcarrier, where each subcarrier frequency corresponds to the center frequency of the “bin” associated with a Discrete Fourier Transform (DFT). The modulated time-domain signal corresponding to all of the subcarriers can then be generated in parallel by the use of well-known DFT algorithm called Inverse Fast Fourier Transforms (IFFT). 
     The symbol period is relatively long compared to single carrier systems because the bandwidth available to each carrier is restricted. However, a large number of symbols is transmitted simultaneously, one on each subcarrier. The number of discrete signal points that may be distinguished on a single carrier is a function of the noise level. Thus, the signal set, or constellation, of each subcarrier is determined based on the noise level within the relevant subcarrier frequency band. 
     Because the symbol time is relatively long and is followed by a guard band, intersymbol interference is a less severe problem than with single carrier, high symbol rate systems. Furthermore, because each carrier has a narrow bandwidth, the channel impulse response is relatively flat across each subcarrier frequency band. The DMT standard for ADSL, ANSI T1.413, specifies 256 subcarriers, each with a 4 kHz bandwidth. Each sub-carrier can be independently modulated from zero to a maximum of 15 bits/sec/Hz. This allows up to 60 kbps per tone. DMT transmission allows modulation and coding techniques to be employed independently for each of the sub-channels. 
     The sub-channels overlap spectrally, but as a consequence of the orthogonality of the transform, if the distortion in the channel is mild relative to the bandwidth of a sub-channel, the data in each sub-channel can be demodulated with a small amount of interference from the other sub-channels. For high-speed wide-band applications, it is common to use a cyclic-prefix at the beginning, or a periodic extension appended at the end of each symbol to maintain orthogonality. Because of the periodic nature of the FFT, no discontinuity in the time-domain channel is generated between the symbol and the extension. It has been shown that if the channel impulse response is shorter than the length of the periodic extension, sub-channel isolation is achieved. 
     5. Residential Phone Wiring 
     Plain Ordinary Telephone Service (POTS) operates over numerous types of existing wiring layouts. Typically, the topology is a star configuration, combined with daisy chained connections for some phones. The type of wiring is also random—twisted pair, untwisted, various gauges, various numbers of wires (with possible cross-talk)—which creates a wide variation in the channel characteristics. Furthermore, the topology changes from time to time as phones are connected, disconnected, etc. 
     Thus the channel characteristics are very noisy and distorted, including phase distortion (group delay) and severe reflections (echoes from signals bouncing off unterminated wiring segments) and spectral dips due to unterminated wiring stubs. In addition, signals associated with the analog phone service include 48 vdc, and 100 volt ring signals. Ring signals are not zero-crossing switched, so high-frequency noise is produced. Any system operating over existing telephone wiring must contend with this environment. 
     SUMMARY OF THE INVENTION 
     There exists a need for a communication system that operates reliably over standard telephone wiring found in residential environments, thereby eliminating the need for re-wiring a residence. Furthermore, the system must operate in conjunction with existing phone service. 
     A LAN Adapter device having an interpolative equalizer is provided. The LAN adapter devices allow LAN computing devices to connect to the LAN medium. The adapter devices are internal or external to the LAN devices and provide a transceiver and protocol stacks for the LAN devices to communicate with each other. The physical layer of the transceivers includes transmitter having a DMT modulator and demodulator that is dynamically configurable with respect to data rate and spectrum usage. Within the receiver is an equalizer for equalizing the received signal with a simple yet effective equalizer training method, thereby obviating the need for extensive equalizer training and long-term coefficient storage. 
     The equalizer trains on known symbols transmitted on specific carriers that are spaced apart in the frequency domain and then determines the channel response at those frequencies. The remaining carriers that are modulated with data are equalized based on an estimate of the channel response determined from interpolating between the points previously determined from the known symbols. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The foregoing and other objects, features and advantages of the present invention will be more readily appreciated upon reference to the following disclosure when considered in conjunction with the accompanying drawings, in which: 
         FIG. 1  depicts a preferred embodiment of the communication system; 
         FIG. 2  shows a preferred embodiment of the protocol stacks within the gateway and the DMT LAN devices; 
         FIG. 3  shows a preferred embodiment of the simplified protocol stacks within the gateway device; 
         FIG. 4  is a block diagram of a preferred embodiment of the transceiver; 
         FIG. 5  shows the DMT LAN; 
         FIG. 6  shows a representative modulation signal point constellation; 
         FIG. 7  depicts transmit data; 
         FIG. 8  shows a correlation signal at the receive correlator, and 
         FIG. 9  shows the equalizer. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     The system described herein, depicted in  FIG. 1 , consists of two major components: the DMT LAN  20  and the DMT LAN gateway  100  that is connected between the DMT LAN  20  and the access infrastructure  10 . The access infrastructure  10  includes the voice and data carrier networks as well as connections to the access points. The physical connection  80  to infrastructure  10  may be a standard land-line connection over twisted pair cable or may be a wireless service to the gateway  100 . The gateway  100  includes an infrastructure protocol stack  40  that communicates via physical connection  80  to the access infrastructure  10  and to the network  90 . The gateway  100  also includes a DMT LAN protocol stack  50  that communicates with the DMT LAN  20 , and a forwarder  60  that bridges the top layers of these two stacks. 
     The DMT LAN  20  includes a plurality of devices similar to DMT LAN device  70  depicted in FIG.  1 . The DMT LAN device  70  may also be referred to herein as a client device, or a node. As shown in  FIG. 5 , the DMT LAN devices connected to the DMT LAN  20  may include computers, computer peripherals such as printers and modems, copiers, fax machines and personal digital assistants. Also suitable for connection are TVs, audio-visual equipment, security systems, as well as less intelligent devices such as appliances, thermostats, and lighting fixtures. Less intelligent devices may have a simplified transceiver, or may be connected to the DMT LAN  20  via a separate control device or bridge. The DMT LAN medium, or physical electrical connection between devices, is preferably standard residential telephone wiring—typically pairs of twisted wires. 
     The preferred embodiment of the gateway  100 , including forwarder  60 , is shown in FIG.  2 . It includes a DMT LAN adapter  74  having a transceiver  400  within the physical layer  262 , and a second transceiver to implement the physical layer of access infrastructure stack  40  for communication with the access infrastructure  10 . The second transceiver utilizes one of a number of communication protocols, such as those generally referred to as xDSL (e.g., ADSL, HDSL, etc.), simplified DSL known as DSL Lite or G.Lite, ISDN, cable modems, and the like. 
     Importantly, connection  200  (as shown in  FIGS. 1 and 2 ) may only be included under certain circumstances, depending on the type of link from gateway  100  to access infrastructure  10 . Many services such as ADSL require a point to point link and require relatively high line quality. In such a case, no other devices may be directly connected over the same medium. Link  200  must be therefore be omitted to keep LAN devices  70  isolated from the access infrastructure link. Other transmission technologies, however, are more flexible and will operate over a physical medium having numerous wiring segments, some of which may be unterminated. In embodiments where link  200  is appropriate, both stacks  40  and  50  of gateway  100  may be connected to the shared medium over a single electrical connection, as opposed to the two separate connections as actually depicted in  FIGS. 1 and 2 . Such a connection is shown for illustrative purposes in  FIG. 3  with the understanding that the gateway device  100  depicted in  FIG. 1 ,  2 , or  3  may be connected in either fashion when appropriate. In  FIG. 3 , link  202  connects LAN stack  50  and infrastructure stack  40  internal to the gateway device  100 , which is in turn connected to the shared LAN medium, while the access infrastructure is also connected directly to the shared LAN medium. As discussed below, when link  200  (or its equivalent) is present, the operating frequency range of devices  70  on LAN  20  must adapted so as not to interfere with communication between the gateway  100  and the access infrastructure  10 . 
     The forwarder  60  is a multi-point forwarder that runs a single-PPP-session module  224  to provide the necessary routing functionality for multi-point connectivity between devices  70  in the DMT LAN  20  and the external WAN environment  90 . 
     The multi-point forwarder  60  includes a name address translation protocol module  212  that allows a single PPP session (i.e., a single Internet IP address) to serve multiple DMT LAN devices  70 , each having separate IP addresses on the local DMT LAN  20 . 
       FIG. 2  shows the protocol layers and the multi-point forwarder  60  that supports multiple IP sessions on behalf of the DMT LAN devices  70  with just one PPP session terminated in module  224  of the forwarder  60 . Any client device  70  may initiate the session, and it remains active until terminated. Termination may either be manual or automatic (timed). The PPP session module supports external network access for the local DMT LAN devices  70 . A Network Address Translation (NAT) module  212  runs NAT services in an upper layer of the multi-point forwarder  60  thereby allowing a port number to be associated with the client&#39;s local IP address. NAT module  212  makes devices on the DMT LAN  20  appear as a single IP address, thus allowing the devices to communicate with external networks  90 , including, for example, the Internet. Internally, the DMT LAN  20  uses private addressing. When a device  70 , which is known locally by its private address, desires to communicate with a device on network  90 , it sends the request to gateway  100 . Before gateway  100  transmits the request to the network  90 , forwarder  60  translates the private address to a common IP address assigned to gateway  100 . Further details of NAT are disclosed in U.S. patent application Ser. No. 09/035,600, filed Mar. 5, 1998, entitled Method and Protocol for Distributed Network Address Translation, the contents of which are hereby incorporated herein by reference. All remote access is therefore handled via the multi-point forwarder&#39;s  60  PPP module  224 . 
     The multi-point forwarder  60  includes a routing module  210  utilizing dynamic host configuration protocol (DHCP) and management module  220  (MNGT), which implements management functionality via, e.g., an SNMP agent, and a PPP module  224  for PPP session management. A PPP session can be initiated by any local device&#39;s  70  request for remote access (e.g., a Web browser). The multi-point forwarder  60  also provides session authentication and security  216  (AUTH). The routing module  210  enables local IP addresses to be assigned to any local client device  70 . Network address translator  212  allows the single PPP session to provide remote connectivity to any number of client devices  70  simultaneously. 
     Additional services in the multi-point forwarder  60  include a domain name server  214  (DNS), firewall  218  (FW), and a simple client agent  222  (SCA). The SCA service allows simple clients, which do not have full TCP/IP capabilities, to send and receive IP data. The SCA  222  does the additional protocol processing on behalf of the simple client. 
     The multi-point forwarder  60  communicates with client devices  70  by way of network adapters  74 . The adapters  74  include MAC resolution layer  260  and physical layer  262 , which is a multi-carrier transceiver that operates in half-duplex mode. That is, one adapter  74  at a time transmits data to other adapters  74  on the DMT LAN  20 . Other devices  70  then take turns transmitting via adapters  74  as determined by the MAC protocol layer  260  as described herein. Adapter devices  74  preferably support both high-speed devices or Full-Rate Device (FRD), and low-speed devices or Sub-Rate Devices (SRD) on the single DMT LAN medium. 
     As shown in  FIG. 2 , the adapter device  74  also provides the bottom layers of the protocol stack for communication on the DMT LAN-side of the gateway  100 . The top layers of the stack typically run on a computing device, rather than the internal (or external) DMT LAN adapter, and ultimately terminate in applications running on a computer, or on a slow-speed SRD client. Existing host computers may require a minimal high-level configuration modification (e.g., via a standard operating system configuration tool) in order to bypass local PPP management because this functionality preferably resides in the gateway&#39;s  100  multi-point forwarder  60 . 
     The MAC layer  260  and the physical layer  262  are implemented in all adapter devices. Data delivered down to the MAC layer are tagged with source and destination MAC addresses and priority. Data arriving to the physical layer  262  from the medium are passed to the MAC layer  260  for processing. All devices decode the destination address of incoming data and discard frames that do not correspond to their own MAC address. Any client device  70  connected to the DMT LAN network (via an adapter) can communicate with any other similarly connected device  70 , or with the gateway  100 . 
     The internal adapter  74  shown in  FIGS. 2 and 3 , provides the MAC layer  260  and physical layer  262  preferably in an application specific integrated circuit, or ASIC. The adapter may alternatively be configured to provide higher level protocol processing, but the LAN device  70  preferably provides the layers above the MAC layer  260 . In addition, an address resolution protocol (ARP) module  256  is layered above the MAC layer  260 . This allows the device  70  to query for the MAC address of any other device  70 , given that device&#39;s (local) IP address. The ARP module  256  also includes a method for querying for the MAC address of the gateway  100 . 
     If multiple devices  70  simultaneously attempt to communicate over DMT LAN medium  110 , data collisions will occur. One suitable method of resolving media access contention in a multi-carrier network medium is set forth in patent application Ser. No. 09/003,844, entitled “Method And Protocol For A Medium Access Control Layer For Local Area Networks With Multiple-Priority Traffic” filed Jan. 7, 1998, the contents of which are incorporated herein by reference. 
     As shown in  FIG. 3 , the adapter device may be either an internal adapter device  74  or an external adapter device  76 . Internal devices  74  are preferred primarily due to speed advantages of its, interface, e.g., a standard parallel bus and external adapter devices  76  are used primarily to provide backward compatibility. They differ mainly in that the external adapter  76  requires additional hardware (a port, or interface) and software layers (port driver  270 ) for communication between the adapter  76  and the client device  70  over a data bus. The internal bus may be a PCI, ISA EISA, or equivalent bus, while the external bus may be e.g., RS-232, parallel port, or USB port as shown in FIG.  3 . These buses are known in the art and have been widely used by general-purpose computers. The external device adapter also provides the MAC  260  and physical layer  262  in an ASIC. 
     All data, whether between a remote server (via PPP) and a local client device  70  or strictly between local client devices  70 , are passed between nodes on the DMT LAN network as local data. Preferably, the data are transferred using TCP/IP protocols where the addresses are assigned locally. The gateway  100  has a local IP address so that DMT LAN devices  70  can access it for forwarding data to the public network  90 . The gateway  100  also has a public IP address for communication with the public network  90 , where the communication is typically performed on behalf of a DMT LAN device  70 . 
     In an alternative preferred embodiment depicted in  FIG. 3 , the forwarder  60 ′ provides simplified single-point connectivity per PPP session. That is, only one PPP session is permitted at any given time, even if multiple PPP-capable clients are connected to the DMT LAN  20 . The simplified forwarder  60 ′ performs MAC address resolution when passing data to the DMT LAN  20  and acts as a data relay when passing data from the DMT LAN  20  to the access infrastructure  10 . Client PPP sessions are terminated in the device  70  (e.g., personal computer) running the client that initiates the session. The simplified forwarder  60 ′ acts primarily as a data relay between the DMT LAN protocol stack  50  and the access infrastructure protocol stack  40 . 
     The simplified forwarder  60 ′ differentiates only high- or low-speed traffic. The simplified forwarder  60 ′ preferably maintains at most three MAC addresses at any given time: its own, the current high-speed PPP session owner, which is a Full-Rate Device (FRD), and the current active slow-speed device, which is a Sub-Rate Device (SRD). Only a single PPP session may be active at any given time, and the session owner (e.g., a personal computer) initiates and manages the session. The forwarder  60 ′ is notified when the session begins and ends, and maintains the MAC address of the session owner for the duration in Link ID buffer  212 . Inbound data (i.e., from the access infrastructure) from the high-speed channel is forwarded to the MAC address of the current PPP session owner stored in Link ID  212 , and the MAC priority is set to high. Outbound PPP data, identified by the source MAC address, is associated with the high-speed channel. Slow speed data is processed in a similar manner, but with a low MAC priority. 
     In the simplified forwarder  60 ′, protocol processing for a PPP session on a high-speed device, such as a home PC, requires that the device be configured to run and manage PPP. Only one PPP-capable client can run a PPP session at any given time. On session startup, the forwarder  60 ′ is notified that a PPP session has become active, and is supplied with the MAC address of the PPP client on the DMT LAN network. With a PPP session active as indicated in PPP state monitor  210 , no new (additional) PPP sessions may be initiated. When the session is ended, the forwarder is notified, and sets a local PPP state monitor  210  to inactive. This allows a new PPP session to be started. 
     The multi-point protocol stack of the client device  70  described above is compatible with the simplified forwarder  60 ′. Thus, the client adapters can work with either type of forwarder—simplified or multi-point. Because the client device  70  manages the PPP connection when used with the simplified forwarder  60 ′, portions of the multi-point protocol stack are unused. The outbound PPP frames are passed directly to the adapter device driver, in a manner similar to a dialup adapter device (e.g., modem). The frame is tagged with the MAC address of the forwarder device, then banded to the MAC layer. The MAC layer hands the data to the physical layer once contention is resolved. Inbound data are passed up the stack in the reverse direction. Data recognized as a PPP frame are passed to the PPP session layer. 
     In the outbound direction, local data are handled the same way as in the multi-point implementation—they simply bypass the PPP layer  264 , as shown in FIG.  3 . The destination MAC address used is that of any local device  70  (except the gateway device  100 ). Inbound data are passed up the stack in the reverse direction. All adapters  74 ,  76  decode the destination address of incoming data, discarding frames that do not correspond to their MAC address. 
     The simplified forwarder  60 ′ also distinguishes between two types of incoming data (from the access infrastructure): high-speed and low-speed. Incoming high-speed data are forwarded to the DMT LAN network with a destination derived from the MAC address of the current PPP session owner; the data are flagged as high priority, which ensures more bandwidth allocation on the DMT LAN network  20 . Incoming low-speed data are forwarded to the currently active low-speed SRD device in a similar manner; these data are flagged as low priority. Outbound data (to the access infrastructure  10 ) are similarly directed to the high- and low-bandwidth channels according to the MAC address of the source DMT LAN device  70 . The applications running on client devices  70  are responsible for setting the appropriate priority for data sent to the forwarder  60 ′. 
     The table below summarizes and compares the features and capabilities of the simplified forwarder  60 ′ and multi-point forwarder  60 . 
     
       
         
               
               
             
           
               
                   
               
               
                 SIMPLIFIED FORWARDER 
                 MULTI-POINT FORWARDER 
               
               
                   
               
             
             
               
                 One PPP session per connection, 
                 One PPP session for entire DMT 
               
               
                 managed in PC 
                 LAN network managed in the 
               
               
                 Requires one IP session per 
                 gateway 
               
               
                 session, per FRD device 
                 Multiple local IP addresses supported 
               
               
                 Only one PPP session at a given 
                 by single gateway PPP session 
               
               
                 time 
                 Multiple simultaneous IP sessions 
               
               
                 Any computer on the DMT LAN 
                 possible 
               
               
                 can start a PPP session, but only 
                 Any number of computers can start 
               
               
                 one at a time 
                 an IP session at any time 
               
               
                 Full local DMT LAN support (file 
                 Full local DMT LAN support (file 
               
               
                 and printer sharing) 
                 and printer sharing) 
               
               
                 Minimal protocol processing in 
                 Gateway becomes a router with NAT 
               
               
                 the RU 
               
               
                   
               
             
          
         
       
     
     Gateway  100  may be configured initially with a simplified forwarder  60 ′, and upgraded to multi-point forwarder  60  by a software download of the multi-point forwarder  60  to the gateway  100 . Upon upgrading, local devices  70  must be configured to turn off local PPP management. Preferably, local device configuration is controlled with a software switch in the forwarder  60 ,  60 ′. Thus, no user intervention is required at all to affect the migration from a simplified forwarder  60 ′ to a multi-point forwarder  60 . 
     Gateway  100  is therefore able to interconnect numerous devices residing on a local area network  20  to a public network  90 . As shown in  FIG. 4 , the gateway  100  includes a transceiver  400  for connection to the local LAN via a first interface, where the transceiver includes a transmitter  402  and receiver  420  for transmitting and receiving data on a number of predetermined frequency ranges over the local LAN  20  to and from any one of the devices  70 . The gateway  100  also includes a second transceiver (not shown) for connection to the public network  90  via a second interface and for transmitting and receiving data to and from the public network. Depending upon the type of connection to the access infrastructure, the first and second interfaces may share a physical connection to the shared medium. The second transceiver implements the physical layer of the access infrastructure stack  40 , and is also connected via stack  40 , and forwarder  60 / 60 ′ to the first transceiver  400  for exchanging data with the first transceiver  400 , thereby connecting any one of the devices  70  on the local shared LAN  20  to the public network  90 . 
       FIG. 5  depicts numerous devices connected to DMT LAN  20 . The access infrastructure  10  is also connected directly to the DMT LAN via physical connection  80  without the use of a gateway device except for modem server  530 . Of course, the DMT LAN of  FIG. 5  may be connected to access infrastructure  10  using a gateway device  100  as shown in FIG.  1 . When a gateway device  100  is not used to provide communication between DMT LAN devices  70  and access infrastructure  10 , a filter or other isolation device may be included between the access infrastructure and the DMT LAN medium to prevent internal DMT LAN signals from reaching access infrastructure  10 . In the implementation as shown in  FIG. 5 , access infrastructure  10  provides voice band POTS service, rather than a high speed xDSL, ISDN, G.Lite, etc., service. The devices  70  on DMT LAN  20  operate at a frequency range higher than the POTS devices such as telephone  510 , fax machine  520  or modem  530 , and as such, do not interfere with the POTS services provided by access infrastructure  10 . 
       FIG. 5  depicts a number of representative devices connected to DMT LAN  20  over a typical wire medium found in a residence or small business. Many of the wiring runs originate at a central node  500 , which also connects to wiring  80  from the access infrastructure  10 . It is understood that wiring  80  from the access infrastructure may connect at any other point to the shared medium. Other wiring runs may diverge into separate runs such as at nodes  502  and  504 . Other runs may not be connected to any device and result in an unterminated wire pair such as nodes  506  and  508 . Telephone  510  and fax machine  520  are standard POTS devices, whereas the remaining devices are connected to the DMT LAN  20  via a DMT LAN adapter device. Copier  570  has an internal DMT LAN adapter, while PDA  550 , printer  560  and modem  530  use external DMT LAN adapters  515 ,  517 ,  519 , respectively. PC  540  may have an internal (or external) DMT LAN adapter, a POTS modem, or both, connecting it to the local shared medium. 
     PC  540  may communicate with, e.g., printer  560  or copier  570  over the DMT LAN while simultaneously communicating with an external device over access infrastructure  10  using an internal POTS modem (not shown). 
     Similarly, POTS modem  530  together with adapter  519  act as a gateway modem server that is accessible to any DMT LAN device via DMT LAN adapter  519  such that modem  530  can provide POTS modem service to any device on DMT LAN  20 . Modem  530  may be configured with an internal LAN adapter, in which case a shared electrical connection may be used for both the adapter  519  and the modem  530 . Multiple devices such as PCs may access DMT LAN adapter  519  using the higher frequency DMT modulation format described herein, while modem  530  simultaneously retransmits the data via voice frequency, i.c., POTS frequency, modem techniques (e.g., as specified in ITU Recommendation V.34 or V.90) to a remote user connected to network  90  via the access infrastructure  10 . Modem  530  may be configured to provide a plurality of simultaneous dial-up POTS connections by connecting to the access infrastructure  10  over additional physical connections, i.e., a second POTS line. 
     The gateway modem server  530  thus resides on the local area network and provides POTS modem service to the devices  70  on the local area network. The devices  70  communicate with the modem server  530  and modem server  530  communicates to access infrastructure  10 , i.e., the POTS service provider, over the same medium. The modem server  530  includes a POTS modem transceiver for sending and receiving modulated signals to and from the POTS service provider using the POTS frequency band. Modem server  530  also includes a multi-point transceiver  400  connected to the POTS modem transceiver in order to transmit and receive multi-carrier data bursts to and from devices  70 . In this manner, any one of devices  70  on the LAN  20  may transmit to the public network  90  by way of the multi-point transceiver  400  and the POTS modem transceiver. 
     The adapter devices  74 ,  76  include a transceiver  400  that implements the physical layer  262 , for communicating with other adapter devices  74 ,  76  and/or the multi-point transceiver  400  of the gateway device  100 . As shown in  FIG. 4 , transceiver  400  of the preferred embodiment includes a transmitter portion  402  and a receiver portion  420 . Transceiver  400  uses wide-band multi-carrier modulation, preferably in the range of 4-6 MHz. When the gateway  100  provides isolation from communications with access infrastructure (or if there are otherwise no conflicting services), the transceiver is easily scalable to operate down to approximately 100 KHz, and up to 8 MHz. It is to be understood that the frequency ranges actually used may extend beyond the presently preferred ranges given above. The adapter devices  74 ,  76  include an adapter interface, or port, for connection to a LAN device  70 . 
     The preferred modulation technique used in the adapter devices  74 ,  76  is a Discrete Multi-Tone (DMT) multi-carrier method, where each channel is split into a number of sub-channels, each with its own carrier. Preferably the number of carriers is sixty-four, but the transceiver is easily scalable to use an additional one hundred twenty eight, or more, carriers. The data bits are mapped to frames of complex frequency domain symbols and transformed digitally using a frequency-domain to time-domain transformation on each frame. A discrete Fourier transform (DFT) provides a computationally efficient implementation of such a transformation. 
     Alternatively, one of many well-known wavelet transformations may be used to generate a modulated time-domain signal. In such a case, the information symbols are modulated onto a family of wavelets where each wavelet occupies a different frequency range. Typically, each wavelet is a time-scaled version of the other wavelets in the family such that the wavelets are orthogonal. Typically, the wavelets also occupy different bandwidths, with, e.g., the longer wavelets occupying the smaller bandwidths at the lower frequency bands, and the shorter wavelets occupying larger bandwidths at the higher frequencies. In this sense, the wavelet transformer also results in a multi-carrier signal similar to a DMT signal, with each wavelet acting as a separate “carrier”. 
     The transmitter  402  of the DMT LAN adapter transceiver  400  includes a scrambler  404  that ensures continuous data transitions. In the MOD modulator  406 , the data is mapped to signal points chosen from a constellation of complex signal points. The IFFT transformer  408  performs an inverse Fourier transform on the complex points to generate a time-domain sequence. The periodic extension is appended in the periodic extension block  410  to the signal to allow for channel impulse response and to enable receiver symbol timing recovery and clock tracking. Finally, the scaler  412  adjusts the amplitude of the digital signal according to the range of the D/A converter  414 , and the data is sent through the channel(s) to the receiver(s)  420 . The scaler  412  may be incorporated into the IFFT module  408 . 
     The scrambler  404  de-correlates the data such that the energy of the time-domain transmit signal is spread evenly across the spectrun. This also ensures a proper peak-to-average signal. A suitable scrambler algorithm is that used in the V.series modems, specifically ITU Recommendation V.34. The performance and complexity of this algorithm are well known, and code exists for its implementation on common DSP platforms. Altematively, a block based scrambler using a lookup table may be used. 
     The MOD modulator block  406  maps input data to complex points in a signal constellation for each sub-channel. One of a number of modulation techniques may be used. Quadrature Amplitude Modulation (QAM), Multiple Phase Shift Keying (MPSK) (including QPSK), Differential Phase Shift Keying (DPSK) (including DQPSK) and the like, are all possible modulation schemes. DQPSK is presently preferred. The phase of each carrier is compared to its previous phase from symbol to symbol. This has the advantage of resolving phase ambiguities between the transmitter and receiver. 
     For illustrative purpose, a typical QAM constellation consisting of sixteen points is shown in FIG.  6 . The points represent the magnitude of a sine and cosine signal component of the modulated carrier. Alternatively, the points may be considered as the magnitude and phase of a given modulated carrier. Integer-valued signal points are merely a convenience, and result in efficient processing in the modulator as well as in the receive demodulator and decision feedback equalizer loop. 
     The constellation in  FIG. 6  is representative of a QAM constellation having four points (labeled in  FIG. 6  as points  600 ) per quadrant. In practice, the size is generally determined by the signal-to-noise ratio encountered on the channel in a standard rate-negotiation period. Thus, a given carrier may have a smaller or larger constellation, depending on the noise present in that band. The scaling of the function is arranged such that each constellation point results in a real and imaginary part, both being integers. The modulator  406  may include a trellis decoder and other Forward Error Correcting (FEC) coders as are well known in the art. 
     In the presently preferred embodiment, modulator  406  is a 64 tone DMT modulator utilizing the bandwidth between approximately 4-6 MHz. Preferably, the sample rate is 16 MHz, and in accordance with the well-known Nyquist sampling theorem, this implies that the total possible usable bandwidth is the range from 0-8 MHz. However, in the preferred embodiment, only a portion of the total capacity is used. This is accomplished in the following manner: a 512 point inverse DFT is used, resulting in a total capacity of 256 carriers across the 8 MHz bandwidth, but only the 64 carriers between 4-6 MHz are modulated while the remaining carriers are set to zero. 
     To generate a real-valued time-domain signal using an inverse DFT, a 512 point Inverse Fast Fourier Transform (IFFT) is performed, where the last 256 points are reverse-ordered complex conjugates of the first 256 points. It is a well-known property of discrete Fourier transforms that real-valued time domain signals have conjugate-symmetric Fourier transforms. In the preferred embodiment, the bin numbers of approximately 128 to 191 (and bins 321 to 384 being the reverse order complex conjugates) are used, with the remainder of bins set to zero. Thus, the real-valued time domain signal will have 512 values. 
     Voice-band frequency content is eliminated in the modulated signal because the frequency bins corresponding to the voice-band are set to zero. The modulated DMT signal does therefore not interfere with POTS devices that are operating over the same wiring. Similarly, the frequency bins corresponding to any other data services present on the LAN medium are set to zero. In this manner, the gateway device  100 , adapter devices  74 ,  76 , and the access infrastructure may all be connected to the shared medium, while allowing communication among DMT LAN devices without interfering with data services over link  80 . If no such data services arc present, or if the gateway device is connected as discussed above such that the LAN  20  is isolated from access infrastructure  10 , then the lower frequency bins may be used. 
     The formula for the IFFT inverse transform is: 
           d   j     =       1     n       ⁢       ∑     k   =   1       n   -   1       ⁢           ⁢       w   k     ⁢     e       -   2     ⁢   π   ⁢           ⁢     i   ⁡     (     j   n     )       ⁢   k               ,       
 
for 0≦j&gt;128, where the d j  are the time domain data points, n is the length of the IFFT, w k  are the complex-valued symbols, and i=√{square root over (−1)}. The w k  are set to a zero value for bins corresponding to frequencies that are not used. This is the case for at least the first, or DC, bin so as to pre vent interference with the operation of POTS devices connected to the DMT LAN  20 . Hence, the above summation begins at k=1 because w 0  is preferably always zero. If the DMT LAN does not operate over the same medium as POTS devices, the DC bin may be used. Setting the appropriate w k  to zero may easily zero out other frequency ranges in use by other devices.
 
     The periodic extension is appended in block  410 . The periodic extension, or cyclic extension as it is often referred to, is a repetition of the beginning samples of the time-domain signal generated by the DFT and is appended to the time-domain signal. One of ordinary skill will recognize that a periodic or cyclic prefix is an equivalent to the periodic extension. The length of the periodic extension is preferably at least as long as the channel impulse response. The model of the channel impulse response includes echoes from unterminated wiring segments within the DMT LAN, which are typical in a home DMT LAN because it is operating over existing twisted pair telephone wiring. The length of the periodic extension is computed based on the worst-case channel impulse response time, the worst case expected reflected echo tails, and the expected symbol (frame) timing error encountered at the receiver. The symbol timing recovery accuracy is mostly a function of the complexity of the receive correlator. Thus the number of periodic extension samples E is based on twice the worst-case channel impulse response as follows: 
       E   =       2   ·   D       1   /     F   s             
 
where D is the worst case maximum group delay in seconds, and F s  is the sampling rate (in seconds) of the A/D (and D/A) converter. For illustrative purposes, with an F s  of 1 MHz, and 40 μsec as the expected channel impulse response time as a result of group delay, an 80 sample extension is required. The calculation for the length of the periodic extension is a one-time calculation. The periodic extension simply appends the required number of samples to the time domain data.
 
     Once the time-domain transmit signal is generated for the current frame of data, it is scaled by the scaler  412  such that it fills as closely as possible the available bits of the D/A converter  414 . Thus the scaling must be by a constant value. To do otherwise would result in an amplitude envelope from frame to frame that would produce undesirable effects. The time domain data to be transmitted is shown in  FIG. 7 , depicting approximately ten frames of data. 
     The receiver  420  is shown in FIG.  4 . The receiver  420  must first obtain frame synchronization. It performs this task by processing the received samples from A/D converter  422  in a correlator  424 . The Frame Synchronizer  426  re-arranges the samples such that the leading-edge samples in each frame are replaced by samples from the periodic extension. Alternatively, a cyclic prefix may be used, where the end of the data sequence is copied and pre-pended to the transmit sequence. An FFT transformer  428  performs a transform of the real valued time domain signal and generates a complex frequency domain signal. The first frame contains known data and is used to determine the equalizer coefficients in the equalizer  430 . Equalizer  430  processes subsequent blocks of received data using these coefficients and updates the coefficients based on an error signal. 
     The frame synchronizer  426  and the correlator  424  accomplish symbol timing. The receive time-domain samples are passed to the correlator  424 . The correlator  424  performs a sliding correlation of the samples spaced the length of the time-domain FFT period, which, in the preferred case, is one hundred twenty eight samples. The correlator  424  then provides a correlation output  800  as shown in FIG.  8 . The length of the periodic extension was intentionally generated as a very long time (80 samples) so it is easy to visualize the correlation function in FIG.  8 . In practice the extension depends upon the physical medium as described herein, but typically is shorter than 80 samples. Alternatively, a method using pilot tones may be used. In such a scheme the phases of two adjacent pilot tones transmitted in the first frame are compared to determine the frame index. Because a sampling offset results in a progressive phase offset from bin to bin of a DFT, an examination of the extent of the phase offset between two known symbols will yield the sampling offset, and thus the frame index. 
     The frame synchronizer  426  examines the correlator output  800  and searches it for a high correlation output that is at least half the length of the periodic extension. The process is adaptive, starting with some threshold  802 , then adjusting down until it finds the correlation. Analysis of the correlation output may be facilitated by the application of a filter such as a moving average filter, or other low-pass filter. The synchronizer  426  provides the index into the data buffer containing the first sample of the first frame of receive time-domain data, and also provides the threshold at which the correlation was found for the purposes of algorithm validation. 
     FFT transformer  428  operates on the synchronized time-domain data to generate the frequency domain spectrum. The Fourier transform used within block  428  takes the real time-domain receive samples that have been properly framed and produces an output consisting of complex values containing real and imaginary components. The function used is equivalent to: 
           w   i     =       1     n       ⁢       ∑     k   =   1       n   -   1       ⁢           ⁢       d   k     ⁢     e     2   ⁢   π   ⁢           ⁢     i   ⁡     (     k   n     )       ⁢   j               ,       
 
for 0≦j &lt;64, where the d k  are the time domain data points, n is the length of the FFT, w j  are the complex-valued symbols, and i=√{square root over (−1)}.
 
     The transformed frequency domain data represents the magnitude and phase of the carriers. The FFT points are commonly referred to as “frequency bins.” The length of the output will contain half as many points as the real valued time-domain receive signal because only the first 64 points are calculated. As stated previously, the other frequency domain points are merely complex conjugates of the desired points, and are therefore not needed. 
     The data is then equalized in block  430  and demodulated in block  432 . The equalizer  430  is a frequency domain complex equalizer that simultaneously solves the problems of symbol timing error, clock error and drift, channel phase and attenuation distortion, and removes any number of echoes caused by reflections of unterminated wiring segments. This is accomplished in one mathematical step of low complexity. 
     A block diagram of the equalizer  430  is shown in FIG.  9 . In the preferred embodiment, the transmitted data is arranged in packets, with each packet consisting of concatenated frames transmitted in serial fashion. The first frame of a packet is an equalization frame of known symbols that is used to provide a coarse estimate of the channel. The receiver&#39;s equalizer  430  is trained to the channel using this frame by forming the ratio of the expected symbol to the received symbol for each frequency bin within the frame. The ratios for each tap of equalizer filter  920  are formed using the complex-valued frequency domain values that are readily available from the FFT block  428 . The result is a sequence of points (e.g., a vector) where each point corresponds to a frequency bin, and each value is an estimate of the inverse of the channel response at that frequency. 
     Multiplication of the frequency domain representation of the incoming frames by the equalizer taps results in a circular de-convolution of the channel impulse response. The circular de-convolution is made possible by the periodic extension, which makes the receive data to appear as if it had been circularly convolved by the channel impulse response. Thus the single step of multiplying the transformed data frames by the equalizer coefficients prior to demodulation corrects for channel impulse response distortion, sampling offset, clock/timing error, etc. 
     In the preferred embodiment of transceiver  400 , the use of only a single frame for the initial training of the equalizer  430  results in a lower signal to noise ratio than if the equalizer  430  is trained over a longer sequence of symbols. While this reduces the number of available constellation points (given a desired bit error rate), and hence reduces the data rate, the overall reduction of complexity in transceiver  400  is highly advantageous. For example, because the equalizer  430  is trained very quickly on a single frame of known symbols, the equalizer  430  need not retain channel information corresponding to a particular transmitter  402  of device  70  on LAN  20 . This is desirable, especially when numerous devices  70  are transmitting on the DMT LAN  20 . Without the rapid equalization scheme described herein, the equalizer  430  would typically either have to store large amounts of equalization data for each of the other transmitting devices  70  on the DMT LAN and retrain every device when the LAN characteristics change, or it would have to perform a lengthy retraining procedure each time a DMT LAN transmitter  402  initiated a session. To retrain the equalizers for every device would require much additional protocol functionality to implement the retraining procedure. Such a scheme would also create undesirable transmission delays on LAN  20 . 
     An alternative frequency domain equalization scheme is also provided. In the alternative embodiment, known equalization symbols are inserted into the data stream such that every N th  bin contains a known symbol. The known symbols are referred to herein as equalization symbols, and the bins are referred to as equalization bins. Preferably, every eighth bin is an equalization bin, and is used in every data frame. The frequency response of the channel at frequencies corresponding to the remaining bins is then estimated by interpolating between the received equalization symbols in the equalization bins. The equalization symbols may vary from equalization bin to equalization bin, but preferably the same set of predetermined estimation symbols is sent in every data frame. A running average for each equalization bin is calculated upon the receipt of a new frame once frame has been transformed to the frequency domain. The equalizer taps are updated using the averaged points corresponding to the equalization bins in addition to the points interpolated there-between. Standard interpolation techniques may be used to obtain the entire channel estimate from the running average of the equalization bins. 
     A decision feedback loop is used after the QAM demodulator to generate an error vector in DFE block  930  that is used to update the equalizer taps after each frame is processed. The DFE block  930  allows the equalizer to track slow changes in the channel and to track clock error between the transmitter and receiver. The DFE structure is not used, however, in the second preferred equalizer that utilizes the interpolative techniques discussed above. 
     The demodulator block  432  takes the complex frequency domain points for each bin after equalization, then demodulates those points back to real data. Demodulator  432  includes data slicers to determine the nearest constellation point to the received (and equalized) point. The demodulator may include a trellis decoder and other FEC decoders. The descrambler  434  reverses the scrambling of the data as described in the transmitter section based on the V.34 scrambler, or a block-based lookup table. Of course, DFE block  830  may update the taps based upon the decoded data decisions instead of the slicer outputs because the data decisions may be more accurate due to FEC processing. 
     A preferred embodiment of the present invention has been described herein. It is to be understood, of course, that changes and modifications may be made in the embodiment without departing from the true scope of the present invention, as defined by the appended claims.