Abstract:
A wireless data network, including a base station and multiple mobile wireless terminals, implements a synchronous 2-way communication protocol in which the availability of a communication channel is signalled by the base station in a control packet. The multiple mobile wireless terminals compete for acquisition of the communication channel by sending a request packet, upon detecting from the control packet that the communication channel is available. The base station grants the channel by acknowledging the request packet of a selected one of the mobile wireless data terminal. The mobile wireless data terminal communicates with the base station over the communication channel subsequent to acquisition. Upon the wireless data terminal relinquishing the communication channel, the base station sends out the next control packet indicating that the communication channel is again available.

Description:
CROSS REFERENCE TO RELATED PATENT APPLICATIONS 
     The present application is related to copending and commonly assigned U.S. patent application (“Copending Application”), Ser. No. 08/915,078, entitled “COMMUNICATION PROTOCOL FOR A WIRELESS DATA SYSTEM”, by Kwok Choi, now abandoned in favor of U.S. patent application, Ser. No. 09/574,686, which is a divisional application of the Copending Application. The Copending Application and the &#39;686 application are incorporated herein by reference in their entireties. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention generally relates to wireless data communication; and in particular, the present invention relates to network access protocols used in wireless data communication. 
     2. Background of the Invention 
     An example of a network access protocol for a two-way wireless data network is shown in FIG.  1  and described in detail in commonly assigned U.S. Patent Application (“&#39;860 Application”), Ser. No. 08/542,860, entitled “TWO-WAY WIRELESS DATA NETWORK”, by Weijia Wang, now U.S. Pat. No. 5,898,904, issued on Apr. 27, 1999. The &#39;760 Application is incorporated herein by reference in its entirety. 
     As shown in FIG. 1, a wireless data network  100  includes a wireless data terminal  101 , a cellularized base station  102 , a message control center  103 , interfaces  105 - 109  to information and communication applications, and radio links  115  and  117 . Message control center  103  provides a high power transmitter capable of broadcasting over a paging channel to wireless data terminals within the entire service area of wireless data network  100 . In this manner, wireless data network  100  is compatible with existing one-way paging services. Message control center  103  also communicates with base station  102  through radio link  117  which is a high power transmission (e.g. 3 watts). 
     Base station  102  is one of a number of base stations that are distributed throughout the service area of wireless data network  100 . Each base station serves a portion of the service area of wireless data network  100  within its immediate vicinity. The local service area of a base station is sometimes called a “cell”. The base stations broadcast to wireless data terminals in their respective cells through a local channel. Typically, the base stations cumulatively serve all locations within the service area of the wireless data network  100 . Wireless data terminal  101  communicates with one or more of the base stations, e.g., base station  102 , through radio link  115 . Radio link  115  needs only provide a low power transmission (e.g. 100 mW) to service the local service area. When wireless data terminal is outside the local service area of any base station, wireless data terminal  101  is restricted only to receiving messages from the 1-way paging channel. Other details of the operation of the two-way wireless data network can be found in the aforementioned &#39;860 Application and incorporated by reference above. 
     Enhancements to the network access scheme described above have been suggested so as to increase the network bandwidth and channel utilization of a two-way wireless data network. One such enhancement is described the Copending Application incorporated by reference above. The Copending Application discloses a network access protocol, referred to as the capture division packet access (CDPA) protocol, which allows a wireless data terminal to camp onto a local base station for two-way communication whenever the wireless data terminal is within the service range of the base station, thus bypassing the message control center all together. Under the “local override” mode of the CDPA protocol, a wireless data terminal registers with a local base station upon entering the local service area of the local base station, and subsequently communicates with that base station over the local channel. The local override mode avoids the undesired latency associated with high message traffic conditions in the paging channel. The CDPA protocol also supports a “local-only” mode in which the wireless data terminal ignores communication over the paging channel and camps onto the local channel at all times for two-way communication. 
     The CDPA protocol described in the Copending Application also includes other performance enhancement features. For instance, the CDPA protocol supports asymmetric downlink and uplink coverage areas in the local channel. The coverage pattern, i.e., the downlink coverage area is larger than the uplink coverage area, allows incremental start-up of a new 2-2-way wireless communication system. Under an asymmetric coverage area scheme, a wireless data terminal carries out two-way communication with a base station while within the uplink coverage area, and receives messages from the base station when outside the uplink coverage area but still within the downlink coverage area. 
     The CDPA protocol described in the Copending Application uses a variable speed retry scheme to avoid collision in the uplink and to extend the range of uplink communication. Under the variable speed retry scheme, when a previous transmission is unsuccessful, a wireless data terminal retransmits the data packet using alternatively a high data rate transmission and a low data rate transmission. Furthermore, the retransmissions are attempted at randomized time intervals so as to minimize collision with other wireless data terminals competing for the uplink. Other aspects of the CDPA protocol are described in details in the Copending Application incorporated by reference above. 
     The CDPA protocol described above is particularly useful when a wireless data terminal communicates primarily with a small number of local base stations. As described in the Copending Application, an example of such an application is found in a hospital where a user typically receives messages originated and received within the hospital during the normal work day. In that application, the hospital usually can be serviced by at most a small number of base stations under the control of a local message control center, forming what is sometimes referred to as a “private” system. The CDPA protocol supports wireless network communication both within the private system and outside the private system, sometimes referred to as the “public” system. 
     Performance problems can arise in the application described above where two-way communication is provided by a private system operating within a public system. For example, to provide coverage within a private system, base stations are sometimes installed closer together than they need to be in the counterpart public system. Such close deployment of base stations can result in overlapping coverage areas in the open space within the service area of the private system. When a wireless data terminal is operating in a coverage area of overlap, the wireless data terminal receives communications from more than one base station. However, interference among these base stations (“co-channel interference”) can occur, causing the wireless data terminal to receive no packet at all or unable to transmit to any of the base stations. A network access scheme capable of minimizing co-channel interference improves efficiency of the two-way wireless data network. 
     In the two-way wireless data network described above, wireless data terminals compete for the uplink bandwidth. Collisions resulting from simultaneous transmissions by two or more wireless data terminals can occur to significantly reduce the channel utilization rate. For instance, packet collisions are very common under the ALOHA protocol because wireless data terminals transmit without regard to channel availability. Collision avoidance schemes, such as the variable speed retry scheme described above, increase channel utilization. 
     SUMMARY OF THE INVENTION 
     The present invention provides a network access protocol which minimizes collisions in the uplink communication and reduces the number of retries needed for a successful uplink transmission. The network access protocol implemented according to the present invention, referred to as the digital sensing multiple access protocol with request-to-send and clear-to-send (the “DSMA/RC protocol”), improves channel utilization rate and transmission performance. Under the DSMA/RC protocol, collision is possible only in the first uplink data packet of a message. Subsequent data packets are transmitted without collision. 
     In one embodiment of the present invention, a method is provided in a two-way communication network. The method includes the steps of: (a) providing a communication channel having first and second sets of time slots; (b) providing a base station serving a service area and having a transceiver for communicating by radio in the communication channel; (c) sending a message from the base station in a first set of time slots indicating whether the communication channel is available; (d) allowing one or more mobile data terminals to transmit a request for access to the communication channel upon receiving the message from the base station; and (e) receiving at the base station the request for access to the communication channel in the second set of time slots, and (f) granting the request for access to the communication channel to the requesting mobile wireless data terminal. 
     In one implementation of the method, the message indicating communication channel availability is provided in a field within a header of a control packet. The indication of channel availability can be piggy-backed onto an acknowledgment message sent by the base station in response to a previous message received from the mobile wireless data terminal. 
     A protocol under the present invention can be implemented in both dual-frequency and single-frequency communication channels. Further, the protocol can be implemented in a full-duplex mode or a half-duplex mode. 
     In one implementation, the communication channel is divided into multiple logical channels. In one implementation, one of the logical channels provides for transmission of data packets from the wireless data terminal to the base station, and transmission of control packets from the base station to the mobile wireless data terminal. The data packets are sent in a predetermined sequence by the mobile wireless data terminal, and acknowledged one by one by the base station in one of the control packets. Each acknowledgment from the base station signals to the mobile wireless data terminal that the base station is ready to receive the next data packet in the predetermined sequence from the mobile wireless data terminal. 
    
    
     The present invention is better understood upon consideration of the detailed description below and the accompanying drawings. 
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 illustrates a wireless data network  100  to which the present invention is applicable. 
     FIG. 2 a  illustrates, in service area  220 A of a wireless data network  200 A, private system  222 A supporting a local override mode of a DSMA/RC protocol, in accordance with the present invention. 
     FIG. 2 b  illustrates, in service area  220 B of a wireless data network  200 B, private system  222 B operating according to an asymmetric area coverage scheme, in accordance with the present invention. 
     FIG. 3 shows the data fields of a downlink data packet under the DSMA/RC protocol of the present invention. 
     FIG. 4 shows the data fields of a uplink data packet under the DSMA/RC protocol of the present invention. 
     FIG. 5 shows the data fields of a downlink control packet under the DSMA/RC protocol of the present invention. 
     FIG. 6 shows the data fields of a uplink control packet under the DSMA/RC protocol of the present invention. 
     FIG. 7 a  illustrates a full duplex operation of the DSMA/RC protocol using physical channels  751   a  and  751   b.    
     FIG. 7 b  illustrates a half-duplex operation of the DSMA/RC protocol using physical channels  701   a  and  751   b.    
     FIG. 7 c  shows a frame for transmitting a data or control packet  701  under the DSMA/RC protocol. 
    
    
     In the detailed description below, like objects which appear in more than one figure are provided with like reference numerals. 
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The present invention provides a synchronous wireless communication protocol, referred to as “digital-sensing multi-access with request-to-send and clear-to-send control” (“DSMA/RC”), for use within two-way communication systems, including a two-communication system served by both a broadcast communication system (e.g. paging) and a “cellular” two-way communication system. In a cellular two-way communication system, the total service area is provided a number of base stations each serving a designated local service area within the total service area. Communication between a wireless data terminal and a base station can be half-duplex, full-duplex, single-frequency, or dual-frequency. In a dual-frequency system, the uplink (i.e., wireless terminal to base station) and the downlink (i.e., base station to wireless data terminal) use different frequencies. The uplink and the downlink can also operate at different data rates (e.g., 1200 baud for the uplink and 9600 baud for the downlink). 
     The DSMA/RC protocol supports a wireless data network operating under either a normal mode, a local override mode, or a local-only mode. In the normal mode, the wireless data network includes a message control center broadcasting over the paging channel and local stations communicating with wireless terminals over the local channel. A wireless data terminal camps on the paging channel for a wake-up message from a message control center. Upon receipt of such a wake-up message, the wireless data terminal switches to a local base station for two-way communication. A wireless data network using DSMA/RC can use one or more frequencies for communication between a base station and wireless data terminals. 
     One embodiment of the present invention is illustrated by a wireless data network  200 A operating under DSMA/RC shown in FIG.  2 A. Wireless data network  200 A includes both a public system and a private system within service area  220 A. For the purpose of this description, the private system is referred to by its service area as private system  222 A. The service area of private system  222 A is served by a base station  202 A. Private system  222 A is controlled by a private message control center  204 A and one or more base stations. Wireless data network  200 A also includes areas served by the public system, such as service area  221 A served by base station  201 A. According to the DSMA/RC protocol of the present invention, when a wireless data terminal enters private system  222 A, the wireless data terminal registers with private base station  202 A and camps onto the local channel for two-way communication. When the wireless data terminal exists private system  222 A, the wireless data terminal registers with other public systems within wireless data network  200 A for two-way communication. A local registration scheme such as that disclosed in the aforementioned Copending Application can be adopted in the DSMA/RC protocol for operating the local override mode in wireless data network  200 A. Wireless data network  200 A can also operate, under the DSMA/RC protocol, in the local-only mode. Under the local-only mode, a wireless data terminal does not receive communication from the paging channel. Instead, a wireless data terminal within wireless data network  200 A camps at all times on the local channel to which the wireless data terminal is registered. In one embodiment, the local modes (i.e., local override and local-only) under DSMA/RC are selected by setting a “LOC” bit in the downlink data or control packet. The formats of the data and control packets under DSMA/RC are described in more details below. 
     A wireless data network under DSMA/RC can support an asymmetric downlink and uplink coverage area pattern. Such a wireless data network is illustrated by wireless data network  200 B of FIG.  2 B. In FIG. 2B, a private system  222 B within service area  220 B includes base station  202 B. As shown in FIG. 2B, base station  202 B has an uplink range  222 B, within which a wireless terminal communicates bidirectionally with base station  202 B, and a downlink range  224 B, within which the wireless data terminal only receives messages from base station  202 B via the local channel. 
     A wireless data network under DSMA/RC can also support asymmetric data transmission rates. Asymmetric data rates can be advantageously employed to extend the uplink distance without correspondingly increasing the power requirements for a wireless data terminal. In accordance with the packet format described below, an “Asym” bit in the downlink packet indicates whether a wireless data terminal should use asymmetric communication. When the Asym bit is reset, the uplink and the downlink data rates are the same (i.e. symmetric), typically at 9600 baud. Alternately, when Asym bit is set, the downlink transmits at a higher data rate (e.g. 9600 baud) while the uplink transmits at a lower data rate, such as 1200 baud. The lower uplink data rate makes possible a longer uplink distance. 
     The DSMA/RC protocol of the present invention uses time-division multiplexing (TDM) to allocate bandwidths of one or more physicals channel among one or more logical channels. One example of full-duplex operations under the DSMA/RC using two physical channels  751   a  and  751   b  is illustrated by FIG. 7 a.  As shown in FIG. 7 a,  physical channels  751   a  and  751   b,  each assigned a different frequency, are allocated to provide a downlink and an uplink, respectively. Under full duplex operation, both physical channels can be simultaneously active (i.e., transmitting simultaneously). In FIG. 7 a,  a control packet, which is typically a short packet used to provide such control information as acknowledgment or non-acknowledgment (“ACK packet” or “NACK packet”), is transmitted in a frame represented by a small rectangle, and a data packet is transmitted in a frame represented by a large rectangle. The space between rectangles represents “guard” times (i.e., time periods of brief predetermined durations provided to prevent interference between transmissions). As discussed below with respect to FIG. 7 c,  a frame includes either a control packet or a data packet, along with certain preamble and synchronization information. 
     As shown in FIG. 7 a,  physical channels  751   a  and  751   b  are divided among five logical channels: message channels  752 ,  754 ,  755  and  756  and a “command” channel  753 . Message channels  752  and  756  are uplink message channels which allows data packets to be sent from wireless data terminals to a base station, and control packets to be sent from the base station to the wireless data terminals. For example, as shown in the FIG. 7 a,  data packets in frames  756   a  and  756   c  are data packets of message channel  756  from one or more wireless data terminals assigned to message channel  756  to a base station, and control packets in frames  756   b  and  756   d  are control packets of message channel  756  from the base station to the wireless data terminals. Similarly, control packets in frames  752   a  and  752   c  are control packets of message channel  752  sent from the base station to the wireless data terminals assigned to message channel  752 , and data packets in frames  752   b  and  752   d  are data packets from the wireless data terminals of message channel  752  to the base station. Message channels  754  and  755  are downlink message channels, which operate analogously as described above for message channels  752  and  756 , except that data packets are sent from the base station to the wireless terminals of message channels  754  and  755 , and control packets are sent from the wireless data terminals of message channels  754  and  755  to the base station. 
     Command channel  753  is used for transmitting control packets only, and the command channel time slots in both physical channels  751   a  and  751   b  are assigned to the base station and the wireless data terminals in a predetermined pattern. The pattern shown in FIG. 7 a,  for example, assigns the time slots for control packets in frames  753   a - 753   h  alternately between downlink communication (physical channel  751   a ) and uplink communication (physical channel  751   b ). 
     The DSMA/RC protocol can also be implemented for half-duplex operations (i.e., downlink communication and uplink communication cannot occur simultaneously). FIG. 7 b  shows an implementation of half-duplex operations of the DSMA/RC protocol using physical channels  751   a  and  751   b.  As shown in FIG. 7 b,  under a half-duplex operation, physical channels  751   a  and  751   b  are divided among three logical channels: message channels  771  and  772 , and command channel  773 . As shown in FIG. 7 b,  message channel  771  allows transmission of uplink data packets (e.g., data packets in frames  771   b  and  771   d ) from the wireless data terminals to a base station, and transmission of downlink control data packets (e.g. control data packets in frames  771   a  and  771   c ) from the base to the wireless data terminals. In an analogous manner, message channel  772  allows transmission of downlink data packets (e.g., data packets in frames  772   a  and  772   c ) from base station to the wireless data terminals, and transmission of uplink control data packets (e.g. data packets in frames  772   b  and  772   d ) from the wireless data terminals to the base station. Command channel  773  sends control packets (e.g., control packets in frames  773   a - 773   d ) in both physical channels  751   a  and  751   b  under a predetermined TDM pattern. As shown in FIG. 7 b,  time slots are allocated in physical channels  751   a  and  751   b  in an alternating pattern. The command channel is typically used for such function as local registration (i.e. notification by a wireless data terminal to a base station that the wireless data terminal is in the base station&#39;s local service area). In this embodiment, local registration is initiated by a wireless data terminal sending a “local registration request” (LRR) control packet (e.g., control packet in frame  753   a ) of command channel  753  in physical channel  751   b,  and completes when the base station returns a “local registration granted” (LRG) in a control packet (e.g., control packet in frame  753   b ) of command channel  753  in physical channel  751   a.    
     Half-duplex operation under the DSMA/RC protocol can also be implemented using a single frequency (i.e., a single physical channel). For example, if physical channels  751   a  and  751   b  use the same frequency, the implementation shown in FIG. 7 b  would illustrate operations of three logical channels 771-773 under the single-frequency half-duplex mode of the DSMA/RC protocol. 
     Although FIGS. 7 a  and  7   b  each illustrate physical channels  751   a  and  751   b  to be symmetrical (i.e., providing the same data rate), asymmetric implementations, i.e., different data rates in the uplink and downlink physical channels can be implemented. In asymmetric implementations, since the packet time in each physical channel is different, the number of message channels, or the number of time slots allocated to each logical channels, assigned to each physical channel is often different to ensure efficient use of the respective bandwidths. In the present embodiment, synchronization between the base station and the wireless data terminals for implementing any mode of the DSMA/RC protocol is provided by the base station. The base station can acquire its time base in a number of ways, for example, through signals received from the Global Positioning System (GPS) or by monitoring a broadcast source. The wireless data terminal acquires its time base by monitoring a synchronization sequence in the downlink communication. 
     FIG. 7 c  shows a frame  700  for transmitting a control or data packet  701  under the DSMA/RC protocol. As shown in FIG. 7 c,  in frame  700 , control or data packet  701  is preceded by a 16-bit preamble pattern  702  and a 16-bit synchronization pattern  703 . As mentioned above, to prevent transmissions from interfering with each other, sufficient guard time, indicated by ramp-up time  704   a  and ramp-down time  704   b  are provided before and after transmission of each packet, respectively. Preamble pattern  702  identifies whether the data or control packet is sent from a base station to a wireless data terminal, or from a wireless data terminal to a base station. Synchronization pattern  703  indicates the beginning of control or data packet  701 . In the present embodiment, data packets and control packets are each provided with different formats for uplink and downlink communications. The downlink data packet format and the uplink data packet format under the DSMA/RC protocol of the present invention are illustrated respectively by downlink data packet  300  in FIG.  3  and uplink data packet  400  in FIG.  4 . Similarly, the downlink control packet and the uplink control packet under the DSMA/RC protocol of the present invention are illustrated respectively by downlink control packet  500  in FIG.  5  and uplink control packet  600  in FIG.  6 . 
     Under the DSMA/RC protocol, the wireless data terminals compete for the uplink. To acquire the uplink, each wireless data terminal monitors control packets in the message channel to which it is assigned. A base station signals the availability of an uplink message channel by setting in each of the data and control packets an asserted end-of-transmission (EOT) flag. A deasserted EOT flag signals that the uplink message channel is busy. This method of monitoring and signalling the availability of a communication channel is known in the art as “digital sensing.” The base station sends “beacon” data and control packets when a message channel remains available. A beacon data or control packet is a dummy packet provided to signal that an active base station is present. In this embodiment, where local service areas of two or more base stations overlap, beacon data or control packets may not be sent to avoid co-channel interference. 
     To acquire an uplink, subsequent to detecting that a message channel is available, a wireless data terminal sends a first data packet at the next data packet time slot of the message channel. When the first data packet from a wireless data terminal is received, the base station grants the uplink message channel to the requesting wireless data terminal by designating a requesting wireless data terminal in an ACK control packet corresponding to the data packet received by the base station. This ACK control packet acts as a “clear-to-send” (CTS) control signal. If the ACK control packet is then received by the designated wireless data terminal, that wireless data terminal can send additional data packets in the uplink message channel. If the wireless data terminal does not receive an ACK control packet corresponding to its first data packet, the wireless data terminal retries the first data packet up to a programmable number of times at later times whenever the message channel becomes idle (i.e., using a “p-persistent” retry strategy). Upon granted the message channel, a wireless data terminal camps on the uplink and downlink physical channels until communication completes and the uplink is relinquished. Under the DSMA/RC protocol, each data packet is acknowledged by an ACK packet, which also serves as a “request-to-send” (RTS) signal for the next data packet. 
     Digital sensing avoids collision because a wireless data terminal will attempt to acquire an uplink only after detecting that a message channel is idle. Thus, collision occurs only among wireless data terminals in competing for an uplink message channel. Once an uplink message channel has been granted to a wireless data terminal, the uplink message channel is provided for the exclusive use by the wireless data terminal until the message channel is relinquished. Under the DSMA/RC protocol, traffic from the base station does not collide because the control or data packets are transmitted in reserved time slots. 
     In the case in which beacons are not sent when an uplink message channel is idle, to avoid co-channel interference, the DSMA/RC protocol operates in a manner similar to the Aloha protocol known in the art. Under that scheme, a wireless data terminal requests a message channel after detecting the absence of activity in the physical channel for a predetermined time duration. 
     As shown in FIG. 3, downlink data packet  300  includes a header portion  310 , a data payload portion  340 , and an error correction portion  350 . Header portion  310  includes (i) a one-byte “Base Station Color code” field  311 , identifying the base station to which the packet relates; (ii) a 1-bit “Data/Control” (D/C) field  312  set to “1”, identifying packet  300  as a data packet; (iii) a 1-bit “Packet Sync” (PS) field  313 , indicating whether the uplink packet is synchronous with the current packet; (iv) a 1-bit “FEC” field  314 , indicating that the wireless data terminal shall perform Reed-Solomon forward error correction function; (v) a 1-bit “No ACK” (NA) field  315 , indicating whether an acknowledgment (ACK) packet is required in response to the current data packet; (vi) a 1-bit “Asym” field  316 , indicating whether the wireless data terminal should use asymmetric data rates as described above; (vii) a 1-bit “Full” field  317 , indicating whether the channel is full duplex or half duplex; (viii) a 1-bit “Local” (LOC) field  318 , indicating whether a data packet is send under the local modes described above; (ix) a start bit  319 , indicating whether the current packet is the first packet of a message; (x) a stop bit  320 , indicating whether the current packet is the last packet of a message; (xi) a 4-byte “Pager ESN” field  321 , uniquely identifying the wireless data terminal; (xii) a 1-byte “Packet Sequence Number” field  322 , identifying the packet order information to be used to re-assemble the message; and (xiii) a 1-byte “length” field  323 , specifying the number of bytes in the data packet. Header portion  310  further includes seven “Reserved” (RES) bits whose functions are reserved for future expansion. All RES bits are set to the value of “0”. Error correction portion  350  includes (i) a 2-byte “Checksum” field  351 , representing the cyclic redundancy checksum of the packet from the beginning of data packet  300  to the last byte of data in payload  340 ; and (ii) 22 check bytes of RS code,  352  for error correction 
     Uplink data packet  400  (FIG. 4) includes a header portion  410 , a data payload  440 , and an error correction portion  450 . Uplink data packet  400  includes fields that are similar to those in downlink data packet  300 . Hence, similarly named fields in uplink data packet  400  are not further described. Header portion  410  of uplink data packet  400  includes (i) a one-byte “Base Station Color code” field  411 ; (ii) a 1-bit “Data/Control” (D/C) field  412  set to “1”, identifying packet  400  as a data packet; (iii) a 1-bit “FEC” field  414 ; (iv) a 1-bit “End-of-Transmission” (EOT) field  415 ; (v) a start bit  419 ; (vi) a stop bit  420 ; (vii) a 4-byte “Pager ESN” field  421 ; (viii) a 1-byte “Packet Sequence Number” field  422 ; and (ix) a 1-byte “length” field  423 . EOT field  415  of data packet  400  indicates whether the current packet is the last packet of the last message of a connection. 
     In this embodiment, the following types of control packets are provided: (i) an ACK packet used in both the uplink and the downlink; (ii) a NACK packet used in both the uplink and the downlink; (iii) a Wake-Up Response packet from a wireless data terminal to a base station; (iv) an LRR packet from a wireless data terminal to a base station requesting registration; and (v) an LRG packet from a base station to a wireless data terminal indicating that local registration is completed. The wake-up response, LRR and LRG packets are used in a command channel. 
     FIG. 5 illustrates the format of a downlink control packet  500 . Downlink control packet  500  includes a header portion  510 , a fixed-length data payload  540 , and an error correction portion  550 . Fields in downlink control packet  500  that are the same as in downlink data packet  300  are not further described. Header portion  510  includes (i) a one-byte “Base Station Color code” field  511 ; (ii) a 1-bit “Data/Control” (D/C) field  512  set to “0”, identifying packet  500  as a control packet; (iii) a 1-bit “Packet Sync” (PS) field  513 ; (iv) a 1-bit “FEC” field  514 ; (v) a 1-bit “EOT” field  515 ; (vi) a 1-bit “Asym” field  516 ; (vii) a 1-bit “Full” field  517 ; (viii) a 1-bit “Local” (LOC) field  518 ; and (ix) a 2-bit “Op-code”  519 , identifying the function of control packet  500 . Data payload  540  of downlink control packet  500  contains two bytes of data, first data byte  541  and second data byte  542 , which are associated with the functions of downlink control packet  500 . For instance, in an ACK packet, first data byte  541  contains the value of the received data packet&#39;s sequence number (PSN) plus 1. In an NACK packet, first data byte  541  contains the value of the received data packet&#39;s PSN. In an LRG packet, first data byte  541  is set to “1” to indicate that local registration request is granted. For the other types of control packet, such as Wake-Up Response and LRR, first data byte  541  is not used. In all types of control packets, second data byte  542  contains the System ID, identifying the wireless data network. Following data payload  540 , error correction portion  550  includes (i) a 2-byte “Checksum” field  551 , representing the cyclic redundancy checksum of the packet from the beginning of data packet  500  to the last byte of data in payload  540 ; and (ii) a 6-byte Reed Solomon code  552  for error correction function. 
     Uplink control packet  600 , as shown in FIG. 6, is similar to downlink control packet  500 . Similarly named fields in uplink control packet  600  are not further described. Header portion  610  of uplink control packet  600  includes (i) a one-byte “Base Station Color code” field  611 ; (ii) a 1-bit “Data/Control” (D/C) field  612  set to “0”, identifying packet  600  as a control packet; (iii) a 1-bit “FEC” field  614 ; and (iv) a 2-bit “Op-code”  619 . Data payload  640  of uplink control packet  600  is defined in the same manner as data payload  540  of downlink control packet  500 . 
     Data integrity can be enhanced by the use of error correcting codes, such as Reed-Solomon (RS) code. In this embodiment, a 17-byte RS code, including 6 “check” bytes is used for a control packet, and a 66-byte RS code, including 22 check bytes is used for a data packet. Other techniques, such as interleaving or bit scrambling, can be used to further enhance transmission integrity by avoiding long runs of 1&#39;s or 0&#39;s. 
     The above detailed description are provided to illustrate the specific embodiments of the present invention and is not intended to be limiting. Numerous modifications and variations within the scope of the present invention are possible. The present invention is defined by the appended claims thereto.