Abstract:
There is disclosed for use in a wireless communications device, a medium access control (MAC) message acknowledgment system for acknowledging MAC messages transmitted in an RF control channel between the wireless communications device and a remote communications unit. The MAC message acknowledgment system comprises a control processor capable of receiving an outbound MAC message unit from a MAC layer device in the wireless communications device and attaching a header to the outbound MAC message unit to thereby form an outbound MAC message suitable for transmission to the remote communications unit, the header comprising logic bits identifying the outbound MAC message unit to the remote communications device to thereby enable the remote communications unit to acknowledge receipt of the outbound MAC message.

Description:
TECHNICAL FIELD OF THE INVENTION 
     The present invention is directed, in general, to wireless communication networks and, more specifically, to a system for providing reliable delivery of medium access control (MAC) messages over the air interface of a wireless communication network. 
     BACKGROUND OF THE INVENTION 
     Reliable predictions indicate that there will be over 300 million cellular telephone customers by the year 2000. Within the United States, cellular service is offered not only by dedicated cellular service providers, but also by the regional Bell companies, such as U.S. West, Bell Atlantic and Southwestern Bell, and the national long distance companies, such as AT&amp;T and Sprint. The enhanced competition has driven the price of cellular service down to the point where it is affordable to a large segment of the population. As a result, wireless subscribers use a wide variety of wireless devices, including cellular phones, personal communication services (PCS) devices, and wireless modem-equipped personal computer (PCs), among others. 
     To maximize usage of the available bandwidth, a number of multiple access technologies have been implemented to allow more than one subscriber to communicate simultaneously in a wireless system. These multiple access technologies include time division multiple access (TDMA), frequency division multiple access (FDMA), and code division multiple access (CDMA). These technologies assign each system subscriber to a specific traffic channel that transmits and receives subscriber voice/data signals via a selected time slot, a selected frequency, a selected unique code, or a combination thereof. 
     Generally, a multiple access wireless system uses dedicated control channels to establish, to maintain, and to break down a connection between a subscriber&#39;s mobile device (also called “mobile unit” or “mobile station”) and the wireless system. The control channel signals transmitted between the base transceiver stations of the wireless system and the mobile units are generally referred to as medium access control (MAC) messages. The MAC layers of the base transceiver stations and the mobile units are the lower half of the data link layer that defines topology dependent access control protocols in a communication network. The MAC layer specifies the message frame formats as well as the conditions for accessing the traffic channels of the wireless network. 
     MAC messages are transmitted in a forward control channel from a base transceiver station to one or more mobile units and in a reverse control channel from the mobile station to the base transceiver station. At the start of a wireless telephone call or data transmission, the MAC messages establish a connection and assign the subscriber to a selected traffic channel. Once the connection is established, the subscriber and the base transceiver station exchange voice and/or data signals via the selected traffic channel. MAC messages are used to maintain the connection and to handle any hand-offs that are performed between base transceiver stations. 
     The performance and reliability of a wireless network is at least partially determined by the reliability with which MAC messages are exchanged between mobile units and base transceiver stations in the wireless network. If a MAC,message is lost in transmission or received out-of-sequence due to the loss of another MAC message, the wireless network typically must re-transmit at least one, and usually several, of the MAC messages in order to compensate for the one or more lost MAC messages. 
     There is therefore a need in the art for wireless networks that provide more reliable communications between mobile units and base transceiver stations in the wireless network. In particular, there is a need in the art for wireless networks that exchange MAC messages in a more reliable manner. 
     SUMMARY OF THE INVENTION 
     To address the above-discussed deficiencies of the prior art, it is a primary object of the present invention to provide, for use in a wireless communications device, a medium access control (MAC) message acknowledgment system for acknowledging MAC messages transmitted in an RF control channel between the wireless communications device and a remote communications unit. In one embodiment of the present invention, the MAC message acknowledgment system comprises a control processor capable of receiving an outbound MAC message unit from a MAC layer device in the wireless communications device and attaching a header to the outbound MAC message unit to thereby form an outbound MAC message suitable for transmission to the remote communications unit, the header comprising logic bits identifying the outbound MAC message unit to the remote communications device to thereby enable the remote communications unit to acknowledge receipt of the outbound MAC message. 
     In another embodiment of the present invention, the MAC message acknowledgment system further comprises a timer coupled to and controllable by the control processor, wherein the timer stores a delay period the MAC message acknowledgment system will wait before re-transmitting the outbound MAC message. 
     In still another embodiment of the present invention, the MAC message acknowledgment system further comprises a memory coupled to and controllable by the control processor, wherein the control processor stores the outbound MAC message in the memory if a receipt of a previously transmitted MAC message has not been acknowledged by the remote communications unit. 
     In yet another embodiment of the present invention, the wireless communications device is a mobile device. 
     In a further embodiment of the present invention, the wireless communications device is a base transceiver station in a wireless network. 
     In a yet further embodiment of the present invention, the control processor is capable of receiving from a transceiver an inbound MAC message transmitted by the remote communications unit. 
     In a still further embodiment of the present invention, the control processor detects an embedded header in the received inbound MAC message and uses the embedded header to acknowledge to the remote communications unit a receipt of an embedded MAC message unit in the received inbound MAC message. 
     In yet another embodiment of the present invention, the control processor modifies a header in a second outbound MAC message in order to acknowledge receipt of the received inbound MAC message. 
     The foregoing has outlined rather broadly the features and technical advantages of the present invention so that those skilled in the art may better understand the detailed description of the invention that follows. Additional features and advantages of the invention will be described hereinafter that form the subject of the claims of the invention. Those skilled in the art should appreciate that they may readily use the conception and the specific embodiment disclosed as a basis for modifying or designing other structures for carrying out the same purposes of the present invention. Those skilled in the art should also realize that such equivalent constructions do not depart from the spirit and scope of the invention in its broadest form. 
     Before undertaking the DETAILED DESCRIPTION, it may be advantageous to set forth definitions of certain words and phrases used throughout this patent document: the terms “include” and “comprise,” as well as derivatives thereof, mean inclusion without limitation; the term “or,” is inclusive, meaning and/or; the phrases “associated with” and “associated therewith,” as well as derivatives thereof, may mean to include, be included within, interconnect with, contain, be contained within, connect to or with, couple to or with, be communicable with, cooperate with, interleave, juxtapose, be proximate to, be bound to or with, have, have a property of, or the like; and the term “controller” means any device, system or part thereof that controls at least one operation, such a device may be implemented in hardware, firmware or software, or some combination of at least two of the same. It should be noted that the functionality associated with any particular controller may be centralized or distributed, whether locally or remotely. Definitions for certain words and phrases are provided throughout this patent document, those of ordinary skill in the art should understand that in many, if not most instances, such definitions apply to prior, as well as future uses of such defined words and phrases. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     For a more complete understanding of the present invention, and the advantages thereof, reference is now made to the following descriptions taken in conjunction with the accompanying drawings, wherein like numbers designate like objects, and in which: 
     FIG. 1 illustrates an exemplary wireless network according to one embodiment of the present invention; 
     FIG. 2 illustrates an exemplary MAC acknowledgment request systems in a base transceiver station and a mobile unit according to one embodiment of the present invention; 
     FIG. 3 illustrates an exemplary MAC message packet suitable for transmission between a base transceiver station and a mobile unit according to one embodiment of the present invention; 
     FIG. 4 is a flow diagram illustrating the operation of an exemplary MAC acknowledgment request system according to one embodiment of the present invention; and 
     FIG. 5 is a flow diagram  500  illustrating the operation of an exemplary MAC acknowledgment request system during receipt of a MAC message packet from another wireless device according to one embodiment of the present invention. 
    
    
     DETAILED DESCRIPTION 
     FIGS. 1 through 5, discussed below, and the various embodiments used to describe the principles of the present invention in this patent document are by way of illustration only and should not be construed in any way to limit the scope of the invention. Those skilled in the art will understand that the principles of the present invention may be implemented in any suitably arranged communications network. 
     FIG. 1 illustrates an exemplary wireless network  100  according to one embodiment of the present invention. The wireless telephone network  100  comprises a plurality of cell sites  121 - 123 , each containing one of the base transceiver stations, BTS  101 , BTS  102 , or BTS  103 . In a preferred embodiment of the present invention, the wireless telephone network  100  is a CDMA-based network. Base transceiver stations  101 - 103  are operable to communicate with a plurality of mobile units (M)  111 - 114 . Mobile units  111 - 114  may be any suitable cellular devices, including conventional cellular telephones, PCS handset devices, portable computers, metering devices, and the like. 
     Dotted lines show the approximate boundaries of the cells sites  121 - 123  in which base transceiver stations  101 - 103  are located. The cell sites are shown approximately circular for the purposes of illustration and explanation only. It should be clearly understood that the cell sites may have other shapes, such as hexagonal, depending on the cell configuration selected and natural and man-made obstructions. 
     BTS  101 , BTS  102  and BTS  103  transfer voice and data signals between each other and the public telephone system (not shown) via communications line  131 . Communications line  131  may be any suitable connection means, including a T 1  line, a T 3  line, a fiber optic link, a network backbone connection, and the like. In some embodiments, BTS  101 , BTS  102  and BTS  103  may be wirelessly linked to one another and/or the public telephone network by a satellite link. 
     In the exemplary wireless network  100 , mobile unit  111  is located in cell site  121  and is in communication with BTS  101 , mobile unit  113  is located in cell site  122  and is in communication with BTS  102 , and mobile unit  114  is located in cell site  123  and is in communication with BTS  103 . The mobile unit  112  is located in cell site  121 , close to the edge of cell site  123 . The mobile unit  112  towards cell site  123 . At some point as mobile unit  112  moves into cell site  123  and out of cell site  121 , a “handoff” will occur. 
     A “handoff” is a well-known process for transferring control of a call from a first cell to a second cell. For example, if mobile unit  112  is in communication with BTS  101  and senses that the signal from BTS  101  is becoming unacceptably weak, mobile  112  may then switch to a BTS that has a stronger signal, such as the signal transmitted by BTS  103 . Mobile unit  112  and BTS  103  establish a new communication link and a signal is sent to BTS  101  and the public telephone network to transfer the on-going voice and/or data signals through the BTS  103 . The call is thereby seamlessly transferred from BTS  101  to BTS  103 . 
     BTS  101 , BTS  102 , and BTS  103  transmit MAC messages in a forward control channel to the respective ones of mobile units  111 ,  112 ,  113  or  114  and receive MAC messages in a reverse control channel from the mobile units. The MAC messages are transmitted in MAC control channels and are used to establish, to maintain, and to break down the traffic channel communication links carrying the voice and/or data signals between the base transceiver stations and the mobile units. To ensure the reliability of the transfer of MAC messages between the base transceiver stations and the mobile units, MAC acknowledgment request systems in accordance with the principles of the present invention are implemented in both the base transceiver stations and the mobile units. 
     FIG. 2 illustrates exemplary MAC acknowledgment request systems  210  and  260  in base transceiver station  101  and mobile unit  112  according to one embodiment of the present invention. FIG. 3 illustrates an exemplary MAC message packet  300  suitable for transmission between base transceiver station  101  and mobile unit  112  according to one embodiment of the present invention. MAC message packet  300 , which may be either an outgoing or an incoming packet, comprises control field  301 , sequence number  302 , and MAC message unit (MMU)  303 , as explained below in greater detail. MAC acknowledgment request system  210  and  260  are responsible for reliably delivering MAC message packets between BTS  101  and mobile unit  112 . MAC messages may originate in MAC layers  201  and  251  of either mobile unit  112  or BTS  101  and are transferred to transceivers  230  and  280  for transmission. 
     MAC acknowledgment request system  210  in BTS  101  comprises MAC acknowledgment control processor  211 , timer  212 , and memory  213 . Memory  213  contains internal received sequence number (IRSN)  221 , internal sent sequence number (ISSN)  222 , send buffer  223 , and queue  225 . As explained below in greater detail, MAC acknowledgment request system  210  transfers MAC message units between the medium access control (MAC) layer  201  of BTS  101  and the transceiver system  230  of BTS  101 . 
     Mobile unit  112  is comprised of similar components. MAC acknowledgment request system  260  in mobile unit  112  comprises MAC acknowledgment control processor  261 , timer  262 , and memory  263 . Memory  263  contains internal received sequence number (IRSN)  271 , internal sent sequence number (ISSN)  272 , send buffer  273 , and queue  275 . MAC acknowledgment request system  260  transfers MAC message units between the medium access control (MAC) layer  251  of mobile unit  112  and the transceiver system  280  of mobile unit  112 . 
     MAC acknowledgment request system  210  and MAC acknowledgment request system  260  perform mirror image functions in BTS  101  and mobile unit  112 , respectively. This being the case, the operations of MAC acknowledgment request system  210  and MAC acknowledgment request system  260  are functionally identical. The operations of both systems can therefore be explained by explaining the operation of only one. For the sake of clarity and simplicity, the present invention will hereafter be explained principally, but not exclusively, in terms of the operation of MAC acknowledgment request system  210 . It will be understood by those skilled in the art, however, that these descriptions also apply to MAC acknowledgment request system  260 . 
     MAC acknowledgment request system  210  is an acknowledgment (ACK) based system that uses timer  212  to control the retransmission of MAC messages that are not acknowledged. In a preferred embodiment of the present invention, MAC acknowledgment request system  210  allows only one outstanding message (per instance) at any one time. 
     MAC acknowledgment control processor  211 , which controls the operations of MAC acknowledgment request system  210 , receives outgoing MAC message unit (MMU)  303  from MAC layer  201 . MMU  303 , whether incoming or outgoing, may contain a variable number of bits. MAC acknowledgment request system  210  attaches a header, comprising control field  301  and sequence number  302 , to outgoing MMU  303 , thus creating an outgoing MAC message packet similar to exemplary MAC message packet  300  in FIG.  3 . Outgoing MAC message packet  300  is then sent to transceiver system  230  for transmission to mobile unit  112 . 
     When MAC acknowledgment request system  210  receives incoming MAC message packet  300  from transceiver system  230 , MAC acknowledgment control processor  211  strips off the header and sends the resulting incoming MAC message unit  303  to MAC layer  201 . 
     In one embodiment of the present invention, the header consists of three (3) bits: a two-bit control field  301  and a one-bit sequence number  302 . MMU  303  may be a variable number of bits in length, however, in a preferred embodiment of the present invention, MMU  303  is twenty-one (21) bits long. MAC acknowledgment control processor  211  maintains a 1-bit internal received sequence number (IRSN)  221  in memory  213  that holds the sequence number  302  of the last incoming MMU  303  that was sent to MAC layer  201 . When MAC acknowledgment request system  210  receives incoming MAC message packet  300  containing incoming MMU  303 , MAC acknowledgment control processor  211  compares sequence number  302  of new incoming MMU  303  to IRSN  221 . Incoming MMU  303  is sent to MAC layer  201  only if sequence number  302  does not match IRSN  221 . In this manner, if MAC message packet  300  is retransmitted because of a lost acknowledgment message, the same MAC message packet  300  will not be sent twice to MAC layer  201  by receiving MAC acknowledgment request system  210 . 
     MAC acknowledgment control processor  211  also maintains a 1-bit internal sent sequence number (ISSN)  222  which indicates the sequence number  302  of the last message sent. In a preferred embodiment of the present invention of the present invention in which MAC acknowledgment request system  210  can have only one outstanding MAC message packet  300  at a time, a 1-bit sequence number  302  is sufficient to perform the foregoing operation. In alternate embodiments of the present invention, sequence number  302  may contain more than one bit in order to maintain a count of a larger number of outstanding MAC message packets  300 . 
     The value in control field  301  determines the meaning of MMU  303  in each MAC message packet  300 . A control field  301  value of “10” (binary) indicates to a receiving device that the current incoming MAC message packet  300  constitutes an acknowledgment from a transmitting device that the transmitting device has successfully received an MMU  303  previously transmitted by the receiving device and also indicates that the current incoming MAC message packet  300  does not contain a new incoming MMU  303  (the rest of MAC message packet  300  is padded with 0&#39;s) 
     A control field  301  value of “01” (binary) indicates to a receiving device that the current incoming MAC message packet  300  contains a new incoming MMU  303 , but does not constitute an acknowledgment of an MMU  303  previously transmitted by the receiving device. 
     A control field  301  value of “11” (binary) indicates to a receiving device that the current incoming MAC message packet  300  contains a new incoming MMU  303  and also constitutes an acknowledgment from a transmitting device that the transmitting device has successfully received an MMU  303  previously transmitted by the receiving device. 
     A control field  301  value of “00” (binary) is a reserved value. 
     In general, MAC acknowledgment request system  210  may send/receive two types of acknowledgments: an acknowledgment may be sent just by itself (a “pure” acknowledgment having control field  301  set to “10”), or it may be “piggybacked” with a new MMU  303  (control field  301  set to “11”). If the acknowledgment is piggybacked on a new MMU  303 , the sequence number  302  of the MAC message packet  300  refers to the sequence number  302  of the new MMU  303 . 
     Since MAC acknowledgment request system  210  may receive an outgoing MMU  303  from MAC layer  201  while it already has an outgoing MAC message packet  300  outstanding (i.e., waiting for an acknowledgment), MAC acknowledgment control processor  211  uses queue  225  to store each outgoing MMU  303  until it can be serviced. In addition, MAC acknowledgment control processor  211  uses send buffer  223  to store the current outgoing MAC message packet  300  (i.e., a MAC message packet  300  that has not been sent nor acknowledged yet). Finally, MAC acknowledgment request system  210  uses timer  212  to dictate retransmissions of outstanding MAC message packets  300 . 
     MAC acknowledgment request system  210  has two operating modes: Idle and Packet Outstanding. If MAC acknowledgment request system  210  is in Idle mode and receives an incoming MAC message packet  300  from transceiver  230 , MAC acknowledgment control processor  211  strips the header off the received packet and checks the sequence number  302 . If the sequence number  302  is different than IRSN  221 , the MAC message packet  300  is a new message and the incoming MMU  303  contained therein is sent to MAC layer  201 . IRSN  221  is then updated to be the sequence number  302  of the MAC message packet  300  just received. 
     If the sequence number  302  of the received MAC message packet  300  matches IRSN  221 , the MAC message packet  300  is a retransmission of a message packet that has already been successfully received in BTS  101 , and the MAC message packet  300  is discarded. The likely cause of the retransmission is the failure of mobile unit  112  to receive an acknowledgment of the first transmission of the message packet. ISSN  222  is not updated in this case. 
     In either case of the foregoing situations, MAC acknowledgment request system  210  sends a pure acknowledgment message (control field  301 =10) to transceiver  230  for transmission to mobile unit  112 . 
     If MAC acknowledgment request system  210  is in Idle mode and receives an outgoing MMU  303  from MAC layer  201 , it updates ISSN  222  (flips the bit), and copies this value into the sequence number  302  of the header for the outgoing MAC message packet  300 . Control field  301  of the header is set to “01” (no acknowledgment) After attaching the header to the outgoing MAC message packet  300 , MAC acknowledgment control processor  211  copies the outgoing MAC message packet  300  into send buffer  223  and sends the outgoing MAC message packet  300  to transceiver  230  for transmission. MAC acknowledgment control processor  211  then starts the retransmission timer  212  and transitions to the Packet Outstanding State. 
     If, while in Packet Outstanding State, MAC acknowledgment request system  210  receives another outgoing MMU  303  from MAC layer  201 , the outgoing MMU  303  is put in queue  225 . If the retransmission timer  212  expires while MAC acknowledgment request system  210  is in the Packet Outstanding State, MAC acknowledgment control processor  211  retrieves the outgoing MAC message packet  300  currently in send buffer  223  and sends it to transceiver  230  for retransmission. In some embodiments of the present invention, whenever timer  212  expires, a counter in memory  213  may be incremented and, if the counter exceeds a pre-determined system value, error recovery procedures are started (reset and initialization). 
     If MAC acknowledgment request system  210  receives an acknowledgment while in Packet Outstanding State, MAC acknowledgment request system  210  flushes send buffer  223 . Subsequent operations of MAC acknowledgment request system  210  depend on whether or not queue  225  is empty. 
     If queue  225  is empty, and MAC acknowledgment request system  210  receives a pure acknowledgment (control field  301 =10), MAC acknowledgment request system  210  transitions to the Idle State. If the acknowledgment was piggybacked on a new incoming MMU  303  (control field  301 =11), MAC acknowledgment control processor  211  strips the header off the incoming MAC message packet  300  and compares the sequence number  302  with IRSN  221 . If the sequence numbers do not match, the new incoming MMU  303  is sent to MAC layer  201  and IRSN  221  is updated (the bit is flipped). If the numbers match, the MAC message packet  300  is discarded. In either case, MAC acknowledgment request system  210  then sends a pure acknowledgment message (control field  301 =10) to transceiver  230 . MAC acknowledgment request system  210  then transitions to the Idle State. 
     If queue  225  is not empty, and MAC acknowledgment request system  210  receives a pure acknowledgment message (control field  301 =10), MAC acknowledgment control processor  211  retrieves the next outgoing MMU  303  from queue  225 . After updating ISSN  222  (flipping the bit), the sequence number  302  in the header is set to the value of ISSN  222 . Control field  301  is set to “01”, the new outgoing MAC message packet  300  is stored in send buffer  223  and sent to transceiver  230 , and timer  212  is reset. 
     If the acknowledgment was piggybacked on a new incoming MMU (control field  301 =11), MAC acknowledgment control processor  211  first processes the incoming MAC message packet  300  as described above. MAC acknowledgment control processor  211  then retrieves the next outgoing MMU  303  from queue  225  and processes it as above, except that control field  301  is set to “11”. The new outgoing MAC message packet  300  is stored in send buffer  223  and sent to transceiver  230 , and timer  212  is reset. MAC acknowledgment request system  210  remains in the Packet Outstanding State. 
     If, while in Packet Outstanding State, MAC acknowledgment request system  210  receives an incoming MAC message packet  300  that does not contain an acknowledgment, MAC acknowledgment control processor  211  strips the header from the incoming MAC message packet  300  and sends the incoming MMU  303  to MAC layer  201  (after checking sequence number  302  against IRSN  221 ). A pure MAC acknowledgment message (control field  301 =10) is then sent to transceiver  230 . No further action is taken in this case. 
     FIG. 4 is a flow diagram  400  illustrating the operation of an exemplary MAC acknowledgment request system  210  during receipt of an incoming MMU from MAC layer  201  according to one embodiment of the present invention. Both Idle mode and Packet Outstanding mode are depicted. While MAC acknowledgment request system  210  is in Idle mode, an MMU  303  is initially received from MAC layer  201  (process step  401 ). MAC acknowledgment request system  210  attaches a header to newly received MMU  303  and sets control field  301  to “01” (process step  402 ). Next, MAC acknowledgment request system  210  stores a copy of the outbound MAC message packet  300  in send buffer  223  (process step  403 ) and also sends the outbound MAC message packet  300  to transceiver  230  (process step  404 ). 
     MAC acknowledgment request system  210  then sets timer  212  to a predetermined system value that establishes the time period that MAC acknowledgment request system  210  will wait for a response to the outbound MAC message packet  300  (process step  405 ). Once the outbound MAC message packet has been passed to transceiver  230 , MAC acknowledgment request system  210  enters the Packet Outstanding mode. 
     While MAC acknowledgment request system  210  is in Packet Outstanding mode, an MMU  303  may be received from MAC layer  201  (process step  451 ). Because MAC acknowledgment request system  210  is still waiting to receive an acknowledgment message for the presently outstanding MAC message packet, the newly received MMU  303  is stored in queue  225  (process step  452 ). At this point, MAC acknowledgment request system  210  waits until an acknowledgment message is received from mobile unit  112 . 
     If an acknowledgment message only is received, MAC acknowledgment request system  210  takes the following actions. MAC acknowledgment request system  210  fetches MMU  303  from queue  225 , attaches a header with control field=“01”, updates ISSN  222 , and sets sequence number  302  equal to ISSN  222 . MAC acknowledgment request system  210  then stores a copy of the outbound MAC message packet  300  in send buffer  223  and sends the outbound MAC message packet  300  to transceiver  230 . Finally, MAC acknowledgment request system  210  sets timer  212  to wait for a response to the outbound MAC message packet  300  (process step  453 ). 
     If MAC acknowledgment request system  210  receives an acknowledgment message that also contains a new inbound MMU  303  from mobile unit  112 , MAC acknowledgment request system  210  takes the following actions. MAC acknowledgment request system  210  first processes the newly received inbound MMU  303  from mobile unit  112  as in FIG.  5 . Next, MAC acknowledgment request system  210  fetches the outbound MMU  303  from queue  225  and attaches a header with control field=“11”. MAC acknowledgment request system  210  then stores a copy of the outbound MAC message packet  300  in send buffer  223  and also sends the outbound MAC message packet  300  to transceiver  230 . Finally, MAC acknowledgment request system  210  sets timer  212  to establish the waiting period for a response acknowledgment from mobile unit  112  (process step  454 ). 
     While MAC acknowledgment request system  210  is in Packet Outstanding mode, timer  212  may expire (process step  455 ). MAC acknowledgment request system  210  responds by re-transmitting the outbound MAC message packet  300  in send buffer  223  (process step  456 ). MAC acknowledgment request system  210  then increments the error counter. If the error counter exceeds a maximum allowed value, error procedures are initiated (process step  457 ). 
     If an incoming MMU  303  is received without an acknowledgment message while MAC acknowledgment request system  210  is in Packet Outstanding mode, MAC acknowledgment request system  210  responds by sending back an acknowledgment message with control field  301  set to “10” and processes the incoming MMU  303  as in FIG. 5 (process step  458 ). 
     FIG. 5 is a flow diagram  500  illustrating the operation of an exemplary MAC acknowledgment request system  210  during reception of a MAC message packet  300  from mobile unit  112  according to one embodiment of the present invention. Initially, MAC acknowledgment request system  210  is in Idle mode. Idle mode is interrupted when MAC acknowledgment request system.  210  receives a MAC message packet  300  from transceiver  230  (process step  501 ). MAC acknowledgment request system  210  reads the header of the received MAC message packet  300  and compares the sequence number  302  therein to the IRSN  221  in memory  213  (process step  502 ). 
     If the sequence number  302  and IRSN  221  are different, MAC acknowledgment request system  210  sends the newly received MMU  303  to MAC layer  201 , and updates the value of IRSN  221 . MAC acknowledgment request system  210  then sends an acknowledgment message back to mobile unit  112  via transceiver  230  (process step  503 ). If the sequence number  302  and IRSN  221  are the same, MAC acknowledgment request system  210  discards the redundant MMU  303  and does not update IRSN  221 . MAC acknowledgment request system  210  then sends an acknowledgment message back to mobile unit  112  via transceiver  230  (process step  504 ) 
     Although the present invention has been described in detail, those skilled in the art should understand that they can make various changes, substitutions and alterations herein without departing from the spirit and scope of the invention in its broadest form.