Abstract:
A first router receives an original IS-634 standard frame formatted message over an A-bis interface from a base transceiver station. The first router identifies header information bits in the message for compression. The first router suppresses selected ones of the identified header information bits and encapsulates the message into a packet. A second router receives the packet from the first router and extracts the message from the packet. The second router reconstructs the selected ones of the header information bits that were suppressed by the first router. The second router regenerates the original IS-634 standard frame formatted message and transports the message to a base station controller over the A-bis interface.

Description:
TECHNICAL FIELD OF THE INVENTION 
     The present invention relates in general to communications signal processing and more particularly to a system and method for optimizing data transport in a communications system. 
     BACKGROUND OF THE INVENTION 
     Code Division Multiple Access (CDMA) technology has made use of backhaul transports by optimizing the L3 transport encapsulation to a point where it is difficult to obtain any additional bandwidth optimization. Since operational expense reduction is a key component to securing a wireless market space, a technique is needed to further optimize the transport in order to provide a competitive edge. More voice call and data flow capacity per transport link is need to obtain the competitive edge in the market. 
     SUMMARY OF THE INVENTION 
     From the foregoing, it may be appreciated by those skilled in the art that a need has arisen for a technique to obtain additional bandwidth optimization in a communications system. In accordance with the present invention, a system and method for optimizing data transport in a communications system are provided that substantially eliminate or greatly reduce disadvantages and problems associated with conventional bandwidth optimizations schemes. 
     According to an embodiment of the present invention there is provided a system for optimizing data transport in a communications system that includes a first router that receives an original IS-634 standard frame formatted message over an A-bis interface from a base transceiver station. The first router identifies header information bits in the message for compression and suppresses selected ones of the identified header information bits. The message is encapsulated into a packet for transport to a second router. The second router receives the packet from the first router and extracts the message from the packet. The second router reconstructs the selected ones of the header information bits that were suppressed by the first router. The second router regenerates the original IS-634 standard frame formatted message and transports the message to a base station controller over the A-bis interface. 
     The present invention provides various technical advantages over conventional bandwidth optimization schemes. Some of these technical advantages are shown and described in the description of the present invention. Embodiments of the present invention may enjoy some, all, or none of these advantages. Other technical advantages may be readily apparent to one skilled in the art from the following figures, description, and claims. 
    
    
     
       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 description taken in conjunction with the accompanying drawings, wherein like reference numerals represent like parts, in which: 
         FIG. 1  illustrates a block diagram of a communications system; 
         FIGS. 2A-2B  illustrate example message structures transported within the communications system; 
         FIG. 3  illustrates a block diagram of a router implementation within the communications system. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       FIG. 1  shows a simplified block diagram of a communications system  10 . Communications system  10  includes one or more mobile or fixed users  12  that are capable of wireless communications. Users  12  receive traffic from and provide traffic to one or more base transceiver stations  14 . Base transceiver stations  14  provide traffic to and receive traffic from one or more base station controllers  16 . Base station controllers  16  in turn communicate with a mobile station controller  18  to facilitate a communication session initiated by or for users  12 . 
     Between base transceiver station  14  and base station controller  16  is a communication link  20  that provides an A-bis interface for communications system  10 . To optimize bandwidth on the A-bis interface, certain information bits in the traffic stream may be compressed and/or eliminated prior to transport between base transceiver station  14  and base station controller  16 . The goal of the present invention is to go beyond L3 transport encapsulation optimization on a CDMA transport and, by using the approaches discussed below, systematically compress as much of the L2 and L3 frame as possible. The optimization techniques of the present invention adapt to the type of frame received and applies as much compression as possible on the various header fields of the traffic stream without interfering with other payload compression techniques. 
       FIGS. 2A-2B  show the structure of messages transported between base transceiver station  14  and base station controller  16  over the A-bis interface.  FIG. 2A  shows a forward message structure of traffic passed from base transceiver station  14  to base station controller  16 .  FIG. 2B  shows a reverse message structure of traffic passed from base station controller  16  to base transceiver station  14 . Both message structures include a message type field, a payload field, and a cyclic redundancy check (CRC) field. 
     The message type field defines the function and format for each message. The bearer messages are Type II messages. Other Type II messages include connect, connect acknowledge, remove, remove acknowledge, drop, and propagation delay measurement report. The message field can be compressed to zero bits for optimization purposes for Type II bearer messages and the direction of the message can be inferred. The values for Type II bearer messages are defined as 07H for forward messages and 08H for reverse messages and can be detected and then compressed. All other values of Type II messages are not compressed and are forwarded unchanged. 
     The payload field includes the header and traffic information carried by the message. In the forward message structure, the payload field includes the following sub-fields: first reserved, frame type, sequence number, forward traffic channel gain, reverse traffic channel, rate set indicator, forward traffic channel rate, second reserved, power control sub-channel count and forward traffic channel information plus layer 3 filler. In the reverse message structure, the payload field includes the following sub-fields: first reserved, frame type, sequence number, reverse traffic channel quality, scaling, packet arrival time error, rate set indicator, reverse traffic channel rate, second reserved, erasure indicator bit, and reverse traffic channel information plus layer 3 filler. 
     From the forward and reverse message structure, various bits can be compressed to none in order to optimize the amount of bandwidth space required to transport these messages. For example, the first reserved bit is always set to 0 and is thus not needed for transport purposes. The frame type field is used to indicate the version number associated with the present frame format. At present, the frame type field has only one valid value identifying either a standard IS-634 or proprietary frame format. Since this identification can be provisioned, the frame type field can be compressed to zero bits. The sequence number field is set to CDMA system time in frames. If there is one High level Data Link Control multiplexed compressed User Datagram Protocol (HDLCmux/cUDP) flow per bearer message, the sequence number field becomes redundant with the cUDP sequence and can be thus compressed to zero bits. The sequence number is copied to the cUDP sequence. The decompressor takes the sequence as is since it is possible that base transceiver station  14  and base station controller  16  send frames out of sequence. 
     The forward and reverse traffic channel gain fields can be reduced from one byte to four bits. Though these fields can take on any value, differential updates can be sent in order to perform the bit reduction. The second reserved fields may be compressed away since there is no established function for them at this time. The power control sub-channel count field can also be removed and added when needed as it represents the number of soft handoff legs associated with the bearer message, which does not change often during a call. A differential technique may be implemented to send these four bits when there is a change in its value. 
     The rate set indicator fields corresponds to a rate set of the traffic channel frame. The rate set indicator field has two possible values (rate set 1 and rate set 2). For a given bearer message flow, only one value can exist so the rate set for the HDLCmux/cUDP flow can be negotiated. As a result, this field can be compressed from its four bit allocation to zero bits. 
     Tables I and II show the reverse and forward traffic channel rate field values. As shown, one of four rates can be used for each frame. In the HDLCmux scheme, each data rate is treated as a separate cUDP flow and the four bit field is removed and folded into the cUDP CID space. The original sequence numbers are maintained as it cannot be assumed that each of these flows will increase by one monotonically. 
     
       
         
               
               
               
             
           
               
                 TABLE I 
               
               
                   
               
               
                   
                 Rate Set 1 
                 Rate Set 2 
               
               
                 Field Value 
                 Transmission Rate 
                 Transmission Rate 
               
               
                   
               
             
             
               
                 0000 
                 9600 bps (Full Rate) 
                 14400 bps (Full Rate) 
               
               
                 0001 
                 4800 bps (Half Rate) 
                  7200 bps (Half Rate) 
               
               
                 0010 
                 2400 bps (Quarter 
                  3600 bps (Quarter Rate) 
               
               
                   
                 Rate) 
               
               
                 0011 
                 1200 bps (Eighth 
                  1800 bps (Eighth Rate) 
               
               
                   
                 Rate) 
               
               
                 0100 
                 Erasure 
                 Erasure 
               
               
                 0101 
                 Idle 
                 Idle 
               
               
                 0110 
                 Rate Set 1 Full Rate 
                 Reserved 
               
               
                   
                 Likely 
               
               
                   
               
               
                 All other values are reserved 
               
             
          
         
       
     
     
       
         
               
               
               
             
           
               
                 TABLE II 
               
               
                   
               
               
                   
                 Rate Set 1 
                 Rate Set 2 
               
               
                 Field Value 
                 Transmission Rate 
                 Transmission Rate 
               
               
                   
               
             
             
               
                 0000 
                 9600 bps (Full Rate) 
                 14400 bps (Full Rate) 
               
               
                 0001 
                 4800 bps (Half Rate) 
                  7200 bps (Half Rate) 
               
               
                 0010 
                 2400 bps (Quarter 
                  3600 bps (Quarter 
               
               
                   
                 Rate) 
                 Rate) 
               
               
                 0011 
                 1200 bps (Eighth 
                  1800 bps (Eighth Rate) 
               
               
                   
                 Rate) 
               
               
                   
               
               
                 All other values are reserved. 
               
             
          
         
       
     
     The rate set indicator is a four bit field that can be suppressed at the compressor side since this value will most of the time not change in mid-flow. Its possible values are 0 and 1. If an update to the rate set indicator is needed, the compressor will send a delta that indicates to the decompressor an update is required. The compressor and decompressor context tables are then updated with the new value in order to further suppress these four bits until the call ends or the field is again updated. The forward traffic channel rate is a four bit field that can be completely suppressed at all times based on the size of the frame payload. The compressor merely strips out these four bits. The decompressor reads the frame&#39;s payload size and derives the forward traffic channel rate value to forward to the A-bis interface. These possible values are 0000, 0001, 0010, and 0011. 
       FIG. 3  shows an implementation to perform further compression. A pair of routers  22 , or other type of unit, provide a conversion capability on the A-bis interface between base transceiver station  14  and base station controller  16 . Though shown as separate units, the functionality of routers  22  may be incorporated into base transceiver station  14  and/or base station controller  16 . Routers  22 , base transceiver station  14 , and/or base station controller  16  may perform bit suppression through hardware, firmware, and/or software modules with executable code. Through this implementation, it is also possible to eliminate the CRC field of the bearer message. The CRC is a standard 16-bit value computed over the message Type II and forward/reverse payload fields. The CRC is based on a standard generated polynomial g(x)=x 16 +x 12 +x 5 +1. By converting frames from the A-bis interface into an HDLC packetized protocol at the routers  22 , there is no longer a need to retain this CRC field since the HDLC protocol also has a two byte layer 2 CRC that will protect against data corruption. The receiver router  22  from the A-bis interface removes the CRC field as part of the compression process. The sender router  22  to the A-bis interface will regenerate the CRC field for the 320-bit layer 2 frame. The HDLC PPP/IP format is as follows: 
                                                                               L1   L2   L3   L2   L1            0x7E   0xFF00 0x0021   L3   IP   Packet   0xCRCX   0x7E . . .                               0x7E . . .       0x7E   0xFF00 0x0067   L3   cUDP   Packet   0xCRCX   0x7E . . .                               0x7E . . .                    
The L2 0xFF00 0X0067 can be compressed to 0x67 by using ACFC and PFC. If 16-bit CIDs are used, it compresses to 0x2067.
 
     Based on the various compression possibilities, overall about 5 bytes of layer 3 header can be compressed. In a best case scenario, all but one of these bytes can be compressed at any given time. In a worst case scenario, only two bytes of compression are achieved. As a result, anywhere from two to four bytes of header per frame can be eliminated from the transmission. 
     Assuming an IP/cUDP transport mechanism at routers  22 , an un-compressed traffic transport is as follows: 
     [PID=1][cUDP=2][IS-634hdr=5][L2pld=11]=19 bytes
         &lt;- - - - -L3 payload- - - - - -&gt;
 
Based on the above optimization, the traffic transport is reduced as follows:
       

     Best Case Scenario (Yields an efficiency of 21%) 
     [PID=1][cUDP=2][IS-634hdr=1][L2pld=11]=19 bytes
         &lt;- - - - -L3 payload- - - - - -&gt;       

     Worst case Scenario (Yields an efficiency of 11%) 
     [PID=1][cUDP=2][IS-634hdr=3][L2pld=11]=19 bytes
         &lt;- - - - -L3 payload- - - - - -&gt;
 
The exact optimization percentage will vary slightly depending on the transport overhead. However, the compression method is independent of transport and is meant to be applied to the incoming IS-634 frames that are destined for a type of transport. Thus, by removing the CRC field and other selected header bits, additional bandwidth optimization can be achieved.
       

     In summary, the IS-634 full frame transport optimization technique discussed above is designed to further optimize the CDMA A-bis interface between base transceiver station  14  and base station controller  16 . The technique targets the L2 and L3 header information in three separate and distinct compression opportunities. The optimization technique takes transport optimization to a new level by not only compressing the L3 transport but also the A-bis L3 and L2 headers. Bandwidth is saved so that call and flow capacity is increased. Operational expenses are decreased through a reduction in the number of backhaul links needed for traffic transport. 
     Thus, it is apparent that there has been provided, in accordance with the present invention, a system and method for optimizing data transport in a communications system that satisfies the advantages set forth above. Although the present invention has been described in detail, it should be understood that various changes, substitutions, and alterations may be readily ascertainable by those skilled in the art and made herein without departing from the spirit and scope of the present invention as defined by the following claims. Moreover, the present invention is not intended to be limited in any way by any statement made herein that is not otherwise reflected in the following claims.