Patent Publication Number: US-7899025-B2

Title: Header suppression in a wireless communication network

Description:
BACKGROUND OF THE INVENTION 
     The Internet Protocol (IP) has become the dominant transport protocol in both wireline and wireless networks, which has led to the convergence of telecommunication and data networks. In many services and applications (e.g., Voice over IP (VoIP), interactive games, instant messaging, etc.), the payload of an IP packet is almost of the same size or even smaller than the header. In addition to the IP network protocol, other protocols (e.g., real-time protocol (RTP), user datagram protocol (UDP), etc.) are added to the original information bits for effective transport in a packet data network. 
     Fortunately, it is not necessary to send the enormous RTP/UDP/IP header for each packet all the time. Instead a header compression algorithm such as the robust header compression (RoHC) may be used. The principle behind header compression is that most of the fields in the RTP/UDP/IP header are static; hence they can be sent once uncompressed during a first communication (e.g., the initial transmitted packets in a wireless system) from the compressor at the transmission side to the decompressor at the reception side. Once the decompressor has reliably acquired the static information, the compressor starts sending compressed headers carrying information regarding the dynamic parts of the header. From the compressed header, the decompressor is able to fully reconstruct the RTP/UDP/IP header and pass the packet on. In this way, the large headers are not transmitted for each packet, leading to tremendous savings in capacity. 
     However, current header compression schemes do have some drawbacks. For ease of explanation, these drawbacks will be described with respect to header compression implemented in a conventional wireless communication system. 
       FIG. 1  illustrates a general architecture of a well-known wireless communication network. As shown, an access terminal (AT)  10  communicates with a base station (BTS)  12  over an air interface. Examples of an AT include a mobile station, a mobile unit, a wireless phone, wireless equipped PDA or computer, etc. Multiple base stations  12  communicate with a radio network controller (RNC)  14 , which provides signaling and traffic processing for each wireless data session.  FIG. 1  shows the AT  10 , BTS  12 , RNC  14  and the interfaces between these components form what is known as a radio access network (RAN). The RAN communicates with a core network to access, for example, the internet. In the example of  FIG. 1 , the core network includes one or more packet data service nodes (PDSNs)  16  connected between the RNCs  14  and, for example, the internet (not shown). 
     As examples, header compression may occur between the AT  10  and the PDSN  16 , between the AT  10  and the RNC  14 , etc. When the AT  10  establishes a connection with the network, for example, a VoIP call, the application layer packet will be carried over the RTP/UDP/IP protocol stacks. The RTP/UDP/IP headers will be compressed by a compressor at the AT  10  using, for example, the RoHC algorithm mentioned above. The compressed packet will be sent uplink from the BTS  12  to the RNC  14  and from the RNC  14  to the PDSN  16 . The decompressor at the RNC  14  or the PDSN  16  decompresses the RoHC header to re-establish the RTP/UDP/IP header. Similarly, on the downlink direction, the PDSN  16  and RNC  14  receive packets and the compressor at the PDSN  16  or RNC  14  compresses the RTP/UDP/IP headers to generate the RoHC or compressed header. The packet with compressed header is sent to the BTS  12  and on to the AT  10 . A decompressor at the AT  10  decompresses the ROHC header to obtain the original RTP/UDP/IP header, and passes the packet onto the application layer. 
       FIG. 2  illustrates another architecture of a wireless communication network—the so called flat IP network architecture. As shown, the AT  10  communicates with a base station (BS)  20  over an air interface. The BS  20  converges multiple mobile network elements into a single entity and combines the signaling and bearer into one IP connection. In this flat IP architecture, the BS  20  contains all the radio access technology based functionalities. In other words, the functionalities in BTS, RNC and PDSN of  FIG. 1  are be converged to the BS. The BS  20  functions like a router in the network and communicates with other BSs and network elements. Compared to  FIG. 1 , there are no separate RNC and PDSN elements anymore. The BS may also communicate with an access gateway  22 , which provides for external connection to other networks such as the internet. 
     In the architecture of  FIG. 2 , header compression may occur between the AT  10  and the BS  20 , or between the AT  10  and the access gateway  22 . When the AT  10  establishes a connection with the network, for example, a VoIP call, the application layer packet will be carried over the RTP/UDP/IP protocol stacks. The RTP/UDP/IP headers will be compressed by a compressor at the AT  10  using, for example, the RoHC algorithm mentioned above. The compressed packet will be sent uplink from the AT  10  to the BS  20 , or from the AT  10  to the BS  20  and from the BS  20  to the access gateway  22 . The decompressor at the BS  20  or the access gateway  22  decompresses the RoHC header to re-establish the RTP/UDP/IP header. Similarly, on the downlink direction, the access gateway  22  and BS  20  receive packets, and the compressor at the access gateway  22  or the BS  22  compresses the RTP/UDP/IP headers to generate the RoHC or compressed header. The packet with the compressed header is sent to the AT  10 . A decompressor at the AT  10  decompresses the ROHC header to obtain the original RTP/UDP/IP header, and passes the packet onto the application layer. 
     The robust header compression (RoHC) algorithm uses several encoding methods, including the window-based least significant bits encoding algorithm, for the compression of the dynamic fields in the protocol headers. The RoHC compression algorithm also incorporates a feedback mechanism. The RoHC compression algorithm is very efficient on wireless links with high error rates and/or long round trip time. Because of its efficiency and robustness, the RoHC compression algorithm is suitable on wireless networks where the radio resource is costly. 
     However, even with RoHC compression, non-static or dynamic fields in the header must be sent. The most dynamically changed fields in the RTP/UDP/IP header are the RTP sequence number and RTP time stamp. Each RTP header includes a RTP sequence number to permit proper ordering of the RTP packets. Accordingly, each RTP packet header has a different, generally, incrementally higher sequence number. The RTP time stamp indicates the sampling time of the first byte in the RTP packet. 
     SUMMARY OF THE INVENTION 
     The present invention relates to improving header compression/decompression over the radio link. In particular, the present invention provides a method for suppressing at least one of the dynamic fields of the RTP header. 
     In an embodiment, a relationship is determined between radio link protocol (RLP) sequence numbers in received RLP packets and real-time protocol (RTP) sequence numbers of RTP packets represented by the received RLP packets. A RTP sequence number associated with a compressed RTP packet is determined based on the determined relationship and at least one of the RLP sequence numbers of the received RLP packet or packets forming the compressed RTP packet. The compressed RTP packet does not include a RTP sequence number. 
     In another embodiment, a relationship is determined between protocol time stamps associated with received radio link protocol (RLP) packets and real-time protocol (RTP) time stamps in RTP packets represented by the received RLP packets. The protocol time stamp is a time stamp added by a network elements interface protocol (e.g., a BTS/RNC interface protocol). A RTP time stamp associated with a compressed RTP packet is determined based on the determined relationship and at least one of the protocol time stamps of the received RLP packet or packets forming the compressed RTP packet. The compressed RTP packet does not include a RTP time stamp. 
     In yet another embodiment, a real-time protocol (RTP) time stamp associated with a compressed RTP packet is determined based on at least one protocol time stamp of a received radio link protocol (RLP) packet or packets forming a compressed RTP packet, the protocol time stamp of a previously received RLP packet, and the RTP time stamp of a previously received RTP packet. The protocol time stamp is a time stamp added by a network elements interface protocol. The compressed RTP packet does not include a RTP time stamp. 
     A still further embodiment includes suppressing at least one of RTP sequence numbers and RTP time stamps in compressed RTP packets. 
     In yet other embodiments, when the interface between the network elements (e.g., between BTS and RNC) is simplified and converged (e.g., in the BS architecture), the use of the protocol time stamps in the above described embodiments may be replaced with a time stamp derived from at least one of the transmission packet information and local timing information. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The present invention will become more fully understood from the detail description given herein below and the accompanying drawings which are given by way of illustration only, wherein like reference numerals designate corresponding parts in the various drawings, and wherein: 
         FIG. 1  illustrates a well-known wireless communication network; 
         FIG. 2  illustrates a so called flat IP wireless communication network; 
         FIG. 3  illustrates the architecture of a portion of a wireless communication system for reverse link communication according to an embodiment of the present invention; and 
         FIG. 4  illustrates the architecture of a portion of a wireless communication system for forward link communication according to an embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF EXAMPLE EMBODIMENTS 
     Referring to  FIG. 1  discussed above, data packet transmissions over the link layer of the wireless network contain information to ensure packet delivery and processing at the RAN. For example, the radio link protocol (RLP) layer at a transmitter encapsulates the RoHC packet and provides a sequence number (SN) in an RLP header for packet delivery. The sequence number is incremented for each RLP packet transmission, and therefore, provides a mechanism for properly ordering packets received out-of-order at the receiver. This mechanism may also be used to recognize missing data packets. 
     Furthermore, the interface over the backhaul between the BTS  12  and RNC  14  adds another protocol (hereinafter “BTS/RNC interface protocol”) for encapsulating the RLP packets. More specifically, the BTS/RNC interface protocol maintains timing information (e.g., a time stamp) on when RLP packets are sent between the BTS and the RNC. On the reverse link, this timing information represents the transmitting time the RLP packets are sent from the AT and passed by the BTS to the RNC. On the forward link, there is a time stamp field in the BTS/RNC interface protocol, representing the time the RLP packet was generated at the RNC. When the BTS receives the packet, it can make use of the timing information in its transmission scheduling decision. 
     Referring to  FIG. 2  discussed before, in a flat IP architecture where the network elements BTS and RNC are converged to a single element BS, the interface protocol between the functional entities BTS and RNC can be simplified. The protocol time stamps can be replaced by a time stamp derived from the transmission time of the packet and the local time at the BS. Namely, the protocol time stamp is one of the values in the interface protocol field, and the BS has the perfect knowledge of this information, since it generates the ROHC packets, and also receives the packets from the AT. On the forward link, the BS obtains the time stamp based on the time instance when a ROHC packet is generated. On the reverse link, the BS receives a packet from the AT and knows exactly when the packet is sent from the AT based on the transmission format and MAC header information. The following descriptions with regard to the use of protocol time stamps are equally applicable to the derived time stamps in the flat IP architecture of  FIG. 2 . 
     The inventors have recognized that when each RLP packet contains one or more ROHC packets, there is a unique mapping between the RLP sequence number (SN) and the compressed RTP SN in the RoHC header. Furthermore, when a RLP packet contains a portion of a RoHC packet, the “first data” and “last data” field in the RLP header can be used to reassemble a complete upper layer packet. In other words, if the “first data” field in the RLP header is set to 1, it means that this RLP packet contains the first portion of an upper layer packet. If the “last data” field is set to 1, it means that this RLP packet contains the last portion of an upper layer packet. If both “first data” and “last data” field is set to 1, it means that this RLP packet contains a complete upper layer packet. In these different cases, there exists a unique mapping between the RLP SN and the RTP SN. Therefore, the inventors have recognized that the RLP SN can be used to recover the RTP SN for a RTP packet such that the RTP SN may be suppressed (i.e., not sent) in the header. 
     In addition, because the BTS/RNC interface protocol time stamp represents the transmission timing information, the inventors have recognized that the BTS/RNC interface protocol time stamp has a unique relationship with the RTP time stamp. This relationship may be used, according to embodiments of the present invention, to suppress the RTP time stamp from the header. This relationship especially pertains to the reverse, or uplink, where the RTP/UDP/IP packets are generated by the application layer at the AT  10 , compressed with the RoHC, and sent out sequentially. In the architecture of  FIG. 2 , it is the derived time stamp that has the unique relationship with the RTP time stamp. 
     If the received RTP/UDP/IP packets are in order, and the RTP time stamp has a distinct pattern, then the time stamp in the BTS/RNC interface protocol header correlates to the RTP time stamp. For example, a VoIP packet&#39;s RTP time stamp is incremented by a fixed interval (usually 20 ms or 160 samples). The time stamp in the BTS/RNC interface protocol is also incremented by approximately the same interval if the arrival time of each VoIP packet does not vary too much. In general, the VoIP packets arriving at the RNC have been passed through the networks. Each packet will experience delay and delay jitter when it arrives at the RNC. On the other hand, the RTP time stamp in each VoIP packet represents the sampling time generated at the originating source (i.e., voice codec). Unless each VoIP packet experiences the same delay and thus there is no delay jitter between the VoIP packets, the arrival time of the packets at the RNC will not have the same distinct pattern as the original RTP packets. Therefore, an estimation method according to embodiments of the present invention may be used to find the mapping between the RTP time stamps and the time stamps in the BTS/RNC interface protocol. 
     Let TS RTP1  (converted to ms from number of samples) be the time stamp of a RTP packet, and TS Interface1  (in ms) be the time stamp of the corresponding RLP packet in the BTS/RNC interface protocol. Let TS RTP2  (converted to ms from number of samples) be the time stamp of the next consecutive RTP packet, and TS Interface2  (in ms) be the time stamp of the corresponding RLP packet in the BTS/RNC interface protocol. And let TS RTP     —     interval  be the time stamp interval between the RTP packets (i.e., 20 ms). The mapping between the time stamps in the interface protocol and the RTP time stamps may be determined as follows: 
                       TS     RTP   ⁢           ⁢   2       -     TS     RTP   ⁢           ⁢   1         ≅       int   ⁡     (           TS     interface   ⁢           ⁢   2       -     TS     interface   ⁢           ⁢   1           TS   RTP_interval       ,   1     )       ·     TS   RTP_interval               (   1   )               
where the int( ) is integer operation in units of 1. Therefore, if TS RTP2  cannot be decoded correctly, the time stamps in the interface protocol may be used to estimate TS RTP2     —     estimated  as follows:
 
     
       
         
           
             
               
                 
                   
                     TS 
                     
                       RTP 
                       ⁢ 
                       
                           
                       
                       ⁢ 
                       2 
                       ⁢ 
                       _estimated 
                     
                   
                   ≅ 
                   
                     
                       TS 
                       
                         RTP 
                         ⁢ 
                         
                             
                         
                         ⁢ 
                         1 
                       
                     
                     + 
                     
                       
                         int 
                         ⁡ 
                         
                           ( 
                           
                             
                               
                                 
                                   TS 
                                   
                                     interface 
                                     ⁢ 
                                     
                                         
                                     
                                     ⁢ 
                                     2 
                                   
                                 
                                 - 
                                 
                                   TS 
                                   
                                     interface 
                                     ⁢ 
                                     
                                         
                                     
                                     ⁢ 
                                     1 
                                   
                                 
                               
                               
                                 TS 
                                 RTP_interval 
                               
                             
                             , 
                             1 
                           
                           ) 
                         
                       
                       · 
                       
                         TS 
                         RTP_interval 
                       
                     
                   
                 
               
               
                 
                   ( 
                   2 
                   ) 
                 
               
             
           
         
       
     
     When there is a jump in the RTP time stamp during the silence compression, the above method may still be used to estimate the RTP time stamp. 
     In the architecture of  FIG. 2 , the same method may be applied by using the derived time stamps in place of the protocol time stamps. 
     When the received RTP/UDP/IP packets are out of order, if the ROHC compressor resides in the RNC, the ROHC compressor can inform the RLP layer about the reordering situation and the time stamps in the interface protocol can be set promptly. If the ROHC compressor resides in PDSN, the PDSN passes the reordering situation to the RNC so that the RNC can use it promptly. Therefore the same estimation method may be used to estimate the RTP time stamp. Therefore, the RTP time stamp can be suppressed (i.e., not sent) in the header. 
     Next, embodiments of the present invention will be described in detail. First, an embodiment of an architecture and a method for header suppression on a reverse link will be described. Then, an embodiment of an architecture and a method for header suppression on a forward link will be described. For ease of explanation only, these embodiments will be described as employed on the wireless communication network of  FIG. 1 . However, it will be understood that the architecture and methods of the present invention are not limited to this wireless system or even to wireless systems. For example, the descriptions below may also apply to the architecture of  FIG. 2  with the derived time stamp used instead of the protocol time stamp. 
     REVERSE LINK 
       FIG. 3  illustrates a functional block diagram of the AT  10 , BTS  12  and the RNC  14  operating on a reverse link, or uplink, according to an embodiment of the present invention. It will be understood that specific details of the AT  10 , BTS  12  and RNC  14  well-known in the art have not been shown for the sake of clarity. 
     Robust Header Compression (RoHC) channels may be established between the AT  10  and the BTS  12 , the AT  10  and the RNC  14  or the AT  10  and the PDSN  16 . For the reverse link, a compressor  104  and decompressor  144  may reside at the AT  10  and RNC  14  (as shown in  FIG. 3 ), respectively. This embodiment will be described using this implementation. However, the description is equally applicable to the decompressor residing at the BTS  12  (albeit, the BTS  12  extracts or generates the link layer information) or the PDSN  16  (albeit, the RNC  14  passes the link layer information to the PDSN  16 ). 
     As shown, the AT  10  includes an application layer IP generator  102  that generates IP packets for a particular application. For example, a VoIP call consists of voice frame(s) encapsulated into a RTP/UDP/IP packet. With the establishment of a connection, the application layer generator  102  generates an application layer packet, which via the protocol stack becomes a RTP/UDP/IP packet. The header compressor  104  initially sends one or more RTP/UDP/IP packets on without compression in order to establish the compression/decompression context. Namely, the compression/decompression context is the static fields of the header. This is accomplished in any well-known manner. After establishing the context, the header compressor  104  compresses the RTP/UDP/IP packets into RoHC packets, for example, using the ROHC algorithm. However, in this embodiment, the compressor  104  also removes or suppresses the RTP SN and/or the RTP time stamp. 
     Subsequently, a link layer packet generator  106  generates a link layer packet by placing ROHC packets into RLP packets. The link layer generator  106  may perform concatenation or fragmentation on the upper layer packets. In this example, a RLP packet can consist of one or multiple ROHC packets. It can also contain only a portion of a ROHC packet. The size of the RLP packet is determined based on the available transmission rate the AT can use at the time. As mentioned above, the RLP layer provides its own SN in the RLP header for packet delivery to provide a mechanism for recognizing missing data packets. 
     The BTS  12  receives the RLP packet, and an interface protocol  108  adds a header to the RLP packet. More specifically, the BTS/RNC interface protocol  108  encapsulates the RLP packets, and the header of the interface protocol packet includes a packet ID (or a time stamp) representing the transmission timing information of the RLP packet at the AT  10 . The BTS  12  passes the RLP packet to the RNC  14  or PDSN  16  (not shown) for decompression. Other protocols in addition to the RLP and interface protocol may be added depending on the system design. 
     The RNC  14  includes a RLP processing module  142  and a decompressor  144 . The RLP processing module  142  receives the RLP packet, obtains the RTP/UDP/IP packet there from, and passes the RLP SN and transmission timing information (hereinafter interchangeably referred to as “link layer information”) to the decompressor  144  along with the RTP/UDP/IP packet. 
     The decompressor  144  determines the RTP SN and/or the RTP time stamp from the context and the link layer information, and then decompresses the RTP/UDP/IP packets in the well-known manner according to, for example, the ROHC algorithm. 
     When the compression/decompressor context is established; namely, when uncompressed RTP/UDP/IP packets are sent, the decompressor  144  obtains, among other things, the RTP sequence number and the RTP time stamp. The decompressor  144  maps the RLP sequence number to the RTP sequence number, and maps the BTS/RNC interface protocol timing information to the RTP time stamp. For example, for three consecutive packets, the RLP sequence numbers may be 20, 21 and 22, and the associated three RTP sequence numbers may be 8, 9 and 10. Accordingly, RLP sequence number 20 is mapped (e.g., stored in association with) to RTP sequence number 8, RLP sequence number 21 is mapped to RIP sequence number 9, etc. as shown in Table 1 below. 
     
       
         
           
               
               
               
             
               
                   
                 TABLE 1 
               
               
                   
                   
               
               
                   
                 RLP Sequence Number 
                 RTP Sequence Number 
               
               
                   
                   
               
             
            
               
                   
               
            
           
           
               
               
               
            
               
                   
                 20 
                 8 
               
               
                   
                 21 
                 9 
               
               
                   
                 22 
                 10 
               
               
                   
                   
               
            
           
         
       
     
     Alternatively, if the RLP and RTP sequence numbers increment by a fixed amount and a same fixed offset exists between associated RLP and RTP sequence numbers, then the decompressor  144  may instead, or in addition, store the offset amount. 
     In the case where an upper layer ROHC packet is fragmented into multiple RLP packets, the “first data” and “last data” field in the RLP header can be used to determine the frame boundary (the first portion and the last portion) of the ROHC packet. For example, for six consecutive packets, the RLP sequence numbers may be 20, 21, 22, 23, 24, and 25. The “first data” field in RLP packet  20  is 1, but the “last data” field is 0. This indicates that RLP packet  20  is the first portion of a ROHC packet. The “first data” and “last data” in RLP packet  21  are both zero. The “first data” in RLP packet  22  is zero, and the “last data” in RLP packet  22  is 1. Therefore, RLP packets  20 ,  21 , and  22  form a complete ROHC packet, with a corresponding RTP SN of 8. The “first data” field in RLP packet  23  is 1 again, indicating the start of another upper layer packet. Accordingly, RLP sequence number 20 is mapped (e.g., stored in association with) to RTP sequence number 8, RLP sequence number 23 is mapped to RTP sequence number 9, etc. as shown in Table 2 below. 
                                 TABLE 2                       RLP Sequence Number   RTP Sequence Number                          20   8           21   (8)           22   (8)           23   9           24   (9)           25   (9)           26   10                         
In this example, the RLP SN to RTP SN mapping is unique but is 3 to 1 mapping.
 
     The interface protocol timing information and the RTP time stamp are mapped in the same manner as the RLP to RTP sequence number mapping. 
     Eventually, compressed RTP/UDP/IP packets having no RTP SN and/or no RTP time stamp are received. The received packets will be passed by the RLP processing module  142  to the decompressor  144 . When this occurs, the decompressor  144  invokes a rebuilding mechanism using the link layer information and the context, and attempts to rebuild the RTP SN and/or RTP time stamp. 
     The rebuilding mechanism involves determining a RTP sequence number and RTP time stamp for the packet. To determine the RTP sequence number, the decompressor  144  uses the RLP sequence number for the packet, and the RLP-to-RTP sequence number map. Using the RLP-to-RTP sequence number map of Table 1 given above, an example of a RTP sequence number determination operation will be described. If the RLP sequence number of the next received packet is 23, then the decompressor  144  recognizes that the RTP SN is 11—this follows the sequence set forth in Table 1. Accordingly, the decompressor  144  would similarly determine 13 as the RTP sequence number for the received packet having 25 as the RLP sequence number. 
     Alternatively, or additionally, the decompressor  144  may have determined the RLP-to-RTP mapping offset. In the example of Table 1, this offset would be −12. Accordingly, using this mapping offset, the RLP sequence number of 23 is mapped to a RTP sequence number of 11 (=23+(−12)). 
     Once the RTP SN is determined, the RLP SN and associated, determined RTP SN are added to the mapping table for use in future determinations. 
     In the same manner, the decompressor  144  may also determine the RTP time stamp from the BTS/RNC timing information. For example, Table 3 shows the value of packet ID (or time stamps) in the interface protocol (in unit of 4 slots, or 6.67 ms), the corresponding transmission time (in ms), the RTP time stamp value (in unit of samples), and the corresponding sampling time. 
                                         TABLE 3                           Corresponding       Corresponding               transmission   RTP Time   sampling time           Packet ID   time (ms)   Stamp   (ms)                                                            6   40   160   20           9   60   320   40           12   80   480   60                        
The transmission time can be correlated with the RTP time stamp. In this example, the transmission time is mapped to the RTP packet sampling time by an offset of 20 ms, i.e., 40 ms is mapped to 20 ms. Assuming the next successfully received packet has an ID of 15, the decompressor  144  can first map the value of packet ID  15  to 100 ms (i.e., 15*6.67=100 ms) transmission time, then map 160 ms to 140 ms (i.e., 100−20=80 ms) RTP sampling time, and last map the RTP sampling time to 640 (i.e., 80*8=640) as the RTP time stamp value.
 
     The RLP time stamp and the associated determined RTP time stamp may then be added to the mapping table for use in future determinations. 
     The example with respect to Table 3 assumes the transmission time of each packet at the AT experiences the same delay after they are generated at the encoder and passed to the transmitter. However, this is not guaranteed and often a very small delay and delay variation may exist. Accordingly, instead as discussed in detail above, the estimation method of equation (2) may be used to determine a RTP time stamp from the BTS/RNC interface protocol time stamp. For an example, Table 4 shows the same RTP time stamp and corresponding sampling time as those in Table 3. However the packet ID and the corresponding transmission time has some delay variation. Packet ID  6  corresponds to transmission time of 40 ms (6*6.67 ms=40 ms). The packet ID for the next RTP packet is 10, corresponds to transmission time of 66.7 ms. And packet ID  12  corresponds to transmission time of 80 ms. The transmission time interval between consecutive packets is not exactly 20 ms as represented by the RTP TS. However, the transmission time can still be correlated with the RTP time stamp using the estimation method of equation (2). 
                                         TABLE 4                           Corresponding       Corresponding               transmission   RTP Time   sampling time           Packet ID   time (ms)   Stamp   (ms)                                                            6   40   160   20           10   66.7   320   40           12   80   480   60                        
In this example, assuming the next successfully received packet has an ID of 23, the decompressor  144  can first map the value of packet ID  23  to 153.33 ms (i.e., 23*6.67=153.33 ms) transmission time, then use equation (2) to get the estimated RTP sampling time from packet ID  12 , i.e., 60+int((153.33−80)/20,1)*20=140 ms. And, then map the RTP sampling time to 1120 (i.e., 140*8=1120) as the RTP time stamp value.
 
     The determined RTP time stamp and the associated RLP time stamp are then added to the table for use in future determinations. 
     Having conducted the determination operation, the decompressor  144  decompresses the ROHC packet in the conventional manner, and uses the determined RTP sequence number and RTP time stamp as the RTP sequence number and time stamp of the received packet after decompression. 
     By being able to remove the RTP SN and/or RTP time stamp from the compressed RTP/UDP/IP packets, further savings in bandwidth over the air interface is achieved. 
     Forward Link 
     While the detailed discussion above concerned compression and decompression on the reverse link, the method of the present invention discussed above, may also be applied on the forward link.  FIG. 4  illustrates a functional block diagram of the AT  10 , BTS  12  and the RNC  14  operating over the forward link, or downlink, according to an embodiment of the present invention. It will be understood that specific details of the AT  10 , BTS  12  and RNC  14  well-known in the art (e.g., the RTP/UDP/IP protocol stacks, etc.) have not been shown for the sake of clarity. 
     Robust Header Compression (RoHC) channels may be established between the AT  10  and the BTS  12 , the AT  10  and RNC  14  or the AT  10  and the PDSN  16 . For the forward link, a compressor  204  and decompressor  244  may reside at the RNC  14  and AT  10  (as shown in  FIG. 4 ), respectively. This embodiment will be described using this implementation. However, the description is equally applicable to the compressor residing at the BTS  12  or the PDSN  16 . 
     As shown, the RNC  14  includes an application layer IP generator  202  that generates IP packets for a particular application. For example, a VoIP call consists of voice frame(s) encapsulated into a RTP/UDP/IP packet. With the establishment of a connection, the application layer generator  202  generates an application layer packet, which via the protocol stack becomes a RTP/UDP/IP packet. The header compressor  204  initially sends one or more RTP/UDP/IP packets on without compression in order to establish the compression/decompression context. Namely, the compression/decompression context is the static fields of the header. This is accomplished in any well-known manner. After establishing the context, the header compressor  204  compresses the RTP/UDP/IP packets into ROHC packets, for example, using the ROHC algorithm. However, in this embodiment, the compressor  204  also removes or suppresses the RTP SN and/or the RTP time stamp. 
     Subsequently, a link layer packet generator  206  generates a link layer packet by placing each RoHC packet into a RLP packet. As mentioned above, the RLP layer provides its own SN in the RLP header for packet delivery to provide a mechanism for recognizing missing data packets. Furthermore, as mentioned with respect to the forward link, the RoHC packet may be fragmented into more than one RLP packet. 
     A BTS/RNC interface protocol  208  at the RNC adds an interface protocol header to the RLP packet before sending the RLP packet to the BTS. More specifically, the BTS/RNC interface protocol encapsulates the RLP packets, and the header of the interface protocol packet includes a sequence number representing the transmission timing information of the RLP packet. The BTS  12  removes the BTS/RNC interface protocol header and passes the RLP packet to the AT  10  for decompression. Other protocols in addition to the RLP and interface protocol may be added depending on the system design. 
     The AT  10  includes a RLP processing module  242  and a decompressor  244 . The RLP processing module  242  receives the RLP packet, obtains the RTP/UDP/IP packet there from, and passes the RLP SN and, possibly, transmission timing information (hereinafter interchangeably referred to as “link layer information”) to the decompressor  244  along with the RTP/UDP/IP packet. The BTS/RNC interface protocol header is not sent with the RLP packets from the BTS to the AT. Accordingly, the BTS/RNC interface protocol timing information is not available to the AT for use in the local repair mechanism. 
     In one embodiment, this timing information may be sent to the AT, albeit not in the BTS/RNC interface protocol header. For example, this timing information may be sent by BTS to AT via additional signaling, either with an added field in the RLP header or with an added message sent separately to the AT. 
     The decompressor  244  determines the RTP SN and/or the RTP time stamp from the context and the link layer information, and then decompresses the RTP/UDP/IP packets in the well-known manner according to, for example, the ROHC algorithm. 
     When the compression/decompressor context is established; namely, when uncompressed RTP/UDP/IP packets are sent, the decompressor  244  obtains, among other things, the RTP sequence number and the RTP time stamp. The decompressor  244  maps the RLP sequence number to the RTP sequence number, and maps the timing information to the RTP time stamp. 
     Eventually, compressed RTP/UDP/IP packets having no RTP SN and/or no RTP time stamp are received. The received packets will be passed by the RLP processing module  142  to the decompressor  144 . When this occurs, the decompressor  144  invokes a rebuilding mechanism using the link layer information and the context, and attempts to rebuild the RTP SN and/or RTP time stamp. 
     The rebuilding mechanism involves determining a RTP sequence number and RTP time stamp for the packet. To determine the RTP sequence number, the decompressor  144  uses the RLP sequence number for the packet, and the RLP-to-RTP sequence number map. Namely, the RTP SN and RTP time stamp may be rebuilt in the same manner or according to the same embodiments discussed above with respect to the reverse link. 
     In another embodiment, the RTP time stamp (TS) is inferred using the determined RTP sequence number (SN). The RTP time stamp is defined to identify the number of samples used to generate the RTP packet payload. When RTP packets carry payloads corresponding to a fixed sampling interval, and the sample rate is fixed, there is a unique mapping between the TS and SN. For example, for conversational speech, a fixed sampling rate of 8 kHz is often used. The speech payload is generated every 20 ms. This is equivalent to an increment of 160 in the RTP TS domain for consecutive packets. In other words, the RTP SN number is incremented by 1, while the TS is incremented by 160 during a talk spurt (where no silence suppression is applied.) In this case, from the method of recovering the RTP SN, the TS may be recovered indirectly. 
     As will be appreciated, the rebuilding mechanism in accordance with the system and methods provided may increase savings in bandwidth. 
     The invention being thus described, it will be obvious that the same may be varied in many ways. Such variations are not to be regarded as a departure from the invention, and all such modifications are intended to be included within the scope of the invention.