Abstract:
A method and apparatus for transmitting/receiving a Voice over Internet Protocol (VoIP) packet on a radio link in a mobile communication system which provides a voice service over a packet network connected to the Internet. To transmit a VoIP packet, a VoIP packet comprising a user datagram protocol (UDP) checksum field is received, the UDP checksum field is eliminated from the received VoIP packet, a cyclic redundancy check (CRC) is added to the UDP checksum field-free VoIP packet, for error detection in the radio link, and the VoIP packet having the CRC is transmitted on the radio link.

Description:
PRIORITY 
   This application claims the benefit under 35 U.S.C. §119(a) to an application entitled “Method And Apparatus For Transmitting/Receiving Voice Over Internet Protocol Packet With User Datagram Protocol Checksum In A Mobile Communication System” filed in the Korean Intellectual Property Office on Aug. 9, 2004 and assigned Serial No. 2004-62544, the entire contents of which are hereby incorporated by reference. 
   BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates generally to a mobile communication system that interacts with the Internet. In particular, the present invention relates to a method and apparatus for efficiently processing a User Datagram Protocol (UDP) Checksum field when a UDP packet is transmitted on a radio channel. 
   2. Description of the Related Art 
   Today&#39;s mobile communication technology has evolved from a traditional voice service into a high-speed, high-quality wireless data packet communication system. A 3 rd  generation mobile communication system such as a Universal Mobile Telecommunication Service (UMTS) is based on Global System for Mobile communication (GSM) and General Packet Radio Services (GPRS). It provides a uniform service that transmits packetized text, digital audio and video, and multimedia data at a 2 Mbps or higher rate to mobile subscribers or computer users around the world. With the introduction of the concept of packet-switched access using a packet protocol like Internet Protocol (IP), UMTS allows access to any end point in a network at any time. 
   The 3 rd  Generation Partnership Project (3GPP), in charge of standardizing the UMTS communication system, has discussed support for Voice over Internet Protocol (VoIP) communications. VoIP is a communication technique in which a voice frame generated from a CODEC (Coder-Decoder) is converted to an IP/User Datagram Protocol (UDP)/Realtime Transport Protocol (RTP) packet, for transmission. Thus, VoIP provides a voice service over a packet network. 
     FIG. 1  illustrates the configuration of a VoIP-enabled User Equipment (UE) in a conventional mobile communication system. 
   Referring to  FIG. 1 , a VoIP-enabled UE  100  comprises a CODEC  106  for converting human voice to a voice frame, an IP/UDP/RTP layer  105  for converting the voice frame to an IP/UDP/RTP packet, a Packet Data Convergence Protocol (PDCP) layer  104  for compressing the header of the IP/UDP/RTP packet, a Radio Link Control (RLC) layer  103  for converting the IP/UDP/RTP packet to a form suitable for transmission on a radio channel, a Medium Access Control (MAC) layer  102 , and a Physical PHY layer  101  for transmitting the packet data on the radio channel. 
   The voice packet data from the UE  100  is delivered to a Radio Network Controller (RNC)  120  via a Node B  110 . Like the UE  100 , the RNC  120  has a MAC layer  122 , a RLC layer  123 , and a PDCP layer  124 . The RNC  120  converts the received data to the original IP/UDP/RTP packet and transmits it to a Core Network (CN)  130 . The IP/UDP/RTP packet is transmitted to the other party over an IP network  140 . The UE of the other party processes the voice data in the reverse order of the above-described operation and provides the voice data to the other party. 
   Now, a description is made of the functionalities of the RLC layer. 
   Typically, the RLC layer operates in one of three ways: unacknowledged mode (UM), acknowledged mode (AM), and transparent mode (TM). VoIP is expected to operate in the RLC UM mode. 
   In a transmitter, a RLC UM entity generates a RLC Protocol Data Unit (PDU) by segmenting, concatenating, or padding a RLC Service Data Unit (SDU) received from a higher layer to a size suitable for transmission on a radio channel and then inserts information about the segmentation/concatenation/padding and a Serial Number (SN) into the resulting data. The RLC PDU is delivered to a lower layer. A RLC UM entity in a receiver recovers the RLC SDU by interpreting the SN and the segmentation/concatenation/padding information of the RLC PDU received from its lower layer. For reference, in the TM mode, the RLC entity simply delivers an RLC SDU received from the higher layer to the lower layer and a RLC PDU received from the lower layer to the higher layer. 
   The MAC layer serves as an interface between the RLC layer and the PHY layer. It inserts a MAC header into the RLC PDU. Data that the MAC layer delivers to the PHY layer is called a Transport Block (TB). 
   The PHY layer processes the TB and transmits/receives the TB on a radio channel. The PHY layer also inserts a Cyclic Redundancy Check (CRC) to the TB for transmission and performs an error check on received data using a CRC. 
   As described above, the voice data generated from the CODEC  106  of the UE  110  is converted to a VoIP packet by the IP/UDP/RTP stack  105 . The header of the VoIP packet is compressed in the PDCP layer  104  configured for uplink transmission, processed to a size suitable for transmission on a radio channel in the RLC layer  103 , and reconstructed to an RLC PDU in the MAC and PHY layers  102  and  101 . The RLC PDU is channel-encoded and then transmitted on the radio channel. In the Node B  110 , the RLC PDU or a TB resulting from processing the RLC PDU in the PHY layer is channel-decoded in a PHY layer  111  and then transmitted to the RNC  120 . The RNC  120  recovers the RLC PDU to the VoIP packet and transmits it to the CN  130 . The CN  130  transmits the VoIP packet to the other party via the IP network  140  or a Public-Switched Telephone Network (PSTN)  150 . Downlink data transmission is performed in the reverse order. 
   For VoIP communications, both the caller and the called party must use the same type of CODECs  106  and  144 . If the UE  100  operating in UMTS communicates with a standard landline phone, a specific device between the PSTN  150  and the UMTS CN  130  performs CODEC conversion for the CODEC  154 . 
   CODECs approved by the 3GPP include an Adaptive Multi-Rate (AMR) CODEC. The AMR CODEC is characterized by Unequal Error Protection/Unequal Error Detection (UEP/UED). 
   The structure of a VoIP packet for transmission on a radio channel will be described in detail. 
   Referring to  FIG. 1 , a VoIP packet  160  is comprised of a Radio Protocol (RP) header  161 , a Robust Header Compression (ROHC) header  162 , a UDP checksum  163 , a Payload  164 , and a CRC  165  inserted in the PHY layer. The RP header  161  is inserted in the PDCP, RLC and MAC layers. Since a PDCP header and a MAC header are not used in many cases in VoIP communications, the RP header  161  can be a 2-3 byte RLC header. 
   The ROHC header  162  is a header compressed by a header compression protocol called ROHC in the PDCP layer. Compressible parts of an IP/UDP/RTP header are converted to the ROHC header. The ROHC header  162  is variable in size, generally 2 to 3 bytes. 
   The UDP header comprises the UDP checksum  163 . The UDP checksum  163  is not compressed and positioned after the ROHC header  162 . Errors are detected from IP/UDP/RTP data by means of the UDP checksum  163  of about 2 bytes. 
   The Payload  164  is the data generated from the CODEC. In the case of the AMR CODEC, the Payload  164  is 32 bytes or 9 bytes. The CRC  165 , inserted in the PHY layer, is a CRC operation value for the entire packet. The size of the CRC  165  is determined on a per call basis. It is usually 12 bits or 16 bits. The PHY layer of the receiver performs an error check using the CRC  165 . 
   As described above, two error detection schemes, that is, the UDP checksum  163  and the CRC  165  are redundantly applied to the conventional VoIP packet  160  in order to detect errors from the VoIP packet  160  transmitted on the radio channel, resulting in an unnecessary overhead. 
   SUMMARY OF THE INVENTION 
   An exemplary object of the present invention is to substantially solve at least the above problems and/or disadvantages and to provide at least the advantages below. Accordingly, an exemplary object of the present invention is to provide a method and apparatus for efficiently using transmission resources by eliminating a User Datagram Protocol (UDP) Checksum field from a Voice over Internet Protocol (VoIP) packet on a radio link that provides an error detection function. 
   Another exemplary object of the present invention is to provide a method and apparatus for eliminating/attaching a UDP Checksum field from/to a VoIP packet between a Packet Data Convergence Protocol (PDCP) layer and a Radio Link Control (RLC) layer. 
   The above exemplary objects are achieved by providing a method and apparatus for transmitting/receiving a VoIP packet on a radio link in a mobile communication system. 
   According to one exemplary aspect of the present invention, in a method of transmitting a VoIP packet on a radio link in a mobile communication system which provides a voice service over a packet network connected to the Internet, a VoIP packet comprising a UDP checksum field is received, the UDP checksum field is eliminated from the received VoIP packet, a CRC is added to the UDP checksum field-free VoIP packet, for error detection in the radio link, and the VoIP packet having the CRC is transmitted on the radio link. 
   According to another exemplary aspect of the present invention, in a method of receiving a VoIP packet on a radio link in a mobile communication system which provides a voice service over a packet network connected to the Internet, a VoIP packet without a UDP checksum field is received on a radio link, and the UDP checksum field is recovered for the received VoIP packet. 
   According to a further exemplary aspect of the present invention, in an apparatus for transmitting a VoIP packet on a radio link in a mobile communication system which provides a voice service over a packet network connected to the Internet, a checksum inserter receives a VoIP packet comprising a UDP checksum field and replaces a UDP checksum in a UDP checksum field of the VoIP packet with a pseudo checksum preset between a transmitter and a receiver. A header compressor compresses a header of the VoIP packet having the pseudo checksum. A checksum deleter eliminates the UDP checksum field from the header-compressed VoIP packet. A RP transmitter adds a CRC to the UDP checksum field-free VoIP packet, for error detection in the radio link, and transmits the VoIP packet having the CRC on the radio link. 
   According to still another exemplary aspect of the present invention, in an apparatus for receiving a VoIP packet on a radio link in a mobile communication system which provides a voice service over a packet network connected to the Internet, a RP receiver receives a VoIP packet without a UDP checksum field on a radio link. A checksum inserter inserts a pseudo checksum preset between a transmitter and a receiver into the received VoIP packet. A header decompressor decompresses a header of the VoIP packet with the pseudo checksum. A checksum calculator calculates a UDP checksum for the header-decompressed VoIP packet and replaces the pseudo checksum with the calculated UDP checksum. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The above and other objects, features and advantages of the present invention will become more apparent from the following detailed description when taken in conjunction with the accompanying drawings in which: 
       FIG. 1  illustrates an operation for providing a Voice over Internet Protocol (VoIP) service in a conventional mobile communication system; 
       FIG. 2  illustrates transmission and reception of an exemplary VoIP packet according to an exemplary embodiment of the present invention; 
       FIG. 3  illustrates an exemplary structure of a user equipment (UE) and a Radio Network Controller (RNC) according to an exemplary embodiment of the present invention; 
       FIG. 4  illustrates exemplary operations of a User Datagram Protocol (UDP) checksum field transmission handler and a UDP checksum field reception handler according to an exemplary embodiment of the present invention; 
       FIG. 5  is a flowchart illustrating an exemplary Packet Data Convergence Protocol (PDCP) layer operation of the UE for uplink VoIP packet transmission according to an exemplary embodiment of the present invention; 
       FIG. 6  is a flowchart illustrating an exemplary PDCP layer operation of the RNC for downlink VoIP packet transmission according to an exemplary embodiment of the present invention; 
       FIG. 7  is a flowchart illustrating an exemplary PDCP layer operation of the UE for downlink VoIP packet reception according to an exemplary embodiment of the present invention; and 
       FIG. 8  is a flowchart illustrating an exemplary PDCP layer operation of the RNC for uplink VoIP packet reception according to an exemplary embodiment of the present invention. 
   

   Throughout the drawings, the same or similar elements, features and structures are represented by the same reference numerals. 
   DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS 
   Exemplary embodiments of the present invention will be described herein below with reference to the accompanying drawings. In the following description, well-known functions or constructions are not described for conciseness. 
   The exemplary embodiments of the present invention are intended to provide a method of eliminating a User Datagram Protocol (UDP) checksum from a Voice over Internet Protocol (VoIP) packet for transmission on a radio channel that provides an error detection function and calculating the UDP checksum at a receiver that receives the radio channel. For example, a UE eliminates a UDP Checksum field from a VoIP packet prior to transmission. An RNC recovers the VoIP packet, calculates a UDP checksum, and inserts it into the VoIP packet. If the VoIP packet has errors on the radio channel, the errors are detected by a CRC in the PHY layer. Therefore, the elimination of the UDP checksum from the VoIP packet scarcely leads to error detection failure. 
   With reference to  FIG. 2 , a comprehensive description will be made of a procedure for eliminating/attaching a UDP Checksum field from/to a VoIP packet between the PDCP layer and the RLC layer according to the present invention. 
     FIG. 2  illustrates transmission and reception of a VoIP packet according to a preferred embodiment of the present invention. 
   Referring to  FIG. 2 , reference numeral  205  denotes a VoIP packet before header compression, generated from the CODEC in the UE or received from the core network (CN), and reference numeral  250  denotes a VoIP packet having a header compressed in the PDCP layer (hereinafter, referred to as a header-compressed VoIP packet). Reference numeral  270  denotes a VoIP packet (hereinafter, referred to as a transmission VoIP packet) produced by eliminating a UDP Checksum  260  from the header-compressed VoIP packet  250  and then transmitted through the Radio Link Control (RLC), Medium Access Control (MAC) and PHY layers. 
   Regarding the VoIP packet  205  before header compression, it comprises an IP Header  275 , a UDP Header  280 , a Realtime Transport Protocol (RTP) Header  240 , and a Payload  245 . 
   The IP Header  275  comprises a Destination IP Address  210 , a Source IP Address  215 , and other IP Header fields  220 . The Destination IP Address  210  indicates the IP address of a final destination for the VoIP packet  205 . The Source IP Address  215  is the IP address of a node that has generated the VoIP packet  205 . In IPv6, the Destination IP Address  210  and the Source IP Address  215  each have a length of 16 bytes, for example. The other IP Header fields  220  comprise information about a protocol version, the protocol number of a higher layer, a flow ID, and so on. The protocol number of the higher layer indicates the type of protocol used in the transport layer above the IP layer. The protocol number of the higher layer is UDP in the VoIP service. 
   The UDP Header  280  comprises a Destination UDP Port Number  225 , a Source UDP Port Number  230 , a UDP Length  232 , and a UDP Checksum  235 . The Destination UDP Port Number  225  is a 2-byte logical identifier (ID) assigned to a specific application (or a specific higher layer) in a node, indicating the higher layer to which the VoIP packet is destined. The Source UDP Port Number  230  indicates a higher layer from which the VoIP packet has been generated. The UDP Length  232  indicates the length of a packet within the UDP Header  280 . The UDP Checksum  235  is a checksum for detecting errors in an IP Pseudo Header, the UDP Header  280 , the RTP Header  240 , and the Payload  245 . It is 2 bytes long. The IP Pseudo Header refers to the Destination IP Address  210 , the Source IP Address  215 , and the protocol number of a higher layer in the IP Header  275 . 
   The RTP Header  240  is 8 bytes long and provides information needed for real-time traffic recovery, such as a SN and a Time Stamp (TS). The Payload  245  comprises the voice data generated from the CODEC. 
   Regarding the header-compressed VoIP packet  250 , it is comprised of a ROHC header  255 , a UDP Checksum  260 , and a Payload  245 . While the ROHC header  255  is variable in configuration and size depending on header compression status, it typically comprises a Connection ID (CID), a SN, and a CRC. The CID identifies a context having header compression-related information, and the SN is the last few bits of the SN of the RTP header  240 . The CRC is calculated over the IP/UDP/RTP headers which are not yet compressed. It is optionally included in the ROHC header  255  to verify that header decompression is correct. 
   In the header compression, “fixed header field values” such as the IP addresses and the UDP port numbers are stored in the context and not transmitted. “Sometimes changing header field values” are transmitted whenever they are changed. For “regularly changing header field values” such as the RTP SN, estimates of their changes are transmitted. “Always irregularly changing header field values” are transmitted all the time. Since the UDP Checksum field has an always irregularly changing value, a UDP checksum is transmitted without compression. 
   The transmission VoIP packet  270  is constructed by eliminating the UDP Checksum field  260  from the header-compressed VoIP packet  250 . Thus, it comprises the ROHC header  255  and the Payload  245 . 
   The receiver receives the transmission VoIP packet  270  and re-inserts the UDP Checksum field into it. Because the receiver has no knowledge of the true value of the UDP checksum eliminated by the transmitter, a header decompressor determines whether header decompression is correct using the CRC of the ROHC header, as described before. In case the receiver inserts an arbitrary UDP checksum, the CRC check will fail, which implies header decompression failure. 
   To overcome the problem, a pseudo UDP checksum preset between the transmitter and the receiver is filled in the UDP Checksum field before header compression of a VoIP packet in an embodiment of the present invention. The pseudo UDP checksum does not affect the header decompression irrespective of its value. It can be, for example, ‘1010 1010 1010 1010’. 
   Upon generation of the VoIP packet  205 , the transmitter fills the pseudo UDP check sum ‘1010 1010 1010 1010’ in the UDP Checksum field  235  and compresses the IP/UDP/RTP header, thereby creating the header-compressed VoIP packet  250 . Thus, the UDP Checksum field  260  of the header-compressed VoIP packet  250  has the value ‘1010 1010 1010 1010’. The transmitter eliminates the UDP Checksum field  260  from the header-compressed VoIP packet  250  and transmits the resulting transmission VoIP packet  270  to the receiver. 
   The receiver recovers the header-compressed VoIP packet  250  by inserting ‘1010 1010 1010 1010’ as the UDP Checksum field  260  in the transmission VoIP packet  270 , and decompresses the header of the header-compressed VoIP packet  250 . The receiver calculates the UDP checksum of the decompressed VoIP packet  205 , neglecting the pseudo UDP checksum, and then inserts the calculated UDP checksum into the UDP Checksum field  235 , thereby recovering the original VoIP packet  205 . 
   In this manner, the use of the pseudo UDP checksum eliminates the need for transmitting a UDP Checksum field on a radio channel. 
   The transmission and reception operations of the UE and the RNC according to an embodiment of the present invention will now be described below with reference to  FIG. 3 . 
     FIG. 3  illustrates exemplary structures of the UE and the RNC according to an exemplary embodiment of the present invention. 
   Both the UE and the RNC perform transmission and reception. That is, the UE transmits on the uplink and receives on the downlink, while the RNC receives on the uplink and transmits on the downlink. The transmission operation is the process of receiving an IP/UDP/RTP packet from a higher layer, setting a pseudo UDP checksum in the UDP Checksum field of the IP/UDP/RTP packet, compressing an IP/UDP/RTP header, and eliminating the UDP Checksum field with the pseudo UDP checksum. The reception operation is the process of receiving the header-compressed IP/UDP/RTP packet from a lower layer, inserting the pseudo UDP checksum in the UDP Checksum field behind the ROHC header, decompressing the IP/UDP/RTP header, calculating the UDP checksum of the decompressed header and the payload, and inserting the calculated UDP checksum into the UDP Checksum field of the decompressed IP/UDP/RTP header. 
   Referring to  FIG. 3 , a checksum field swapper  305  and  355 , a header compressor  310  and  360 , and a checksum field deleter  315  and  365  collectively form a UDP checksum field transmission handler  393  for the UE and the RNC. 
   During transmission, the checksum field swapper  305  and  355  substitutes a pseudo UDP checksum for a UDP checksum, the header compressor  310  and  360  compresses an IP/UDP/RTP header, and the checksum field deleter  315  and  365  deletes the pseudo checksum. 
   A pseudo checksum inserter  380  and  340 , a header decompressor  385  and  345 , and a UDP checksum calculator  390  and  350  collectively form a UDP checksum field reception handler  395  for the UE and the RNC. 
   During reception, the pseudo checksum inserter  380  and  340  inserts a pseudo checksum in a receive packet, the header decompressor  385  and  345  decompresses a header, and the UDP checksum calculator  390  and  340  substitutes a true checksum for the pseudo checksum. The header compressors  310  and  360 , and the header decompressors  385  and  345  are provided with a typical header compression protocol such as ROHC and reside in, for example, the PDCP layer. 
   The UE is provided with the UDP checksum field transmission handler  393  for uplink transmission, and the UDP checksum field reception handler  395  for downlink reception. The UE also has RLC entities  378  and  320 , MAC entities  377  and  323 , and PHY entities  375  and  325 , for both the uplink and the downlink operations. 
   The RNC is provided with the UDP checksum field transmission handler  393  for downlink transmission, and the UDP checksum field reception handler  395  for uplink reception. The RNC also has RLC entities  335  and  370 , MAC entities  333  and  372 , and PHY entities  330  and  373 , for both the uplink and the downlink operations. 
     FIG. 4  illustrates exemplary operations of the UDP checksum field transmission handler and the UDP checksum field reception handler according to an exemplary embodiment of the present invention. These operations will be described in the context of both the UE and the RNC. 
   Referring to  FIG. 4 , a checksum field swapper  410  receives a VoIP packet  405  from a Serving GPRS Support Node (SGSN) if it is in the RNC and from a higher layer if it is in the UE. The VoIP packet  405  is comprised of an IP Header, a UDP Header with a UDP Checksum field, and a Payload. The checksum field swapper  410  replaces the UDP checksum of the UDP header with a predetermined pseudo checksum in the VoIP packet  405 , thereby producing a pseudo VoIP packet  415 . The pseudo VoIP packet  415  is comprised of the IP Header, the UDP Header with the pseudo checksum, and the Payload. 
   A header compressor  420  compresses the IP Header, the UDP Header, and the RTP Header. Thus, the header-compressed VoIP packet  425  is comprised of an ROHC Header, the pseudo checksum, and the Payload. The header compression is a conventional ROHC header compression. 
   A checksum field deleter  430  eliminates the UDP Checksum field with the pseudo checksum from the header-compressed VoIP packet  425 , thereby producing a checksum-free VoIP packet  435 . The checksum-free VoIP packet  435  is comprised of the ROHC header and the Payload. 
   The checksum-free VoIP packet  435  is transmitted through RLC/MAC/PHY layers  440 . The VoIP packet  445  transmitted on the radio channel comprises an RP Header, the ROHC header, the Payload, and a CRC. 
   In the receiver, RLC/MAC/PHY layers  450  processes the checksum-free VoIP packet  445  and reconstructs an RP-free VoIP packet  455  by eliminating the RP header from the checksum-free VoIP packet  445 . The RP-free VoIP packet  455  is comprised of the ROHC header and the Payload. 
   A pseudo checksum inserter  460  inserts the pseudo checksum into the RP-free VoIP packet  455 , thus constructing a pseudo-checksum VoIP packet  465 . The pseudo-checksum VoIP packet  465  comprises the ROHC header, the pseudo checksum, and the Payload. The pseudo checksum is identical or substantially identical to that used in the checksum field swapper  410 . The transmitter and the receiver can preset the value of the pseudo checksum at a call setup, and the same pseudo checksum can be applied to all calls. 
   A header decompressor  470  decompresses the header of the pseudo-checksum VoIP packet  465 , thus reconstructing a header-decompressed VoIP packet  475 . The header-decompressed VoIP packet  475  is comprised of the original IP, UDP, and RTP headers, the pseudo checksum being set in the UDP Checksum field of UDP header. 
   A UDP checksum calculator  480  calculates the UDP checksum of the VoIP packet  475  in the same manner as done by UDP and replaces the pseudo checksum with the resulting UDP checksum, thereby constructing a VoIP packet  485 . The VoIP packet  485  has the UDP checksum of the IP Header, the UDP Header, and the Payload in the UDP Checksum field. If the receiver is the UE, the UE delivers the VoIP packet  485  to the higher layer, and if the receiver is the RNC, the RNC delivers the VoIP packet  485  to the SGSN. 
   With the header compression illustrated in  FIG. 4 , the UDP Checksum field is not transmitted on a radio channel. As a result, 2 bytes per packet are saved. 
   Aside from the UDP, UDP-lite can be used as the transport protocol of VoIP communications. This exemplary embodiment of the present invention is also applicable to UDP-lite. 
   In UDP-lite, UDP checksum coverage information is filled in the UDP Header Length field, and the UDP checksum of a part indicated by the UDP checksum coverage is set in the UDP Checksum field. This exemplary embodiment of the present invention is implemented for UDP-lite in the same manner as for UDP, except that the UDP checksum calculator calculates the UDP checksum in the manner defined by UDP-lite. That is, the UDP checksum calculator  480  calculates a checksum referring to the UDP checksum coverage information of the header-decompressed VoIP packet  475  and sets the checksum in the UDP Checksum field. 
   With reference to  FIGS. 5 through 8 , the PDCP layer operations of the UE and the RNC on the uplink and downlink according to an embodiment of the present invention will be described below. 
     FIG. 5  is a flowchart illustrating an exemplary PDCP layer operation of the UE for the uplink transmission according to an exemplary embodiment of the present invention. 
   Referring to  FIG. 5 , the UE receives a VoIP packet from the higher layer in step  500 . 
   In step  505 , the UE replaces the checksum of the VoIP packet with a predetermined pseudo checksum. The UE then compresses the IP/UDP/RTP header of the resulting VoIP packet in step  510 , eliminates the UDP checksum field from the header-compressed VoIP packet in step  515 , and delivers the UDP checksum-free VoIP packet to the lower layer in step  520 . 
     FIG. 6  is a flowchart illustrating an exemplary PDCP layer operation of the RNC for the downlink transmission according to an exemplary embodiment of the present invention. 
   Referring to  FIG. 6 , the RNC receives a VoIP packet from the SGSN in step  600 . 
   In step  605 , the RNC replaces the checksum of the VoIP packet with a predetermined pseudo checksum. Thus, the VoIP packet is comprised of an IP Header, a UDP Header with the pseudo checksum, and a Payload. 
   The RNC then compresses the IP/UDP/RTP header of the resulting VoIP packet in step  610 , eliminates the UDP Checksum field from the header-compressed VoIP packet in step  615 , and delivers the UDP checksum-free VoIP packet to the lower layer in step  620 . 
     FIG. 7  is a flowchart illustrating an exemplary PDCP layer operation of the UE for the downlink reception according to an exemplary embodiment of the present invention. 
   Referring to  FIG. 7 , the UE receives a header-compressed VoIP packet from the lower layer in step  700 . In step  705 , the UE inserts a predetermined pseudo checksum into the VoIP packet. The UE then decompresses the IP/UDP/RTP header of the VoIP packet with the pseudo checksum in step  710 . 
   In step  715 , the UE calculates the UDP checksum of the header-decompressed VoIP packet in the same manner as done by UDP. The UE substitutes the UDP checksum for the pseudo checksum in the header-decompressed VoIP packet in step  720  and delivers the resulting VoIP packet to the higher layer in step  725 . 
     FIG. 8  is a flowchart illustrating an exemplary PDCP layer operation of the RNC for the uplink reception according to an exemplary embodiment of the present invention. 
   Referring to  FIG. 8 , the RNC receives a header-compressed VoIP packet from the lower layer in step  800 . In step  805 , the RNC inserts a predetermined pseudo checksum into the VoIP packet. 
   The RNC then decompresses the IP/UDP/RTP header of the VoIP packet with the pseudo checksum in step  810 . In step  815 , the RNC calculates the UDP checksum of the header-decompressed VoIP packet in the same manner as done by the UDP. The RNC substitutes the UDP checksum for the pseudo checksum in the header-decompressed VoIP packet in step  820  and delivers the resulting VoIP packet to the SGSN in step  825 . 
   In accordance with the exemplary embodiments of the present invention as described above, the use of a pseudo UDP checksum at both a transmitter and a receiver obviates the need for transmitting a UDP Checksum field. Therefore, transmission resources are saved. 
   While the invention has been shown and described with reference to certain exemplary embodiments thereof, it will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the spirit and scope of the invention as defined by the appended claims.