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    Nolia1 Nolia1 Document Transcript

    • Feature Description Functional FeaturesHUAWEI UMTS Circuit-Switched Core Network Table of Contents Table of ContentsChapter 5 TrFO and TFO ................................ ................................ ................................ .......... 5-1 5.1 Overview of TrFO and TFO ................................ ................................ ........................... 5-1 5.2 TrFO Functions ................................ ................................ ................................ ................. 1 5.3 TFO Functions ................................ ................................ ................................ .................. 3 5.4 TrFO and TFO ................................ ................................ ................................ .................. 4 5.5 Function of MGW during TrFO/TFO ................................ ................................ .................. 4 i
    • Feature Description Functional FeaturesHUAWEI UMTS Circuit-Switched Core Network Chapter 5 TrFO and TFO Chapter 5 TrFO and TFO5.1 Overview of TrFO and TFO In the radio network, in order to improved network utility efficiency and user capacity, it is necessary to compress voice. Two terminations in the network supporting different codec types cannot interconnect with each other until two times of transcoding are implemented. During the transcoding process, the voice codec type needs to be decoded and then re-encoded, so voice is damaged and voice quality is lowered. Transcoder Free Operation (TrFO) and Tandem Free Operation (TFO) are two negotiation mechanisms to ensure voice quality in wireless networks. TrFO is applicable to UMTS and compliant with the specification 3GPP TS 25.953; while TFO is applicable to GSM and UMTS and compliant with the specifications 3GPP TS 22.053, 3GPP TS 23.053 and 3GPP TS 28.062. UMG8900 supports both TrFO and TFO to reduce transcoding time and improve voice quality by cooperating with (G)MSC Server in the core network and equipments of the access network. In TrFO, the point-to point out-of-band negotiation about voice coding/decoding is implemented based on Out of Band Transparent Control (OoBTC). Out-of-band negotiation means that the channel used for the negotiation between two terminations is independent from the service flow channel, and the point-to-point codec type is selected based on the BICC protocol. TFO adopts in-band negotiation. The in-band negotiation means the service flow channel is used for negotiation. TC participates in the whole negotiation process.5.2 TrFO Functions The application of TrFO in the entire radio system network is shown in Figure 5-1. 1
    • Feature Description Functional FeaturesHUAWEI UMTS Circuit-Switched Core Network Chapter 5 TrFO and TFO Oo Codec ¤ Negotiation Control MSC MSC Plane   Se e   Se e     ¡ T ¡ Oo Codec ¤ Oo Codec r ¢ Negotiation ¤ ¢ RANA RANA a Negotiation M W £ M W £ n Control Control s earer earer earer ¦ i ¦ ¦ Req Req Req ME RNC MGW t MGW RNC ME N e User t Plane w ¥ ¥ ¥ CN earer o ¥ ¥ Radio earer Iu earer Iu earer Radio earer r k End to end connection ME: Mobile Equipment OoB: Out of Band RANAP: Radio Access Network Application RNC: Radio Network Controller Part Figure 5-1 TrFO application network structure In the TrFO mode, the calling ME performs out-of-band negotiation with the called ME. If they support the same codec type, transcoding is not needed in the service bearer set-up process, that is, transcoders are not necessary to add. In the TrFO mode, voice codec type negotiation is implemented by (G)MSC Server nodes. A typical negotiation process involving more than two (G)MSC Servers is detailed as below. 1) The originating (G)MSC Server (O-MSC) sends the codec types and an option list (IAM messages) supported, which are arranged as preference. After receiving the information from the originating (G)MSC Server node, the intermediate (G)MSC Server node deletes the options it does not support and then continue to send the list. The preference parameter in the list is not changed. The termination (G)MSC Server node (T-MSC) analyzes the list, then selects the codec type with the highest preference (an APM message, containing the selected codec type and optional codec types list), and finally returns its selection result. Till now, the negotiation process is completed. If no commonly supported code type can be selected during negotiation, transcoders need to be inserted for transcoding. The OoBTC negotiation can be classified into three types. And different negotiation processes correspond to different services:  Basic negotiation process (originating A, intermediate B and remote C): In this process, basic calling services are involved. 2
    • Feature Description Functional FeaturesHUAWEI UMTS Circuit-Switched Core Network Chapter 5 TrFO and TFO  Bearer modification process (originating A, remote B and remote C): C needs to modify B selection and bearer. The services concerned are CFU, CFB, CFNRy, CFNRc, CD, ECT and IN (with a more completed process).  Three-party negotiation process (originating A, remote B and remote C): CCD (Conference Call Device) is located at A. Firstly set up the link between A and B (A, B, Codec x), and then add C, which is only connected with A (A, C, Codec y).The concerned services are CW, CH and MPTY.5.3 TFO Functions In GSM or UMTS, two times of transcoding must be implemented when an MS accesses another MS or an MS accesses a UE as shown in Figure 5-2: Tandem Transcoding  LM ¨ © LM ¨  © § Transcoding Transcoding § MS UE MS UE Function Function Encoding e coding  Encoding  e coding compressed ITU-T G.711 compressed  § speech -law U-law speech Figure 5-2 Typical cascaded operations of TCs When an MS accesses another MS, the voice transcoding process is described as follows:  The originating MS encodes voice signals, and then sends them to the P MN A it resides in through radio links. The local TC decodes the compressed voice and according to ITU-T G.711, converts the decompressed voice to A-law or U-law PCM  voice that is then transferred to the radio network P MN B across the fixed network.   The TC of P MN B encodes the voice and the P MN B sends the compressed voice to another MS. In the above networking application, the two TCs are in cascading operation status. As the voice is encoded and decoded twice in the call process, the voice signals shall be damaged especially in a low-rate TC. To minimize the damage on voice and improve voice quality, the TC functions are   disabled at the P MN A and P MN B so that the voice signals are transparently transmitted between the originating and terminating MSs provided that the originating and terminating MS uses the same TC. This is the function of the TFO protocol. In this case, voice signals are only coded at the originating MS and decoded the terminating MS once, thus improving the voice quality. TFO affords the following functions: 3
    • Feature Description Functional FeaturesHUAWEI UMTS Circuit-Switched Core Network Chapter 5 TrFO and TFO  Establishing a channel for transparent transmission between two TCs.  Providing in-band signaling between two TCs for TFO operation control.  Transferring information of TC configurations such as ACS and SCS between two TCs.  Making codec types matched.  Enabling and implementing TFO if the same codec type is selected by both sides.  Restoring the TC cascading operation status quickly and seamlessly from TFO status if necessary. To make codec types matched, the method of optimising TC can be used. The optimising results may require TFO. In this case, other functional entities such as RNC may be required to participate in enabling TFO. TFO offers multiple benefits including:  Avoid repeated TC operations and improved voice quality. The effect is especially obvious in the presence of low-rate transcoding.  Occupy less P MN resources. During TFO, 16kbit/s or 8kbit/s multiplexed voice  signals that are compressed are transferred across networks.  Save the power of network equipments. During TFO, TC is disabled.  Possibly reduce the point-to-point transmission delay. At present, UMG8900 supports TFO of AMR transcoding only. For details about TFO message sequence, please see 3GPP TS 28.062 specifications.5.4 TrFO and TFO The association and difference between TrFO and TFO are:  If the OoBTC fails to establish the TrFO and transcoders are required, then in-band TFO may be used for negotiation.  TFO is activated either at set-up or during the communication phase.  In-band TFO shall be used for interworking with the 2G systems, and TrFO shall used for pure 3G network application.  In-band TFO shall be the fallback mechanism when transcoders cannot be avoided,   R99 set limitations of TrFO. The BICC protocol set1 only support AA 1 and AA 2, and does not support bearer modification or repeated codec negotiation. For example, if a node has no BC modification capability, you have to release the bearer link already set up and then set up a new one.5.5 Function of MGW during TrFO/TFO TrFO is firstly originated by the user ME. And only when there is no commonly supported code type during negotiation, is MGW used. (G)MSC Server initiates transcoder insertion, and under the control of the (G)MSC Server, MGW implements transcoder insertion according to the H.248 messages received. 4
    • Feature Description Functional FeaturesHUAWEI UMTS Circuit-Switched Core Network Chapter 5 TrFO and TFO During TFO, TC is responsible for the whole negotiation process, including making the decision whether to perform transcoding. 5