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Content for  TR 43.903  Word version:  16.0.0

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0  IntroductionWord‑p. 5

The present document captures the results of the feasibility study for introducing support for A-interface over IP.

1  ScopeWord‑p. 6

The present document contains the result from the study of introduction of support for A-interface over IP. High level areas that are studied are e.g. potential placement of transcoders in the core network, effective bandwidth utilisation at the A-interface, impact on call related messages, payload formats.
The following items shall be covered in the study:
  • In the target solution it is wanted to transfer compressed speech as far as possible end-to-end to achieve efficient transport and speech quality. The possibility to free GERAN from handling all kind of transcoders shall be studied, and the architecture might place Codecs in the core network.
  • Impacts/changes on current A-interface procedures resulting from placing transcoders in the core network as well as in the BSS shall be studied, e.g. impacts on the assignment and handover procedures.
  • In addition to allow compressed speech over the A-interface the study shall provide further solution for effective bandwidth utilisation at the A interface, which means it shall describe multiplexing of RTP flows and how this will be negotiated between the BSS and CN nodes.
  • The study shall describe a solution for "true end-to-end Codec negotiation", which considers on a call basis the preference/situation of the radio network.
  • It shall be studied how call related messages have to be adapted, e.g. transfer of Codec related information, identification of calls/sessions.
  • The study shall describe the wanted payload formats and other relevant user plane parameters like packetization time etc.

2  References

The following documents contain provisions which, through reference in this text, constitute provisions of the present document.
  • References are either specific (identified by date of publication, edition number, version number, etc.) or non specific.
  • For a specific reference, subsequent revisions do not apply.
  • For a non-specific reference, the latest version applies. In the case of a reference to a 3GPP document (including a GSM document), a non-specific reference implicitly refers to the latest version of that document in the same Release as the present document.
TR 21.905: "Vocabulary for 3GPP Specifications".
TS 23.205: "Bearer-independent circuit-switched core network".
TS 25.415: "UTRAN Iu Interface User Plane Protocols".
TS 48.006: "Signaling Transport Mechanism Specification for the Base Station System - Mobile-services Switching Centre (BSS - MSC) Interface".
TS 23.002: "Network architecture (Release 8)".
TS 48.008: "Mobile Switching Centre - Base Station System (MSC-BSS) interface Layer 3 specification".
TS 48.060: "In-band control for remote transcoders and rate adaptors for full rate traffic channels".
TS 48.061: "In-band control for remote transcoders and rate adaptors for half rate traffic channels".
TS 26.103: "Speech Codec list for GSM and UMTS".
TS 29.802: "(G)MSC-S - (G)MSC-S Nc Interface based on the SIP-I protocol".
TS 23.153: "Out of band transcoder control; Stage 2".
TS 23.231: "SIP-I based circuit-switched core network; Stage 2".
TS 29.414: "Core network Nb data transport and transport signalling".
TS 25.415: "UTRAN Iu interface user plane protocols".
TS 29.415: "Core Network Nb Interface User Plane Protocols".
TS 28.062: "Inband Tandem Free Operation (TFO) of speech codecs; Service description; Stage 3".
TS 26.093: "Mandatory speech codec speech processing functions Adaptive Multi-Rate (AMR) speech codec; Source controlled rate operation".
RFC 4040:  "RTP Payload Format for a 64 kbit/s Transparent Call".
RFC 3551:  "RTP Profile for Audio and Video Conferences with Minimal Control".
RFC 768:  "User Datagram Protocol".
RFC 791:  "Internet Protocol".
RFC 792:  "Internet Control Message Protocol".
RFC 2460:  "Internet Protocol, Version 6 (IPv6) Specification".
RFC 4443:  "Internet Control Message Protocol (ICMPv6) for the Internet Protocol Version 6 (IPv6) Specification".
RFC 4867:  "RTP Payload Format and File Storage Format for the Adaptive Multi-Rate (AMR) and Adaptive Multi-Rate Wideband (AMR-WB) Audio Codecs".

3  Definitions, symbols and abbreviationsWord‑p. 7

3.1  Definitions

For the purposes of the present document, the terms and definitions given in TR 21.905 and the following apply. A term defined in the present document takes precedence over the definition of the same term, if any, in TR 21.905.
For the sake of easy explanation the following short terms are defined:
Legacy MSC Server:
The Legacy MSC server does not support AoIP.
Legacy MGW:
The Legacy MGW does not support AoIP.
Legacy BSS:
The Legacy BSS does not support AoIP.
New MSC Server:
The New MSC Server supports only the AoIP-interface. Legacy BSSes are not supported by a New MSC Server.
Upgraded MSC Server:
The Upgraded MSC Server supports both, the TDM A-interface and the IP A-interface. Both kinds of interfaces could work simultaneously for different BSSs. It is claimed by some companies (e.g. Ericsson) that it is necessary to support AoTDM and AoIP also for the same, Upgraded BSS. Also legacy BSS, i.e. without any change, is supported by an Upgraded MSC Server.
New MGW:
The New MGW supports all UMTS and GSM Codecs as specified in TS 26.103 and has only an IP interface towards the BSS. The New MGW does not support AoTDM, not TFO and not PCMoIP.
Upgraded MGW:
The Upgraded MGW supports most or all UMTS and GSM Codecs as specified in TS 26.103 and has an IP interface towards the BSS. The Upgraded MGW supports both, AoIP and AoTDM. It supports PCMoIP and optionally TFO on any PCM link.
New Core Network:
A New Core Network has only New MSC Servers and New MGWs.
Upgraded Core Network:
A Core Network, where at least one MSC-Server or one MGW is upgraded to handle AoIP, while AoTDM, TFO or PCMoIP may be handled by some MSC-Servers or MGWs still.
Transcoder-less BSS:
A Transcoder-less BSS supports only AoIP, not AoTDM any longer. There is no way to use transcoders in a Transcoder-less BSS. It is not compatible to legacy core networks.
Upgraded BSS:
An Upgraded BSS starts from AoTDM with transcoders in BSS and ends potentially in AoIP without any transcoders in BSS and without AoTDM, i.e. as "Transcoder-less BSS". But several intermediate deployment scenarios are allowed for a safe and flexible migration. In order to be able to interwork with any kind of core network it seems obvious that AoTDM and AoIP will be needed in parallel for some time in most BSS vendors development strategies. The Upgraded BSS has the option to report its capability to the CN.
For the purposes of the present document, the following concepts apply:
Codec Type
Any of the existing GSM Codec Types, like GSM_FR, GSM_HR, GSM_EFR, FR_AMR, HR_AMR, FR_AMR-WB, see TS 26.103.
Codec Configuration
mainly used in context of AMR and AMR-WB to specify the mode set to be used during the call, e.g.
  • NB-Set1 = {(12.2) - 7.4 - 5.9 - 4.75}
  • WB-Set0 = {12.65 - 8.85 - 6.60}
Compatible Codec Configurations
Codec Configurations that do not require transcoding, although the Codec Types and Configurations may be different, e.g. FR_AMR(set 1) to HR_AMR (set 1), i.e. .
  • FR_AMR {12.2 - 7.4 - 5.9 - 4.75} to
  • HR_AMR { 7.4 - 5.9 - 4.75}
Interface Type
The A-Interface will exist in various types, e.g. as AoTDM or AoIP (target)

3.2  SymbolsWord‑p. 8


3.3  AbbreviationsWord‑p. 9

For the purposes of the present document, the abbreviations given in TR 21.905 and the following apply. An abbreviation defined in the present document takes precedence over the definition of the same abbreviation, if any, in TR 21.905.
A over IP
A over TDM
Call Identifier Code or Circuit Identifier Code
Gateway Control Protocol (H.248)
Mobile Station - Supported Codec List
(MSC-) Preferred Codec List
Supported Codec List (in OoBTC)
Speech Coder Version List

4  Requirements

  1. The transport protocol for the BSC-MGW interface (user-plane) shall be IP based.
  2. There shall be no impact on legacy and all GERAN MS/UE.
  3. Legacy BSCs with TDM interface shall be supported.
  4. TrFO shall be supported.
  5. Any proposed solution shall not preclude the use of any existing speech Codec (this includes GSM EFR, GSM FR, GSM HR, AMR-WB, AMR-FR and AMR-HR) supported by GERAN in Rel-8.
  6. It shall be possible to re/use 2G/3G MGW/MSC hardware.
  7. All teleservices, bearer services, VGCS and supplementary services defined for GSM shall be supported on the BSC-MGW interface.
  8. There shall be no impact on the GERAN radio interface (Um interface).
  9. There shall be no impact on the BTS hardware and software. An exception could be in the case of TC is removed from the BSC (FFS), then there may be impact to the BTS software.
  10. A-flex shall be supported.
  11. TFO shall not be mandated. An exception is for the case of the TC remains in the BSC (FFS).
  12. Multiplexing of user-plane data shall be possible.
  13. GSM/AMR Codec adaptation shall be possible, e.g. due to overloading of the BSC or radio conditions. The GSM/AMR Codec adaptation delay shall be in the same order as in the current A-interface solution.
  14. End-to-end speech delay shall not be increased. Congestion in the IP transport may introduce additional delay; however the end-to-end delay shall not exceed the ITU recommendation [G.114].
  15. It shall be possible to secure the BSC-MGW interface (see item e) below).
  16. It shall be possible to automatically configure IP addresses and transport layer ports (e.g. RTCP, UDP port numbers). Whether manual configuration is possible is FFS.
  17. Speech interruption times during handovers shall be in the same order as in the current TDM implementations.
  18. The interaction of dynamic AMR Codec change and TrFO shall not degrade the overall quality of the speech in the case of MS to MS calls.
  19. BTS synchronization requirements as stated in 45.010 clause 5 shall be fulfilled. The means to achieve this are implementation specific.
For further investigation in feasibility study:
  1. The location of the TC (in BSC and/or MGW).
  2. Bandwidth efficiency improvements through use of compressed Codec (GSM EFR, GSM FR, GSM HR, AMR-WB, AMR-FR and AMR-HR) on the BSC-MGW interface.
  3. Smooth migration from the legacy A-interface to the new BSC-MGW IP-based interface.
  4. The manual configuration of IP addresses and any transport layer ports, e.g. RTCP or UDP port numbers.
  5. Since IP transport is vulnerable to unauthorised intrusions, security aspects shall be investigated.
  6. Whether to align the support of IPv4 or IPv6 for the U-plane according to the C-plane.
  7. Support for GAN.

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