Contribution to 3GPP SA WG2
Work Item: Speech Transcoder Location and Control at the PLMN Border

Title: Transcoder location scenario and basic requirements

Source:	Siemens

Related to:	Work Item on ”Speech Transcoder Location and Control at the PLMN border

Purpose:	Discussion and inclusion into WI output document

1.	Introduction

The Work Item on ”Speech Transcoder location at the PLMN border” shall investigate the benefit, implications and requirements on the UMTS transmission efficiency, signalling and procedures when the transcoding function is located at the PLMN boarder. 
To see the full picture when discussing these issues, the scenarios must be identified first. In the following the scenarios of transcoder location in a PLMN, supporting transcoding at the PLMN border are summarised in figure 1.
Also we propose basic requirements for any solution in chapter 2. The list does not claim to be complete and should be amended if required.

2.	Network Scenario

To discuss the implications of transcoding located at the border of the UMTS core network, the resulting network configuration has to be described and the basic requirements have to be listed. Figure 1 shows the involved network elements. 

Figure 1: Scenarios of transcoder location in a PLMN supporting transcoding at the PLMN border

•	Network element type � - UMSC CVS (Compressed Voice Switch):
This UMSC CVS is compliant with the feature that is subject of this feasability study. It has the capability to perform compressed voice switching. It does not include a transcoding unit and passes the received speech channel unmodified to the UTRAN or to a Core Network entity (transit UMSC or Gateway UMSC). This implies that traffic to other networks has to be transcoded latest at the Gateway UMSC. Calls between UMSC users in the same network are not transcoded. 

•	Network element type ‚ - UMSC R99:
UMSC R99 supports the transcoding function according to the Release 99 specifications including TFO (if standardised by 3GPP SA WG4 for Release 99).

•	Network element type ƒ - GSM MSC:
Beside the UMTS MSC also GSM MSC coexist in a typical third generation PLMN to serve the GSM user and provide a satisfying coverage for UMTS users in the starting phase of UMTS. According to the GSM standard the TRAU is part of the GSM BSS and permanent assigned to a speech channel. Physically the TRAU is located close to the MSC to save transmission capacity.

•	Network element type „ - Gateway UMSC:
The Gateway UMSC provides the transcoding of the compressed speech inside to PLMN to the speech format required in the other networks (PSTN/ISDN/other GSM PLMN), which is most likely PCM A-law. It is assumed that the transcoding function can be switched on respectively off by control signalling at call setup and during the call.

•	There are two options to perform handover and mobile to mobile calls between UMSC CVS and legacy MSC’s (‚ or ƒ), either option 1a via the G-UMSC („) or that every UMSC CVS also has codec functionality (1b). This is ffs.

3.	Basic Requirements
Based on the scenario identified above, the studied solution shall comply with the following requirements

Migration:
-	Type ‚ UMSC R99 must be supported
-	If possible, solution shall avoid the necessity to update of the GSM MSC

Transmission Network:
-	It must be possible to transmit compressed speech unmodified through the UMTS core network

Features:
-	UMTS/GSM Handover shall be possible both ways within one PLMN
-	Calls between UMSC users in the same UMTS network shall not be transcoded
-	Calls to other networks shall be transcoded as close as possible to the network border, e.g. in the Gateway UMSC
-	It must be possible to control the transcoder unit in such a way to switch it on or off at call setup and during a speech call

4.	Proposal
1.	Chapter 2 and chapter 3 shall be included in the Technical Report of the Study UMTS Core Network, Speech Transcoder Location and Control - at the PLMN Border.
2.	A liaison statement to SA WG4 should be drafted to ask about the procedures for Tandem Free Operations in UMTS R99.