F.4 Balance between Speech Quality and Network Capacity

28.0623GPPInband Tandem Free Operation (TFO) of speech codecsService descriptionStage 3TS

The preference order for the Codec Type Optimisation and Codec Mismatch Resolution is based on speech quality and does not take into account the load in the network.

Guideline 8:

In capacity limited networks, the operator should only include Codec Types using half rate channels in the supported Codec List (GSM_HR, HR_AMR).

Annex G (informative):
Call flows for AMR TFO

Some example TFO protocol flows are shown for GSM (2G: left side) and UMTS (3G: right side) for a GSM-UMTS (2G-3G) TFO call. Other scenarios, like for GSM-GSM or UMTS-UMTS TFO calls can be derived by mirroring the relevant side. In cases where this is not directly obvious the other side is shown, too.

Configuration Frames (Con_Req, Con_Ack, etc) exist in GSM between BTS and TRAU (Abis Interface) as well as between TRAU and TRAU (the TFO Interface). They are used for delay measurements and for fast exchange of configuration parameters.

These Configuration Frames exist in UMTS between TC and TC or TC and TRAU (the TFO interface), but not on the Iu Interface. Instead the TFO Configuration is exchanged between SMSC and TC directly, e.g. via H.248 protocol. Optionally a proprietary interface between BSC and TRAU may also be used in GSM. In that case the Configuration Frames on the BTS-TRAU interface may be irrelevant.

The example in G.4 shows the version where the complete distant configuration is sent down to the BTS and further on to the BSC. In another version, G.5, only the Optimal Codec Type and Configuration is sent down to the BTS and BSC.

The protocol flows on the TFO interface (TRAU-TRAU, TRAU-TC, TC-TC) is in all cases identical.

Notations:

The TFO_Protocol States and the States of TFO_BTS are marked in yellow. The messages are shown as they appear on the interfaces, i.e. after the TFO_Protocol has already entered the new State.

The colours of the TFO Messages and Comments have no further meaning than highlighting the important parts and indicating what belongs together.

Some of the closed boxes contain comments or global descriptions of the ongoing procedures.

The TFO_Messages require a relatively long transmission time, up to several hundreds of milliseconds. These transmission times are not reflected within these call flow charts. But please consider that some sequences that appear in chronological sequence within the charts are in practise occurring in parallel and are overlapping in time.

The left side in the flow charts is arbitrarily called "local" side and is show as GSM, while the right side is called "distant" and is mainly shown as UMTS. But that is in most scenarios not relevant and the opposite is as true. The hand-over is per definition on the local side, just to simplify the discussion.

The mapping of the messages shown in the flow charts to the BTS-BSC messages is:

TFO_Report (Distant Configuration) is "RemoteCodecConfigurationReport (Distant Parameters)"

TFO_Report (Optimal Configuration) is "MultiRateCodecModeReq (Optimal Parameters)"

TFO_Report (Delay) is "RoundTripDelayReport (delay)"

TFO_Report (Status) is "TFO_Report (Status)"

ChannelActivation () is "ChannelActivation (Configuration, Handover_Indication)"