Tech-invite3GPPspecsSIPRFCs
Overview21222324252627282931323334353637384‑5x

Content for  TS 28.062  Word version:  16.0.0

Top   Top   Up   Prev   Next
1…   4…   5…   A…

 

4  General DescriptionWord‑p. 15

4.1  Background Information

Tandem Free Operation (TFO) is intended to avoid the traditional double speech encoding/decoding in MS to MS (GSM), MS to UE (GSM/3G) or UE to UE (3G) call configurations. In the following paragraphs the term "MS" is used for MS and UE, the term UE only if a 3G terminal is explicitly addressed.
In a normal MS-MS call configuration the Speech Signal is first encoded in the originating MS, sent over the Air Interface, converted to A-law or μ-law ITU-T Recommendation G.711 [13] in the local transcoder, carried over the fixed network, transcoded again in the distant transcoder, sent over the distant Air Interface and finally decoded in the terminating MS (see Figure 4.1-1). In this configuration, the two speech codecs (coder/decoder pairs) are in "Tandem Operation". The key inconvenience of a tandem configuration is the speech quality degradation introduced by the double transcoding. This degradation is usually more noticeable when the speech codecs are operating at low rates.
Reproduction of 3GPP TS 28.062, Figure 4.1-1: Typical Speech Codec Tandem Operation
Up
When the originating and terminating connections are using the same speech codec, it is possible to transmit transparently the speech frames received from the originating MS to the terminating MS without activating the transcoding functions in the originating and terminating networks (see figure 4.1-2). In this configuration, "Tandem Free Operation" is on-going.
Reproduction of 3GPP TS 28.062, Figure 4.1-2: Tandem Free Operation of Speech Codec
Up
The key advantages of Tandem Free Operation are:
  • Improvement in speech quality by avoiding the double transcoding in the network;
  • Possible savings on the inter-PLMN transmission links, which are carrying compressed speech compatible with a 32 kbit/s or 16 kbit/s or 8 kbit/s sub-multiplexing scheme, including packet switched transmission;
  • Possible savings in processing power in the network equipment since the transcoding functions in the Transcoder Units are bypassed;
  • Possible reduction in the end-to-end transmission delay.
The major constraint of Tandem Free Operation is that the inter-PLMN transmission links must be transparent to the compressed speech frames. This means that any device located in the transmission path (IPE: in path equipment) between the originating and the terminating transcoders must be disabled, switched-off, or made aware of the TFO situation to keep unaltered any compressed speech frame sent over the transmission path. Examples of such devices are listed in Annex B.
The TFO Protocol defined in the present document provides the following services:
  • Establishment of a transparent path between transcoders;
  • Provision of an In-band signalling link between transcoders;
  • Exchange of information on the active speech codec type and supported speech codec types at both ends of the call configuration;
  • Codec Mismatch Resolution;
  • Establishment and Maintenance of Tandem Free Operation when identical codec types are used at both ends of the call configuration;
  • Fast and seamless fall back to Tandem Operation in case of necessary or unexpected TFO interruption (i.e. activation of supplementary services);
  • Support for cost efficient transmission.
The present document defines Tandem Free Operation for the different Speech Codec Types used in GSM and GSM evolved 3G systems. This includes the GSM_FR, GSM_HR, GSM_EFR and FR_AMR, HR_AMR, OHR_AMR, UMTS_AMR, UMTS_AMR_2, FR_AMR-WB, UMTS_AMR-WB, OFR_AMR-WB, OHR_AMR-WB codec types. However, the procedures used to establish TFO are considered system independent and could be extended to call configurations involving other systems like ISDN phones, speech servers, IP Multimedia or other wireless systems.
For non-AMR Speech Codec Types (i.e. GSM_FR, GSM_EFR and GSM_HR), Tandem Free Operation is fully compatible with the installed equipment base. The feature is fully supported by the Transcoder Units. The additional processing complexity is small compared to the encoding/decoding functions. Other network elements are not affected and possibly not aware of the establishment of Tandem Free Operation.
For the support of AMR Tandem Free Operation in GSM, the BTS and possibly the BSC may be involved in addition to the TRAU.
The resolution of a possible codec mismatch is defined as an optional feature. A codec mismatch occurs when incompatible speech codecs are used at both ends of the call configuration at call set-up. The resolution consists in finding an optimal speech codec on which TFO may be established. For that purpose, other elements in the Radio Access Network (BSS in GSM or RNC in 3G) might be involved.
Up

4.2  Principle of TFO OperationWord‑p. 16

4.2.1  Principle for TFO Operation for Narrow-Band speech codec types

Tandem Free Operation is activated and controlled by the Transcoder Units after the completion of the call set-up phase at both ends of an MS-MS, MS-UE, or UE-UE call configuration. The TFO protocol is fully handled and terminated in the Transcoder Units. For this reason, the Transcoder Units cannot be bypassed in Tandem Free Operation. This is the key difference with the feature called Transcoder Free Operation (TrFO) defined in TS 23.153.
In return, the Transcoder Units continuously monitor the normal Tandem Free Operation and can terminate TFO as soon as necessary with limited impact on the speech quality.
Before TFO is activated, the Transcoder Units exchange conventional 64 kbit/s PCM speech samples coded according to the ITU-T Recommendation G.711 [13] A-Law or μ-Law. The Transcoders can also exchange TFO messages by stealing the least significant bit in every 16th speech sample (see Annex A for the specification of the TFO message transmission rule and clauses 6 to 8 for the description of the TFO procedures and messages content).
If compatible Speech Codec Types and Configurations are used at both ends of the MS-MS, MS-UE, or UE-UE call configuration, the Transcoders automatically activate TFO. If incompatible Speech Codec Types and/or Configurations are used at both ends, then a codec mismatch situation exists. TFO cannot be activated until the codec mismatch is resolved. This capability is an optional feature involving other network elements of the Radio Access Network. The rules for finding a common codec type and solve the codec mismatch are defined in clause 11 and clause 12.
Once TFO is activated, the Transcoder Units exchange TFO Frames carrying compressed speech and in-band signalling, which structure is derived from the GSM TRAU Frames defined in the TS 48.060 and TS 48.061 (see clause 5). The exchange of TFO messages is still possible while TFO is active. In this case, the stealing process will result in embedding a message in the synchronisation pattern of the TFO Frame.
When TFO is activated between two end connections using the GSM_HR speech codec, the TFO Frames are carried over 8 kbit/s channels mapped onto the least significant bit (LSB) of the 64 kbit/s PCM speech samples.
When TFO is activated between two end connections using the GSM_FR or GSM_EFR speech codecs, the TFO Frames are carried over 16 kbit/s channels mapped onto the two least significant bits of the 64 kbit/s PCM speech samples.
When TFO is activated between two end connections using the AMR speech codec, the TFO Frames are carried over 8 or 16 kbit/s channels mapped onto the least or two least significant bits of the 64 kbit/s PCM speech samples. The format depends on the codec configuration (Optimized Active Codec Set).
To facilitate a seamless TFO interruption, the six or seven MSB of the PCM speech samples (not compressed) are transmitted to the far end unchanged.
Like GSM TRAU Frames, the TFO Frames have a fixed size (and duration) of:
  • 160 bits (20 ms) for the 8 kbit/s format;
  • 320 bits (20 ms) for the 16 kbit/s format.
Figure 4.2-1 provides a reference model for the functional entities handling Tandem Free Operation. The TFO Protocol is fully described in clauses 9 (State Machine) and 10 (Detailed Protocol).
Reproduction of 3GPP TS 28.062, Figure 4.2.1-1: Functional Entities Handling Tandem Free Operation
Up
The same TFO protocol and Frame Format is used irrespective of the PLMN types at both ends of the call configuration. Figure 4.2-2 shows a normal TFO configuration involving the same or two different GSM networks.
Reproduction of 3GPP TS 28.062, Figure 4.2.1-2: TFO Configuration between GSM Networks
Up
Figure 4.2-3 presents a TFO configuration involving two GSM-evolved 3G Networks. Note that the same protocol and Frame Structure are also used irrespective of the type of Transmission Network connecting the two 3G networks (ATM or STM).
Reproduction of 3GPP TS 28.062, Figure 4.2.1-3: TFO Configuration between 3G Networks
Up
Finally, figure 4.2-4 presents a TFO configuration involving two different network types (GSM and 3G). Similar configurations could be derived with any network supporting a TFO protocol compatible with the present document.
Reproduction of 3GPP TS 28.062, Figure 4.2.1-4: TFO Configuration between a GSM and a 3G Network
Up

4.2.2  Principle for TFO Operation for Wide-Band speech codec types (i.e. AMR-WB)Word‑p. 19
In case of AMR-WB the TRAU/TC performs in uplink direction the wideband decoding and a successive lowpass-filtering, downsampling to 8kHz sampling rate and PCM (G.711) encoding, before its sends the narrowband version of the speech signal towards its destination. This downsampled speech signal in PCM (G.711) representation allows interworking with the narrowband world (PSTN etc.). If a 64kbit/s channel is used, then a transcoded wideband signal (7 kHz speech bandwidth and 16kHz sampling rate) would anyway not fit into it. An efficient way to transport the wideband signal via such a channel is to use TFO (or TrFO) which delivers the compressed (encoded) speech. The encoded speech has a bandwidth significantly lower than 64kbit/s. In TFO_State OPERATION the TRAU/TC sends the AMR-WB TFO Frames within the LSBs of this PCM signal.
In the other, downlink direction the TRAU/TC performs G.711 decoding, upsampling to 16 kHz sampling rate, lowpass- filtering and wideband encoding before it sends the AMR-WB parameters down to the A/Iu interface. In TFO_State OPERATION the TRAU sends the AMR-WB parameters as received via the TFO Frames downlink.
A listener on the A/Iu interface will always hear the narrowband version of the speech conversation, while both ends send and receive the wideband version.
The basic principle for TFO operation for WB speec codec tpyes is the same as for narrow-band speech codec types (see section before). The following items must additionally be considered:
  • A new size of 640 bits for the 32 kbit/s TFO Frames format is needed in case the highest AMR-WB modes shall be used (the related TRAU format is defined in TS 48.060).
  • The scenario in figure 4.2.2-1 shows the situation when AMR-WB TFO has not yet been established while the call started with a narrowband codec. This is a likely starting scenario, because it it not desirable to occupy radio ressources unnecessarily with wide-band signals, until TFO is operational.
  • Figure 4.2.2-2 describes the situation after AMR-WB TFO establishment
  • Because of the higher speech signal bandwidth (up to almost 24 kbit/s for AMR-WB) up to the four LSBs must be stolen by TFO franes.
  • In case of TFO interruption, the remaining MSBs of the PCM speech samples (not compressed) might not only be less than for narrowband TFO, the transcoded bits carry a different kind of signal: The downsampled signal has narrowband properties (as depicted in figure 4.2.2-3). Because of the significant difference of the narrowband speech signal's impression (possibly even distorted by the stealing of four LSBs) to the wideband signal's quality, AMR-WB TFO interruptions should be avoided as best as possible.
Reproduction of 3GPP TS 28.062, Figure 4.2.2-1: Pre-TFO scenario for AMR-WB (for subsequent codec optimisation)
Up
Reproduction of 3GPP TS 28.062, Figure 4.2.2-2: AMR-WB with TFO Ongoing
Up
Reproduction of 3GPP TS 28.062, Figure 4.2.2-3: AMR-WB after TFO interruption
Up

4.3  TFO Standard Version HandlingWord‑p. 20
In TFO Specifications before REL-4 no TFO version handling is defined.
In TFO Specifications of REL-4 an "AMR TFO version number" is defined in the Ver (Version number) field of the AMR_ACS and AMR_SCS Extension Blocks (see clause 7) and the ATVN field in AMR Configuration frames (see Annex C). Only one REL-4 AMR TFO version is defined: version "0".
From REL-5 onwards the "TFO Version number" contained in the "TFO Version" extension block (section 7) and in "Generic Configuration Frames" (Annex H) shall reflect the Version and Subversion of the corresponding TS 28.062 (first and second digit of the TS version number, see foreword). The AMR TFO version number (Ver, AVTN, as in REL-4) shall be treated as "undefined" in case the TFO Version Number (as in REL-5 and onwards) is indicated in the TFO Messages.
The current TFO Version supports the GSM_FR, GSM_HR, GSM_EFR ,five AMR (Narrow Band) speech codec types (FR_AMR, HR_AMR, UMTS_AMR, UMTS_AMR_2, OHR_AMR: AMR-NB family) and four AMR Wide Band speech codec types (FR_AMR-WB, UMTS_AMR-WB, OFR_AMR-WB, OHR_AMR-WB: AMR-WB family).
The smallest defined TFO Version number is 0.0. It stands for all TFO Versions before 5.3. All numbers between 0.0 and 5.3 are reserved for future use. If the Local and Distant version numbers differ, the smallest version number shall have precedence and shall be applied on both sides. The following features (table 4.3-1) are optional or mandatory for the different Codec Types, depending on the applicable version number:
Feature
TFO Version Immediate Codec Type Optimisation Generic Configuration Frames
Codec Type ↓
GSM_FR
GSM_HR
GSM_EFR
Optional.
The TFO Version extension block need not to be sent. If not contained in TFO Messages, or is lower than 5.3, then Pre-REL-5 handling shall apply
Mandatory,
if TFO Version is 5.3 or higher.
If the TFO Version is lower than 5.3 then Generic Configuration Frames shall not be used. Only TFO_REQ_L and (TFO_ACK_L) shall be used.
If the TFO Version is 5.3 or higher, then Generic Configuration Frames shall be used. TFO_REQ_L and TFO_ACK_L shall not be used embedded into TFO Frames.
FR_AMR
HR_AMR
UMTS_AMR
UMTS_AMR2
OHR_AMR
Optional. The TFO Version extension block need not to be sent. If not contained in TFO Messages, or is lower than 5.3, then Pre-REL-5 handling shall apply Mandatory,
if TFO Version is 5.3 or higher.
If the TFO Version is lower than 5.3, then Generic ConfigurationFrames shall not be used. If the TFO Version is 5.3 or higher, then
Generic Configuration Frames shall be used. The parameter field in REL-4 AMR Configuration frames shall be treated as undefined. TFO_REQ_L and TFO_ACK_L shall not be used embedded into TFO Frames.
FR_AMR‑WB
UMTS_AMR‑WB
OFR_AMR‑WB
OHR_AMR‑WB
Mandatory.
The TFO Version extension block shall always be sent.
Mandatory. Generic Configuration Frames shall be used. TFO_REQ_L and TFO_ACK_L shall not be used embedded into TFO Frames.
Up

4.4  Document ContentWord‑p. 21
In the following, Clause 5 defines the structure of the TFO Frames exchanged between the Transcoder Units. The TFO Frames carry the compressed speech (payload) and some control bits for the inter-transcoder in-band signalling. Clause 6 introduces the elementary procedures used for the establishment and maintenance of Tandem Free Operation. Clause 7 defines the detailed content of the TFO messages associated with the TFO procedures. The TFO Message Structure follows the generic format defined in Annex A. Clause 8 defines how the TFO messages are mapped onto the TFO Frames. Clause 9 defines the TFO State Machine. Clause 10 contains the detailed TFO protocol. Clause 11 and Clause 12 specify the TFO Decision algorithm and the optional Codec Mismatch Resolution.
Annex B is an informative annex defining the expected behaviour of In-Path Equipment (IPE) for compatibility with Tandem Free Operation.
Annex C and Annex D define specific TFO processes for GSM and 3G systems.
Annex E contains a reference implementation for the TFO decision algorithm (C-code) described in Clause 11 and Clause 12.
Annex F is an informative Implementer's Guide containing recommendations in the implementation and introduction of AMR TFO.
Annex G provides basic Message Flow sequences for the TFO protocol.
Up


Up   Top   ToC