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30 TFOAbout This Chapter

30.1 Overview

This topic describes the application of the TFO in improving voice quality.

30.2 Availability

This topic describes the availability of TFO. The realization of TFO depends on the cooperation

of relevant NEs, software releases, and speech coding schemes.

30.3 Impact

The topic describes the impact of the TFO on system performance and other features.

30.4 Technical Description

Once a call connection is established, a TRAU frame is used to activate a traffic channel so that

the TFO message response mechanism is established. The rearranged speech frames form TFO

frames and the transcoding process is reduced to such a level that the speech quality is

significantly improved.

30.5 Capabilities

This topic describes the TFO capabilities in different services.

30.6 Implementation

The implementation of TFO refer to configuring and verifying TFO.

30.7 Maintenance Information

There is no maintenance information related to TFO.

30.8 References

HUAWEI BSC6000 Base Station Subsystem

BSS Feature Description 30 TFO

Issue 01 (2007-09-05) Huawei Technologies Proprietary 30-1

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30.1 Overview

This topic describes the application of the TFO in improving voice quality.

Definition

There is a symptom called the Tandem Operation during the calling process in which the speech

is transcoded two times. Speech signals are degraded due to Tandem Operation, as shown in

Figure 30-1.

Figure 30-1 Tandem Operation

A-law/u-law PCM

BSC

(TC)BTS MSC

BSC

(TC)BTS

Encoding Decoding

Speech

compression

PCM PCM

Speech

frameSpeech

frame

Decoding Encoding

Speech

compression

Figure 30-2 shows the Tandem Free Operation in which the speech signals are transparently

transmitted between two MSs.

Figure 30-2 TFO

BSC

(TC)BTS MSC

BSC

(TC)BTS

Endcoding DecodingSpeech compression

PCM PCM

Speech

frameSpeech

frame

TFO TFODisable

Tandem

operation

Disable

Tandem

operation

Disable

Tandem

operation

Disable

Tandem

operation

Purposes

TFO eliminates one encoding/decoding process to improve the speech quality.

Terms

Terms Definition

TFO Tandem Free Operation eliminates one encoding/decoding process

during an MS-MS call.

30 TFO


BSS Feature Description

30-2 Huawei Technologies Proprietary Issue 01 (2007-09-05)

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Terms Definition

IPE The in-path equipment in a network is classified into echo canceller,

digital communication multiplexer equipment (DCME), dual tone

multi-frequency (DTMF) generator, A-law/u-law converter, and so

on.

TFO MESSAGE TFO MESSAGE is the in-band signaling used for TFO negotiation

between TCs.

TFO FRAME As shown in Figure 30-2, TRAU frame is the speech data format

used between the MS and the TC. TFO frame is the speech data

format used between the TCs once the TFO is established.

Abbreviation

Abbreviation Full Spelling

AEC Acoustic Echo Cancellation

ALC Adaptive Level Control

EFR Enhance Full Rate speech codec

FR Full Rate speech codec

HR Half Rate speech codec

IPE In Path Equipment

MOS Mean Opinion Score

TFO Tandem Free Operation

TRAU Transcoding and Rate Adaptation Unit

30.2 Availability

This topic describes the availability of TFO. The realization of TFO depends on the cooperation

of relevant NEs, software releases, and speech coding schemes.

Network Elements Involved

Table 30-1 lists the network elements involved in TFO.

Table 30-1 NEs involved in TFO

MS BTS BSC MSC MGW SGSN GGSN HLR

NOTE

l : not involved

l : involved




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Software Releases

Table 30-2 lists the software versions of GBSS products that support TFO.

Table 30-2 GBSS products and related versions

Product Version

BSC BSC6000 V900R001C01 and later releases

Miscellaneous

The BSC6000 supports the TFO in the FR, HR, EFR coding schemes. It dos not support the

TFO in the AMR HR and AMR FR coding schemes.

30.3 Impact

The topic describes the impact of the TFO on system performance and other features.

Impact on System Performance

None.

Impact on Other Features

Once TFO is enabled, the system disables the AEC/ALC in the FR/EFR/HR services.

30.4 Technical Description

Once a call connection is established, a TRAU frame is used to activate a traffic channel so that

the TFO message response mechanism is established. The rearranged speech frames form TFO

frames and the transcoding process is reduced to such a level that the speech quality is

significantly improved.

30.4.1 TFO Messages

This topic describes the TFO messages. There are three types of TFO messages: TFO_REQ,TFO_ACK, and IS_IPE.

30.4.2 TFO Frame

This part describes the TFO frame, which is equivalent to the TRAU frame (see GSM Rec.

08.60). Actually, the TFO frame is a bit conversion format of the TRAU frame.

30.4.3 TRAU Operating Process in TFO Mode

This topic describes the TRAU operating process in ideal and abnormal cases.

30.4.4 Realization of TFO

This topic describes the realization of TFO. The TRAU implements the TFO by activating a

channel on the Abis interface, transferring TFO messages, and establishing acknowledgement

mechanism.

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30.4.1 TFO Messages

This topic describes the TFO messages. There are three types of TFO messages: TFO_REQ,

TFO_ACK, and IS_IPE.

Transmission of TFO Messages

TFO messages are transmitted in bit spacing schemes. In other words, the least significant bit

in the first sample of the 16 PCM samples is replaced. This reduces the impact of bit replacing

to a minimum level.

Classification of TFO Messages

l TFO_REQ

This command requests for activating the TFO feature.

l TFO_ACK

This command responses to the TFO_REQ commands.

l IS_IPE

The IS_IPE is classified into FO_TRANS and TFO_NORMAL.

TFO_TRANS is used to activate and maintain the ability of the IPE equipment to

support the TFO feature.

TFO_NORMAL is used to revert the IPEs to normal operation.

NOTE

TFO_FILL does not belong to IS_IPE in terms of classification. Its enables the IPEs to synchronize

with the TFO message in short. The TFO_FILL is mainly used to re-synchronize the IPEs in abnormalcases.

l Other commands

TFO_DUP is mainly used in intra-cell and inter-cell handovers. It enables the newly

activated TRAU channel to establish a TFO connection with the original TRAU

channel.

TFO_SYL reports that the synchronization information of the TFO frame is lost. It is

mainly used for channel handover.

30.4.2 TFO Frame

This part describes the TFO frame, which is equivalent to the TRAU frame (see GSM Rec.

08.60). Actually, the TFO frame is a bit conversion format of the TRAU frame.

The TFO frame occupies the two least significant bits of the 8-bit PCM frame. The remaining

six bits are still used to transmit the decoded speech signals. In other words, when the TFO is

activated, the decoder of the TRAU is still in work. On receiving the TRAU frame through the

Abis interface, the decoder performs decoding and A-law conversion of the TRAU frame, and

generates a new PCM frame by combining the six most significant bits of the TRAU frame with

the TFO frame.

30.4.3 TRAU Operating Process in TFO Mode

This topic describes the TRAU operating process in ideal and abnormal cases.




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TRAU Operating Process in Idea Case (Simple Mode)

1. On receiving a speech frame, the TRAU is activated. The TRAU also activates the encoder

and decoder.

2. The TRAU activates the TFO protocol processor, sends the TFO_REQ on the A interface,and monitors the TFO messages on the A interface (check whether there is TFO_REQ or

TFO_ACK sent from the remote end).

3. If the coding format of the TRAUs matches at two ends, the TRAU initiates the uplink TFO

and begins to send TFO frames. The TRAU also embeds TFO_TRANS messages into the

TFO frames to maintain the "direct pass-through" function of the IPEs.

4. On receiving the remote TFO frames through the A interface, the TRAU initiates the

downlink TFO and deactivates the encoder. The TFO protocol processor is then responsible

for the construction and transmission of downlink TRAU frames.

5. If the synchronization with the BTS fails in 10 successive frames, the current channel is

regarded being released. The TRAU deactivates the decoder and the TFO protocol

processor.

Handling in Abnormal Cases

l After a channel is activated, the TRAU sends consecutive TFO_REQ commands. If no

response is received when the number of TFO_REQ commands has exceeded the threshold,

the TRAU stops sending TFO messages but keeps monitoring the TFO messages on the A

interface until the channel is released.

l If the coding format of the TRAUs does not match at two ends, the TRAU aborts the

establishment of TFO links and returns to normal operation.

l Bit errors occur during the transmission of TFO frames and TFO messages.

Bit errors are detected through comparison between synchronization bits and Cyclic

Redundancy Check (CRC).

The TFO messages can do certain error correction of the bit errors. However, the bits to be

corrected differ. For details, refer to the protocols listed in 30.8 References.

If the TFO frames cannot be normally received due to bit errors, only synchronization bit

errors are allowed based on the DTX status. If there are error parameters, the current frame

should be discarded and replaced with the preceding good one.

l Synchronization lost of the TFO frames

Synchronization lost of the TFO frames is normally caused by sudden channel handover.

As far as the TRAU is concerned, the synchronization lost of the TFO frames occurs when

no channel handover information is available for the TRAU. Therefore, if there is

synchronization lost of TFO frames, the TRAU does not quit the TFO activation status but

tries to establish a connection with the remote TRAU through the TFO_SYL. The remote

TRAU response with the TFO_DUP. If the connection is established within a predefined

period, the current status of the call remains; if the connection is not established within the

predefined period, the local TRAU quits the TFO status.

30.4.4 Realization of TFO

This topic describes the realization of TFO. The TRAU implements the TFO by activating a

channel on the Abis interface, transferring TFO messages, and establishing acknowledgement

mechanism.

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1. During the call setup process, the TRAU searches for valid TRAU frames on the Abis

interface to activate the current channel.

2. When the channel is activated, the TRAU activates the encoder and decoder, and inserts

TFO messages in the PCM samples on the A interface by using the bit stealing scheme.

l If the TFO message acknowledgment mechanism is not established within a predefinedperiod, the TRAU performs transcoding in a normal way.

l If the TFO message acknowledgment mechanism is established, the TRAU rearranges the

speech frames received on the Abis interface, forms the TFO frames, and then transmits

the TFO frames to the remote TRAU. On receiving the TFO frames, the remote TRAU

processes the bits that constitute the TFO frames, forms the TRAU frames, and then

transmits the TRAU frames to the Abis interface. As the speech signals are encoded and

decoded only in the MS, this prevents repeated transcoding and can thus improve speech

quality.

30.5 CapabilitiesThis topic describes the TFO capabilities in different services.

In the TFO, the Mean Opinion Score (MOS) of the HR services increases by 0.30.5, the FR

services by 0.20.4, and the EFR services by 0.10.3.

30.6 Implementation

The implementation of TFO refer to configuring and verifying TFO.

30.6.1 Configuring TFOThis topic describes how to configure TFO through the BSC6000 Local Maintenance

Terminal.

30.6.2 Verifying TFO

This topic describes how to verify TFO through the BSC6000 Local Maintenance Terminal.

30.6.1 Configuring TFO

This topic describes how to configure TFO through the BSC6000 Local Maintenance

Terminal.

ProcedureStep 1 Right-click a GDPUC on the BSC Device Panel of the BSC6000 Local Maintenance

Terminal and select Configure Board Attributes on the short-cut menu. A dialog box is

displayed, as shown in Figure 30-3.




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Figure 30-3 Configure the attributes for the GDPUC

Step 2 ClickNext. A dialog box is displayed, as shown in Figure 30-4.

Figure 30-4 Configuring the TFO feature

Step 3 Select Configure All, and set the TFO Switch to Open, as shown inFigure 30-5.

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Figure 30-5 Configuring the TFO switch

Step 4 ClickFinish. The TFO is activated.

----End

30.6.2 Verifying TFO

This topic describes how to verify TFO through the BSC6000 Local Maintenance Terminal.

Procedure

Step 1 Right-click a GDPUC on the BSC Device Panel of the BSC6000 Local MaintenanceTerminal and select Query TC Resources on the short-cut menu. A dialog box is displayed,

as shown in Figure 30-6.




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Figure 30-6 Querying TC resources

Step 2 Click the DSP state tab. The DSP State page is displayed, as shown in Figure 30-7.

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Figure 30-7 Querying DSP state

Step 3 Select the corresponding Subrack No., Slot No., and DSP No., and then clickQuery. As shownin Figure 30-8, all the channels with TFO activated are displayed. The TFO verification

succeeds.




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Figure 30-8 Querying TFO state

----End

30.7 Maintenance Information

There is no maintenance information related to TFO.

Alarms

None.

Counters

None.

30.8 References

l

3GPP 02.53: "Digital cellular telecommunication system (Phase 2+); Tandem FreeOperation (TFO) of Speech Codecs; Service Description; Stage 1"

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l 3GPP 03.53: "Digital cellular telecommunication system (Phase 2+); Tandem Free

Operation (TFO) of Speech Codecs; Service Description; Stage 2"

l 3GPP 08.62: "Inband Tandem Free Operation (TFO) of Speech Codecs; Service

Description; Stage 3"




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