Huawei TFO

30 TFO

About This Chapter

30.1 OverviewThis describes the application of the TFO in improving voice quality.

30.2 AvailabilityThis lists the NEs, software, and voice coding schemes required for the implementation of TFO.

30.3 ImpactThis describes the impact of TFO on other system features.

30.4 Technical DescriptionThis describes the application of the TFO. Once a call connection is established, the TRAUframe is used for channel activation so that the TFO message response mechanism is established.The rearranged speech frames form the TFO frames so that the transcoding process is reducedto such a level that the speech quality is significantly improved.

30.5 CapabilitiesThis describes the TFO capabilities with different service types.

30.6 ImplementationThis describes how to configure and verify TFO.

30.7 Maintenance InformationNone.

30.8 References

HUAWEI BSC6000 Base Station SubsystemBSS Feature Description 30 TFO

Issue 02 (2007-12-17) Huawei Technologies Proprietary 30-1

30.1 OverviewThis describes the application of the TFO in improving voice quality.

Definition

There is a process called the Tandem Operation during a conversation in which the speech istranscoded two times. Speech signals are degraded due to Tandem Operation, as shown in Figure30-1.

Figure 30-1 Tandem operation

A-law/u-law PCM

BSC(TC)BTS MSC BSC

(TC) BTS

Encoding Decoding

Speechcompression

PCM PCMSpeechframe

Speechframe

Decoding Encoding

Speechcompression

Figure 30-2 shows the Tandem Free Operation in which the speech signals are transparentlytransferred between two MSs.

Figure 30-2 TFO

BSC(TC)BTS MSC BSC

(TC) BTS

Endcoding DecodingSpeech compression

PCM PCMSpeechframe

Speechframe

TFO TFODisableTandem

operation

DisableTandem

operation

DisableTandem

operation

DisableTandem

operation

Purposes

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

Terms

Terms Definition

TFO Tandem Free Operation: It eliminates one encoding/decodingprocess during an MS-MS call.

30 TFOHUAWEI BSC6000 Base Station Subsystem

BSS Feature Description

30-2 Huawei Technologies Proprietary Issue 02 (2007-12-17)

Terms Definition

IPE In Path Equipment: The IPE can be classified into echo canceller,digital communication multiplexer equipment (DCME), dual tonemulti-frequency (DTMF) generator, A-law/u-law converter, and soforth.

TFO MESSAGE The TFO MESSAGE is the in-band signaling used for TFOnegotiation between TCs.

TFO FRAME As shown in Figure 30-2, TRAU frame is the speech data formatused between the MS and the TC. TFO frame is the speech dataformat used between the TCs once the TFO is established.

AcronymsAcronyms 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 AvailabilityThis lists the NEs, software, and voice coding schemes required for the implementation of TFO.

NEs InvolvedTable 30-1 lists the network elements involved in TFO.

Table 30-1 NEs involved in TFO

MS BTS BSC MSC MGW SGSN GGSN HLR

– – √ – – – – –



MS BTS BSC MSC MGW SGSN GGSN HLR

NOTE

l –: not involved

l √: involved

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

Table 30-2 GBSS products and software versions

Product Version

BSC BSC6000 V900R003C01 and later releases

MiscellaneousThe BSC supports the TFO in the FR, HR, EFR coding schemes. It dos not support the TFO inthe AMR HR and AMR FR coding schemes.

30.3 ImpactThis describes the impact of TFO on other system features.

Impact on System PerformanceNone.

Impact on Other FeaturesOnce TFO is enabled, the system disables the AEC/ALC in the FR/EFR/HR services.

30.4 Technical DescriptionThis describes the application of the TFO. Once a call connection is established, the TRAUframe is used for channel activation so that the TFO message response mechanism is established.The rearranged speech frames form the TFO frames so that the transcoding process is reducedto such a level that the speech quality is significantly improved.

30.4.1 TFO MessagesThis describes the TFO message types, namely, TFO_REQ, TFO_ACK, and IS_IPE.

30.4.2 TFO FrameThis describes the TFO frame, which is actually a bit conversion format of the TRAU (see GSMRec.08.60).

30.4.3 TRAU Operating Process in TFO Mode




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

30.4.4 Realization of TFOThis describes how the TRAU activates a channel on the Abis interface, transfers TFO messages,and establishes acknowledgement mechanism.

30.4.1 TFO MessagesThis describes the TFO message types, namely, TFO_REQ, TFO_ACK, and IS_IPE.

Transmission of TFO MessagesTFO messages are transmitted in bit spacing schemes. In other words, the least significant bitin the first sample of the 16 PCM samples is replaced. This minimizes the impact caused by bitreplacement.

Classification of TFO Messagesl TFO_REQ messages

This command type requests to activate the TFO feature.l TFO_ACK messages

This command type responds to the TFO_REQ commands.l IS_IPE messages

The IS_IPE messages are classified into FO_TRANS messages and TFO_NORMALmessages.– The TFO_TRANS messages are used to activate and maintain the ability of the IPE

equipment to support the TFO feature.– The TFO_NORMAL messages are used to revert the IPEs to normal operation.

NOTE

The TFO_FILL does not belong to IS_IPE in terms of classification. Its enables the IPE to synchronizewith the TFO message in short. The TFO_FILL is mainly used to re-synchronize the IPEs in abnormalcases.

l Other commands– The TFO_DUP command is mainly used in intra-cell and inter-cell handovers. It enables

the newly activated TRAU channels to establish a TFO connection with the originalTRAU channels.

– The TFO_SYL command reports that the synchronization information of the TFO frameis lost. It is mainly used for channel handover.

30.4.2 TFO FrameThis describes the TFO frame, which is actually a bit conversion format of the TRAU (see GSMRec.08.60).

The TFO frame occupies the two least significant bits of the 8-bit PCM frame. The remainingsix bits are still used to transfer the decoded speech signals. In other words, when the TFO isactivated, the decoder of the TRAU is still in work. On receiving the TRAU frame through theAbis interface, the decoder performs decoding and A-law conversion of the TRAU frame, andgenerates a new PCM frame by combining the six most significant bits of the TRAU frame withthe TFO frame.



30.4.3 TRAU Operating Process in TFO ModeThis describes the TRAU operating process in ideal and abnormal cases.

TRAU Operating Process in Idea Case (Simple Mode)1. The TRAU is activated when it receives a speech frame. The encoder and decoder are

activated at the same time.2. The TRAU activates the TFO protocol processor, sends the TFO_REQ message on the A

interface, and monitors the TFO message on the A interface (check whether there isTFO_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 TFOand begins to send TFO frames. The TRAU also embeds TFO_TRANS messages into theTFO frames to maintain the "direct pass-through" function of the IPE.

4. On receiving the remote TFO frames through the A interface, the TRAU initiates thedownlink TFO and deactivates the encoder. The TFO protocol processor is then responsiblefor the construction and transmission of downlink TRAU frames.

5. If the synchronization with the BTS fails in 10 successive frames, the current channel isregarded being released. The TRAU deactivates the decoder and the TFO protocolprocessor.

Handling in Abnormal Casesl 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 Ainterface until the channel is released.

l If the coding format of the TRAUs does not match at two ends, the TRAU aborts theestablishment 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 CyclicRedundancy Check (CRC).The TFO messages can do certain error correction of the bit errors. However, the bits to becorrected 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 biterrors are allowed based on the DTX status. If there are error parameters, the current frameshould be discarded and replaced with the preceding good one.

l Synchronization lost of the TFO framesIn normal situations, the synchronization lost of the TFO frames are mainly caused by theinstant channel handover. The TRAU has no knowledge of the channel handoverinformation. Therefore, if there is synchronization lost of TFO frames, the TRAU does notquit the TFO activation status but tries to establish a connection with the remote TRAUthrough the TFO_SYL message. The remote TRAU responses with the TFO_DUP. If theconnection 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 theTFO status.

30.4.4 Realization of TFOThis describes how the TRAU activates a channel on the Abis interface, transfers TFO messages,and establishes acknowledgement mechanism.




1. During the call setup process, the TRAU searches for valid TRAU frames on the Abisinterface to activate the current channel.

2. When the channel is activated, the TRAU activates the encoder and decoder, and insertsTFO 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 the transcoding in a normal way.

l If the TFO message acknowledgment mechanism is established, the TRAU rearranges thespeech frames received on the Abis interface, forms the TFO frames, and then transmitsthe TFO frames to the remote TRAU. On receiving the TFO frames, the remote TRAUprocesses the bits that constitute the TFO frames, forms the TRAU frames, and thentransmits the TRAU frames over the Abis interface. As the speech signals are encoded anddecoded only in the MS, this prevents repeated transcoding and can thus improve speechquality.

30.5 CapabilitiesThis describes the TFO capabilities with different service types.

In the TFO, the Mean Opinion Score (MOS) of the HR services increases by 0.3–0.5, the FRservices by 0.2–0.4, and the EFR services by 0.1–0.3.

30.6 ImplementationThis describes how to configure and verify TFO.

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

30.6.2 Verifying TFOThis describes how to verify TFO on the BSC6000 Local Maintenance Terminal.

30.6.1 Configuring TFOThis describes how to configure 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 Configure Board Attributes on the short-cut menu. A dialog box isdisplayed, as shown in Figure 30-3.



Figure 30-3 Configuring the attributes of the GDPUC

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

Figure 30-4 Configuring the TFO feature

Step 3 Select Configure All on the DSPTC tab page, and set the AEC switch to Open, as shown inFigure 30-5.




Figure 30-5 Configuring the TFO switch

Step 4 Click Finish. The TFO is activated.

----End

30.6.2 Verifying TFOThis describes how to verify TFO on 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.



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.




Figure 30-7 Querying DSP state

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



Figure 30-8 Querying TFO state

----End

30.7 Maintenance InformationNone.

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"




l 3GPP 03.53: "Digital cellular telecommunication system (Phase 2+); Tandem FreeOperation (TFO) of Speech Codecs; Service Description; Stage 2"

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



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Huawei TFO