Echo Cancelling

Echo Cancelling

DN98909313Issue 7-0

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The information in this document is subject to change without notice and describes only the product defined in the introduction of this documentation. This documentation is intended for the use of Nokia Siemens Networks customers only for the purposes of the agreement under which the document is submitted, and no part of it may be used, reproduced, modified or transmitted in any form or means without the prior written permission of Nokia Siemens Networks. The documentation has been prepared to be used by professional and properly trained personnel, and the customer assumes full responsibility when using it. Nokia Siemens Networks welcomes customer comments as part of the process of continuous development and improvement of the documentation.

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f Important Notice on Product Safety Elevated voltages are inevitably present at specific points in this electrical equipment. Some of the parts may also have elevated operating temperatures.

Non-observance of these conditions and the safety instructions can result in personal injury or in property damage.

Therefore, only trained and qualified personnel may install and maintain the system.

The system complies with the standard EN 60950 / IEC 60950. All equipment connected has to comply with the applicable safety standards.

The same text in German:

Wichtiger Hinweis zur Produktsicherheit

In elektrischen Anlagen stehen zwangsläufig bestimmte Teile der Geräte unter Span-nung. Einige Teile können auch eine hohe Betriebstemperatur aufweisen.

Eine Nichtbeachtung dieser Situation und der Warnungshinweise kann zu Körperverlet-zungen und Sachschäden führen.

Deshalb wird vorausgesetzt, dass nur geschultes und qualifiziertes Personal die Anlagen installiert und wartet.

Das System entspricht den Anforderungen der EN 60950 / IEC 60950. Angeschlossene Geräte müssen die zutreffenden Sicherheitsbestimmungen erfüllen.

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Table of ContentsThis document has 43 pages.

1 Summary of changes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7

2 Echo cancelling. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 92.1 Echo cancelling overview . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 92.2 Sources of echo in the GSM network . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 92.3 Ways of cancelling echo in the network . . . . . . . . . . . . . . . . . . . . . . . . . . . 102.4 Echo in PSTN – PSTN calls . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 122.5 Echo in MS – PSTN calls . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 122.6 Echo in GSM MS – GSM MS calls . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 132.7 Echo in MS – PSTN calls (speaker phone). . . . . . . . . . . . . . . . . . . . . . . . . 142.8 Echo in GSM MS – analog MS calls . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 152.9 Echo in UE - PSTN calls according to 3GPP Rel–4 . . . . . . . . . . . . . . . . . . 152.10 Echo canceller control in the network . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 162.11 Internal control of echo cancellers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 182.12 External control of echo cancellers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23

3 Echo canceller units . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 243.1 Structure of a digital echo canceller . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 243.2 Echo canceller units in MSC . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 263.3 Echo cancellers in MSC and Integrated MSS. . . . . . . . . . . . . . . . . . . . . . . 273.4 Summary of echo canceller unit types . . . . . . . . . . . . . . . . . . . . . . . . . . . . 303.5 Echo cancelling specifications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31

4 Operation and maintenance of echo cancellers . . . . . . . . . . . . . . . . . . . . . 334.1 Echo canceller hardware configuration management. . . . . . . . . . . . . . . . . 334.2 Echo canceller supervision . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 334.3 Echo canceller alarm functions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 344.4 Echo canceller recovery . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 354.5 Echo canceller fault handling . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 354.6 ECU routine testing (EC08) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 364.7 ECU time-slot testing (EC08 and EC1P/EC1P–S) . . . . . . . . . . . . . . . . . . . 374.8 Echo cancelling parameters (ET/EC) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 374.9 EC2ET/EC2ET–A daughterboard SW loading . . . . . . . . . . . . . . . . . . . . . . 374.10 Related topics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 38

5 Echo canceller troubleshooting. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 395.1 Checking the cancelling of electric echo. . . . . . . . . . . . . . . . . . . . . . . . . . . 39

6 Checking echo canceller versions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 426.1 Checking EC9 EPROM variant of EC1P/EC1P–S . . . . . . . . . . . . . . . . . . . 426.2 Checking the SW versions of ET/EC . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 426.3 Checking daughterboard type and SW versions. . . . . . . . . . . . . . . . . . . . . 43

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List of FiguresFigure 1 Placing of echo cancellers in GSM network . . . . . . . . . . . . . . . . . . . . . . 10Figure 2 Echo in a PSTN–PSTN call through International Switching Centre (ISC).

12Figure 3 Echo in a PSTN–PSTN call through Rel-4 Multimedia Gateway . . . . . . 12Figure 4 Echo in an MS–PSTN call. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13Figure 5 Echo in an MS–PSTN call through an International Switching Centre (ISC)

13Figure 6 Echo in a GSM MS–GSM MS call through a single MSC. . . . . . . . . . . . 14Figure 7 Echo in an MS–MS call through different MSCs. . . . . . . . . . . . . . . . . . . 14Figure 8 Echo in an MS–PSTN call (speaker phone) . . . . . . . . . . . . . . . . . . . . . . 15Figure 9 Echo in a GSM MS–analog cellular call . . . . . . . . . . . . . . . . . . . . . . . . . 15Figure 10 Echo in UE – PSTN calls according to 3GPP Rel-4 . . . . . . . . . . . . . . . . 16Figure 11 Echo in UE – PBX calls according to 3GPP Rel-4 . . . . . . . . . . . . . . . . . 16Figure 12 Example of call establishment signalling in a PSTN–MS call . . . . . . . . . 17Figure 13 Example of call establishment signalling in a PSTN–PSTN call in Rel-4 net-

work . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18Figure 14 Incoming call from PSTN to GMSC, OEC and IEC disconnected . . . . . 18Figure 15 Incoming call from PSTN to GMSC, IEC connected. . . . . . . . . . . . . . . . 19Figure 16 Incoming call from PSTN to GMSC, OEC and IEC disconnected . . . . . 19Figure 17 Incoming call from PSTN to GMSC, OEC connected . . . . . . . . . . . . . . . 19Figure 18 Mobile–originated speech call toward trunk, IEC and OEC connected . 20Figure 19 Mobile–originated speech call toward trunk, IEC disconnected . . . . . . . 20Figure 20 Mobile–originated data call . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21Figure 21 Mobile–terminated speech call, IEC connected . . . . . . . . . . . . . . . . . . . 21Figure 22 Mobile–terminated speech call, OEC and IEC connected . . . . . . . . . . . 22Figure 23 Mobile–terminated data call . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22Figure 24 Structure of a digital echo canceller . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24Figure 25 Impulse response of echo. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25Figure 26 Operating environment of EC1P . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27Figure 27 Operating environment of EC2ET/ECE2 . . . . . . . . . . . . . . . . . . . . . . . . 29Figure 28 Operating environment of EC2ET–A/ECE2–A . . . . . . . . . . . . . . . . . . . . 30

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List of TablesTable 1 Echo cancelling overview . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9Table 2 ET/EC and PIU configuration . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28Table 3 DX 200 echo cancellers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30Table 4 EC2ET daughterboards . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31Table 5 EC2ET–A daughterboards . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31Table 6 ECE2, ECE2–C, ECE2–A daughterboard . . . . . . . . . . . . . . . . . . . . . . 31Table 7 EC9 variants of EC1P and EC1P–S . . . . . . . . . . . . . . . . . . . . . . . . . . . 42

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1 Summary of changesChanges made between issues 7-0 and 6-0The company and product names have been changed according to the official Nokia Siemens Networks portfolio naming.

Changes made between issues 6–0 and 5-0Integrated Gateway Control Server (GCS) has been removed as it no longer exists as a separate product. In M14.0 it is a functionality of the MSC Server.

Echo cancelling

Section Echo in UE - PSTN calls according to 3GPP Rel.99 has been removed as Rel.99 is not supported in M14.0.

Echo canceller units

The following plug-in unit- types have been added to the document: ECE2-B, ECE2-CB and ECE2-AB.

Table ET/EC and PIU configuration has been updated with the ROHS plug-in units.

Changes made between issues 5–0 and 4–3Echo in UE - PSTN calls according to 3GPP Rel-4

Information added concerning the use of a PBX connection with an Integrated MSS.

Echo canceller troubleshooting

Checking the state of the echo canceller in the MGW added (as step 12).

Changes made between issues 4–3 and 4–2Echo in PSTN-PSTN calls

Rel-4 information added.

Echo in UE - PSTN calls according to 3GPP Rel-4

New section.

Echo canceller control in the network

Section on Echo canceller control in Rel-4 network architecture added.

Internal control of echo cancellers

Section on Echo canceller control in Rel-4 network architecture added.

External control of echo cancellers

Information on Tone Disablers updated.

Echo canceller units in MSC

The EC1P-S conforms to ITU-T Rec. G.165.

Echo cancellers in MSCi, Integrated MSS and Integrated GCS

ECE2 can be used with the ECE2–C motherboard and EC2E-NA daughterboard. Table 1 corrected accordingly.

Summary of echo canceller unit types

Table 2, DX 200 echo cancellers, updated. E1 and T1 added.


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Summary of changes

Checking of functional modes added in Checking the cancelling of electric echo (as item 8).

Checking of the ECHO parameter on the given circuit group or outgoing route added.

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2 Echo cancellingWe speak of echo when someone talking via a telephone hears, after a certain delay, their own voice reflected back to them. Echo becomes noticeable to end users when the delay is 20 – 25 ms or longer. When the delay becomes noticeable, it can be one of the main sources of end-user dissatisfaction. The longer the delay, the more irritable it can be, and it can at worst make it impossible to carry out a telephone conversation.

In practice, all connections where the one-way delay is longer than 25 ms demand some form of echo cancelling.

2.1 Echo cancelling overview

2.2 Sources of echo in the GSM networkEcho is generated at two points in the GSM system. The first is the handset itself, where the acoustic coupling between the earpiece and microphone produces acoustic echo.

The second major source of echo is the impedance mismatch in hybrids (usually located at the local exchange), which generates network echo. While the local channel from the phone to the switching centre is a two–wire bidirectional channel, the channel between the switching centres is a four–wire channel comprising two unidirectional two–wire channels, a hybrid is the device that couples the two–wire and four–wire channels. The hybrid should transfer all the energy from the incoming branch of the four–wire channel to the two–wire channel. No coupling should take place between the outgoing and incoming branches of the four–wire channel, but in practice, a part of the energy on the incoming branch is transferred to the outgoing branch. This produces electric echo.

Echo is also present in normal PSTN–to–PSTN connections but that presents no great difficulty as long as the delays are reasonable. National networks do not need echo can-celling, unless they are very large. In international networks and satellite connections, long distances create additional delays in the order of 4 – 6 ms for 1000 km cable length or about 520 ms for one satellite hop. This is why International Switching Centres (ISC) and satellite connections always need echo cancelling.

In the GSM system the problem is caused not so much by echo itself but the long delays occurring in a digital network. For instance, when a GSM subscriber calls a PSTN sub-scriber, the call is first transmitted to the Base Station Subsystem (BSS), where it expe-riences a 90 ms speech coding and interleaving delay before continuing through the Mobile Switching Centre (MSC)/Visitor Location Register (VLR) to the local PSTN switch. At the 4–to–2 wire hybrid in the local PSTN switch, an electric echo is created and transmitted back along the path of the call. When the echo reaches the BSS, it again experiences a 90 ms delay before continuing to the mobile phone. Thus, the coding and

Echo cancelling descriptions Echo cancelling instructions

Echo cancelling


Operation and maintenance of echocancel-lers


Checking echo canceller versions

Table 1 Echo cancelling overview

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Echo cancelling

interleaving processes along with other transmission delays will always generate a delay of about 100 ms in both uplink and downlink directions, and, without echo cancel-ling in the network, the mobile user hears his own voice after a delay of about 200 ms.

When two mobile phone users talk through the network, the echo delay is twice as long as in an MS to PSTN call, 360 – 400 ms, because there are two BSSs in the signal path.

2.3 Ways of cancelling echo in the networkWays of cancelling echo in the networkTo deal with echo in the downlink direction, that is, the PSTN electrical echo generated in a hybrid, there is an echo canceller in the MSC as a mandatory part. Basically, the echo canceller memorises the voice samples sent to the PSTN and then compares the samples to the voice samples received back from the PSTN. These speech samples (containing the echo) are modified by the echo canceller to prevent the echo effect from being passed back to the mobile.

In the uplink direction, the PSTN phone user hears an acoustic echo as the mobile phone user's voice is transmitted back from the mobile phone and experiences the same delay as the electric echo. To deal with this echo, the mobile phone is equipped with a built–in acoustic echo canceller. This, too, is a standard part of the GSM system.

Figure 1 Placing of echo cancellers in GSM network

Ideally, the built–in echo cancellers in the mobile stations should make further cancelling of acoustic echo unnecessary. The echo canceller design of some mobile phone types is not, however, efficient enough, and the network may be equipped with an additional echo canceller, for instance, the Acoustic Echo Cancellation (AEC), located in the transcoder in the BSS.

Nokia Siemens Networks Acoustic Echo Cancellation (AEC)The Nokia Siemens Networks Acoustic Echo Cancellation (AEC) is a software feature integrated in the transcoder (TCSM/TCSM2) and designed to remove echo originated from mobile phones in the uplink direction. It replaces speech frames containing echo with comfort noise resembling the actual background noise while maintaining the full duplexity of the call. The AEC is automatically disabled during data or fax calls. It is com-patible with High Speed Circuit Switched Data (HSCSD), all GSM codecs and can be used with or without the uplink Discontinuous Transmission (DTX) activated.

The AEC is not a part of the DX 200 MSC and therefore it is not within the scope of this document. For more information, contact your local Nokia Siemens Networks represen-tative.

Avoiding non–linearities in the echo pathNon–linearities in the echo path tend to affect the performance of echo cancellers. Therefore, the following features should be avoided in PSTN connections (see G.168, 3GPP TS 43.050):

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• Excessive signal levels. The clipping of signal in the echo path causes distortion, which affects the performance of the echo canceller.

• Poor hybrids. Attenuation in the echo path should be at least 6 dB (Rec. G. 168). Otherwise, echo canceller may not work properly.

• Speech compression (for example, ADPCM, LD–CELP) introduces distortion, which affects the performance of the echo canceller.

• Significant differences in clock rates between non–synchronised network compo-nents. The resulting phase roll and slips are likely to affect the performance of the echo canceller.

• Analog frequency–division multiplexing(FDM) routings which exhibit phase roll.

Typically these non-linearities cause a somewhat higher echo level during double talk, but this does not necessarily disturb the subscriber. In the worst case, however, combined with a poor hybrid, the echo canceller cannot remove the echo at all.

Cancelling of acoustic echoGSM recommendation 3.50 states that a handset and a handsfree Mobile Station (MS) should perform acoustic echo cancellation. In other words, the mobile should have a built–in echo suppressor or canceller and no acoustic echo cancellation is needed on the network side.

However, it seems that some mobiles are not capable of removing the acoustic echo sufficiently and the subscriber may sometimes hear the mobile–originated echo.

Additionally, according to field tests done by Nokia Siemens Networks, an extra gain used in the transcoder may produce a more noticeable acoustic echo. The echo level is higher in MS – MS calls than in PSTN – MS calls, because the echo path has double gain compared to PSTN – MS calls. Therefore, the extra gain in the transcoders is a compromise between the loudness of the network and the level of the acoustic echo.

From the point of view of echo levels, the 0 dB gain should be used in the transcoders. If more gain is needed, for example because of mobiles having earpiece volume levels that are too low, it is preferable to use a downlink gain with uplink attenuation, because the total gain in the echo path remains 0dB. The only drawbacks are that the PSTN sub-scriber hears the mobile at a lower level, and the lack of downlink gain in MS – MS calls.

The use of uplink Discontinuous Transmission (DTX) can somewhat prevent the trans-mission of mobile–originated acoustic echo. The reason is that the uplink DTX cuts off the echo path if the mobile's Voice Activity Detection (VAD) algorithm does not interpret the acoustic echo as speech. Instead of echo, the mobile sends Silence Description (SID) frames. Therefore, the uplink DTX can decrease the echo if the downlink signal level is low enough. This also depends on the mobile's VAD sensitivity, on the volume setting, and on the properties of the mobile's echo canceller. Lowering the volume setting of the MS usually helps the echo problem.

Means of preventing the acoustic echo originating from the MS:

• Activate uplink DTX. It removes low–level echo. The best results can be achieved by using the uplink DTX together with 0dB gain in the transcoders.

• Lower the volume level setting of the MS. If a PSTN or MS subscriber hears his own voice echoing from the MS direction, he can ask the mobile subscriber to lower the volume level of the MS. This is probably the most effective way to suppress the acoustic echo.

• Equip transcoders in the network with the Acoustic Echo Cancellation (AEC) to improve the echo cancelling ability of the MSs.

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2.4 Echo in PSTN – PSTN callsWhen a PSTN subscriber calls another PSTN subscriber, the signal is transmitted through 2–wire circuits in the local access network, but through 4–wire circuits between the exchanges. Transfer to a 2–wire circuit in the local access network requires conver-sion in hybrid transformers. Although some delay occurs in the conversion, in a call within a PSTN the delay is generally shorter than 25 ms and requires no echo cancelling.

In a PSTN–to–PSTN call through International Switching Centres (ISC), delay occurs at both ends of the connection. To improve speech quality, the ISC is equipped with an Echo Canceller (EC).

Because the EC removes the echo only in one direction, ECs are needed at both ends. The maximum delay that the international switching centre has to handle is up to 12 + (0.004 * distance/km) ms when digital transmission is used in national connections (ITU–T G.114).

Figure 2 Echo in a PSTN–PSTN call through International Switching Centre (ISC)

In a PSTN-to-PSTN call through a Rel–4 Multimedia Gateway (MGW), delays are caused by the Multimedia Gateway. If there is a backbone between the Multimedia Gateways, the delay becomes even longer. Because of this effect the Multimedia Gateway is always equipped with an EC if a PSTN is connected to it.

Figure 3 Echo in a PSTN–PSTN call through Rel-4 Multimedia Gateway

2.5 Echo in MS – PSTN callsOn the PSTN side, the 4–wire to 2–wire connection will always create some echo. In an MS–to–PSTN call, the mobile user's voice is reflected from this hybrid back downlink to the mobile user at an approximate delay of 200 ms.

An echo delay of 200 ms would make speaking extremely uncomfortable, if not impos-sible. Removing this echo is the task of the echo cancellers in the Mobile Switching Centre (MSC). These echo cancellers store the information in the MS–to–PSTN direc-

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tion and recognise when the same information is present in the PSTN–to–MS direction and reduce it to a tolerable level.

ETSI Recommendation GSM 3.50 states that echo cancellation equipment used in the GSM system should be able to handle delays up to 60 ms. Echo cancellers used in the DX 200 MSC can handle delays up to 64 ms.

The folllowing figure shows how echo is generated in an MS–to–PSTN call.

Figure 4 Echo in an MS–PSTN call

When the call between the GSM subscriber and the PSTN subscriber goes through an ISC, echo cancellers located at the MSC and those located at the ISC are both in the path of the call. Simultaneous use of these echo cancellers (a tandem connection) may result in clipping of the speech signal during double talk, that is, when the participants are talking at the same time. Another undesirable effect may be modulation of the back-ground noise, which would further affect the quality of the call. To avoid this, the echo cancellers are controlled through signalling. In this case the ISC is equipped with an EC because the echo path delay exceeds 22 ms, and the ISC's EC being the nearest, it is used, whereas the MSC's EC is switched off.

Figure 5 Echo in an MS–PSTN call through an International Switching Centre (ISC)

The call may also be routed to a subscriber through another MSC. In this case the EC closest to the echo path is used and other ECs on the call path are not needed.

2.6 Echo in GSM MS – GSM MS callsOn the Mobile Station (MS) side, the voice coming from the earpiece of the MS is also picked up by the microphone, which means that there will be an acoustic echo travelling by air and along the body of the MS. In an MS–to–PSTN call (see Figure Echo in an MS–PSTN call) this would result in the PSTN subscriber hearing his own voice as an echo with a delay of about 200 ms, if the acoustic echo cancelling is not successful.

Acoustic echo generated in the MS is heard by a PSTN subscriber when receiving a MS–to–PSTN call.

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Echo cancelling

3GPP TS 43.050 states that a handset and a handsfree MS should provide a Terminal Coupling Loss (TCL), that is, the loss of acoustic echo, of at least 46 dB and 33 dB, respectively. This cannot, however, be achieved by the acoustical design of the MS alone. The MS should have a built–in echo suppressor or canceller so that no acoustic echo cancellation is needed on the network side.

Echo in a GSM MS – GSM MS call in the same MSC areaIf both mobile stations are in the area of the same MSC, the call is completely digital. As there are no hybrids in the signal path, no electrical echo is generated. The only echo generated is acoustic, and present in both directions. The total delay is even greater than in an MS–PSTN call because the transfer delay is doubled: the total delay is at least 360 ms. In this case it is the mobile stations that must take care of the echo cancelling.

The following figure shows a situation where a GSM subscriber calls another GSM sub-scriber in the same MSC area.

Figure 6 Echo in a GSM MS–GSM MS call through a single MSC

Echo in a GSM MS – GSM MS call through different MSCsWhen the call between two GSM subscribers takes place through two MSCs, the echo cancellers of both MSCs are in the signal path. As the acoustic echo is cancelled by the MSs themselves, the MSC echo cancellers are disconnected by appropriate signalling.

Figure 7 Echo in an MS–MS call through different MSCs

2.7 Echo in MS – PSTN calls (speaker phone)If the PSTN phone has a very low Terminal Coupling Loss (TCL), which means that the phone is a speaker phone or a handset with increased receiver volume, the situation shown in the figure below may arise. The internal voice–switching devices of the speaker phones do not always cancel the acoustic echo properly, and the MSC per-ceives two types of echo, electric and acoustic. This is called a composite echo. If the level of the acoustic echo is low, as it typically is, the composite echo can be considered

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as a dispersion of the electric echo. The Echo Canceller (EC) in the MSC–PSTN inter-face can usually reduce the level of the composite echo if the delay path of the acoustic echo does not exceed 64 ms and if the echo path loss is high enough. It should be noted that excessive non–linearities caused by the speakers, microphones, or voice–switching devices of some speaker phones may considerably affect the EC's performance.

Figure 8 Echo in an MS–PSTN call (speaker phone)

2.8 Echo in GSM MS – analog MS callsWhen the call is between a GSM MS and an analog mobile (for example Advanced Mobile Phone Service (AMPS), Total Access Communication Systems (TACS), or NMT) phone, the connection is a 4–wire connection. Thus, the MSC perceives an acoustic echo because the analog mobile phone does not have internal echo suppression. The EC in the MSC can remove the acoustic echo if the delay path of the acoustic echo does not exceed 64 ms and the echo path loss is at least 6 dB. In the figure below, BSS is the GSM base station subsystem and BS is the base station subsystem in the analog network.

Figure 9 Echo in a GSM MS–analog cellular call

2.9 Echo in UE - PSTN calls according to 3GPP Rel–4When the call is between an UE and a PSTN phone and the PSTN is connected to a Rel–4 Multimedia Gateway (MGW), echo is generated at the 4–wire to 2–wire conver-sion as usual. The Rel–4 MGW has its own echo canceller, which is controlled by the MSC Server (MSS).

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Echo cancelling

Figure 10 Echo in UE – PSTN calls according to 3GPP Rel-4

Note that if a PBX connection is used with an Integrated MSS, the user plane is routed via the Integrated MSS and echo cancelling is carried out in the Integrated MSS.

Figure 11 Echo in UE – PBX calls according to 3GPP Rel-4

2.10 Echo canceller control in the networkThe basic principle is to place the echo canceller as close as possible to the near end at the point where the echo is generated, and to avoid tandem connections. The network elements themselves do not know when to connect echo cancellers in different call cases. Information on included and not included echo cancellers is conveyed by call establishment signalling.

Let us consider, for example, a call from PSTN to a Mobile Station (MS) through a gateway exchange (GMSC) and visited exchange (VMSC), where echo is generated both in the PSTN and in the mobile. There is an Echo Canceller (EC) in the MSC elimi-nating echo in the downlink direction. Unless cancelled, this echo would be heard by the mobile user. There is also an echo canceller in the mobile station eliminating the acoustic echo generated in the MS itself. This echo would be heard by the PSTN user. There could be echo cancellers between the GMSC and the VMSC, as shown in the figure below. In a real network, however, echo cancellers between the GMSC and the VMSC would create a tandem connection and are therefore not installed.

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Figure 12 Example of call establishment signalling in a PSTN–MS call

The EC at the PSTN interface is called the Outgoing Echo Canceller (OEC), while the EC in the MS is called the Incoming Echo Canceller (IEC).

The hypothetical ECs in the GMSC and VMSC are marked as OEC' and IEC', respec-tively.

The IEC must be in the mobile station instead of the GMSC. This is because IEC', whose typical tail length is 64 ms, would not be able to deal with the 180 ms two–way delay caused by speech and channel coding in the GSM network.

Using, for instance, ISDN signalling, the network negotiates the use of echo cancellers using Initial Address Messages (IAM) and Address Complete Messages (ACM). The sit-uation in the figure is handled as follows:

• IAM with OEC=0 is received by the GMSC which receives the OEC for the connec-tion.

• IAM with OEC=1 is sent to VMSC which disables OEC' if it is installed (in a real network it is not).

• VMSC returns an ACM with IEC=1 indicating that an incoming echo canceller is enabled. This IEC is in the MS.

• GMSC receives the ACM with IEC=1 and enables OEC.

If the VMSC would not include the IEC=1 information in the ACM, IEC' in GMSC would be enabled. However, with the tail length of more than 180 ms, echo cancelling would not work.

Enabling IEC' in the GMSC would additionally lead to a tandem situation where there would be one EC in the MS and another in the GMSC. Such a connection would seri-ously affect speech quality.

If OEC in the GMSC were always enabled, there would again be an EC in every call from the PSTN forwarded from the GMSC back to the PSTN or the VMSC affecting speech quality.

Besides being controlled internally by the exchange's call control, echo cancellers can be controlled externally by modem and FAX equipment.

Echo canceller control in Rel–4 network architectureRel–4 network architecture introduces BICC signalling between MSC Servers. The logic of sending of OEC indication is changed if the outgoing signalling is BICC.

Let us consider a PSTN-PSTN call in which the bearer is connected to a Rel–4 Multime-dia Gateway and the interconnecting network consists of two MSC Servers with BICC signalling between them.

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Echo cancelling

Figure 13 Example of call establishment signalling in a PSTN–PSTN call in Rel-4 network

Using BICC signalling, the network negotiates the use of echo cancellers using Initial Address Messages (IAM) and Address Complete Messages (ACM). The situation in the figure is handled as follows:

• IAM with OEC=0 is received by the MSS 1. • IAM with OEC=0 is sent to MSS 2. • MSS 2 returns an ACM with IEC=0. MSS 2 enables IEC and sends the ACM with

IEC=1. • MSS 1 receives the ACM with IEC=1 and enables OEC.

2.11 Internal control of echo cancellersEcho canceller control in an incoming call from PSTN to GMSCIn the first example the gateway exchange (GMSC) receives an IAM message from the PSTN. The message indicates that the echo canceller at the far side of the PSTN is not connected (EC = 0). OEC in the GMSC remains connected until it receives the ACM message. The received ACM message indicates that the incoming echo canceller is not connected (EC = 0), and as a result, the GMSC disconnects both echo cancellers, OEC and IEC. Note that in Rel-4 Architecture, the OEC and IEC of the MGW are connected due to a longer processing delay in the MGW.

Figure 14 Incoming call from PSTN to GMSC, OEC and IEC disconnected

In the next figure the GMSC receives an IAM message from the PSTN. The message indicates that the echo canceller at the far side of the PSTN is connected (EC = 1). The OEC in the GMSC is disconnected. Because the received ACM indicates that the incoming echo canceller is not connected, the GMSC leaves its echo canceller IEC con-nected. The ACM sent to the PSTN indicates that the incoming echo canceller is con-nected (EC = 1).

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Figure 15 Incoming call from PSTN to GMSC, IEC connected

When the GMSC receives an IAM indicating that the echo canceller at the far side of the PSTN is connected (EC = 1), OEC in the GMSC is disconnected. Because the received ACM indicates that the incoming echo canceller is connected (EC = 1), the GMSC dis-connects its echo canceller IEC. The ACM is passed unchanged to the PSTN and indi-cates that the incoming echo canceller is connected.

Figure 16 Incoming call from PSTN to GMSC, OEC and IEC disconnected

In the fourth example of a PSTN–to–GMSC call, the GMSC receives an IAM indicating that the echo canceller on the PSTN side is not connected (EC = 0) and OEC in the GMSC is left connected. The GMSC receives an ACM indicating that the incoming echo canceller is connected, and as a result, disconnects its echo canceller IEC. The ACM is passed unchanged to the PSTN and indicates that the incoming echo canceller is con-nected.

Figure 17 Incoming call from PSTN to GMSC, OEC connected

Echo canceller control in mobile–originated callsThe following examples show the echo canceller control in different cases of a mobile–originated call. Because the MS contains an echo canceller, the IAM message for speech calls should always have echo canceller information 'outgoing echo canceller connected' (EC = 1).

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Echo cancelling

In the call situation below the IAM message indicates that the outgoing echo canceller OEC is connected. The received ACM indicates that the incoming echo canceller is not connected and the MSC leaves its echo canceller IEC connected.

Figure 18 Mobile–originated speech call toward trunk, IEC and OEC connected

The MSC disconnects its echo canceller IEC when the received ACM message indi-cates that the incoming echo canceller is connected (EC = 1).

Figure 19 Mobile–originated speech call toward trunk, IEC disconnected

In mobile–originated data calls the echo canceller of the MS is not used and the MSC disconnects its echo canceller IEC regardless of the EC information in the ACM message (EC =1 or EC = 0).

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Figure 20 Mobile–originated data call

Echo canceller control in mobile–terminated callsThe following examples show how the echo canceller control works in different cases of a mobile–terminated call. Because the MS itself contains an echo canceller, the ACM message sent by the MSC should always have echo canceller information 'outgoing echo canceller connected' for speech calls.

In the first example, the MSC receives an IAM message indicating that the echo cancel-ler at the far end is connected (EC = 1). As a result, the MSC disconnects its own echo canceller OEC. The ACM message sent from the MSC indicates that the incoming echo canceller is connected (EC = 1).

Figure 21 Mobile–terminated speech call, IEC connected

If, in the same situation, the EC information in the IAM message is 0 indicating that the echo canceller at the far end is not connected, the MSC leaves its echo canceller OEC connected and returns an ACM indicating that the incoming echo canceller is connected (EC = 1).

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Echo cancelling

Figure 22 Mobile–terminated speech call, OEC and IEC connected

In data calls the echo canceller of the MS is not used and the MSC disconnects its echo canceller IEC regardless of the EC information in the IAM message (EC =1 or EC = 0). ACM is always sent with the EC information 0 indicating that the incoming echo cancel-ler is not connected.

Figure 23 Mobile–terminated data call

Echo canceller control in Rel–4 network architectureRel–4 network architecture introduces the Rel–4 Multimedia Gateway with which the user plane is separated from the control plane. The control plane is handled by the MSC Server, the user plane is controlled by the Multimedia Gateway. In this network architec-ture it is possible to connect TDM resources to the Multimedia Gateway and as a con-sequence the Multimedia Gateway provides EC equipment for these connections. However, it is still the control plane which controls the EC equipment logic.

If TDM resources are connected to the Multimedia Gateway, the control plane has a new parameter in the EC equipment handling logic. The ECHO parameter of the incoming circuit group or outgoing route is used for checking whether the EC equipment can be connected to the given resource. If ECHO=Y, then the EC equipment is connected according to the legacy logic. If ECHO=N, the EC equipment is not utilised even if it would be necessary.

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2.12 External control of echo cancellersThe ITU–T has two recommendations concerning data transmission in echo suppres-sion and cancelling environment.

ITU–T Recommendation G.164 recommends the use of a preceding 2100 Hz tone before starting data transmission. This tone is recognised by the echo suppressor which then disables the suppressor for the duration of the data transfer. G.164 covers modems operating at speeds up to and including 9600 bps because this was the maximum speed achievable at the time the recommendation was written. All modems and facsimile equipment manufactures include this capability and most echo suppressors are fitted with Tone Disablers complying to G.164.

High data rates make it necessary to disable echo cancelling during the transmission of the tones (or digital data) in the channel.

To avoid the need for both types of Tone Disablers, the ITU–T issued, as part of Rec. G.165 and G.168, new recommendations covering the disabling of cancellers in the presence of the same 2100 Hz tone but subject to periodic phase reversals to distin-guish it from the G.164 tone.

The G.165 and G.168 Tone Disabler does not respond to a 2100 Hz tone without the phase reversals and therefore is not compatible with G.164. The G.164 Tone Disabler does, however, respond to any 2100 Hz tone within the specified tolerances whether it contains phase reversals or not. Nokia Siemens Networks echo cancellers support only G.165 or G.168 Tone Disablers.

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3 Echo canceller unitsEcho cancellers (EC) are voice-operated devices that are designed to reduce the echo caused by reflections in hybrids. An adaptive filter distinguishes echo cancellers from more simple echo suppressors, which are devices that simply suppress signals that are below a certain threshold level. The adaptive filter produces an echo estimate, which is then subtracted from the actual echo signal. This makes it possible to decrease the echo without attenuating the near–end speech and to achieve good full-duplex speech quality.

Echo cancellers used in the GSM network, for instance in the MSC, are digital echo can-cellers.

For background information, see Section Echo cancelling.

3.1 Structure of a digital echo cancellerDigital echo cancellers are designed in accordance with ITU–T Recommendation G.165 or G.168. The main components of a digital echo canceller are an echo estimator, a digital subtractor, a non-linear processor (NLP), and control circuitry. The echo estima-tor and digital subtractor form an adaptive linear filter controlled by an echo canceller algorithm.

Ideally, the incoming signal Lrin entering the receive path of the echo canceller would pass through the receive path to the PSTN side without any reflections, but the hybrid on the PSTN access side generally reflects some of the signal back to the speaker. The reflected signal is an attenuated and possibly distorted version of the signal and is heard by the speaker at a delay of up to 64 ms.

Digital echo cancellers used in the MSC are designed to handle echo path delays of up to 64 ms. This requirement is based on 3GPP TS 43.050, according to which the disper-sion may be up to 8 ms and the one–way delay is supposed to be less than 22 ms, resulting in the maximum expected echo path delay, 60 ms.

The attenuation of the returned echo is described by echo loss (ERL) between the Rout and Sin ports of the echo canceller.

Figure 24 Structure of a digital echo canceller

The characteristics of the echo path are described by its impulse response, which consists of pure delay tr and dispersion. The sum of these is the echo path delay td (ITU–T Rec. G.168).

The impulse response is specific to each echo path, and therefore to each call. Now, the task of the linear filter is to analyse the impulse response of the incoming signal and create a replica of it. The filter stores the samples taken from the incoming signal and

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compares them to the signals coming in from Sin port on the send path. When the linear filter finds that the impulse response of the signal matches the sample taken from the signal on the receive path, it subtracts the replica from the returning signal, thus reducing the level of echo. The output from the linear filter may still contain some residual echo (Lres).

Figure 25 Impulse response of echo

The properties of the echo path differ in different networks. In addition, the echo path can include more than one source of echo. For example, the network configuration may have multiple 2-wire to 4-wire conversions in the end path of an echo canceller. In order to function under these circumstances, the linear filter must be able to continuously adapt to different echo paths. How well and how rapidly it does this depends on the quality of the algorithm governing the filter's operation.

Depending on how well the linear filter works, the echo reduction of the filter is 10 – 35 dB. The echo reduction level is also affected by any non–linearity of the echo path between the Rout and Sin ports. Non-linearities result, for example, from acoustic echo at the near end or PCM codecs on the echo path. The A or µ law conversion in a PCM codec alone limits the maximum achievable attenuation to 35 dB. In networks conform-ing to Recommendation G.113, in which up to five pairs of PCM codecs are permitted in the echo path, the accumulated non-linear distortion may further affect the echo reduc-tion level. As a result, the residual echo level Lres may remain high.

To reduce the residual echo, the echo canceller has a non-linear processor (NLP), located on the send path between the subtractor and the Sout port. Based on the fairly low level of residual echo at the output of the subtractor, the task of the NLP is to block signals that are below a defined threshold level and pass signals that are above it.

The NLP cannot distinguish between echo and near-end speech, and it would, if contin-uously applied, also suppress the near-end speech as well as background noise in the near-end signal. To avoid this, the NLP is controlled by a double-talk detector that switches off the NLP when near-end speech is present. The double-talk detector, which is a part of the controlling algorithm, recognises near-end speech by comparing the Rin and Lres signals. The NLP is switched on only when the algorithm has estimated the echo path delay and the delay difference between the Rin and Lres signals. When active, the NLP functions as intended to reduce the residual echo, and when inactive, it does not perform any non–linear processing on any signal passing through the echo canceller.

The fact that NLP also tends to suppress background noise from the near end would cause distracting variation of silence and noise during a call unless dealt with. This effect is removed by adding comfort noise to the output when the NLP is blocking the actual background noise. The injection of comfort noise is closely associated with the control of the NLP, so that it is only injected when the NLP is active, that is, when there is no double talk.

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In addition, the echo canceller contains a tone disabler. The tone disabler monitors both the Rin and Sin signals and disables the echo canceller operation if a fax or modem terminal equipment sends a predefined disabling tone.

Echo canceller requirementsIn summary, a digital echo canceller must be able to

• produce low returned echo Lret, preferably at or below the level of noise floor for all speech levels

• adjust rapidly at the beginning of a call to avoid transient echo • cancel the echo with minimal delay • keep its operation stable for the duration of the call, even during double talk or echo

path change • operate without producing any impairments to the signal • handle voice-band data of modems and facsimile equipment

3.2 Echo canceller units in MSCIn the MSC, echo cancellers are handled as ECU functional units and are based on the plug-in unit types EC08, EC1P and EC1P–S. Only EC1P–S is currently in production.

Echo cancelling functions in the MSC are handled by the Echo Cancelling Unit (ECU). The ECU is a functional unit whose purpose is to cancel the electric echo returning from the PSTN. The unit can handle the echo cancelling of one PCM circuit. It is located on the trunk PCM between the Group Switch (GSW) and the Exchange Terminal (ET). The ECU is controlled by the Interworking Control Unit (IWCU) via a semipermanent LAPD link.

The ECU monitors each channel to detect echo in the transmission path. When echo is detected, the ECU formulates a mathematical model of the transmission circuit produc-ing the echo to obtain an approximation of it. The echo is cancelled by subtracting the approximation from the transmission signal.

The hardware of ECU consists of:

• one COCA plug-in unit controlling up to four EC08 plug–in units • or one EC1P or EC1P–S plug–in unit

Depending on the equipment level, the unit is housed in the following cartridges and racks:

• in the ECUC cartridge in the ECE rack, with maximum capacity of 24 PCM lines, 16 of which are provided with the echo cancelling function

• in the ECUC–T cartridge in the ECTE rack (a later and more compact design). The maximum capacity of an ECTE rack is 48 PCM lines, 24 of which are provided with the echo cancelling capacity.

EC08The 8–channel echo canceller EC08 is the earliest echo canceller type in the MSC. It is installed in the Echo Canceller Unit (ECU) and controlled by the Communication and Control Adapter (COCA).

The COCA is a plug-in unit whose main functions are transmitting the messages from the Interworking Control Unit (IWCU) to the signal processors and performing mainte-

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nance tasks. Communication to the IWCU is implemented with a microcontroller using the LAPD protocol.

Each COCA controls up to four EC08 units.

EC08 is the plug-in unit performing the echo cancelling. It contains four signal processor blocks, each of which handles two channels. Communication with the signal processors and COCA takes place through the memory of the signal processors. Part of the memory is RAM to which COCA has access. Supervision is handled through time-slot 0 connected to the Common Channel Signalling Unit (CCSU).

The echo canceller has the ITU–T Rec. G.165 type tone disabler.

Compact Echo Canceller (EC1P/EC1P–S)EC1P consists of a connected motherboard and daughterboard (EC1N). The EC1N is the actual echo canceller, while the motherboard forms the interface to the environment. EC1P is connected by internal PCM circuits to the ET and the switch. Via the ET, it is connected to the PSTN and via the switch to the AS7 plug-in unit in the IWCU. The daughterboard cancels the echo from 31 voice time–slots

The echo canceller conforms to ITU–T Rec. G.165 and has a tone disabler and the comfort noise injection capability.

EC1P–S consists of a motherboard and a daughterboard. The motherboard is the same as in EC1P, but the daughterboard type is IDEC21. The unit connects to an external 2 Mbit/s PCM line through an Exchange Terminal (ET1E) and to an internal 4 Mbit/s PCM of the GSW. Supervision is handled through time-slot 0 connected to the CCSU. Echo cancelling control is done through a LAPD link to the AS7 in the IWCU.


Figure 26 Operating environment of EC1P

3.3 Echo cancellers in MSC and Integrated MSS In the MSC, Compact MSC, and Integrated MSS echo cancelling and exchange terminal functions are integrated into a single plug-in unit, EC2ET/ECE2. From the point of view of hardware configuration, the unit is handled as an Exchange Terminal (ET).

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The ET/EC plug–in units are EC2ET, EC2ET–C, EC2ET–A, ECE2, ECE2–C and ECE2–A. In addition they also include the following ROHS plug-in unit types: ECE2-B, ECE2-CB and ECE2-AB. They are the ROHS versions of similar PIUs: ECE2, ECE2-C and ECE2-A.

Note that the ECE2-B, ECE2-CB and ECE2-AB are new ROHS compliant PIUs and they use the same SW as non-ROHS PIUs ECE2, ECE2-C and ECE2-A.

The table below lists the motherboard and daughterboard names and the corresponding Plug-In Unit (PIU) configuration. EC2ET–A and ECE2–A are for the T1 environment, other PIUs are for the E1 environment. Hereafter the ET/EC term is used for covering all the PIUs listed below in Table ET/EC and PIU configuration.

Integrated ET and EC (EC2ET/ECE2)EC2ET/ECE2 is a combined digital echo canceller and exchange terminal. It can control echo cancelling on 62 voice channels and provide an ET interface for two external 2 Mbit/s PCMs. The unit is connected to the GSW with a 4 Mbit/s PCM. Echo cancelling and ET control functions are handled via a LAPD link connected to the AS7 plug-in unit of the controlling signalling computer units (General Signalling Unit (GSU) in Compact MSC and Common Channel Signalling Unit (CCSU), Channel Associated Signalling Unit (CASU), or Primary Rate Access Unit (PAU) in MSC). The EC2ET/ECE2 provides echo cancelling control for speech and data calls independent of the signalling system.


The EC2ET/ECE2 also supports Tandem Free Operation (TFO) detection. When the echo canceller detects TFO signalling, it disables echo cancellation for the duration of the TFO call (see 3GPP TS 28.062).

The hardware consists of the EC2ET/ECE2 plug-in unit housed in the ET4C cartridge. Each cartridge can contain 16 EC2ET/ECE2 plug-in units. This equals the capacity of 32 PCMs. Another cartridge type is ET5C, housing 2 EC2ETs. In the MSC the cartridges are placed in the ETC cabinets, while in a Compact MSC, the cabinet type is MMBC.

The plug-in unit consists of a two boards:

• motherboard containing the exchange terminal and echo cancelling control func-tions

• daughterboard containing the echo cancelling functions

In operation and maintenance, the EC2ET/ECE2 is considered as two separate ETs.

Motherboard Daughterboard PIU configuration

EC2ET EC2E or EC2E–N EC2ET

EC2ET–C EC2E or EC2E–N EC2ET

EC2ET–A EC2T or EC2E–N EC2ET_A

ECE2 and ECE2-B EC2E–NA ECE2

ECE2–C and ECE2-CB

EC2E–NA ECE2_C or ECE2

ECE2–A and ECE2-AB

EC2E–NA ECE2_A

Table 2 ET/EC and PIU configuration

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The figure below shows the operation environment of EC2ET/ECE2 in the MSC, Compact MSC and Integrated MSS.

Figure 27 Operating environment of EC2ET/ECE2

Integrated ET and EC (EC2ET–A/ECE2–A)EC2ET–A/ECE2–A is a combined digital echo canceller and exchange terminal. It can control echo cancelling on 48 voice channels and provide an ET interface for two external 1.544 Mbit/s PCMs. The unit is connected to the GSW with a 4 Mbit/s PCM. Echo cancelling and ET control functions are handled via a LAPD link connected to the AS7 plug–in unit of the controlling signalling computer units (GSU in Compact MSC and CCSU, CASU, or PAU in MSC). The EC2ET–A/ECE2–A provides echo cancelling control for speech and data calls independent of the signalling system.


The EC2ET–A/ECE2–A also supports Tandem Free Operation (TFO) detection. When the echo canceller detects TFO signalling, it disables echo cancellation for the duration of the TFO call (see 3GPP TS 28.062).

The hardware consists of the EC2ET–A/ECE2–A plug-in unit housed in the ET4C car-tridge. Each cartridge can contain 16 plug–in units. This equals the capacity of 32 PCMs. Another cartridge type is ET5C, housing 2 EC2ETs/ECE2s. In the MSC the cartridges are placed in the ETC cabinets, while in a Compact MSC, the cabinet type is MMBC.

The plug–in unit consists of two boards:

• motherboard containing the exchange terminal and echo cancelling control func-tions,

• daughterboard containing the echo cancelling functions.

There are two types of daughterboards for both EC2ET and EC2ET–A. The new EC2E-N daughterboard has replaced the EC2E and EC2T daughterboards in deliveries. ECE2 and ECE2–A will replace EC2ET and EC2ET–A in deliveries. The daughterboard of ECE2/ECE2–A is EC2E–NA. See Section Summary of echo canceller unit types.

In operation and maintenance, the EC2ET–A/ECE2–A is considered as two separate ETs and handled in the same way as EC2ET/ECE2.

The figure below shows the operation environment of EC2ET–A/ECE2–A in the MSC.

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Figure 28 Operating environment of EC2ET–A/ECE2–A

3.4 Summary of echo canceller unit typesThe table below summarises the DX 200 echo canceller types.

EC NE Technology Channels Release Construction

EC08 MSC DSP 8 ch. M5 COCA+ 4 EC08, 1 PCM

ECET ISC DSP 1 PCM – –

EC1P MSC ASIC 1 PCM M5 Coherent

EC1P–S MSC ASIC 1 PCM M7 Tellabs / Coherent

EC2ET MSC, ISC

ASIC 2 PCM (E1, ETSI)

M9 For more infor-mation on EC2ET daugh-terboards, see Table EC2ET daughter-boards below.

EC2ET–A MSC ASIC 2 PCM (T1, ANSI)

M9 For more infor-mation on EC2ET–A daughter-boards, see Table EC2ET daughter-boards.

ECE2 MSC, ISC

DSP 2PCM (E1, ETSI)

M11 Nokia Siemens Networks

Table 3 DX 200 echo cancellers

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The table below shows the two EC2ET daughterboard types:

The table below shows the two EC2ET–A daughterboard types:

The table below shows the ECE2, ECE2–C, ECE2–A daughterboard type:

3.5 Echo cancelling specifications • 3GPP TS 43.050, Transmission planning aspects of the speech service in the GSM

PLMN system • 3GPP TS 28.062. Recommendation, Inband Tandem Free Operation (TFO) of

Speech Codecs • ITU–T Recommendation G.113, Transmission systems and media, General charac-

teristics of international telephone connections and international telephone circuit; Transmission impairments, 1993

• ITU–T Recommendation G.114, Transmission systems and media, General charac-teristics of international telephone connections and international telephone circuits; One–way transmission time, 1996

• ITU–T Recommendation G.131, Stability and echo, Malaga–Torremolinos; 1984 • ITU–T Recommendation G.121, Transmission systems and media, General charac-

teristics of national systems forming part of international connection; Loudness Ratings (LRs) of national systems, 1993

• ITU–T Recommendation G.122, Transmission systems and media, General charac-teristics of national systems forming part of international connection; Influence of national systems on stability and talker echo in international connections, 1993

ECE2–A MSC DSP 2PCM (T1, ANSI)

M11 Nokia Siemens Networks

Daughterboard type Technology Manufacturer

EC2E ASIC Tellabs/Coherent

EC2E–N DSP Nokia Siemens Networks

Table 4 EC2ET daughterboards


EC2T ASIC Tellabs/Coherent

EC2E–N DSP Nokia Siemens Networks

Table 5 EC2ET–A daughterboards


EC2E–NA DSP Nokia Siemens Networks

Table 6 ECE2, ECE2–C, ECE2–A daughterboard

EC NE Technology Channels Release Construction

Table 3 DX 200 echo cancellers (Cont.)

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• ITU–T Recommendation G.164, Transmission and media, Apparatus associated with long–distance telephone circuits and other terminal equipments; Echo sup-pressors, 1993

• ITU–T Recommendation G.165, General characteristics of international telephone connections and international telephone circuits; Echo cancellers, 1993

• ITU–T Recommendation G.167, General characteristics of international telephone connections and international telephone circuits; Acoustic echo cancellers, 1993

• ITU–T Recommendation G.168, Transmission systems and media, Digital systems and networks, International telephone connections and circuits – Apparatus associ-ated with long–distance telephone circuits; Digital network echo cancellers, 2000

• ITU–T Recommendation G.711, General aspects of digital transmission systems terminal equipments; Pulse code modulation (PCM) of voice frequencies, 1993

• ITU–T Recommendation P.830, Telephone transmission quality, Methods for objec-tive and subjective assessment of quality; Subjective performance assessment of telephone–band and wideband digital codecs, 1996

• ITU–T Recommendation P.831 Telephone transmission quality, telephone installa-tions, local line networks, Methods for objective and subjective assessment of quality, Subjective performance evaluation of network echo cancellers, 1998

• ITU–T Recommendation Q.115, Switching and Signalling, Clauses applicable to ITU–T standard systems – Control of echo suppressors, 1999

• ITU–T Recommendation Q.762, Switching and Signalling, Signalling system No. 7 – ISDN User Part general functions of messages and signals, 1999

• ITU–T Recommendation Q.763, Switching and Signalling, Signalling system No. 7 – ISDN User Part format and codes, 1999

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4 Operation and maintenance of echo cancel-lers

4.1 Echo canceller hardware configuration managementECUThe hardware configuration management of ECU consists of the following tasks:

• creating or deleting the echo canceller as the ECU functional unit • creating or deleting the plug-in units COCA with up to 4 EC08 plug-in units, or the

EC1P/EC1P–S • connecting or disconnecting the COCA or EC1P/EC1P–S to the PCM • printing out the configuration information (equipment and PCM data)

ET/ECThe configuration of ET/EC is handled as two separate ETs. One ET/EC forms two ET functional units so that the indexing of the PCMs is identical. One ET/EC has only one LAPD channel.

The hardware configuration management of ET/EC consists of the following tasks:

• creating two ET functional units to the corresponding PCM indexes using two separate commands

• creating the ET/EC plug-in unit for the even-numbered ET • creating the same ET/EC plug-in unit for the odd-numbered ET • connecting the ETs to the PCM

State management commands must be separately given to both ETs.

4.2 Echo canceller supervisionSupervision of the controlling computerThe controlling computer (such as IWCU in the MSC) is supervised according to the normal supervision principles of DX 200 control computers.

Supervision of COCA, EC1P/EC1P–S, and ET/ECThe hardware and software of the COCA, EC1P/EC1P–S, and ET/EC is supervised by the following means:

• testing the read/write memory • checking the check sums of programs and fixed files • observing the clock signals • sending supervision messages between program blocks in COCA, EC1P/EC1P–S,

or ET/EC • watchdog timers

Supervision of EC1P/EC1P–S and ET/EC is divided into supervision of the programs on the motherboard and supervision of the programs on the daughterboard and it is carried out using periodic supervision messages.

An alarm is generated for each faulty time-slot and the time-slot is blocked.

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Operation and maintenance of echo cancellers

Supervision of EC08In the EC08, the operation of the signal processor software is supervised by a response capability test. Each of the signal processors has a special byte in its dual port memory to which COCA constantly writes fixed data. The signal processor changes this byte at certain intervals. If a signal processor fails to change the byte, the supervising program loads new software to the signal processor and sets an alarm. If restarting fails contin-uously, another alarm is set. In addition, the ASIC in EC08 is continuously supervised by a program block in COCA. It checks that interrupts are given by the ASIC and sets an alarm if a malfunction is detected.

Supervision of data linkThe data link between the controlling computer unit and the EC is supervised by the LAPD protocol software in the AS7 and state administration in the EC. Supervision frames are constantly transmitted through the data link even if no data is sent. If the supervision program finds that the data link is out of use, the program initiates a restart in the EC. Restart is needed for the recovery of certain clock disturbances. Alarms caused by a data link failure are set in the controlling computer unit. At the restart of the unit, a test is executed for the hardware of the data link at the EC end. If establishing the connection fails, a red LED is lit on the plug-in unit and a wired alarm is set.

Note that if the EC type is EC08, supervision tasks are handled by or through the con-trolling plug-in unit, COCA.

Supervision of trunk networkFault conditions in the EC can often be detected through trunk network supervision per-formed by the AFS plug-in units in a signalling computer unit. AFS receives the 0 time-slots of all the trunk PCMs and observes if the PCM as a whole is functioning correctly.

4.3 Echo canceller alarm functionsAlarm handling of the EC consists of analysing the fault information received from differ-ent sources and informing the user of the faults.

Alarms are handled on three levels:

system level All the wired alarms are set in the Operation and Maintenance Unit (OMU) and transmitted directly to the Alarm System Program Block (Centralised Part) which stores them, controls the alarm outputs, and starts the recovery actions if needed. In a unit failure, the program block starts recovery actions.

control computer level Alarms of the data link are set in the controlling computer and transmitted directly to the Alarm System Program Block (Distributed Part). Alarms from the trunk network supervision are set in the signalling computer units and also transmitted to the program block, which then stores the alarms and filters them further. If needed, the distributed part send the alarms to the centralized part in the OMU.

preprocessor level Preprocessor alarms of ECU are gathered by the program block responsible for echo canceller and modem pool supervision. In ET/EC this task is done by Preprocessor Alarm Collection program block. The program block prioritises and filters the alarms and sends them to the distributed part.

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4.4 Echo canceller recoveryThe recovery system controls the states of the functional units. Recovery functions are fault recovery or elimination of the effects of faults, restart control, and control of the user interface.

Recovery receives real-time data on the states of the functional units. Using this data, recovery controls the restarting of the system and of its functional units.

Recovery of echo cancellers operates according to the general principles of the system.

4.5 Echo canceller fault handlingThe task of the diagnostic system is to locate hardware faults. The diagnostic system uses the hardware configuration data to determine which diagnostic programs to run and how they should be run and to determine which plug–in units are faulty.

The diagnosis can be activated by the recovery system or by the user.

The fault location of the EC is mainly based on the diagnostic programs in the EC.

If the diagnostic result is NO RESPONSE FROM UNIT, diagnostics on the controlling computer is needed. The HDCL channel between the EC and the AS7 may be faulty. You should activate diagnostics on the controlling computer and if this finds no fault, the EC is probably faulty.

Note that if the EC type is EC08, the diagnostic programs are in the controlling plug–in unit, COCA.

Fault location of ECUThe diagnosis of ECU consists of the following:

• test of COCA • test of EC08 • loop test of ECU • test of EC1P/EC1P–S

Fault location of COCAThe diagnosis of COCA consists of the following:

• check sum test of read-only memory • test of read/write memory • watchdog test

Fault location for EC08The diagnosis of EC08 consists of the following:

• test of dual port memory and data bus • test of DSP • test for ASIC chip

Fault location of EC1P/EC1P–SThe diagnosis of EC1P/EC1P–S consists of the following:

• check sum test of read-only memory • test of read/write memory • watchdog test

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• internal and external tests for daughterboard

Loop test for ECUThe ECU diagnostic loop test is implemented in the diagnostic programs of the OMU. The test is based on an ET diagnostic loop test. An ET connected to the ECU is looped using a tone generator (TG), and test data is sent to the time-slots of the ECU from an ASS plug-in unit in the OMU. The DSPs in the ECU are restarted at the beginning of the test. The loop test can be run on EC08, EC1P, and EC1P–S.

Fault location of ET/ECWhen running diagnostics for one functional unit (ET) of the ET/EC the other functional unit must also be placed to the test state (TE) before diagnostics can be executed. In fault situations PCM circuits are blocked and an alarm is set.

If a failure is detected in ET/EC diagnostics are not automatically run for the unit.

The diagnostics of ET/EC consist of the following:

• internal test and internal loop 1 test • diagnostics of the motherboard, consisting of a check sum test, test of read/write

memory, and a watchdog test • diagnostics of the daughterboard, consisting of internal and external tests

Testing command to either ET of the even-odd pair will automatically start diagnosis on both ETs. The internal test and internal loop test are, however, run on the ET specified in the command only. Therefore, to completely test the ET/EC give the diagnosis command to both ETs.

While the diagnosis is running in one ET, do not make any changes to the other ET in the same plug-in unit (for example, by using the YE or US command groups).

4.6 ECU routine testing (EC08)For ECUs implemented with COCA and EC08 plug-in units, there is a user–initiated routine test. This test checks the connectivity of the channels as well as the operation of the signal processors, but not the echo cancelling function. The test is directed to time-slots of ECUs that are in the operational state (WO–EX).

The time-slot to be tested is looped in the Group Switch (GSW) and in the ET so that a closed loop is composed. Both the GSW side and the ET side are tested with test data sent by a signal processor in the EC08. The time-slot is looped in the ET by a signal pro-cessor sending management data to the ET in time-slot 0. Because of the ET looping, the time-slot is unavailable for traffic for a short duration during the test. Before blocking the time-slot, the signal processor ensures that the time-slot is idle by checking that there is a valid loop, in the GSW.

If the test finds a faulty time-slot, it immediately tests the rest of the time-slots in the ECU to check the overall condition of the ECU. If the routine test locates faulty time-slots and units, it sets an alarm. It also blocks and separates the faulty time-slots. This test is run at night at regular intervals. Depending on the number of units to be tested, completing the test may last one or several nights.

Use the UE command group to activate and modify the routine test. If there are EC08 echo cancellers in the exchanges, set the test to ON and set the timing so that the test is run at night. If there are no EC08s, set the test to OFF. The command group is ECU and DSU Routine Test Handling (UE).

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The ECU routine test is run automatically when the AFS CONNECTION FAILURE alarm is set. The test checks the time-slots of the PCM with an ECU in the WO–EX state. The test only checks the idle time-slots on the PCM and blocks and separates them for the duration of the test. Reserved time-slots are not tested. When the alarm is cancelled, the test is repeated for the blocked circuits, and if the time-slot proves to be in order, it is put back in use.

If the test finds a faulty time–slot, it immediately checks the rest of the time-slots in the ECU to see if the ECU is totally out of use.

Using an internal loop test, the routine test checks that the time-slots can transfer test data in both directions between the GSW and the ET. An attempt is made to put the failed ECU back to operation.

4.7 ECU time-slot testing (EC08 and EC1P/EC1P–S)For ECUs implemented with EC08 and EC1P/EC1P–S, there is a time-slot test. If allowed, the test is done whenever the AFS CONNECTION FAILURE alarm is set and the ECU is in the WO–EX state. The test checks the condition of the PCM. If the first tested time-slot is in order, the test ends, but if it is faulty, all the time-slots are tested. If a fault is found, an alarm is set.

Depending on the parameter settings, an ECU whose time-slots are all faulty is restarted and the time-slot test is run again.

You can enable and disable both the ECU time-slot testing and ECU restarting after failure using the UE command group.

4.8 Echo cancelling parameters (ET/EC)The following echo canceller parameters can be interrogated:

• echo cancellation • non-linear processor • comfort noise • tone disabler • disabling tone • tandem free operation • echo direction • daughterboard software versions and daughterboard type

The following echo canceller parameters can be modified:

• non-linear processor • comfort noise

Modification of these parameters affect speech quality.

Interrogate the settings with the UFI command and change the settings with the UFM command of the Echo Canceller Handling (UF) command group.

4.9 EC2ET/EC2ET–A daughterboard SW loadingIf a software upgrade is needed, you can load the daughterboard software COE_S1M2 of EC2ET and EC2ET–A with the UFLcommand. Refer to SW Change Notes for infor-

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mation about the needed upgrades. To interrogate ongoing loadings, use the UFT command.

If the versions of the board and the switch do not match, the daughterboard software of EC2E–N is automatically loaded at reset of the unit. The daughterboard SW modules of EC2E–N are ECDROMQT, ECXPRBMX and E2CRAMQT.

You can check the daughterboard type and software versions with the UFI command.

4.10 Related topicsEcho cancelling



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5 Echo canceller troubleshooting

5.1 Checking the cancelling of electric echoPurposeWhen echo is encountered in a network, it is often difficult to quickly find out the reason for it. This is because the echo is not generated and cancelled in a single place. Even if the echo cancellers are working properly, there may still be echo in the network because of non-linearities or high levels in PCM signals.

Electrical echo originates from the PSTN network and is encountered in a call from a GSM mobile to a PSTN phone when the echo cancellation in the MSC is not used.

If there is any reason to suspect that electrical echo is not properly cancelled in the MSC, do the checks listed below.

If none of the checks indicate a fault, then the echo canceller itself is in order and its control via the LAPD channel works. The problem lies somewhere between the MSC and the PSTN. There may be, for instance, incorrect signal levels or some equipment in the echo path causes non-linearities.

Steps

1 Check that the echo canceller is configured correctly (WTI).

ZWTI:U;

ZWTI:P:ET,:;

2 Check the working state of the echo canceller (USI)

ZUSI:<unit type>,<unit index>;

3 Check the LAPD channels (DTF)

Make sure the LAPD channels through which the echo cancellers are controlled are up and running.

ZDTF;

4 Check the alarms (AHO)

ZAHO;

5 Diagnose the echo canceller (UDU)

Change the working state to TE–EX:

ZUSC:<unit type>,<unit index>:<working state>;

Diagnose the unit:

ZUDU:<unit type>,<unit index>;

6 Check that jumpers in the ET/EC unit are set according to the Jumper Settings of the Plug–in Units manual

7 Check and modify the ET/EC setup parameters

Stepsa. Check that no daughterboard SW loading is taking place (UFT)

ZUFT;

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Do not change echo canceller setup parameters when daughterboard software loading is taking place.

b. Check the echo canceller setup parameters (UFI)

Check the parameters for all time-slots interrogated from the echo canceller:

ZUFI:<pcm number>:TSL;

c. If necessary, modify the echo canceller setup parameters (UFM)

ZUFM:<pcm number>:<setup parameter>;

8 Check the functional modes (YEH)

Check T1 and JT1 functional modes of EC2ET-A and ECE2-A:

ZYEH:ET,<unit index>;

9 Check the echo canceller SW versions

If the echo canceller is EC1P/EC1P–S, check the EC9 PROM version. If the echo canceller is ET/EC, check the SW versions of RAM and ROM (PROM) in the unit and RAM in the controlling computer.

If necessary, update the EC9 PROM version.

If the software version of ET/EC plug-in unit and the version in the controlling computer are different, reset ET/EC to update the software version.

10 Check the direction of poor echo cancellation

Check if poor echo cancelling occurs in calls from the MS's direction or from the PSTN's direction, or possibly from the same geographical area, even from the same phone number.

It may also happen, particularly with some PBXs, that wrong signalling information about echo canceller equipment usage is sent between adjacent networks. The result of wrong signalling information can be that the echo canceller equipment is not connected at all and a strong echo is heard throughout the call. In the MSC, you can avoid this situation by setting the value of the FECHO parameter of the CPF and CPM commands to Y (echo canceller is forcibly connected). This setting keeps echo cancellation on in all speech calls no matter what the signalling information is.

11 Check ECHO parameter on the given circuit group or outgoing route

If the TDM resource is connected to a Rel–4 Multimedia Gateway, the ECHO param-eter of the corresponding incoming circuit group or outgoing route must be set to Y to get the EC equipment connected.

ZRRI:GSW:ROU=<route number>;

ZRCI:SEA=3:CGR=<circuit group number>:PRINT=3;

12 Check the state of the echo canceller in the MGW

First you must enter the MCJ command in the MSC Server to obtain the TDMTER ID. Then enter the JKH command in the MGW.

ExampleZMCJ:IMSI=<international mobile subscriber identity>:;

Printout:

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The state of the echo cancellor in the MGW can then be viewed with the following command:

ZJKH:VID=2,:TTYP=2,:PCM=2,TSL=25,;

SUBSCRIBER A SUBSCRIBER B IMSI : 216017011110000 CGR/CRCT/PCM-TSL/STATE : 00512/00024/ - /R 00388/00028/ - /R MGW : MGW2 MGW2 TDMTERMID : 0002-25 0006-29 TERMID : 40000059 400000DD BACK/FORW : / HO LINK : ROAM LINK :

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Checking echo canceller versions

6 Checking echo canceller versions

6.1 Checking EC9 EPROM variant of EC1P/EC1P–SPurposeEC1P and EC1P–S plug–in units have the following variants:

Steps

1 Check the EC9 EPROM variant of the EC1P/EC1P–S (DDX)

ZDDX:ECU,<unit index>:”ZSX”;

6.2 Checking the SW versions of ET/ECBefore you startET/EC PIUs have the following RAM and ROM package variants:

Steps

1 Check the echo canceller motherboard SW versions (ROM and RAM) with the command

ZDPP:<control unit>:ET,<unit index>:CID;

where <control unit> is the controlling unit of the ET and <unit index> is the ET index.

2 Check the SW version in the controlling computer (WQV)

ZWQV::E2T;

Further Information

Plug–in unit Variants

EC1P EC9_PXMX.PAC for EC1P and normal echo cancellation direction

EC9_P1MX.PAC for EC1P and inverted echo cancellation direction for optimised Transit MSC

EC1P–S EC9_P2MX.PAC for EC1P–S and normal echo cancella-tion direction

EC9_P3MX.PAC for EC1P–S and inverted echo cancella-tion direction for optimised Transit MSC

Table 7 EC9 variants of EC1P and EC1P–S

PIU ROM package RAM package

EC2ET and EC2ET–C E2TROMGX.PAC E2TRAMGX.PAC

EC2ET–A E2AROMGX.PAC E2ARAMGX.PAC

ECE2 and ECE2–C EA2ROMQX.PAC EA2RAMQ8.PAC

ECE2–A EA2ROMQX.PAC EA2RAMQ9.PAC

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ExampleChecking the SW versions of ET–256ZDPP:CCSU,0:ET,256:CID;

6.3 Checking daughterboard type and SW versionsPurposeTo check the daughterboard type and SW version, refer to Change Notes for instruc-tions.

The echo canceller daughterboard SW variant names are as follows:

EC1P COE_S1MX.PAC

EC1P–S COE_S1M1.PAC

EC2ET/EC2E COE_S1M2.PAC

EC2ET–A/EC2T COE_S1MA.PAC

Steps

1 Check daughterboard SW version (UFI)

ZUFI:<pcm number>:VER;

The SW modules for EC2E–N and EC2E–NA are ECDROMQT.PAC, E2CRAMQT.PAC and ECXPRBMX.PAC. The ZUFI command automatically shows the SW module versions for different types of daughterboards.

If the software versions in EC2E/EC2T turn out to be old according to the latest Change Notes, load a new version using the ZUFL command. If the software versions in the board and the switch do not match, the daughterboard software of EC2E–N and EC2E–NA is automatically loaded at reset of the unit.

Echo Cancelling

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