09-troubleshooting for voice

7/30/2019 09-Troubleshooting for Voice

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Troubleshooting ManualM900/M1800 Base Station Subsystem Table of Contents

Huawei Technologies Proprietary

i

Table of Contents

Chapter 9 Troubleshooting for Voice.......................................................................................... 9-19.1 Overview............................................................................................................................ 9-19.2 Fundamental Knowledge................................................................................................... 9-1

9.2.1 Transmission Format of Voice Signal ..................................................................... 9-19.2.2 Transmission Path of Voice Signal ......................................................................... 9-29.2.3 Concepts ................................................................................................................. 9-49.2.4 Common Operations ............................................................................................... 9-59.2.5 Supplement ............................................................................................................. 9-7

9.3 Processing of Voice Troubles............................................................................................ 9-89.3.1 Analysis................................................................................................................... 9-89.3.2 Procedures to locate faults...................................................................................... 9-9

9.4 Locating Trouble .............................................................................................................. 9-109.4.1 Single Pass and No pass...................................................................................... 9-109.4.2 Echo ...................................................................................................................... 9-129.4.3 Voice Discontinuity................................................................................................ 9-149.4.4 Noise ..................................................................................................................... 9-159.4.5 Cross-talking ......................................................................................................... 9-17

9.5 Examples ......................................................................................................................... 9-189.5.1 Cross-talking Resulting from Improper Data Configuration .................................. 9-189.5.2 Voice Discontinuity Resulting from BCCH Carrier Mutual-assistance.................. 9-199.5.3 Single Pass Resulting from MS Fault.................................................................... 9-209.5.4 Noise Resulting from Poor Contact of E1 ............................................................. 9-219.5.5 Voice Loopback Resulting from Outgoing Cabling ............................................... 9-22


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Troubleshooting ManualM900/M1800 Base Station Subsystem Chapter 9 Troubleshooting for Voice


9-1

Chapter 9 Troubleshooting for Voice

9.1 Overview

This chapter introduces the fundamental knowledge, common operations and usual

processing of GSM voice troubles, including single pass, no pass (indicating both

parties cannot hear each other.), echo, noise and cross-talking etc.

The operations and methods that are involved in this chapter are basic modes for

locating voice troubles. To handle the specific troubles, refer to this chapter on the

basis of large quantities of dials-and-tests for accurate location.

9.2 Fundamental Knowledge

9.2.1 Transmission Format of Voice Signal

FTC is responsible for implementing format conversion of voice signals in GSM

system. That is to say, in the direction from FTC to MSC, all the voice signals are at

the rate of 64Kbit/s and in PCM format, and their contents are PCM samples got after

these voice signals undergo 8kHz sampling and A-law compression and expansion

prior to 8-bit encoding. In the direction from FTC to BTS, the voice signals are at the

rate of 16kbit/s and in compressed format (TRAU frame), with contents of abstracted

characteristic parameters of these voice signals. Generally, the direction from FTC

upwards is called 64kbit/s link, and that from FTC downwards is called 16kbit/s link

for convenience.

The uplink and downlink voice signals over 64kbit/s link are symmetrical, but the

formats of the uplink and downlink TRAU frames over 16kbit/s link are different. As a

result, the voice signals over 16kbit/s link cannot be looped back, but those over

64kbit/s link can.

By combining fault scope and voice format, the transmission format of voice signals

can be concluded as shown in Figure 9-1.


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9-2

Figure 9-1 Transmission format of voice signal

9.2.2 Transmission Path of Voice Signal

The transmission path of the GSM voice signal is given below (the key points are

highlighted in bold):

MSRadio line (including antenna system)

BTS(E1)

BTS_DDFTrunktransmissionBSC_DDF(E1)BIE(HW)GNETGOPT(Optical fiber)

GFBIGCTNE3M(E1 or transmission equipment)MSMFTCMSC

Note:The transmission path is for multi-module environment. For the single-module

environment, no board on the AM/CM module is provided, such as E3M etc.

MSC (when a signal doesnt go out of a SM, it is destined to the corresponding GNET.

Here, the SM and GNET are parts of MSC):

DT(HW)GNETGOPT(Optical fiber)GFBIGCTN

See Figure 9-2 for details:


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9-3

BTSE1

BTSE1

BTSE1

BTSE1

BTSE1

BTSE1

E1

BIE

E1

BIE

E1

BIE

E1

BIE

GNET

GNET

HW

HW

HW

HW

GOPT

GFBI

GCTN

Radio

Link

TransmissionEquipment



E3MHW

E3MHW

E3MHW

E3MHWT

ransm

issio

n

Eq

uip

m

ent

E1E1

E1

E1

E1

TCSM

GNET

GNET

HW

HW

HW

HW

GCTN

DT

DT

DT

DT

GOPT

GFBI

E1LinkorThroug

hDDF

E1

E1

E1

E1

E1

E1

E1

E1HWDTOther MSC / HLR / GMSC

PSTN EC

MSC Side

BSC Side

(BM1)

(BM2)

(SM1)

(SM2)

Figure 9-2 Transmission path of GSM voice signal

1) Timeslot interchange is implemented in BTS, which is not illustrated in the figure

above. BTS2X BIE or BTS3X TMU implements switch between TRX time slots

and E1 time slots, which are in a definite one-to-one relationship.

2) BIE implements mapping between E1 time slots and HW time slots, which are in

a definite correspondence relationship.

3) GNET at BSC BM implements mapping between HW time slots and A interface

time slots, which is dynamic. The mapping of each call is different from others .

4) GCTN at BSC implements interchanges between 2.048%16Mbit/s HW time slots

in GFBI and those in E3M, which are not in a definite correspondence

relationship, and each call is different from others.

5) E3M implements interchanges between HW time slots and E1 time slots, which

are in a definite one-to-one relationship.

6) GNET at MSC implements interchanges between two A interface circuits (i.e.,

calls between two MSs in the same SM), between HW time slots in DT and those

in GOPT (i.e., calls between two SMs in the same MSC) or between local circuit

and outgoing circuit (e.g., calls between MSs in the local SM and local fixed-line

phones, or between out-of-town fixed-line phones and MSs).

7) GCTN at MSC can implement interchanges between HW time slots in two FBIs

only when the call between two SMs is originated.


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9-4

Note:The above analysis doesn't include the prepaid MS. For such MS, the outgoing route

is needed even if the call between two local MSs is originated.

GMPU is not illustrated in the above figure. However, actually the allocation of circuits

GMPU accomplishes and request for layout of GCTN transferred by GMCCS both are

relevant with the interchange between voice signals.

A E3M or a TCSM may not serve the BM controlling and maintaining it, but for the A

interface circuit occupied by the call under the BM, such relationship doesnt apply.

That is, the call under BM1 doesnt always occupy the TCSM circuit configured by

BM1. Actually, the TCSMs configured by a BM usually belongs to different SMs. Ainterface circuits are allocated by MSC, consequently, it is possible the A interface

circuit occupied by the call under BM2 corresponds to the TCSM belonging to BM1,

and at this time, GCTN takes the responsibility of implementing the overlap between

HW in GFBI of BM2 and that in E3M of BM1.

9.2.3 Concepts

Full rate service: Full Rate encoding/decoding (FR), and the algorithm Regular Pulse

Excitation-Long Term Prediction (RPE LTP) is adopted. Encoding process: TC frames

the voice signals received from MSC based on the principle of 20ms per frame. A

frame of data contains 160 PCM sampling points, each of which is 8 bits long. The

parameters output after encoding contain 260 bits totally, which plus the

synchronization header and control parameter is a 320-bit TRAU frame. Decoding is

contrary to encoding: After receiving a TRAU frame sent from BSC, TC restores it into

the voice data according to the decoding algorithm and sends it to MSC.

DTX: Voice Activity Detection (called VAD for short) and Silence Descriptor (SID) are

applied to Discontinuous Transmission (DTX) in GSM. When TRAU detects the data

received from MSC is a non-voice message via the VAD functional module, the voice

flag bit in the TRAU frame formed after encoding shall be cleared. After discriminating

the flag bit, BTS disconnects the downlink until the flag bit is set. Similarly, when

receiving an uplink frame, TRAU discriminates the SID flag and set of the flag means

the MS is in an interim state. To make the receiving end feel the GSM network is

keeping serving it, TRAU injects a comfortable noise into the uplink through the

substitute technology so as not to make the subscriber consider the conversation is

interrupted. DTX is applied to TRAU, which may lower the transmission power of BTS

and MS, reduce intra-frequency interference among radio interfaces and weaken the

sensitivity of the GSM voice signal to exceptions occurring to radio interfaces.


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9-5

9.2.4 Common Operations

I. Dial-and-test on A interface circuit

Here only the interconnection between Huawei MSC and Huawei BSC is described.

For route selection rule of other manufacturers' MSC, refer to their corresponding

documents. If the signaling connected with the MS lies in module N, MSC shall assign

the circuit of module N preferentially. As a result, if a trunk is to be tested, the No. of

the module corresponding to the trunk shall be found first, then reserve the A interface

link of this module and block A interface links of other modules, and afterwards block

the circuits excepting the circuit to be tested piece by piece.

For MSC from other manufacturers, as BSC does not provide the user interface

tracing function, a signaling analyzer e.g., K1205 or MA10 should be attached to the Ainterface for tracing SS7. (If there are lots of links used for such purpose, most of

them should be blocked at nights and only one or two links are reserved to facilitate

the tracking). To distinguish fault messages, when a fault occurs, push any button of

the MS when the conversation holds, and then search the START DTMF message

before performing Call TRACE. In this way, the assignment request for this call can

be found, thus CIC shall be found.

II. CIC in assignment request message of interface tracking

On the window What It Means, click Assignment Request and click hexadecimaldigits at the bottom to make them framed in red. Click Circuit Identity Code, and the

hexadecimal digits framed in red indicate a CIC, which can be converted into the CIC

in decimal format, via which the corresponding module No. and trunk circuit No. can

be found in the trunk circuit table. (For MSC, the trunk circuit table is also needed).

006E shown in Figure 9-3 indicates a hexadecimal CIC.


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9-6

Figure 9-3 CIC format description table

III. Monitoring system in real time

Since blocking and unblocking operations are frequently implemented for links and

circuits during the dial-and-test, it is necessary to monitor the system running in real

time to avoid mis-operations. For example, check whether large quantities of

subscribers cannot assign circuits owing to mis-operations. The real time counter

query function of traffic measurement console can be used to satisfy such purpose.

The procedures are as follows: Before operating links and circuits, enable the real

time counter query function and select the item number of assignment failures. If the

result is 10000 (indicating the accumulated value since BSC runs), query the resultsof the real time counter again after operating links and circuits (you may click the

button oblige to query in the query window), hence the current number of

assignment failures can be got. If it is still 10000, it means no new assignment failures

occur and the above operation is safe. If the number becomes 11000, it indicates the

above operation resulted in lots of assignment failures and should be restored

immediately.

No sooner had the system been upgraded or data been modified than the real time

counter query function can be performed to facilitate the monitoring to the system.


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9-7

IV. Allocation principle of A interface circuit

Provided that the call signaling from BSC arrives at a module in MSC, this module

shall be allocated with circuits preferentially. Only when the circuit occupancy in the

module reaches the stated threshold can the signaling be transferred to other

modules for circuit allocation. Therefore, for a single-module MSC, A interface circuits

are allocated sequentially, while for a multi-module MSC, A interface circuits are

preferentially allocated to the module the call signaling gets to before other modules

in the MSC are allocated with A interface circuits sequentially. If the A interface circuits

are not congested too much, the signaling is hardly transferred among modules and

generally the A interface circuits in the module are allocated sequentially.

9.2.5 Supplement

1) Only when a valid uplink TRAU frame is received can TC be started. Therefore,

in case a fault or a severe bit error occurs to the uplink 16kbit/s link, which

causes the situation that TC is not able to receive the valid TRAU frame, TC

cannot be started. And no matter a conversation over the faulty link is originated

between two MSs or between an MS and a fixed-line phone, both parties cannot

hear each other. If a downlink 16kbit/s link gets faulty, it only impacts on the

downlink voice signal. When a MS over the faulty link communicates with a

fixed-line phone or another MS over a normal link, the MS over the faulty link

cannot hear the opposite party. Such situation is called signal pass. If both the

downlink 16kbit/s links of the channels occupied by the two MSs are faulty, they

shall not be able to hear one another.

2) As what is transmitted over the transport line is the signal multiplexed by many

time slots, all the time slots over the line shall get faulty if the line is faulty or

connection error occurs. Only in the devices where processing is made by time

slots may parts of time slots fail, such as the boards implementing switching:

BTS2X BIE, BTS3X TMU, BTS3X BIE, BTS3X GNET, BTS3X GCTN, BTS3X

E3M, MSC GNET and MSC GCTN etc. Such faults may be caused by board

failure, incorrect data configuration, host program error, poor contact of the line,

bad quality of stub or electromagnetic interference. If the EMC protectioncapability of a board is not good enough, parts of its time slots may get faulty.

3) Same BER may impact the 64kbit/s link less than the 16kbit/s link. If bit error

occurs to the 64kbit/s link, the generated noise shall be relatively even without

obvious fluctuation. Even when the noise submerges the voice due to overhigh

BER, it still can remain even. However, for the 16kbit/s link, even if the original

bit error is symmetrical, since the 16kbit/s compressed signal should be decoded

to become the voice signal, the bit error shall become uneven for the voice signal

after decoding, and bubble, voice discontinuity and metallic sound may occur.


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9-8

9.3 Processing of Voice Troubles

9.3.1 Analysis

For a voice fault, judge whether it occurs inside MSC or only out of MSC. Only the

path via which the call between two MSs (not including prepaid MS) in a MSC goes

can be ensured to be in the MSC. For a fault out of MSC, check relevant outgoing

equipment and data. If correct, it shall be a fault of outgoing equipment.

For a fault inside MSC, it can be located according to the following procedures:

1) Judge whether the fault occurs to one site or multiple sites.

2) If the fault only occurs to one site, perform dial-and-test on all carriers of the site

to further check whether a time slot, a frequency or the whole site has such

problem. If it is a frequency fault, it may be caused by interference. If it is a site

fault, check the transmission path from this site to GNET (including boards, lines,

trunk transmission equipment etc.).

3) If the fault occurs to multiple sites, check how these sites are distributed as per

data configuration and see whether they share the same transmission path, the

same BIE, the same BM, the same SM or the same MSC.

4) If a special transmission path gets faulty, check the corresponding transmission

equipment, cables and optical fibers.

5) If the faulty sites share the same BIE, check the BIE and HW between BIE and

GNET.

6) If the fault occurs to a BM, check the boards and cables between GNET and

GCTN

7) If the fault occurs to multiple BMs that correspond to a SM, and the faulty call

belongs to the SM, check all boards and cables between GCTN of BSC and

GNET of the SM. If the fault occurs only when the SM calls other SMs, check the

boards and cables between GNET of the SM and GCTN of MSC.

8) If multiple SMs fail to converse and these conversations are implemented in the

MSC, check the boards and cables between GNETs of these SMs and GCTN of

the MSC.

Note:Besides the transmission equipment, boards and cables, the slots accommodating

these boards, backplanes, stubs and connectors should be checked as well. For

convenience of description, all above are called boards and cables hereafter.


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9-9

9.3.2 Procedures to locate faults

Use two test MSs and activate the functions Call Hold and Call Wait for them. Lock

their frequencies to a BTS and record the version No. of the BTS. Perform thedial-and-test at a night and enable user interface tracking of MSC to trace the A

interface message. When single pass occurs, dont hook on and execute the following

operations:

1) Record the CICs of the calling and the called parties as per the tracked A

interface message, and the corresponding frequency and channel time slot

according to the message displayed on the two test MSs. Check whether the

recorded CICs and the corresponding time slots are on a FTC and a TRX

respectively.

2) Hand over GCTN of MSC during conversation. If single pass remains, proceed

the following operations. If it disappears, the GCTN may be faulty, and switch

over it back to check whether single pass still exists.

3) If the functions Call Hold and Call Wait are activated for the two test MSs, this

step shall be executed, otherwise go to the next step. When MS A fails to

communicate with MS B, use MS C to originate a call to the faulty MS (assuming

it is MS A). MS A accepts the new call, and at this time the original call is held.

Since the resource at radio side (including radio resource and AIE) used by MS

A didnt change at all during the two calls, if noise still exists during the second

conversation, MS A might be faulty (you may use C to originate a call to B for

further confirmation). Contrarily, if the trouble disappears during the secondconversation, it could be concluded that MS B might be faulty.

4) According to the CIC recorded in the A interface message find the corresponding

MSC module No. in trunk circuit table of the MSC data management console.

Switch over the GNET of the MSC module (if the two MSs correspond to

different modules, the two relevant GNETs shall be switched over). If single pass

still exists, proceed the following operations. If the trouble disappears, the GNET

of MSC or the HW connected with the active GNET gets faulty. Switch over the

board back to check whether single pass still remains.

5) Switch over GCTN of BSC during conversation. If single pass remains, proceed

the following operations. If the trouble disappears, the GCTN may be faulty.

Switch over the board back to check whether the trouble still exists.

6) According to the CIC recorded in the A interface message find the corresponding

BSC module No. in the trunk circuit table of the BSC data management console.

Switch over the GNET of the corresponding BSC module. If single pass remains,

proceed the following operations. If the trouble disappears, the GNET of BSC or

the HW connected with the active GNET gets faulty. Switch over the board back

to check whether the trouble still exists.

7) Search the corresponding trunk circuit No. of any channel of the BTS to be

tested in the radio channel configuration table. If the number is N, N/256 is no


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other than the BIE group No., and switch over the group of BIEs. If single pass

remains, proceed the following operations. If the trouble disappears, the trouble

may be caused by BIE. Switch over them back for further test.

8) Use the test MS to implement forced handover. Hand over the MS to the cell ofanother adjacent site controls and check whether single pass disappears. If

possible, inter-BSC switchover test can be implemented.

9.4 Locating Trouble

9.4.1 Single Pass and No pass

I. Description

Single pass indicates during conversation only one party can hear the voices sent

from the opposite, but the other can hear nothing. No pass means both parties cannot

hear each other.

II. Analysis

As per voice circuit procedure in the system, the two type troubles may be caused by:

1) Radio problem

Radio environment, e.g., imbalance between uplink/downlink levels resulting in voice

of poor quality and interference to one party.

2) BTS fault

Hardware: Board (e.g., CDU, TRX, TMU etc.) fault, error of the switching network

table of TMU.

Software: Data configuration error. For instance, TS number in [Radio Channel

Configuration Table] are configured incorrectly. Site BIE mode ID in [Site BIE Trunk

Mode Description table] are inconsistent with those in [Site BIE Configuration Table],

which causes the situation that the cascaded BTS cannot converse normally.

3) Abis interface fault

Poor quality devices between BTS and BIE (including the trunk transmission

equipment in the middle), connectors and cables as well as bit error of transmission

line may cause voice of poor quality to one party.

4) BSC fault

Hardware: All boards and cables between BIE and GCTN (including the backplane).

Software: Time slot and HW configuration of BIE.

5) A interface fault


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9-11

Hardware:

z Board fault: Including the boards such as E3M, MSM, FTC, DT at MSC etc.

z Cable fault: Including the cables such as crossover cable and crossed pair of

cables.z Setting error of DIP switch: There are DIP switches on 12FTC and 13FTC setting

whether these FTCs are multiplexed, and MSM also carries a DIP switch used

for setting the time slot occupied by the maintenance control message of FTC. If

these DIP switches are set incorrectly, it may cause single pass or no pass.

Software: CIC configuration. CIC is for setting whether A interface trunk circuit is

available. If 12FTC is put into use, EFR service cannot be configured, otherwise

single pass may occur when a MS originates a call to a fixed-line phone or no pass

may occur when a MS originates a call to another MS. For the group of TCSMs

during multiplexing, the four time slots that are used for bearing signaling andcorrespond to four FTCs should be set to be unavailable. And the last time slot of the

last FTC that is used as the maintenance time slot should be set to be unavailable as

well, otherwise both parties involved in a conversation may not be able to hear each

other.

6) MSC

Hardware:

z The boards such as DT, GNET and GCTN are faulty or in poor contact with

backplanes, or the backplanes or slots get faulty.

z Cable damage or in poor contact. For instance, the HW cable between DT andGNET, optical fiber between SM and AM and outgoing trunk cables are

damaged or in poor contact.

Software: [Semi-permanent Connection Table] or data of outgoing trunk is configured

incorrectly.

Sometimes a faulty MS may cause such troubles.

III. Handling process

Judge whether parts of BTSs under a BM get faulty or the whole BM even moredevices fail. If the fault only occurs in the outgoing process, check the associated

outgoing trunk and data. For the fault occurring in the office, check all parts within the

fault scope.


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9-13

The following method can be used confirm the opposite MS generates acoustic echo:

Adjust the tone of the opposite MS and the local party can feel the change of echo

tone apparently.

2) Echo occurring in case of conversation between MS and fixed-line phone

During conversation between a fixed-line phone and a digital MS, if the MS can hear

large quantities of echoes, such kind of echo is called acoustic echo, which may

result from lack of Echo Cancellor (EC). Search the corresponding route data of the

call to ensure the data is correct. If correct, check whether the EC of the

corresponding mobile network equipment is configured accurately according to the

principle that EC is placed near PSTN, so as to guarantee EC works normally.

Occasionally, echoes of high volume may occur when a digital MS originates a call to

a fixed-line phone. That is because the hybrid coil at the fixed network doesntmeasure up and the volume of echoes generated exceeds the processing capability

of EC, which results in echoes of high volume occurring at the digital mobile network.

As soon as a call is set up, EC shall perform adaptive matching of echo cancellation

dependent parameters. If parameter search is too slow or unsuccessful, temporary

echo or continuous echo may occur just after the call was set up. Unfrequent

occurrence of such phenomenon is normal.

3) Voice loopback

According to the numbers of the calling and the called parties and time when a

loopback occurs, search the corresponding CDRs in MSC and confirm whether the

calls looped back went through the same route, and then check whether the trunk

cable corresponding to the route is connected incorrectly.

When a intra-office call is looped back, block the A interface circuit and make 32

circuits of only one trunk idle. Then perform dial-and-test on each trunk of the A

interface in sequence to check whether loopback exists. If it exists, check whether the

corresponding trunk is connected incorrectly as per the No. of the CIC occupied by

the call.

If only the outgoing call is looped back, perform dial-and-test on the outgoing trunk tocheck whether loopback exists. If it exists, check the corresponding trunk is

connected incorrectly as per the No. of the CIC occupied by the call.

If all trunks are connected correctly after the above operations, (however, it cannot be

ensured that other offices corresponding to the outgoing route are connected

correctly), the trouble still remains. Try to switch over GNET and GCTN at MSC.

If the trouble still cannot be eliminated after the above operations and it is confirmed

the outgoing call was looped back, check whether the equipment and cables of other

offices involved in the outgoing route are in a normal state.


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9.4.3 Voice Discontinuity

I. Description

The voice in the conversation is frustrated. Some instances of the conversation are

lost, or in worse condition, the entire conversation becomes difficult for both parties.

II. Analysis

The causes resulting in voice discontinuity are as follows:

1) Frequent handover

Since GSM system supports hard handover. The handover from a source channel to

the destination channel can cause loss of Abis interface downlink voice frames, so

voice discontinuity resulting from handover during conversation is inevitable. Sporadic

handover is not attention-getting, while frequent handover occurring at cell edges or

due to cell overlap may cause discontinuity of conversation. Such trouble can be

avoided by optimizing the network, adjusting obliquity and height of the antenna and

configuring the parameters interference handover uplink/downlink quality threshold

and emergency handover uplink/downlink quality limit threshold.

Location method: Use a test MS to check the channel occupied by the MS changes

continuously during a conversation.

2) Radio link interference

Interference may increase the BER over the radio link and cause voice discontinuity.

In addition, the conversation may be of poor quality owing to signal fluctuation at cell

edges.

Location method: Perform a road test through a test MS to analyze and check

whether radio link interference exists with the help of Ant Pilot network optimization

software.

3) BTS transmission fault

Check whether all connectors (including connectors on DDF) are in good condition.

For fiber optical transmission, check whether fiber connectors are clean and whether

BER is high, and microwave transmission may be impacted by climate. It should be

noted that the 75: coaxial cable laid from the transmission interface boards (such as

42BIE, TMU) to the BTS cabinet top in BTS cabinet may be of poor contact due to

long time use. If the connectors in the cabinet are dusty, it may impact the

conversation. If both microwave transmission and fiber transmission are adopted,

make sure these types of equipment are matched in interface transmission

impedance.


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Location method: Check whether the transmission alarms, such as BTS BIE remote

alarm, LAPD link alarm, are given from the alarm system. Furthermore, testing

whether bit error occurs to the transmission path is the most effective and convenient

method.

Caution:

Sending pseudo codes and using the clock in the BER tester could guarantee the

result of the BER test completely reflects the quality of the link.

4) Carrier board fault

Location method: Use a test MS to check whether the channel or frequency occupied

by the MS changes continuously during a conversation and whether the frequency

and time slot are fixed in case that no voices are transmitted.


To locate a voice discontinuity trouble, perform dial-and-test to find the position where

the trouble occurs and then judge which kind of fault it is according to the above

methods.

9.4.4 Noise

I. Description

Bubbles, clicks and metallic sounds heard during a conversation are called noises. In

worse condition, only noises instead of voices can be heard.

II. Analysis

Generally, noises are caused owing to bit error. Besides the fault of any board,

connector or cable on the path via which voice signals go through, grounding error,

interference, clock fault or wrong setting of DIP switch may result in bit error.

Interference on radio link can cause bit error, while clock unsynchronization shall lead

up to slip frame or loss of frames. Wrong setting of DIP switch may bring about

errored bits, although such mistake happens occasionally.

Different errored bits may cause different impacts. The errored bits on the line from A

interface to MSC impact on PCM sample, as a result, the noises generated are

relatively even because the noises and voices are in a overlap relationship. The


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errored bits on the line from A interface to BSC impact on the compressed voice

signals which should be decoded before being heard although these bits are also

well-proportioned. Consequently, the noises generated, such as bubbles, sense of

discontinuity and metallic sounds, fluctuate greatly, and make some sentences cannotbe distinguished clearly. Slip frame or loss of frame caused by clock

unsynchronization is regular in time, therefore, noises appear regularly during a

conversation.


Find the cause resulting in noises first according to the features of noises, then

determine the scope for location test in terms of the place where noises occur.

1) BTS fault

Possible causes:

z Trunk transmission bit error

z TRX fault, including version incompatibility of TRX software and hardware.

z FPU fault

z CDU fault

z MCK fault, which may cause instability of BTS clock frequency, thus impact the

quality of conversation.

z Interference on radio channel

z Antenna fault

Location:

z Perform dial-and-test by using a test MS

z Check whether the related alarm is given

z Tracing messages or check signal quality and whether inference exists from the

MS

z Use an antenna tester to test the antenna system

z Check whether the grounding system is wrong.

z Perform a transmission bit error test

z Test the clock signal of the faulty BTS.

2) BIE related noise

Possible causes:

z BIE fault

z HW fault between BIE and GNET

z Transmission error from BIE to BTS.

Location:

z Check whether the related alarm is given (PCM alarm)

z Perform a transmission bit error test

z Replace the BIE


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z Connect the BTS to another BIE

z Check the HW and E1 cable of the BIE

z Switch over the GNET

3) BM related noisePossible causes:

z Switching network board (GNET and GCTN) fault

z Optical fiber interface circuit fault.

Location:

z Check the connection of optical fibers and related connectors

z Check HW of GNET

z Switch over GNET or GCTN.

4) Intra-office noise

Possible causes:

z A interface dependent board & cable fault (E3ME1MSMinternal

HWFTCE1or trunkDTHWGNET at MSC)

z Inter-module circuit fault: Optical fiber interface circuit, GCTN at MSC

Location:

z Enable GSM user interface trace at MSC and perform dial-and-test on A

interface circuit. Analyze the trunk occupied by the faulty call based on the

interface message and calculate the corresponding board, and then check the

related boards and cables (including all boards, cables and backplanes fromE3M to GNET

z If trunk equipment is in the way between BSC and MSC, test the equipment.

(Check whether errored bits exist in it and whether the grounding system is

correct)

z Check whether the related alarm is given

z If inter-module forwarding at MSC is implemented for the faulty call, check the

associated connections and connectors. Switch over GCTN at MSC if necessary.

5) Outgoing noise

Based on the outgoing route, check the related outgoing trunk equipment and cables.

9.4.5 Cross-talking

I. Description

During a conversation, the voices from the third party, besides those from the

opposite party can be heard, or only the voices from the third party can be heard.

Such phenomenon is called cross-talking.


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II. Analysis

Generally cross-talking occurs when the call passes through the outgoing route.

Wrong data configuration (e.g., CIC) or incorrect hard connection (e.g., E1 cable of A

interface) may cause cross-talking accompanied with single pass or no pass.


Similar to the location method of single pass/no pass.

9.5 Examples

9.5.1 Cross-talking Resulting from Improper Data Configuration

I. Description

Single pass/no pass accompanied with cross-talking occurred during conversations

near a BTS.

II. Handling process

1) Based on lots of dials-and-tests that were performed on site, maintenance

personnel found that when the TCH time slot at frequency 37 was assigned,

single pass and no pass occurred. After many dials-and-tests, the maintenancepersonnel found single pass would occur to a MS as long as it is assigned to the

master BCCH carrier (at frequency 37), and no pass would occur so far as both

MSs are assigned to the master BCCH carrier. After a while, cross-talking occurs,

and only the voices from one party can be heard.

2) The associated BIE was switched over as soon as the trouble appeared, but the

trouble still remained.

3) The maintenance personnel checked the [Radio Channel Configuration Table] of

the faulty TRX12 and found the Circuit number be 1110 and 1117, which were

the same as the TRX9 Circuit number.

4) Repeat of trunk circuit No. may bring about single pass, no pass even

cross-talking.

5) No single pass or cross-talking happened any more during on-site

dials-and-tests after the dynamic setting of the related data had been modified.

III. Analysis

The Circuit number in the [Radio Channel Configuration Table] is used to deliver

TMUs of BTS (BIEs of BTS20) for switching of time slots of a carrier. Since the carrier

signaling adopts the data in the radio channel connection table, it is normal. However,


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as TCHs for bearing voices are configured repeatedly, the TCH assigned to the

carrier shall be switched to the corresponding TCH of TRX9, thus result in single pass.

If the corresponding TCH of TRX9 is being used for conversation, the uplink voices of

TRX 9 may be switched to the corresponding time slot of the TRX, thus cross-talkingoccurs.

9.5.2 Voice Discontinuity Resulting from BCCH Carrier Mutual-assistance

I. Description

The user near a BTS complained the conversation was discontinuous.

Alarm information:

TRX hardware alarm 21:59;41

Mutual-assistance occurred to BCCH of the cell 22:00:15

TRX hardware alarm recovery 22:09;42

BCCH mutual-assistance switchback 22:10:12

TRX hardware alarm 22;10:19


1) To prevent such circular switchover, the faulty TRX was to be blocked so as to

exit the service before it was replaced.

2) As the TRX was in a Disable state when the under-power alarm occurred, the

remote operation didnt take effect.

3) As the faulty TRX was concerned with carrier mutual-assistance, the alarm

would disappear temporarily in ten minutes. Because the faulty carrier also

served as the TRX where the master BCCH was located, the power amplifier

should be activated. After the above operation, an alarm occurred, and carrier

mutual-assistance was enabled again.

4) The remote has no time to block the TRX due to the short duration of displaying

normal working status. Therefore, the faulty carrier should be replaced as soon

as possible.

5) If the carrier cannot be replaced immediately, it is recommended to switch off the

power of the faulty TRX.

III. Analysis

The channel configuration of the first carrier is changed because carrier

mutual-assistance was implemented for the under-power alarm, and the power

amplifier is disabled as well. The second carrier has been configured with BCCH and


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SDCCH and the conversation is handed over to the second carrier. Based on the

judgement principle of under-power alarm, the power amplifier shall be enabled every

ten minutes after the alarm occurs and will implement detection in 10 ms. If no alarm

signal is detected, it should be considered the alarm disappeared. In this way, as thefirst carrier has disabled the power amplifier after ten minutes and no conversation is

made, the first carrier fault will be considered to be recovered after the detection 10

ms later.

As per the cell BCCH mutual-assistance procedure, the original BCCH carrier will

make BCCH mutual-assistance switchback to reconfigure channels after recovery.

However, the active TRX will serve to send power, generate under-power alarm and

implement cell BCCH mutual-assistance, consequently handover between the two

carriers will be ceaseless. In this way, as it has no time to hand over a call that has

been handed over to the carrier whose power amplifier was enabled just now back tothe second carrier during second-to-first handover, conversation discontinuity will

occur.

9.5.3 Single Pass Resulting from MS Fault

I. Description

Single pass always occurred every time when a MS of certain type served as the

calling party to originate a call to a fixed-line phone or a MS.


1) When several MSs of other types were used to originate calls to the problem MS,

single pass always occurred to the problem MS used as the called party. But

when dials-and-tests were made between these MSs of other types, single pass

didnt occur at all. Therefore, the MS of this type may be faulty.

2) Afterwards, dials-and-tests were performed in the equipment room, meanwhile

the user interface tracking function was enabled, and the tracking signaling was

found in a normal state. Therefore, it could be concluded the MS of this type got

faulty.3) After flapped several times, the MS was put into use again for dial-and-test. No

matter it served as the calling party or the called party, single pass didnt occur

any longer. Then, MSs of other types were used for several dials-and-tests, and

single pass still didnt appear.

III. Analysis

After used for a long while, especially crashed, the parts in the MS may be loose.

Therefore, once single pass occurs, check the quality of the MS first.


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9.5.4 Noise Resulting from Poor Contact of E1

I. Description

Loud noises always occurred when the user supported by a BM of a BSC called. After

dial-and-test, the noise was confirmed to have the following features:

1) Noise description

Voice signals became faint (without discontinuity or distortion. These signals can be

considered to be normal if these noises could be regardless), while noises got loud

(the type of these noises are similar to that of white noise), and the voice signal and

the noise were overlapped. RQ in the test MS was 0. When the MS served as the

calling party, loud noises appeared as soon as a ringback tone was returned.

2) Popularity

The result of dial-and-test on the faulty BTS showed that the noise was irrelevant to

the place where the BTS resided, BIE the BTS belonged to and the transmission path

between the BTS and the BIE. And such noise existed in all cells of all BTSs.

3) Downlink

4) Random occurrence. The occurrence probability increases with the augment of

traffic.


1) Based on the above features, it can be concluded that:z According to features (1) and (4), it could be known that the noise was caused by

fault of the circuit from A interface to MSC. (The noise and the voice signal were

overlapped. The noise could be caused by bit error of PCM sample or of 64kbit/s

link, and its occurrence probability is relevant with polling of A interface circuits.

z As per feature (2), the faulty point should be in the range from BIE of BM to

MSC.

2) GNET and GOPT were switched over, and optical fibers and fiber connectors

were unplugged/plugged and cleaned.

GCTN was switched over again for the line from A interface to BSC, and the

noise didnt disappear at all.

A interface circuit allocation principle: Provided that the call signaling from BSC

arrives at a module in MSC, this module shall be allocated with circuits

preferentially. Only when the circuit occupancy in the module reaches the stated

threshold can the signaling be transferred to other modules for circuit allocation.

If the A interface circuits are not congested too much, the signaling is hardly

transferred among modules and the A interface circuits are almost allocated

sequentially in the module. For the single-module MSC, A interface circuits are

allocated sequentially.


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BSC and MSC have four modules respectively, whose A interfaces are in a

one-to-one relationship, as a result, it is impossible the signaling from multiple

BMs reaches the same SM. Furthermore, as the A interface circuit has sufficient

space, usually signaling transfer doesnt occur at MSC. Also, dial-and-test shouldbe performed between two MSs under the coverage of the same BTS, therefore,

it almost can be concluded that the A interface circuit from BM to MSC, namely

FTCDTGNET of SM1, gets faulty.

By performing dial-and-test on the A interface circuit between BM1 and MSC, the

corresponding trunk circuit (with DT No. of 11) of FTC No. 87 was confirmed to

be faulty. After FTC and DT were switched over in turns, the trouble still

remained. A trunk alarm was given by tightly touching the E1 cable near the

connectors at the back of the two boards to indicate the E1 cable is of poor

contact with the plug of the DTM backplane. After unplugged and the plugged,

the E1 cable was tested again, and the trouble disappeared.

III. Analysis

If the noise is superimposed, usually it indicates the line from A interface to MSC is

faulty. If it is random and ruleless, the line from TC to BSC may be faulty.

9.5.5 Voice Loopback Resulting from Outgoing Cabling

I. Description

During conversation, the local party only can hear its own voices instead of voices

from the opposite party, while the opposite one can hear nothing.


1) By performing traversal dial-and-test on the trunk circuits corresponding to all

FTCs, it was confirmed that loopback didnt occur to the trunk of local A

interface.

2) Dial-and-test was performed on the trunk between local MSC and tandem

exchange.3) Physical line loopback was found in the trunk between local MSC and tandem

exchange, which caused voice loopback of the outgoing call.

4) Relaying the trunk between local MSC and tandem exchange solved this trouble.

III. Analysis

As the physical uplink and the physical downlink of radio interface are separate by

frequency, and those of Abis interface are unsymmetrical, it is impossible that

loopback occurs between Abis interface and BTS. Also, frequency hopping cannot

cause loopback.


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The data configuration was correct after scrutiny.

Voice loopback is possibly caused by loopback on the physical layer, and may occur

in the following cases:

1) As most of voice loopback exists in the outgoing call, it may occur to the trunk

between local MSC and tandem exchange.

2) Loopback may occur to the trunk between local MSC and TMSC.

3) Loopback may occur to transmission equipment.

09-troubleshooting for voice

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