GSM Network Speech Quality Problems & Solutions ZTE university

Training goals To know the causes of speech quality problems and problem handling procedures.

Contents Monologue Cross-talk Bidirectional Silence Poor Speech Quality Echo

Monologue－Causes Causes of monologue MS Wireless part BTS (software& hardware) MSC (software& hardware BSC(software& hardware) Abis interface MS External interference A interface (software & hardware) Causes of monologue

Monologue－Problem handling flow Monologue appears in the range of some BSCs or a certain number of BTSs under the BSC Confirm monologue problem area Monologue problem only occurs in outgoing calls Monologue exists in the whole MSC range Monologue problem only exists in the coverage of a BTS We can tell if it’s an inter-office or intra-office problem from subscriber’s feedback and call test . If it’s an intra-office problem, find out if the problem is with some BSCs or all BSCs under the MSC Find out if the problem is with some sites or all sites under a BSC.

Monologue－Problem handling flow Monologue appears in the range of some BSCs or a certain number of BTSs under the BSC Monologue exists in the whole MSC range Confirm monologue problem area Monologue problem only occurs in outgoing calls Monologue problem only exists in the coverage of a BTS Check corresponding outgoing trunk and data. As for problems inside the office, check all parts in the problem area.

Monologue－Problem handling flow Monologue appears in the range of some BSCs or a certain number of BTSs under the BSC Confirm monologue problem area Monologue problem only occurs in outgoing calls Monologue exists in the whole MSC range Monologue problem only exists in the coverage of a BTS Try to find out if customer or other relevant personnel made changes on MSC data or performed operations during site swap.

Monologue－Problem handling flow Monologue appears in the range of some BSCs or a certain number of BTSs under the BSC Monologue exists in the whole MSC range Confirm monologue problem area Monologue problem only occurs in outgoing calls Monologue problem only exists in the coverage of a BTS first check if the version of BSC DRT is upgraded to be able to solve monologue problem if the version is right, change over the active and standby DSN boards of BSC; make test after the changeover to see if the problem still exists. When analyzing and locating this problem, we can open the “GSM subscriber interface trace” at MSC maintenance platform, make circuit dialing test at A interface; with the interface message, analyze the trunk occupied by the problem call, calculate the corresponding single board to check the boards and connection wires (all single boards and connection wires between E3M and NET, including backboards).

Monologue－Problem handling flow Monologue appears in the range of some BSCs or a certain number of BTSs under the BSC Confirm monologue problem area Monologue problem only occurs in outgoing calls Monologue exists in the whole MSC range Monologue problem only exists in the coverage of a BTS check cell’s radio parameters, whether UL/DL power link is balanced or not (collect data through signaling tracing), whether MS max transmission power is set correct or not. make call test and check hardware to find out if hardware problems exist, such as equipment connection problem, TRX fault, combiner fault, antenna fault, etc.. further confirm the problem is about UL or DL with MS calling landline phone.

Inter-office monologue cases Problem description The operation and maintenance department of an operator complained to ZTE engineers that monologue existed in Meilong town. In communication, ZTE engineers found that most sites under BSC4 have the problem, but BSC11 in the town didn’t have the problem. Problem analysis ZTE engineers went to the site and made call test. CALL TEST between MSs inside the office, both MOC and MTC were under the same BSC (BSC4) ;MOC was under BSC4, and MTC was under BSC11; no monologue problem occurred in both situations. Intra-office call test made calls to local landline phone, monologue happened, and the probability of monologue happening was about 40%. it was inferred that the monologue problem only existed in calls between different offices (our operator’s office – China Telecom), and the cause of problem should be with the outgoing trunk and data. Inter-office call test Call test method

Inter-office monologue case Problem handling Checked: if Any changes on MSC data or operations were performed during site swap? Problem location: inter-office Start Confirmed: trunk circuit data were configured wrong during the expansion and swap of Telecom outgoing circuits. End Process: adjusted configuration mistake Problem solved

Typical case-Monologue due to TRX fault Problem description A great number of subscribers complained about monologue problem in a town . Problem analysis Engineers made DT and CALL TEST on site; Among the 50 times of CALL TEST, monologue or two-way silence occurred about 15 times.; Through observing the situation of MS seizing frequency and timeslot, engineers found that monologue occurred when MS seized a frequency of a cell; The MS receive level fluctuated greatly, which even reached 30dB, as shown in the following figure.

Typical case-Monologue due to TRX fault Problem description Engineers concluded that the monologue problem was mainly caused by the defective TRX; monologue occurred when MS occupied this TRX. Problem handling When the defective TRX was replaced, engineers made CALL TEST again and monologue problem disappeared, and the problem didn’t occur in the following days.

Typical case-Monologue due to repeater fault Problem description In a network, subscribers in the coverage area of a BTS (BTS A) complained about monologue problem, which was even serious at the prefectural government building area and a normal school (covered by BTS A Cell 1) . Problem handling Engineers went to the site and made call test, finding the monologue problem was related to the level. When DL level was under －80dBm, metallic noise and monologue appeared in calls. It was also found in call test that UL speech quality was poor, while DL speech quality was good, so the monologue was due to UL problem. After checking OMCR data, engineers found that interference band of BCCH 113 and TCH 99 in BTS A was very serious, the interference class even reached 5. Serious monologue existed Repeater

Typical case-Monologue due to repeater fault

Typical case-Monologue due to repeater fault Problem handling The monologue at BTS A was confirmed to be caused by repeater interference. Closed repeater and made call test again, metallic noise and monologue disappeared in calls.

Contents Monologue Cross-talk Bidirectional Silence Poor Speech Quality Echo

Cross-talk － Causes & handling of problem Crossed-wires at A interface; incorrect exchange of timeslots made by timeslot multiplexing equipment when the equipment is used in transmission; wrong configuration of BTS data . Causes of cross-talk Locate problem area; If cross-talk appears during intra-office calls, check if there are crossed wires at A interface first; If cross-talk appears during inter-office calls, check if there are crossed wires in outgoing office circuits ; Block the E1 equipment that uses timeslot multiplexing device, and make test, so as to check if the problem is caused by timeslot multiplexing equipment fault; As for dealing with cross-talk at one BTS, focus on checking of BTS transmission, hardware and data configuration. Methods for handling cross-talk

Typical case - Cross-talk due to intra-office fault Problem description Frequent monologue and a few cross-talks occurred in a network after it started commissioning; In many times of call test, we caught several times of cross-talk; One phenomenon was that monologue happened to MOC or MTC, and the counterparty could not hear MOC or MTC but a third person’s voice or the talk between another two people; The other was that when call drop happened to MOC or MTC, the counterparty didn’t release the call but heard a third person’s voice after more than 10 seconds.

Typical case - Cross-talk due to intra-office fault Problem analysis From the complaint letters collected on site and many times of call test, it was discovered that monologue happened to the 12th and 13th E1 in BSC1. When MOC seized timeslots on the two E1 to originate a call, the following phenomena appeared: Sometimes, the MTC didn’t hear anything at the beginning, but heard cross-talk after a while, and this continued until hang-up. When MTC answers the call, it heard a third person’s voice, and this continued until hang-up. Sometimes, both the MOC and MTC could hear nothing, and after a while they both heard the talk between another two people, and this continued until hang-up.

Typical case - Cross-talk due to intra-office fault Problem handling Made call test to timeslot of A interface, finding that the problem centered on the A interface frame of the first layer in BSC1. After the backboard of the frame was replaced with a new one, the problem was solved. 6 5 1 Put the original EDRT board back to position, the problem phenomena didn’t return, calls were established normally without monologue or cross-talk Checked E1 head and connection, there was no problem and no crossed wires. To handle the two faulty E1 4 2 Changed EDRT board of the corresponding slot in BSC1 with a new one, the problem disappeared, no monologue or cross-talk happened; Connected the faulty E1 to BSC1 and MSCA, the cross-talk problem continued. 3 Changed DTI board of the corresponding slot in MSCA with a new one, the problem continued

Typical case - Cross-talk due to wrong data configuration Problem description Many subscribers complained about cross-talk problem in a town, the problem centered in the area covered by a site. Problem analysis Made call test on site; It’s discovered that cross-talk occurred when MS occupied BCCH timeslot2 of a cell under the site; while no similar problem happened to other timeslots of other TRXs; Changed the faulty TRX, but cross-talk still existed.

Typical case - Cross-talk due to wrong data configuration Problem handling Engineers checked BTS data, and it was discovered that there was something wrong with the sub-timeslot ID configured to the channel corresponding to timeslot2 on the BCCH TRX, which was the same as that configured to the channel corresponding to timeslot3, hence cross-talk was resulted. After further investigation, it was found that data configuration mistake happened when OMCR engineers changed timeslot2 from SDCCH to TCH in radio resource management. it is suggested that the operation (change of channel types) be performed in integrated configuration management, so as to avoid data configuration mistakes.

Contents Monologue Cross-talk Bidirectional Silence Poor Speech Quality Echo

Two-way silence－Causes & handling of problem Methods for handling two-way silence Causes of two-way silence First locate the problem area, make certain if the problem is prevalent in most sites or it just appears in some sites; If the problem is prevalent, we can refer to cause 1, 2 and 5, and make investigation accordingly; If the problem just exists in some sites, we can refer to cause 3 and 4, and make investigation accordingly. Transmission circuits are self-looped; The CIC codes at the two sides of the circuit do not meet with each other; Some timeslots of the transmission circuit adopted are not good; In the shared E1, timeslot multiplexing equipment is not configured with timeslots correctly; DRT/EDRT or TIC fault.

Typical case- Bidirectional silence due to A interface circuit fault Problem description An operator that subscribers in an area complained about silence, monologue and echo problems in calls, which returned to normal after a while (keep hang-on). These problems also happened to BSC18, BSC19 (Siemens) and BSC9 (ZTE) under ZTE MSC04. Problem analysis From the problem phenomena, it was certain that the problem existed widely in most sites. Engineers made call test on site.

Typical case- Bidirectional silence due to A interface circuit fault Analysis of test phenomena: After handover from Siemens cell to ZTE cell, testers couldn’t hear each other.

Typical case- Bidirectional silence due to A interface circuit fault Analysis of test phenomena: Speech was normal after establishment of call, but when inter-BSC handover happened, testers could not hear each other but echo (their own voice), which lasted for 15 seconds. When inter-BSC handover happened again, the call returned to normal. Third handover, from ZTE 24112 to Siemens 11581, the call returned to normal. After handover from ZTE 28722to ZTE 24122, testers still couldn’t hear each other.

Typical case- Bidirectional silence due to A interface circuit fault Analysis of test phenomena: Afterwards, test of handover during calls was made between cells under ZTE BSC9, but the problem mentioned above didn’t happen. Test summary: The call was normal at the beginning (both parties could hear each other). When inter-BSC handover occurred, two-way silence emerged immediately;; when inter-BSC handover happened again, the call returned to normal. Inter-cell handover within one BSC wouldn’t bring such problem. Because inter-BSC handover involves the participation of MSC and the circuit exchange connection at A interface, engineers suspected that there was something wrong with A interface circuit or MSC.

Typical case- Bidirectional silence due to A interface circuit fault Found problem Confirmed single board fault, check the board and wire connection 2 pairs of PCM were detected selflooped, amend the mistake Continued call test Problem disappeared Kept tracing test phone Call test engineer MSC engineer Made call test to catch problem Kept tracing test phone Problem solved Flow chart of problem handling The root cause of the problem is maintenance people’s incorrect manipulation (during expansion and swap of BSC A interface).

Contents Monologue Cross-talk Bidirectional Silence Poor Speech Quality Echo

Poor call quality－Causes & handling of problem Poor speech quality is mainly caused by the high speech error rate at radio interface. The phenomenon of poor speech quality is: both MOC and MOC can get through, but the speech quality is too bad, and there is clear noise in calls. Methods of handling poor call quality Causes of poor call quality First locate the problem area, make certain if the problem is prevalent in most sites or it just appears in some sites; If the problem is prevalent, we can refer to cause 2, 4 and 5, and make investigation accordingly; If the problem just exists in some sites, we can refer to cause 1 and 3, and make investigation accordingly. Low receive level; internal or external interference; BTS hardware fault, including clock, TRX, and antenna, etc.; and for BTS (V2.0) equipment, if the toggle switch of the matched resistance of E1 on CMM single board isn’t set correct, speech quality will be affected; BSC EDRT single board fault; Some timeslots of the transmission circuit adopted are not good.

Typical case- Poor call quality due to EDRT fault Problem description Subscriber’s complaint about poor speech quality in a swapped network gradually increased after Spring Festival. Problem analysis Subscribers who lodged complaint were scattered in different counties, which meant that the problem was prevalent in most BTSs. Phenomena of on-site test: When dialing or receiving calls, the party at the BTS area could hear the counterparty, but couldn’t be heard. This meant that there was something wrong with the BTS UL signal, which was reflected in on-and-off speech, trembling voice accompanied with high current hum or background noise.

Typical case- Poor call quality due to EDRT fault Result of primary analysis: The problem was caused by BSC hardware fault, like EDRT board fault, etc.. Problem handling: Made call test to all DSP of each EDRT in the BSC, found out and replaced the faulty EDRT, the problem was solved. Problem location: Faulty DSP chips of BSC EDRT single boards resulted in disorder in code conversion and rate adaptation and poor speech quality.

Typical case- Poor call quality due to clock cable fault Problem description An operator reported subscriber’s complaint about poor speech quality in the serving area of a site, metallic noise appeared in calls. Problem analysis Since the problem subscriber complained was in a single site area, engineers tentatively confirmed the cause was BTS fault. Problem handling During the call test metallic knocking suddenly appeared and it lasted for 30 seconds and then disappeared. In this process, the MS remained busy. Engineers carried out call test on TRXs one by one, and finally found one faulty TRX. While the problem didn’t disappear even after the faulty TRX was replaced. When clock cable of the TRX was replaced, the problem disappeared.

Typical case- Poor call quality due to DTX fault Problem description An operator reported that in the area of a site near the capital city, background noise often emerged during calls, which has affected normal communication. Problem analysis No BTS hardware fault: in test with YBT250, no UL interference was detected. Result of call test: when both MOC and MTC were under the site, the two parties could hear the same noise. When making calls with PSTN, only the PSTN terminal could hear the noise, it was normal with MS side. At V1 sites, no noise appeared. While the same problem happened to calls at V2 sites. Summary: Cause of background noise should be wrong data configuration or discrepancy in BTS or BSC versions. Problem handling After blocking UL power control and UL DTX of V2 site in OMCR, no background noise was detected. BTS V2 and BS30 (12M) could lead to the appearance of background noise. In the future version 12O, this kind of problem will be solved.

Contents Monologue Cross-talk Bidirectional Silence Poor Speech Quality Echo

Echo problem－Phenomena Two types of echo: Generally speaking, echo means one party can hear his/her own voice apart from the counterparty’s voice during calls between digital MSs or between digital MS and landline phone. Another kind of echo-the loopback of voice means one party can hear himself/herself but not the counterparty’s voice, while the counterparty can not hear anything during calls between digital MSs or between digital MS and landline phone.

Echo problem — Causes of echo Causes of echo problem MS isolation performance isn’t up to standard in protocol Echo in calls between MSs Echo in calls between MS and landline phone Echo canceller isn’t added. Wrong connection of A interface trunk, hardware self-looped. Voice loopback

Echo problem — Problem handling flow Voice loopback Judge echo type Calls between MSs Calls between MS and landline phone Judge echo type from customer feedback and call test.

Echo problem — Problem handling flow Calls between MS and landline phone Judge echo type Calls between MSs Voice loopback Collect the MS type; Use speech monitor; Judge MS isolation performance, export explanatory document.

Echo problem — Problem handling flow Calls between MS and landline phone Judge echo type Calls between MSs Voice loopback focus on finding out route data of the calls and make certain that the data are correct; when route data are configured right, check the EC configuration of mobile network equipment to make sure the EC is correct.; If the problem occurs during inter-office calls, we need to check if there are crossed-wires in outgoing office circuits.

Echo problem — Problem handling flow Calls between MS and landline phone Judge echo type Calls between MSs Voice loopback Find out MSC call list according to number and time of call; Make call test to all intra-office and inter-office trunks one by one; Check the faulty trunk circuit according to CIC.

Typical case- Echo due to self-looped A interface trunk Problem description Subscribers at an area complained about echo problem in calls. 。 Problem analysis After checking large number of complaint letters and performing call tests, it’s certain the problem was about voice loopback. One party could only his/her own voice, but the counterparty’s voice; the counterparty could hear nothing. The problem only appeared under MSC 04.

Typical case- Echo due to self-looped A interface trunk Solution to problem: A group of engineers made call test on site to recreate the problem; A group of MSC engineers traced test phone. Problem location: voice Loopback appeared under MSC04 Start Figured out corresponding single board according to the A interface trunk CIC taken by MS, checked relevant single board and wire connection, finding PCM was self-looped. End Handled problem: adjusted connection mistake. Problem disappeared in retest.

Questions for thinking What is the relation between call quality and MOS value? Please explain the common flow for handling monologue problem.