Presentation on Troubleshooting and Management in GSM networks

Troubleshooting & Management

In GSM Networks

The Idea of Unbounded CommunicationCommunication everywhere, with everybody, and at any time – that was the dream and goal Of researchers, engineers and users, since the advent of the first wireless communication systems. Today it feels like we have almost reached that goal. Digitalization of communication systems, enormous progress in microelectronics, computers and software technology, the invention of efficient algorithms and procedures for compression, security and processing of all kinds of signals, as well as the development of flexible communication protocols have all played an important role for this progress. Today, technologies are available that enable the realization of high-performance and cost-effective communication systems for many application areas.

Last few years have witnessed a phenomenal growth in the wireless industry, both in terms of mobile technology and subscribers. Mobile network operators and vendors have recognized the importance of efficient networks with equally efficient design processes.

This has resulted in specialized need for monitoring and troubleshooting services related to network Management came into sharp focus.

A Brief Introduction of GSM Network Architecture & Technology

Before we go into further detail of different aspects that are applied in monitoring and Troubleshooting GSM Networks. We would take a brief look at the Architecture of GSM Network and technological aspects that are applied in the network.

The Next few slides will focus on that particular aspect.

GSM Architecture Overview

Conti…

Elements of GSM Network A GSM network is made up of three subsystems:

• The Mobile Station (MS)

• The Base Station Sub-system (BSS) – comprising a BSC and several BTSs • The Network and Switching Sub-system (NSS) – comprising an MSC and associated registers

Several interfaces are defined between different parts of the system: • 'A' interface between MSC and BSC • 'Abis' interface between BSC and BTS • 'Um' air interface between the BTS (antenna) and the MS.

Abbreviations:

MSC – Mobile Switching Centre BSS – Base Station Sub-system

BSC – Base Station Controller HLR – Home Location Register

BTS – Base Transceiver Station VLR – Visitor Location Register

TRX – Transceiver AuC – Authentication Centre

MS – Mobile Station EIR – Equipment Identity Register

OMC – Operations and Maintenance Centre PSTN – Public Switched Telephone Network

The Mobile Station (MS)

The mobile station consists of:

• Mobile equipment (ME)

• Subscriber Identity Module (SIM).

• The SIM stores permanent and temporary data about the mobile, the subscriber and the network, including:

• The International Mobile Subscribers Identity (IMSI)

• MS ISDN number of subscriber

• Authentication key and algorithms for authentication check.

• The mobile equipment has a unique International Mobile

Equipment Identity (IMEI), which is used by the EIR.

The Base Station Sub System (BSS)

The BSS comprises:• Base Transceiver Station (BTS)

• One or more Base Station Controllers (BSC).

BTS contains: • Radio Transmitter/Receiver (TRX).

• Signal processing and control equipment.

• Antennas and feeder cables.

The purpose of the BTS is to:• Provide radio access to the mobile stations

• Manage the radio access aspects of the system

The Purpose of the BSC is to:• Allocates a channel for the duration of a call.

• Maintains the call.

• Monitoring quality.

• Controlling the power transmitted by the BTS or MS.

• Generating a handover to another cell when required.

Network Switching Subsystem (NSS)

The network subsystem acts as an interface between the GSM network and the public networks, PSTN/ISDN. The main components of the NSS are MSC, HLR, VLR, AUC, and EIR.

Multiple Access Scheme Technology In GSM Networks

• The objective of a cellular radio operation is to provide a service where mobile subscribers can communicate with any subscriber in the PSTN, where any subscriber in the PSTN can communicate with any mobile subscriber, and where mobile subscribers can communicate among themselves via the cellular radio system. In all cases the service is full duplex. Ideally, for full-duplex service, a portion of the bandwidth is assigned for transmission from a cell site to mobile subscriber, and another portion is assigned for transmission from a mobile user to a cell site.

• In the next slide we will look into the bandwidth allocations assigned to GSM Network.

GSM uses Paired Radio Channels

UPLINK

DOWNLINK

0 124 0 124

Feature /Bandwidth GSM 900 GSM 1800

Uplink (MHz) 890–915 1710–1785Downlink (MHz) 935–960 1805–1880

Number of available channels

124 374

Multiple Access Mechanism

There are three main types of Access mechanisms that are applied in Cellular Networks specially Networks comprising of 1st and 2nd Generation.

• Frequency Division Multiple Access (FDMA).• Time Division Multiple Access (TDMA).• Frequency and Time Division Multiple Access.

Frequency Division multiplex Access

• Separation of the whole spectrum into smaller frequency bands• A channel gets a certain band of the

spectrum for the whole time.• Advantages:

– no dynamic coordination necessary.

– works also for analog signals.• Disadvantages:

– waste of bandwidth guard spaces

– Prone to frequency

Interference.

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k2 k3 k4 k5 k6k1

Time Division Multiple Access(TDMA)

• A channel gets the whole spectrum for a certain amount of time

• Advantages:– only one carrier in the

medium at any time.– throughput high even

for many users.

• Disadvantages:– precise

synchronization necessary.

f

Time and Frequency Multiplex

• Combination of both methods• A channel gets a certain frequency band for a certain

amount of time

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k2 k3 k4 k5 k6k1

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f

Time and Frequency Multiplex

• Example: GSM • Advantages:

– Better protection against tapping.

– Protection against frequency selective interference.

– Higher data rates compared tocode multiplex.

• But: precise coordinationrequired

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c

k2 k3 k4 k5 k6k1

GSM combines FDMA and TDMA Bandwidth is subdivided into channels of 200khz, shared by up to eight stations,

assigning slots for transmission on demand.

GSM Frame Structure

Normal Burst

Flag Bits Tail BitsTail Bits

ONE FRAME duration 4.615 m Sec

577 microseconds

FRAME 1 FRAME 2 FRAME 3

Guard Period Guard Period

Key Performance Indicators (KPI’s)

Its Significance and Importance in Troubleshooting and optimizing Network

Key Performance IndicatorsWhen we discuss about active monitoring of the GSM Network we cannot neglect the importance of Indicators.

Definition:

A key performance indicator is a value based on one or more observation counters and that is representative either of system performance (BSS) or subscriber’s point of view. Counters:

Counters are the record of events that took place during initiation, failure or cancellation of any event that took place in TRX, Cell and BSC Level.

Key Performance Indicator (KPI)

Call Setup Success RateIts Importance in monitoring and Troubleshooting in GSM Network

Call Setup Success Rate

Definition:

Call Setup Success Rate is define as the ability of a cell/network to provide traffic channel to call originating mobiles. Any failure in the assignment phase is charged to the CSSR.

Formula:CSSR= No. of TCH Assign/No. of Channel requests Speech

Factors That Effect CSSR and its Troubleshooting

List of Factors that Effect CSSR.• Faulty Radio (Transceiver)• Faulty Combiner/ jumper/Cable.• High rate of interference either Co-Channel or Adjacent Channel.• General increase in traffic beyond cell capacity resulting in

congestion and Degraded CSSR.

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Factors That Effect CSSR• Faulty TransceiverAs its name suggests Transceiver is a hardware equipment which is responsible for transmission & receiving of signal b/w

Mobile and Bts. It is connected to Antenna network combiner and Antenna via transmission cables.

Analysis:• Call efficiency on a particular transceiver is highly degraded.• Increase in Call drop observed with the degradation in CCSR.• Increase in Path loss observed on Transceiver level > 5 dB.

Troubleshooting Faulty Transceiver

• On Analyzing Transceiver level stats we come up with the following results.

• As can be seen from above table that call efficiency got degraded during last days in the month of September.

BSC_NAME CELL_NAMETRX_

IDDATE

IncHO Attempts

CallSetup Attempts

Call Efficiency

Radio_Drops(Number)

Drop Handover

Moro DKM5844E 2 25-Sep-10 418 2518 99.56314535 7 7

Moro DKM5844E 2 26-Sep-10 822 5584 99.51647564 36 17

Moro DKM5844E 2 27-Sep-10 593 3271 99.60256802 10 9

Moro DKM5844E 2 28-Sep-10 875 6521 99.646096 18 13

Moro DKM5844E 2 29-Sep-10 1443 7889 77.89326911 56 41

Moro DKM5844E 2 30-Sep-10 320 2069 75.10874819 61 45

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Troubleshooting Faulty Transceiver

At this stage we have successfully analyze the problem. Now the step needs to be taken to rectify the problem.• The first action taken by operations and maintenance radio is to reset

the Transceiver remotely because some times Transceiver got stuck/hang due to its software related issues. We can take the same example as some times our computer got stuck/hang due to the same reason so we would reset it in order to make it operational again.

• If the problem persists the best possible solution is to replace the effected Transceiver as soon as possible.

• If it is not possible to replace the Transceiver timely due to the fact that site lies in a very remote area of which physical Access to site is not easy. Then the effected Transceiver needs to be locked to avoid further degradation in Call Setup Success Rate.

Factors That Effect CSSR• Faulty CombinerAs its name suggests Combiner is a hardware equipment which is responsible for coupling transmission & receiving signals of different TRX’s connected to it.

At one end it is connected to set of TRX’s and at other end it is connected to antenna via feeder cables.

Analysis:• Call efficiency on all transceivers connected to cell is highly

degraded.• Increase in Call drop observed with the degradation in CCSR

for whole cell.• At O&M center VSWR (Voltage Standing Wave Ratio) alarm is

appearing on cell Level.

Troubleshooting Faulty Combiner• On Analyzing Transceiver level stats we come up with the following

results.

• As can be seen from above table that both TRX’s Call efficiency got degraded during last days in the month of September.

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BSC_NAME CELL_NAME TRX_ID DATE IncHO Attempts

CallSetup Attempts Call Efficiency Radio_Drops(

No)Drop

HandoverMoro MOR6843A 1 25-Sep-10 718 2518 99.563145 7 7

Moro MOR6843A 2 25-Sep-10 418 2518 99.563145 7 7

Moro MOR6843A 1 26-Sep-10 934 5584 99.516476 36 17

Moro MOR6843A 2 26-Sep-10 822 5584 99.516476 36 17

Moro MOR6843A 1 27-Sep-10 593 3271 99.602568 10 9

Moro MOR6843A 2 27-Sep-10 593 3271 99.602568 10 9Moro MOR6843A 1 28-Sep-10 875 6521 99.646096 18 13Moro MOR6843A 2 28-Sep-10 875 6521 99.646096 18 13

Moro MOR6843A 1 29-Sep-10 1443 7889 77.893269 56 41

Moro MOR6843A 2 29-Sep-10 1565 1082 53.529957 680 539

Moro MOR6843A 1 30-Sep-10 320 1069 75.108748 61 45

Moro MOR6843A 2 30-Sep-10 1934 2167 41.974439 952 601

Troubleshooting Faulty Combiner At this stage we have successfully analyze the problem. Now the step needs to be taken to rectify the problem.• The first action taken by operations and maintenance radio is to reset the

Combiner remotely because some times combiner got stuck/hang due to its software related issues. We can take the same example as some times our computer got stuck/hang due to the same reason so we would reset it in order to make it operational again.

• If the problem persist the best possible solution is to replace the effected Combiner as soon as possible.

• The important thing while dealing with combiner related problem is that the degradation that it cause is far more critical than with the single Transceiver having low efficiency because the problem in combiner would effect the whole Cell Key Performance Indicators rather than single Transceiver level Stats. The other possible solution is to avoid further degradation it is suggested to lock the affected Combiner but the drawback is that it would lock the whole cell/sector because the combiner acts as interface between cell/Transceiver. So the only possible solution is to replace the combiner as soon as possible.

Factors That Effect CSSR• High rate of interference either Co-Channel or

Adjacent Channel Channel Interference also plays an important role in degrading Call Setup Success Rate. As mentioned earlier interference are of two types Co-Channel and Adjacent Channel of which Co-channel interference effects highly our concerned cell as compared to adjacent Channel interference.• Co-Channel Interference:

Interference caused by second wireless station which tuned on the same frequency.

• Adjacent Channel Interference:Adjacent-channel interference or is a interference caused by extraneous power from a signal in an adjacent channel.

Analysis for Investigating Co and Adjacent Channel Interference

There are number of methods been applied while investigating Interference.• Degradation in level and Quality observed both in uplink and downlink.• High Rate of Quality and Level Handovers observed specially in downlink

direction.• On Transceiver level the effected assigned interfered frequency Transceiver

shows High rate of call setup failures, High rate of drop calls and Handover Failures.

• In case of interference the interferer cell shows same level of degradation as our concerned cell it can be further confirmed by observing interferer cell stats as well.

• It can be further confirmed by conducting Drive Test.

Troubleshooting Interference Issue

The only and best solution is to perform frequency Audit with the most updated frequency plan available. While assigning different frequency it should be keep under consideration that the new assign frequency does not have Co and Adjacent Channel interference to the existing assign neighboring cell frequencies.

Factors That Effect CSSR

Degraded CSSR due to General Increase in Traffic beyond Cell Capacity.

Analysis:• High “TCH Congestion rate”.• Increase in TCH Assignment Preparation Failures.• High “Directed Retry Rate” if activated.• No neighboring site/cell was found down or in-operative.• No down or inoperative Transceivers found in concerned cell

or adjacent cell/site.• Significant Increase in traffic observed while comparing from

previous days.

Troubleshooting degraded CSSR due to Increase in Traffic

• Firstly Check on Transceiver level that Half Rate is enabled on all Corresponding Transceivers connected to the cell because depending on traffic behavior of the cell half rate been sometimes disabled.

• On Concentric Cells make sure that Traffic is equally distributed in inner (DCS) and outer (GSM) Transceivers equally.

• Cell level parameter called Traffic load Handover can set true to avoid degradation in CSSR.

• The best possible solution is to add new Dual rate Transceivers to accommodate high increase in traffic which is called Expansion in the cell.


Call Drop RateIts Importance in monitoring and

Troubleshooting GSM Network

Call Drop Rate

Definition:

The CDR is logically defined as the ratio between the TCH drops occurring during the conversation phase to the number of successful seizures on the cell or area.Formula:Number of TCH drops after assignment/Total no. of TCH Successful assignments.

Factors That Effect CDR and its Troubleshooting

The Call Drop Rate, as a KPI, has a great impact on customer service. Therefore, it must be traced in routine network optimization. If a cell is having CDR (above 1%), the cell is classified as a cell that needs to be carefully investigated and successfully optimized.

List of Factors that Effect Call Drop Rate (CDR)• Low signaling Level (Lack of coverage).• Sector/Feeder Swap in between Adjacent Cells of Site.• Overshooting Cells.

Factors That Effect CDR

Low signaling Level (Lack of coverage)Lack of coverage plays a vital role in increasing Call drop rate. It can be due to coverage holes in a remote region with no dominant server or it can be in an urban area where signal tends to weaken due to entering in premises like tunnel, underground subway and in some cases a very thick walled concrete

building.

Analysis:• Degradation in signal observed with increase in Bad Rx Level and Rx Qual.• When investigating call level trends it was found that call duration was

significantly low.• On Air interface frame-level high rate of SACCH frames got corrupted.• On conducting drive it was found average level of a call was found to be

above -95 dBm which follows with the degraded voice quality.

Troubleshooting low Signal Level

When Troubleshooting the cases facing with low signal level the best possible solution is to commissioned new Sites/cells to overcome coverage issues in regions specially remote . When dealing with urban area environment the possible solution is to add micro sites in an area facing with coverage issues like mentioning Subways, tunnels and buildings with thick concrete and metal structure. Multilayer environment in which introducing dual band frequencies also improves the drop call rate of the area.

Sector/Feeder Swap in between Adjacent Cells of Site

As the name suggests, this happens when the feeder cables of two different sectors are completely crossed, which in turn leads to the fact that the coverage areas of the two adjacent cells are swapped. Drive tester may observe a lot of HO failures and call drops.

A better understanding can be done while observing coverage level b/w Swap Sectors E and F of Site NFZ0378 in next slide.

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As can be observed from above snap that on the coverage area of sector F of NFZ0378 its adjacent cell NFZ0378 of Sector E is serving whereas same condition applies for Sector E where NFZ0378F is serving instead of Sector E of NFZ0378.

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Troubleshooting Sector Swap Issue

After confirming Sector Swap issue through drive results the mentioned case was escalated to operations departments to physically intervene the site and resolve issue. In the mean while when team was attempting to visit the site if the KPIs become highly degraded. It was highly suggested to lock the affected cells to avoid further degradation.

Over Shooting Cell This is a very important problem that’s need be addressed carefully and efficiently to counter increasing Drop Call Radio issue in a cell which does not face any coverage related issues, Hardware Failures, Down Sites/Cells and no External interference. The cause of this problem arises when a dominant cell provides a lot of scattered coverage’s inside other cells territory, due to propagation conditions of the radio waves. The consequence of this spurious coverage’s is the probable production of a high level of interference.

It may happen for example that some peculiar propagation conditions exist at one point in time that provide exceptional quality and level although the serving BTS is far and another is closer and should be the one the mobile should be connected to if the conditions were normal.

It may then happen that these exceptional conditions suddenly drop and the link is lost, which would not have happened if the mobile had been connected to the closest cell and therefore call drops suddenly.

Analysis for Investigating Degraded CDR of Over Shooting Cell

• No degradation observed on Transceivers Efficiency connected to the concerned cell which follows with no Hardware Failures combiner, splitter etc. No Down Sites/Cells either Adjacent or neighbour.

• High increase in drop radio due to sudden drop in radio conditions observed on cell level.

• It can be further confirmed by observing TA Samples distribution report. Whereas TA stands for Timing Advance. TA is defined as the distance from Mobile Station to Base Transceiver Station. It is been instructed to MS by BSS to send TA measurements reports accordingly so as to avoid radio end propagation delays.

• It can be further proved by conducting Drive Test on that area.

Troubleshooting Over Shooting Cell problem

To avoid further degradation in Call Drop Rate we can reduce Timing advance values so now the BTS can only camp on to the Mobile Stations which are lying in a dominant coverage area of the serving BTS. Therefore by decreasing the upper timing advance values the overshooting cell after changing Timing advance value just does not allow an MS to talk to a BTS if it is too far away with no effect on loss of coverage.


Handover Success RateIts Importance in monitoring and

Troubleshooting GSM Network

Handover Success RateDefinition:

Handover is define as Switching over of a Mobile Station from one BTS cell to other one in dedicated mode without call drop or subscriber irritation.Formula:(NB_OUT_EXT_HO_SUCC+NB_OUT_INT_HO_SUC

C)/ (NB_OUT_EXT_HO_REQ+NB_OUT_INT_HO_REQ)

As mentioned in its formula the Handover success rate is a combination of two important values that is number of outgoing handover success rate to the neighbour cells lies in the boundaries of other BSC and number of outgoing success rate to the cells lies within the boundaries of the same BSC. Divided by combination of both Handover requests External (Requests generated for those cells lies in different BSC) and Internal (Requests generated for those cells lies in same BSC). Therefore any degradation in any of the values would result in degrading the HSR value.

Factors That Effect HSR and its Troubleshooting

The process of handover within any cellular system is of great importance. It is a critical process and if performed incorrectly handover can result in the loss of the call. Dropped calls are particularly annoying to users and if the number of dropped calls rises, customer dissatisfaction increases and they are likely to change to another network.

List of Factors that Effect Handover Success Rate(HSR)

• Missing Adjacencies/Neighbors.• Non Symmetrical (One Way Neighbors)• Degradation in HSR due to Sector/Feeder Swap in between

Adjacent Cells of a Single Site.

Factors That Effect CSSR

Missing Adjacencies/Neighbors. Missing Adjacencies/Neighbors is considered as common issue while monitoring network on day to day basis in which the problem arises of which serving Site/Cell neighbors are not properly assigned. By assigning neighbors it means that certain adjacencies should be defined at OMC in order to carry out successful handover. Correct adjacency definitions are the basic requirement for mobility. Optimization of neighbour cell lists saves BS and MS transmission powers, since MSs are connected to optimal cells. Also, the number of dropped calls is reduced.

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Missing Adjacencies/Neighbors

As can be observed from above snap that our concerned cell shown in red is defined neighbors/adjacencies to the cells shown in blue so whenever there is a requirement for handover generated the call can easily be transferred to a defined neighbor cell shown in blue on the basis of level or quality but what would happen if a particular relation not defined like consider the example shown above our concerned cell shown in red try to initiate outgoing Handover to its immediate neighbor shown in yellow but was unable to initiate handover simply because it was not defined a potential candidate at OMC end to accept Handover which in turn results in high rate of HO failures between the adjacencies resulting in increase in call drop of the serving cell.

Troubleshooting Missing Adjacencies/Neighbors

• The best possible way to troubleshoot Handover failures due to missing neighbor is to perform visual neighbor audit and add any missing neighbor on both ways which were found during neighbor audit.

Non Symmetrical (One Way Neighbors)

• Non Symmetrical neighbors are those neighbors which were defined at OMC end in one direction only which means that our serving cell can successfully handover its call to adjacent neighbor successfully but at the same time not been able to accept any handover request from its adjacent neighbour. A better understanding can be done by observing the following snap.

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Non Symmetrical (One Way Neighbors)

As can be seen from above snap that our serving cell NRP6361A is defined one way neighbor to its Adjacent Neighbor cell NRP6360A at OMC end that means any subscriber camp on NRP6361A will successfully execute out going Handover to its adjacent cell NRP6360A but on the other hand NRP6361A cannot allow any reception of Handover from NRP6360A because it not defined as a potential neighbor for NRP6361A to receive incoming request from NRP6360A. This kind of situation would result increase in Incoming HO Failures to NRP6361A and Out Going HO Failures to NRP6360A which cause general degradation on both cells.

Troubleshooting Non Symmetrical (One Way Neighbors)

• The best possible way to troubleshoot Handover failures due to Non Symmetrical (One Way Neighbors) is to check neighbor level stats of which it was found that b/w two adjacencies requests are generating on one way afterwards perform visual neighbor audit and add any missing neighbor on both ways which were found during neighbor audit.


As the name suggests, this happens when the feeder cables of two different sectors are completely crossed, which in turn leads to the fact that the coverage areas of the two adjacent cells are swapped. Drive tester may observe a lot of HO failures and call drops.

A better understanding can be done while observing coverage level b/w Swap Sectors E and F of Site NFZ0378 in next slide.

Continue…


As can be observed from above snap that on the coverage area of sector F of NFZ0378 its adjacent cell NFZ0378 of Sector E is serving whereas same condition applies for Sector E where NFZ0378F is serving instead of Sector E of NFZ0378.

Continue…

Troubleshooting Sector Swap Issue

After confirming Sector Swap issue through drive results the mentioned case was escalated to operations departments to physically intervene the site and resolve issue. In the mean while when team was attempting to visit the site if the KPIs become highly degraded. It was highly suggested to lock the affected cells to avoid further degradation.

Analysis for Investigating Degraded CDR of Over Shooting Cell

• No degradation observed on Transceivers Efficiency connected to the concerned cell which follows with no Hardware Failures combiner, splitter etc. No Down Sites/Cells either Adjacent or neighbour.

• High increase in drop radio due to sudden drop in radio conditions observed on cell level.

• It can be further confirmed by observing TA Samples distribution report. Whereas TA stands for Timing Advance. TA is defined as the distance from Mobile Station to Base Transceiver Station. It is been instructed to MS by BSS to send TA measurements reports accordingly so as to avoid radio end propagation delays.

• It can be further proved by conducting Drive Test on that area.

Thank you