• GSM Services

Objectives

On completion of this module you will be able to ...

• Explain what a service is in GSM.
• Classify and explain the various types of service.
• Describe the services' technical functionality from the network operators' and the customers' point of view.

Content

2.1	Classification
2.2	Bearer Services
2.3	Teleservices
2.4	Supplementary Services
2.4.1	Line Identification Services
2.4.2	Call Offering Services
2.4.3	Call Completion Services
2.4.4	Multi Party Supplementary Services
2.4.5	Closed User Groups (CUG)
2.4.6	Advice of Charge
2.4.7	Call Restriction Services
2.5	Value Added Services

2.1 Classification

The GSM services subdivide into the Bearer Services and the Teleservices. These base services are supplemented by the Supplementary Services. In addition, we have the Value Added Services.

The Bearer Services are telecommunication services that guarantee the transmission of signals between access points in telecommunication networks. They are used exclusively for transport and define the services a network can offer, for example data transmission between a corporate network and a mobile notebook, or data download from the web.

The Teleservices are telecommunication services as well as functions that enable communication between users, and are based on protocols agreed on by the network operators. Teleservices affect the final link in the communication process - the end user. They include telephone services, emergency calls, the Short Message Service, E-mail and fax.

Supplementary Services modify or add to the basic telecommunication services. They are only offered in combination with a Bearer Service or a Tele service. The same Supplementary Service can be applied to a range of Telecommunication Services. Most of these services have been inherited directly from fixed networks, with some concessions as regards their adjustment to the mobile radio network. Supplementary Services include e.g. line identification and call forwarding. Value-Added Services depend on the service provider or network operator they are supplied by. They are not predefined in specifications, and include e.g. reservation or information services.

Let's have a closer look at the services

2.2 Bearer Services

GSM specifies a huge variety of Bearer Services for data transmission. Bit rates between 300 and 9,600bps can be chosen according to data terminal equipment and the interference situation of the air interface. With HSCSD, the data rate may increase to 57.6 kbps.

All Bearer Services are handled as circuit-switched services between the mobile data equipment and the NSS requiring a permanent link between the involved components.

A converter, called Packet Assembler-Dis-assembler or PAD, is used to adapt these signals to the packet-switched mode in public data networks, e.g. the Internet. With GPRS, a total packet-switched transmission is provided between all data terminal equipment at a maximum bit rate of 171.2 kbps.

2.3 Teleservices

Teleservices provide voice and non-voice services. Voice services include normal speech telephony and emergency calls. Non-voice services include the Short Message Service, for example, that is the sending, receipt and broadcast of short messages.

Short messages can be received at any time, even during a call, and are transmitted over signalling channels. Whereas normal short messages are stored temporarily in the Short Message Service Center SMSC, with Cell Broadcast they are stored in the Base Station Controller BSC.

The fax services, also referred to as Group 3 facsimile, provide automatic fax service between a mobile fax and a fax machine within a PSTN, or between 2 mobile fax terminals, each with its own MSISDN at a maximum speed of 9.6 kbps.

2.4 Supplementary Services

Supplementary Services can be controlled by the subscriber as well as by the network. There are two groups of services.

First, there are those services which are provided by the service provider and made available to a customer.

Then there are the services which are invoked by a customer. The desired service is, for example, activated by a customer pressing a particular key, as is the case with line identification.

The supplementary services subdivide into several types.

2.4.1 Line Identification Services

The Line Identification Services include the Calling Line Identification Presentation (CLIP), that is the display of the caller's number on the called party's mobile station. The network transmits the call number at call set-up, provided that both parties are ISDN or GSM customers.

Using Calling Line Identification Restriction (CLIR), the calling party can restrict its call number, so it cannot be identified by the called party. An appropriate instruction is sent from the caller's network to the network of the recipient. Some networks allow particular user groups, such as the police, to identify a call number even if the CLIR service is activated.

In accordance with CLIP and CLIR, there is the Connected Line Identification Presentation, COLP, for identification of the called party by the caller, and Connected Line Identification Restriction, COLR, if the called party does not want to be identified by the caller. COLP and COLR come into play when the called party has diverted its number to another number.

2.4.2 Call Offering Services

Call Offering Services are services which allow a customer to forward incoming calls to other target numbers.

Call Forwarding Unconditional CFU ensures that all incoming calls are diverted to a number specified by the customer, independent of the conditions of the call.

Call Forwarding on mobile Subscriber Busy CFB diverts calls only if the called subscriber is making a call and the line is busy. Calls can also be diverted if the called party can't be reached - because it currently has no network access, because the phone is switched off, or because the call is not accepted.

Supported by the Optimal Routing Service SOR - a GSM Phase 2+ feature - calls are routed directly to the subscriber's current location. This service is based on the information provided by the called subscriber's home network. As an option, the network may permit or deny SOR on a per call basis. SOR has not yet been implemented.

2.4.3 Call Completion Services

Call Completion Services allow the customer to postpone incoming calls which he is currently not able to accept, as well as to hold calls while simultaneously making another call.

Call Waiting CW notifies the customer of an incoming call when no traffic channel is available, and the customer is engaged in an active or held call. The customer now has from half a minute to two minutes time to decide whether to accept, reject or ignore the postponed call.

The Call Holding Service HOLD permits the customer to interrupt communication on the active call for another call, without losing the connection to the first call. This is because the traffic channel remains assigned to the customer even after interruption of the active call, so he can resume the communication.

2.4.4 Multi Party Supplementary Services

Multi Party Services, also called conferencing services, allow the customer to communicate with more than one party simultaneously. The Multi Party Service can be initiated if a customer is in control of at least one active call and one call on hold. After it has been initiated, parties an be added, disconnected or separated. Thus the subscriber initiating the service can add more parties to the Multiparty Call and exclude others from conversation. At the same time, he can have a private conversation with another party, which the other participants cannot hear. During all this, the participants of the Multiparty Call can still communicate with each other.

2.4.5 Closed User Groups (CUG)

Closed User Group (CUG) gives a particular group of customers the opportunity to communicate with each other in PLMN or ISDN networks. The members of a Closed User Group can't communicate with subscribers outside the group. If desired, one or more subscribers in this group can be authorised to call subscribers outside, or to receive calls from them.

Emergency calls are not subject to the restrictions of this service.

2.4.6 Advice of Charge

The service Advice of Charge (AOC) is a supplementary service that allows a mobile subscriber who uses services to receive service-related billing details.

With Reverse Charging, on the other hand, a called subscriber can be charged for certain calls. For this purpose, each individual call is analyzed and evaluated. Reverse Charging is activated at request by the calling subscriber, and must be permitted by the called subscriber. At present, Reverse Charging is not implemented in GSM networks.

2.4.7 Call Restriction Services

There are basically two categories of Call Restriction Services: one refers to outgoing calls, and the other to incoming calls. The service offers several possibilities, such as:

• Barring all outgoing and incoming calls
• Barring all outgoing international calls

or

• Barring all incoming calls except those coming from the country of the home network.

The calls can be barred manually by the subscriber, or by network configuration.

2.5 Value Added Services

Value Added Services are supplied by the respective Service Provider or network operator, and can be transmitted either via a normal telephone call or via SMS. These services offer the supplier a further opportunity - apart from price - to attract customers.

• Value Added Services are, for example:
• Information on cultural events or traffic announcements via Cell Broadcast
• Reservation of a hotel, a flight or a hire car
• Help in the event of a car breakdown
• Information services
• Medical advice
• Ticket reservation
• Sports news
• Delivery services, like sending flowers to a particular person and many more.

• GSM Essentials

Content

1.1	Development
1.2	GSM Today
1.3	Evolution from GSM to the 3rd Generation
1.3.1	HSCSD
1.3.2	GPRS
1.3.3	EDGE
1.3.4	UMTS Objectives On completion of this module you will be able to ... Explain the basic functionality of the GSM network. Judge the global importance of GSM as a digital mobile radio standard. List the technologies and standards of 3rd generation mobile radio and outline their essential features. 1.1  Development By 1979, several mobile radio networks already existed worldwide. They operated on different frequency bands and were based on different standards. A mobile phone user in Britain could not use his mobile phone in Sweden, because the British and the Swedish mobile radio networks were each based on their own standards, with different signaling protocols, and were incompatible with each other. In view of the European unification process, it became clear that mobile telephony, too, had to overcome national borders. A new European standard was due! For this purpose, the "Conference Europeans des Administrations des Pastes et des Telecommunications” established a workgroup, whose task was to work out specifications for a standard Western European mobile system. This was the Group Special Mobile, GSM, after which the new standard - the Global System for Mobile Communications - was named. In 1988, the European Telecommunications Standards Institute (ETSI) was founded. Its task was to work out the GSM standard for a digital radio telephone network. In the GSM 900 standard, a frequency range between 890 and 915 MHz was assigned to the uplink, and a range between 935 and 960 MHz was assigned to the downlink. In GSM 1800, the frequency ranges 1710 - 1785 MHz were added in the uplink and 1805 - 1880 MHz in the downlink. In 1995, GSM 1900, with its own frequency range from 1850 to 1910 MHz in the uplink and 1930 to 1990 MHz in the downlink, was implemented in America. At the end of 1996, there were already 120 GSM networks in operation, and within the year 2000 there were already 150 million GSM users all over the world. The GSM specification had to consider the following: The system had to Function Europe-wide Provide a high speech quality Use the available frequencies as efficiently as possible Provide ISDN service characteristics Be compatible with other ways of data transmission and Meet high security requirements as far as the customers and the transmitted information were concerned. Many of these conditions have been turned into advantages for the users of today: The available frequencies are used efficiently. The average speech quality is higher than in analog mobile networks. Speech encryption guarantees the security standards. A wider range of services is provided compared to analog cell networks, like voice-, fax-, data- or Internet services. International roaming between all the GSM countries is possible. Furthermore, international competition reduces the prices. 1.2  GSM Today Today, GSM is the first digital cellular mobile communication system to enable international roaming and ISDN service characteristics. GSM is an open standard for services, infrastructure and communication - independent of the individual countries, network operators and producers, and flexible to the requirements of the individual user. All this has boosted the development of GSM: Now, over 300 network operators in 130 countries are offering GSM services. Over 150 million customers use GSM. A considerable share of the world-wide telecommunication market goes to GSM products and -services. The cellular structure in GSM provides an almost complete radio coverage. The system allows a maximum distance of 35 kilometers between the mobile station and the base station. Therefore, the geographical area where GSM is used must be subdivided into smaller areas, which are known as cells. The cell size must be adjusted to the subscriber density and to the environment. If a subscriber moves from one cell to another during a call, the connection is handed over from the old radio station to the new radio station, without any interruptions. This procedure is called "handover". GSM distinguishes different channels on the basis of their frequency. Each cell is assigned one or more frequencies, which it uses to serve active subscribers. An uplink/downlink frequency pair can be used by up to 8 mobile stations practically simultaneously. This is done with the help of the Time Division Multiple Access (TDMA). Since there is only a limited amount of available traffic channels, high subscriber numbers require that the frequencies be used several times. This is not an insoluble problem, since several cells can use the same frequency, provided that the distance between the cells is sufficient. But it sometimes makes network planning a very complex task. Thus GSM offers capacity for more subscribers than one would expect in view of the very limited supply of frequency resources. To guarantee flexibility, open interfaces are specified in GSM between particular network elements. This way, network operators can be supplied by different producers. Nevertheless, the interfaces' functionality is very well specified, to guarantee a smooth data transmission. In GSM, there are two truly open interfaces. The first one is the air interface, located between the mobile and the base station. The other one is the A-interface between the Base Station Subsystem (BSS) and the Network Subsystem (NSS). The GSM network structure is decentralized, and consists of three separate subsystems communicating with each other over a series of interfaces. Apart from the Network Subsystem for routing, and subscriber localization, and the Base Station Subsystem (BSS) for radio coverage and radio resource management, there is also the Operation & Maintenance Subsystem (OMS), which guarantees network management and administration. 1.3  Evolution from GSM to the 3rd Generation GSM has been designed, above all, for speech communication. Although the standard also offers data services, their possibilities are limited by a data rate of 9.6 kbps. To meet the growing demands of increasingly complex data applications, e.g. in the multimedia or Internet sector, higher data rates are necessary. Therefore, the system's capacity is constantly being enhanced. 1.3.1        HSCSD One extension of the GSM standard is High Speed Circuit Switched Data (HSCSD). This innovation gives the bit rates an enormous boost. HSCSD is circuit-switched and optimizes the existing transmission rates in two respects: Firstly, a new channel coding method, available in GSM Phase 2+ in single circuit-switched data, increases the data rate from 9.6 kbps to 14.4. Secondly, the bundling of up to 4 timeslots can increase the user data rate to 57.6 kbps. This allows, for example, Faster e-mail transfer Faster file transfer, and Speedier and thus cheaper web browsing and data download from the Internet. As most of the services used require higher data rates in the downlink than in the uplink, HSCSD really shows an asymmetrical implementation, e.g. 3 timeslots in the downlink and 1 in the uplink. This also facilitates mobile station design and avoids battery capacity problems. 1.3.2        GPRS In contrast to HSCSD, the General Packet Radio Service GPRS is packet-switched instead of circuit-switched. The radio network resources are only used if data is actually being transmitted. Thus, billing is no longer based on the duration of the call, but on the amount of transmitted data. Additionally, the type of data service can be charged, for example web browsing or WAP access. By bundling up to 8 channels, a data rate of up to 171.2 kbps can be achieved, with up to 8 subscribers per channel. The more subscribers transmit data within a cell, the more the datarate available to each individual user is reduced. GPRS requires some modifications to the existing GSM infrastructure. Together with HSCSD, GPRS is a further step towards mobile multimedia. 1.3.3        EDGE Enhanced Data Rates for Global Evolution (EDGE) is a technology concentrating on the air interface between the mobile and the base station. Based on a new modulation process, 8 Phase Shift Keying, or 8-PSK, EDGE achieves three times the data rates of HSCSD and GPRS at the air interface. Furthermore, with EDGE, the subscriber can use all 8 timeslots at the air interface. In contrast to GSM, a digital data unit at the air interface, a symbol, is not just one bit, but three bits. This technology allows data rates of almost 474 kbps per user. On the other hand, EDGE is very bit error sensitive and requires careful planning and a sufficient number of base stations. 1.3.4        UMTS The Universal Mobile Telecommunications System (UMTS), represents the 3rd generation of mobile communication. It achieves bit rates of up to 2 Mbit/s if the mobile is static and no other user is transferring any data. Realistic situations will allow a maximum of 144 or 384 kbps. The introduction of UMTS won't make GSM unnecessary. Simple services like speech will be dealt with by GSM in the future, whereas high data rate multimedia services and mobile office applications will be handled by UMTS. UMTS mainly affects the air interface, where a broadband transmission method achieves high transmission speeds. This requires considerable changes to the network architecture. Thus, especially at the beginning, a close cooperation with the existing GSM network structure is necessary to guarantee that services are supplied area-widde.

• Global Telecoms Revenue to Reach $2.2 Trillion in 2013

The global telecommunications industry was not immune to economic forces in 2012 that slowed growth from earlier predictions, according to a new market analysis report from Insight Research. Spending for wireline services contracted in 2012, while spending on wireless services grew modestly.

According to the new industry market study, telecommunications services revenue worldwide will grow from $2.2 trillion in 2012 to $2.7 trillion in 2018 at a combined average growth rate of 3.8 percent.

The report notes that wireless subscriber growth compounded with rising usage will raise wireless revenues by 31 percent from current levels, yet wireline revenues will remain flat until substantial economic recovery kicks in. Despite these modest gains, there are some sectors, such as Ethernet, Cloud, and Mobile Solutions, that will show double-digit annual percentage growth.

• What is 3GPP?

The 3rd Generation Partnership Project (3GPP) is a collaboration between groups of telecommunication associations, to make a globally applicable third generation 3G mobile phone system specification within the scope of the International Mobile Telecommunication-2000 project of the International Telecommunication Union (ITU). 3GPP specifications are based on evolved Global Systel for Mobile Communication (GSM) specifications.

3GPP standardization encompasses Radio, Core Network and Service architecture.

The groups are the European Telecommunications Standarts Institute, Association of Radio Industries and Businesses/Telecommunication Technology Committee (ARIB/TTC) (Japan), Alliance for Telecommunications

Industry Solutions (North America) and (South Korea). The project was established in December 1998.

3GPP should not be confused with 3rd Generation Partnership Project 2 (3GGP2), which specifies standards for another 3G technology based on IS-95 (CDMA), commonly known as CDMA2000.

The 3GPP has specified the following standards:

• GSM
• GPRS
• GERAN
• WCDMA
• HSPA (HSDPA and HSUPA)

3GPP2 was born out of the International Telecommunication Union’s (ITU) International Mobile Telecommunications “IMT-2000” initiative, covering high speed, broadband, and Internet Protocol (IP)-based

mobile systems featuring:

• network-to-network interconnection,
• feature/service transparency,
• global roaming,
• seamless services independent of location.

IMT-2000 is intended to bring high-quality mobile multimedia telecommunications to a worldwide mass market by achieving the goals of increasing the speed and ease of wireless communications, responding to the

problems faced by the increased demand to pass data via telecommunications, and providing “anytime, anywhere” services.

Source:www.teletopix.org

• TCH Drop Rate – Parameter Related

The parameter settings on the BSC side and MSC side may affect the TCH call drop rate. You should check the settings of the following parameters for a cell with a high TCH call drop rate. See Case 5: Reduction of Call Drops by Optimizing Handover Parameters and Case 12: Increase in Call Drop Rate Due to Change of TR1N on the MSC Side.
1. SACCH Multi-Frames
This parameter determines whether an uplink radio link is faulty. Each time the BTS fails to decode the measurement report on the SACCH from the MS, the counter decreases by 1. Each time the BTS successfully decodes the measurement report on the SACCH, the counter increases by 2. When the value of this counter is 0, the BTS regards the radio link as faulty. In the traffic measurement, if there are many call drops (M3101A) related to radio link failure, you can infer that the radio propagation conditions are poor. In this case, you can set this parameter to a greater value.
2. Radio Link Timeout
This parameter determines whether a downlink radio link is faulty. Each time the BTS fails to decode the measurement report sent over the SACCH by the MS, the counter decreases by 1. Each time the BTS successfully decodes the measurement report sent over the SACCH, the counter increases by 2. When the value of this parameter is 0, the BTS regards the radio link as faulty. In the traffic measurement, if there are many call drops (M3101A) related to radio link failure, you can infer that the radio propagation conditions are poor. In this case, you can set this parameter to a greater value.

3. RXLEV_ACCESS_MIN
This parameter specifies the minimum receive level of an MS to access the BSS. If this parameter is set to a too small value, some MSs with low receive levels may access the network and call drops are likely to occur. You can set this parameter to a great value to reduce the TCH call drop rate. The counters such as call setup success rate and the counters related to traffic volume, however, are accordingly affected.
4. RACH Min.Access Level
This parameter determines whether an MS can access the network over the RACH. If this parameter is set to a too small value, some MSs with low signal levels may access the network and call drops are likely to occur. You can set this parameter to a great value to reduce the TCH call drop rate. The counters such as call setup success rate and paging success rate, however, are affected.
5. Min DL Power on HO Candidate Cell and Min Access Level Offset
The sum of the values of the two parameters specifies the minimum downlink receive level of a candidate neighboring cell for a handover. If this parameter is set to a too great value, some desired cells may be excluded from the candidate cells; if this parameter is set to a too small value, an unwanted cell may become the candidate cell. Both conditions may lead to the increase of call
drops.
6. Timer T3103 series
Timer T3101 series consists of T3103A, T3103C, and T8. These timers are started to wait for a handover complete message. If the lengths of the timers are set to small values, probably no message is received when timer T3103 series expires. In this case, the BSC considers that the radio link in the originating cell is faulty. Then, the BSC releases the channel in the originating cell. Thus, call drops occur. In the traffic measurement, if many call drops are related to handovers (CM331: Call Drops on Radio Interface in Handover State), you can set this parameter to a greater value. If this parameter is set to a too great value, channel resources are wasted and
TCH congestion occurs.

7. Timer T3109
This parameter specifies the period for waiting for a Release Indication message after the BSC sends a Channel Release message to the BTS. If this parameter is set to a too small value, the link may be released before the Release Indication message is received. As a result, a call drop occurs. You can set this parameter to a greater value to reduce the TCH call drop rate. It is recommended that timer T3109 be set to 1–2 seconds longer than timer Radio Link Timeout.
8. Timer T3111
This parameter specifies the interval between the time that the main signaling link is disconnected and the time that a channel is deactivated. The purpose is to reserve a period of time for repeated link disconnections. If this timer is set to a too small value, a channel may be deactivated too early. Thus, call drops increase.

9. Timers T305 and T308
Timers T305 and T308 are used on the MSC side. Timer T305 specifies the period during which the MSC monitors the on-hook procedure. Timer T308 specifies the period during which the MSC monitors the resource release procedure. You should set the two parameters when adding BSC data. Note that the modification of the data in the timer table does not take effect. If timers T305 and T308 are set to invalid or great values, the MSC clears the call a long time after the MS hangs up. After the T3103 and Radio Link
Timeout timers expire, the number of call drops is increased and thus the TCH call drop rate is significantly affected.

10. TCH Traffic Busy Threshold
If the current channel seizure ratio exceeds the value of this parameter, the BSC preferentially assigns a half-rate channel to a dualrate-enabled call. Otherwise, the BSC assigns a full-rate channel to the dualrate-enabled call. Compared with a full-rate channel, a half-rate channel has weak antiinterference capabilities. Therefore, if a large number of half-rate channels are assigned, the TCH call drop rate increases. It is recommended that this parameter should not be set to a too small value if congestion is unlikely to
occur.

11. Call Reestablishment Forbidden
This parameter specifies whether to allow call reestablishment. In case of burst interference or radio link failure due to blind areas caused by high buildings, call drops occur. In this case, MSs can initiate the call reestablishment procedure to restore communication. To reduce the TCH call drop rate, you can set this parameter to No to allow call reestablishment. In certain conditions, allowing call reestablishment greatly reduces the TCH call drop rate. Call reestablishment lasts for a long time, and therefore some subscribers cannot wait and hang up. This affects user experience.

12. Parameters related to edge handover

When the receive level drops greatly, an edge handover cannot be performed in time in any of the following conditions: The parameter Edge HO UL RX_LEV Threshold or Edge HO DL RX_LEV Threshold is set to a small value; the parameter Inter-cell HO Hysteresis is set to a great value; the parameters Edge HO Watch Time and Edge HO AdjCell Watch Time
are set to great values; the parameters Edge HO Valid Time and Edge HO AdjCell Valid Time are set to great values. As a result, a call drop occurs. To reduce the TCH call drop rate, you can appropriately set these parameters so that edge handovers can be performed in time to avoid call drops.

13. Parameters related to BQ handover
When the signal quality deteriorates, a BQ handover cannot be performed in time in any of the following conditions: The parameters
ULQuaLimitAMRFR, ULQuaLimitAMRHR, UL Qual. Threshold, DLQuaLimitAMRFR, DLQuaLimitAMRHR, and DL Qual. Threshold are
set to great values; the parameter BQ HO Margin is set to a small value; the parameter Inter-cell HO Hysteresis is set to a great value. As a result, call drops occur. To reduce the TCH call drop rate, you should appropriately set these parameters so that BQ handovers can be performed in time to avoid call drops.

14. Parameters related to interference handover
If the parameters RXQUAL1 to RXQUAL12 are set to great values or if the RXLEVOff parameter is set to a great value, strong interference may occur. In this case, if interference handovers are not performed in time, call drops occur. To reduce the TCH call drop rate, you can appropriately set these parameters so that interference handovers can be performed in time to avoid call drops. If the parameters RXQUAL1 to RXQUAL12 are set to small values, the number of handovers due to other causes increases greatly, thus affecting the handover success rate.

15. Parameters related to concentric cell handover
A call at the edge of the overlaid subcell cannot be handed over to the underlaid subcell in any of the following conditions: In the case of a normal concentric cell, the parameters RX_LEV Threshold and RX_LEV Hysteresis are set to great values; in the case of an enhanced concentric cell, the parameter OtoU HO Received Level Threshold is set to a great value. As a result, a call drop is likely to occur. If the Call Drop Ratio on TCH on the TRX in the OverLaid Subcell (RM330a) is high, you can appropriately set these parameters so that calls at the edge of the overlaid subcell can be handed over to the underlaid subcell in time. When a call in the underlaid subcell has interference, the call cannot be handed over to the overlaid subcell if the RX_QUAL for UO HO Allowed parameter is set to Yes and the RX_QUAL Threshold parameter is set to a great value. Thus, a call drop occurs. If the Call Drop Ratio on TCH on the TRX in the Underlaid Subcell (RM330) is high, you can set these parameters properly so that the call can be handed over to the overlaid
subcell at the earliest.

16. Parameters related to power control
If the power control level and quality threshold are set to small values, call drops are likely to occur because of low signal level or bad voice quality.

17. T200 and N200
If the parameters T200 FACCH/F, T200 FACCH/H, N200 of FACCH/Full rate, and N200 of FACCH/Half rate are set to small values, data links are disconnected too early. Thus, all drops are likely to occur. If call drops occur because of T200 expiry, you can increase the values of T200 and N200 properly.

18. Neighboring cell relations
If the neighboring cells configured in the BA2 table are incomplete, call drops are likely to occur in the case of no suitable neighboring cell for handover and progressive deterioration in the voice quality. Neighboring cell relations should be configured completely on the basis of the drive test data and electronic map (for example, Nastar) to minimize the call drops due to no available neighboring cells.

19. MAIO
If frequency hopping (FH) is applied in a cell and the MAIO is set inappropriately (for example, different TRXs serving the same cell have the same MAIO), frequency collision may occur during FH. Thus, the TCH call drop rate increases.

20. Disconnect Handover Protect Timer
This parameter is a software parameter of the BSC. After receiving a DISCONNECT message from an MS, the BSC cannot hand over the MS within the period specified by this parameter. Therefore, the following case can be avoided: After being handed over to the target cell, the MS cannot be put on hook because it does not receive a release acknowledgement message. You are advised to set this parameter properly.

21. TR1N
This parameter should be set on the MSC side. It is used to avoid the retransmission of short messages. When this parameter is set to a too great value, the MSC does not send a CLEAR CMD message if the MS receives a short message during link disconnection. As a result, the MS sends the BTS a DISC message to disconnect layer 2 connection. After receiving the DISC message, the BTS sends a REL_IND message to the BSC. Then, the BSC sends a CLEAR REQ message to the MSC and the number of call drops is incremented by one.

22. Software Parameter 13 and MAX TA
When the parameter Software Parameter 13 is enabled and the parameter MAX TA is set to a too small value, the channel is released when the TA of a call exceeds the MAX TA. In this case, call drops occur. It is recommended that the parameter Software Parameter 13 should not be enabled.

23. Directly Magnifier Site Flag
If a BTS is installed with repeaters, the handover between repeaters can only be asynchronous because the distance between repeaters is long. If synchronous handovers are performed, the handovers may fail and thus many call drops occur. Therefore, when a BTS is installed with repeaters, the parameter Directly Magnifier Site Flag should be set to Yes to avoid asynchronous handovers between cells under the same BTS.