2- OMO111020 BSC6000 GSM V9R8C01 Cell Parameters ISSUE1.01.pdf

N-0GSM Cell Parameters

Confidential Information of Huawei. No Spreading Without Permission


By reading cell parameters, MS can access the network, perform cell selection and reselection, fully utilize various services provided by the


selection and reselection, fully utilize various services provided by the network, and achieve favorable cooperation with the network.


Cell parameters can be divided into two parts:


Cell parameters is sent on BCCH, which are used in idle mode.

Cell parameters is sent on SACCH, which are used in dedicated mode.


As a global cellular mobile communication system, GSM conducts strict coding for each GSM network in every country, and even every location area, BTS, and cell, so as to


GSM network in every country, and even every location area, BTS, and cell, so as to ensure that each cell corresponds to a unique number all over the world. The adoption of this coding scheme can achieve the following objectives:

1. Ensure that MS can correctly identify the current network, so that MS can accuratelyselect the network expected by subscribers and operators.

2. Ensure that the network can know the real-time position of MS, so that the network canprovide various service requests from the MS.

3. Ensure that the MS can report correct neighbor cells information to the network duringconversation, so that network can perform handover when necessary to keep continuousconversation for the mobile subscribers.


MCC is used in international mobile subscriber identity (IMSI) andlocation area identity (LAI).


location area identity (LAI).

1. LAI. It is periodically transmitted in cell parameters of each cell.MCC indicates the home country of GSM PLMN. MS uses thereceived information as the important basis for network selection.2. IMSI of MS. MSs IMSI also contains MCC. It shows the residentcountry of the mobile subscriber. When MS logs on the network orapplies for a certain service, it must report its IMSI to the network(When TMSI is unavailable.). The network uses the MCC in IMSI tojudge whether this subscriber is an international roaming subscriber.

As the unique country identity standard, MCCs are allocated andmanaged by the International Telecommunication Union (ITU). ITUDefault Value E.212 (blue book) stipulated the MCC number forevery country. The MCC of China is 460 (decimal). Due to thespecial meaning of MCC, modification is prohibited once it has beenset in the network.


MNC is used in international mobile subscriber identity (IMSI) and location area identity (LAI).


identity (LAI).

LAI. It is periodically transmitted in each cell. Here, MNC indicates the network number of GSM PLMN. MS uses the received information as an important basis for network selection.

IMSI also contains MNC. It shows the home GSM PLMN network of the subscriber. When MS logs on the network or applies for a certain service, it must report IMSI to the network (When TMSI is unavailable.). The network judges whether this subscriber is a roaming subscriber according to the MNC in IMSI, and uses it as one of the important parameters for addressing to subscriber HLR.

If a country has more than one GSM PLMN, different networks must have different MNC. MNC is allocated by relevant telecommunication management department of the country. One operator can have one or more MNC (which regards to the scale provided by the service, usually one operator has one MNC.). Different operators can share the same MNC. Currently, China have two GSM networks, which are operated by China Mobile and China Unicom. Their MNC are 00 and 01 respectively. Due to the special meaning of MNC, modification is prohibited once it has been set in the network.


When MS is powered on or LAC of current cell is found to be different from its originally stored Descriptions, MS will inform network of the current location area via location update,


stored Descriptions, MS will inform network of the current location area via location update, and the network uses the LAI for paging. Generally the allocation and coding of LAC is set at the early stage of network construction, and seldom modified during the operation.

The size of location area (LA) is one of key factors in the system. If the LA coverage is too small, the chances for MS to update location increase, and this will increase the signaling load in the system. If the LA coverage is too large, when network conducts paging to the MS, the same paging information will be transmitted in a large number of cells, and this will lead to the heavy load on CCCH. The adjustment of LA size has no unified standard. Operating departments can decide whether to adjust the size according to the currently running network. If the CCCH signaling load is heavy because of too big LA coverage, then reduce the size of LA, and vice versa. It is generally recommended to set the LA as large as possible. The calculation of LA is related with the paging strategies of different manufacturers. If Huawei equipment is employed, it is recommended to set the TRX number within the range of 300 in one location area. In the early stage of network construction, the traffic is not heavy, so the TRX number in one LA can be larger than this value. It is necessary to monitor the PCH load and the increase of traffic in a long term. If necessary, PCH capacity can be increased by adding one extended BCCH channel.

While making LA planning, try to make use of the geographical distribution and behaviors of mobile subscribers to allocate the LA, so as to achieve the objective of reducing the times of location update at the boundary between location areas. Please note that LAC in cell parameters must be in consistent with that in MSC. Otherwise, call setup failure will occur.


Cell Identity (CI) is one part of Cell Global Identity (CGI), transmitted in each cell.


in each cell.

There is generally no restriction for the allocation of CI. Value from 0 to 65535 (decimal) can be obtained. But it should be ensured that one location area cannot have two cells with the same CI.

CI is usually determined in the network design. Except for some special cases, CI value should not be changed during the operation of the system.

Please note that one location area is not permitted to have two or more cells using the same CI. CI on MSC should be the same as that on BSC. Otherwise, MS cannot make calls in this cell.


BSIC is transmitted on Synchronous Channel (SCH) of each cell. Its functions are asbelow:


below:

1. If MS have read SCH, it is considered as being synchronous with that cell. However, tocorrectly read the information on the downlink common signaling channel, MS must getthe TSC (Training Sequent Code) that is adopted by the common signaling channel.According to GSM specification, TS (Training Sequent) has eight fixed formats, which arerepresented by TSC ranged 0~7 respectively. TSC number adopted by common signalingchannel of each cell is just the BCC of the cell. So one of the functions of BSIC is toinform MS of the TSC adopted by the common signaling channel of the cell.2. Since BSIC attends the coding process of information bits in random access burst, itcan be used to prevent the BTS from accepting a RACH transmitted from MS in aneighbor cell as the access signal from the MS of the serving cell.3. When MS is in dedicated mode, it must measure the BCCH level of the neighbor cellsand report it to BTS according to BA2 that is sent on SACCH, including their respectiveBSIC. In special circumstance, when there are two or more cells using the same BCCH inthe neighbor cells, BSC can use BSIC to distinguish these cells and avoid wronghandover or even handover failure.4. MS must measure the BCCH signals of neighbor cells in dedicated mode, and reportthe results to the network. Since MS sends measurement report which contain theDescriptions of a maximum of 6 neighbor cells each time, it is necessary to control MS toreport only the cells which have neighbor relationships with the serving cell. The NCC isused for the above purpose. Network operators can use parameter NCC Permitted tocontrol MS to report the neighbor cells with NCC permitted in the serving cell only.


NCC and BCC together form the base station identity code (BSIC), transmitted on synchronous channel of each cell.


transmitted on synchronous channel of each cell.

In many cases, different GSM PLMNs have the identical coverage in many But their network planning are independent from each other. To ensure that adjacent BTSs have different BSICs, it is generally regulated that adjacent GSM PLMN select different NCC.

Adjacent or close cells with the same BCCH frequency must have different BSIC. Special attention should be paid to the configuration of cells in boundary areas.


BSIC includes BCC and NCC which is transmitted on SCH. BCC is a part of BSIC, used to identify different cell with the same BCCH in the same GSM system.


used to identify different cell with the same BCCH in the same GSM system. According to the requirements of GSM specification, TSC of BCCH in each cell should be the same with BCC of the cell. Generally this consistency must be ensured by manufacturers. Adjacent or close cells using the same BCCH must have different BSIC, otherwise, inter cell handover might be unsuccessful.

BCC planning has three solutions. All of them have taken distance principle into consideration to avoid collisions of adjacent cells with the same BCCH and same BSIC.

1. Based on the existing BCC set, select one of the BCC that has been used by other cell, ensure at the same time that BCC selected will not cause BSIC/BCCH collision with adjacent cells. The advantage of this solution is that it can ensure BCC be evenly distributed in the whole network. However, if done manually, this solution is time-consuming and troublesome, we can use automatic distribution tools. 2. When defining BCC, try to assign the value from 0. When causes BSIC/BCCH collision, expand the value range. The advantage is that the number of BCC used is kept to the smallest. So when adding a new BTS, in order to avoid the BSIC/BCCH collision, a new BCC can be selected without modifying the BCC of original cells around.

3. Allocate BCC according to its reuse model. That is to use the same BCC within one cluster. It means that adjacent cells cannot use the same BCCH with the service cell. This solution is frequently used, and also the simplest one.


There are a lot of parameters in GSM system, they are usually transmitted to MS from BTS via Um interface. It aims to maintain


transmitted to MS from BTS via Um interface. It aims to maintain favorable cooperation between MS and BTS. On the other hand, the values of these parameters directly affect the traffic load and signaling flow of each part of the system. Therefore, proper configuration of these parameters is important to the favorable and stable operation of the system. The following will elaborate on the definitions, value ranges, and effects on the system of these system control parameters.


Detach process (IMSI) refers to the process that MS informs the network that it is shifting from working state to non-working state (usually a power-off process), or the SIM card is


from working state to non-working state (usually a power-off process), or the SIM card is being taken out from MS. Upon receiving the notice from MS, the network knows that the IMSI subscriber is in non-working state. Therefore, if the MS is called, the call connection will be implemented.

IMSI attach process is opposite to detach process. It is the process that MS informs the network it has entered the service area (usually a power-on process) or SIM card has been inserted into MS. After entering service state again, MS will test whether the current location area (LAI) is the same with the latest LAI recorded in MS. If yes, MS will start IMSI attach process. Otherwise MS will start location update process, upon receiving the location update or IMSI attach process, the network will indicate that this IMSI subscriber is in working state.

Note that ATT configuration of different cells in the same LAI must be the same. It is because IMSI detach process will be started when MS is power-off in the cell with ATT set as yes. The network will record that this subscriber is in non-working state and reject all the called connection requests to this subscriber. When MS is power-on again, if it is in the same LAI as it was power-off (thus the LAI update process will not be started) but in another cell, and ATT of the cell is set as no, then the MS will not start IMSI attach process. In this case, this subscriber can not be called normally until the MS starts the location update process.


In GSM system, the downlink common control channel mainly includes Access Granted Channel (AGCH) and Paging Channel (PCH). It serves to send the access granted


Channel (AGCH) and Paging Channel (PCH). It serves to send the access granted (immediate assignment) and paging messages. CCCH is shared. According to the configuration of traffic channel and traffic model, CCCH can be carried by either one or multiple physical channels. Moreover, CCCH and SDCCH can share one physical channel. The MS needs to know how the CCCH(s) is/are configured, so that it can find and select one to listen to. The CCCH Conf is just used to tell the MS about this matter.

When CCCH is a physical channel which combined with SDCCH, the capacity of CCCH is the lowest. When CCCH is a physical channel which is not combined with SDCCH, the capacity is higher. For other cases, the more the physical channels are used as CCCH, the higher the CCCH capacity is.

Configuration of CCCH Conf is specified according to the traffic model. This model is closely related to the cell location and environment. According to experiences, when TRX quantity in the cell is 1 or 2, it is recommended to use a combined CCCH as the common control channel. When TRX quantity in the cell is 3 or 4, it is recommended to use a non-combined CCCH as the common control channel.

Currently CCCH can be configured according to actual traffic load. If the paging load is very heavy, the paging traffic of cell should be distributed via multiple CCCH physical channels other way. Special attention should be paid to PCH in CCCH. Generally PCH capacities of various cells under one LAC must be the same.


As downlink CCCH includes both AGCH and PCH, it is necessary to set the number of blocks, which are reserved for AGCH among CCCH message blocks. To let MS know


blocks, which are reserved for AGCH among CCCH message blocks. To let MS know such configuration information, the cell parameter of each cell includes a configuration parameter, which is the number of access granted blocks reserved (BS_AG_BLKS_RES). This parameter actually assigns the proportion of AGCH and PCH on CCCH. It affects the time of MSs response to the paging.

The network operator can adjust this parameter to balance the traffic of AGCH and PCH by referring to the following principles:

1. Principle for BS_AG_BLKS_RES: make this parameter as small as possible without causing overload of AGCH, so as to increase the capability of paging and improve the system performance.2. Generally it is recommended to select 1 (when CCCH Conf is 1 combined CCCH), 2 or 3 (when CCCH Conf is one of other values) for BS_AG_BLKS_RES.3. During operation, observe the statistics of AGCH overload and adjust BS_AG_BLKS_RES properly.

Note: In Huawei system, when AGCH has been all occupied, if PCH is free, it can be used to send the immediate assignment command. If AGCH blocks reserved is set as 0, the immediate assignment would be sent only when there is free PCH channel. Therefore, a fixed capacity reserved for AGCH is necessary.


This parameter specifies the number of paging sub-channels that are assigned in a cell. In the network, MS only monitors the paging sub-channel it belongs and ignores the


In the network, MS only monitors the paging sub-channel it belongs and ignores the Description of the others. When this parameter is set larger, there will be more paging sub-channels in the cell and accordingly there will be less MS in each paging sub-channel. Therefore, the bearing capability of PCH will be more (theoretically the capacity of each PCH does not increase, but the buffer that buffers paging message in each BTS is increased, which makes the sending of paging messages more even in the time domain), and the lifetime of MS battery will be longer. The value of this parameter should be as small as possible under the condition that the overload on PCH does not occur. In the operation, the PCH load should be measured regularly and the value of this parameter should be adjusted properly according to the PCH load. In a location area, paging is sent in all the cells. Therefore, all cells in the same location area should have the same or nearly the same PCH capacity (number of paging sub-channels). In the area where the PCH bears a medium or large load, it is suggested to be set as 6 or 7 (6 or 7 multi-frames are used as a cycle of paging). For the area with a small load, it is set as 4 or 5. Besides, it is often set as 2.

Note:

1. One CCCH block (four consecutive CCCH timeslots) can bear the information of two IMSI pagings or four TMSI pagings or two AGCH immediate assignments.

2. In idle mode MS camps in a cell. The DSC is initialized to the integer part of 90/N (N is BS_PA_MFARMES, with the value range: 2~9). when MS can successfully decode the message on paging sub-channel, DSC will increase by 1, but it will not exceed initially value. If decoding fails, DSC will decrease by 4. If DSC


MS will make location update when detecting the change of location. Besides, MS will make periodic location update controlled by parameter T3212. Once MS read T3212 from cell


periodic location update controlled by parameter T3212. Once MS read T3212 from cell parameter, it will store it in SIM card. When the time reaches T3212 value, the location update process will be triggered. The shorter the period is, the better the performance is. But it will bring more signaling load for system. On setting of this parameter, the processing capability of MSC and BSC, the flux of A interface, Abis interface and Um interface, the flux of HLR and VLR should be considered. Generally this parameter is set as a larger value for urban area and smaller for suburb, countryside or the place with poor coverage.

Large T3212( 16 hours 20 hours) is recommended for the area with heavy traffic, and small T3212 (3 hours, 2 hours) for the area with normal traffic. For the area where the traffic exceeds the system capacity, it is recommended to set T3212 as 0 (no periodic location update). To set the value of T3212 properly, its necessary to conduct long-term measurement on the processing capability and flux of each entity in the system. If any overload occurs, increase the value T3212.

Note that this value should be smaller than the period by which the network queries the IMSI attached subscriber. Otherwise, the following situation occurs: When MS has not done any operation in a certain time, and it is not yet the time for periodic location update, the network will set IMSI flag of MS as detached, because its query result shows that MS has not done any operation. Thus, the network will not process the paging of this MS. So, before MS initiates another round of periodic location update, once there is a call to the MS, the network will voice the calling party that the called MS is has been powered off. As usual, the T3212 is set smaller than one third of the MSCs check time.

When MS reselects a cell in a different location area, it will make a non-periodic location update and reset T3212 in the new cell. If it reselects in the same location area, then the timer value will be remainder of the original one divided by the new T3212.


Let us make the following assumption. The system query time is set to 1 hour, T3212 value of cell A is set to 4 (0.4 hour), and T3212 value of the adjacent cell B is set to 8 (0.8


value of cell A is set to 4 (0.4 hour), and T3212 value of the adjacent cell B is set to 8 (0.8 hour), and they are in the same location area. MS reselects B when the periodic location update status in A is 3/4, and the MS periodic location update status in B changes to 3/8. If MS stays in B for some time and the location update status reaches 7/8, then MS reselects A. At this time, it can be seen that in cell A, MS reselection status changes to 7/4, i.e. 3/4. If MS reselects to B at this time, the status will change to 3/8 instead of 7/8. The above analysis shows that if the above case occurs (the probability is high), though T3212 values of both cells are smaller than the system query time, the MSs frequent cell reselection leads to the final equivalent time is greater than system query time. Thus the subscriber will be considered as a power-off subscriber within certain time even it is in normal idle mode.


MS keeps on measuring the BCCH signal level of the serving cell and the neighbor cells. In order to know the adjacent cells, neighbor cell description information will be broadcast


order to know the adjacent cells, neighbor cell description information will be broadcast periodically in cell parameter of each cell. This information lists the BCCH of all neighbor cells. MS extracts the information from cell parameter and use it as basis for neighbor cell measurement.

For GSM network, the neighbor relationship between cells is accomplished when designing the network topology. During the network construction, the neighbor cell relationship must be configured in accordance with the topology design that has been planned. Moreover, after the commission of network, neighbor relationship should be modified according to the data of drive test and traffic measurement. When the networks architecture is changed (e.g. adding BTSs or changing the network frequency configuration.), the network operator must strictly follow the changed-cell-neighbor-relationship, re-set and verify it. Improper neighbor cell description is usually one of the main reasons of call drop. Besides, since the actual network topology structure is often greatly different from the theoretical calculation result, and network is in ever-changing environment, the network operator must configure the neighbor cell description according to the actual situation.


In theoretical calculated neighbor cell relationship, cell A and cell C are not adjacent cells. Assume that one MS moves from cell A to cell C during the conversation, theoretically,


Assume that one MS moves from cell A to cell C during the conversation, theoretically, MS needs twice of inter-cell handovers. Assume that the interference in cell D is rather serious, call drop is may occur during this period. But in fact, the coverage of A, B, C, and D is not the case as the theory. A and C have overlapping coverage. If A and C are regarded as adjacent cells here, that is to say, add the BCCH of C and A respectively to the neighbor cell description of A and C, then when MS passes from A to C, only one handover happens. Whats more, call drop could be avoided because of the good quality of cell C.


Part of the signals from cell A leaks out and covers some areas far away from this cell. It is overshooting. If MS is in dedicated mode in the shady area and moves from this area


is overshooting. If MS is in dedicated mode in the shady area and moves from this area towards B and C, since there are no BCCH of cell B and C in cell As neighbor cell description, call drop is unavoidable. If the antenna of BTS is located too high, or the transmitting power is too large, overshooting will occur. BTSs built at the early stage of GSM construction usually have this problem, because coverage is the major purpose at that time and the antenna height is very high. The best solution for this phenomenon is to adjust the location and downtilt of the antenna, or to adjust the transmitting power of the BTS to eliminate the BTSs over-covered area. In real situation, it is hard or even impossible to change the location of antenna. So one more simple and applicable method is to add BCCH of B and C to the neighbor cell description of cell A (no need to add As BCCH to B and C). But it must be ensured that there are no cells which are all neighbor cells of cell A and using the same frequency and same BSIC with cell B and C. Generally, this method is not recommended.


There are a lot of parameters in GSM system, they are usually transmitted to MS from BTS via Um interface. It aims to maintain


transmitted to MS from BTS via Um interface. It aims to maintain favorable cooperation between MS and BTS. On the other hand, the values of these parameters directly affect the traffic load and signaling flow of each part of the system. Therefore, proper configuration of these parameters is important to the favorable and stable operation of the system. The following will elaborate on the definitions, value ranges, and effects on the system of these system control parameters.


After initiating immediate assignment process, MS keeps monitoring messages on BCCH and CCCH group it belongs to. If the network does not send Immediate Assignment or Immediate Assignment Extend message, MS will


belongs to. If the network does not send Immediate Assignment or Immediate Assignment Extend message, MS will resend the channel request message at a certain time interval. The larger this parameter is, the higher the call setup success rate is, but also the heavier the load of RACH and SDCCH is.

When MS initiates immediate assignment, it will send the channel request message to the network via RACH. As RACH is an ALOHA channel, the network is incapable of controlling the access time of MS. Thus in heavy traffic spot, it is unavoidable that several MSs may simultaneously make access request and cause collision which will lead to two results: one is when one request signal level is obviously higher than the others access signals, the access request with higher level will be handled; the other is the network can recognize none of them due to mutual-interference. As the traffic is increasing, access request loss due to collision will increase, too. To make sure that the system can correctly receive the access request and increase the access success rate, the network allows MS to send several channel requests before receiving an immediate assignment message so as to achieve a higher access success probability. MS will return to idle mode if it fails to receive an immediate assignment command after the MAX Retrans exceeds. Once MS sends a channel request, it will start timer T3120 and wait on the downlink CCCH. When T3120 times out and RACH resend times are not more than MAX Retrans., MS will resend channel request message (containing one new random reference), and restart T3120 with a random value. When T3120 is times out and MAX Retrans is reached, MS will start T3126. If MS still fails to receive a response from the network after T3126 times out, it will give up the access. If MS receives the access rejection, it will stop T3120 and start T3122. Within T3122, no new access attempt will be allowed.

Recommendation:Set to 7 for areas with low traffic (suburban or rural area) and the cell radius more than 3 km.Set to 4 for areas with ordinary traffic (non-busy area in the city) and the cell radius is less than 3km. Set to 2 for micro-cell

Set to 1 for micro-cell with heavy traffic or with obvious congestion.

Set to 4 or 7 for satellite transmission BTS.


It is set to reduce the collisions on RACH. It mainly affects the execution


efficiency of the immediate assignment process. The value of this parameter is related to CCCH configuration mode, both of them together determine the parameter S (see the next page). The MS sends the first random access burst at a random TS in the set {0, 1, , MAX(T, 8)--1}. And the TS number between any two adjacent channel request messages is a random value in the set {S, S+1, , S+T-1}.

Generally, parameter T+S should be as small as possible (in order to shorten the access time of MS), but AGCH and SDCCH must not be overloaded. If AGCH or SDCCH of the cell is overloaded, then parameter T can be changed to make parameter S larger, until AGCH or SDCCH of the cell is not overloaded.

When the RACH collisions is serious, value T should be large. When the number of RACH collisions is small, value T should be as small as possible.


When T becomes larger, the interval range between channel request messages sent by different MSs will increase and RACH collisions will be reduced. When value S becomes


different MSs will increase and RACH collisions will be reduced. When value S becomes larger, the interval between channel request messages sent by the same MS will increase, collisions on RACH will be reduced and the availability of AGCH and SDCCH will increase. But the increase of either will prolong the access duration of MS, resulting in deterioration of access performance of the entire network. Generally, value T should be selected to make S as small as possible (in order to shorten MS access time), but AGCH and SDCCH must not be overloaded.


To avoid MS being unable to set up call even it is in the coverage area, consideration should be given to BTS sensitivity and MS


consideration should be given to BTS sensitivity and MS RXLEV_ACCESS_MIN during the setting of this parameter.

A too small value for this parameter makes MSs easily to access but the call drop rate may rise.

A too big value for this parameter may cause some MSs unable to make calls.


Once assigned with a dedicated channel, MS will start counter S. From then on, S will decrease by 1 when a SACCH message fails to be decoded, and will increase by 2 when


decrease by 1 when a SACCH message fails to be decoded, and will increase by 2 when decoded correctly. When S decreases to 0, there will be a radio link failure. This allows either re-establishment or release of the connection. If the value of this parameter is too small, the radio link will easily get failed which will result in call drops. If it is too large, MS will not release for a long time which will lower the availability of resources (this parameter functions for the downlink).

For area with little traffic (remote area), it is recommended to be between 52~64. For area with light traffic and large coverage(suburb or countryside), it is recommended to be between 36~48.

For area with heavy traffic (urban), it is recommended to be between 20~32.For the area with very heavy traffic (area covered by microcell), it is recommended to be between 4~16.

For the cell with obvious coverage hole or the area where the call drops is serious during movement, this parameter can be increased appropriately in order to increase the possibility to resume the conversation.

Note: Radio link timeout is the parameter used to judge the downlink failure. Likewise, the uplink will be monitored at BTS, either based on the uplink SACCH error or based on the receiving level and quality of the uplink.


If cell A and B are adjacent to each other, assume that one MS moves from point P to point Q during a conversation, usually an outgoing cell handover will occur. If the value of


point Q during a conversation, usually an outgoing cell handover will occur. If the value of parameter radio link timeout is too small and the quality of signal at the edge of cells A and B is poor, the radio link will time out before the handover occurs, thus resulting in call drops.


Because the robustness of the AMR voice frames is not at the same level with that of SACCH frames, the GSM specification only


level with that of SACCH frames, the GSM specification only improves the robustness of the AMR voice frames but not that of the SACCH frames. Therefore, the actual coverage capability of AMR is determined by the coverage capability of the SACCH.

In application, the data[RLT] and [SACCH Multi-Frames] should be set to higher values for AMR channels to increase the robustness of the SACCH. Thus the network coverage performance of AMR is improved and the call drop rate is reduced.

"Robustness" indicates the anti-interference capability. Protocols in 3GPP R6 version provide several new technologies to enhance the robustness of the SACCH. The problem about low robustness of the SACCH is expected to be solved with the evolution of technologies.


In working status, MS needs to measure adjacent cells BCCH signals and


report them to network. But each report can only include a maximum of six adjacent cells. Thus it is necessary to make MS only report the potential target cells for handover, instead of reporting all according to the signal level (usually MS does not report the signals of cells from other GSM PLMN). The above function can be implemented by making MS only measure the cells whose NCC are selected. Parameter NCC Permitted lists the NCCs of the cells that the MS needs to measure.

BSIC is transmitted continuously on SCH of each cell and the higher three bits of BSIC are NCC. MS only needs to compare the measured NCC of the adjacent cell with parameter NCC Allowed. If it is allowed, MS will report it to BTS, otherwise it will discard the measurement result.

Note: Improper setting of this parameter will lead to lots of call drops.


In some special case, the operator expects to prohibit all or part of the MSs from sending access requests or paging response. For example, emergency status occurs or a serious


access requests or paging response. For example, emergency status occurs or a serious fault occurs to a GSM PLMN. Therefore, GSM specification 0211 requires to assign an access level for each common GSM subscriber. The common access level is divided into level 0~9, which is stored in the SIM card of MS, and has nothing to do with access priority.

Some cells with extremely heavy traffic may be congested during busy hour, resulting in a large number of RACH collisions, AGCH overload, Abis interface overload, etc. GSM specifications provide a variety of ways for dealing with the overload and congestion, but most of them will lower the availability of equipment resources. The network operator can set the access control parameter (C0-C9) properly to control the traffic inside the cell. For example, when the cell suffers traffic overload or congestion, Ci can be set as 0 to prohibit MS with this access level from accessing this cell (change of Ci will not affect MS in dedicated mode), thus reducing the traffic of the cell. To solve this problem, values of C0-C9 in the cell can be changed periodically. For example, at intervals of five minutes, alternatively allow the access of MS with odd access levels and those with even access levels.

For example, 1000000000 indicates to allow the access of subscribers with the levels other than 0. During installation and commissioning of BTS or during maintenance test for some cells, they all can be set to 1 to prohibit the access of subscribers.


For some special subscribers, GSM specifications have reserved five special access levels 11-15, which usually have higher access priority. A special subscriber can have one or multiple access


which usually have higher access priority. A special subscriber can have one or multiple access levels (between 11 and 15) at the same time, which are also stored in the SIM card of the subscriber.

Class15PLMN Staff;

Class14Emergency Services;

Class13Public Utilities (e.g. water/gas suppliers);Class12Security Services;

Class11For PLMN Use.

For subscribers with the access levels 0~9, their access right is also applicable to the home PLMN and visit PLMN. For subscribers with the access levels 11-15, their access right is only applicable to the home PLMN. For subscribers with the access levels 12, 13 and 14, their access right is applicable to the area of the country to which the PLMN belongs.

Subscribers with the access level 11~15 have a higher access priority than those with the access levels 0~9.

The access level control parameter consists of 16 bits: C0-C15, which respectively corresponds to 15 access levels in bit mapping mode (C10 is used for permitting emergency call). When a bit is 1, it indicates not to allow MS with the corresponding level to access then cell. Otherwise it indicates to allow the access.


When the value is 0, MS will report measurement results of six strongest adjacent cells no matter which band they are in. When it is X(X


S is a GSM900 cell, cells A~H are adjacent to cell S. Of them, A and B are GSM1800 cells and others are GSM900 cells. The above diagram shows the influences of different


cells and others are GSM900 cells. The above diagram shows the influences of different MBR parameters as follows:

1)When MBR = 0, MS will report six adjacent cells with strongest signals without considering the bands, the report result is: C, E, B, H, F, D.2)When MBR = 1, the result is: C, B, E, H, F, D.3)When MBR = 2, the result is: C, E, B, A, H, F.4)When MBR = 3, the result is: C, E, H, B, A, F (3 GSM1800 cells should be reported. But there are only 2 currently, so 2 GSM1800 cells are reported. For the rest, GSM900 cells will be reported).


In GSM network, MSs service capability, supported band, power capability,


encryption capability and so on are described by classmarks. There are three classmarks which are classmark1, classmark2 and classmark3. The network can know the MSs capability by checking the classmarks of the MS. After receiving the class mark enquiry message, MS will send classmark change message to the network as soon as possible. CM3 (Classmark 3) includes the information about MS power, multiband and/or multislot capability. To perform handover between different bands, the power level must be described correctly. In the process of paging and sending of the BA2 information between different bands, the CM3 message must be known.

Note:

1) ECSC is invalid for single-band MS. For dual-band MS, when ECSC is not used, after the MS sends EST IND , MSC will still send the CLASSMARK REQUEST message, and MS will response with the CLASSMARK UPDATE message, and other functions are not affected. For the dual-band MS, when this parameter is set to Yes, the connection time between different MS will be obviously shortened.

2) When the encryption function is enable, The parameter must be set to "Yes".

3) M900/M1800 hybrid cells sharing BCCH are advised to be configured as "yes", and M1800 cells in dual-band network are advised to be configured as yes".


Class mark update flow.


For dual-band MS, after the SCCP connection is established andbefore receiving message CM service Accepted, there will be aclassmark update flow. Classmark is originated by MSC. MSC willsend one Classmark Request message, then BSC will convert thismessage into Classmark Enquiry. This is classmark request. ThenMS will send one Classmark Change, and convert it intoClassmark Update at BSS. MSC can get the MS Classmark 3 viathis message. Here the Classmark Update is to make MSC get MSClassmark 3. CR message reports the information of Classmark 2.And in the classmark update flow, MS will report Classmark 3information which is related to the dual-band capability. Therefore,MSC needs classmark update to get the MS Classmark 3.


For MS with common access control class 0~9, when Emergent Call Disable is Yes, it indicates not to allow an emergency call.


indicates not to allow an emergency call.

For MS with access levels 11~15, the emergency call will not be allowed only when the corresponding access control class bit is set to 0 and Emergent Call Disable is set to Yes at the same time.


When MS is powered on, it will try to find a GSM PLMN. MS will select an appropriate cell and read cell parameter. This process is called cell selection. The appropriate cell is


and read cell parameter. This process is called cell selection. The appropriate cell is restricted by many factors. For example, whether this cell belongs to the selected network (under manual network selection mode), whether the cell is barred, the cell selection priority of the cell, whether the access level of MS is prohibited by the cell, whether the quality of radio channel meets the requirement of communication, etc. Among them, the quality of radio channel is one important factor of cell selection. GSM specification stipulates one parameter called path loss principle C1. The appropriate cell must ensure the C1>0. C1 is obtained by the calculation of receiving level and the cell selection parameters.

After MS selects cell, MS will settle in the selected cell. At the same time it begins to measure the signal level of BCCH of neighbor cells. It records six neighbor cells with the strongest signals (refresh at least every 60s), extracts various cell parameter and control information of each neighbor cell. (MS must conduct data block decoding for all the six strongest BCCH of neighbor cells within 5 minutes, including parameters affecting cell reselection. When MS regards a new neighbor BCCH as one of the six strongest BCCHs, it will conduct data block decoding for this new BCCH at least every 30s). Moreover, MS must check one of the six strongest at least every 30s. If BSIC has any changes, it will be regarded as a new BCCH and data decoding will be conducted again. During this process, MS will not stop monitoring PCH. When a certain condition is satisfied, MS will move from current cell to anther cell. This process is called cell reselection. The condition includes many factors, and they are all related to the quality of radio channels. When a neighbor cells radio channel quality is better than current cells, cell reselection occurs. The channel quality standard for cell reselection is C2. C2 is obtained by calculation of the receiving level and a number of parameters.


If the SIM card of MS has not stored any BCCH (usually it is a new SIM card), it will search all the 124 RF channels and 374 more GSM1800 channels for dual-band MS, and measures the receiving signal


RF channels and 374 more GSM1800 channels for dual-band MS, and measures the receiving signal level of each. The whole process lasts about 3s to 5s. During this period, MS obtains at least 5 measurement samples from each RF channel. Then MS tunes to the carrier with strongest receiving level, and judge whether it is BCCH (by searching for FCCH). If yes, MS tries to decode SCH and makes itself synchronous with that BCCH, then the MS reads cell parameter on it. If MS can correctly read the cell parameter and verify this cell belongs to the selected PLMN and its C1 is larger than 0, and also its cell selection priority is normal, then MS conducts location update. After passing, MS resides in that cell. Otherwise, MS will tune to second strongest BCCH and go on with the same procedure. If after trying the strongest 30 (single-band) or 40 (dual-band) carriers it still cant find a suitable cell to reside in, the MS will try to access the cells with low cell selection priority. If still unsuccessful, the MS will try the cells of other PLMNs which are allowed by the SIM card. If failed again, MS will stay at a cell (signal is the strongest, C1 is larger than 0, cell selection priority is not prohibited.) without considering the PLMN and enter the emergency call mode (service bar mode). Meanwhile, the MS keeps on monitoring all RF channels.

Note:

1. When MS access level is prohibited by this cell, cell selection algorithm is not affected. If the condition is fulfilled, MS will still try to reside in this cell.

2. MS belongs to the PLMN selected, but is prohibited from access, or C1


When MS is powered off, it will store some BCCH carrier information. When MS is powered


on, it will first search the BCCHs which have been stored. If MS can decode the BCCH data of this cell but cannot reside, it will check the BA table of this cell and try these BCCHs. If it still cannot pass, MS will start the cell selection process without BCCH list.


Search all frequency


Judge whether it is BCCH

If MS can correctly read the cell parameter and verify this cell belongs to the selected PLMN and its C1 is larger than 0, and also its cell selection priority is normal, then MS conducts location update. After passing, MS resides in that cell.

Otherwise, MS will tune to second strongest BCCH and go on with the same procedure. If after trying the strongest 30 (single-band) or 40 (dual-band) carriers it still cant find a suitable cell to reside in, the MS will try to access the cells with low cell selection priority.

If still unsuccessful, the MS will try the cells of other PLMNs which are allowed by the SIM card.

If failed again, MS will stay at a cell (signal is the strongest, C1 is larger than 0, cell selection priority is not prohibited.) without considering the PLMN and enter the emergency call mode (service bar mode).


To prevent MS from accessing the system when the receiving signal level is


very low (this will make unsatisfactory communication quality and waste the radio resources of the network), GSM specifications require that the MSs receiving level must be greater than a threshold when it needs to register in the network. The threshold is the RXLEV_ACCESS_MIN.

For some cells with high traffic, this parameter can be increased appropriately to lower the values of C1 and C2 of this cell. Accordingly the effective coverage range of the cell will be reduced. But the value of RXLEV_ACCESS_MIN should not be too big, otherwise coverage hole (with regard to idle mode MS) will be created at the edge of the cell. When this method is used to balance the traffic, the value of RXLEV_ACCESS_MIN is recommended to be no more than 20.

Except for the areas with densely distributed BTS and good coverage, generally it is not recommended to use RXLEV_ACCESS_MIN to adjust the traffic of the cell. For isolated BTS or BTS with poor coverage, this value should be set properly, otherwise the call drop rate may increase and QoS will be affected.


During MSs communication with BTS, its transmitting power is controlled by the network via power control command. This command is transmit in SACCH (There are two head


via power control command. This command is transmit in SACCH (There are two head bytes. One is power control byte, the other is time advance). MS must get the power control head from the downlink SACCH, and output the power as is indicated in the in the power control header. If the MS cannot support the power level in the power control header, it will use the nearest power level it supports.

Since SACCH is associated signaling channel, it must be combined with other channels, such as SDCCH or TCH. Therefore, the control of MSs power begins after MS receives SACCH. While the power level used by MS before it receives SACCH (power used when RACH is sent) is determined by the MS TXPWR_MAX_CCH (maximum power level of control channel).This parameter will affect cell selection and cell reselection.

C1 = RLA_C - RXLEV_ACCESS_MIN - MAX((MS_TXPWR_MAX_CCH - P), 0)RLA_C: mean receiving level of MS

RXLEV_ACCESS_MIN: minimum receiving level of MS permitted to access

P: maximum physical supported transmitting power of MS.


The network operator can set the cell access is permitted or not by the parameter. Usually all cells allow MS to access, thus it is set as No. But in the special cases, the


Usually all cells allow MS to access, thus it is set as No. But in the special cases, the operator may want a cell to be used for handover service only, which can be realized by setting the parameter as Yes (CBQ should be No in this case).


For the area overlapped by cells, the operator often wants MS to


preferably select certain cell during cell selection according to the cell capacity, traffic and cell functions, i.e. setting the cell priority. This function can be implemented by setting parameter cell bar quality. It works with parameters cell bar access together to determine the cell selection and cell reselection priority of the cell.

Usually the priorities of all cells should be set as Normal. But in some special cases such as micro-cell and dual-band network, the operator may expect MS to preferably enter the cells of a certain type. In this case, the network operator can set the priority of this type of cell as Normal while setting the other cells as Low. MS will select the cell with lower priority only when there is no appropriate cell with the priority as Normal. During the network optimization by means of cell priority, it is necessary to note that CBQ only influences the cell selection. Therefore, in order to achieve the target, C2 (cell reselection parameter) must be taken into consideration.


Each circle in the diagram indicates a cell. For some causes, the traffic in cell A and that of cell B are obviously higher than those of the adjacent cells. To make the traffic of the


of cell B are obviously higher than those of the adjacent cells. To make the traffic of the entire area distributed on average, set the priorities of cell A and cell B as Prohibited and those of other cells as Normal. In this way, the services in the shadow areas in the diagram will be shared by the adjacent cells. It must be pointed out that this setting will reduce the actual coverage areas of cell A and cell B, which is different from decreasing the transmitting powers of cell A and cell B.


Assume that micro-cell B and macro-cell A together cover an area. In order to make micro-cell B share more traffic of macro-cell A, the priority of cell B can be set as Normal


micro-cell B share more traffic of macro-cell A, the priority of cell B can be set as Normal and that of cell A as Low. Thus in the coverage area of cell B, MS will select cell B as long as the level of cell B reaches the RXLEV_ACCESS_MIN, no matter cell B has a lower signal level than cell As or not. And then reselection parameters can be set appropriately to make MS not to reselect cell A.


C2=C1+CRO-TO*H(PT-T) When PT is not equal to 31:


C2=C1-CRO When PT is equal to 31.

Wherein:

1. Function H(x)=0, when x0.2. T is a timer with initial value 0. When a certain cell becomes one of the six strongest neighbor cells, T corresponding to this cell begins to count. When the cell is out of the six strongest neighbor cells, the corresponding timer is reset.

3. CRO is used to revise the C2 intentionally.

4. The function of TO is to reduce the value of C2 from T begins to T reaches the stipulated PENALTY_TIME.

5. PT is the time that TO functions on C2. But if PT=31, the C2 formula is changed as C2=C1-CRO.


After cell selection, MS will reselect another better cell in idle mode. It is C2 parameter that determines cell reselection. The principle for MS reselection is: select the cell with


that determines cell reselection. The principle for MS reselection is: select the cell with the maximum C2 as the serving cell. C2 is determined by the following factors:

C2=C1+CRO-TO*H(PT-T) (PT


TO indicates the temporary modification on C2. Temporary means that it functions for C2 only within a duration which is determined by PT parameter. Setting of this parameter


only within a duration which is determined by PT parameter. Setting of this parameter only affects MS of GSM Phase II and above.


If the communication in a cell is affected due to very heavy traffic or some other reasons, this cell should not be the preferable cell that MS works in (a repulse should be made for


this cell should not be the preferable cell that MS works in (a repulse should be made for this cell). In this case, PT can be set as 31, which causes TO invalid and C2=C1 CRO. Therefore, C2 value of this cell is decreased. MS will reselect this cell with little possibility. Besides, the network operator can set CRO according to the repulse degree to this cell. The higher the repulse degree, the larger CRO.

For the cell with very low traffic, MS should prefer to work in this cell. In this case, CRO is recommended to be between 0-20dB. It can be set according to the preference degree to this cell. The higher the preference degree and the larger CRO. Generally TO is recommended to be the same as or a little more than CRO. PT is mainly used to prevent MSs too frequent cell reselections. Generally it is recommended to be 0 (20s) or 1 (40s).

For the cell with medium traffic, generally CRO is recommended to be 0 and PT be 31 as a result of C2=C1.

Setting of PT can effectively prevent the fast moving MS from accessing the micro-cell. This parameter can be set according to the size of micro-cell. And it is recommended to be 20s for the ordinary micro-cells. When PT is set as 31, it is used to change the direction of CRO.


This parameter is to inform MS whether C2 is used as the standard for cell reselection. The minimum interval between cell reselections caused by C2 parameter is 15s to avoid


The minimum interval between cell reselections caused by C2 parameter is 15s to avoid too frequent cell reselection.

If PI=Yes, C2 is used for cell reselection standard; if PI=No, C1 is used for cell reselection.


This parameter aims to prevent the frequent location update that may increase the network signaling flow and to reduce the possibility of paging message loss. If the value of this


signaling flow and to reduce the possibility of paging message loss. If the value of this parameter is too small, the location update will have ping-pong effect and the signaling load on SDCCH will increase. Moreover, the call setup successful rate of the system will become lower because MS will not respond paging during location update. When it is too large, the cell where MS resides for a long time may not be the best when MS enters a new location area. When MS reselects a cell from a different location, MS will start a location update. Due to the fading of radio channel, C2 values of two cells at the their edges will fluctuate, which causes MS to reselect frequently. To reduce the influence, GSM specifications define a parameter called cell reselection hysteresis. It is required that MS start cell reselection only when C2 value of the adjacent cell (in a different location area) is greater than that of serving cell and their difference is greater than the value of reselection hysteresis.

Appropriate cell reselection hysteresis is important for network optimization. Usually it is recommended to be 8~10dB and can be adjusted in the following cases:When the traffic of an area is very heavy and the signaling overload often occurs, it is recommended to increase the value of cell reselection hysteresis of the adjacent cells belonging to different LACs.

When the overlapping coverage of the adjacent cells belonging to different LAC are wide, it is recommended to increase the value of cell reselection hysteresis.


In case of call drop, MS can start call re-establish process to resume the conversation. The network has the right to decide whether to re-establish or not. This function is


The network has the right to decide whether to re-establish or not. This function is achieved by setting Call Reestablishment Forbidden.

In some special cases (e.g. the cell has a dead zone at a fixed position), call drop will occur if MS passes through the dead zone during the conversation. If call re-establishment is permitted, the dropped call be resumed. However, call re-establishment takes relatively long time. Most of the subscribers have already released manually before the re-establishment is completed. Therefore, the re-establish of call not only fails to achieve the target, but also waste a lot of radio resources. So it is recommended that except for some special cells, it is not permitted to re-establish the call on the network.


This parameter is used to concurrently deliver the channel activation message and immediate assignment command message to quicken the processing of


and immediate assignment command message to quicken the processing of signaling. Thus the response speed of the network is ensured.

This function is used in satellite transmission mode. In satellite transmission mode, the Immediate Assignment Opt parameter must be set to Yes to minimize the effect of the satellite transmission on time delay. In terrestrial transmission mode, the default value of this parameter is No.


In TCH assignment process, if there is no TCH available, hand over can take place instead of assignment, and this is called direct retry.


place instead of assignment, and this is called direct retry.

Direct retry is not a main measure to solve traffic congestion. Instead, it is only an emergency operation during traffic peak in local radio network.

If direct retry takes place frequently in a certain part of the radio network, it is necessary to adjust BTS TRX configuration and even the network layout.


In TCH assignment process, if there is no TCH available, hand over can take place instead of assignment, and this is called direct retry. Direct retry is not a


place instead of assignment, and this is called direct retry. Direct retry is not a main measure to solve traffic congestion. Instead, it is only an emergency operation during traffic peak in local radio network. If direct retry takes place frequently in a certain part of the radio network, it is necessary to adjust BTS TRX configuration and even the network layout.


During the asynchronous handover, MS constantly sends the handover access Burst to BTS. When BTS detects the Burst, BTS


handover access Burst to BTS. When BTS detects the Burst, BTS send physical information to the MS on the main DCCH/FACCH, and starts timer T3105. At the same time, it sends the MSG_ABIS_HO_DETECT message to BSC. The physical information contains related information of different physical layers so as to guarantee the correct access of MS. If the timer T3105 times out before receiving the SAMB frame from MS, BTS re-sends physical information to MS.


When BTS sends physical information to the MS, the BTS starts the timer T3105. If the timer T3105 times out before receive the SAMB frame from


T3105. If the timer T3105 times out before receive the SAMB frame from MS, BTS re-sends physical information to MS and restarts the timer T3105. The maximum times for re-sending physical information is Ny1.

The physical information is sent through FACCH, which is sent every four TDMA frames and takes 18 ms. If the value of T3105 is smaller than or equal to 18 ms, then BTS should re-send the physical information to the MS when the timer T3105 times out for the first time. This timeout is meaningless if the physical information sent at the previous time is not sent on the FACCH because the time is shorter than a FACCH period. Considering the previous factors, 20 ms is the reasonable minimum value for this parameter. At present, the default value of this parameter is 70 ms.


In order to monitor the communication quality of radio link and conduct


power control, both MS and BTS must have measuring function. But the measurement will encounter some problems when several independent functions of GSM specification are combined for use. First, GSM specification allows BCCH frequency to attend frequency hopping (excluding the BCCH timeslot); secondly, GSM specification allows to conduct downlink power control for the frequency hopping channel; lastly, the power of BCCH frequency is not allow to be changed because MS needs to measure the signal level of the adjacent cells. In the above conditions, the downlink power control of a channel can only be applicable to a subassembly of the frequency assembly that the channel is using, that is, the BCCH frequency during the frequency hopping process cannot be power-controlled. If MS measures the downlink channel level in ordinary mode, the measurement result involving the BCCH frequency will be inaccurate for the power control. To reduce the influence of this problem upon power control, MS is required to deduct the receiving level value obtained from the timeslot of BCCH carrier when calculating the average value of receiving level during frequency hopping. To make MS execute the above operation, the parameter PWRC should be set as yes in the system.


The dual timeslot extended cell combines two timeslots to provide sufficient delay. Theoretically, a maximum TA value of 219 (63+156) is supported, i.e.,


delay. Theoretically, a maximum TA value of 219 (63+156) is supported, i.e., the maximum coverage radius can be 120km theoretically. Dual timeslot extended cell can be divided into two types: Cell level and TRX level. Versions after 03.1120A (BTS 00.1130A and latter) support TRX level dual timeslot extended cell.

For dual timeslot extended cell, MAX TA should be set as 219, or calls may fail even there are good signals. For the TRX where the main BCCH/SDCCH/GPRS are located, the concentric circle (HW_IUO Property) must be set as overlaid subcell, or a prompt of error is displayed. For dual timeslot TRXs in the dual timeslot extended cell, only the even timeslots are effective, while the normal TRXs in the dual timeslot extended cell are the same with the TRXs in normal cell, i.e. eight timeslots are all effective. The RF hopping of dual timeslot extended cell is similar to that of normal cell. But if the hopping mode of the dual timeslot extended cell is baseband hopping, the identical MA group should be applied to TRXs of the same type (either all dual timeslot TRXs or all normal TRXs), or a prompt of error is displayed.


When the TCH is idle mode, BTS measures the uplink signal strength and sort the results into different interference bands and then sends them to BSC. BSC


the results into different interference bands and then sends them to BSC. BSC will use these data as a basic criteria to allocate the channels. Interference can be divided into six grades according to the interference signal strength. The signal level used to define the grades is called inference band threshold. Based on these interference band thresholds, BTS can know which band the current interference belongs to, and send it to BSC via radio resources indication messages.

In the traffic statistics system, we can register interference band in cell measurement function. Interference band 1 to 5 are supported. The definition of the interference bands are listed below.

Interference band 1 means the interference intensity is below interference band threshold 1;

Interference band 2 means the interference intensity is between interference band threshold 1 and 2;



Interference band 5 means the interference intensity is above interference band threshold 4.

Because interference band threshold 0 is too weak and interference band


DTX has very limited influence on the conversation quality. But its application has two advantages: one is that the interference on radio channel is effectively reduced and a


advantages: one is that the interference on radio channel is effectively reduced and a better average conversation quality can be achieved; the other is that DTX can considerably reduce the power consumption of MS. Therefore DTX is recommended on the network.

According to the protocol, MS reports BTS with two kinds of measurement report. One is called full measurement report. It will average the levels and qualities of 100 timeslots in the whole measurement cycle (one measurement cycle involves 4 TCH-multi-frames except idle frames). The other is called sub measurement. It averages the levels and qualities of 12 timeslots, including eight consecutive TCH bursts and four SACCH bursts.

According to GSM specification, both BTS and MS should conduct this two kind of measurement (FULL and SUB) no matter whether the uplink/downlink DTX of the system is activated or not. Each measurement report of SACCH indicates whether DTX is active or not. According to this indication, BTS can make the right selection, either FULL or SUB.


Procedure analysis:


The place is located in a highway section in suburb and is about three kilometers away from downtown. The terrain structure does not fluctuate seriously. Theoretically, the signal strength should be about -80dBm, which is very different from the actual value measured. After conducting frequency scanning here, we find that cell A, B and C are covering here. Their frequencies are Fa, Fb, Fc and signal strength is -95, -80dBm, -92dBm respectively. After checking the data, it is found that cell B is not configured as cell As neighbor, thus an island is formed between cell A and B. When MS comes from cell A goes towards B, it cannot have cell reselection or handover to B and this leads to as if the coverage near cell B is not good.


Procedure analysis:


This BTS site originally worked normally. The problem arose recently. Since the subscriber complained that the problem were most serious near BTS A and B, we decided to check these two BTS first. First we checked the alarm on BTS A and B, it was normal. Then we checked the traffic statistics data and found out that the SDCCH requests in cell 2 of BTS A rose from about 3000 in 17th May to about 6000 in 18th May. The obvious change began from 7:00 AM on 18th May. At the same time, SDCCH attempt failures rose suddenly to more than 3000. It was obviously abnormal. And SDCCH congestion rate in cell 3 of BTS A was about 30%. But the traffic volume, congestion rate, and call drop rate of BTS A is normal. No abnormal situation is found in BTS Bs traffic measurement. After checking the SDCCH traffic measurement of all the adjacent cell of BTS A, we found a cell in which times of SDCCH attempt decreased dramatically. The SDCCH statistics result showed the problem should exist in cell2 and 3 of BTS A.

Where did the 3000 times of SDCCH requests come from? Analyzed from the failure, we suspected that the data was modified, so we checked the BSC data operation log. Using MA10 to trace the network access flow of MS, and we found a lot of LOCATION UPDATE REJECTED messages. Since there was no record of modification of BSC data, we checked MSCs related cell parameters, and found that CGI of cell 2 and 3 of BTS A were modified. All the three cells in BTS A were using exactly the same CGI. After correcting the CGI of cell 2 and 3, everything was back to normal.


Cause analysis


We check the parameter RXLEV_ACCESS_MIN in data management system, and find out that RXLEV_ACCESS_MIN of correlative cells are not consistent, and some even have great differences. Then in this area, MS is quite likely to reselect the cell with poor signals. And subscribers will feel the obvious fluctuation of signals.


Cause analysis:


Recently, no modification is conducted on the BSC. The current network only differs from original network in the number of subscribers. In VLR, there are more than 4000 local subscribers and 5000 roaming subscribers. Therefore, the problem might be caused by the sudden increase of subscribers.

The pressure brought by subscriber increase on network is mainly shown in two aspects: (1) TCH (traffic channel) congestion rate increases; (2) SDCCH (signaling channel) congestion rate increases. MS maybe cannot make MOC or MTC, and at the same time cannot conduct location updating successfully. The direct result of location update failure is MS loses connection with the network. Check BSC cell parameter data table. It is found that T3212 is set to 2 (unit: 6 minutes), i.e. 12 minutes, and MSC corresponding time is set to 30 minuets. This kind of configuration will cause the result that all activated MS will originate a periodic location update every 12 minutes. When the number of subscriber reaches a certain limit, SDCCH will be fully seized. If at this time some MS originates periodic location update, it will fail because there is no spare SDCCH available. Thus MS loses the connection with the network.

Documents

2- OMO111020 BSC6000 GSM V9R8C01 Cell Parameters ISSUE1.01.pdf