Call Setup Failure

Prepared: MEX/TIM Luis Batallan TEM/T- Approved Ericsson: Approved Telcel:

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Subdirección de Calidad de Servicio y Evaluación del Desempeño de la Red

CALL SETUP FAILURE Date: 2005-06-22 Rev. A

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CALL SETUP FAILURE

REVISION RECORD Date Revision Sections affected Revised by

2005-06-22 A First version Luis Ricardo Batallan


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CALL SETUP FAILURE

Table of contents

1 ABSTRACT 0 2 PURPOSE OF THE DOCUMENT 0 3 SCOPE 0 4 INTRODUCTION 0 4.1 RR CONNECTION ESTABLISHMENT (RR) 0 4.2 SERVICE REQUEST (RR) 0 4.3 AUTHENTICATION (MM) 0 4.4 CIPHERING MODE SETTING (RR) 0 4.5 IMEI CHECK (MM) 0 4.6 CALL INITIATION (CC) 0 4.7 TRAFFIC CHANNEL ASSIGNMENT 0 4.8 CALL CONFIRMATION AND CALL ACCEPTED 0 5 STATISTICAL ASSESSMENT 0 5.1 RR Connection establishment 0 5.2 Service Request 0 5.3 Security Functions 0 5.4 Call Initiation 0 5.5 Traffic Channel Assignment 0 6 CONCLUSIONS AND RECOMMENDATIONS 0 7 REFERENCES 0

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1 ABSTRACT

After drive test evaluation by “Calidad de Servicio Región 9” has been found some problems during Call setups in the GSM system. The problem can be assessed from statistics point of view to determine in which part of the process the Call Setup is failing.

2 PURPOSE OF THE DOCUMENT

The purpose of this document is to describe: the procedure used by the GSM system for a Call Setup and how the performance for this process can be assessed.

3 SCOPE

The TELCEL RF optimization team, which is troubleshooting Call Setup failures.

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INTRODUCTION

Call set-up within GSM consists of the following main steps:

• RR connection establishment

• Service request

• Authentication

• Ciphering mode setting

• TMSI-Reallocation

• IMEI Check

• Call initiation

• Assignment of a traffic channel

• User alerting

• Call accepted

The above steps are taken for both mobile originating and mobile terminating calls. However, for mobile terminating call they must be preceded by an interrogation phase (the system must first find out where the subscriber is located at that moment)

The process is outlined in Figure - 1 for mobile terminated call setup, being the process for mobile originated call setup very similar with the following differences:

• In RR Connection Establishment, there is no paging message. Only channel request and immediate assignment.

• In Service Request, the MM message “CM Service Request” is used instead of the RR message “Paging Response”.

• In Call Initiation, the “Setup” message is sent by the mobile and the network replies with “Call Proceeding”.

• In Call Confirmation and Call Accepted, the messages go in the opposite direction (compared to Mobile Terminated Call Setup)

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MS BTS BSC MSC

Paging Request (PCH)Paging Command

Paging

Channel Request (RACH)Channel Required

Channel Activation

Channel Activation Ack

Immediate Assign Cmd(Immediate Assignment)Immediate Assignment

(AGCH)

SABM (Paging Response)

UA (Paging Response)Est Ind (Paging Resp)

Paging Response

Authentication RequestData Req (Auth Req)

Authentication Req

Authentication ResponseData Ind (Auth Resp)

Authentication Resp

Ciphering Mode CmdEncryption Cmd(Ciph Mode Cmd)

Ciphering Mode Cmd

Ciphering Mode CompleteData Ind (CipheringMode Complete) Ciphering Mode Comp

Identity RequestData Req (Identity Req)

Identity Request

Identity ResponseData Ind (Identity Resp)

Identity Resp

SetupData Req (Setup)

Setup

Call ConfirmedData Ind (Call Confirmed)

Call Confirmed

Assignment ReqChannel Activation

Channel Activation Ack

Assignment Cmd

SABMUA

Est IndAssignment Complete

Data Req (Ass Cmd)

Data Ind (Assign Comp)(Assignment Complete

RF Chan Release

RF Chan Release Ack

Connect Data Ind (Connect)

Connect

Connect AckData Req (Connect Ack)Connect Ack

RR ConnectionEstablishment (RR)

Done on CCCH

Service Request (RR)

Done on SDCCH

Authentication (MM)

Done on SDCCH

Cipher Mode Setting (RR)

Done on SDCCH

Identity Check (MM)

Done on SDCCH

Call Initiation (CC)

Done on SDCCH

Assignment of a TCH (RR)

Done on SDCCH andFACCH/TCH

Establishlayer 2 linkon SDCCH

Establishlayer 2 linkon FACCH

DeactivateSDCCH inBTS

MS

Yes

BzzAlerting Data Ind (Alerting)

Alerting

Call Confirmation and CallAccepted (CC)

Done on FACCH/TCH

ActivateSDCCH inBTS

ActivateTCH inBTS

Figure 1- Call Setup Process

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4.1 RR CONNECTION ESTABLISHMENT (RR)

MSC/VLR initiates the call set-up procedure by sending the Paging message to the relevant BSCs. The actual information contained in the Paging message depends on whether IMSI or TMSI is used in the Paging Request message.

The BSC detects the Paging message and translates the LAI into a Cell Global Identity (CGI) if the cell has not been provided in the paging message. The BSC sends the Paging Command message to the relevant BTSs. This message contains the IMSI or TMSI, channel number (channel type and time slot number), and paging group. In this case, the channel type is downlink CCCH (PCH). The paging group is determined by the IMSI and two other parameters defined in BSC and broadcast as system information.

Finally, the BTS sends the Paging Request message to the MS. It is sent on the PCH, and it occupies four bursts.

When the paged MS receives a Paging Request message it responds by sending a Channel Request message, thereby requesting a signaling channel for call set-up. The Channel Request message is sent on the Random Access Channel (RACH) and consists of 8 information bits: a random number (5 bits) and an establish cause (3 bits).

In GSM specifications phase 2, the number of bits used to establish cause can be 3 through 6. Accordingly, the random number will be described using 5 down to 2 bits. The establish cause can be:

• Answer to page

• Originating call

• Location updating

• Emergency call

• Others, for example IMSI detach, SMS, supplementary services management

When the BTS detects an access burst, it sends a Channel Required message to the BSC to request a signaling channel for the MS. This message contains the access delay of the access burst and the request reference parameter. BSC compares the access delay value with the maximum permitted timing advance parameter value to ensure that the MS is within the allowed range. BSC then determines which channel to use and sends the identity of this channel to the BTS in the Channel Activation message. The contents of this message are:

• Reason for allocation

• A complete description of the channel containing the channel

• Description information element

• DTX applied or not

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• MS power

• BS power

• Timing Advance (TA)

The BTS then activates the channel and confirms this by sending the Channel Activation Acknowledge message.

The BSC then sends the Immediate Assignment command message containing an Immediate Assign message to the mobile station.

The message tells the MS to switch to the allocated signaling channel (SDCCH+SACCH). The message contains:

• The channel description information element (Channel Type, ARFCN, TN, TSC, MAIO, HSN)

• The frame number when the Channel Required message was detected

• The random reference from the access burst

• The TA that the MS will use

The MS compares the random reference and the frame number with those stored in the MS. If they correspond, the MS goes to the service request step.

4.1.1 Timers for RR Connection Establishment and Service Request (ALOHA)

Figure 2 – Timers for RR connection establishment

To avoid collisions in the BTS when more than one mobile attempt to access the system, for example when they are paged in one message; the Aloha procedure is used to spread out the accesses. This procedure makes use of timer T3120 (T3126 Ph2 MS) in the MS.

T3126 for Phase 2 MS

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When the MS receives a Paging message or when it decides to access the system for other reasons, the MS generates a random number between 0 and 8 and an integral value defined by the parameter called TX minus 1. Then it starts timer T3120 (T3126 Ph2 MS) that expires after this random number of TDMA–frames. When the timer expires, the MS sends the random access burst with the Channel Request.

The MS restarts T3120 (T3126 Ph2 MS) and waits for an order (Immediate Assignment) from the network to go to an SDCCH where the signalling can continue. The new random number is generated in the MS, this time ranging between 0 and TX. The parameter TX, which is broadcast on the BCCH, has a maximum value of 50. In GSM specification phase 2, the time that the MS must wait after the restart depends on the non-combined or combined SDCCH and the value of TX. Note: For TELCEL network most of MS are phase 2, so for the current parameter setting (TX=20) the time between channel requests retries will be approx. 0.5 seconds. T3126 as explained has a random value, but the maximum is 5 seconds.

4.1.2 Random Access Failure

If the MS does not receive an Immediate Assignment from the network before T3120 (T3126 Ph2 MS) expires, it sends a new random access burst with a channel request. The number of times the MS can attempt to access the system is limited. The MS knows what the limit is from the system information read on the BCCH. Note: The number of times the can attempt to access is determined for the cell parameter MAXRET, for TELCEL network the current parameter setting is between 2 and 4.

T3120 (T3126 Ph2 MS) is stopped when the Immediate Assignment message is received.

On the network side there are two timers:

• T3113, which starts when Paging Request is sent and stops when Paging Response is received (PAGTIMEFRST1LA, PAGTIMEREP1LA, PAGTIMEFRSTGLOB, PAGTIMEREPGLOB)

• T3101 (default 15 decisecs.), which supervises the allocation of a channel and hence starts when the Immediate Assignment message is sent and stops when the SABM frame is received.

If T3113 expires before the Paging Response is received, the Paging Request message can be sent again according to an exchange parameter set by the operator. If T3101 expires however, the request is cancelled and the channel released.

4.1.3

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Immediate assignment reject

When the immediate assignment is rejected by the system, on receipt of the IMMEDIATE ASSIGNMENT REJECT message, the MS stops T3120, starts timer T3122 with the indicated value, ("wait indication" information element set by BSC timer 'WAIT_INDICATION') and returns to CCCH idle mode (listening to the paging channel). The MS is not allowed to make a new attempt to establish an RR-connection in the same cell until T3122 expires.

The IMMEDIATE ASSIGNMENT REJECT message is sent in the following situations:

a) The random access is a request for 'Location Updating' or 'other procedures that can be completed with an SDCCH and NECI set to 1' and no channel available (default value for T3122=10 sec.)

b) The Central Processor in BSC is overloaded (default value for T3122=20 sec.)

c) Too many random accesses are received within 100 ms (more than 35 for APZ 212 11, more than 90 for APZ 212 25, more than 150 for APZ 212 20 and to be defined for APZ 212 30). (Default value for T3122=200 sec.)

Figure 3 – Immediate Assignment Reject

4.1.4 Assignment Successful

For the successful assignment there are two “guard timers” for the establishment of MS data kink connection and the establishment of MS layer 3 connection respectively as outlined on the figure 4:

• T3107a (default 6 sec.): Guard time for establishment of MS data link connection (assignment procedure). Started as message 'ASS_CMD' is sent to MS and stopped by reception of 'EST_IND' from BTS.

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• T3107b (default 6 sec.): Guard time for establishment of MS layer 3

connection (assignment procedure). Started as message 'EST_IND' is received from BTS and stopped by reception of 'ASS_CMP' or 'ASS_FAIL' from MS.

Figure 4 – Assignment Successful

If the timers expires and the ASSIGNMENT COMPLETE has not be sent, then a CLEAR REQUEST will be send from the BSC to the MSC invoking clear procedure.

4.2 SERVICE REQUEST (RR)

The layer 2 message Set Asynchronous Balanced Mode (SABM) is sent to the BTS as soon as the MS has tuned to the new channel. SABM may contain a layer 3 service request message. In this case, the information field in SABM contains the Paging Response message.

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The SABM message is used to start the acknowledged mode on the SDCCH channel. In practice, this means that the counters for send and receive sequence numbers are reset to zero.

The Paging Response message received from the MS contains the following information:

• MS identity (IMSI or TMSI and possibly IMEI)

• The ciphering key sequence number (CKSN)

• The mobile station class mark 2, that is, the type of MS station being used

The CKSN can be used by the MSC to start ciphering without authentication.

When the Paging Response arrives at the BTS, it is forwarded to the BSC in an Establish Indication message. This message tells the BSC that a change to acknowledged mode has been made. BSC activates radio connection quality supervision in the Transceiver Handler (TRH), which starts the locating function on SDCCH. It also initiates the power control algorithm for dynamic MS power control.

Paging Response is an Initial MS message, which means that it was initially meant to be sent transparently to the MSC. However, the BSC adds CGI before forwarding the Paging Response to the MSC as a Complete L3 message.

The Paging Response is also looped back to the MS in an Unnumbered Acknowledgment (UA) frame. This confirms that only one MS is using the signaling channel. If the paging response is not returned the following case might occur:

Two MSs access the system at the very same frame number, with the same establish cause and the same random reference (there are only 32). If both MSs are strong, they must send another access burst in accordance with the Aloha procedure, which is a way to spread out random accesses in order to avoid consecutive collisions. However, if one of the MSs is weak, the signal does not reach the BTS. Unaware of this, the MS receives the reply sent to the other MS a short while after its own access attempt. The MS tunes to the signaling channel, sends the Paging Response message and then listens for the Paging Response to be looped back. Now the MS hears that the MS identity given is not its own and leaves the channel.

The BSC sets up a logical SCCP connection towards the MSC by sending the Connection Request message, including complete Layer 3 information, the Paging Response message piggybacked. The SCCP Connection Confirm message is returned to the BSC which means that the connection oriented signaling is established on the A interface.

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4.3 AUTHENTICATION (MM)

Figure 5 – Authentication Successful

An exchange property in MSC/VLR defines whether authentication is to take place or not. An Authentication Request is sent transparently to the MS.

The parameters in the Auth. Req. message are the random (RAND) number, 128 bits, and CKSN, 3 bits. When the MS receives the Auth. Req. message it stores the CKSN, which is to be sent in the next service request message. It then calculates the authentication parameter SRES by inserting Ki, the subscriber authentication key that is stored in the SIM card; and RAND in algorithm number A3 as well as Kc by inserting Ki and RAND in algorithm A8.

The SRES parameter is returned to MSC/VLR in the Authentication Response message. The Data Indication (DI) frame is used to send layer 3 messages transparently in the direction from BTS to BSC. In the opposite direction, the Data Request (DR) frame is used.

Timer T3260 (default 12 sec.) on the network side supervises authentication. It is set in the MSC when Auth. Req. is sent and stopped when Auth Resp is received. On the first expire Auth. Req. is sent again, but on the second expire the channel is released (CLEAR COMMAND).

4.3.1 Unsuccessful Authentication

If authentication fails that is, SRES from the MS does not correspond with the SRES value stored in MSC/VLR; the network can distinguish between two different ways of identification used by the MS:

• TMSI

• IMSI

If TMSI is used, the network may initiate the identification procedure, where the MS is told to send its IMSI value. If the IMSI given by the MS differs from that which is stored in the network, the authentication is restarted using the correct parameters. If the IMSI provided is the expected one, the MSC should proceed as described below.

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If the IMSI has been used or the network decides not to use the identification procedure, an Authentication Reject message is sent to the MS. All connections in progress are then released.

The MS receiving the Authentication Reject message deletes its TMSI, LAI, Kc, and CKSN and switches to the “IDLE no IMSI" state. In this state, the MS is only allowed to perform default cell selection, and only emergency calls are permitted.

This is a serious case because it means barring the SIM card from use. To prevent this from happening because of dues to bit errors or software errors, the GSM systems have an exchange parameter controlling unsuccessful authentication. If this parameter is set, then the MSC does not send the Authentication Reject message, but instead rejects the MS access to the system by sending one of the following messages depending on the establish cause (in brackets):

• “IMSI unknown in VLR" (Mobile originating call)

• No message, only disconnection (Mobile terminating call)

• “PLMN not allowed" (Location updating)

• “IMSI unknown in VLR" (Support services control operations)

4.4 CIPHERING MODE SETTING (RR)

Figure 6 – Ciphering successful case

If the authentication procedure is successfully performed, the ciphering mode setting procedure can be initiated by the MSC/VLR (depending on the exchange property setting in the MSC/VLR), which then sends the Ciphering Mode Command to the BSC. This is a BSSMAP message, which contains the Kc.

Kc is forwarded by BSC to the BTS. BTS stores Kc and tells the MS to start ciphering. BTS starts the deciphering. The Ciphering Mode Command message sent from BTS to MS is non-ciphered.

The MS inserts Kc and TDMA frame number into algorithm A5, which yields a ciphering sequence that is added to the message to be sent. The message is called Ciphering Mode Complete, and it does not have any parameters. It only tells the BTS that ciphering has started.

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When the BTS receives the Ciphering Mode Complete message or any other correctly deciphered layer 2 frame, ciphering is started on the network side. Ciphering Mode Complete is then sent in a DI frame to the MSC.

The default value for the Ciphering Timer is 30 seconds, after that if not Ciphering Mode Complete is received on the MSC it will send a CLEAR COMMAND. The default value for T3230 is 15 sec.

4.5 IMEI CHECK (MM)

Figure 7 – IMEI check

The exchange properties in the MSC/VLR define whether or not the IMEI check is performed. If it is, the MSC/VLR orders the mobile station to send its IMEI number. The downlink message, Identity Request, contains information about what the mobile should send back. In case of an IMEI check, the mobile receives a request to send back its IMEI number.

The same Identity Request message can be used in other situations to order the mobile to send back its IMSI number. The IMEI sent by the mobile is checked against information in the network’s Equipment Identity Register (EIR). If the EIR returns the answer “white listed” the mobile is allowed to use the network. If the mobile is “black listed” it will be rejected.

The EIR can also return the answer “gray listed” or “unknown”. In these cases the operator can decide whether the mobile is permitted access the network.

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4.6 CALL INITIATION (CC)

Figure 8 – Call Initiation

Call initiation is started when the MSC sends the Setup message to the MS. This message contains a request for bearer services (GSM - bearer capabilities). That is, speech, data, fax, bit rate, etc. If the MS can handle the required service, it sends a Call Confirmed message to the MSC. If the MS cannot handle the requested bearer services, it sends the release complete message to the MSC with reject cause No. 88, “Incompatible destination". The MSC then releases the connection to the MS and also releases the connection to the calling subscriber with cause number 88.

The call initiation procedure is supervised at the network side with the timer T303 (default value 30 sec.). This timer is set when the Setup message is sent and stopped when the Call Confirmed message is received. If the network does not receive the Call Confirmed message before timer T303 expires it releases the connection to the calling subscriber with cause No. 18, “No user responding". It also releases the connection to the MS. The Setup message is only sent once.

T303

4.7 TRAFFIC CHANNEL ASSIGNMENT

The assignment procedure is initiated by the MSC, which sends the Assignment Request message to BSC. This message contains:

• Priority (a value between 1 and 14)

• CIC for trunk select to carry the traffic (speech or data) between MSC and BSC

• DTX flag indicating whether DTX should be used on the downlink

• Optionally, radio channel identity. By using this parameter the MSC can force the BSC to choose one particular radio channel for O&M reasons.

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Figure 9 – Assignment of traffic channel

The list from the locating function is then evaluated in BSC and depending on the result, one of the following assignment procedures is started:

• Assignment to serving cell

• Assignment to another cell in the same BSC

• Assignment to an external cell

Only assignment to serving cell is described below.

The BSC software reads the value of the timing advance and calculates MS power level from the locating function, selects an idle traffic channel and sends the Channel Activation to the BTS.

Channel Activation is the same type of the message that is used for assigning the SDDCH + SACCH in the RR connection establishment. But in this case the channel type is set to Bm + ACCH meaning full rate TCH + SACCH + FACCH.

The BTS acknowledges the allocation of the speech channel by sending a Channel Activation Acknowledge message.

The BSC then sets up a path through the group switch, and ensures that there will be no disconnection due to missing Measurement Reports from the MS when the MS disappears from the old channel.

The BSC sends the Assignment Command towards the MS, telling the mobile station to switch to the new channel (TCH + SACCH + FACCH). This message is sent on the SDCCH, and it consists of a complete channel description.

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The MS tunes to the new physical channel and sends SABM (on FACCH) to indicate that the channel is correctly seized and that SACCH is then started in acknowledged mode. As the BTS receives this message, it returns UA to the MS and sends an Establish Indication message to the BSC.

The MS then sends an Assignment Complete message to the MSC indicating that the traffic channel is up and running.

Finally, the BSC tells the BTS that the signaling channel is no longer needed by sending the RF Channel Release message. This message is acknowledged as soon as the connection is released.

4.8 CALL CONFIRMATION AND CALL ACCEPTED

Figure 10 – Call confirmation and call accepted

The call confirmation procedure starts as the MS sends the Alerting message to the MSC. This message indicates that a ringing tone has been generated in the MS and it can be used for user-to-user signaling or for invocation of supplementary services. The message is sent transparently.

When the Alerting message is received, the MSC/VLR sends the TUP Address Complete (ACM) message to the calling subscriber, who can now hear the ringing tone generated in the MSC.

When the MS user answers, the Connect message is sent to the MSC. This message contains facility, user-user signaling and a progress indicator. When the message is received at the MSC, a Connect Acknowledgment is sent back to the MS, and the originating exchange is informed by means of a TUP or ISUP Answering message and also charging information.

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5 STATISTICAL ASSESSMENT

5.1 RR CONNECTION ESTABLISHMENT

Figure 11 – RR Connection establishment counters

On the RR connection establishment there are counters at BSC level for Channel Required and the result of this request: NOFFIEX: is incremented when capacity is requested for a received CHANNEL REQUIRED message in BSC if establishment cause is equal to ’Answer to paging’. NFTDIEX: Number of fetched incoming calls that has been accepted by Processor Load Control function since there is enough capacity for the call for which the counter Number of offered incoming calls (NOFFIEX) has been previously incremented. NREJIEX: is incremented if an answer to paging is rejected by the Processor Load Control in BSC/TRC function. NOFFPRIO: Number of offered normal originating connections incremented at reception of the of the CHANNEL REQUIRED message in BSC except if establishment cause is equal to ’Emergency call’, ’Location update’, ’Answer to paging’ or the received Timing Advanced value is too high. NFTDPRIO: Number of fetched normal originating calls is incremented when the Processor Load Control function has accepted a request since there is enough capacity for the call for which the counter Number of offered normal originating calls (NOFFPRIO) has been previously incremented. NREJPRIO: is incremented if normal originating connections, such as originating speech or data calls, SMS and IMSI detach, are rejected by the Processor Load Control in BSC/TRC function. NOFFNPRIO: Number of offered location updates is incremented at reception of location update at the Exchange Input Load Supervision function. NFTDNPRIO: Number of fetched location updates incremented when location updates are fetched by the Exchange Input Load Supervision function.

BSC LEVEL COUNTERSNOFFIEX NFTDIEX NREJIEX NOFFPRIO NFTDPRIO NREJPRIO NOFFNPRIO NREJNPRIO

CELL LEVEL COUNTERS CSIMMASS REJCSIMMASS

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NREJNPRIO: is incremented if the Processor Load Control in BSC/TRC function rejects a location update. After Channel Activation there two counters at Cell Level counting the Immediate Assignment and Immediate Assignment Reject: CSIMMASS: States the number of CS IMMEDIATE ASSIGNMENT commands sent by the BSC.

REJCSIMMASS: States the number of IMMEDIATE ASSIGNMENT REJECT commands sent by the BSC.

Using counters from BSC level and Cell level is possible to check the difference between the “channels required” and “immediate assignment commands sent by the BSC”. Due to use of different level counters some inconsistent results can be obtained (for low traffic cases), but for most of cases still useful.

( )

( ) [ ]%*100_NOFFNPRIONOFFPRIONOFFIEX

SREJCSIMMASCSIMMASSNOFFNPRIONOFFPRIONOFFIEXASSIGNLOST++

−−++=

Figure 12 – Lost Assignments

The CS IMMEDIATE ASSIGNMENT REJECT message will be sent when no channel is available for the assignment. Using the immediate assignment counters is possible to obtain the percentage of immediate assignment rejected due to lack of radio resources.

( ) [ ]%__SREJCSIMMASCSIMMASS

SREJCSIMMASREJASSIGNIMM+

=

Figure 13 – Immediate Assignment Reject

Figure 14 – POPTR1 Assignment failure

POPTR1 ASSIGNMENT FAILURE

0.000.501.001.502.002.503.003.504.004.505.00

13:00 14:00 15:00 16:00 17:00 18:00 19:00 20:00 21:00 22:00 23:00

IMM_ASSIGN_REJ LOST_ASSIGN

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5.2 SERVICE REQUEST

Figure 15 – Service Request

When the connection is established between the MS and the BTS, it send Establish Indication message to the BSC that could contain either “page response” (for mobile terminated calls) or “CM service request” (for mobile originated calls). Then the BSC will start the establishment of link to the MSC by sending a “SCCP connection request”. The “SCCP connection confirm” message is returned to the BSC, which means that the connection oriented signaling is established on the A interface and then the CMSESTAB counter is stepped.

In order to check the “lost attempts” (i.e. not successful and not congested) in this process is possible through STS counters count the total of calls attempts which ones were “lost”.

[ ]%*100__ ⎥⎦⎤

⎢⎣⎡ −−−

=CCALLS

CCONGSUBCCONGSCMSESTABCCALLSATTEMPTSSDCCHLOST

Figure 16 – Lost SDCCH Attempts

Figure 17 – POPTR1 LOST ATTEMPTS

(CM Service Request)

CMSESTAB

POPTR1 LOST SDCCH ATTEMPTS

0.001.002.003.004.005.006.007.008.009.00

10.00

17/0

5/05

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Note: Due to the system will try more than one attempt to allocate a channel in the BTS, it is possible to obtain a little bit higher percentage of “lost attempts” than we actually have using the above formula.

5.3 SECURITY FUNCTIONS

For security functions (Authentication and Ciphering) there are available counters at MSC level on the Object Type Security Handling (SECHAND) in order to check failures in these processes. NAUTREQTOT: Number of sent authentication requests NAUTREQSUCC: Number of successful authentications including check of SRES or XRES NCIPATTTOT: Number of ciphering mode setting attempts NCIPSETSUCC: Number of successful ciphering mode settings

[ ]%*100_ ⎥⎦

⎤⎢⎣

⎡=

CNAUTREQSUCNAUTREQTOTSUCCAUTHENT

Figure 18 – Authentication Success

[ ]%*100_ ⎥⎦⎤

⎢⎣⎡=

NCIPATTTOTCNCIPSETSUCSUCCCIPHER

Figure 19 – Ciphering Success

Figure 20 – POPTR1 SECURITY FUNCTIONS

POPTR1 SECURITY FUNCTIONS

98.8098.9099.0099.1099.2099.3099.4099.5099.60

18/0

5/20

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05

07/0

6/20

05

09/0

6/20

05

11/0

6/20

05

13/0

6/20

05

15/0

6/20

05

17/0

6/20

05

19/0

6/20

05

AUTHENT_SUCC CIPHER_SUCC

0


2424222222222224



5.4 CALL INITIATION

For the Call Initiation there are two counters belonging to the object type “Layer 3 Call Control Counters for MSC/VLR Server” (L3CCMSG): NL3CPROC: Number of sent Layer 3 Call Control CALL PROCCEEDING messages NL3CCONF: Number of received Layer 3 Call Control CALL CONFIRMED messages

[ ]%33*100_ ⎥⎦

⎤⎢⎣⎡=

CPROCNLCCONFNLCONFIRMCALL

Figure 21 – Call Initiation Success

Note: L3CCMSG has currently no data on TELCEL network (probably the Object Type is not activated)

5.5 TRAFFIC CHANNEL ASSIGNMENT

Figure 22 – TRAFFIC CHANNEL ASSIGNMENT

Using the counters for “Assignment Request” and for “Assignment Complete” is possible to obtain the failure in the Traffic Channel Assignment process. TASSALL: Assignment attempts for all MS power classes. Successful attempts are counted in the target cell and failed attempts are counted in the serving cell

TFCASSALL: Assignment complete for all MS power classes

TFCASSALL TFCASSALLSUB THCASSALL THCASSALLSUB

TASSALL

(⎟⎠⎞

⎜⎝⎛ +++−

×=TASSALL

UBTHCASSALLSTHCASSALLUBTFCASSALLSTFCASSALLTASSALLiedServiceDen 100

0


2424232323232324



POPTR1 Service Denied

0.000.050.100.150.200.250.300.350.400.45

15/06

/2005

16/06

/2005

17/06

/2005

18/06

/2005

19/06

/2005

20/06

/2005

Figure 23 – POPTR1 Service Denied

Figure 24 – Service Denied

Using the Measurements for Traffic Channels is possible to detect failures in the channel activation on the BTS. TFCALLS: Call attempt counter

TFMSESTB: Successful MS channel establishment on TCHs

TFCONGSAS: Number of congestion at assignment

TFCONGSHO: Number of congestion at handover

Figure 25 – POPTR1 Channel Activation Failure

[ ]%*100__ ⎥⎦⎤

⎢⎣⎡ −−−

=TFCALLS

TFCONGSHOTFCONGSASTFMSESTBTFCALLSFAILACTCH

Figure 24 – Channel Activation Failure

(⎟⎠⎞

⎜⎝⎛ +++−

×=TASSALL

UBTHCASSALLSTHCASSALLUBTFCASSALLSTFCASSALLTASSALLiedServiceDen 100

POPTR1 Channel Activation Failure

0.00

0.50

1.00

1.50

2.00

16/06/2005 17/06/2005 18/06/2005 19/06/2005 20/06/2005

0


2424242424242424



6 CONCLUSIONS AND RECOMMENDATIONS

From the statistics for POPTR1 we can conclude the following:

- On the RR connection establishment we can see that the IMMEDIATE ASSIGMENT REJECT is a quite high.

- There are some failures on the Channel Activation, but values below 1 % are normal.

- For the Security Functions the rate of failure is low, so this is not an issue

- On the Traffic Channel Assignment the statistics doesn’t show too much failures

7 REFERENCES

- 3GPP TS 04.008 (3rd Generation Partnership Project; Mobile Switching Centre - Base Station system (MSC-BSS) Interface Layer 3 Specification)

- BSC Object Types and Counters, User Guide

- GSM Advanced System Technique Course

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