bss call phases

8/2/2019 BSS Call Phases

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Malaysia - GSM 900 Network BSS Call Phases

ALCATEL 2002 File Offer Reference Date Edition Page

BSS Call Phases.doc 01MYS0617 14/02/02 1.0 1/15

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OFFER

TOCELCOM

Malaysia GSM 900 Network

BSS Call Phases

VOLUME IV SECTION 4.1.3


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TABLE OF CONTENTS

INTRODUCTION..................................................................................................................................................4

INTRODUCTION..................................................................................................................................................4

1. MOBILE TERMINATED CS CALL ...............................................................................................................5

1.1 Paging, Radio and Link Establishment...............................................................................................5

1.2 Authentication and Ciphering..............................................................................................................8

1.3 Call Setup and Normal TCH assignment ............................................................................................9

2. MOBILE ORIGINATED CS CALL ..............................................................................................................11

2.1 Radio and Link Establishment...........................................................................................................11

2.2 Authentication and Ciphering............................................................................................................13

2.3 Call Setup and TCH normal assignment ..........................................................................................14


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TABLE OF FIGURES

Figure 1: Paging, Radio and Link Establishment for Mobile Terminated CS Call ...............................................7

Figure 2: Authentication and Ciphering for Mobile Terminated CS Call ..............................................................8

Figure 3: Call Setup and Normal TCH Assignment for Mobile Terminated CS Call..........................................10

Figure 4: Radio and Link Establishment for Mobile Originated CS Call ............................................................12

Figure 5: Authentication and Ciphering for Mobile Originated CS Call..............................................................13

Figure 6: Call Setup and Normal TCH assignment for Mobile Terminated CS Call ..........................................15


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INTRODUCTION

This document aims at describing the BSS Call Phases of the Celcom offer for the GSM 900 Network

concerning the Base Station Subsystem part. This document deals with the BSS Call Phases at the Mobile

Terminated Calls and Mobile Originated Call levels. The description is compliant with the Celcom requests.

The document is organised as following:

Chapter 1 describes the Call Phases for Mobile Terminated Calls Chapter 2 presents the description of Call Phases for Mobile Originated Calls.


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1. MOBILE TERMINATED CS CALL

1.1 Paging, Radio and Link Establishment

The MSC has an incoming call for an MS. It initiates the paging procedure by sending the PAGING message

to a BSS which controls the cell from the location area in which the MS last reported its presence using the

location updating procedure (at this point in time, MSC starts timer T3113 which guards the reception of

PAGING RESPONSE message).

The BSC sends a PAGING COMMAND message to each one of the BTSs supporting the cells indicated in

the PAGING message.

The BTS formats a PAGING REQUEST message (type 1, 2 or 3) and sends this out at the appropriate time

on the PCH on the air interface.

The MS receives the PAGING REQUEST message which contains either its IMSI or TMSI.

The MS starts the immediate assignment procedure by sending the CHANNEL REQUEST message on the

RACH with a timing advance of '0'.

The BTS sends a CHANNEL REQUIRED to the BSC, the message contains the random number sent by the

MS, the frame number on which the message was received and the measured timing advance within that

frame.

The BSC, on reception of the CHANNEL REQUIRED message, reserves and activates an SDCCH channel

(to perform the immediate assignment procedure), the CHANNEL ACTIVATION message contains the

measured timing advance received in the CHANNEL REQUIRED message, the MS and BS powers are set

to the maximum allowed in the cell. The BSC starts the timer T9103 to supervise the channel activation

procedure.

The BTS allocates the channel, commands the LAPDm layer 2 entity to perform the contention resolution

establishment procedure, starts sending on SACCH the dedicated SYSTEM INFORMATION messages and

sends back the CHANNEL ACTIVATION ACKNOWLEDGE message to the BSC.

The BSC builds an IMMEDIATE ASSIGN COMMAND message and sends it to the BTS. The message is

sent on the same CCCH slot from which the CHANNEL REQUEST originated. The BSC starts the timer

T3101 to supervise the establishment of the link by the MS.

The BTS sends this out as an IMMEDIATE ASSIGNMENT or IMMEDIATE ASSIGNMENT EXTENDED

message on the AGCH or on the PCH.

The MS receives the IMMEDIATE ASSIGNMENT message and checks that the random number and the

TDMA frame number match those previously sent.

If this condition is found true, the MS switches to the indicated channel, sets the timing advance as specified

in the IMMEDIATE ASSIGNMENT or IMMEDIATE ASSIGNMENT EXTENDED message and starts to

establish the LAPDm connection by use of a SABM layer 2 frame which contains a PAGING RESPONSE

layer 3 message.


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When the BTS receives the SABM layer 2 frame with the layer 3 message, it notes the contents of the layer

3 message, sends a UA layer 2 frame back to the MS with the same layer 3 message and sends to the BSC

an ESTABLISH INDICATION containing the layer 3 message.

The BSC immediate assignment entity stops the timer T3101, extracts the MS classmark information

element from the message (this information may be used in subsequent procedure), sends to the BSC

SCCP entity a SCCP N-CONNECT REQUEST containing the layer 3 message and starts T9105 to

supervise the response from the BSC SCCP entity.

The BSC SCCP entity sends a CR to the peer entity containing the PAGING RESPONSE layer 3 message,

and starts T(conn_est) to supervise the response from the MSC.

On the reception of the ESTABLISH INDICATION message, the BSC :

starts queuing any uplink DTAP message received from the MS (the 2 first DTAP messages may bequeued). If any CLASSMARK CHANGE message is received from the MS, the BSC processes itand remembers to send a CLASSMARK UPDATE message to the MSC as soon as the SCCP

connection is established.

The MSC SCCP entity stops timer T3113 and confirms the establishment of the SCCP connection by

sending back the CC message.

At this point the BSC SCCP entity stops the timer T(conn_est) and informs the BSC immediate assignment

entity with the SCCP N-CONNECT CONFIRM. It is at this point that the BSC immediate assignment entity

stops the timer T9105.


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Figure 1: Paging, Radio and Link Establishment for Mobile Terminated CS Call

Mobile terminated CS call

MS BTS BSC MSC

PAGINGPAGING COMMAND

PAGING REQUEST (PCH)

CHANNEL REQUEST

(RACH) CHANNEL REQUIRED

CHANNEL ACTIVATION

CHANNEL ACTIVATION ACK

IMMEDIATE ASSIGNMENT

IMMEDIATE ASSIGN COMMAND

SABM (Paging Response)

UA

ESTABLISH INDICATION

(paging response)

MULTIPLE SACCH INFO MODIFYCR (COMPLETE L3 INFO)

CC

C01

Paging, radio and link establishment

C148


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1.2 Authentication and Ciphering

Authentication checks that Mobile Station is the required station an not an intruder.

All Information (Signalling, Speech and Data) is sent in cipher mode, to avoid intruders and for

monitoring.

Each time the network is accessed for whatever reason, a different number is used to identify a

subscriber, so that the subscriber identity cannot be used by an intruder. On each access operation, a

new temporary number is sent in cipher mode, and provides identification for the next access operation.

Figure 2: Authentication and Ciphering for Mobile Terminated CS Call


MS BTS BSC MSC

Authentication and Ciphering

MM_AUTHENTIFICATION_REQSDCCH

SDCCH MM_AUTHENTIFICATION_RESP

CIPHER_MODE_COMENCRYPTION

SDCCH

SDDCH

CIPHERING_COMP CIPHER_MODE_COMP

RR CIPHERING_COM

RR CIPHERING_COMP


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1.3 Call Setup and Normal TCH assignment

1 The MSC sends the ASSIGNMENT REQUEST message to the BSS. After checking successfully thereceived ASSIGNMENT REQUEST message.If provided by the MSC, the BSC may store the Classmark

information (type 2)

2 The BSC activates the TCH

The BTS starts:

ciphering and deciphering immediately (if required), sending SACCH messages immediatelywith a Layer 1 header reflecting the TA provided in the CHANNEL ACTIVATION message.

The power of transmission on the Downlink is adjusted to the value contained in the CHANNEL

ACTIVATION message.

Upon receipt of the CHANNEL ACTIVATION message, the BTSs behaviour is as follows:

starts the remote transcoder supervision, enables the Radio link failure detection algorithm andstarts measurement reporting.

3 The BSC sends the ASSIGNMENT COMMAND message to the MS on the SDCCH and starts timer

T3107.

4 The MS disconnects the Physical layer of the old channel and releases the LapDm using a local end

release. It then connects to the new channel and establishes the SAPI 0 connection at Layer 2.

The BTS then sends the LapDm establishment event of SAPI 0 to the BSC in an ESTABLISH INDICATION

message and starts T_CFI_TR.

The BSC makes the "switch through" at this point for all TSs involved.

5 The MS then sends the ASSIGNMENT COMPLETE message to the BTS which transfers it transparently

to the BSC.

The BSC stops timer T3107 and sends the ASSIGNMENT COMPLETE message to the MSC, including the

chosen channel, chosen codec if relevant and chosen encryption algorithm if relevant (see the detailed

section for more information). It is at this point that the BSC releases the old SDCCH


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Figure 3: Call Setup and Normal TCH Assignment for Mobile Terminated CS Call


Call setup and normal TCH assignmentMS BTS BSC MSC

CHANNEL ACTIVATION




SABM FACCH

UA


C01

CC CALL SET_UP

CC CALL CONFIRM

ASSIGNMENT REQUESTPHYSICAL CONTEXT REQUEST

PHYSICAL CONTEXT CONFIRM

RR ASSIGNMENT COMPLETE ASSIGNMENT COMPLETE

C16

CC CONNECT

CC ALERTING

CC CONNECT ACK

C161b

C18


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2. MOBILE ORIGINATED CS CALL

2.1 Radio and Link Establishment

The MS initiates a call by sending a CHANNEL REQUEST message to the BTS.

The BTS forwards this onto the BSC using the CHANNEL REQUIRED message.

The BSC discards the CHANNEL REQUIRED message if "RACH filtering" is enabled and filtering conditions

are encountered, as described in section "".

The BSC activates an SDCCH channel in the BTS.

The BSC then sends an IMMEDIATE ASSIGN COMMAND to the BTS which sends an IMMEDIATE

ASSIGNMENT or IMMEDIATE ASSIGNMENT EXTENDED message on the Air interface to the MS. TheBSC runs the timer T3101 to supervise the establishment of the SDCCH channel.

The MS upon reception of the IMMEDIATE ASSIGNMENT or IMMEDIATE ASSIGNMENT EXTENDED

message switches to the indicated channel and establishes it by sending an SABM layer 2 frame with a

layer 3 message contained.

When the BTS receives the SABM layer 2 frame with the layer 3 message, it notes the contents of the layer

3 message, sends a UA layer 2 frame back to the MS with the same layer 3 message and sends to the BSC

an ESTABLISH INDICATION containing the layer 3 message.

The BSC immediate assignment entity stops the timer T3101, extracts the MS classmark information

element (this information may be used in subsequent procedure), sends to the BSC SCCP entity a SCCP N-CONNECT-REQUEST containing the layer 3 message and starts T9105 to supervise the SCCP

establishment.

The BSC SCCP entity sends a CR to its peer entity with the layer 3 message, and starts timer T(conn_est)

to supervise the response from the MSC.

On the reception of the ESTABLISH INDICATION message, the BSC :- starts queuing any uplink DTAP

message received from the MS (the 2 first DTAP messages may be queued). If any CLASSMARK CHANGE

message is received from the MS, the BSC processes it and remembers to send a CLASSMARK UPDATE

message to the MSC as soon as the SCCP connection is established.

Modifies the sequence of the dedicated SYSTEM INFORMATION messages to be sent onSACCH if needed, if so a SACCH INFO MODIFY message is sent to the BTS

The MSC SCCP entity confirms the establishment of the SCCP connection by sending back the CC

message.

At this point, the BSC SCCP entity stops timer T(conn_est) and sends a SCCP N-CONNECT CONFIRM to

the BSC immediate assignment entity. It is at this point that the BSC immediate assignment entity stops the

timer T9105.


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Figure 4: Radio and Link Establishment for Mobile Originated CS Call

Mobile originated CS call

MS BTS BSC MSC

CHANNEL REQUEST

(RACH) CHANNEL REQUIRED

CHANNEL ACTIVATION




SABM

UA


MULTIPLE SACCH INFO MODIFY CR (COMPLETE L3 INFO)

CC

C02h

Radio and link establishment

C148


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2.2 Authentication and Ciphering

Authentication checks that Mobile Station is the required station an not an intruder.

All Information (Signalling, Speech and Data) is sent in cipher mode, to avoid intruders and for monitoring.

Each time the network is accessed for whatever reason, a different number is used to identify a subscriber,

so that the subscriber identity cannot be used by an intruder. On each access operation, a new temporary

number is sent in cipher mode, and provides identification for the next access operation.

Figure 5: Authentication and Ciphering for Mobile Originated CS Call


MS BTS BSC MSC

Authentication and Ciphering

MM_AUTHENTIFICATION_REQSDCCH

SDCCH MM_AUTHENTIFICATION_RESP

CIPHER_MODE_COMENCRYPTION

SDCCH

SDDCH

CIPHERING_COMP CIPHER_MODE_COMP

RR CIPHERING_COM

RR CIPHERING_COMP


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2.3 Call Setup and TCH normal assignment

1 The MSC sends the ASSIGNMENT REQUEST message to the BSS. After checking successfullythe received ASSIGNMENT REQUEST message.If provided by the MSC, the BSC may store the

Classmark information (type 2)

2 The BSC activates the TCH

The BTS starts:

ciphering and deciphering immediately (if required), sending SACCH messages immediately with a Layer 1 header reflecting the TA provided in

the CHANNEL ACTIVATION message.

The power of transmission on the Downlink is adjusted to the value contained in the CHANNELACTIVATION message.

Upon receipt of the CHANNEL ACTIVATION message, the BTSs behaviour is as follows:

starts the remote transcoder supervision, enables the Radio link failure detection algorithm andstarts measurement reporting.

3 The BSC sends the ASSIGNMENT COMMAND message to the MS on the SDCCH and starts timer

T3107.

4 The MS disconnects the Physical layer of the old channel and releases the LapDm using a local end

release. It then connects to the new channel and establishes the SAPI 0 connection at Layer 2.

The BTS then sends the LapDm establishment event of SAPI 0 to the BSC in an ESTABLISH INDICATION

message and starts T_CFI_TR.

The BSC makes the "switch through" at this point for all TSs involved.

5 The MS then sends the ASSIGNMENT COMPLETE message to the BTS which transfers it

transparently to the BSC.

The BSC stops timer T3107 and sends the ASSIGNMENT COMPLETE message to the MSC, including the

chosen channel, chosen codec if relevant and chosen encryption algorithm if relevant (see the detailed

section for more information). It is at this point that the BSC releases the old SDCCH


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Figure 6: Call Setup and Normal TCH assignment for Mobile Terminated CS Call

END OF DOCUMENT


Call setup and TCH normal assignment

MS BTS BSC MSC

CHANNEL ACTIVATION




SABM FACCH

UA

ESTABLISH INDICATIONC01

CC CALL SET_UP

CC CALL CONFIRM

ASSIGNMENT REQUESTPHYSICAL CONTEXT REQUEST

PHYSICAL CONTEXT CONFIRM

RR ASSIGNMENT COMPLETE ASSIGNMENT COMPLETE

C16

CC CONNECT

CC ALERTING

CC CONNECT ACK

C161a

C18

bss call phases

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