Call

Example Network Diagram

Call Flow - Mobile to Mobile Call

MS A BTS1 BSC1 MSC/VLR1

Air Interface/LAPDm Abis Interface/LAPD A Interface

First Step: RR CONNECTION ESTABLISHEMENT

rr_t3120 start RR CHANNEL REQUEST

RACH

CHAN RQD

Subscriber A types Subscriber B's MSISDN number and presses "send button" on his mobile. RR Connection Establishment begins and the called number digits needs to be transported from Mobile A to MSC. For this first

phase, Radio Resource (RR) connection is established to MSC

RR connection establishment is triggered by sending the RR CHANNEL REQUEST message. This message requests BSS for allocation for radio resources for the RR Connection setup. The mobile now waits for an

Assignment on Access Grant Channel (AGCH). At this point the mobile is listening to AGCH for a reply. RR CHANNEL REQUEST is sent on RACH and slotted aloha channel that can be used at random without any co

ordination between the mobiles, any mobile can transmit on this channel whenever it wishes. If two mobiles transmit on the channel at the same time, thier messages will be lost ina collision. The mobile will detect the collision via a

timeout and retransmit the messa after a random back off

Channel Activate

Channel Act Ack

IMM ASS CMD

RR IMM ASSIGNMENT rr_t3101 startrr_t3120 stop AGCH

(SDCCH Freq, TS,Freq and Time)

rr_t3101 stopSDCCH, SAPI=0

CHANnel ReQuireD procedure may occur on the Abis interface at any time, on an asynchronous basis. The procedure is initiated by the TRX carrying the so called CCCH channel, upon detection of a random access from an

MS. The TRX then sends a CHANnel ReQuireD message to BSC containing the Request Reference Parameter (random number selected by MS plus some low order bits of the TDMA frame number for the access) and teh access

delay

The IMMEDIATE ASSIGN COMMAND procedure may occur at any time, in an asynchronous basis, to initiate the immediate assignment, the BSC sends an IMMEDIATE ASSIGN COMMAND message, including an RR Immediate

Assign message. The BSC will transmit this message on the AGCH channel which is mapped on the same tmeslot as the received Channel Required message. Upon receipt of this message the BTS transmits the immediate assignment

as received. This procedure can also be used to return an Immediate Assign Reject message to MS

The BSS shall allocate a SDCCH to the mobile. The SDCCH allocation assigns a specific frequency and a timeslot on that frequency. After the mobile receives this messa, the mobile shall only use the specified resourcs for

communicaion with the mobile network. BTS transmits the radio resource assignment to the mobile via the AGCH channel. The message also conatins the time and frequency corrections. The time corrections allow the mobile to

time it's transmisions so that they reach the BSS only in the specified timeslot. The frequency corrections correct for the doppler shift caused by the mobile's motion. After adjusting frequency and timing, the mobile detunes from AGCH

and tunes to the specified radio channel. BTS starts "rr_t3101" RR timer waiting replay from MS

RR SABM+MM CM SERVICE REQUEST

This is the first message that is sent after tuning to the channel. The Mobile initiates a LAPDm connection with the BSS by sending a Set Asynchronous Balanced Mode (SABM) message. SABM used to set up a logical channel SDCCH in acknowledged Multi Frame Mode. The service request message meant for the MS is also sent in this

messge. On receipt of this message BTS will stop rr_t3101 timer

RR UASDCCH, SAPI=0

MS Power ControlSACCH

sccp_tconn est startSS7

sccp_tconn est stopCONNECTION CONFIRM (CC)

SS7

DTAP, AUT REQSS7

LAP D

SDCCH

SDCCH

dtap, aut respLAP D

DTAP, AUT RESPSS7

ID Request (Type:IMEI)SS7

ID Request (Type:IMEI)LAPD

The BSS replies with Unnumbered Acknowledge (UA) to complete the LAPDm setup handshake. UA acknowledges SABM

SCCP CONNECTION REQUEST +

BSSMAP COMPLETE LAYER

3 INFORMATION

dtap, I(aut req) N(S)=0;N(R)=0

Data REQ, I(AUT REQ) N(S)=0;N(R)=0

Send I frame containing the L3 info authentication request

DATA IND, I(AUT RESP) N(S)=0;N(R)=1

ID Request (Type:IMEI)SDCCH

ID Response IMEI)

ID Response IMEI)

ID Response IMEI)

CLASSMARK REQ

REQ CLASSMARK

CLASSMARK ENQ

CLASSMARK SEND

UPDATE CLASSMARK

CLASSMARK UPDATE

SS7

cipher reqLAP D

SDCCH

SDCCH

BSSMAP, CIPH MODE CMD

Since the subscriber has been successfully authenticated, the MSC initates ciphering of the data being sent on the channel. The channel is ciphered so as to protect the call from evesdropping. Ciphering on the radio link is enabled in

three steps. As a first step, the BSS starts expecting ciphered data from the mobile but continues to send data in CLEAR, since the mobile has not been informed about the ciphering, all data received from the mobile will be in error.

ENCRYPT CMD, CIPHERING MODE CMD,Mode=Clear

The BSS sends the CIPHERING MODE COMMAND to the Mobile. The mobile will be able to reeive this message as the transmission from the BSS is still in CLEAR. As a second step, the Mobile receives the message and enables

ciphering in transmit and receive directions. This action will result in all BSS data being received in error The BSS is still transmitting data in CLEAR

CIPHERING MODE COMPLETE, Mode=Ciphered

ciper sucLAPD

SS7

BSS replies back to the MSC, including that ciphering has been successfully enabled

DTAP: TMSI REALLOCATION COMMAND

DTAP: TMSI REALLOCATIOM COMPLETE

SECOND STEP: CALL SETUP

DATA REQ, SET UPSDCCH

dtap, setupLAP D

DTAP SETUP

Ciphering has already been enabled, so this message is transmitted with ciphering. The BSS will receive this message as it is already expecting ciphred data in the receive direction. The third and final step in the ciphering handshake. The BSS enables the ciphreing in transmit direction. From this point on ciphering is enabled in both

directions.

BSSMAP, CIPH CMPLT

VLR1 generates a new TMSI and forwards to MSC1 for this MS. MS stores TMSI in its non-volatile store

MSC informs VLR that TMSI reallocaation was succesful. VLR associates the TMSI with IMSI and old TMSI is deallocated if it is allocated by it previously

RR Connection Establishemnt completed - At this point a connection has been setup between the mobile and the MSC. From this point onward, the BSS is just acting as a conduit for transporting the signalling

messages between the mobile and the MSC

The mobile sends the setup message to establish a Voice Call. The message contains the dialled digits and other information needed for call establishement

DTAP, CALL PROC

DT1, dtap, call proc

DATA REQ, CALL PROC

BSSMAP_T11 starts

Physical Context Req

Physical Context Conf

Channel Activation

Chnal Act ACK

Initiate Assignment

Assignment CommandSDCCH

SABMFACCH

UAFACCH

Assignment CompleteFACCH

The mobile is informed that the call setup is in progress. At this point the mobile phone displays a meesage "Connecting….." on the screen to indicate that call setup is being attempted.

BSSMAP: Assignment Request

The MSC Allocates a voice circuit on one of the digital trunks between the MSC and the BSS. MSC informs the BSS about the allocated voice channel.

The call is also switched from signalling circuit to voice circuit.

BSSMAP_T10(T3107)starts

BSSMAP_T10(T3107)stops

Assignment Successful

BSSMAP_T11 StopsAssignment Complete

SD RF Chanl Release

SD RF Chanl Rel Ack

MSC1 Checks the class of service from VLR1 database. MSC1 sends VLR1 the message "Send Info O/G Call". VLR1 verifies its database and if MSA is eligible for making O/G call,

VLR1 reples MSC1 with the message "Complete Call". If MSA is not eligible for making O/G call announcement will be fed to MSA. It will be transmitted to MSA by FACCH. If MSC1 receives

"Complete Call" message from VLR1. MSC1 analyzes the dialled digits using GT analysis defined in GT analyses with NP=1 and routes it to GMSC

Since MSISDN-B is a mobile subscriber, MSC/VLR1 has to go for HLR Enquire for subscriber B. There is no direct connection between MSC1 and HLR. MSC/VLR1 will send Send Routing Information to GMSC. It

refers to GT analysis with NP=1

Alerting

Connect

Connect Ack

s/w s/w

Conversation

END Button

CC Disconnect

CC Release

CLEARBSSMAP

RR Channel Release

RR Channel ReleaseCLEAR

BSSMAP

RR DISC

RR UA

Release indication

Example Network Diagram

Call Flow - Mobile to Mobile Call

MSC/VLR1 GMSC HLR 1 HLR 2

MAP - E Interface MAP-D Interface

First Step: RR CONNECTION ESTABLISHEMENT

Subscriber A types Subscriber B's MSISDN number and presses "send button" on his mobile. RR Connection Establishment begins and the called number digits needs to be transported from Mobile A to MSC. For this first

phase, Radio Resource (RR) connection is established to MSC

RR connection establishment is triggered by sending the RR CHANNEL REQUEST message. This message requests BSS for allocation for radio resources for the RR Connection setup. The mobile now waits for an

Assignment on Access Grant Channel (AGCH). At this point the mobile is listening to AGCH for a reply. RR CHANNEL REQUEST is sent on RACH and slotted aloha channel that can be used at random without any co

ordination between the mobiles, any mobile can transmit on this channel whenever it wishes. If two mobiles transmit on the channel at the same time, thier messages will be lost ina collision. The mobile will detect the collision via a

timeout and retransmit the messa after a random back off

CHANnel ReQuireD procedure may occur on the Abis interface at any time, on an asynchronous basis. The procedure is initiated by the TRX carrying the so called CCCH channel, upon detection of a random access from an

MS. The TRX then sends a CHANnel ReQuireD message to BSC containing the Request Reference Parameter (random number selected by MS plus some low order bits of the TDMA frame number for the access) and teh access

The IMMEDIATE ASSIGN COMMAND procedure may occur at any time, in an asynchronous basis, to initiate the immediate assignment, the BSC sends an IMMEDIATE ASSIGN COMMAND message, including an RR Immediate

Assign message. The BSC will transmit this message on the AGCH channel which is mapped on the same tmeslot as the received Channel Required message. Upon receipt of this message the BTS transmits the immediate assignment

as received. This procedure can also be used to return an Immediate Assign Reject message to MS

The BSS shall allocate a SDCCH to the mobile. The SDCCH allocation assigns a specific frequency and a timeslot on that frequency. After the mobile receives this messa, the mobile shall only use the specified resourcs for

communicaion with the mobile network. BTS transmits the radio resource assignment to the mobile via the AGCH channel. The message also conatins the time and frequency corrections. The time corrections allow the mobile to

time it's transmisions so that they reach the BSS only in the specified timeslot. The frequency corrections correct for the doppler shift caused by the mobile's motion. After adjusting frequency and timing, the mobile detunes from AGCH

and tunes to the specified radio channel. BTS starts "rr_t3101" RR timer waiting replay from MS

This is the first message that is sent after tuning to the channel. The Mobile initiates a LAPDm connection with the BSS by sending a Set Asynchronous Balanced Mode (SABM) message. SABM used to set up a logical channel SDCCH in acknowledged Multi Frame Mode. The service request message meant for the MS is also sent in this

messge. On receipt of this message BTS will stop rr_t3101 timer

The BSS replies with Unnumbered Acknowledge (UA) to complete the LAPDm setup handshake. UA acknowledges

Since the subscriber has been successfully authenticated, the MSC initates ciphering of the data being sent on the channel. The channel is ciphered so as to protect the call from evesdropping. Ciphering on the radio link is enabled in

three steps. As a first step, the BSS starts expecting ciphered data from the mobile but continues to send data in CLEAR, since the mobile has not been informed about the ciphering, all data received from the mobile will be in error.

The BSS sends the CIPHERING MODE COMMAND to the Mobile. The mobile will be able to reeive this message as the transmission from the BSS is still in CLEAR. As a second step, the Mobile receives the message and enables

ciphering in transmit and receive directions. This action will result in all BSS data being received in error The BSS is

BSS replies back to the MSC, including that ciphering has been successfully enabled

SECOND STEP: CALL SETUP

Ciphering has already been enabled, so this message is transmitted with ciphering. The BSS will receive this message as it is already expecting ciphred data in the receive direction. The third and final step in the ciphering handshake. The BSS enables the ciphreing in transmit direction. From this point on ciphering is enabled in both

VLR1 generates a new TMSI and forwards to MSC1 for this MS. MS stores TMSI in its non-volatile store

MSC informs VLR that TMSI reallocaation was succesful. VLR associates the TMSI with IMSI and old TMSI is deallocated if it is allocated by it previously

RR Connection Establishemnt completed - At this point a connection has been setup between the mobile and the MSC. From this point onward, the BSS is just acting as a conduit for transporting the signalling

messages between the mobile and the MSC

The mobile sends the setup message to establish a Voice Call. The message contains the dialled digits and other information needed for call establishement

At this point the mobile phone displays a meesage "Connecting….." on the screen to indicate that call setup is being attempted.

The MSC Allocates a voice circuit on one of the digital trunks between the MSC and the BSS. MSC informs the BSS about the allocated voice channel.

The call is also switched from signalling circuit to voice circuit.

SRI (MSISDN-B)ISUP

TC-INV[Send_Routing_Info (MSISDN-B)]MAP/C

TC-BEGINSCCP

TC-INV[Provide Routing Number (MSRN) {PRN(IMSI)}]

TC-BEGIN

MSC1 Checks the class of service from VLR1 database. MSC1 sends VLR1 the message "Send Info O/G Call". VLR1 verifies its database and if MSA is eligible for making O/G call,

VLR1 reples MSC1 with the message "Complete Call". If MSA is not eligible for making O/G call announcement will be fed to MSA. It will be transmitted to MSA by FACCH. If MSC1 receives

"Complete Call" message from VLR1. MSC1 analyzes the dialled digits using GT analysis defined in GT analyses with NP=1 and routes it to GMSC

Since MSISDN-B is a mobile subscriber, MSC/VLR1 has to go for HLR Enquire for subscriber B. There is no direct connection between MSC1 and HLR. MSC/VLR1 will send Send Routing Information to GMSC. It

refers to GT analysis with NP=1

The GMSC refers to GT analysis for NP=1, TON = NAT or INT, depends on MSISDN format received by GMSC. GT analsis in GMSC refers to HLR2. GMSC requests routing information for the GSM subscriber

from HLR2.

HLR2 uses MSISDN-B and finds out IMSI and the SS7 address of MSC/VLR which is serving MS-B at that moment. SS7 address of the serving MSC/VLR was obtained by HLR during LU of MS-B. In this case, MS-B is present in

MSC/VLR2 area. HLR 2 sends Provide Routing Number message (PRN) to MSC/VLR2. HLR 2 sends IMSI number of MS-B to VLR2 as data

TC-RES [Provide_MSRN_Ack] TC-ENDSCCP[MAP/D]

MSC/VLR2 allocates a temporary roaming number (MSRN). The MSRN is then passed to the HLR

SCCP[MAP/C]

SRI_ACKTCAP/SCCP

IAM (MSRN)

Send Info I/C call [IAM(MSRN)]ISUP

SAM (mor digits if any will be sent)



TC-RES [Send_Routing_Info_Ack(MSRN)] TC-END

HLR 2 Passes MSRN to GMSC

GMSC uses MSRN and refers to digit analysis and finds out the route to MSC/VLR 2. GMSC 2 GMSC 2 sends MSRN as the reference number for the call to establish and sends ISUP messae IAM to MSC/VLR2. MSC/VLR2 receives the call. At this point, the MSRN is marked free and may be reassigned for

other calls.

MSC/VLR2 needs to locate the subscriber in the loction area (which BTS?), since location area might span several cells, paging mechanism is used to locate the subscriber. The MSC/VLR2 uses TMSI of MS-B which is allocated to MS-B earlier to address MS-B.

Note that, MSC/VLR2 will send PAGING messge to all BSCs that handle location area

ISUP ACM

ISUP ACM

ISUP ANM

ISUP ANM

Charging Starts

s/w s/w

Conversation

RELISUP

ISUP REL

MSC/VLR2 BSC1

TC-INV[Provide Routing Number (MSRN) {PRN(IMSI)}]MAP/D

TC-BEGINSCCP

The GMSC refers to GT analysis for NP=1, TON = NAT or INT, depends on MSISDN format received by GMSC. GT analsis in GMSC refers to HLR2. GMSC requests routing information for the GSM subscriber

from HLR2.



TC-RES [Provide_MSRN_Ack] TC-ENDSCCP[MAP/D]

MSC/VLR2 allocates a temporary roaming number (MSRN). The MSRN is then passed to the HLR

Check Subscriber B Class of serviceBSSMAP Page (UDT)

SCCP-CL

LAPD



GMSC uses MSRN and refers to digit analysis and finds out the route to MSC/VLR 2. GMSC 2 GMSC 2 sends MSRN as the reference number for the call to establish and sends ISUP messae IAM to MSC/VLR2. MSC/VLR2 receives the call. At this point, the MSRN is marked free and may be reassigned for

other calls.



RR Paging Request Type 3 (TMSI)

BSC1 of MSC2 receives the Page command. BSC will send the Page message to all the cells that serve the subscriber's location area. Note that, all the BSCs,

serving the location area will send page message to all the cells

All cells in the location area will broadcast the Page message on PCH.

Channel Required

Channel ACT

Channel ACT Ack

IMMEDIATE Assign CMD

LAPD

UA (Page Response)

SCCP CR+RR Paging Response

BSSMAP: CC

All the mobiles belonging pageing group of MS-B listen to paging message in BTS1. MS-B finds that the TMSI specified in the page message matches its own

TMSI. The MS-B decides to respond to the page. An RR Connection establishmentis initiated by sending a channel request to the network

MS-B tunes to the assigned SDCCH channels and transmits page response and piggybacks SABM to intiate RR sesssion

Establish Ind (Page Response,CM Service

Request)

When BTS and MS are in contact over SDCCH, Establishment Indication messageis sent from BTS1 to BSC1. Paging Response and SABM are piggybacked on this message.

BSC1 sends UA(Page Response) to BTS1 and BTS1 sends UA(Page Response) to MS-B

BSC1 sends a SCCP Connection Request (CR) to the MSC/VLR2. The Page response message is piggybacked with the request.

SCCP

Assignment Req

Initiate Assignment

Assignment Success

Assignment Complete

CC Setup

Call Confirmed

CC Alerting

SRBT (IP Network/IP Trunks)ISUP ACM

CC Connect

Charging Starts

s/w s/w

MSC/VLR2 initiates authentication check, IMEI Check, Ciphering activation, IMEI chek, Classmark request, TMSI Reallocation procedures to MS-B. These procedures are similar to MS-A. Ref MS-A

procedures given above and are not printed here again.

Conversation

CC Release

BTS1 MSB




BSC1 of MSC2 receives the Page command. BSC will send the Page message to all the cells that serve the subscriber's location area. Note that, all the BSCs,

serving the location area will send page message to all the cells

PCH

All cells in the location area will broadcast the Page message on PCH.

RR CHANNEL REQ

RACH

Channel Required

Channel ACT

Channel ACT Ack

IMMEDIATE Assign CMD

RR IMMEDIATE Assgn

AGCH

UA (Page Response)UA (Page Response)


All the mobiles belonging pageing group of MS-B listen to paging message in BTS1. MS-B finds that the TMSI specified in the page message matches its own

TMSI. The MS-B decides to respond to the page. An RR Connection establishmentis initiated by sending a channel request to the network

RR SABM + RR PAGING RESPONSE

MS-B tunes to the assigned SDCCH channels and transmits page response and piggybacks SABM to intiate RR sesssion

Establish Ind (Page Response,CM Service

Request)

When BTS and MS are in contact over SDCCH, Establishment Indication messageis sent from BTS1 to BSC1. Paging Response and SABM are piggybacked on this message.

BSC1 sends UA(Page Response) to BTS1 and BTS1 sends UA(Page Response) to MS-B

BSC1 sends a SCCP Connection Request (CR) to the MSC/VLR2. The Page response message is piggybacked with the request.

Initiate Assignment

Assignment Command

Assignment Complete

Assignment Success

Call ConfirmedRinging

CC Alerting

ERS (External Ringtone Server)SRBT (IP Network/IP Trunks)

Answers the callCC Connect

s/w

MSC/VLR2 initiates authentication check, IMEI Check, Ciphering activation, IMEI chek, Classmark request, TMSI Reallocation procedures to MS-B. These procedures are similar to MS-A. Ref MS-A

procedures given above and are not printed here again.

CC Release

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