call flow
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Transcript of call flow
Example Network Diagram
Call Flow - Mobile to Mobile Call
MS A BTS1 BSC1 MSC/VLR1
Air Interface/LAPDm Abis Interface/LAPD A Interface MAP - E Interface
First Step: RR CONNECTION ESTABLISHEMENT
RR CHANNEL REQUEST
RACH
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
CHAN RQD
Channel Activate
Channel Act Ack
IMM ASS CMD
RR IMM ASSIGNMENT rr_t3101 startAGCH
(SDCCH Freq, TS,Freq and Time)
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 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_t3101 stopSDCCH, SAPI=0
RR UASDCCH, SAPI=0
MS Power ControlSACCH
sccp_tconn est startSS7
sccp_tconn est stopCONNECTION CONFIRM (CC)
SS7
DTAP, AUT REQSS7
LAP D
SDCCH
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
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
SDCCH
dtap, aut respLAP D
DTAP, AUT RESPSS7
ID Request (Type:IMEI)SS7
ID Request (Type:IMEI)LAPD
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
DATA IND, I(AUT RESP) N(S)=0;N(R)=1
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.
cipher reqLAP D
SDCCH
SDCCH
ciper sucLAPD
BSSMAP, CIPH CMPLTSS7
BSS replies back to the MSC, including that ciphering has been successfully enabled
DTAP: TMSI REALLOCATION COMMAND
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
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.
VLR1 generates a new TMSI and forwards to MSC1 for this MS. MS stores TMSI in its non-volatile store
DTAP: TMSI REALLOCATIOM COMPLETE
SECOND STEP: CALL SETUP
DATA REQ, SET UPSDCCH
dtap, setupLAP D
DTAP SETUP
DTAP, CALL PROC
DT1, dtap, call proc
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
DATA REQ, CALL PROC
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 RequestBSSMAP_T11
starts
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.
Physical Context Req
Physical Context Conf
Channel Activation
Chnal Act ACK
Initiate Assignment
Assignment CommandSDCCH
SABMFACCH
UAFACCH
Assignment CompleteFACCH
Assignment Successful
Assignment Complete
SD RF Chanl Release
SD RF Chanl Rel Ack
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
BSSMAP_T11 Stops
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
SRI (MSISDN-B)ISUP
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.
SRI_ACKTCAP/SCCP
IAM (MSRN)
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.
ISUP ACM
Alerting
ISUP ANM
ConnectCharging Starts
Connect Ack
s/w s/w s/wConversation
END Button
CC Disconnect
RELISUP
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
GMSC HLR 1 HLR 2 MSC/VLR2 BSC1
MAP - E Interface MAP-D Interface
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
SRI (MSISDN-B)
TC-INV[Send_Routing_Info (MSISDN-B)]MAP/C
TC-BEGINSCCP
MAP/DTC-BEGIN
SCCP
SCCP[MAP/D]
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.
TC-INV[Provide Routing Number (MSRN)
{PRN(IMSI)}]
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-END
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)
Check Subscriber B Class of serviceBSSMAP Page (UDT)
SCCP-CL
MSC/VLR2 allocates a temporary roaming number (MSRN). The MSRN is then passed to the HLR
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
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
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
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
SCCP CR+RR Paging Response
BSC1 sends a SCCP Connection Request (CR) to the MSC/VLR2. The Page response message is piggybacked with the request.
BSSMAP: CCSCCP
Assignment Req
Initiate Assignment
Assignment Success
Assignment Complete
CC Setup
Call Confirmed
CC Alerting
SRBT (IP Network/IP Trunks)ISUP ACM
ISUP ACM
CC Connect
Charging StartsISUP ANM
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.
ISUP ANM
s/w s/w s/wConversation Conversation
REL
ISUP REL
CC Release
BTS1 MSB
LAPD
PCH
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
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
RR Paging Request Type 3 (TMSI)
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
LAPDRR 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
CC Setup
Call ConfirmedRinging
CC Alerting
ERS (External Ringtone Server)SRBT (IP Network/IP Trunks)
Answers the callCC Connect
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.
s/w
CC Release