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Signalling And CallProcessing
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GSM Functional Planes
OAM
CM
MM
RR
transmission
operator user
CM
MM
RR
trans
MS BTS BSC MSC/
VLR
HLR
GMSC
BTS BSCMSC
VLRHLR GMSC
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gna ng ro ocoArchitecture
BSSAP
MM
CM
BSSAP
CM
BTS
MS BSC MSC
AUm Abis B/C/D/E/F
MTP
SCCP
MTP
SCCPSCCP
MM
RRM
RRM
RRM
LAPDLAPDLAPDmLAPDmSignaling
Layer1
Signaling
Layer1
Signaling
Layer1
Signaling
Layer1
BTSM BTSM
I
S
U
P
M
A
P
TCAP
Relay
MTP
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y Signaling View
VLR
VLR
MAP/G
MAP/BMAP/C
MAP/
D
MAP
MAP/F
MAP/E / ISUP
EIR
MAP /
ISUP
MAP /
ISUP
MSC
BSC
BTS
MS
PSTN/
ISDN
PLMN
BSSAP /
Q.931
BSS
LAPD / Q.931
LAPDm / Q.931
MSC
HLR AUC
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What Are RRM Functions?
RRM functions cover all activities relatedto physical layer (air interface)
These relate functions like
Channel assignment
Channel release
Channel change and handover
Change of channel frequencies, hoppingsequences (algorithms) and frequency tables
Measurement reports from the MS
Power control and Time Advance
Modification of channel modes (speech and
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Some RRM Messages
Name of Message Sent by &
sent to
Carried on
Channel
Purpose of Message
Assignment
Command
BSC to MS SDCCH Allocation of TCH
(ARFCN & TS#)
Assignment
Complete
MS to BSC SDCCH Acknowledge for
Assignment Command
Channel Request MS to BSC RACH Request for channel
Channel Release BSC to MS SDCCH orTCH
Release of SDCCH orTCH
Cipher Mode
Command
BSC to MS SDCCH To indicate to MS that
ciphering will start
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Some RRM Messages (Contd.)Name of Message Sent by &
sent to
Carried on
Channel
Purpose of Message
Cipher Mode
Complete
MS to BSC SDCCH Acknowledge that MS
will also start ciphering
Immediate
Assignment
Command
BSC to MS AGCH Allocation of TCH
(ARFCN & TS#)
Paging Request BSC to MS PCH To inform MS about
incoming call / SMS
Paging Response MS to BSC SDCCH To respond to paging by
revealing MS identity(TMSI)
System Information
Messages
BSC to all
MSs
BCCH To broadcast CGI,
System information,
neighbouring cell
description
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Some RRM Messages (Contd.)
Name of Message Sent by &sent to Carried onChannel Purpose of Message
Measurement
Report
MS to BSC SACCH MS reporting on signal
strength measurement
Handover
Command
BSC to MS FACCH /
SACCH
Change of MS channel
allocation (will includenew ARFCN & TS#)
Handover Access MS to BSC
(new)
FACCH /
SACCH
Initial message sent on
the new TCH about
presence of MS
HandoverComplete
MS to BSC(new)
FACCH /SACCH
MS informs new BSCthat handover has been
completed
Handover Failure MS to BSC
(old)
SACCH /
FACCH
MS indicates that
handover has failed
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What Are MM Functions?
All activities connected with mobility ofMS are MM functions
Location updating
Periodic registration
Authentication procedure
IMSI attach procedure (on power up a MS
will present its IMSI to network and get aTMSI)
IMSI detach ( on power off of a MS, detachprocedure to tell network it is no longer in
service.
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Some MM Messages
Name of Message Sent by &sent to Carried onChannel Purpose of Message
Authentication
Request
MSC to MS SDCCH Sending RAND for
authenticating MS
Authentication
Response
MS to MSC SDCCH MS response by sending
SRES
Authentication
Reject
MSC to MS SDCCH MSC informing MS that
authentication has failed
Location Updating
Request
MS to MSC SDCCH To update location by
including TMSI and LAI
Location Update
Accept Command
MSC to MS SDCCH To indicate that location
updating has been done
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Some MM Messages (Contd.)Name of
Message
Sent by &
sent to
Carried on
Channel
Purpose of Message
TMSI
Reallocation
Command
MSC to MS SDCCH To indicate that location
updating has been done and
will include new TMSI
TMSI
ReallocationComplete
MS to MSC SDCCH Acknowledgement of TMSI
Reallocation Command byMS
IMSI Detach
Indication
MS to MSC SDCCH Indication to MSC that its
identity be removed from
MSC/VLR
IMSI AttachCommand
MS to MSC SDCCH To inform MSC that MS isagain powered up
Identity
Request
MSC to MS SDCCH MSC requesting MS to send
its identity (IMSI)
Identity
Response
MS to MSC SDCCH MS sends its identity (IMSI)
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What are CM Functions?
CM functions relate to establishing,releasing call and status indication
Call establishment for mobile-originatedcalls
Call establishment for mobile-terminatedcalls
Indication of status
Changes of transmission mode during anongoing call
Call reestablishment after interruption of an
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Flow of CM Messages
MSC /GMSC
PSTN /PLMN
MS A
NNI
ISUP messages
UNI
Q.931 messages
Interface between MS and MSC is called User toNetwork Interface (UNI)
Q.931 messages are used in this interface
Interface between MSC / GMSC and externalnetwork is called Network to Network Interface
(NNI)ISUP messages are used in this interface
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Message name
Starting a call
Transmitted by
SETUPEMERGENCY SETUP
CALL PROCEEDING
PROGRESS
CALL CONFIRMED
ALERTING
CONNECT
MS/MSCMS
MSC
MSC
MS
MS/MSC
MS/MSC
During a call
START DTMF
STOP DTMF
MODIFY
USER INFORMATION
MS
MS
MS/MSC
MS/MSC
Ending a call
DISCONNECT
RELEASE
RELEASE COMPLET
MS/MSC
MS/MSC
MS/MSC
Abnormal conditions
STATUS
STATUS ENQUIRYCONGESTION CONROL
MS/MSC
MS/MSCMS/MSC
Call Management Messages
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Important ISUP Messages
There are 3 important ISDN User Part(ISUP) messages
IAM (Initial Address Message) is sent by the
calling network to initiate a call sendingcomplete information about the call callingnumber, called number, type of call and otherinformation
ACM (Address Complete Message) is sent byterminating network to confirm that the callednumber is free and is being alerted
ANM (ANswer with Metering) is sent by
terminating network once the called
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Important Q.931 Messages
There are 3 important ISDN User Part
(ISUP) messages
Setup message is sent by user or network
(originating side) to initiate a call by sendinginformation about the call calling number,
called number, type of call and other
information this is equivalent to IAM
Alerting is sent by the other side (terminating
side) to confirm that the called number is free
and is being alerted this is equivalent to
ACM
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Mobile Originated Call
MS BSC MSC VLR PSTN
Setup (dialled digits + encryption)
Assignment of TCH (SDCCH)
Assignment Complete
Assignment Complete
(voice path from MS to MSC)
Assignment of trunk
on A channel
Call Proceed (on SDCCH)
Send info for Outgoing Call(call restriction query)l
Alerting
Connect
Connect Acknowledge
Complete Call
Route Establishment
Answer
1
2
3
4
5
6
7
8
9
10
11
12
13
14
ringback heard by MS
Call Establishment to PSTN no. (IAM)
(ACM)
(ANM)
M I M bil
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Messages In Mobile
Originated Call1. When user dials the number and presses ok button MS sends Setup
message transferring this info to MSC2. MSC sends a message to VLR for accessing subscription information
of MS
3. VLR checks the database of MS to determine whether the call can beallowed or not and returns a message
4. MSC send Call Proceedingmessage to MS5. MSC allocates a trunk between BSC &MSC and asks BSC to assign
TCH for MS
6. BSC allocates TCH, sends Ass ignmentmessage to MS
7. MS tunes to the TCH and sends Ass ignment Completemessage toBSC
essages n o e r g na e
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essages n o e r g na eCall
8. BSC deallocates SDCCH, connects TCH to the trunk
and sends Ass ignment Completemessage to MSC9. MSC sends Initial Address Message (IAM) to PSTN
containing the digits dialed
10. PSTN sets up the call and returns Address Complete
Message(ACM)11. MSC informs MS that destination number is being
alerted, subscriber hears ringing tone
12. When called subscriber answers PSTN sends Answer
message13. MSC informs MS that connection has been established
through the Connectmessage
14. MS returns a Connect Ack now ledgemessage and
conversation can proceed
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Page Request (TMSI)
Alerting
Call Confirmed
Page Message to
appropriate BSCs
Setup
Connect
Connect Acknowledge
Setup Complete
Answer
Channel Request (on RACH)
SDCCH Assignment (on AGCH)
Page Response on SDCCHPage Response
Process Access Request
Complete Call
Page
LA & TMSI
Send info for Incoming Call
Routing Information
Send Routing Information (VLR)12
3
4
5
6
7
8
9
10
11
12
13
14
15
1617
18
1920
21
MS BSC VLR PSTNHLR
end of MSC-VLR
dialogue
Call Establishment Message
Mobile Terminated Call
MSC
GMSC
(IAM)
(ACM)
(ANM)
M I M bil
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Messages In Mobile
Terminated Call
1. GMSC receives IAMfor an incoming call from PSTN to MS withdirectory no MSISDN
2. MSC requests routing information from HLR through RoutingInformation Message by including MSISDN
3. HLR acknowledges by returning MSRN / MSISDN and address ofserving VLR
4. MSC queries VLR with Send Info fo r Incom ing Cal lmessage
5. VLR responds with a Pagemessage that specifies the LAI & TMSI ofMS
If incoming calls are barred VLR will inform the MSC and MSCwould connect to an appropriate announcement
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Call (Contd.)
6. MSC determines the BSCs covering LAI and
sends the Pagemessage to each of theBSCs
7. Each BSC broadcasts TMSI of MS in Page
Requestmessage in PCH8. MS responds with Channel Request
message in RACH
9. BSC allocates SDCCH, sends ChannelAss ignmentmessage to MS over AGCH
10.MS sends Page Responsemessage
(containing MSs TMSI & LAI) over SDCCH
essages n o e erm na e
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essages n o e erm na eCall (Contd.)13.VLR responds with a Complete Call
message14.MSC sends Setupmessage to MS
15.MS responds with Call Con firmed
message16.MS sends Alert ingmessage to MSCindicating the subscriber is being alertedabout incoming call
17.MSC returns ACMto PSTN
18.When user answers, MS sends Connectmessage to MSC
19 MSC returns Connect Acknow ledge