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IMSI Detach
The MS must inform the network when it is entering an
inactive state (detach).
1. At power off or when the SIM card is taken out, the MS asks for a signaling channel
2. The MS uses this signaling channel to send the IMSI detach message to the MSC/VLR.
3. In the VLR, an IMSI detach flag is set for the subscriber which is used to reject incoming calls to the MS.
The detach will not be acknowledged.
Section 5 – Call Handling
Only the VLR is updated with the “detached” information.
Section 5 – Call Handling
IMSI Attach
The attach procedure is performed only when the MS is turned on and is in the same LA as it was when it sent the detach message. If the MS changes location area while being switched off, it is forced to do a normal location update. The procedure is as follows
1. The MS requesting a signaling channel.
2. The MSC/VLR receives the IMSI attach message from the MS.
3. The MSC/VLR sets the IMSI attach in the VLR, that is, the mobile is ready for normal call handling.
4. The VLR returns an acknowledgment to the MS.
Section 5 – Call Handling
Section 5 – Call Handling
Periodic Location Update
To avoid unnecessary paging of the MS in case the MSC never got the IMSI detach message, there is another type of location updating called periodic registration.
The procedure is controlled by timers both in the MS and in the MSC.
If the MS does not register within the determined interval plus a guard time, then the scanning function in the MSC detects this and the MS will be marked detached.
Section 5 – Call Handling
Paging
A call to MS is routed to the MSC/ VLR and send a paging message to the MS. This message is broadcast all over the Location Area (LA), which means that all BTSs with in the LA will send a paging message to the mobile. The MS, moving in the LA and listening to the CCCH information, will hear the paging message and answer it immediately.
Section 5 – Call Handling
Paging Capacity
Paging capacity is the number of mobiles that can be paged per second
This depends on: • CCCH configuration • AGCH blocks reservation • Type of paging message used • Paging message takes 4 bursts
(1 CCCH block) • This can page up to 4 mobiles
depending on the message type used
Section 5 – Call Handling
Paging Message Types
Type 1: can address up to two mobiles using either IMSI or TMSI
Type 2: can address up to 3 mobiles, one by IMSI and other 2 by TMSI.
Type 3: can address up to 4 mobiles using the TMSI only.
If the network does not use TMSI then only type 1 is used in the network.
Section 5 – Call Handling
Calculation Of Paging Capacity
X = number of mobiles paged per paging message (1 to 4)
Y = number of possible paging messages per multiframe
Duration of channel multiframe = 0.235 seconds (235 ms)
• X depends on paging message type
• Y depends on CCCH configuration in the multiframe (e.g. 3 or 9) and the number of AGCH blocks reserved
Section 5 – Call Handling
PCH Dimensioning
Paging channel requirement in blocks per multiframe is given by:
Calls = Number of calls predicted for the location area during busy hour
MT = Fraction of calls which are mobile terminated
PF = Paging Factor = number of pages required per call
M = safety margin
PMF = Paging Message Factor = number of pages per message
Number of control channel multiframes per second = 4.25
Section 5 – Call Handling
Example
A particular location area contains 50 000 subscribers. It is predicted that 30% of these will receive a call during the busy hour. On average 2 pages are needed per call and only type 3 paging messages (TMSI) are used.
This gives the following data: Calls = 50 000 MT = 0.3 PF = 2 PMF = 4
Section 5 – Call Handling
A typical safety margin for peak variations in number of calls is 1.2
• 1 PCH block per multiframe will be adequate
Section 5 – Call Handling
Paging Control
The MSC has to initiate the paging procedure, as it holds the information on the last MS location update.
MSC sends a paging message to BSC and sets a timer for response from the MS, which is send as a part of service request message. The paging message from the MSC contains a cell list identifier, identifying the cells in which paging message is to be transmitted.
Section 5 – Call Handling
Call From MS (Mobile to PSTN)
cb
VLR
MSC Exch PSTN
PLMN
Section 5 – Call Handling
Call From MS Overview
• Mobile is active and idle, wants to set up a call
• User dial the number and press send, at first moment it sends on RACH
• MSC/VLR assigns a dedicated channel
• If the calling MS is allowed to make a call MSC/VLR acknowledges the access request
• Depending on whether a fixed or a mobile subscriber is called, number is analyzed directly in the MSC/VLR.
• Call setup message is acknowledged as soon as the link is ready.
• MS is also assigned to move to a dedicated traffic channel TCH.
Section 5 – Call Handling
Signaling Interfaces
BSC
VLR
MSC
LAPDm
LAPD
DTAP
BSSMAP
PLMN ISDN/PSTN
ISUP/TUP
Section 5 – Call Handling
Point Of Interconnect (POI) Location
In case of long distance mobile to PSTN call, circuits define to route a call in the switch should be such that, call can travel maximum distance on the airtime and minimum on the land line to enhance the revenue.
Call should handover to the POI as near as possible to the subscriber location.
Section 5 – Call Handling
Call to MS (PSTN to Mobile)
GMSC
GSM/PLMN
PSTN MSISDN
Link is setup from local exchange to the GMSC
Section 5 – Call Handling
GMSC
GSM/PLMN
PSTN
HLR
signaling No.7: Interrogation function used by GMSC
MSISDN + MSRN request
Section 5 – Call Handling
GMSC
GSM/PLMN
PSTN
HLR
VLR
MSC
signaling No7: Request for MSRN to VLR
IMSI
Section 5 – Call Handling
GMSC
GSM/PLMN
PSTN
HLR
VLR
MSC
IMSI
MSRN in VLR. signaling No 7: MSRN send to GMSC
MSRN request + IMSI MSRN
Section 5 – Call Handling
GSM/PLMN
PSTN
VLR
MSC GMSC
Link is setup from GMSC to MSC/VLR
HLR
Section 5 – Call Handling
GSM/PLMN
PSTN
VLR
MSC GMSC HLR BSC
signaling No7: Paging message is sent to the BSS
Section 5 – Call Handling
GSM/PLMN
PSTN
VLR
MSC GMSC
HLR
BSC
Air path signaling: Paging message is sent over the air path to MS. The MS answers.
Section 5 – Call Handling
GSM/PLMN
PSTN
VLR
MSC GMSC
HLR
BSC
The link is setup from the MSC/VLR to the MS, completing the connection from subscriber to subscriber
Section 5 – Call Handling
Signaling Interfaces
BSC
VLR
MSC
LAPDm
LAPD
DTAP
BSSMAP
PLMN ISDN/PSTN
ISUP/TUP
HLR
GMSC
MAP
ISUP/TUP
Section 5 – Call Handling
Mobile to Mobile (Mobile Originated)
MS BTS BSC MSC Channel Request
rn Channel Request
rn+fn+TA
SDDCH Allocation
TA+SDDCH+power
Channel Activation
SCCP Connection Req
Immediate assign (AGCH) Immediate assign commd
Switch to SDDCH
rn+fn+TA+ SDCCH
Channel Activation Ack
SABM
Establish Indication
SCCP Connection Confirm
Service Request
Service Request
UA
Cm+Service Request
Section 5 – Call Handling
MS BTS BSC MSC
Setup (SDCCH)
Tele/bearer service called party no.
Layer 3CC Layer 3CC
Layer 3CC Layer 3CC Call proceeding
Assignment request
Channel type+cm TCH allocation
Physical context request
Physical context confirm
Power+TA Channel activation
TCH+TA+cipher+DTX+power
SACCH
TA+power updates
Channel activation ack
Assignment command (SDCCH)
Release SDCCH
Section 5 – Call Handling
MS BTS BSC MSC
SABM (FACCH)
Establish indication
UA (FACCH) Set transcoder
Assignment complete (FACCH)
Set switching path
alerting
connect
Layer 3CC
Layer 3CC
Layer 3CC
Layer 3CC
Layer 3CC
Connect ack
Initiate SDCCH release
Section 5 – Call Handling
Mobile to Mobile (Mobile Terminated)
MS BTS BSC MSC
Paging
TMSI/IMSI+cell list Paging command
TMSI/IMSI paging group+ channel no TMSI/IMSI
Paging request (PCH)
Paging request (RACH)
Channel required
Radio and Link Establishment Procedure
Section 5 – Call Handling
MS BTS BSC MSC
Layer 3CC Layer 3CC Setup
Tele/bearer service
Layer 3CC Layer 3CC
Call confirmed (SDCCH) bearer service
Normal Assignment Procedure for TCH
alerting Layer 3CC
Layer 3CC
Layer 3CC Layer 3CC
connect
Layer 3CC Layer 3CC Connect acknowledge
Ring tone
User answer
Section 5 – Call Handling
SMS Point to Point
The Short Message Service, SMS, provides means of sending text messages, to and from GSM mobile station. SMS makes use of service centre, which acts a store and forward center for short messages.
Section 5 – Call Handling
Mobile Terminated SMS
SMS - C SMS - GMSC MSC/VLR
HLR
Section 5 – Call Handling
SMS –C has the capability to transfer the short messages and also
provides the information about the delivery.
1. A user sends a message to an SMS – C
2. SMS – C sends the message to the SMS – GMSC
3. SMS – GMSC interrogates the HLR for routing information
4. HLR interrogates MSC/VLR for a roaming number
5. MSC/VLR returns a MSRN to the SMS – GMSC via HLR
6. SMS – GMSC reroutes the message to MSC/VLR
Continued…..
Section 5 – Call Handling
7. MS is paged and a connection is setup between MS and the
network.
8. If authentication was successful the MSC/VLR delivers the message to the MS. It is transmitted on the allocated SDCCH
9. If the delivery was successful a delivery report is sent from MSC/VLR to the SMS – C.
In the case of an unsuccessful delivery the service messages waiting will provide the HLR and VLR with the information that there is a message in the originating SMS – C waiting to be delivered to the MS.
Section 5 – Call Handling
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