IMSI Detach - Discount Coupons

Section 5 – Call Handling

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

PLMN

Exch VLR
PSTN MSC

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

ISDN/ PLMN
PSTN

LAPDm

VLR
BSSMAP

LAPD

MSC BSC

DTAP

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)

MSISDN

PSTN

Link is setup from local GSM/PLMN
exchange to the GMSC
GMSC

Section 5 – Call Handling

PSTN

GSM/PLMN

signaling No.7: Interrogation GMSC
function used by GMSC

MSISDN + HLR
MSRN request

Section 5 – Call Handling

PSTN

GSM/PLMN

VLR signaling No7: Request GMSC
MSC for MSRN to VLR

IMSI HLR

Section 5 – Call Handling

PSTN

GSM/PLMN

VLR IMSI GMSC
MSC
MSRN in VLR.
signaling No 7: MSRN
send to GMSC

HLR

MSRN MSRN request + IMSI

Section 5 – Call Handling

PSTN
GSM/PLMN

VLR GMSC
MSC
HLR

Link is setup from GMSC to
MSC/VLR

Section 5 – Call Handling

PSTN

GSM/PLMN

VLR GMSC HLR
BSC MSC

signaling No7: Paging
message is sent to the BSS

Section 5 – Call Handling

PSTN
GSM/PLMN

HLR

VLR GMSC Air path signaling:
BSC MSC Paging message is
sent over the air
path to MS. The MS
answers.

Section 5 – Call Handling

PSTN
GSM/PLMN

HLR

VLR GMSC The link is setup from
BSC MSC the MSC/VLR to the
MS, completing the
connection from
subscriber to
subscriber

Section 5 – Call Handling

Signaling Interfaces

ISDN/ PLMN
PSTN
HLR
LAPDm

MAP VLR BSSMAP LAPD
GMSC
MSC BSC

ISUP/TUP DTAP

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
Channel Activation

TA+SDDCH+power
Channel Activation Ack

Immediate assign (AGCH) Immediate assign commd

Switch to rn+fn+TA+ SDCCH Establish Indication
Service Request
SDDCH SABM

Cm+Service Request

UA SCCP Connection Req
Service Request SCCP Connection Confirm

Section 5 – Call Handling

MS BTS Layer 3CC BSC MSC
Layer 3CC
Setup (SDCCH) Layer 3CC
Call proceeding
Tele/bearer service
called party no.

Layer 3CC

Assignment request

Channel type+cm
TCH allocation
Physical context request

SACCH Physical context confirm
TA+power updates
Power+TA
Channel activation
TCH+TA+cipher+DTX+

power

Channel activation ack

Assignment command (SDCCH)

Release
SDCCH

MS BTS BSC Section 5 – Call Handling

MSC

SABM (FACCH)

Establish indication

UA (FACCH) Set
transcoder

Set switching
path

Assignment complete (FACCH)

Initiate SDCCH release

Layer 3CC alerting

Layer 3CC Layer 3CC connect
Layer 3CC
Connect ack Layer 3CC

Section 5 – Call Handling

Mobile to Mobile (Mobile Terminated)

MS BTS BSC MSC

Paging command Paging
TMSI/IMSI+cell list
TMSI/IMSI paging
Paging request (PCH) group+ channel no
TMSI/IMSI

Paging request (RACH)

Channel required

Radio and Link Establishment Procedure

Section 5 – Call Handling

MS BTS BSC MSC

Layer 3CC Setup

Layer 3CC Tele/bearer service

Call confirmed (SDCCH) Layer 3CC Layer 3CC
bearer service

Normal Assignment Procedure for TCH

Ring tone alerting Layer 3CC Layer 3CC
User answer connect Layer 3CC Layer 3CC

Layer 3CC Connect acknowledge

Layer 3CC

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

HLR

SMS - C SMS - GMSC MSC/VLR

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.