9C. GSM - Roaming and Switching

7/28/2019 9C. GSM - Roaming and Switching

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ROAMING ANDSWITCHING



Mobile Application Part (MAP)

Interface

Key thing in GSM is that subscribers can be called

globally. This is because of roaming or mobility functions

These rely mostly on GSM extensions of SS#7

Mobile Application Part procedures relevant to roaming

are: Location Registration/update, IMSI Attach/Detach,requesting subscriber data for call se-up, and paging.

Additionally in MAP: functions for supplementary

services and handover, subscriber management, IMEI

management, authentication/identification management,

data transfer for SMS



Mobile Application Part (MAP)

Interface

MAP entities for roaming services are in MSC,

HLR and VLR.

At subscriber interface MAP functions

correspond to functions of Mobility management(MM)

Most important functions Location registration

and location update –enables to route the calls

where the subscriber is



Overview of Location

Registration

Before mobile user can get GSM services the MS has t be

registered to the network

Registration can happen in Home Network (Home PLMN) or to a

visited network provided there is roaming agreement between the

network operators

New registration needed when user changes network and a TMSI

has not been issued yet. In this case the user has to report to the

network using IMSI and receives TMSI when doing location

registration

The registration data is stored to the SIM card. So after power up

only location update is needed if uses stays within same network A5 = encryption algorithm in air interface, TMSI is transmission is

encrypted



Overview of Location

Registration



Location Update

MS already has TMSI in the current network.

This and LAI identify the user. User also has

encryption key Kc.

After authentication new TMSI is generated, andit is transmitted in encrypted form (Kc).

After the process the new: 3-tuple RAND,

SRES, Kc also called SIM-Triplet are updated to

VLR and consumed old ones are deleted. Authentication center usually has pre-calculated

triplets available to make process faster.



Location Update



Location Update After

Changing VLR Area

In this case user changes to a new VLR area,

and in location update the new VLR has to ask

registration parameters (IMSI, RAND, SRES,

Kc) from old VLR. In case user data (TMSI etc.) is lost from

database, the authentication needs to be done

by using IMSI.

After that a new TMSI is assigned and it is nowtransferred in encrypted form.



Location Update After

Changing VLR Area



Routing Call to MS

Due to mobility users cannot be called directly.

In location update The MS obtains temporary

routing number MSRN according to which MS is

reachable. In case of incoming call from outside the MS is

dialed by MSISDN. Call is routed to Gateway

switch GMSC based by MSISDN. GMSC

interrogates the MSRN from HLR. GMSC routes

the call using MSRN. MSC then searches

corresponding TMSI from VLR and routes the

call to destination



Routing Call to MS



Call Routing Variants for

National MSISDN



Call Routing Variants for

International MSISDN



Routing Through GSMC for

International MSISDN



Outgoing Call Setup



Interrogating of Routing

Information for Incoming Call



Incoming Call Setup



Mobile Initiated Call

Termination



Forwarding Short Message

In the air interface a connectionless protocol

(Short Message Transfer Protocol, SM-TP) used

for transfer short messages

Short messages are transferred using signalingchannels (SACCH or SDCCH). Messages are

stored in Short Message Center (SMS-SC) and

delivered by store and forwarded basis

(transmission when receiver available)




Incoming message

Arrives from Short message Center SMS-SC to a Short Message

Gateway MSC (SMS-GMSC). Location MS need to be

determined, then authentication etc.

MAP procedures used for transfer. Success / failure reported to

SMS-GMSC (MAP message Forward Acknowledgement / Error

Indication). GMSC then informs SMS-SC.

Outgoing message

SMS from MS is straightforward sins SMS-GMSC is know to all

MSC, os message can be immediately passed to SMS-GMSC



Handover

Handover is the process where the connection is

set to a new base station as signal goes too low.

This ensures continuous communication despite

of user movement.

Several possibilities depending on location at

the network.

Controlled by protocols and algorithms based on

measurement data.



Handover



Internal and External

Handover



Signaling for Intra MSC

Handover



Handover Decision



Handover Causes and

Threshold

Handover is based on measurements. When

predefined limits are exceeded handover is

commanded.

Limits can be fine-tuned by operator to optimizenetwork



Handover Causes and

Threshold



Detection of Mandatory

Handover

Typical situations for a mandatory handover

The received signal level in the uplink or downlink

(RXLEVEL_UP / RXLEVEL_DL) drops below the respective

handover threshold value (L_RXLEVEL_UP_H /

L_RXLEVEL_DL_H) and power control range has been

exhausted, i.e. MS or BSS have reached their maximaltransmitted power

The bit error rate ratio as a measure of signal quality in uplink

and/or downlink (RXQUAL_UP / RXQUAL_DL) exceeds the

respective handover threshold value (L_RXQUAL_UP_H /

L_RXQUAL_DL_H) while at the same time the received signal

level drops into the neighbourhood of threshold value

The maximum distance to the base station (MAX_MS_RANGE)

has been reach



Detection of Mandatory

Handover



Handover



Basic Handover Between Two

MSCs



Subsequent Handover

(Handback) MSC-A MSC-B

S SC



Subsequent Handover MSC-B

MSC-B’

9C. GSM - Roaming and Switching

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