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7/28/2019 9C. GSM - Roaming and Switching
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ROAMING ANDSWITCHING
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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
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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
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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
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Overview of Location
Registration
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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.
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Location Update
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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.
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Location Update After
Changing VLR Area
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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
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Routing Call to MS
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Call Routing Variants for
National MSISDN
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Call Routing Variants for
International MSISDN
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Routing Through GSMC for
International MSISDN
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Outgoing Call Setup
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Interrogating of Routing
Information for Incoming Call
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Incoming Call Setup
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Mobile Initiated Call
Termination
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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)
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Forwarding Short Message
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
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Forwarding Short Message
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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.
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Handover
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Internal and External
Handover
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Signaling for Intra MSC
Handover
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Handover Decision
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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
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Handover Causes and
Threshold
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Handover Causes and
Threshold
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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
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Detection of Mandatory
Handover
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Handover
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Basic Handover Between Two
MSCs
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Subsequent Handover
(Handback) MSC-A MSC-B
S SC
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Subsequent Handover MSC-B
MSC-B’
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