gsm_call_routing.pdf
Transcript of gsm_call_routing.pdf
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CALL ROUTING IN GSM
UNIT - 2
Prepared By:- NITIN PANDYA
Assistant Professor,
SVBIT.
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Call Routing
Call Originating from MS
Call termination to MS
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Outgoing Call
1. MS sends dialled number to BSS
2. BSS sends dialled number to MSC
3,4 MSC checks VLR if MS is allowed the requested service. If so,MSC asks BSS to allocate resources for call.
5 MSC routes the call to GMSC
6 GMSC routes the call to local exchange of called user
7, 8,
9,10 Answer back(ring back) tone is routed from called user to MS via GMSC,MSC,BSS
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Incoming Call 1. Calling a GSM subscribers 2. Forwarding call to GSMC
3. Signal Setup to HLR
4. 5. Request MSRN from VLR
6. Forward responsible MSC to GMSC
7. Forward Call to current MSC
8. 9. Get current status of MS
10.11. Paging of MS
12.13. MS answers
14.15. Security checks
16.17. Set up connection
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PLMN INTERFACE (private land mobile network)
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NETWORK ASPECTS IN GSM
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For roaming purpose a mobile need registration, authentication, call routing and location update.
The layer dependencies in GSM is structured into three general layers :
Layer 1 is physical layer
Layer 2 is DLL used for interface, LAPD(link access procedure-D) protocol is used.
Layer 3 is used for signaling purpose, it includes
Radio resources Management : setup control, maintenance
Mobility Management : registration procedures
Connection Management : includes services like SMS
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GSM FREQUENCY ALLOCATION
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Frequency Resource
GSM900 :
up: 890~915MHz
down: 935~960MHz
duplex interval: 45MHz
bandwidth: 25MHz
frequency interval: 200KHz
GSM1800 :
up: 1710-1785MHz down:
1805-1880MHz
duplex interval: 95MHz working bandwidth: 75MHz frequency interval: 200KHz
EGSM900 :
up: 880~890MHz
down: 925~935MHz
duplex interval: 45MHz
bandwidth: 10MHz
frequency interval: 200KHz
GSM1900MHz:
up:1850~1910MHz
down:1930~1990MHz
duplex interval: 80MHz working bandwidth: 60MHz frequency interval: 200KHz
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Mobile to base : Ft(n)=890.2 + 0.2(n-1)MHz
Base to mobile : Fr(n)=Fr(n) + 45 MHz
To share bandwidth for multiple users, GSM uses a combination of Time
Division Multiplexing and Frequency Division Multiplexing
encoding.
One or more carrier frequency is assigned to each base station.
Each of these carrier frequencies divided in time using TDMA scheme.
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CARRIER FREQUENCIES AND TDMA FRAMES 13 NITIN PANDYA
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ORGANIZIATION OF BURSTS,TDMA FRAMES
SACCH-slow associated control channel
A TCH (traffic channel) carry 26 multiple frames at one instance.
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Mobility Management
Paging
Location Update
Handover
Authentication & Security Issues during Handover
Roaming
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Paging
For a mobile terminated call the MS needs to be traced,
located and then the call connected.
The MS is traced through the Paging process within a
location.
Mobile call reception is the process of the GSM
system sending paging messages on a paging control
channel to alert mobile devices that they are receiving a
call. Mobile devices listen for paging messages with their
identification code on a paging channel.
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Location Update
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As a MS moves around it is constantly monitoring the signal
strength of its current BTS, as well as neighboring BTS's to
determine if the neighbors have a stronger signal.
When the MS is in idle mode (not in a call), it will determine
for itself when to move from its current BTS to a more
attractive one.
When the MS switches from a BTS in one VLR to a BTS in a
different VLR, it must do an location update, so the network
knows which MSC/VLR the MS is currently using.
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Temporarily Mobile Subscriber Identification Number
(TMSI)
To insure the IMSI security, the VLR will assign an
unique TMSI number for the accessed subscriber. It is
used locally only and is a 4-byte TMSI number BCD
code.
Numbering Arrangement
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Handovers
Between 1 and 2 Inter BTS /
Intra BSC
Between 1 and 3
Inter BSC/ Intra MSC
Between 1 and 4
Inter MSC
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Security in GSM
On air interface, GSM uses encryption and TMSI instead of IMSI.
SIM is provided 4-8 digit PIN to validate the ownership of SIM
3 algorithms are specified :
- A3 algorithm for authentication
- A5 algorithm for encryption
- A8 algorithm for key generation
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Authentication Concept
Random Number
Generator
Authentication
Algorithm Authentication
Algorithm
Secret Data Secret Data
Random Number
Authentication
Response
Yes
No
Authentication
Response
=
Mobile Station Serving Network
Grant
Access
Deny
Access
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Authentication in GSM (A3 Algo.)
During the authentication process the MSC challenges the
MS with a random number (RAND).
In SIM the RAND number is stored with a secret key Ki.
Both RAND & Ki are 128 bits long.
Using A3 algorithm with RAND & Ki a 32 bit output is
generated by SIM called SRES(signature response).
Using same algorithm AuC also generates a SRES as same as
old one, to compare with SIM (SRES & Ki).
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Authentication in GSM (A3 Algo.)
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Algorithm for Encryption (A5 Algo.)
Uses same cryptography methods as used in Internet
Network.
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Key generation and Encryption(A8)
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Characteristics of GSM Standard
Fully digital system using 900,1800 MHz frequency band.
TDMA over radio carriers(200 KHz carrier spacing.
8 full rate or 16 half rate TDMA channels per carrier.
User/terminal authentication for fraud control.
Encryption of speech and data transmission over the radio path.
Full international roaming capability.
Low speed data services (upto 9.6 Kb/s).
Compatibility with ISDN.
Support of Short Message Service (SMS).
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Advantages of GSM over Analog system
Capacity increases
Reduced RF transmission power and longer battery life.
International roaming capability.
Better security against fraud (through terminal validation and user
authentication).
Encryption capability for information security and privacy.
Compatibility with ISDN,leading to wider range of services
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GSM Applications
Mobile telephony
GSM-R
Telemetry System
- Fleet management
- Automatic meter reading
- Toll Collection
- Remote control and fault reporting of DG sets
Value Added Services
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Future Of GSM
2nd Generation
GSM -9.6 Kbps (data rate)
2.5 Generation ( Future of GSM)
HSCSD (High Speed ckt Switched data)
Data rate : 76.8 Kbps (9.6 x 8 kbps)
GPRS (General Packet Radio service)
Data rate: 14.4 - 115.2 Kbps
EDGE (Enhanced data rate for GSM Evolution)
Data rate: 547.2 Kbps (max)
3 Generation
WCDMA(Wide band CDMA)
Data rate : 0.348 2.0 Mbps
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