Gsm in detail

1. GSMGSM
Functionalities of
BTS,BSC,MSC,VLR,HLR,MGW,SDP
SYSTEM

2. Compared to a fixed networkCompared to a fixed network
Gateway MSC
MSC
BSC
MS
Local switches
Transit exchang
Subscriber stage
Fixed phone

3. Going furtherGoing further
• 1946 – First commercial mobile radio-telephone
service was invented by Bell and AT&T. That was Half
duplex(PTT)
• 1973 – First handheld cellular phone – Motorola.

4. But what’s cellular?But what’s cellular?
HLR,
VLR
MSC
PSTN
BS

5. GSMGSM
• More than 800 million end users
in 190 countries and
representing over 70% of
today's digital wireless market.
o source: GSM Association

6. What’s new?What’s new?
• Radio access network
• Encryption
• Voice coding
• Locate a mobile station
• Identify and authenticate
• Handle mobility during a call

7. GSM OverviewGSM Overview
BSC:Base Station Controller MSC:Mobile Switching Centre

8. A bird eye’s viewA bird eye’s view
BTS
MS
BSC
MSC
BSC
BSC
BTS
BTS
GMSC
Access Network
databases

9. Base Transceiver StationBase Transceiver Station
• Controls the radio link
o encryption
o error control
o signal strength
• 1 - 6 duplex carriers, for
example:
o two layers
o 120o
sectors
o (6+8)*3 = 42 voice calls
• Cost ~ 100 – 200 k$

10. Base Station ControllerBase Station Controller
• communication with the mobile
terminal
o local hand-over
o voice codecs
o carrier allocation
o paging to terminals
• talks to the MSC
• serves up to 40 BTSs
• could be co-located with MSC

11. Mobile Swicthing CenterMobile Swicthing Center
• Mobility management
o identifies and authenticates
o locates
o switches between BSCs
o Handover to other MSCs
• 64 kbps regular voice channels

12. Keeping track of aKeeping track of a
mobile stationmobile station
VLR
visiting location registerLocation area
Location area
update the VLR with new
location area

13. Moving to a new MSCMoving to a new MSC
Location area
Location area
VLR
which LA which MSC ?
update the VLR with new
location area and new MSC

14. but also….but also….
VLR
HLR
home location register
update the home location register!

15. Home Location RegisterHome Location Register
MSC
GMSC
PSTN
incomming call
where is the mobile?
HLR
How do we reach him?
VLR

16. VLR & HLRVLR & HLR
• Visitor Location Register (VLR): AVLR is acurrent subset of
HLR for a particular cell.
• Whenever a subscriber enters a new cell, its current
location is stored in VLR representing that cell.
• The entry is removed when the subscriber leaves that cell.
• The information about a subscriber is replicated in HLR
and VLR to achieve faster location search which begins
from VLR and ends in HLR if the entry is not found in the
VLR.
• The VLR data are not used for any administrative purpose;
it merely serves as a location identifier.

17. VLR & HLRVLR & HLR
• HLR and VLR are huge databases maintained by high-
power servers, often a Unix workstation.
• Tandem, which is part of Compaq (a part of HP), made the
server and called it HLR when used for cellular system.
• A large number of mobile units use the same HLR, and there
are usually more than one HLR for nationwide cellular
system.

18. MEDIA GATEWAYMEDIA GATEWAY
• A media gateway is a translation device or service that
converts digital media streams between disparate
telecommunications networks such as PSTN, SS7,
Next Generation Networks (2G, 2.5G and 3G radio
access networks) or PBX.
• Media gateways enable multimedia communications
across Next Generation Networks over multiple transport
protocols such as Asynchronous Transfer Mode (ATM)
and Internet Protocol (IP).
• Because the media gateway connects different types of
networks, one of its main functions is to convert between
different transmission and coding techniques. Media
streaming functions such as echo cancellation, DTMF,
and tone sender are also located in the media gateway.

19. SDPSDP
• A service data point is the node in
the IN GSM which is responsible for providing
subscriber information. A GSM network element that
needs information about subscriber will interrogate
the SDP about such information.
• The operator has to define all the rate plans in SDP.
The SDP is being interrogated on a periodical
interval to evaluate the validity of the current call
by the subscriber. The validity is being measured
according to several criteria:
• The subscriber balance cover the charge of the
service being requested by him/her.
• The subscriber is eligible to use the service
according to his rate plan