Ministry of Higher Education and Scientific
Research Al-Furat Al-Awsat Technical University
Engineering Technical College / Najaf
GSM Architecture Part 2
‫الحسن‬
‫قص جليل‬
-: ‫مدرس المادة‬
‫ي‬
‫ي‬
Base Station Sub-System
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The BSS is the fixed end of the radio interface that provides
control and radio coverage functions for one or more cells and
their associated MSs.
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The BSS comprises one or more Base Transceiver Stations
(BTSs), each containing the radio components that
communicate with MSs in a given area, and a Base Site
Controller (BSC) which supports call processing functions and
the interfaces to the MSC.
Digital radio techniques are used for the radio communications
link, known as the Air Interface, between the BSS and the MS.
The BSS consists of three basic Network Elements (NEs).
1. Transcoder (XCDR) or Remote transcoder (RXCDR) .
2. Base Station Controller (BSC).
3. Base Transceiver Stations (BTSs) assigned to the BSC. .
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Base Station Sub-System
Base Station Controller
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The BSC network element provides the control for the BSS.
It controls and manages the associated BTSs, and interfaces with the (MSC)
key component for handling and routing information
The purpose of the BSC is to perform a variety of functions. The following
comprise the functions provided by the BSC:
1. Controls the BTS components.2. Performs Call Processing.
3. It is a high-capacity switch that provides functions such as handover, cell
configuration data, and control of radio frequency(RF) power levels in base
transceiver stations
4. The databases for all the sites, including information such as carrier
frequencies, power reduction levels, receiving levels for cell border
calculation, are stored in the BSC
5. Provides the A Interface between the BSS and the MSC.
6. Manages the radio channels.
7. Transfers signalling information to and from MSs.
NETWORK COMPONENTS
Base Transceiver Station (BTS)
• The BTS is the radio equipment (transceivers and antennas) needed
to service each cell in the network. A group of BTSs are controlled
by a BSC.
• BTS is typically able to handle (3-5) radio carries, carrying between
(24 – 40) simultaneous communication. Reducing the BTS volume is
important to keeping down the cost of the cell sites.
• The BTS network element consists of the hardware components,
such as radios, interface modules and antenna systems that
provide the Air Interface between the BSS and the MSs.
• The BTS provides radio channels (RF carriers) for a specific RF
coverage area.
• The radio channel (RF) is the communication link between the MSs
within an RF coverage area and the BSS.
• The BTS also has a limited amount of control functionality which
reduces the amount of traffic between the BTS and BSC.
BSS
BSS Network Topologies
Transcoder and Rate
Adaptation Unit (TRAU)
Transcoder Concept
1. allows for the maximum of flexibility and innovation in
optimizing the transmission between MSC and BTS.
2. Compression of Digital data is required to adapt high data
rate (PSTN/MSC) to low data rate Traffic Channel (TCH)/BSC
3. Transcoder converts the data rate from 64 kbps to 16 kbps
and vice versa
4. TRAU (Transcoding Rate and Adaptation Unit) is located
between BSC and MSC. The main task of GSM TRAU is to
compress and de-compress the speech data
5. Transcoders provide gain in system capacity but reduce voice
quality
Transcoder Concept
• The transcoding function may be located at the MSC, BSC, or BTS.
• Transcoding is used where two interfaces do not support the same
encoding scheme.
• During a voice call in a GSM network, both mobile devices perform
voice encodings to make user voice suitable for GSM radio network
transmission.
• Incorporate voice compression necessary in order to assure better
use of the limited-bandwidth radio channel. Voice frames are then
typically decompressed and re-encoded for transport over the 64
kbps circuit switched links through the core network
• The reason networks were designed in such a way is simple
connections to other networks (e.g. Public Switched Telephone
Network, PSTN) and possible additional voice processing in the core
network itself, like for example echo cancelation
Mobile Switching Centre (MSC)
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The Mobile services Switching Centre (MSC) co-ordinates the setting
up of calls to and from GSM users.
It is the telephone switching office for MS originated or terminated
traffic and provides the appropriate bearer services, teleservices and
supplementary services.
It controls a number of Base Station Sites (BSSs) within a specified
geographical coverage area and gives the radio subsystem access to
the subscriber and equipment databases.
It controls calls to and from other telephone and data systems. It also
performs such functions as.
When the MSC provides the interface between PSTN and the BSS in
the GSM network it is called the Gateway MSC.
Mobile Switching Centre (MSC)
Tasks of the MSC
The main tasks of the MSC include:
1. Delivering calls to subscribers as they arrive based on
information from the VLR.
2. Connecting outgoing calls to other mobile subscribers or the
PSTN.
3. Delivering SMSs from subscribers to the short message service
centre (SMSC) and vice versa.
4. Arranging handovers from BSC to BSC.
5. Carrying out handovers from this MSC to another.
6. Supporting supplementary services such as conference calls or
call hold.
7. Generating billing information(Fee).
8. managing the interface between GSM & PSTN.
NETWORK COMPONENTS(VLR)
Each MSC has a VLR
• The Visited Location Register (VLR) is a local subscriber database, holding
details on those subscribers who enter the area of the network that it
covers.
• The details are held in the VLR until the subscriber moves into the area
serviced by another VLR.
• The data includes most of the information stored at the HLR (Home
location Registration), as well as more precise location and status
information.
• The additional data stored in VLR are
1. Mobile status ( Busy / Free / No answer etc. )
2. Location Area Identity ( LAI )
3. Temporary Mobile Subscribers Identity ( TMSI )
4. Mobile Station Roaming Number ( MSRN )
NETWORK COMPONENTS
HOME LOCATION REGISTER( HLR )
• The HLR contains the master database of all subscribers in the PLMN.
• This data is remotely accessed by the MSC´´s .
• There are logically one HLR in the network, although it may consist of
several separate computers.
• The parameters stored in HLR are
1. Subscribers ID (IMSI )
2. Current subscriber VLR.
3. Supplementary services subscribed .
4. Supplementary services information (eg. Current forwarding
address )
5. Authentication key and AUC functionality.
Authentication Centre ( AUC )
• Is a function in a GSM network used for the authentication a mobile
subscriber that wants to be connected to the network.
• Once the subscriber is authenticated, the AUC is responsible for the
generation of the parameters used for the privacy and the ciphering of
the radio link.
• Each subscriber is assigned an authentication key (Ki) which is stored in
the SIM and at the AUC
• The trick is to compare the K i stored in the mobile with the one stored
in the network without actually having to transmit it over the radio air
interface.
• The Authentication Centre generates information that can be used for
all the security purposes during one transaction. This information can
be generated using A3 and A8 algorithm which is located in both SIM
module and in the Authentication Centre (AC). This information is
called an Authentication Triplet. The authentication triplet consists of
three numbers: RAND ,SRES and Kc.
A3- Authentication
A3 Input:
– 128-bit RAND random
– Ki 128-bit private key
A3 Output:
– 32-bit SRES signed response
A8 Key Generator
A8 Input:
– 128-bit RAND random
– Ki 128-bit private key
A8 Output:
– 64-bit Kc Cipher Key
GSM Authentication
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the MS will send either an IMSI or a TMSI to the BSS.
The BSS forwards the MSC/VLR
The MSC/VLR forwards the IMSI to the HLR and requests verification of the IMSI as well
as Authentication Triplets.
The HLR will forward the IMSI to the Authentication Center (AUC) and request
authentication triplets.
The AUC using the IMSI, extracts the subscriber’s authentication key (Ki). The AUC then
The AUC then generates a random number (RAND), applies the Ki and RAND to both the
authentication algorithm (A3) and the cipher key generation algorithm (A8) to produce
an authentication Signed Response (SRES) and a Cipher Key (Kc). The AUC then
sends them along with the IMSI, back to the HLR.
The HLR validates the IMSI by ensuring it is allowed on the network and is allowed
subscriber services. It then forwards the IMSI and Triplets to the MSC/VLR.
The MSC/VLR stores the SRES and the Kc and forwards the RAND to the BSS and orders
the BSS to authenticate the MS
The MS uses the RAND to calculate the SRES and sends the SRES back to the BS
The BSS forwards the SRES up to the MSC/VLR.
The MSC/VLR compares the SRES generated by the AUC with the SRES generated by the
MS. If they match, then authentication is completed successfully
Gateway Mobile Switching Center (GMSC)