GSM Architecture

1. GSM System Overview

2. • Global System for Mobile Communications
(GSM) is a digital wireless network standard
designed by standardization committees from
major European telecommunications
operators and manufacturers.
• The GSM standard provides a common set of
compatible services and capabilities to all
mobile users across Europe and several million
customers worldwide.
• The basic requirements of GSM have been
described in five aspects:

3. • Services. The system will provide service portability; that is,
mobile stations (MSs) or mobile phones can be used in all
participating countries. The system will offer services that
exist in the wireline network, as well as services specific to
mobile communications. In addition to vehicle-mounted
stations, the system will provide service to MSs used by
pedestrians and/or onboard ships.
• Quality of services and security. The quality for voice
telephony of GSM will be at least as good as the previous
analog systems over the practical operating range. The
system will be capable of offering information encryption
without significantly affecting the costs to users who do not
require such facility.
• Radio frequency utilization. The system will permit a high
level of spectrum efficiency and state-of-the-art subscriber
facilities. The system will be capable of operating in the
entire allocated frequency band, and coexist with the
earlier systems in the same frequency band.

4. • Network. The identification and numbering
plans will be based on relevant ITU
recommendations. An international
standardized signaling system will be used for
switching and mobility management. The
existing fixed public networks should not be
significantly modified.
• Cost. The system parameters will be chosen
with a view to limiting the cost of the
complete system, in particular the MSS.

5. GSM Architecture
• Figure 9.1 illustrates the GSM architecture. In
this architecture, a mobile station (MS)
communicates with a base station system
(BSS) through the radio interface. The BSS is
connected to the network and switching
subsystem (NSS) by communicating with a
mobile switching center (MSC) using the A
interface.

7. Mobile Station
• The MS consists of two parts: the subscriber
identity module (SIM) and the mobile
equipment (ME). In a broader definition, the
MS also includes a third part called terminal
equipment (TE), which can be a PDA or PC
connected to the ME. In this case, the first two
parts (i.e., ME and SIM) are called the mobile
terminal (MT). An SIM can be:
– A smart card, usually the size of a credit card
– A smaller-sized "plug-in SIM"
– A smart card that can be perforated, which
contains a plug-in SIM that can be broken out of it

8. • The SIM is protected by a personal identity
number (PIN) between four to eight digits in
length. The PIN is initially loaded by the
network operator at the subscription time.
This PIN can be deactivated or changed by the
user. To use the MS, the user is asked to enter
the PIN. If the number is not correctly entered
in three consecutive attempts, the SIM is
blocked and the MS cannot be used. To
unblock the SIM, the user is asked to enter the
eight-digit PIN unblocking key (PUK).

9. • A SIM contains the subscriber-related information,
including the PIN and PUK codes. The subscriber-
related data also include a list of abbreviated and
customized short dialing numbers, short messages
received when the subscriber is not present, and
names of preferred networks to provide service,
and so on.
• Parts of the SIM information can be modified by
the subscriber either by using the keypad of an
MS or a personal computer using an RS232
connection.
• Subscriber-related data are sent to the ME during
operation, which are deleted after the removal of
the SIM or deactivation of the MS.

10. • The SIM card can be updated over the air
through SIM Toolkit, with which network
operators can remotely upgrade an MS by
sending codes through short messages.
• These messages are issued from a SimCard
server and are received by MSS equipped with
SIM-Toolkit capability.
• SIM Toolkit provides security-related functions
so that SIM cards are not falsely modified. In
some networks, for example D1 T-Mobil in
Germany, every new MS connected to the
network is SIM Toolkit-compliant, and will be
used for high-security applications such as
mobile banking.

11. • The ME contains the noncustomer-related
hardware and software specific to the radio
interface. When the SIM is removed from an MS,
the remaining ME cannot be used for reaching
the service, except for emergency calls. SIMs may
be attached to MEs with different characteristics.
• At every new connection between MS (SIM) and
the network, the characteristic indication of the
ME, called classmark, is given to the network.
• This SIM-ME design supports portability, as well
as enhancing security.
• Usually, the ME is the property of the subscriber.
• The SIM, although loaned to the subscriber, is the
property of the service provider.

12. Base Station System
• The BSS connects the MS and the NSS. The BSS
consists of two parts: the base transceiver station
(BTS) and the base station controller (BSC).
Figures 9.3 and 9.4 illustrate GSM BTS and BSC
products, respectively.
• The BTS contains transmitter, receiver, and
signaling equipment specific to the radio
interface in order to contact the MSs.
• An important part of the BTS is the
transcoder/rate adapter unit (TRAU) that carries
out GSM-specific speech encoding/decoding and
rate adaption in data transmission.
• The BSC is responsible for the switching functions
in the BSS, and is in turn connected to an MSC in
the NSS.

13. • The BSC supports radio channel
allocation/release and handoff management.
• A BSC may connect to several BTSS and
maintain cell configuration data of these BTSs.
• The BSC communicates with the BTSS using
ISDN protocols via the A-bis interface.
• In GSM BSS design, a BSC may connect to only
one BTS, in which case they are likely to be
colocated.
• In this scenario, the BSC and the BTS may be
integrated without the A-bis interface.

14. • Capacity planning for BSC is very important. In
busy hours, the processor load of a BSC is
roughly distributed over call activities (around
20-25%), paging and short message service
(around 10-15 percent), mobility management
(handoff and location update; around 20-25
%), and hardware checking/network-triggered
events (around 15-20 %), A BSC is typically
engineered at 8 percent utilization. When a
BSC is overloaded, it first rejects location
update, next MS originating calls, then
handoffs.

15. Network and Switching Subsystem
• The NSS supports the switching functions, subscriber
profiles, and mobility management.
• The basic switching function in the NSS is performed by the
MSC.
• This interface follows a signaling protocol used in the
telephone network.
• The MSC also communicates with other network elements
external to GSM utilizing the same signaling protocol.
• The current location of an MS is usually maintained by the
HLR and VLR.
• When an MS moves from the home system to a visited
system, its location is registered at the VLR of the visited
system.
• The VLR then informs the MS's HLR of its current location.

16. • The authentication center (AuC) is used in the security
data management for the authentication of
subscribers.
• The AuC may be colocated with the HLR.
• An incoming call is routed to an MSC, unless the fixed
network is able to interrogate the HLR directly.
• That MSC is called the gateway MSC (GMSC).
• An MSC can function as a GMSC by including
appropriate software and HLR interrogation functions,
and by provisioning interface and the signaling link to
the HLR.
• The GMSC obtains the location information and routes
the calls to the visited MSC of the subscribers to
receive the calls.

17. Radio Interface
• The GSM radio link uses both FDMA and TDMA
technologies.
• The 900 MHz frequency bands for the GSM downlink
signal and the uplink signal are 935-960 MHz and 890-
915 MHz, respectively.
• The frequency band is divided into 124 pairs of
frequency duplex channels with 200-KHz carrier
spacing.
• Note that, for a given distance, less power is required
to transmit signal over a lower frequency.
• To save MS power, uplink frequencies in mobile
systems are always the lower band of frequencies.
• Discontinuous transmission is used in GSM to save the
power consumption of the MS.

18. • With this function, an MS turns the
transmitter on only while voice is present.
• When there is no voice input, the transmitter
is turned off.
• GSM also supports discontinuous reception
where the MS needs to listen only to its
subchannel for paging.

19. • The length of a GSM frame in a frequency channel is 4.615
msec.
• The frame is divided into eight bursts (time slots) of length
0.577 msec.
• The time slots in the uplink are derived from the downlink
by a delay of three time slots.
• This arrangement prevents an MS from transmitting and
receiving at the same time.
• However, due to propagation delays, especially when the
MS is far away from the BTS, the three time-slot delay
cannot be accurately maintained.
• The solution is to compute the timing advance value so that
the exact shift between downlink and uplink seen by the
MS is three time slots minus the timing advance value.
• This timing advance value is calculated by the BSS, based
on the bursts received from the MS, and is signaled to the
MS twice per second to inform the MS of the appropriate
timing value.

21. • Two types of logical channels are defined: traffic
channels (TCHs) and control channels (CCHS).
• TCHS are intended to carry user information
(speech or data).
• Two kinds of TCHS are defined:
– Full-rate TCH (TCH/F). Provides transmission speed of
13 Kbps for speech or 9.6, 4.8, or 2.4 Kbps for data.
Enhanced full-rate (EFR) speech coders have been
implemented to improve the speech quality of TCH/F.
– Half-rate TCH (TCH/H). Allows transmission of 6.5
Kbps speech, or 4.8 or 2.4 Kbps of data.
• The CCHS are intended to carry signaling
information. Three types of CCHS are defined in
GSM:

22. • Common control channels (CCCHs). Include
the following channel types:
– Paging channel (PCH), used by the network to
page the destination MS in call termination.
– Access grant channel (AGCH), used by the network
to indicate radio link allocation upon prime access
of an MS.
– Random access channel (RACH), used by the MSs
for initial access to the network.

23. • Dedicated control channels. Supported in GSM
for dedicated use by a specific MS.
– Standalone dedicated control channel (SDCCH),
used only for signaling and for short messages.
– Slow associated control channel (SACCH),
associated with either a TCH or an SDCCH. The
SACCH is used for nonurgent procedures, mainly
the transmission of power and time alignment
control information over the downlink, and
measurement reports from the MS over the
uplink. A TCH is always allocated with a control
channel SACCH to transport both user information
and signaling data in parallel.

24. – Fast associated control channel (FACCH), used for
time-critical signaling, such as call-establishing
progress, authentication of subscriber, or handoff
. The FACCH makes use of the TCH during a call;
thus, there is a loss of user data because the
FACCH "steals" the bandwidth of the TCH.
– Cell broadcast channel (CBCH), carries only the
short message service cell broadcast messages,
which use the same time slot as the SDCCH.

25. • The CBCH is used on the downlink only. SDCCH,
SACCH, and FACCH are used in both downlink and
uplink.
• Broadcast channels (BCHs). Used by the BTS to
broadcast information to the MSs in its coverage
area.
– Frequency correction channel (FCCH) and
synchronization channel (SCH) carry information from
the BSS to the MS. The information allows the MS to
acquire and stay synchronized with the BSS.
– Broadcast control channel (BCCH) provides system
information such as access information for the
selected cell and information related to the
surrounding cells to support cell selection and
location registration procedures in an MS.