GSM Overview
By
B Pavan Kumar
It all started like this
First telephone (photophone) –
Alexander Bell, 1880
The first car mounted radio
telephone – 1921
Going further
1946 – First commercial mobile radio-
telephone service by Bell and AT&T in
Saint Louis, USA. Half duplex(PTT)
1973 – First handheld cellular phone
– Motorola.
First cellular net
Bahrein 1978
But what’s cellular?
MSC
BS
PSTN HLR, VLR,
AC, EIR
Cellular principles
Frequency reuse – same frequency in
many cell sites
Cellular expansion – easy to add new
cells
Handover – moving between cells
Roaming between networks
Generation Gap
1G – Analog [routines for sending
voice]
All systems are incompatible
No international roaming
Little capacity – cannot accommodate
masses of subscribers
Generation Gap(2)
2G – digital [voice encoding]
Increased capacity
More security
Compatibility
Can use TDMA or CDMA for
increasing capacity
TDMA
Time Division Multiple Access
Each channel is divided into timeslots,
each conversation uses one timeslot.
Many conversations are multiplexed
into a single channel.
Used in GSM
CDMA
Code Division Multiple Access
All users share the same frequency all
the time!
To pick out the signal of specific user,
this signal is modulated with a unique
code sequence.
Back to Generations
2.5 G – packet-switching
Connection to the internet is paid by
packets and not by connection time.
Connection to internet is cheaper and
faster [up to 56KBps]
The service name is GPRS – General
Packet Radio Services
The future is now
3G
Permanent web connection at 2Mbps
Internet, phone and media: 3 in 1
The standard based on GSM is called
UMTS.
The EDGE standard is the
development of GSM towards 3G.
GSM
More than 800 million end users in 190
countries and representing over 70% of
today's digital wireless market.
source: GSM Association
Israel
Orange uses GSM
Pelephone and Cellcom are about to use
GSM
GSM Overview
Introduction the architecture
Mobile phone is identified by SIM
card.
Key feature of the GSM
Has the “secret” for authentication
Introduction the architecture contd..,
BTS – houses the radiotransceivers of
the cell and handles the radio-link
protocols with the mobile
BSC – manages radio resources
(channel setup, handover) for one or
more BTSs
Introduction the architecture
contd..,
MSC – Mobile Switching Center
The central component of the network
Like a telephony switch plus everything
for a mobile subscriber: registration,
authentication, handovers, call routing,
connection to fixed networks.
Each switch handles many cells
Introduction the architecture contd..,
HLR – database of all users + current
location. One per network
VLR – database of users + roamers
in some geographic area. Caches the
HLR
EIR – database of valid equipment
AuC – Database of users’ secret keys
More GSM
GSM comes in three flavors(frequency
bands): 900, 1800, 1900 MHz. 900 is
the Orange flavour in Israel.
Voice is digitized using Full-Rate
coding.
20 ms sample => 260 bits . 13 Kbps
bitrate
Sharing
GSM uses TDMA and FDMA to let
everybody talk.
FDMA: 25MHz freq. is divided into
124 carrier frequencies. Each base
station gets few of those.
TDMA: Each carrier frequency is
divided into bursts [0.577 ms]. 8
bursts are a frame.
Channels
The physical channel in GSM is the
timeslot.
The logical channel is the information
which goes through the physical ch.
Both user data and signaling are
logical channels.
Channels contd.,
User data is carried on the traffic
channel (TCH) , which is defined as
26 TDMA frames.
There are lots of control channels for
signaling, base station to mobile,
mobile to base station
