GSM and CDMA
What is it all about ?
You can change your GSM SIM card to
get rid of your girlfriend….
Shewag ki Maa ka phone CDMA pe
How is one better than the other ???
Which one is cheaper ???
Which one is more reliable ???
What is the difference in technology ….if
Is there a single
security code that
will permit me to
make changes to
my phone ???
What role does
bandwidth play ???
How a mobile
subscriber on one
network access his
home network ???
What is the underlying
Are all different versions of digital
Time Division Multiple
Users separated in
compared to FDMA
Used for GSM
Users separated in
Only possible in
Code Division Multiple
Users separated by
compared to TDMA
Time for History….
GSM originated in Europe…
GSM earlier known as Group Special
Mobile, a study group appointed to
study and develop European public
land mobile system
One important criteria that the proposed
system had to meet was ISDN
And now CDMA…
Introduced in 1980
IN North America….
Purpose…. What else
……but Military usage
Used in Cellular Communication
System , in the early 90’s
Finally ….What is
A way to transmit bits of information through
wideband, spread spectrum radio interface.
IS-95 is the transmission protocol that
Evolution from IS-95A to IS-95B to
Better utilization of radio spectrum by allowing
multiple users to access the same physical
How CDMA Works?
Generating a CDMA signal
1.analog to digital conversion
3.encoding and interleaving
4.channelizing the signals
5.conversion of the digital signal to a Radio
Frequency (RF) signal
Code GeneratorClock Multiplier
Working of CDMA Channel Establishment
..How is it
CDMA uses a device called a vocoder
to accomplish voice compression.
The term "vocoder" is a contraction of
the words "voice" and "code."
Vocoders are located in the phone.
The encoded voice data is further encoded to
separate it from other encoded voice data.
The encoded symbols are then spread over
the entire bandwidth of the CDMA channel.
This process is called channelization.
The receiver knows the code and uses it to
recover the voice data.
CDMA uses two important types of
codes to channelize users.
Walsh codes channelize users on the
forward link (BTS to mobile).
Pseudorandom Noise (PN) codes
channelize users on the reverse link
(mobile to BTS).
CDMA Call Processing State
System Idle State
System Initialization State
System Access State
Traffic Channel State
Hand Off ???
Advantage of CDMA is the ability to
communicate with more than one base
station at one time during a call
This functionality allows the CDMA
network to perform soft handoff
How does it all HAPPEN ?
MS BSC/MSCSecondary BS ‘B’Primary BS ‘A’
Pilot Strength Measurement
Pilot Measurement Request
Pilot Strength Measurement
Frame Selector Join
Roaming in CDMA
Base Station Visited MSC VLR Home MSC HLR Originating Switch
Normal Call Processing Continues
Why is roaming charged
The Directory number of the MS is
Originating Switch sends an ss7 initial
address message (IAM) to home MSC
MSC queries the HLR for the location of
The HLR returns the locations of the
The MSC invokes call forwarding to the
MSC in the visited system..
The forwarding MSC switch send an
SS7 IAM to the visited MSC
Base Station MSC VLR Home MSC HLR Originating Switch
ISDN Hold Ack
MSC Applies Call waitingFlash
MSC puts Call1 on Hold connects call2
Physical equipment used by the
Physical equipment that provides radio
Switching functions of GSM
Databases required for subscribers
Done through MSC
When an MS is switched
• It first has to determine if it has access
• It does a location update for the same
informing its PLMN about its location
• Location of an MS is stored in a central
MS MSCBSS PSTNVLR
Send info for outgoing signal
Privacy and Security in
Each subscriber is identified using a
cryptographic security mechanism
Subscriber security information is stored
in the SIM Card
The algorithm are stored in the SIM
card and in the authentication center.
No boundary between telephony,
information and entertainment services
It will range from voice only to voice,
data, and other multimedia application
Wireless users will be able to make
video conference calls and surf the
WHAT IS ALL THIS
How is it related to my
People demand access for information
and services wherever they are
GSM should provide this connectivity.
Internet access and a whole range of
mobile multimedia capability.
Thus GPRS in GSM
Pay for what you use…
GPRS enhances GSM data services
There is no end to end connection to be
established, so setting up a GPRS call
is almost instantaneous and users can
be continuously online
And users pay for the actual data
transmitted, rather than for connection
A GPRS architecture in GSM
MT PDN TE
Finally Everything Together