GSM and CDMA

What is it all about ?

By

Manjit Chandel

Marketing

You can change your GSM SIM card to get rid of your girlfriend….

Shewag ki Maa ka phone CDMA pe aaya tha…

Questions ???

How is one better than the other ???

Which one is cheaper ???

Which one is more reliable ???

What is the difference in technology ….if any ???

More Questions….

Is there a single security code that will permit me to make changes to my phone ???

What role does bandwidth play ???

What provides protection from eavesdropper ???

How a mobile subscriber on one network access his home network ???

What is the underlying Difference ??

TDMA

FDMA

CDMA

Layman’s view…Are all different versions of digital technology…

Time Division Multiple Access

Users separated in time

Requires digital transmission

Normally wider bandwidth compared to FDMA

Used for GSM

Frequency Division Multiple Access(FDMA)

Users separated in frequency

Only possible in analog system

Code Division Multiple Access

Users separated by code

Requires digital transmission

Wider bandwidth 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 systemOne important criteria that the proposed system had to meet was ISDN compatibility..

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 CDMA? …..

A way to transmit bits of information through wideband, spread spectrum radio interface.

IS-95 is the transmission protocol that employs CDMA.

Evolution from IS-95A to IS-95B to CDMA2000Better utilization of radio spectrum by allowing multiple users to access the same physical channel.

How CDMA Works?

Generating a CDMA signal

1.analog to digital conversion

2.vocoding

3.encoding and interleaving

4.channelizing the signals

5.conversion of the digital signal to a Radio

Frequency (RF) signal

How does it works……

Code GeneratorClock Multiplier

FilterModulator

Data Clock

Data

Carrier

TransmitSignal

Working of CDMA Channel Establishment

Voice Compression ..How is it accomplished??

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.

Channelizing

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.

Codes…

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 ?

Soft Handover…

MS BSC/MSCSecondary BS ‘B’Primary BS ‘A’

Pilot Strength Measurement

Pilot Measurement Request

Pilot Strength Measurement

Handoff Request

Handoff Direction

ACK

Handoff Information

ACK

ACK

ACK

Frame Selector Join

Roaming in CDMAMobile Station

Base Station Visited MSC VLR Home MSC HLR Originating Switch

User Dials CallIAM

HLR Query

HLR Response

SS7 IAM

Normal Call Processing Continues

Why is roaming charged more ???

The Directory number of the MS is dialedOriginating Switch sends an ss7 initial address message (IAM) to home MSCMSC queries the HLR for the location of the MSThe HLR returns the locations of the visited System

Roaming Contd..

The MSC invokes call forwarding to the MSC in the visited system..

The forwarding MSC switch send an SS7 IAM to the visited MSC

Call processing……

Call Waiting….Base Station MSC VLR Home MSC HLR Originating Switch

User Dials CallIAM

VLR Query

VLR Response

Mobile Station

ISDN Hold Ack

MSC Applies Call waitingFlashISDN Hold

MSC puts Call1 on Hold connects call2

GSM

NSSBSSMS

OSS

UsersExternalNetworks

Operators

GSM Subsystems

Entities

MS:Physical equipment used by the

subscribersBSS:

Physical equipment that provides radio coverageNSS:

Switching functions of GSMDatabases required for subscribersDone through MSC

When an MS is switched on…What happens??

• It first has to determine if it has access to PLMN

• It does a location update for the same informing its PLMN about its location

• Location of an MS is stored in a central database

MS MSCBSS PSTNVLR

GSM

CALL

FLOW

SetupSend info for outgoing signal

Complete CallCall Proceeding

AssignmentAssignment

Assignment CompleteAssignment Complete

IAM

Assignment Complete (ACM)

Alerting

AnswerConnect

Connect Acknowledge

Privacy and Security in GSM

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.

Security Algorithms for GSM

1.Authentication Algorithm(A3)

• Used by handset to compute a signed response to a random number transmitted by BS

2.Private key generation(A8)

•Uses the same random number and K(i) to generate a private key(K©)that is used for voice and data privacy

A3 A8

K(I) on SIM CardRAND(from BS)

SRES to BS K© privacy mask

Generation Next…3G

No boundary between telephony, information and entertainment servicesIt will range from voice only to voice, data, and other multimedia applicationWireless users will be able to make video conference calls and surf the internet simultaneously

WHAT IS ALL THIS

How is it related to my topic….

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 servicesThere 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 time

A GPRS architecture in GSM

BSS

EIR

SGSN

MSC/VLR

SMS-GMSC

GGSNTE

HLR

MT PDN TE

GGSN Other PLMN

SM-SC

Finally Everything Together

GSM

CDMAone

PDC

TDMA

CDMA2000 IX

GPRS

CDMA200 IXEV

WCDMA

EDGE