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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