Intro to Communication systems Chap 4

8/13/2019 Intro to Communication systems Chap 4

1/23

GSM

GLOBAL SYSTEM FORMOBILE COMMUNICATION


2/23


3/23


4/23


5/23

Bands Of GSM


6/23


7/23


8/23


9/23

Frequency Bands and Bandwidth

Uplink 890 915 MHz 25 MHz

Downlink 935 960 MHz 25 MHz

100 KHz 200 KHz 100 KHz

1 43 1242 .

A 200 KHz carrier spacing has been chosen. Excluding 2x100 KHz edges of

the band, this gives 124 possible carriers for the uplink and downlink. The

use of carrier 1 and 124 are optional for operators.

Multiple Access Technique

FDMA/TDMA. The total band is divided into 124x200 KHz bands (FDMA).

Each group of 8 users transmit through a 200 KHz band sharing

transmission time (TDMA).


10/23


11/23


12/23

Features Of GSM


13/23

1) Compatibility

Before GSM every country has a different Cellular system.Not compatible with each other in terms of equipment&

services.

The need for a common standard for mobile communications

was therefore obvious.

GSMhas been specified and developed by many European

countries working in co-operation with each other.

An additional advantage; large market for GSM equipment.

This means manufacturers can produce equipment in higher

quantitiesand of better quality.

Therefore competitive and aggressive pricing structure

exists


14/23


15/23

2) Noise Robustness

Before GSM Cellular services used analogueradio signals.

Although excellent audio quality but vulnerable to noise.

The noise which interferes with the current system may be

produced by any of the following sources:

1. A powerful or nearby external source (a vehicle ignitionsystem or a lightning bolt, perhaps);

2. Another transmission on the same frequency (co-channel

interference);

3. Another transmission breaking through from a nearbyfrequency (adjacent channel interference);

4. Background radio noise intruding because the required

signal is too weak to exclude it.


16/23

In order to combat these problems, GSM usesdigital technology instead of analogue.

By using digital signals, we can manipulate thedata and include sophisticated errorprotection,

detectionand correctionsoftware.

The overall result is that the signals passedacross the GSM air interface withstand more

errors. Due to this feature, the GSM air interface in

harsh RF environments can produce a usablesignal,

2) Noise Robustness


17/23


18/23

4 Use of Standardized Open Interfaces

The equipment in each of the analogue cellularnetworks tends to be produced by onemanufacturer.

This is because the equipment is only designed to

communicate with other equipment made by thatmanufacturer.

This situation is veryprofitablefor themanufacturers .

Unfortunately for the MS user and the networkprovider, this means high prices.

The situation is very different with GSM, wherestandard interfaces such as C7and X.25are usedthroughout the network.


19/23

With some of the first generation systems, it has beenestimated that up to 20% of cellular phone calls are stolen.

Extensive measures have been taken, when specifying theGSM system.

Subscription check through IMEI& SIM.

GSM also offers the capability to encrypt all signaling overthe air interface.

Different levels of encryptionare available to meet differentsubscriber/country requirements.

With the authentication processes, together with theencryption and the digital encoding of the air interfacesignals, it makes it very difficult for the casual hacker tolisten-in topersonal calls.

In addition to this, the GSM air interface supports frequency

hopping.

5) Improved Security and Confidentiality


20/23

With analogue systems, handoversare frequently aproblem.

Area and the subscriberis often aware that a handoverhas occurred!

In GSM, a great deal of thought went into the designand implementationof handovers.

GSM provides handover processes for the following:

Quality (uplink/downlink).

Interference (uplink/downlink).

RF level (uplink/downlink).

MS distance.

Power budget.

6) Flexible Handover Processes


21/23

Integrated Services Digital Network (ISDN) is astandard that most developedcountriesarecommitted to implement.

This is a new and advanced telecommunication

network designed to carry voiceand user data overstandard telephone lines.

The GSM network has been designed to operatewith the ISDN system and provides features which

are compatiblewith it. GSM can provide a maximum data rate of 9.6 kbit/swhile ISDN provides much higher data rates thanthis.

7) ISDN Compatibility


22/23

Telephony

Emergency Calls (with/without SIM Card

inserted in MS)

Short Message Service Point To Point.

Short Message Cell Broadcast.

Advanced Message Handling Service. Dual Personal and Business Numbers.

Data Services

8) Speech Services


23/23

i Number Identification

ii Call Barring

iii Call Forwarding

iv Call Completion

v Charging

vi Multi-partyvii e-fax

viii Voice Messaging

9) Supplementary Services

Intro to Communication systems Chap 4

Documents

Transcript of Intro to Communication systems Chap 4