Microwave Transmission System

Microwave Spectrum:

Range is approximately 1 GHz to 40 GHz

Total of all usable frequencies under 1 GHz gives a reference on the capacity of in the microwave range
Components of a Microwave System:

1) Digital Modem.

2) Radio Frequency (RF) Unit.

3) Antenna.
Digital Modem:

The digital modem modulates the information signal.

(intermediate frequency or IF)

RF Unit:

IF is fed to the RF unit which is mounted as close physically to the antenna as possible.

(direct connect is optimal)

Waveguides:
Waveguides are hollow channels of low-loss material used to direct the signal from the RF unit to the antenna.  

Advantages & Applications

Advantages of GSM:

1. Capacity increases.
2. Reduced RF transmission power and longer battery life.
3. International roaming capability.
4. Better security against fraud (through terminal validation and user authentication).
5. Encryption capability for information security and privacy.
6. Compatibility with ISDN,leading to wider range of services.

GSM Applications:
   1. Mobile telephony.

   2. GSM-R.

   3. Telemetry System:

- Fleet management.

- Automatic meter reading.

- Toll Collection.

- Remote control and fault reporting of DG sets.

   4. Value Added Services. 

Characteristics of GSM Standard

1. Fully digital system using 900,1800 MHz frequency band.
2. TDMA over radio carriers(200 KHz carrier spacing.
3. 8 full rate or 16 half rate TDMA channels per carrier.
4. User/terminal authentication for fraud control.
5. Encryption of speech and data transmission over the radio path.
6. Full international roaming capability.
7. Low speed data services (upto 9.6 Kb/s).
8. Compatibility with ISDN.
9. Support of Short Message Service (SMS).

Authentication & Encryption

Handovers & Security

Handovers:

Between 1 and 2 – Inter BTS / Intra BSC.

Between 1 and 3 – Inter BSC/ Intra MSC.

Between 1 and 4 – Inter MSC

 

Security in GSM:

On air interface, GSM uses encryption and TMSI instead of IMSI.

SIM is provided 4-8 digit PIN to validate the ownership of SIM.3 algorithms are specified :

- A3 algorithm for authentication
- A5 algorithm for encryption
- A8 algorithm for key generation

 

Call Routing

Call Originating from MS.
Call Termination to MS.

 

Logical Channels

GSM Frame Structure

Physical Channel

GSM Operations

Network Switching SubSystem (NSS)

Mobile Switching Center (MSC):
Heart of the network.

Manages communication between GSM and other networks.

Call setup function and basic switching.

Call routing.

Billing information and collection.

Mobility management
- Registration
- Location Updating
- Inter BSS and inter MSC call handoff

MSC does gateway function while its customer roams to other network by using HLR/VLR.


Home Location Registers (HLR):

Permanent database about mobile subscribers in a large service area (generally one per GSM network operator).

Database contains IMSI, MSISDN, prepaid/postpaid, roaming restrictions, supplementary services.


Visitor Location Registers (VLR):

Temporary database which updates whenever new MS enters its area, by HLR database.

Controls those mobiles roaming in its area.

Reduces number of queries to HLR.

Database contains IMSI, TMSI, MSISDN, MSRN, Location Area, authentication key.


Authentication Center (AUC):

Protects against intruders in air interface.

Maintains authentication keys and algorithms and provides security triplets ( RAND, SRES, Kc).

Generally associated with HLR.


Equipment Identity Register (EIR) :

Database that is used to track handsets using the IMEI (International Mobile Equipment Identity).

Made up of three sub-classes: The White List, The Black List and the Gray List.

Only one EIR per PLMN.

Base Station SubSystem (BSS)

Base Station Subsystem is composed of two parts that communicate across the standardized Abis interface allowing operation between components made by different suppliers:

1. Base Transceiver Station (BTS)
2. Base Station Controller (BSC)


Base Transceiver Station (BTS):

Encodes, Encrypts, Multiplexes, Modulates and Feeds the RF signals to the antenna.

Frequency hopping.

Communicates with Mobile station and BSC.

Consists of Transceivers (TRX) units.


Base Station Controller (BSC):

Manages Radio resources for BTS.

Assigns Frequency and time slots for all MS’s in its area.

Handles call set up.

Transcoding and rate adaptation functionality.

Radio Power control.

It communicates with MSC and BTS.

Mobile Station (MS)

The Mobile Station is made up of two entities:

1. Mobile Equipment (ME)
2. Subscriber Identity Module (SIM)


Mobile Equipment (ME):
Portable,vehicle mounted, hand held device.

Uniquely identified by an IMEI (International Mobile Equipment Identity).

Voice and data transmission.

Monitoring power and signal quality of surrounding cells for optimum handover.

Power level : 0.8W – 20W.


Subscriber Identity Module (SIM):

Smart card contains the International Mobile Subscriber Identity (IMSI).

Allows user to send and receive calls and receive other subscribed services.

Encoded network identification details.
A3,A5 and A8 algorithms.

Protected by a password or PIN.

Can be moved from phone to phone – contains key information to activate the phone.

GSM System Architecture

GSM Services

Tele Services:

Telecommunication services that enable voice communication via mobile phones.

Offered services are 'Mobile telephony' & 'Emergency calling'.


Bearer Services:

Include various data services for information transfer between GSM and other networks like PSTN, ISDN etc.

Short Message Service (SMS).
–up to 160 character alphanumeric data transmission to/from the mobile terminal.

Group 3 fax.

Voice mailbox.

Electronic mail.


Supplementary Services:

Call Waiting- Notification of an incoming call while on the handset.

Call Hold- Put a caller on hold to take another call.

Call Barring- All calls, outgoing calls, or incoming calls.

Call Forwarding- Calls can be sent to various numbers defined by the user.

Call Diverting – All Calls can be Divert on a particular number.

Call Conferencing - Link multiple calls together.

CLIP – Caller line identification presentation.

CLIR – Caller line identification restriction.

GSM History

Developed by Group Spéciale Mobile (founded 1982) which was an initiative of CEPT (Conference of European Post and Telecommunication).

Aim : To replace the incompatible analog system.

Presently the responsibility of GSM standardization resides with special mobile group under ETSI ( European telecommunication Standards Institute).

Full set of specifications phase-I became available in 1990.

Under ETSI, GSM is named as 'Global System for Mobile communication'.

GSM

Global System for Mobile communication (GSM) is a second generation cellular standard developed to cater voice services and data delivery using digital modulation.