18 GSM Architecture: Part I

Suchit Purohit

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Learning Objectives

  • Introduction to GSM
  • Advantages of GSM
  • GSM Specifications
  • Services provided by GSM
  • GSM architecture

Introduction

 

GSM abbreviates for global systems for mobile communications. It was developed as 2G standard for cellular communications in 1982. The purpose of foundation was to establish a European mobile phone communication standard but soon it got widely accepted standard in telecommunication industry and is implemented globally. The foundation of GSM lies on cellular networks founded by Bell laboratories in 1970. GSM owns market share of 80-85% of world’s digital cellular subscriber of 7 billion mobile subscribers, more than 1 billion are GSM subscribers. GSM network is spread in more than 210 countries in the world. Along with voice and data service the ability of GSM which made it so popular is its roaming capability. In this module and subsequent modules, we will learn about GSM standard. We would start with GSM architecture understanding the three layers and underlying entities with their functions. Then we would see how handovers are handled in GSM, how security mechanisms are implemented and what all actions take place when a call is made to and from GSM network. In this module we would start with introduction to GSM, specifications, services provided and first layer of GSM architecture.

Advantages of GSM Network

  • ed Spectrum efficiency
  • International Roaming
  • Low-cost mobile stations
  • High Quality speech
  • Compatibility with ISDN
  • Support for new services

GSM Specifications

 

In this section , we will learn about GSM specifications like frequency used, multiple access techniques used, modulation technique, data rate supported ,frame duration, duplexing distance used etc.

 

Frequency

 

Of different variants of GSM, the most popular ones are GSM 900, GSM 1800 and GSM 1900. The different versions of GSM and the frequency allocations are as follows:

 

TABLE 1 FREQUENCY

GSM band ƒ (MHz) Uplink (MHz) (Mobile to Base) Downlink (MHz) (Base to Mobile) Channel number Equivalent LTE band
T-GSM-380 380 380.2 – 389.8 390.2 – 399.8 dynamic
T-GSM-410 410 410.2 – 419.8 420.2 – 429.8 dynamic
GSM-450 450 450.6 – 457.6 460.6 – 467.6 259 – 293 31
GSM-480 480 479.0 – 486.0 489.0 – 496.0 306 – 340
GSM-710 710 698.2 – 716.2 728.2 – 746.2 dynamic 12
GSM-750 750 777.2 – 792.2 747.2 – 762.2 438 – 511
T-GSM-810 810 806.2 – 821.2 851.2 – 866.2 dynamic 27
GSM-850 850 824.2 – 849.2 869.2 – 893.8 128 – 251 5
P-GSM-900 900 890.0 – 915.0 935.0 – 960.0 1 – 124
E-GSM-900 900 880.0 – 915.0 925.0 – 960.0 975 – 1023, 0 – 124 8
R-GSM-900 900 876.0 – 915.0 921.0 – 960.0 955 – 1023, 0 – 124
T-GSM-900 900 870.4 – 876.0 915.4 – 921.0 dynamic
DCS-1800 1800 1710.2 – 1784.8 1805.2 – 1879.8 512 – 885 3
PCS-1900 1900 1850.2 – 1909.8 1930.2 – 1989.8 512 – 810 2

 

The frequencies used  by GSM network  depends on regulatory requirements for particular country and ITO region in which country is located. Different countries use different bands.

 

To facilitate international roaming, cell phones should be able to cover bands of countries they visit. For e.g. Europe uses GSM 900 & 1800 bands, middle east, Africa, Asia & North America uses 850 bands and 1900 MHz. Canada uses 1900Mhz. Therefore to facilitate international roaming, call phones operate on multiple bands and are called multi band phone. Multi band phones are dual band, tri band & Quad band.

 

Multiple Access methods

 

Combination of SDMA, FDMA and TDMA is employed in GSM. SDMA is implied by division of area into cells and employing frequency reuse.

 

FIGURE 1: SDMA IN CELLULAR SYSTEMS

 

FDMA divides available bandwidth into 124 channels of 200 KHz each with spacing of 100 KHz between adjacent carrier frequencies. To imply TDMA, eight time slots of 577 micro sec each are there. Therefore effectively 124 * 8 = 992 channels are available for transmitters and receiver .

 

FIGURE 2: FDMA IN GSM

Duplexing Distance

 

Voice over phones required a full duplex channel since sending and receiving is required at the same time the transmission from mobile phone to base station is via uplink channel and from base station to mobile phone is via downlink channel. Separate frequencies are allocated for UL and DL  channels  the amount of guard band frequency’s called duplexing distance  in GSM duplexing distance is 80 MHz.

 

FIGURE 3: DUPLEXING DISTANCE BETWEEN UPLINK AND DOWNLINK FREQUENCIES

Modulation

It uses Gaussian minimum shift keying (GMSK) modulation technique

 

Speech coding

 

LPC Linear predictive coding is used

 

Frame Duration: 4.615 ms

Duplexing Technique: FDD frequency Div. Duplexing

Speech channels per RF channel: 8

GSM Services

 

The Services provided by GSM are categorized as

  1. Teleservices Services
  2. Supplementary Services
  3. Bearer Services

Teleservices

  • Telephonic voice at full data rata (13.4 kbps)
  • Fax, sms, emergency number
  • MMS
  • Teletext, Video text

Teleservices are point to point i.e from one equipment to another

 

Supplementary Services

 

These services are enhancements to standard telephony services. These are specific to service provider. These supplementary services are

  • User Identification
  • Call forwarding
  • Closed user GPS formation
  • Multi party groupings
  • Call Waiting
  • Call holding
  • Call Barring for a user or gp of uses
  • Information regarding call charges etc.

Bearer Services

 

Bearer: Set of data transmitted or received from terminal equipment. Bearer services are responsible for transmission of data leading to explosion in use of mobile internet access and mobile data transfer. Voice signals are also transmitted as data.

 

Voice Data: Data obtained after digitizing, coding, encoding appending error detection and correction bits and encrypting of voice signal

 

The Data rates for bearer services vary from service to service. Categories of data transmission and the corresponding data rates are provided in table 2 and table 3.

 

TABLE 2: DATA RATES AS PER BEARER SERVICE

Transparent Non Transparent
The interface uses only physical layer protocol Services uses physical layer, data link layer and flow control layer protocols
Different data rates of 2.4, 4.8 or 9.6 kbit / s are available 300 bps – 9.6 kbps

TABLE 3: DATA RATES AS PER BEARER SERVICE

Synchronous Asynchronous
Data is transmitted from sender to receiver at a fixed rate and constant phase difference is maintained between frames. Data is transmitted from sender to receiver at variable rates and a constant phase different is NOT maintained between consecutive frames or data burnt data rate
Data rate can be 1.2, 2.4 or 4.8 kbs 300 – 9.6 kbps

GSM  Architecture

 

GSM is a network with hierarchical structure with architecture consisting of basic 3 sub – systems namely:

  1. Radio Sub System
  2. Network Sub System
  3. Operation Sub System

In this section we will understand the entities, interfaces and functions involved in radio sub system.

Radio sub system

 

It comprises of all radio specific entities. It is connected to other sub-systems like NSS and OSS by A – interface and O – interface connect MS to the network. The entities belonging to radio sub system are

  • Mobile station (MS)
  • Base Transceiver System (BTS)
  • BSC Base Station Controller

 

The three entities form a hierarchical structure as shown in figure. There are number of MS, which connect to a BTS, Number of BTS are connected to a BSC. Hence,

 

No. of MS > No of BTS > No. of BSC

 

BSC with BTS’s and MS’s form Base station subsystem .BSS are connected to MSC in NSS and OMC in OSS layer.

FIGURE 4: ENTITIES OF RADIO SUB SYSTEM

Mobile Station

 

It can be any hand-held devices like phone, tablet, PDU, laptop which connects to GSM network. It consists of necessary network and software to transmit and receive GSM data. It consists of a user terminal to access the device and SIM card through which user can access its account in GSM, connects to network and uses services of network. The MS for GSM 900 have transmit power of 2W whereas for GSM 1800 1W is enough because the cell sizes are smaller.

 

IMEI or International mobile equipment identity is a 15 digit number identifier associated with the mobile station. It is a useful tool to prevent stolen handset from accessing a network. IMEI numbers are stored on devices beneath the battery printed on a small while label. It can also be retrieved by dialing *#06# from the MS

 

Subscriber Identity Module

 

Through SIM user can access its account in GSM, connects to network and uses services of network. Following information is stored in SIM

  • Card serial number, type and list of subscribed services
  • IMSI number (International mobile Subscriber Identity): A 15-digit number allocated to uniquely identify the subscribers. It does not change when user moves from one network to another. It is used to generate cipher key, TMSI and LAI from service provider. It constitutes of 3 parts
    • 3 digit mobile country code
    • 2 digit mobile network code
    • 10 digit MSIN number (Mobile Subscriber Identity Number) The same format is used by service provider all over the world.

For eg. If IMSI is 310 15 0123456789

310 – Country Code for USA; 15 – AT & T Mobility;  MSIN – 0123456789

  • PIN (Personal Identification Number) used to lock and unlock the MS.
  • PUK – Pin Unblocking Key enables subscribers to unlock the SIM if it is accidentally or deliberately lock
  • Ki – 128 bit authentication key used for authentication and cipher key generation during encryption
  • Kc : Cipher key

BTS (Base Transceiver System)

 

BTS comprises of all radio equipment like antenna, signal processing and amplifiers necessary for radio transmission. The MS connects to BTS via full duplex channels. It manages a cell or number of cells depending on whether the antenna is omnidirectional or sectorized antenna. The size of GSM cell can be from 100m to 35km depending on environment, topography and teledensity. Functions of BTS are:

  • Formation of cells
  • Signal processing and amplification of signals so that the strength of signal stays upto an accepted threshold value because below this it won’t be possible to discern the signal leading to loss of data
  • Channel encoding and decoding for coding voice into data and vice versa
  • Encryption and decryption
  • Paging to identify mobile stations
  • Frequency Hopping so that multiple channels for various mobile stations can operate simultaneously using different channel band frequencies
  • Data rate adaption in case of synchronous data transmission

Base station controller

 

The Base Station Controller (BSC) controls and supervises a number of Base Transceiver Stations (BTS). The BSC is responsible for the allocation of radio resources to a mobile call and for the handovers that are made between base stations under his control.

 

Functions of BSC

  • Radio Channel Setup
  • Signal Processing
  • Reserves radio frequencies for communication
  • Manages handovers among BTSs
  • Control and handover of signals from BSC to MSC
  • Frequency Hopping so that multiple BTSs can operate simultaneously using different frequencies at same time
  • Measures the use of frequency channel spectrum for traffic control
  • Encryption and decryption
  • Updation of location registry

Summary

  • GSM is a first digital 2G telecommunication standard. It has the highest subscriber base worldwide.
  • Amongst many variants most popular are GSM 800/1800/1900
  • Services provided are teleservices, supplementary services and bearer services
  • Architecture is layered architecture with 3 sub-systems namely radio sub system, network subsystem and operation subsystem.
  • Radio sub-system comprises of radio specific entities like mobile station, base station and base station controller
  • Mobile station consists of card, antenna and SIM card. SIM card consists of static and dynamic information like IMSI number, key for authentication and encryption, keys for locking and unlocking phone etc.
  • Base station consists of transmitter as well as receiver. It performs functions like signal processing, encryption, cell formation, handovers etc.
  • Many base stations are controlled by Base station controller. It performs functions like channel allocation, signal processing, intra cell handovers etc.
you can view video on GSM Architecture: Part I

Suggested Reading:

  1. Mobile Communication 2nd edition by Jochen Schiller, Pearson education
  2. Mobile Computing by Asoke Talukder, Roopa Yavagal (Tata McGraw Hill)
  3. “Wireless communication and networking” by William Stallings
  4. Mobile Cellular Telecommunications — W.C.Y. Lee, Mc Graw Hill
  5. Wireless Communications – Theodore. S. Rapport, Pearson Education
  6. Reza B’Far (Ed), “Mobile Computing Principles”, Cambridge University Press.