Wireless Data Communication Networks Lecture 19: Introduction to GSM

1. Wireless Data Communication Networks Lecture 19: Introduction to GSM

2. Outline • Brief history • GSM network architecture • Elements of the air interface organization

3. Global System for Mobile Communication • Most popular cellular standard • Started as Groupe Special Mobile • 2nd generation global digital standard • Important time stamps: • 1982 CEPT created Groupe Special Mobile • 1987 MoU – 13 European countries • 1989 Transferred to ETSI • 1991 Publishing of Phase 1 specs • 1991 Radiolinia Finland – first GSM network • 1993 Telstra Australia – first non-European GSM network • 1995 US implements GSM in PCS band • 1996 Russia and China implement GSM • Since 1996 tremendous growth all over the world Official GSM logo

4. GSM in the World GSM currently holds about 80% of the market share worldwide

5. GSM Network Layout GSM architecture is first standardized cellular system architecture • GSM network can be divided into three main subsystems: • Base Station Subsystem - BSS • Mobile Station Subsystem - MSS • Network Switching Subsystem - NSS

6. GSM Network Components • Network has many functional components • Components are integrated through a network protocol – MAP • Majority of GSM interfaces are standardized – allows interworking of equipment from different vendors • Major equipment vendors: Ericsson, Nokia-Siemens, Alcatel-Lucent, Huawei, Motorola, etc.

7. Overview of GSM - Mobile Station • Offered as a phone for voice services • Data services will bring new devices to the market • Two functional parts: • HW/SW radio interface • SIM GSM Mobile Architectural Diagram • Two types of SIM (Subscriber Identity Module) • Smart Card • Plug-in

8. Overview of GSM - Base Transceiver Station • BTS is a set of transceivers (TX/RX). • GSM BTS can host up to 16 TX/RX. • In GSM one TX/RX is shared by 8 users. • The main role of TX/RX is to provide conversion between traffic data on the network side and RF communication on the MS side. • Depending on the application, it can be configured as megacell, macrocell, microcell, pico and femto cell, omni, sectored, etc. Typical BTS installations

9. Overview of GSM - Base Station Controller • Provides a small digital exchange with some mobility tasks • Connects to one or several BTS on the Abis Interface • Connects to the MSC on the A Interface • Designed to offload most of the radio link related processes from the MSC • Provides clock distribution to BTS • Communicates with the OMC • In GSM, BSC handles majority RRM functions • Handoff • Power control • Channel assignment

10. Overview of GSM - TRAU • If it resides on the MSC side, it provides substantial changes in the backhaul – 4 users over a single T-1/E-1 TDMA channel. • TRAU, BSC and BTSs form Base Station Subsystem (BSS) • TRAU is responsible for trans-coding the user data from 16Kb/sec to standard ISDN rates of 64Kb/sec. • It can physically reside on either BSC side or MSC side. Location of TRAU in GSM

11. Overview of GSM - MSC • One of the MSC has an added functionality for communication with public network – Gateway MSC (GMSC) • All calls from the “outside networks” are routed through GMSC • Responsible for connecting the mobile to the landline side • GSM MSC is commonly designed as a regular ISDN switch with some added functionality for mobility support • GSM Network can have more than one MSC GSM MSC and Gateway MSC

12. Introduction to GPRS Overview of GSM - HLR/AuC • Database for permanent or semi-permanent data associated with the user • Logically, there is only one HLR per network • Typical information stored in HLR: International Mobile Service Identification Number (IMSI), service subscription information, supplementary services, current location of the subscriber, etc. • HLR is usually implemented as an integral part of MSC • AUC is an integral part of HLR responsible for ciphering and encryption. • GSM specifies elaborate encryption schemes. • There are three levels of the encryption: • A5/1 – Used by countries in Europe and USA • A5/2 – Used by countries and the so called COCOM list • No encryption – used by all other countries

13. GSM frequency bands • 14 bands defined • Most dominant • GSM-900 (P-GSM+ E-GSM) • DCS-1800 • GSM-850 (US) • PCS-1900 (US) • It is common that a phone is capable of multiband operation • Dual band - 2 bands • Quad band - 4 bands

14. GSM – Frequency <-> ARFCN calculations ARFCN – Absolute Radio Frequency Channel Number 2 2 Example – US PCS allocation

15. Overview of GSM - Air Interface • GSM is a FDMA/TDMA based technology • Transmissions are discontinuous • Each user is assigned a timeslot • Each frequency is divided into eight timeslots • Each channel has a 200 kHz bandwidth • Overhead signaling is required for coordination and control • Information is sent in bursts • Several types of bursts GSM as a FDMA/TDMA Interface

16. Overview of GSM - Burst Types • Used to carry information on both control and traffic channels • Mixture of data and overhead • GSM defines 8 training sequences assigned in color code mode • Both on the forward and reverse link Normal Burst Synchronization Burst • Facilitates the synchronization of the MS to the network at the base band • Commonly referred to as S-burst • Only on the forward link • The same sync sequence is used in all GSM networks

17. Overview of GSM - Burst Types • Used when the MS is accessing the system • Shorter in length – burst collision avoidance • Extended synchronization sequence • Used only on the reverse link Access Burst • GSM mobiles use slotted ALOHA to access the system • In the case of collision – a hashing algorithm is provided Dummy Burst • Supports MAHO • Used to ensure constant power level of the broadcast control channel • Only on the forward link

18. Overview of GSM - Burst Types • Sometimes referred to as the F-burst • Provides mobile with precise reference to the frequency of the broadcast control channel • Inserting the F-bursts on the control channel produces spectral peak 67.7 KHz above the central frequency of the carrier • Only on the forward link • Spectral characteristics of the control channel. • The peak in the spectrum allows for easier MS network acquisition Frequency synchronization burst

19. Overview of GSM - Physical Channels • A GSM physical channel can carry several different types of logical channels • Can be divided into two categories: traffic and signaling • Signaling channels can be further categorized as: • Broadcast • Common Control • Dedicated Control

20. Overview of GSM - Frame Hierarchy • Different organization on the superframe level for different logical channels

21. GSM Time Division Duplex • Communication on the forward and reverse link does not happen simultaneously • Delay of three slots between TX and RX • Time division duplexing avoids RF duplexer at the RF stage • Reduces the cost of mobile • Saves battery

22. Review questions • What are fourmost prevalent frequency bands for GSM deployment? • Explain GSM FDMA/TDMA access? • How many time slots are on one GSM ARFCN? • What is an ARFCN? • Name 2 basic channel types in GSM? • Name five burst types used in GSM? • What are the elements of GSM system? • What is a BTS? • What is a BSC? • What is a MSC? • What is a HLR? • What is a VLR? • What is TRAU? • What is a SIM card? • What is the Abis?

23. Review questions • Draw GSM TDMA hierarchy for traffic channel. • Draw GSM TDMA hierarchy for control channel. • What is the bandwidth of a GSM channel? • What is the difference between DCS-1800 and PCS-1800? • What is a quad-band phone? • Do GSM mobiles use RF duplexers? Please explain.