1. 1 Lecture 20 EEE 441 Wireless And Mobile Communications

2. 2 GSM Network Overview

3. 3 GSM Interfaces U m Interface  Mobile Station and Base Station Subsystem communicate across Um interface, also known as air interface or radio link A bis interface  Base Transceiver Station (BTS) and Base Station Controller (BSC) communicate across Abis interface A interface  Base Station Subsystem communicates with Mobile Switching Center across A interface

4. 4 BSS (Base Station Subsystem) BSC BTS HLRVLREIRAuC MSC PSTN Um Abis A SS7 TRAU

5. 5 Mobile Switching Center (MSC) Interface between radio system and fixed networks (PSTN and ISDN) Connected to BSS through A interface; usually an E-1, maybe wired or microwave

6. 6 GSM Radio Transmission Combination of FDMA and TDMA Frequencies: 800, 900, 1800, 1900 MHz For example, GSM 900:  Uplink = 890-915 MHz  Downlink = 935-960 MHz Each 25 MHz bandwidth is divided into 124 carrier frequencies spaced 200 KHz with one or more carriers allocated to each base station Each 200kHz then rotated among 8 users in TDMA fashion

7. 7 GSM Channel Types Traffic Channels (TCHs)  TCHs carry digitally encoded user speech or user data;  uses both the forward and reverse links Control Channels (CCHs)  CCHs carry signaling and synchronizing commands between the base station and the mobile station

8. 8 Traffic Channels (TCH) Types of TCH  Full-rate (TCH/F)  Half-rate (TCH/H): double capacity, but half the quality  Eighth-rate or Stand-alone Dedicated Control Channels (SDCCH) 26 TDMA frames  24 traffic channels  1 slow associated control channel (SACCH)  1 unused channel  13 th or 26 frame can SACCH or the idle channel.

9. 9 Control Channels (CCH) Control Channels accessed by:  idle mode mobiles to exchange signaling information required to change to dedicated mode  dedicated mode mobiles to monitor surrounding base stations for handover and other information Broadcast Control Channel (BCCH), Common Control Channel (CCCH), Dedicated Control Channel (DCCH) BCH and CCCH are forward control channels in GSM

10. 10 Control Channels (cont’d) The BCCH also broadcasts a list of channels that are currently in use within the cell. Frequency Correction Channel (FCCH)  The FCCH allows each subscriber unit synchronize its internal frequency standard to the exact frequency of the base channel. Synchronization Channel (SCH)  Each mobile to frame synchronize with the base station. (Use TS0 just after FCCH.)

11. 11 Control Channels (cont’d) Paging Channel (PCH)  Alerts mobile to incoming call (base station to all the mobile in the cell) Random Access Channel (RACH)  Used by mobile to request access to GSM network Access Grant Channel (AGCH)  Allocates an SDCCH to mobile for signaling following a request on the RACH

12. 12 Call Routing

13. 13 GSM Timing A very elaborate timing structure ranging from 1/4 of a bit (900ns) to an encryption hyperframe (3 hours 28 minutes and 53.76s)!

14. 14 GSM Framing

15. 15 Incoming Call

16. 16 GSM Phase 2+ High-Speed Circuit Switched Data (HSCSD) General Packet Radio Service (GPRS) Enhanced Data Rates for GSM Evolution (EDGE)

17. 17 HSCSD Terminal Adaptation Function (TAF) Interworking Functions (IWF) enhanced RLP to handle multilink (aka multiple time slot) operation

18. 18 GPRS

19. 19 GSM/GPRS Architecture/Interfaces

20. 20 GSM Network Protocols

21. 21 GSM Signaling Protocols Layer 1 (physical layer)  Uses many channel structures Layer 2 (data link layer)  Across U m interface uses LAPDm (a slight modification of LAPD protocol used in ISDN)  Across Abis uses LAPD  Across A interface, uses MTP and SCCP of SS7 Layer 3 is sub-divided into 3 sub-layers

22. 22 Layer 3 Signaling Protocols RR: Radio Resources Management Establishment, maintenance, and termination of radio channels that allow point-to-point dialogue between network and mobile stations; including cell selection and handover procedures MM: Mobility Management Manages location updating, registration, security and authentication procedures CM: Connection Management Handles general call control, similar to CCITT Recommendation Q.931, and provides supplementary services & Short Message Services (SMS)

23. 23 Handovers (Handoffs) Four types of handovers:  Channels (time slots) in same cell  Between cells within same BSC  Between BSCs, within same MSC  Between MSCs

24. 24 Notices Read  Rappaport, Ch 11.3