Gsm Networks

GSM Netw or ks: T he Basi cs

Agenda Structure of a GSM network Architecture of a GSM network as a typical cellular network GSM functional entities GSM network interfaces GSM protocols

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GS M Ne tw or ks

GSM Design Goals Good subjective speech quality Low terminal and service cost Support for international roaming Ability to support handheld terminals Support for range of new services and facilities Spectral efficiency ISDN compatibility

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GSM Network Overview :Access Network Base Station Subsystem

:Core Network GSM CS network

HLR

BSC

BTS

Mobile

VLR

MSC

BTS

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Abis

AuC

SS7

Station

Um

EIR

A

PSTN

GSM Interfaces Um Interface Mobile station and base station subsystem communicate across Um interface, also known as air interface or radio link Abis interface Base transceiver station (BTS) and base station controller (BSC) communicate across Abis interface A interface Base station subsystem communicates with mobile service switching center across A interface Slide 6

GS M Functional En tit ies

GSM Network Architecture MS: Mobile Station BSS: Base Station Subsystem MSC: Mobile Switching Center O&M: Operations and Maintenance Center VLR, HLR, AuC, EiR …

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Mobile Station (MS) Mobile Equipment (ME) SIM: Subscriber Identity Module While subscriber roams or is stationary, the MS transmits a radio signal to one of the many BTS using a radio-link protocol via the Um interface

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BSS (Base Station Subsystem) The BSS is composed of two parts: BTS: Base Transceiver Station BSC: Base Station Controller BTS handles: RF interface

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BSS (Base Station Subsystem) BSC handles (through the Abis interface): Radio-channel setup Frequency hopping Handovers BSC also connects MS to MSC using A interface

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BSS (Base Station Subsystem) Sometimes a Transcoder Rate Unit (TRAU) is placed on BTS to perform transcoding between 64 Kbps A-law and 13 Kbps RPE/LTP speech channels HLR BTS

VLR

BSC

BTS

Abis

A TRAU

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AuC

SS7 MSC

Um

EIR

PSTN

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

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MSC (Mobile Switching Center) (2) Each MSC covers several cells (BSSs) BTS

BTS BTS

BTS

BTS BTS

BSC

BSC

MSC Slide 14

BTS

MSC (Mobile Switching Center) (3) Also performs signaling between MSC and other functional entities using SS7: Registration Authentication Location updating Handovers Call routing to a roaming subscriber

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Other GSM Network Entities HLR: Home Location Register VLR: Visitor Location Register EIR: Equipment Identity Register AuC: Authentication Center

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HLR (Home Location Register) Central database for all subscribers: Identity of the subscriber Services accessible to the subscriber Current location of the subscriber Given a Mobile Subscriber ISDN number (MS-ISDN), call is routed to IMSI number-VLR Each subscriber appears only once in database HLR might be physically distributed in several sites (e.g., using first two digits to identify physical HLR) Slide 17

VLR (Visitor Location Register) Database with information on MS within area served by MSC: MS Roaming number TMSI if applicable Location area in which was last registered Supplementary services Used by an MSC to retrieve information for various purposes: Handling of calls to or from a roaming mobile station currently located in its area Typically part of MSC

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Call Routing

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AuC (Authentication Center) Entity associated to HLR for authentication: allow International Mobile Subscriber Identity (IMSI) to be authenticated Allows ciphering of communication over radio path between mobile station and network ciphered Transmits data needed for authentication and ciphering via HLR to VLR, MSC and SGSN which need to authenticate a mobile station (SIM validation)

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EIR (Equipment Identity Register) Logical entity responsible for storing International Mobile Equipment Identities (IMEIs) in network used in GSM system Equipment classified as "white listed", "grey listed” and "black listed” Ensures that MEs being used are valid and authorized to function on the Public Land Mobile Network (PLMN)

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PLMN (Public Land Mobile Network) Functionally may be regarded as independent telecommunications entities A collection of MSC’s areas within a common numbering plan

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BTS

BTS BTS

BTS

BTS

BTS

BTS BTS

BSC

PSTN

BTS BTS

BSC

MSC

BTS

BTS BTS

BTS

BSC

BSC

MSC

GS M Pr otocols

GSM Radio Transmission Combination of FDMA and TDMA to send information 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 frequencies allocated to each base station

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GSM Framing

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TCH (Traffic Channels) Used to carry speech and data 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 1 unused channel Slide 26

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 51 TDMA frame format Broadcast Control Channel (BCCH) Broadcasts on the downlink information such as base station identity, frequency allocation, frequencyhopping sequences Slide 27

Control Channels (2) Frequency Correction Channel (FCCH) and Synchronization Channel (SCH) Synchronize mobile to time slot structure of cell Random Access Channel (RACH) Used by mobile to request access to GSM network Paging Channel (PCH) Alerts mobile to incoming call Access Grant Channel (AGCH) Allocates an SDCCH to mobile for signaling following a request on the RACH Slide 28

GSM Network Protocols

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GSM Signaling Protocols Layer 1 (physical layer) Uses many channel structures Layer 2 (data link layer) Across Um 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 SAPI=0 Identifies radio signaling procedures Layer 3 is sub-divided into 3 sub-layers Slide 30

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)

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Handover Four types of handovers: Channels (time slots) in same cell Between cells within same BSC Between BSCs, within same MSC Between MSCs BTS

BTS BTS

BTS

BTS

BTS BTS

BSC

BTS BTS

BTS

BSC

MSC

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BTS

BTS BTS

BTS

BSC

BSC

MSC

MAP (Mobile Application Part) SS7 top layer protocol Responsible for signaling between different entities in network, such as between HLR and VLR MAP specification is complex Several variants for the big manufacturers (Ericsson, Motorola …)

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Su mmar y

Summary GSM is the dominant cellular technology today GSM offers high quality voice with advanced services GSM is the stepping-stone to 3G networks

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