2G - Global System for Mobile Communications (GSM)

GSM

Defined by CEPT/ETSI.

Requirements in terms of:

• Services -> Portability, =PSTN.

• QoS -> =PSTN.

• Security -> Low cost cipher.

• RF Usage -> Efficiency.

• Network -> Numbering ITU-T, SS-7.

• Cost -> Low

Differences with the first Generation Systems

Digital traffic channels.

• first-generation systems are almost purely analog; second-generation systems are digital.

Encryption.

• all second generation systems provide encryption to prevent eavesdropping.

Error detection and correction.

• second-generation digital traffic allows for detection and correction, giving clear voice reception.

Channel access.

• second-generation systems allow channels to be dynamically shared by a number of users.

Basic Architecture

Defines cells

Defines a Mobile Terminal.

• Mobile Equipment + Subscriber Identity Module (etc...; e.g. International Mobile Station Equipment Identity (IMEI)).

Uses a Network Subsystem.

• MSC; HLR, VLR

Uses a Radio Subsystem.

• BSS; BTransceiverS, BSController

Defines an Operation Support Subsystem.

The Base Station Subsystem (BSS) is structured as Base Station Controllers (BSC) + Base Transceiver Station (BTS ).

BSCs are connected to the Mobile Switching Center (MSC) through physical lines.

MSCs are interconnected to each other.

There are MSCs connected to the public network (PSTN), the Gateway Mobile Switching Center (GMSC).

Mobile Switching Center

MSC = local switching center.

• Contains:

  • Home Location Register (HLR).

  • Visitor Location Register (VLR).

  • Autentication Center (AuC).

  • Equipment Identity Registry (EIR).

Connects the BSS (Base Station Subsystem).

• Master of the cell, define channels and access to them...

Contains the registers for “their” mobile terminals.

Specific signalling channels.

• MT-BS (MSC): location, call setup, received call answer.

• BS (MSC)-MT: cell identification, location update, received call setup.

Network Subsystem DBs

HLR - Home Location Register.

• maintains permanent information about the subscribers of a GSM network (subscriber record).

  • Subscription data: IMSI, MSISDN, subscription type (restrictions, supplementary services, ...).

• tracks the location and state of the mobile terminal within the network.

  • Location information: mobile VLR number.

VLR - Visitor Location Register.

• maintains temporary information about the subscribers registered on a GSM network (including subscribers in roaming).

  • Data: IMSI, MSISDN, TMSI, MSRN, subscription type, location area, ...

• keeps up-to-date information about the location of the user within the network.

AuC – Authentication Center

• service responsible for the authentication of the subscribers.

• maintains the encryption algorithms.

• maintains the secret key (ki) for each subscriber.

• generates the session keys.

EiR – Equipment Identity Register

• provides security mechanisms for the mobile equipments.

• keeps lists of mobile equipments.

  • white list (authorized).

  • gray list (under “observation”).

  • black list (blocked).

Mobile Station

Mobile Station (MS) communicates across Um interface (air interface) with base station transceiver in same cell as mobile unit.

Mobile equipment (ME) – physical terminal, such as a telephone.

• ME includes radio transceiver, digital signal processors and subscriber identity module (SIM).

GSM subscriber units are generic until SIM is inserted.

SIM: Suscriber Identity Module

Informations:

• subscriber identity, password (PIN), subscription information (authorized networks, call restrictions, ...), security algorithms, short numbers, last received/dialed numbers, last visited location area, ...

SIM card + GSM terminal = access to GSM services.

Air interface (Um) – channel allocation

GSM uses:

• FDD (Frequency Division Dupplexing) for dupplexing.

• TDMA (Time Division Multiple Access) with 8 time-slots for multiple access.

  • Three slots delay (up and down) → avoids simultaenous rx/tx.

• 200 kHz frequency channels (124 in GSM 900) for each cell, 124 channels per band ( => maximum 8 users per channel).

GSM Signalling and Transport Layers.

The Intelligent Network (IN).

• Uses Signalling System No. 7 (SS7).

• Charging options.

  • Freephone.

  • Local rates.

  • Premium rates.

• Routing options.

  • Group calls

    • Network call centre options.

    • Virtual Private Networks.

  • Personal calls

    • one number.

    • configurable routing.

Types of handover (GSM)

1. Intra-cell: from a channel to another within the same cell.

2. Inter-cell, Intra-BSC: from a channel in one cell to a channel in another cell, both controlled by the same BSC.

3. Inter-BSC, Intra-MSC: from a channel in one cell to a channel in another cell, controlled by different BSCs, under the same MSC control.

4. Inter-MSC: from a channel in one cell to a channel in another cell connected to different MSCs.

Short Message Service - SMS

Supports the transmission of messages up to 1601 characters, between mobile terminals.

Messages are transmitted through the signalling channels.

Is used for a variety of applications:

• text messages between users (very popular).

• broadcast of location-dependent information (e.g. local restaurants).

• access to computing applications (e.g. home banking and e-mail).

• configuration of mobile terminals over the air.