GSM

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GSM

Global System for Mobile

Communications

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Background & History

•

The development of GSM started in early

1980’s for Europe's Mobile infrastructure.

•

The first was to establish a team with the

title

"Group Special Mobile"

(hence the

term "GSM", which today stands for Global

System for Mobile Communications) to

develop a set of common standards.

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Cellular Mobile Systems

•

1986 - A decision was reached to

implement a digital transmission system.

•

Three Phases of the GSM Standard

GSM Phase1 Completed in 1990 GSM Phase2 Completed in 1994 GSM Phase2+ Being Standardized

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Frequency Bands and Bandwidth

• Uplink 815 – 900 MHz 25 MHz

• Downlink 935 – 960 MHz 25 MHz

• A 200 KHz carrier spacing has been chosen. Excluding 2x100 KHz edges of the band, this gives 124 possible

carriers for the uplink and downlink. The use of carrier 1 and 124 are optional for operators.

100KHz 200 KHz 100KHz

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Second Generation Mobile Network Architecture PSTN / ISDN • • • • • • • • • • MSC MSC BSS BSS • • • BSS • • • BSS

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GSM Subsystems

GSM

Base Station Subsystem

BSS

Network and Switching Subsystem

NSS

Operational Subsystem

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BSS (Base Station Subsystem)

• All radio related functions are performed in the BSS.

BSS

Base Transceiver Stations BTS Base Station Controllers BSC

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Base Transceiver Stations (BTS)

• The BTS handles the radio interface to the mobile

station. The BTS is the radio equipment needed to service each cell in the network. A

group of BTSs are controlled by a

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Base Station Controllers (BSC)

• The BSC provides all the control functions and

physical links between the MSC and BTS. It is a high-capacity switch that provides functions such as handover, cell configuration data, and control of radio frequency (RF) power levels in base transceiver stations. A number of BSCs are served by an MSC.

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Network and Switching Subsystem

• Performs main switching

functions of GSM.

• The switching system (SS)

is responsible for

performing call processing and subscriber-related

functions. The switching system includes the many functional units.

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Mobile Service Switching Center (MSC)

• Within NSS, MSC performs the necessary

switching functions required for the MSs located in an MSC area & to carry out Handover functions

• The MSC is also involved in the internetworking

functions to communicate with other networks such as PSTN and ISDN.

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Home Location Register (HLR)

• A database used for storage and management of

subscriptions.

• The HLR is considered the most important database, as it stores permanent data about subscribers, including a

subscriber's service profile, location information, and activity status.

• Any administrative action by the service provider on subscriber data is also performed in the HLR.

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Visitor Location Register (VLR)

• The VLR is connected to one or more MSCs.

• The VLR is the functional unit that dynamically

stores subscriber information when it is located in the area covered by VLR.

• When a roaming MS enters an MSC area, the

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Operation & Maintenance Subsystem

OMSS

• The OMSS is responsible for handling system

security based on validation of identities of various telecommunication entities.

• The functions are performed in Authentication Center (AuC) & Equipment Identity Register

(EIR).

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Channel Types

• Traffic channels (TCHs)

The traffic channels are intended to carry encoded speech or user data.

• Control Channels (CCHs)

The control channels are intended to carry signalling and Synchronization data between the base station and the Mobile station.

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Logical Control Channels

Downlink Uplink

-Common Control Channels • Random Access - RACH

Dedicated Control Channels

• Stand – alone Dedicated - SDCCH • Slow Associated – SACCH

• Fast Associated - FACCH Broadcast Control Channels

• Frequency Correction - FCCH • Synchronization – SCH

• Broadcast - BCCH

Common Control Channels • Access Grant - AGCH • Paging - PCH

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Downlink Only Channels

• Broadcast Control Channels (BCCH) – Broadcasts to all

mobiles general information regarding their own cell as well as the neighbouring (up to 16) cells, e.g. information used for cell selection and for describing the current control channel structure.

• Frequency Correction Channels (FCCH) – for mobiles for frequency correction.

• Synchronization Channels (SCH) – for frame synchronization of mobiles and identification of the base station.

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Common Control Channels

• Access Grant Channels (AGCH) for assignment of a dedicated Channel after a successful random access.

•Paging Channels (PCH) – for paging to mobiles.

For downlink

• Random Access Channels (RACH) – used for random access Attempts by mobiles.

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One full rate channel (multiframe)

0 1 2 3 4 5 6 7

1 TDMA frame = 8 timeslots (4.615 ms)

SACCH

TC₀ TC₁ TC₁₀ TC₁₁ TC₁₂ TC₁₃ TC₂₃ Idle

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Power Control

• RF power control will be used in the GSM MS and BS to

reduce the transmit power to the minimum required to achieve the minimum quality objective and hence reduce the level of

co-channel interference

•The MS will be capable of varying its transmit power form its maximum output down to 20 mW in steps of nominally 2 dB

•The BS calculates the RF Power level to be used by the MS and sends a 4 bit number instruction to the corresponding MS

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HANDOVER

• A GSM mobile is only active, i.e., either transmitting

or receiving, in 2 or the 8 timeslots in one frame.

• The MS scans transmissions from surrounding BSs in the spare timeslots. It then reports the measured results, together with those for the serving BS, back to the fixed network via the BS, where the handover decision is

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Inter BSS Handover

MS BSS (serving) BSS (target) MSC (1)

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SACCH

(Periodic measurement report)

(Handover needed) (Handover request) (Handover request acknowledgement) (Handover commend) (Handover completion) (Clear command) SACCH FACCH FACCH

(Exchange cell ID, timing advance, power control, information etc.)

(Change to the new channel) (It includes the HO reference and the assigned physical channel)

(Periodic measurements reports)

(It includes a HO reference and an assigned physical channel)

(It includes TMSI)

(2) (9) (8) (7) (6) (5) (4) (3)

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Future Outlook

• With the success of GSM and to meet the

demanding requirements of the subscribers,

GPRS, HSCSD and EDGE has been introduced which offer high data rates for the transmission.

• 3rd Generation (3G) systems will soon be

introduced all over the world and attract large number of subscribers.

• UMTS promises to provide voice, data and

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