GSM & GPRS

By: Tamal Chakraborty

GSM Architecture

Mobile Station Provides access to the GSM network Communicates across Um interface (air interface)

with base station transceiver in same cell as mobile unit

Consists of a mobile equipment (ME) and a subscriber identity module (SIM)

GSM subscriber units are generic until SIM is inserted

SIMs roam, not necessarily the subscriber devices

Components of a GSM network

Base Station Subsystem (BSS) BSS consists of base station controller and one

or more base transceiver stations (BTS) Each BTS defines a single cell Includes radio antenna, radio transceiver and a

link to a base station controller (BSC) BSC reserves radio frequencies, manages

handoff of mobile unit from one cell to another within BSS, and controls paging

The BSC Communicates directly with MSC The BSS also includes a transcoder (XCDR),

used to convert the speech or data output from MSC into the form specified by GSM for transmission over the air-interface.

Components of a GSM network

Base Station Subsystem

Base Station Subsystem

Base Station Subsystem

Network Subsystem NS provides link between cellular network and

public switched telecommunications networks Controls handoffs between cells in different BSSs Authenticates users and validates accounts Enables worldwide roaming of mobile users Central element of NS is the mobile switching center

(MSC) and its associated system-control databases and processors together with the required interfaces.

Components of a GSM network

Home location register (HLR) database stores information about each subscriber that belongs to it

Visitor location register (VLR) database maintains information about subscribers currently physically in the region

Authentication center database (AuC) used for authentication activities, holds encryption keys

Equipment identity register database (EIR) keeps track of the type of equipment that exists at the mobile station

MSC Databases

MSC Databases

The Operation and maintenance Center (OMC) is the centralized maintenance and diagnostic heart of the base station system (BSS).

It allows the network provider to operate, administer and monitor the functioning of the BSS.

Multiple NMCs are managed by a Network Management Centre (NMC)

OMC & NMC

GSM Radio Interface

AIR INTERFACE

UPLI

NK

890 -

915 M

Hz

DOWNLI

NK 935

- 960 M

Hz

MOBILE

BASE TRANSCEIVER STATION

GSM Radio Channels

GSM uses paired radio channels

1 124 1 124

890MHz 915MHz 935MHz 960MHz

124 radio carriers, inter carrier spacing 200khz.

8 channels/carrier

890 to 915mhz mobile to base - UPLINK

935 to 960mhz base to mobile - DOWNLINK

FDMA, TDMA, CDMA

FDMA

Separation of the whole spectrum into smaller frequency bands A channel gets a certain band of the spectrum for the whole time Advantages: no dynamic coordination

necessary works also for analog signals

Disadvantages: waste of bandwidth

if the traffic is distributed unevenly

inflexible guard spaces

k2 k3 k4 k5 k6 k1

f

t

c

TDMA

f

t

c

k2 k3 k4 k5 k6 k1

A channel gets the whole spectrum for a certain amount of time

Advantages: only one carrier in the

medium at any time throughput high even

for many users Disadvantages: precise

synchronization necessary

Frequency-Time Multiplex

A channel gets a certain frequency band for a certain amount of time. Example: GSM

Advantages: Better protection against

tapping Protection against frequency

selective interference Higher data rates compared to

code multiplex But: precise coordination

required

f

t

c

k2 k3 k4 k5 k6 k1

GSM Multiplexing

GSM combines FDM and TDM: bandwidth is subdivided into channels of 200kHz, shared by up to eight stations, assigning slots for transmission on demand.

GSM Frame Format

TRAFFIC CHANNELS SIGNALLING CHANNELS

Trail bits allow synchronization of transmissions from mobile units

Encrypted bits encrypted data

Stealing bit - indicates whether block contains data or is "stolen"

Training sequence used to adapt parameters of receiver to the current path propagation characteristics

Strongest signal selected in case of multipath propagation

Guard bits used to avoid overlapping with other bursts

GSM TDMA Frame

Capacity & Spectrum

The need: Optimum spectrum

usage More capacity High quality of

service Low cost

I wish I could increase capacity without adding NEW BTS!

What can I do?

Network capacity at required QoS with conventional frequency plan

Subscriber growth

Time

Out of

Capacity!!!

Cell size determines number of cells available to cover geographic area and (with frequency reuse) the total capacity available to all users

Capacity within cell limited by available bandwidth and operational requirements

Each network operator has to size cells to handle expected traffic demand

Cell Size & Capacity

Implements space division multiplex: base station covers a certain transmission area (cell)

Mobile stations communicate only via the base station Advantages of cell structures:

higher capacity, higher number of users less transmission power needed more robust, decentralized base station deals with interference, transmission area etc. locally

Problems: fixed network needed for the base stations handover (changing from one cell to another) necessary interference with other cells

Cell structure

The problem with employing Omni-directional cells (Radiate waves to 360 degrees) is that as the number of MSs increases in the same geographical region, we have to increase the number of cells to meet the demand. To gain a further increase in capacity within the geographic

area we can employ a technique called sectorization. Sectorization splits a single site into a number of cells, each cell has transmit and receive antennas and behaves as an independent cell. This has a number of advantages: firstly, as we are now concentrating all the energy from the cell in a smaller area 60, 120, 180 degrees instead of 360 degrees, we get a much stronger signal, which is beneficial in locations

such as in-building coverage. Secondly, we can now use the same frequencies in a much closer re-use pattern, thus allowing more cells in our geographic region which allows us to support more MSs.

Cell-Sectoring

Cell-Sectoring

Split a bigger cell into number of smaller cells Decrease transmission power in base and mobile Results in more and smaller cells Reuse frequencies in non-contiguous cell groups Example: cell radius leads 4 fold capacity increase

Cell-Splitting

Cell-Splitting

Highway

Town Suburb

Rural

Cell Distribution in a Network

BTS BSC MSC

Um Abis A

GSM Signaling Protocol Architecture

GPRS Network Architecture

GBS

GBS consists of: Serving GPRS Support Node (SGSN) Gateway GPRS Support Node GGSN)

GPRS Benefits

Optimal support for packet switched traffic. The operator can join the Internet boom with true IP connectivity

The possibility to offer new, innovative services. New user segments such as telemetry of electric meters will become accessible to the operator

The ability to profit with idle capacity that would otherwise be used only to cover peak-hour traffic. Many users can use one time-slot simultaneously

It is economical to the user as it supports multiple users on the same channel(s)

Thank You!!