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GSM and GPRS details
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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!!