Global System for Mobile Communications

Global System for Mobile Communications
GSM (Global System for Mobile Communications, originally Groupe Spécial Mobile), is a standard set developed by the European Telecommunications Standards Institute (ETSI) to describe protocols for second generation (2G) digital cellular networks used by mobile phones
The GSM standard was developed as a replacement for first generation (1G) analog cellular networks, and originally described a digital, circuit switched network optimized for full duplex voice telephony. This was expanded over time to include data communications, first by circuit switched transport, then packet data transport via GPRS (General Packet Radio Services) and EDGE (Enhanced Data rates for GSM Evolution or EGPRS)
There are five different cell sizes in a GSM network
macro, micro, pico, femto and umbrella cells.
The coverage area of each cell varies according to the implementation environment.
Macro cells can be regarded as cells where the base station antenna is installed on a mast or a building above average roof top level.
Micro cells are cells whose antenna height is under average roof top level; they are typically used in urban areas.
Picocells are small cells whose coverage diameter is a few dozen metres; they are mainly used indoors
Femtocells are cells designed for use in residential or small business environments and connect to the service provider’s network via a broadband internet connection.
Umbrella cells are used to cover shadowed regions of smaller cells and fill in gaps in coverage between those cells.
GSM carrier frequencies
GSM networks operate in a number of different carrier frequency ranges (separated into GSM frequency ranges for 2G and UMTS frequency bands for 3G), with most 2G GSM networks operating in the 900 MHz or 1800 MHz bands.
Most 3G networks in Europe operate in the 2100 MHz frequency band

Base Station Subsystem (BSS)

Base Transceiver Station (BTS)

Radio interface control between BTS and MS
Transmission execution, channel encryption, diversity,
frequency hopping
Base Station Controller (BSC)
Handover control, channel assignments, collection of
cell configuration data etc
Base transceiver station
A base transceiver station (BTS) is a piece of equipment that facilitates wireless communication between user equipment (UE) and a network. UEs are devices like mobile phones (handsets), WLL phones, computers with wireless internet connectivity, WiFi and WiMAX devices and others. The network can be that of any of the wireless communication technologies like GSM, CDMA, Wireless local loop, WAN, WiFi, WiMAX, etc.
BTS is also referred to as the radio base station (RBS), node B (in 3G Networks) or, simply, the base station (BS).

A BTS in general has the following parts
Transceiver (TRX)
It basically does transmission and reception of signals. It also does sending and reception of signals to and from higher network entities (like the base station controller in mobile telephony).

Power amplifier (PA)
Amplifies the signal from DRX for transmission through antenna; may be integrated with DRX.<driver receiver>
Combiner
Combines feeds from several DRXs so that they could be sent out through a single antenna. Allows for a reduction in the number of antenna used.
Duplexer
For separating sending and receiving signals to/from antenna. Does sending and receiving signals through the same antenna ports (cables to antenna).
Antenna
This is the structure that lies underneath the BTS; it can be installed as it is or disguised in some way
Alarm extension system
Collects working status alarms of various units in the BTS and extends them to operations and maintenance (O&M) monitoring stations.
Control function
Controls and manages the various units of BTS, including any software. On-the-spot configurations, status changes, software upgrades, etc. are done through the control function.
Baseband receiver unit (BBxx)
Frequency hopping, signal DSP, etc.
Diversity techniques
To improve the quality of the received signal, often two receiving antennas are used, placed at an equal distance to an uneven multiple of a quarter of wavelength (for 900 MHz the wavelength it is 30 cm). This technique, known as antenna diversity or space diversity, avoids interruption caused by path fading.
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
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.

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.


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
MSC informs the associated VLR about the MS.


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).
• One OMC can serve several MSCs.



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) 

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


Synchronization Channels (SCH)

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