Basic Wireless GSM Fundamentals

Basic Wireless GSM Fundamentals

Understanding GSM.

INTRODUCTION:-The Global System for Mobile Communications (GSM) is a set of recommendations and specifications for a digital cellular telephone network (known as a Public Land Mobile Network, or PLMN).


These recommendations ensure the compatibility of  equipment from different GSM manufacturers, and interconnectivity between different administrations, including operation across international boundaries. GSM networks are digital and can cater for high system capacities.They are consistent with the world-wide digitization of the telephone network, and are an extension of the Integrated Services Digital Network (ISDN), using a digital radio interface between the cellular network and the mobile subscriber equipment.                                                CELLULAR TELEPHONY:-A cellular telephone system links mobile subscribers into the public telephone system or to another cellular subscriber.Information between the mobile unit and the cellular network uses radio communication. Hence the subscriber is able to move around and become fully mobile.The service area in which mobile communication is to be provided is divided into regions called cells.Each cell has the equipment to transmit and receive calls from any subscriber located within the borders of its radio coverage area.

GSM 900 FREQUENCIES:-

GSM systems use radio frequencies between 890-915 MHz for receive and between 935-960 MHz or transmit.RF carriers are spaced every 200 kHz, allowing a total of 124 carriers for use.An RF carrier is a pair of radio frequencies, one used in each direction.Transmit and receive frequencies are always separated by 45 MHz.

GSM 1800 FREQUENCIES:-

GSM systems use radio frequencies between 890-915 MHz for receive and between 935-960 MHz for transmit.RF carriers are spaced every 200 kHz, allowing a total of 124 carriers for use.An RF carrier is a pair of radio frequencies, one used in each direction.Transmit and receive frequencies are always separated by 45 MHz

GSM Network Diagram

Components:-A cellular telephone system links mobile station (MS) subscribers into the public telephone system or to another cellular system’s MS subscriber.Information sent between the MS subscriber and the cellular network uses radio communication.   This removes the necessity for the fixed wiring used in a traditional telephone installation.Due to this, the MS subscriber is able to move around and become fully mobile, perhaps travelling in a vehicle or on foot.

Mobile Switching Centre (MSC):-The Mobile services Switching Centre (MSC) co-ordinates the setting up of calls to and from GSM users. It is the telephone switching office for MS originated or terminated traffic and provides the appropriate bearer services, teleservices and supplementary services.It controls a number of Base Station Sites (BSSs) within a specified geographical coverage area and gives the radio subsystem access to the subscriber and equipment databases.The MSC carries out several different functions depending on its position in the network.When the MSC provides the interface between PSTN and the BSS in the GSM network it is called the Gateway MSC.Some important  functions carried out by MSC are Call processing  including control of data/voice call setup, inter BSS & inter MSC handovers, control of mobility management, Operation & maintenance support including database management, traffic metering and man machine interface & managing the interface between GSM & PSTN.

            Mobile Equipment:-The Mobile Equipment is the hardware used by the subscriber to access the network.The mobile equipment can be Vehicle mounted, with the antenna physically mounted on the outside of the vehicle or portable mobile unit, which can be handheld. Mobiles are classified into five classes according to their power rating.

10W , 8W , 5W , 2W , 0.8W

Equipment Identity Register ( EIR ):-The Equipment Identity Register (EIR) contains a centralized database for validating the international mobile station equipment identity.The IMEI. The database contains three lists:

–The white list contains the number series of equipment identities that have been allocated in the different participating countries. This list does not contain individual numbers but but a range of numbers by identifying the beginning and end of the series.

–The grey list contains IMEIs of equipment to be monitored and observed for location and correct function.

–The black list contains IMEIs of MSs which have been reported stolen or are to be denied service.

The EIR database is remotely accessed by the MSC’s in the Network and can also be accessed by MSC in a different PLMN.

HOME LOCATION REGISTER( HLR ):-The HLR contains the master database of all subscribers in the PLMN.This data is remotely accessed by the MSC´´s and VLRs in the network. The data can also be accessed by an MSC or a VLR in a different PLMN to allow inter-system and inter-country roaming.A PLMN may contain more than one HLR, in which case each HLR contains a portion of the total subscriber database. There is only one database record per subscriber.The subscribers data may be accessed by the IMSI or the MSISDN.

The parameters stored in HLR are

–Subscribers ID (IMSI and MSISDN )

–Current subscriber VLR.

–Supplementary services subscribed to.

–Supplementary services information (eg. Current forwarding address ).

–Authentication key and AUC functionality.

–TMSI and MSRN

VISITOR LOCATION REGISTER ( VLR ):The Visited Location Register (VLR) is a local subscriber database, holding details on those subscribers who enter the area of the network that it covers.The details are held in the VLR until the subscriber moves into the area serviced by another VLR.The data includes most of the information stored at the HLR, as well as more precise location and status information. The additional data stored in VLR are

–Mobile status ( Busy / Free / No answer etc. )

–Location Area Identity ( LAI )

–Temporary Mobile Subscribers Identity ( TMSI )

–Mobile Station Roaming Number ( MSRN )

The VLR provides the system elements local to the subscriber, with basic information on that subscriber, thus removing the need to access the HLR every time subscriber information is required

Base Station Sub-System ( BSS ) :The BSS is the fixed end of the radio interface that provides control and radio coverage functions for one or more cells and their associated MSs.It is the interface between the MS and the MSC.The BSS comprises one or more Base Transceiver Stations (BTSs), each containing the radio components that communicate with MSs in a given area, and a Base Site Controller (BSC) which supports call processing functions and the interfaces to the MSC. Digital radio techniques are used for the radio communications link, known as the Air Interface, between the BSS and the MS. The BSS consists of three basic Network Elements (NEs).

–Transcoder (XCDR) or Remote transcoder (RXCDR) .

–Base Station Controller (BSC).

–Base Transceiver Stations (BTSs) assigned to the BSC.

Base Station Controller (BSC):The BSC network element provides the control for the BSS.It controls and manages the associated BTSs, and interfaces with the Operations and Maintenance Centre (OMC). The purpose of the BSC is to perform a variety of functions. The following comprise the functions provided by the BSC:

–Controls the BTS components.-

–Performs Call Processing.

–Performs Operations and Maintenance (O & M).

–Provides the O & M link (OML) between the BSS and the OMC.

–Provides the A Interface between the BSS and the MSC.

–Manages the radio channels.

–Transfers signalling information to and from MSs.

Base Transceiver Station (BTS):The BTS network element consists of the hardware components, such as radios, interface modules and antenna systems that provide the Air Interface between the BSS and the MSs.The BTS provides radio channels (RF carriers) for a specific RF coverage area.The radio channel is the communication link between the MSs within an RF coverage area and the BSS.The BTS also has a limited amount of control functionality which reduces the amount of traffic between the BTS and BSC

Operation And Maintenance Centre For Radio OMCR:The OMC controls and monitors the Network elements within a region.The OMC also monitors the quality of service being provided by the Network.The following are the main functions performed by the OMC-R
The OMC allows network devices to be manually removed for or restored to service. The status of network devices can be checked from the OMC and tests and diagnostics invoked.
–The alarms generated by the Network elements are reported and logged at the OMC. The OMC-R Engineer can monitor and analyze these alarms and take appropriate action like informing the maintenance personal.

–The OMC keeps on collecting and accumulating traffic statistics from the network elements for analysis.–Software loads can be downloaded to network elements or uploaded to the OMC.

 Transcoder( XCDR ):The speech transcoder is the interface between the 64 kbit/s PCM channel in the land network and the 13 kbit/s vocoder (actually 22.8 kbit/s after channel coding) channel used on the Air Interface.This reduces the amount of information carried on the Air Interface and hence, its bandwidth.If the 64 kbits/s PCM is transmitted on the air interface without occupation, it would occupy an excessive amount of radio bandwidth. This would use the available radio spectrum inefficiently.The required bandwidth is therefore reduced by processing the 64 kbits/s PCM data so that the amount of information required to transmit digitized voice falls to 13kb/s.The XCDR can multiplex 4 traffic channels into a single 64 kbit/s timeslot. Thus a E1/T1 serial link can carry 4 times as many channels.This can reduce the number of E1/T1 leased lines required to connect remotely located equipment.When the transcoder is between the MSC and the BSC it is called a remote transcoder (RXCDR).

Bss & Transcoder Connection

Cell Characteristics:The Basic Union In The System defined as the area where radio coverage is given by one base station.Addressed by Cell Global Identity (CGI).A cell has one or several frequencies, depending on traffic load.Frequencies are reused, but not used in neighboring cells due to interference.

Cell Planning:Reusing frequencies in different geographic areas.7 cell repeat pattern is typical, but other patterns are also common
Cell Patterns:Frequency Utilization v.s. K

•        Frequency can’t be reused in the same pattern
•        Larger K
o   More channels in a pattern
o   Higher subscriber number
o   Worse frequency reusability
           "Smaller K"
o   Less channels in a pattern, high channel distortion
o   Smaller subscriber number
o   Better frequency reusability

GSM NETWORK INTERFACES:-

The network structure is defined within the GSM standards. Additionally each interface

between the different elements of the GSM network is also defined. This facilitates the
information interchanges can take place. It also enables to a large degree that network
elements from different manufacturers can be used. However, as many of these interfaces were not
fully defined until after many networks had been deployed.
1. Um interface The "air" or radio interface standard that is used for exchanges
between a mobile (ME) and a base station (BTS / BSC). For signaling, a modified
version of the ISDN LAPD, known as LAPDm is used.
2. Abis interface This is a BSS internal interface linking the BSC and a BTS, and it
has not been totally standardized. The Abis interface allows control of the radio
equipment and radio frequency allocation in the BTS.
3. A interface The A interface is used to provide communication between the BSS
and the MSC. The interface carries information to enable the channels, timeslots
and the like to be allocated to the mobile equipments being serviced by the BSSs.
The messaging required within the network to enable handover etc to be
undertaken is carried over the interface.
4. B interface The B interface exists between the MSC and the VLR . It uses a
protocol known as the MAP/B protocol. As most VLRs are collocated with an
MSC, this makes the interface purely an "internal" interface. The interface is used
whenever the MSC needs access to data regarding a MS located in its area.
5. C interface The C interface is located between the HLR and a GMSC or a SMSG.
When a call originates from outside the network, i.e. from the PSTN or another
mobile network it ahs to pass through the gateway so that routing information
required to complete the call may be gained. The protocol used for
communication is MAP/C, the letter "C" indicating that the protocol is used for
the "C" interface. In addition to this, the MSC may optionally forward billing
information to the HLR after the call is completed and cleared down.
6. D interface The D interface is situated between the VLR and HLR. It uses the
MAP/D protocol to exchange the data related to the location of the ME and to the
management of the subscriber.
7. E interface The E interface provides communication between two MSCs. The E
interface exchanges data related to handover between the anchor and relay MSCs
using the MAP/E protocol.
8. F interface The F interface is used between an MSC and EIR. It uses the MAP/F
protocol. The communications along this interface are used to confirm the status
of the IMEI of the ME gaining access to the network
9. G interface The G interface interconnects two VLRs of different MSCs and uses
the MAP/G protocol to transfer subscriber information, during e.g. a location
update procedure.
10. H interface The H interface exists between the MSC the SMS-G. It transfers
short messages and uses the MAP/H protocol.