GSM uses narrowband Time Division Multiple Access (TDMA) for voice and Short Messaging Service (SMS).
What is GSM?
· GSM stands for Global System for Mobile Communication and is an open, digital cellular technology used for transmitting mobile voice and data services.
· The GSM emerged from the idea of cell-based mobile radio systems at Bell Laboratories in the early 1970s.
· The GSM is the name of a standardization group established in 1982 to create a common European mobile telephone standard.
· The GSM standard is the most widely accepted standard and is implemented globally.
· The GSM is a circuit-switched system that divides each 200kHz channel into eight 25kHz time-slots. GSM operates in the 900MHz and 1.8GHz bands in Europe and the 1.9GHz and 850MHz bands in the US.
· The GSM is owning a market share of more than 70 percent of the world's digital cellular subscribers.
· The GSM makes use of narrowband Time Division Multiple Access (TDMA) technique for transmitting signals.
· The GSM was developed using digital technology. It has an ability to carry 64 kbps to 120 Mbps of data rates.
· Presently GSM support more than one billion mobile subscribers in more than 210 countries throughout of the world.
· The GSM provides basic to advanced voice and data services including Roaming service. Roaming is the ability to use your GSM phone number in another GSM network. A GSM digitizes and compresses data, then sends it down through a channel with two other streams of user data, each in its own time slot. It operates at either the 900 MHz or 1,800 MHz frequency band.
Why GSM?
· Improved spectrum efficiency.
· International roaming.
· Low-cost mobile sets and base stations (BSs)
· High-quality speech
· Compatibility with Integrated Services Digital Network (ISDN) and other telephone company services.
· Support for new services.
GSM - Architecture
The GSM network can be divided into following broad parts.
· The Mobile Station(MS)
· The Base Station Subsystem (BSS)
· The Network Switching Subsystem (NSS)
· The Operation Support Subsystem(OSS)
The added components of the GSM architecture include the functions of the databases and messaging systems:
· Home Location Register (HLR)
· Visitor Location Register (VLR)
· Equipment Identity Register (EIR)
· Authentication Center (AuC)
· SMS Serving Center (SMS SC)
· Gateway MSC (GMSC)
· Chargeback Center (CBC)
· Transcoder and Adaptation Unit (TRAU)
GSM network areas:
· Cell: Cell is the basic service area: one BTS covers one cell. Each cell is given a Cell Global Identity (CGI), a number that uniquely identifies the cell.
· Location Area: A group of cells form a Location Area. This is the area that is paged when a subscriber gets an incoming call. Each Location Area is assigned a Location Area Identity (LAI). Each Location Area is served by one or more BSCs.
· MSC/VLR Service Area: The area covered by one MSC is called the MSC/VLR service area.
· PLMN: The area covered by one network operator is called PLMN. A PLMN can contain one or more MSCs.
What is SIM?
The SIM provides personal mobility so that the user can have access to all subscribed services irrespective of both the location of the terminal and the use of a specific terminal. You need to insert the SIM card into another GSM cellular phone to receive calls at that phone, make calls from that phone, or receive other subscribed services.
BSS is composed of two parts:
· The Base Transceiver Station (BTS)
· The Base Station Controller (BSC)
The BSS uses the Abis interface between the BTS and the BSC. A separate high-speed line (T1 or E1) is then connected from the BSS to the Mobile MSC.
BTS Functions:
· Encoding, encrypting, multiplexing, modulating, and feeding the RF signals to the antenna.
· Transcoding and rate adaptation
· Time and frequency synchronizing
· Voice through full- or half-rate services
· Decoding, decrypting, and equalizing received signals
· Random access detection
· Timing advances
· Uplink channel measurements
BSC Functions:
· Control of frequency hopping
· Performing traffic concentration to reduce the number of lines from the MSC
· Providing an interface to the Operations and Maintenance Center for the BSS
· Reallocation of frequencies among BTSs
· Time and frequency synchronization
· Power management
· Time-delay measurements of received signals from the MS
It assigns and releases frequencies and time slots for the MS. The BSC also handles intercell handover. It controls the power transmission of the BSS and MS in its area.
NSS performs the switching of calls between the mobile and other fixed or mobile network users, as well as the management of mobile services such as authentication.
Home Location Register (HLR)
The HLR is 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.
Mobile Services Switching Center (MSC)
The MSC performs the
switching of calls between the mobile and other fixed or mobile network users, as well as the management of mobile services such as such as registration, authentication, location updating, handovers, and call routing to a roaming subscriber.
Visitor Location Register (VLR)
The VLR is a database that contains temporary information about subscribers that is needed by the MSC in order to service visiting subscribers. When a mobile station roams into a new MSC area, the VLR connected to that MSC will request data about the mobile station from the HLR. Later, if the mobile station makes a call, the VLR will have the information needed for call setup without having to interrogate the HLR each time.
Authentication Center (AUC)
The Authentication Center is a protected database that stores a copy of the secret key stored in each subscriber's SIM card, which is used for authentication and ciphering of the radio channel.
Equipment Identity Register (EIR)
EIR is a database that contains a list of all valid mobile equipment on the network, where its International Mobile Equipment Identity (IMEI) identifies each MS.
The Operation Support Subsystem(OSS)
The operations and maintenance center (OMC) is connected to all equipment in the switching system and to the BSC. The implementation of OMC is called the operation and support system (OSS).
Here are some of the OMC functions:
· Administration and commercial operation (subscription, end terminals, charging and statistics).
· Security Management.
· Network configuration, Operation and Performance Management.
· Maintenance Tasks.
GSM Specifications
- GSM uses Gaussian Minimum Shift Keying (GMSK) modulation method.
- GSM chose a combination of TDMA/FDMA as its method. The FDMA part involves the division by frequency of the total 25 MHz bandwidth into 124 carrier frequencies of 200 kHz bandwidth. One or more carrier frequencies are then assigned to each BS. Each of these carrier frequencies is then divided in time, using a TDMA scheme, into eight time slots. One time slot is used for transmission by the mobile and one for reception.
- Transmission Rate: 22.8 Kbps and over-the-air bit rate of 270 kbps.
- Frequency Band: uplink frequency range is 933 - 960 MHz and downlink frequency band 890 - 915 MHz.
- Channel Spacing: 200kHz. This indicates separation between adjacent carrier frequencies.
- Speech Coding: GSM uses linear predictive coding (LPC). The purpose of LPC is to reduce the bit rate.
- Duplex Distance: 80 MHz. Duplex distance is the distance between the uplink and downlink frequencies.
International Mobile Station Equipment Identity (IMEI):
It uniquely characterizes a mobile station and gives clues about the manufacturer and the date of manufacturing. IMEI = TAC + FAC + SNR + SP.
· Type Approval Code (TAC): 6 decimal places, centrally assigned.
· Final Assembly Code (FAC): 6 decimal places, assigned by the manufacturer.
· Serial Number (SNR): 6 decimal places, assigned by the manufacturer.
· Spare (SP): 1 decimal place.
International Mobile Subscriber Identity ( IMSI):
Each registered user is uniquely identified by its international mobile subscriber identity (IMSI). It is stored in the subscriber identity module (SIM) A mobile station can only be operated if a SIM with a valid IMSI is inserted into equipment with a valid IMEI.
There are following parts of an IMSI:
· Mobile Country Code (MCC): 3 decimal places, internationally standardized.
· Mobile Network Code (MNC): 2 decimal places, for unique identification of mobile network within the country.
· Mobile Subscriber Identification Number (MSIN): Maximum 10 decimal places, identification number of the subscriber in the home mobile network.
Mobile Subscriber ISDN Number ( MSISDN):
real telephone number of a mobile station. MSISDN Structure:
structure.
· Country Code (CC) : Up to 3 decimal places.
· National Destination Code (NDC): Typically 2-3 decimal places.
· Subscriber Number (SN): Maximum 10 decimal places.
Mobile Station Roaming Number ( MSRN):
a temporary location dependent ISDN number. It is assigned by the locally responsible VLR to each mobile station in its area. Calls are also routed to the MS by using the MSRN.
The MSRN has same structure as the MSISDN.
· Country Code (CC) : of the visited network.
· National Destination Code (NDC): of the visited network.
· Subscriber Number (SN): in the current mobile network.
Temporary Mobile Subscriber Identity (TMSI):
The VLR, which is responsible for the current location of a subscriber, can assign a temporary mobile subscriber identity (TMSI) which has only local significance in the area handled by the VLR. It is stored on the network side only in the VLR and is not passed to the HLR.
Local Mobile Subscriber Identity (LMSI):
an additional searching key to each mobile station within its area to accelerate database access. The LMSI is assigned when the mobile station registers with the VLR and is also sent to the HLR.
Cell Identifier (CI):
Within an LA, the individual cells are uniquely identified with a cell identifier (CI). Together with the global cell identity (LAI + CI) calls are thus also internationally defined in a unique way.
GSM - Operations
Call from Mobile Phone to PSTN:
1. The MSC/VLR receives the message of a call request.
2. The MSC/VLR checks if the mobile station is authorized to access the network. If
so, the mobile station is activated. If the mobile station is not authorized, service
will be denied.
3. MSC/VLR analyzes the number and initiates a call setup with the PSTN.
4. MSC/VLR asks the corresponding BSC to allocate a traffic channel (a radio
channel and a time slot).
5. The BSC allocates the traffic channel and passes the information to the mobile
station.
6. The called party answers the call and the conversation takes place.
7. The mobile station keeps on taking measurements of the radio channels in the
present cell and neighboring cells and passes the information to the BSC. The
BSC decides if handover is required, if so, a new traffic channel is allocated to the
mobile station and the handover is performed. If handover is not required, the
mobile station continues to transmit in the same frequency.
Call from PSTN to Mobile Phone:
1. The Gateway MSC receives the call and queries the HLR for the information
needed to route the call to the serving MSC/VLR.
2. The GMSC routes the call to the MSC/VLR.
3. The MSC checks the VLR for the location area of the MS.
4. The MSC contacts the MS via the BSC through a broadcast message, that is,
through a paging request.
5. The MS responds to the page request.
6. The BSC allocates a traffic channel and sends a message to the MS to tune to the
channel. The MS generates a ringing signal and, after the subscriber answers, the
speech connection is established.
7. Handover, if required, takes place, as discussed in the earlier case.
The MS codes the speech at 13 Kbps for transmission over the radio channel in the given
time slot. The BSC converts (or transcodes) the speech to 64 Kbps and sends it over a
land link or radio link to the MSC. The MSC then forwards the speech data to the PSTN.
In the reverse direction, the speech is received at 64 Kbps rate at the BSC and the BSC
does the transcoding to 13 Kbps for radio transmission.
GSM - User Services
- Telephony Services
- Data Services.
- Supplementary Services
- Voice Calls
- Videotext and Facsmile
- Short Text Messages
Bearer Services or Data Services
GSM currently has a data transfer rate of 9.6k. New developments that will push up data transfer rates for GSM users are HSCSD (high speed circuit switched data) and GPRS (general packet radio service) are now available.
Supplementary Services
- Multiparty Service or conferencing
- Call Waiting
- Call Hold
- Call Forwarding
- Call Barring
- Number Identification
- Calling Line Identification Presentation
- Calling Line Identification Restriction
- Connected Line Identification Presentation
- Connected Line Identification Restriction
- Malicious Call Identification
- Advice of Charge (AoC)
- Closed User Groups (CUGs)
- Unstructured supplementary services data (USSD)
What is Next?
· High Speed Circuit Switched Data (HSCSD): by using several circuit channels.
· General Packet Radio Service (GPRS) to provide packet radio access to external packet data networks (such as X.25 or Internet)
· Enhanced Data rate for GSM Evolution (EDGE): using a new modulation scheme to provide up to three times higher throughput (for HSCSD and GPRS).
· Universal Mobile Telecommunication System (UMTS): a new wireless technology using new infrastructure deployment.
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