GSM - Quick Guide
GSM - Overview
What is GSM?
What is GSM?
If you are in Europe, Asia or Japan and using a mobile phone, then most probably you must be using GSM technology in your mobile phone.
- 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 supports more than one billion mobile subscribers in more than 210 countries throughout 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.
The GSM study group aimed to provide the followings through the 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 Brief History:
Following table shows many of the important events in the rollout of the GSM system; other events were introduced, but had less significant impact on the overall systems.
|1982||CEPT establishes a GSM group in order to develop the standards for a pan-European cellular mobile system.|
|1985||A list of recommendations to be generated by the group is accepted.|
|1986||Field tests are performed to test the different radio techniques proposed for the air interface.|
|1987||Time Division Multiple Access (TDMA) is chosen as the access method (with Frequency Division Multiple Access [FDMA]). The initial Memorandum of Understanding (MoU) is signed by telecommunication operators representing 12 countries.|
|1988||GSM system is validated.|
|1989||The responsibility of the GSM specifications is passed to the European Telecommunications Standards Institute (ETSI).|
|1990||Phase 1 of the GSM specifications is delivered.|
|1991||Commercial launch of the GSM service occurs. The DCS1800 specifications are finalized.|
|1992||The addition of the countries that signed the GSM Memorandum of Understanding takes place. Coverage spreads to larger cities and airports.|
|1993||Coverage of main roads' GSM services starts outside Europe.|
|1994||Data transmission capabilities launched. The number of networks rises to 69 in 43 countries by the end of 1994.|
|1995||Phase 2 of the GSM specifications occurs. Coverage is extended to rural areas.|
|1996||June: 133 network in 81 countries operational.|
|1997||July: 200 network in 109 countries operational, around 44 million subscribers worldwide.|
|1999||Wireless Application Protocol came into existence and 130 countries operational with 260 million subscribers.|
|2000||General Packet Radio Service(GPRS) came into existence.|
|2001||As of May 2001, over 550 million people were subscribers to mobile telecommunications.|
GSM - Architecture
A GSM network consists of several functional entities, whose functions and interfaces are defined. The GSM network can be divided into following broad parts.
Following is the simple architecture diagram of GSM Network:
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)
Following is the diagram of GSM Network along with added elements:
The MS and the BSS communicate across the Um interface, also known as the air interface or radio link. The BSS communicates with the Network Service Switching center across the A interface.
GSM network areas:
In a GSM network, the following areas are defined:
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.
GSM - Specification
Specifications for different Personal Communication Services (PCS) systems vary among the different PCS networks. The GSM specification is listed below with important characteristics.
Modulation is a form of change process where we change the input information into a suitable format for the transmission medium. We also changed the information by demodulating the signal at the receiving end.
The GSM uses Gaussian Minimum Shift Keying (GMSK) modulation method.
Because radio spectrum is a limited resource shared by all users, a method must be devised to divide up the bandwidth among as many users as possible.
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. They are separated in time so that the mobile unit does not receive and transmit at the same time.
The total symbol rate for GSM at 1 bit per symbol in GMSK produces 270.833 K symbols/second. The gross transmission rate of the time slot is 22.8 Kbps.
GSM is a digital system with an over-the-air bit rate of 270 kbps.
The uplink frequency range specified for GSM is 933 - 960 MHz (basic 900 MHz band only). The downlink frequency band 890 - 915 MHz (basic 900 MHz band only).
This indicates separation between adjacent carrier frequencies. In GSM, this is 200 kHz.
GSM uses linear predictive coding (LPC). The purpose of LPC is to reduce the bit rate. The LPC provides parameters for a filter that mimics the vocal tract. The signal passes through this filter, leaving behind a residual signal. Speech is encoded at 13 kbps.
The duplex distance is 80 MHz. Duplex distance is the distance between the uplink and downlink frequencies. A channel has two frequencies, 80 MHz apart.
Frame duration: 4.615 mS
Duplex Technique: Frequency Division Duplexing (FDD) access mode previously known as WCDMA.
Speech channels per RF channel: 8.
GSM - Addressing
GSM distinguishes explicitly between user and equipment and deals with them separately. Besides phone numbers and subscriber and equipment identifiers, several other identifiers have been defined; they are needed for the management of subscriber mobility and for addressing of all the remaining network elements. The most important addresses and identifiers are presented in the following:
International Mobile Station Equipment Identity (IMEI):
The international mobile station equipment identity (IMEI) uniquely identifies a mobile station internationally. It is a kind of serial number. The IMEI is allocated by the equipment manufacturer and registered by the network operator, who stores it in the EIR. By means of IMEI, one recognizes obsolete, stolen or non-functional equipment.
There are following parts of an IMEI:
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.
Thus, IMEI = TAC + FAC + SNR + SP. It uniquely characterizes a mobile station and gives clues about the manufacturer and the date of manufacturing.
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):
The real telephone number of a mobile station is the mobile subscriber ISDN number (MSISDN). It is assigned to the subscriber (his or her SIM, respectively), such that a mobile station set can have several MSISDNs depending on the SIM.
The MSISDN categories follow the international ISDN number plan and therefore have the following 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):
The Mobile Station Roaming Number ( MSRN) is 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.
Location Area Identity (LAI):
Each LA of an PLMN has its own identifier. The Location Area Identifier (LAI) is also structured hierarchically and internationally unique as follows:
Country Code (CC) : 3 decimal places.
Mobile Network Code (MNC): 2 decimal places.
Location Area Code (LAC): maximum 5 decimal places or maximum twice 8 bits coded in hexadecimal (LAC < FFFF).
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.
Together with the current location area, TMSI allows a subscriber to be identified uniquely and it can consist of up to 4x8 bits.
Local Mobile Subscriber Identity (LMSI):
The VLR can assign an additional searching key to each mobile station within its area to accelerate database access. This unique key is called the Local Mobile Subscriber Identity (LMSI). The LMSI is assigned when the mobile station registers with the VLR and is also sent to the HLR.
An LIMSI consists of four octets ( 4 x 8 bits).
Cell Identifier (CI):
Within an LA, the individual cells are uniquely identified with a cell identifier (CI), maximum 2 x 8 bits. Together with the global cell identity (LAI + CI) calls are thus also internationally defined in a unique way.
GSM - Operations
The operation of the GSM system can be understood by studying the sequence of events that takes place when a call is initiated from the Mobile Station.
Call from Mobile Phone to PSTN:
When a mobile subscriber makes a call to a PSTN telephone subscriber, the following sequence of events takes place:
- The MSC/VLR receives the message of a call request.
- 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.
- MSC/VLR analyzes the number and initiates a call setup with the PSTN.
- MSC/VLR asks the corresponding BSC to allocate a traffic channel (a radio channel and a time slot).
- The BSC allocates the traffic channel and passes the information to the mobile station.
- The called party answers the call and the conversation takes place.
- 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:
When a PSTN subscriber calls a mobile station, the sequence of events is as follows:
- The Gateway MSC receives the call and queries the HLR for the information needed to route the call to the serving MSC/VLR.
- The GMSC routes the call to the MSC/VLR.
- The MSC checks the VLR for the location area of the MS.
- The MSC contacts the MS via the BSC through a broadcast message, that is, through a paging request.
- The MS responds to the page request.
- 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.
- 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.
In its original form, GSM supports 9.6 Kbps data, which can be transmitted in one TDMA time slot. Over the last few years, many enhancements were done to the GSM standards (GSM Phase 2 and GSM Phase 2+) to provide higher data rates for data applications.
GSM - Protocol Stack
The layered model of the GSM architecture integrates and links the peer-to-peer communications between two different systems. The underlying layers satisfy the services of the upper-layer protocols. Notifications are passed from layer to layer to ensure that the information has been properly formatted, transmitted, and received.
The GMS protocol stacks diagram is shown below:
The signaling protocol in GSM is structured into three general layers, depending on the interface.
Layer 1: The physical layer, which uses the channel structures over the air interface.
Layer 2: The data-link layer. Across the Um interface, the data-link layer is a modified version of the Link access protocol for the D channel (LAP-D) protocol used in ISDN, called Link access protocol on the Dm channel (LAP-Dm). Across the A interface, the Message Transfer Part (MTP), Layer 2 of SS7 is used.
Layer 3: The third layer of the GSM signaling protocol is divided into three sublayers:
- Radio Resource management (RR)
- Mobility Management (MM) and
- Connection Management (CM).
The MS to BTS Protocols:
The RR layer oversees the establishment of a link, both radio and fixed, between the MS and the MSC. The main functional components involved are the MS, the BSS, and the MSC. The RR layer is concerned with the management of an RR-session, which is the time that a mobile is in dedicated mode, as well as the configuration of radio channels, including the allocation of dedicated channels.
The MM layer is built on top of the RR layer and handles the functions that arise from the mobility of the subscriber, as well as the authentication and security aspects. Location management is concerned with the procedures that enable the system to know the current location of a powered-on MS so that incoming call routing can be completed.
The CM layer is responsible for CC, supplementary service management, and Short Message Service (SMS) management. Each of these may be considered as a separate sublayer within the CM layer. Other functions of the CC sublayer include call establishment, selection of the type of service (including alternating between services during a call), and call release.
After the information is passed from the BTS to the BSC, a different set of interfaces is used. The Abis interface is used between the BTS and BSC. At this level, the radio resources at the lower portion of Layer 3 are changed from the RR to the Base Transceiver Station Management (BTSM). The BTS management layer is a relay function at the BTS to the BSC.
The RR protocols are responsible for the allocation and reallocation of traffic channels between the MS and the BTS. These services include controlling the initial access to the system, paging for MT calls, the handover of calls between cell sites, power control, and call termination. The RR protocols provide the procedures for the use, allocation, reallocation, and release of the GSM channels. The BSC still has some radio resource management in place for the frequency coordination, frequency allocation, and the management of the overall network layer for the Layer 2 interfaces.
From the BSC, the relay is using SS7 protocols so the MTP 1-3 is used as the underlying architecture, and the BSS mobile application part or the direct application part is used to communicate from the BSC to the MSC.
At the MSC, the information is mapped across the A interface to the MTP Layers 1 through 3 from the BSC. Here, the equivalent set of radio resources is called the BSS MAP. The BSS MAP/DTAP and the MM and CM are at the upper layers of Layer 3 protocols. This completes the relay process. Through the control-signaling network, the MSCs interact to locate and connect to users throughout the network. Location registers are included in the MSC databases to assist in the role of determining how and whether connections are to be made to roaming users.
Each user of a GSM MS is assigned a HLR that is used to contain the user's location and subscribed services. A separate register, the VLR, is used to track the location of a user. As the users roam out of the area covered by the HLR, the MS notifies a new VLR of its whereabouts. The VLR in turn uses the control network (which happens to be based on SS7) to signal the HLR of the MS's new location. Through this information, MT calls can be routed to the user by the location information contained in the user's HLR.
GSM - User Services
GSM has much more to offer than voice telephony. Additional services allow you greater flexibility in where and when you use your phone. You should contact your local GSM network operator for information on the specific services available to you.
But there are three basic types of services offered through GSM which you can ask for:
Telephony (also referred to as teleservices) Services
Data (also referred to as bearer services) Services.
Teleservices or Telephony Services:
A Teleservice utilises the capabilities of a Bearer Service to transport data, defining which capabilities are required and how they should be set up.
The most basic Teleservice supported by GSM is telephony. This includes Full-rate speech at 13 Kbps and emergency calls, where the nearest emergency- service provider is notified by dialing three digits. A very basic example of emergency service is 911 service available in USA.
Videotext and Facsmile:
Another group of teleservices includes Videotext access, Teletex transmission, Facsimile alternate speech and facsimile Group 3, Automatic facsimile Group 3 etc.
Short Text Messages:
SMS (Short Messaging Service) service is a text messaging which allow you to send and receive text messages on your GSM Mobile phone. Services available from many of the world's GSM networks today - in addition to simple user generated text message services - include news, sport, financial, language and location based services, as well as many early examples of mobile commerce such as stocks and share prices, mobile banking facilities and leisure booking services.
Bearer Services or Data Services
Using your GSM phone to receive and send data is the essential building block leading to widespread mobile Internet access and mobile data transfer. 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 are provided on top of teleservices or bearer services, and include features such as caller identification, call forwarding, call waiting, multi-party conversations, and barring of outgoing (international) calls, among others. A brief description of supplementary services is given here:
Multiparty Service or conferencing: The multiparty service allows a mobile subscriber to establish a multiparty conversation, i.e., a simultaneous conversation between three or more subscribers to setup a conference call. This service is only applicable to normal telephony.
Call Waiting: This service allows a mobile subscriber to be notified of an incoming call during a conversation. The subscriber can answer, reject, or ignore the incoming call. Call waiting is applicable to all GSM telecommunications services using a circuit-switched connection.
Call Hold: This service allows a subscriber to put an incoming call on hold and then resume this call. The call hold service is only applicable to normal telephony.
Call Forwarding: The Call Forwarding Supplementary Service is used to divert calls from the original recipient to another number, and is normally set up by the subscriber himself. It can be used by the subscriber to divert calls from the Mobile Station when the subscriber is not available, and so to ensure that calls are not lost. A typical scenario would be a salesperson turns off his mobile phone during a meeting with customers, but does not with to lose potential sales leads while he is unavailable.
Call Barring: The concept of barring certain types of calls might seem to be a supplementary disservice rather than service. However, there are times when the subscriber is not the actual user of the Mobile Station, and as a consequence may wish to limit its functionality, so as to limit the charges incurred. Alternatively, if the subscriber and user are one and the same, the Call Barring may be useful to stop calls being routed to international destinations when they are routed. The reason for this is because it is expected that the roaming subscriber will pay the charges incurred for international re-routing of calls. So, GSM devised some flexible services that enable the subscriber to conditionally bar calls.
Number Identification: There are following supplementary services related to number identification:
Calling Line Identification Presentation: This service deals with the presentation of the calling party's telephone number. The concept is for this number to be presented, at the start of the phone ringing, so that the called person can determine who is ringing prior to answering. The person subscribing to the service receives the telephone number of the calling party.
Calling Line Identification Restriction: A person not wishing their number to be presented to others subscribes to this service. In the normal course of event, the restriction service overrides the presentation service.
Connected Line Identification Presentation: This service is provided to give the calling party the telephone number of the person to whom they are connected. This may seem strange since the person making the call should know the number they dialled, but there are situations (such as forwardings) where the number connected is not the number dialled. The person subscribing to the service is the calling party.
Connected Line Identification Restriction: There are times when the person called does not wish to have their number presented and so they would subscribe to this person. Normally, this overrides the presentation service.
Malicious Call Identification: The malicious call identification service was provided to combat the spread of obscene or annoying calls. The victim should subscribe to this service, and then they could cause known malicious calls to be identified in the GSM network, using a simple command. This identified number could then be passed to the appropriate authority for action. The definition for this service is not stable.
Advice of Charge (AoC): This service was designed to give the subscriber an indication of the cost of the services as they are used. Furthermore, those Service Providers who wish to offer rental services to subscribers without their own Subscriber Identity Module (SIM) can also utilize this service in a slightly different form. AoC for data calls is provided on the basis of time measurements.
Closed User Groups (CUGs): This service is provided on GSM to enable groups of subscribers to only call each other. This type of services are being offered with special discount and is limited only to those members who wish to talk to each other.
Unstructured supplementary services data (USSD): This allows operator-defined individual services.
GSM - Security
The security methods standardized for the GSM System make it the most secure cellular telecommunications standard currently available. Although the confidentiality of a call and anonymity of the GSM subscriber is only guaranteed on the radio channel, this is a major step in achieving end-to-end security.
The subscriber's anonymity is ensured through the use of temporary identification numbers. The confidentiality of the communication itself on the radio link is performed by the application of encryption algorithms and frequency hopping, which could only be realized using digital systems and signaling.
This chapter gives an outline of the security measures implemented for GSM subscribers.
Mobile Station Authentication:
The GSM network authenticates the identity of the subscriber through the use of a challenge-response mechanism. A 128-bit random number (RAND) is sent to the MS. The MS computes the 32-bit signed response (SRES) based on the encryption of the random number (RAND) with the authentication algorithm (A3) using the individual subscriber authentication key (Ki). Upon receiving the signed response (SRES) from the subscriber, the GSM network repeats the calculation to verify the identity of the subscriber.
Note that the individual subscriber authentication key (Ki) is never transmitted over the radio channel. It is present in the subscriber's SIM, as well as the AUC, HLR, and VLR databases as previously described. If the received SRES agrees with the calculated value, the MS has been successfully authenticated and may continue. If the values do not match, the connection is terminated and an authentication failure indicated to the MS.
The calculation of the signed response is processed within the SIM. This provides enhanced security, because the confidential subscriber information such as the IMSI or the individual subscriber authentication key (Ki) is never released from the SIM during the authentication process.
Signaling and Data Confidentiality:
The SIM contains the ciphering key generating algorithm (A8), which is used to produce the 64-bit ciphering key (Kc). The ciphering key is computed by applying the same random number (RAND) used in the authentication process to the ciphering key generating algorithm (A8) with the individual subscriber authentication key (Ki). As will be shown in later sections, the ciphering key (Kc) is used to encrypt and decrypt the data between the MS and BS.
An additional level of security is provided by having the means to change the ciphering key, making the system more resistant to eavesdropping. The ciphering key may be changed at regular intervals as required by network design and security considerations. In a similar manner to the authentication process, the computation of the ciphering key (Kc) takes place internally within the SIM. Therefore, sensitive information such as the individual subscriber authentication key (Ki) is never revealed by the SIM.
Encrypted voice and data communications between the MS and the network is accomplished through use of the ciphering algorithm A5. Encrypted communication is initiated by a ciphering mode request command from the GSM network. Upon receipt of this command, the mobile station begins encryption and decryption of data using the ciphering algorithm (A5) and the ciphering key (Kc).
Subscriber Identity Confidentiality:
To ensure subscriber identity confidentiality, the Temporary Mobile Subscriber Identity (TMSI) is used. The TMSI is sent to the mobile station after the authentication and encryption procedures have taken place. The mobile station responds by confirming reception of the TMSI. The TMSI is valid in the location area in which it was issued. For communications outside the location area, the Location Area Identification (LAI) is necessary in addition to the TMSI.
GSM - Billing
GSM service providers are doing billing based on the services they are providing to their customers. All the parameters are simple enough to charge a customer for the provided services.
In this chapter, we will discuss about most frequently used billing techniques and parameters to charge a GSM subscriber.
These services can be charged on per call basis. Only call initiator has to pay the charges, and nowadays, all the incoming charges are free. A customer can be charged based on different parameters like:
- International call or long distance call.
- Local call
- Call made during peak hours.
- Call made during night time
- Discounted call during weekends.
- Call per minute or per second.
- Many more other criteria can be designed by a service provider to charge their customers.
Till the time this tutorial is written, most of the service providers are charging their customer's SMS services based on number of text messages sent from their mobile phone. There are other prime SMS services available where service providers are charging more than normal SMS charge. These services are being used in collaboration of Television Networks or Radio Networks to demand SMS from the audiences.
Most of time, charges are paid by the SMS sender but for some services like stocks and share prices, mobile banking facilities and leisure booking services, etc. Recipient of the SMS has to pay for the service.
Using GPRS service, you can browse Internet and can play games on the Internet, you can download movies or music, etc. So a service provider will charge you based on the data uploaded as well as data downloaded on your mobile phone. These charges will be based on per Kilo Byte data downloaded/uploaded.
Additional parameter could be a Quality of Service provided to you. If you want to watch a movie, then a low quality may work because some data loss may be acceptable to you, but if you are downloading a zip file, then a single byte loss will corrupt your complete downloaded file.
Another parameter could be peak and off peak time to download a data file or to browse the Internet.
Most of the supplementary services are being provided based on monthly rental or absolutely FREE. Like Call Waiting, Call Forwarding, Calling Number Identification, and call on hold are available at very low or zero prices.
Call Baring is a service, which service providers use just to recover their dues, etc., otherwise this service is not being used by any subscriber.
Call conferencing service is a form of simple telephone call where customer will be charged for multiple calls made at a time. No service provider charges extra charge for this service.
Closed User Group (CUG) is very popular and is mainly being used to give special discounts to the users if they are making calls to a particular defined group of subscribers.
Advice of Charge (AoC) can be charged based on number of queries made by a subscriber.
GSM - Summary
Hope, now you are aware of GSM Technology. We have taught you all the basic concepts related to GSM technology.
You have learnt about GSM basic overview, its architecture along with a description about all important GSM elements and a brief GSM specification. You have gone through all the important GSM Addresses and Identifiers also.
Further, we have given a short description of GSM protocol stack and available GSM services and their billing techniques.
A list of all the important GSM Acronyms has been given for your quick reference. So you can bookmark this page for future reference.
What is Next?
To enhance the current data capabilities of GSM, operators and infrastructure providers have specified new extensions to GSM Phase II. These extensions are:
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.
If you are not aware of GPRS technology, then our Simple GPRS tutorial will give you a very good start up.
Now, if you need more details about GSM technology, then I would recommend you to go through other GSM resources listed in GSM Useful Resources chapter.
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