What are the three services offered by GSM?

Answer 1

GSM services GSM services are a standard collection of applications and features available to mobile phone subscribers all over the world. The GSM standards are defined by the 3GPP collaboration and implemented in hardware and software by equipment manufacturers and mobile phone operators. The common standard makes it possible to use the same phones with different companies' services, or even roam into different countries. GSM is the world's most dominant mobile phone standard. The design of the service is moderately complex because it must be able to locate a moving phone anywhere in the world, and accommodate the relatively small battery capacity, limited input/output capabilities, and weak radio transmitters on mobile devices. [hide]

• 1 Accessing a GSM network
• 2 Voice calls
  • 2.1 How outgoing calls are made from a mobile
  • 2.2 How incoming calls are made to a mobile
    • 2.2.1 Step One: Contact the Gateway MSC
    • 2.2.2 Step Two: Determine how to route the call
    • 2.2.3 Step Three: Ringing the phone
  • 2.3 Voice charges
  • 2.4 How speech is encoded during mobile phone calls
• 3 Data transmission
  • 3.1 Circuit-switched data protocols
  • 3.2 General Packet Radio Service (GPRS)
  • 3.3 Short Message Service (SMS)
• 4 Supplementary Services
• 5 External links

In order to gain access to GSM services, a user needs three things:

• A subscription with a mobile phone operator. This is usually either a Pay As You Go arrangement, where all GSM services are paid for in advance, or a Pay Monthly option where a bill is issued each month for line rental, normally paid for a month in advance, and for services used in the previous month.
• A mobile phone which is GSM compliant and operates at the same frequency as the operator. Most phone companies sell phones from third-party manufacturers.
• A SIM card which is issued by the operator once the subscription is granted. The card comes pre-programmed with the subscriber's phone "identity" and will be used to store personal information (like contact numbers of friends and family).

After subscribers sign up, information about their phone's identity and what services they are allowed to access are stored in a "SIM record" in the Home Location Register (HLR). The Home Location Register is a database maintained by the "home" phone company for all of its subscribers. It is used to answer queries like, "Where on the mobile phone network is the device associated with this phone number?" and "What services is this subscriber paying for?" Once the SIM card is loaded into the phone and it is powered on, it will search for the nearest mobile phone mast, also called a Base Transceiver Station or BTS. If a mast can be successfully contacted, then there is said to be coverage in the area. Stationary phones are always connected to the same part of the phone network, but mobile phones can "visit" any part of the network, whether across town or in Another Country via a foreign provider. Each geographic area has a database called the Visitors Location Register (VLR) which contains details of all the local mobiles. Whenever a phone attaches, or visits, a new area, the Visitors Location Register must contact the HomeLocation Register. The Visitors LR will tell the Home LR where the phone is connected to the network (which VLR), and will ask it for a copy of the SIM record (which includes, for example, what services the phone is allowed to access). The current cellular location of the phone (i.e. which BTS it is at) is entered into the VLR record and will be used during a process called paging when the GSM network wishes to locate the mobile phone. Every SIM card contains a secret key, called the Ki, which it uses to prove its identity to the phone network (to prevent theft of services) upon first contact. The network does this by consulting the Authentication Center of the "home" phone company, which also has a copy of the secret key. (Though the authentication is accomplished without transmitting the key directly.) Every phone contains a unique identifier (different from the phone number, which is associated at the HLR with the removable SIM card), called the International Mobile Equipment Identity (IMEI). When a phone contacts the network, its IMEI is supposed to be checked against the global Equipment Identity Register to locate stolen phones and facilitate monitoring. Once a mobile phone has successfully attached to a GSM network as described above, calls may be made from the phone to any other phone on the global Public Switched Telephone Network assuming the subscriber has an arrangement with their "home" phone company to allow the call. The user dials the telephone number, presses the send or talk key, and the mobile phone sends a call setup request message to the mobile phone network via the mobile phone mast (BTS) it is in contact with. The element in the mobile phone network that handles the call request is the Visited Mobile Switching Center (Visited MSC). The MSC will check against the subscriber's temporary record held in the Visitor Location Register to see if the outgoing call is allowed. If so, the MSC then routes the call in the same way that a telephone exchange does in a fixed network. If the subscriber is on a Pay As You Go tariff (sometimes known as Prepaid (for example, in Australia)), then an additional check is made to see if the subscriber has enough credit to proceed. If not, the call is rejected. If the call is allowed to continue, then it is continually monitored and the appropriate amount is decremented from the subscriber's account. When the credit reaches zero, the call is cut off by the network. The systems that monitor and provide the prepaid services are not part of the GSM standard services, but instead an example of intelligent network services that a mobile phone operator may decide to implement in addition to the standard GSM ones. [edit] Step One: Contact the Gateway MSCWhen someone places a call to a mobile phone, they dial the telephone number (also called a MSISDN) associated with the phone user and the call is routed to the mobile phone operator's Gateway Mobile Switching Centre. The Gateway MSC, as the name suggests, acts as the "entrance" from exterior portions of the Public Switched Telephone Network onto the provider's network. As noted above, the phone is free to roam anywhere in the operator's network or on the networks of roaming partners, including in other countries. So the first job of the Gateway MSC is to determine the current location of the mobile phone in order to connect the call. It does this by consulting the Home Location Register (HLR), which, as described above, knows which Visitor Location Register (VLR) the phone is associated with, if any. [edit] Step Two: Determine how to route the callWhen the HLR receives this query message, it determines whether the call should be routed to another number (called a divert), or if it is to be routed directly to the mobile.

• If the owner of the phone has previously requested that all incoming calls be diverted to another number, known as the Call Forward Unconditional (CFU) Number, then this number is stored in the Home Location Register. If that is the case, then the CFU number is returned to the Gateway MSC for immediate routing to that destination.

• If the mobile phone is not currently associated with a Visited Location Register (because the phone has been turned off or is not in range) then the Home Location Register returns a number known as the Call Forward Not Reachable (CFNRc) number to the Gateway MSC, and the call is forwarded there. Many operators may set this value automatically to the phone's voice mail number, so that callers may leave a message. The mobile phone may sometimes override the default setting.

• Finally, if the Home Location Register knows that the phone is in the jurisdiction of a particular Visited Location Register, then it will request a temporary number (called an MSRN) from that VLR. This number is relayed to the Gateway MSC, which uses it to route the call to another Mobile Switching Center, called the Visiting MSC.

[edit] Step Three: Ringing the phoneWhen the call is received by the Visiting MSC, the MSRN is used to find the phone's record in the Visited Location Register. This record identifies the phone's location area. Paging occurs to all mobile phone masts in that area. When the subscriber's mobile responds, the exact location of the mobile is returned to the Visited MSC. The VMSC then forwards the call to the appropriate phone mast, and the phone rings. If the subscriber answers, a speech path is created through the Visiting MSC and Gateway MSC back to the network of the person making the call, and a normal telephone call follows. It is also possible that the phone call is not answered. If the subscriber is busy on another call (and call waiting is not being used) the Visited MSC routes the call to a pre-determined Call Forward Busy (CFB) number. Similarly, if the subscriber does not answer the call after a period of time (typically 30 seconds) then the Visited MSC routes the call to a pre-determined Call Forward No Reply (CFNRy) number. Once again, the operator may decide to set this value by default to the voice mail of the mobile so that callers can leave a message.... In the United States and Canada, callers pay the cost of connecting to the Gateway MSC of the subscriber's phone company, regardless of the actual location of the phone. As mobile numbers are given standard geographic numbers according to the North American Numbering Plan, callers pay the same to reach fixed phones and mobile phones in a given geographic area. Mobile subscribers pay for the connection time (typically using in-plan or prepaid minutes) for both incoming and outgoing calls. For outgoing calls, any long distance charges are billed as if they originate at the GMSC, even though it is the Visiting MSC which completes the connection to the PSTN. Plans that include nationwide long distance and/or nationwide roaming at no additional charge over "local" outgoing calls are popular. Mobile networks in Europe, Asia and Australia only charge their subscribers for outgoing calls. Incoming calls are free to the mobile subscriber; however, callers typically pay a higher rate when calling mobile phones. Special prefixes are used to designate mobile numbers so that callers are aware they are calling a mobile phone and therefore will be charged a higher rate. From the caller's point of view, it does not matter where the mobile subscriber is, as the technical process of connecting the call is the same. If a subscriber is roaming on a different company's network, the subscriber, instead of the caller, may pay a surcharge for the connection time. International roaming calls are often quite expensive, and as a result some companies require subscribers to grant explicit permission to receive calls while roaming to certain countries. When a subscriber is roaming internationally and a call is forwarded to his or her voice mail, such as when his or her phone is off, busy, or not answered, he or she may actually be charged for two simultaneous international phone calls—the first to get from the GMSC to the VMSC and the second to get from the VMSC to the Call Forward Busy or Call Forward No Reply number (typically the voice mailbox) in the subscriber's country. However, some networks' GMSCs connect unanswered calls directly, keeping the voice signal entirely within the home country and thus avoiding the double charge.[citation needed] During a GSM call, speech is converted from analogue sound waves to digital data by the phone itself, and transmitted through the mobile phone network by digital means. (Though older parts of the fixed Public Switched Telephone Network may use analog transmission.) The digital algorithm used to encode speech signals is called a codec. The speech codecs used in GSM are called Half-Rate (HR), Full-Rate (FR), Enhanced Full-Rate (EFR) and Adaptive Multirate (AMR). All codecs except AMR operate with a fixed data rate and error correction level. The Public Switched Telephone Network (PSTN) is essentially a collection of interconnected systems for taking an audio signal from one place and delivering it to another. Older analogue phone networks simply converted sound waves into electrical pulses and back again. The modern phone system digitally encodes audio signals so that they can be combined and transmitted long distances over fiber optic cables and other means, without losing signal quality in the process. When someone uses a computer with a traditional modem, they are encoding a (relatively slow) data stream into a series of audio chirps, which are then relayed by the PSTN in the same way as regular voice calls. This means that computer data is being encoded as phone audio, which is then being re-encoded as phone system data, and then back to phone quality audio, which is finally converted back to computer data at the destination. GSM voice calls are essentially an extension of the PSTN, dealing only with audio signals. Behind the Scenes, we know these audio channels happen to be transmitted as digital radio signals. The GSM standard also provides separate facilities for transmitting digital data directly, without any of the inefficient conversions back and forth to audio form. This allows a mobile "phone" to act like any other computer on the Internet, sending and receiving data via the Internet Protocol or X.25. The mobile may also be connected to a desktop computer, laptop, or PDA, for use as a network interface. (Like a modem or ethernet card, but using a GSM-compatible data protocol instead of a PSTN-compatible audio channel or an ethernet link to transmit data.) Newer GSM phones can be controlled by a standardised Hayes AT command set through a serial cable or a wireless link (using IrDA or Bluetooth). The AT commands can control anything from ring tones to data compression algorithms. In addition to general Internet access, other special services may be provided by the mobile phone operator, such as SMS. A circuit-switched data connection reserves a certain amount of bandwidth between two points for the life of a connection, just as a traditional phone call allocates an audio channel of a certain quality between two phones for the duration of the call. (But remember that in the GSM system, there is no need to use audio signals to create data connections, even circuit-switched ones. The idea of a circuit-switched data connection being like a phone call is just an analogy to help explain the idea.) Two circuit-switched data protocols are defined in the GSM standard, and they have not-very-creative names: Circuit Switched Data (CSD) and High-Speed Circuit-Switched Data (HSCSD). These types of connections are typically charged on a per-second basis, regardless of the amount of data sent over the link. This is because a certain amount of bandwidth is dedicated to the connection regardless of whether or not it is needed. Circuit-switched connections do have the advantage of providing a constant, guaranteed quality of service, which is useful for real-time applications like video conferencing. A packet-switched connection chops data into distinct chunks, known as packets, which may arrive at their destination via different routes, at different times, out of sequence, or (hopefully only occasionally) not at all. An intermediate protocol, like TCP, might be used to ensure the original data stream is reassembled at the destination (by putting packets in order and retransmitting missing ones, if necessary). The General Packet Radio Service (GPRS) is a packet-switched data transmission protocol which was incorporated into the GSM standard in 1997. It is backwards-compatible with systems that use pre-1997 versions of the standard. GPRS does this by sending packets to the local mobile phone mast (BTS) on channels not being used by circuit-switched voice calls or data connections. Multiple GPRS users can share a single unused channel because each of them uses it only for occasional short bursts. The advantage of packet-switched connections is that bandwidth is only used when there is actually data to transmit. This type of connection is thus generally billed by the kilobyte instead of by the second, and is usually a cheaper alternative for applications that only need to send and receive data sporadically, like instant messaging. GPRS is usually described as a 2.5G technology; see the main article for more information. Main article: Short message serviceThe GSM standards first defined the structure of a Short Message, and provide a means of transmitting messages between mobile devices and Short Message Service Centres via the Short message service (SMS). SMS messages may be carried between phones and SMSCs by any of the circuit-switched or packet-switched methods described above or, more typically, by the MAP protocol through the SS7 signaling channel used for call setup. SMSCs can be thought of as central routing hubs for Short Messages. Many mobile service operators use their SMSCs as gateways to external systems, including the Internet, incoming SMS news feeds, and each other (often using the de facto SMPP standard). The SMS standard is also used outside of the GSM system; see the main article for details. GSM supports a comprehensive set of supplementary services that complement and support the telephony and data services described above. They are all defined in GSM standards. (See GSM codes for supplementary services) A partial listing of supplementary services follows.

• Call Forwarding. This service gives the subscriber the ability to forward incoming calls to another number if the called mobile unit is not reachable, if it is busy, if there is no reply, or if call forwarding is allowed unconditionally.

• Barring of Outgoing Calls. This service makes it possible for a mobile subscriber to prevent all outgoing calls.

• Barring of Incoming Calls. This function allows the subscriber to prevent incoming calls. The following two conditions for incoming call barring exist: baring of all incoming calls and barring of incoming calls when roaming outside the home PLMN.

• Advice of Charge (AoC). The AoC service provides the mobile subscriber with an estimate of the call charges. There are two types of AoC information: one that provides the subscriber with an estimate of the bill and one that can be used for immediate charging purposes. AoC for data calls is provided on the basis of time measurements.

• Call Hold. This service enables the subscriber to interrupt an ongoing call and then subsequently reestablish the call. The call hold service is only applicable to normal telephony.

• Call Waiting. This service enables the 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.

• Multiparty service. The multiparty service enables a mobile subscriber to establish a multiparty conversation - that is, a simultaneous conversation between three and six subscribers. This service is only applicable to normal telephony.

• Calling Line Identification presentation/restriction. These services supply the called party with the integrated services digital network (ISDN) number of the calling party. The restriction service enables the calling party to restrict the presentation. The restriction overrides the presentation.

• Closed User Groups (CUGs). CUGs are generally comparable to a PBX. They are a group of subscribers who are capable of only calling themselves and certain numbers.

• Explicit Call Transfer (ECT). This service allows a user who has two calls to connect these two calls together and release its connections to both other parties.

• BBC 3 Investigates video BBC3 programme "investigates" has undertaken the largest ever survey of the proximity to schools of mobile phone masts in the UK.
• GSM Call Flows and Sequence Diagrams Detailed call flow diagrams describing GSM call setup, location update and handover.

Answer 2

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.

• Supplementary 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.

Voice Calls:

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

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.that is, 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.