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 short battery life, limited input/output capabilities, and weak radio transmitters on mobile devices.
Accessing a GSM network
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 Home Location 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.
Voice calls
How outgoing calls are made
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 Centre (Visited MSC). The MSC will check against the subscriber's temporary record held in the Visited 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, 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.
How Mobile terminating are made
Step One: Contact the Gateway MSC
When someone places a call to a mobile phone, they dial the telephone number associated with the phone user (also called a MSISDN) 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 Visited Location Register (VLR) the phone is associated with, if any.
Step Two: Determine how to route the call
When 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.
Step Three: Ringing the phone
When 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.
Voice charges
In the United States, callers pay the cost of connecting to the "home" gateway of the subscriber's phone company, regardless of the actual location of the phone. This is the same as the cost to reach a fixed phone in the same geographic area as the "home" exchange. Mobile subscribers pay for "airtime" for both incoming and outgoing calls, whether by the minute, with a flat rate, or some more complicated billing scheme. Any long distance charges are billed (and dialed) as if they originated at the "home" exchange. (Though some providers offer plans that include nation-wide long distance at no additional charge over "local" outgoing calls.)
If a subscriber is roaming on a different company's network, the subscriber (not the caller) may be charged extra for the connection time. (The technical process of connecting the call is the same, however.) International roaming calls are often even more 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 their voice mail (because their phone is off, busy, or not answered) they may actually be charged for two simultaneous international phone calls. The first to get from the "home" network to their current location to reach their phone in the first place, and the second to get back from their current location to the voice mailbox in the "home" country. (Some operators connect unanswered calls directly, keeping the voice signal entirely within the home country and thus avoiding the double charge.)
How speech is encoded during mobile phone calls
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.
Data transmission
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.
Circuit-switched data protocols
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.
General Packet Radio Service (GPRS)
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.
Short Message Service (SMS)
The 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.
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.
Supplementary Services
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.
External links
BBC3 programme "investigates" has undertaken the largest ever survey of the proximity to schools of mobile phone masts in the UK.