GSM services
Encyclopedia
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.
After subscribers sign up, information about their identity (telephone number) and what services they are allowed to access are stored in a "SIM record" in the Home Location Register
(HLR).
Once the SIM card is loaded into the phone and the phone is powered on, it will search for the nearest mobile phone mast (also called a Base Transceiver Station/BTS
) with the strongest signal in the operator's frequency band. If a mast can be successfully contacted, then there is said to be coverage
in the area. The phone then identifies itself to the network through the control channel. Once this is successfully completed, the phone is said to be attached to the network.
The key feature of a mobile phone is the ability to receive and make calls in any area where coverage is available. This is generally called roaming from a customer perspective, but also called visiting when describing the underlying technical process. Each geographic area has a database called the Visitor Location Register (VLR), which contains details of all the mobiles currently in that area. Whenever a phone attaches, or visits, a new area, the Visitor Location Register must contact the Home Location Register to obtain the details for that phone. 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 is used to provide authentication and encryption services. This is useful to prevent theft of service, and also to prevent "over the air" snooping of a user's activity. The network does this by utilising the Authentication Center and is accomplished without transmitting the key directly.
Every GSM phone contains a unique identifier (different from the phone number), called the International Mobile Equipment Identity
(IMEI). This can be found by dialing *#06#. When a phone contacts the network, its IMEI may be checked against the Equipment Identity Register
to locate stolen phones and facilitate monitoring.
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
.
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 nearest mobile phone base transceiver station (BTS).
The call setup request message is handled next by the Mobile Switching Center, which checks the subscriber's 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 prepaid tariff (sometimes known as Pay As You Go (PAYG) or Pay & Go), 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.
, 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.
all the mobile phone masts
in the area in order to inform the phone that there is an incoming call for it. 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.
If the subscriber does not respond to the paging request, either due to being out of coverage, or their battery has gone flat/removed, then the Visited MSC routes the call to a pre-determined Call Forward Not Reachable (CFNRc) 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.
A roaming user may want to avoid these forwarding services in the visited network as excessive roaming charges may apply.
, 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 that 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 (except Hong Kong, Macau (Macao) and Singapore), Australia & Argentina only charge their subscribers for outgoing calls. Incoming calls are free to the mobile subscriber with the exception of receiving a call while the subscriber is roaming as described below. 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.
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.
, sending and receiving data via the Internet Protocol
.
The mobile may also be connected to a desktop computer, laptop
, or PDA
, for use as a network interface (just like a modem
or Ethernet
card, but using one of the GSM data protocols described below instead of a PSTN-compatible audio channel or an Ethernet link to transmit data). Some GSM phones can also 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
.
Two circuit-switched data protocols are defined in the GSM standard: 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.
(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.
(more commonly known as text messaging
) has become the most used data application on mobile phones, with 74% of all mobile phone users worldwide already as active users of SMS, or 2.4 billion people by the end of 2007.
SMS text messages may be sent by mobile phone users to other mobile users or external services that accept SMS. The messages are usually sent from mobile devices via the Short Message Service Centre using the MAP
protocol.
The SMSC is a 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 other mobile operators (often using the de facto
SMPP standard for SMS exchange).
The SMS
standard is also used outside of the GSM system; see the main article for details.
.
Mobile phone
A mobile phone is a device which can make and receive telephone calls over a radio link whilst moving around a wide geographic area. It does so by connecting to a cellular network provided by a mobile network operator...
subscribers all over the world. The GSM standards are defined by the 3GPP
3GPP
The 3rd Generation Partnership Project is a collaboration between groups of telecommunications associations, known as the Organizational Partners...
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
Roaming
In wireless telecommunications, roaming is a general term referring to the extension of connectivity service in a location that is different from the home location where the service was registered. Roaming ensures that the wireless device is kept connected to the network, without losing the...
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.
Accessing a GSM network
In order to gain access to GSM services, a user needs three things:- A billing relationship with a mobile phone operator. This is usually either where services are paid for in advance of them being consumed (prepaid), or where bills are issued and settled after the service has been consumed (postpaidPostpaid mobile phoneThe post-paid mobile phone is a mobile phone for which service is provided by a prior arrangement with a carriage service provider . The user in this situation is billed after the fact according to their use of mobile services at the end of each month...
). - A mobile phoneMobile phoneA mobile phone is a device which can make and receive telephone calls over a radio link whilst moving around a wide geographic area. It does so by connecting to a cellular network provided by a mobile network operator...
that is GSM compliant and operates at the same frequencyGSM frequency rangesGSM frequency bands or frequency ranges are the cellular frequencies designated by the ITU for the operation of GSM mobile phones.- GSM frequency bands :There are fourteen bands defined in 3GPP TS 45.005, which succeeded 3GPP TS 05.05:...
as the operator. Most phone companies sell phones from third-party manufacturers. - A Subscriber Identity Module (SIM) card, which is activated by the operator once the billing relationship is established. After activation the card is then programmed with the subscriber's Mobile Subscriber Integrated Services Digital Network Number (MSISDN) (the telephone number). Personal information such as contact numbers of friends and family can also be stored on the SIM by the subscriber.
After subscribers sign up, information about their identity (telephone number) and what services they are allowed to access are stored in a "SIM record" in the Home Location Register
Network Switching Subsystem
Network switching subsystem is the component of a GSM system that carries out call switching and mobility management functions for mobile phones roaming on the network of base stations...
(HLR).
Once the SIM card is loaded into the phone and the phone is powered on, it will search for the nearest mobile phone mast (also called a Base Transceiver Station/BTS
Base Transceiver Station
A base transceiver station or cell site is a piece of equipment that facilitates wireless communication between user equipment and a network. UEs are devices like mobile phones , WLL phones, computers with wireless internet connectivity, WiFi and WiMAX gadgets etc...
) with the strongest signal in the operator's frequency band. If a mast can be successfully contacted, then there is said to be coverage
Coverage (telecommunication)
In telecommunications, the coverage of a radio station is the geographic area where the station can communicate. Broadcasters and telecommunications companies frequently produce coverage maps to indicate to users the station's intended service area. Coverage depends on several factors, such as...
in the area. The phone then identifies itself to the network through the control channel. Once this is successfully completed, the phone is said to be attached to the network.
The key feature of a mobile phone is the ability to receive and make calls in any area where coverage is available. This is generally called roaming from a customer perspective, but also called visiting when describing the underlying technical process. Each geographic area has a database called the Visitor Location Register (VLR), which contains details of all the mobiles currently in that area. Whenever a phone attaches, or visits, a new area, the Visitor Location Register must contact the Home Location Register to obtain the details for that phone. 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
Mobile phone
A mobile phone is a device which can make and receive telephone calls over a radio link whilst moving around a wide geographic area. It does so by connecting to a cellular network provided by a mobile network operator...
.
Every SIM card contains a secret key, called the Ki, which is used to provide authentication and encryption services. This is useful to prevent theft of service, and also to prevent "over the air" snooping of a user's activity. The network does this by utilising the Authentication Center and is accomplished without transmitting the key directly.
Every GSM phone contains a unique identifier (different from the phone number), called the International Mobile Equipment Identity
International Mobile Equipment Identity
The International Mobile Equipment Identity or IMEI is a number, usually unique, to identify GSM, WCDMA, and iDEN mobile phones, as well as some satellite phones. It is usually found printed inside the battery compartment of the phone...
(IMEI). This can be found by dialing *#06#. When a phone contacts the network, its IMEI may be checked against the Equipment Identity Register
Network Switching Subsystem
Network switching subsystem is the component of a GSM system that carries out call switching and mobility management functions for mobile phones roaming on the network of base stations...
to locate stolen phones and facilitate monitoring.
Outgoing
Once a mobile phoneMobile phone
A mobile phone is a device which can make and receive telephone calls over a radio link whilst moving around a wide geographic area. It does so by connecting to a cellular network provided by a mobile network operator...
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
Public switched telephone network
The public switched telephone network is the network of the world's public circuit-switched telephone networks. It consists of telephone lines, fiber optic cables, microwave transmission links, cellular networks, communications satellites, and undersea telephone cables, all inter-connected by...
.
The user dials the telephone number
Telephone number
A telephone number or phone number is a sequence of digits used to call from one telephone line to another in a public switched telephone network. When telephone numbers were invented, they were short — as few as one, two or three digits — and were given orally to a switchboard operator...
, presses the send or talk key, and the mobile phone
Mobile phone
A mobile phone is a device which can make and receive telephone calls over a radio link whilst moving around a wide geographic area. It does so by connecting to a cellular network provided by a mobile network operator...
sends a call setup request message to the mobile phone network
Network Switching Subsystem
Network switching subsystem is the component of a GSM system that carries out call switching and mobility management functions for mobile phones roaming on the network of base stations...
via the nearest mobile phone base transceiver station (BTS).
The call setup request message is handled next by the Mobile Switching Center, which checks the subscriber's 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
Telephone exchange
In the field of telecommunications, a telephone exchange or telephone switch is a system of electronic components that connects telephone calls...
does in a fixed network.
If the subscriber is on a prepaid tariff (sometimes known as Pay As You Go (PAYG) or Pay & Go), 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
Intelligent network
The Intelligent Network , is the standard network architecture specified in the . It is intended for fixed as well as mobile telecom networks...
services that a mobile phone operator may decide to implement in addition to the standard GSM ones.
Gateway MSC contact
When someone places a call to a mobile phoneMobile phone
A mobile phone is a device which can make and receive telephone calls over a radio link whilst moving around a wide geographic area. It does so by connecting to a cellular network provided by a mobile network operator...
, they dial the telephone number
Telephone number
A telephone number or phone number is a sequence of digits used to call from one telephone line to another in a public switched telephone network. When telephone numbers were invented, they were short — as few as one, two or three digits — and were given orally to a switchboard operator...
(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
Public switched telephone network
The public switched telephone network is the network of the world's public circuit-switched telephone networks. It consists of telephone lines, fiber optic cables, microwave transmission links, cellular networks, communications satellites, and undersea telephone cables, all inter-connected by...
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
Mobile phone
A mobile phone is a device which can make and receive telephone calls over a radio link whilst moving around a wide geographic area. It does so by connecting to a cellular network provided by a mobile network operator...
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.
Routing 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 phoneMobile phoneA mobile phone is a device which can make and receive telephone calls over a radio link whilst moving around a wide geographic area. It does so by connecting to a cellular network provided by a mobile network operator...
is not currently associated with a Visited Location Register (because the phone has been turned off) 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 roaming in a particular Visited Location Register area, then it will request a temporary number (called an MSRNMSRNMSRN - Mobile Station Roaming NumberThe Mobile Station Roaming Number is an E.214 defined telephone number used to route telephone calls in a mobile network from a GMSC to the target MSC . It can also be defined as a directory number temporarily assigned to a mobile for a mobile terminated call...
) from that VLR. This number is relayed back to the Gateway MSC, and then used to route the call to the MSC where the called phone is roaming.
Locating and ringing the phone
When the call arrives at the Visiting MSC, the MSRN is used to determine which of the phones in this area is being called, that is the MSRN maps back to the SIM of the original phone number dialled. The MSC pagesMobility management
Mobility management is one of the major functions of a GSM ora UMTS network that allows mobile phones to work. The aim of mobility management is to track where the subscribers are, allowing calls, SMS and other mobile phone services to be delivered to them....
all the mobile phone masts
Base Station Subsystem
The base station subsystem is the section of a traditional cellular telephone network which is responsible for handling traffic and signaling between a mobile phone and the network switching subsystem...
in the area in order to inform the phone that there is an incoming call for it. 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
Telephone
The telephone , colloquially referred to as a phone, is a telecommunications device that transmits and receives sounds, usually the human voice. Telephones are a point-to-point communication system whose most basic function is to allow two people separated by large distances to talk to each other...
call follows.
It is also possible that the phone call is not answered. If the subscriber is busy on another call (and call waiting
Call waiting
Call waiting , in telephony, is a feature on some telephone networks. If a calling party places a call to a called party which is otherwise engaged, and the called party has the call waiting feature enabled, the called party is able to suspend the current telephone call and switch to the new...
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.
If the subscriber does not respond to the paging request, either due to being out of coverage, or their battery has gone flat/removed, then the Visited MSC routes the call to a pre-determined Call Forward Not Reachable (CFNRc) 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.
A roaming user may want to avoid these forwarding services in the visited network as excessive roaming charges may apply.
Voice charges
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 PlanNorth American Numbering Plan
The North American Numbering Plan is an integrated telephone numbering plan administered by Neustar which encompasses 24 countries and territories, including the United States and its territories, Canada, Bermuda, and 16 nations of the Caribbean...
, 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 that 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 (except Hong Kong, Macau (Macao) and Singapore), Australia & Argentina only charge their subscribers for outgoing calls. Incoming calls are free to the mobile subscriber with the exception of receiving a call while the subscriber is roaming as described below. 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
Roaming
In wireless telecommunications, roaming is a general term referring to the extension of connectivity service in a location that is different from the home location where the service was registered. Roaming ensures that the wireless device is kept connected to the network, without losing the...
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.
How speech is encoded during mobile phone calls
During a GSM call, speech is converted from analogueAnalog signal
An analog or analogue signal is any continuous signal for which the time varying feature of the signal is a representation of some other time varying quantity, i.e., analogous to another time varying signal. It differs from a digital signal in terms of small fluctuations in the signal which are...
sound waves to digital
Digital
A digital system is a data technology that uses discrete values. By contrast, non-digital systems use a continuous range of values to represent information...
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
Public switched telephone network
The public switched telephone network is the network of the world's public circuit-switched telephone networks. It consists of telephone lines, fiber optic cables, microwave transmission links, cellular networks, communications satellites, and undersea telephone cables, all inter-connected by...
may use analog transmission.)
The digital algorithm used to encode speech signals is called a codec
Codec
A codec is a device or computer program capable of encoding or decoding a digital data stream or signal. The word codec is a portmanteau of "compressor-decompressor" or, more commonly, "coder-decoder"...
. The speech codecs used in GSM are called Half-Rate (HR)
Half Rate
Half Rate is a speech coding system for GSM, developed in the early 1990s.Since the codec, operating at 5.6 kbit/s, requires half the bandwidth of the Full Rate codec, network capacity for voice traffic is doubled, at the expense of audio quality. It is recommended to use this codec when the...
, Full-Rate (FR)
Full Rate
Full Rate or FR or GSM-FR or GSM 06.10 was the first digital speech coding standard used in the GSM digital mobile phone system. The bit rate of the codec is 13 kbit/s, or 1.625 bits/audio sample...
, Enhanced Full-Rate (EFR)
Enhanced Full Rate
Enhanced Full Rate or EFR or GSM-EFR or GSM 06.60 is a speech coding standard that was developed in order to improve the quite poor quality of GSM-Full Rate codec. Working at 12.2 kbit/s the EFR provides wirelike quality in any noise free and background noise conditions...
and Adaptive Multirate (AMR)
Adaptive Multi-Rate
The Adaptive Multi-Rate audio codec is a patented audio data compression scheme optimized for speech coding. AMR was adopted as the standard speech codec by 3GPP in October 1999 and is now widely used in GSM and UMTS...
. All codecs except AMR operate with a fixed data rate and error correction level.
Data transmission
The GSM standard also provides separate facilities for transmitting digital data. This allows a mobile phone to act like any other computer on the InternetInternet
The Internet is a global system of interconnected computer networks that use the standard Internet protocol suite to serve billions of users worldwide...
, sending and receiving data via the Internet Protocol
Internet Protocol
The Internet Protocol is the principal communications protocol used for relaying datagrams across an internetwork using the Internet Protocol Suite...
.
The mobile may also be connected to a desktop computer, laptop
Laptop
A laptop, also called a notebook, is a personal computer for mobile use. A laptop integrates most of the typical components of a desktop computer, including a display, a keyboard, a pointing device and speakers into a single unit...
, or PDA
Personal digital assistant
A personal digital assistant , also known as a palmtop computer, or personal data assistant, is a mobile device that functions as a personal information manager. Current PDAs often have the ability to connect to the Internet...
, for use as a network interface (just like a modem
Modem
A modem is a device that modulates an analog carrier signal to encode digital information, and also demodulates such a carrier signal to decode the transmitted information. The goal is to produce a signal that can be transmitted easily and decoded to reproduce the original digital data...
or Ethernet
Ethernet
Ethernet is a family of computer networking technologies for local area networks commercially introduced in 1980. Standardized in IEEE 802.3, Ethernet has largely replaced competing wired LAN technologies....
card, but using one of the GSM data protocols described below instead of a PSTN-compatible audio channel or an Ethernet link to transmit data). Some GSM phones can also be controlled by a standardised Hayes AT command set through a serial cable or a wireless link (using IRDA
Infrared Data Association
The Infrared Data Association defines physical specifications communications protocol standards for the short-range exchange of data over infrared light, for uses such as personal area networks ....
or Bluetooth
Bluetooth
Bluetooth is a proprietary open wireless technology standard for exchanging data over short distances from fixed and mobile devices, creating personal area networks with high levels of security...
). 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
Short message service
Short Message Service is a text messaging service component of phone, web, or mobile communication systems, using standardized communications protocols that allow the exchange of short text messages between fixed line or mobile phone devices...
.
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.Two circuit-switched data protocols are defined in the GSM standard: Circuit Switched Data
Circuit Switched Data
Circuit Switched Data is the original form of data transmission developed for the time division multiple access -based mobile phone systems like Global System for Mobile Communications...
(CSD) and High-Speed Circuit-Switched Data
High-Speed Circuit-Switched Data
High-speed circuit-switched data , is an enhancement to Circuit Switched Data , the original data transmission mechanism of the GSM mobile phone system, four times faster than GSM, with data rates up to 38.4 kbit/s....
(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
Quality of service
The quality of service refers to several related aspects of telephony and computer networks that allow the transport of traffic with special requirements...
, which is useful for real-time applications like video conferencing.
General Packet Radio Service (GPRS)
The General Packet Radio ServiceGeneral Packet Radio Service
General packet radio service is a packet oriented mobile data service on the 2G and 3G cellular communication system's global system for mobile communications . GPRS was originally standardized by European Telecommunications Standards Institute in response to the earlier CDPD and i-mode...
(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
Base Transceiver Station
A base transceiver station or cell site is a piece of equipment that facilitates wireless communication between user equipment and a network. UEs are devices like mobile phones , WLL phones, computers with wireless internet connectivity, WiFi and WiMAX gadgets etc...
) 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
Instant messaging
Instant Messaging is a form of real-time direct text-based chatting communication in push mode between two or more people using personal computers or other devices, along with shared clients. The user's text is conveyed over a network, such as the Internet...
.
GPRS is usually described as a 2.5G technology; see the main article for more information.
Short Message Service (SMS)
Short Message ServiceSMS
SMS is a form of text messaging communication on phones and mobile phones. The terms SMS or sms may also refer to:- Computer hardware :...
(more commonly known as text messaging
Text messaging
Text messaging, or texting, refers to the exchange of brief written text messages between fixed-line phone or mobile phone and fixed or portable devices over a network...
) has become the most used data application on mobile phones, with 74% of all mobile phone users worldwide already as active users of SMS, or 2.4 billion people by the end of 2007.
SMS text messages may be sent by mobile phone users to other mobile users or external services that accept SMS. The messages are usually sent from mobile devices via the Short Message Service Centre using the MAP
Mobile Application Part
The Mobile Application Part is an SS7 protocol which provides an application layer for the various nodes in GSM and UMTS mobile core networks and GPRS core networks to communicate with each other in order to provide services to mobile phone users...
protocol.
The SMSC is a central routing hubs for Short Messages. Many mobile service operators use their SMSCs as gateways to external systems, including the Internet
Internet
The Internet is a global system of interconnected computer networks that use the standard Internet protocol suite to serve billions of users worldwide...
, incoming SMS news feeds, and other mobile operators (often using the de facto
De facto
De facto is a Latin expression that means "concerning fact." In law, it often means "in practice but not necessarily ordained by law" or "in practice or actuality, but not officially established." It is commonly used in contrast to de jure when referring to matters of law, governance, or...
SMPP standard for SMS exchange).
The SMS
Short message service
Short Message Service is a text messaging service component of phone, web, or mobile communication systems, using standardized communications protocols that allow the exchange of short text messages between fixed line or mobile phone devices...
standard is also used outside of the GSM system; see the main article for details.
Supplementary Services
See also GSM codes for supplementary servicesGSM codes for supplementary services
Global System for Mobile Communications standards define certain codes that make it possible to query and set certain service parameters from mobile devices...
.
- Call forwardingCall forwardingCall forwarding , in telephony, is a feature on some telephone networks allowing an incoming call to a called party to be redirected to a third party. For example, the third party may be a mobile telephone, voicemail box or other telephone number where the desired called party is situated. It was...
. - Barring of Outgoing Calls.
- Barring of Incoming Calls.
- Advice of Charge (AoC). This service lets users find out how much a call will cost or has cost, without having to phone their mobile network operatorMobile network operatorA mobile network operator , also known as mobile phone operator , carrier service provider , wireless service provider, wireless carrier, or cellular company, or mobile network carrier is a telephone company that provides services for mobile phone subscribers.One essential...
's customer service department for information. This makes users satisfied by helping to prevent bill shockBill shockBill shock was popularized in the telecommunications industry as a term for the negative reaction a subscriber can experience if their mobile phone bill has unexpected charges...
. It also reduces customer service departments' workload. Few mobile network operators offer Advice of Charge service. MTN IrancellMTN IrancellIrancell is the second mobile phone network operator of Iran. It is 49% owned by the South Africa-based MTN Group.MTN Irancell operates on GSM 900/1800 and is the first network operator of Iran which provides GPRS and MMS service and will let subscribers choose their desired number from between...
offers an Advice of Charge service to prepaid subscribers, but it is implemented differently than the GSM standard recommends. - Call Hold.
- Call Waiting.
- Multiparty service.
- Calling Line Identification presentation/restriction.
- Closed User Group (CUG)Closed User Group (CUG)Closed User Groups are groups of GSM mobile telephone subscribers who can only make calls and receive calls from members within the group. Any other calls would be rejected...
. - 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
External links
- GSM Call Flows and Sequence Diagrams Detailed call flow diagrams describing GSM call setup, location update and handover.
- GSM Call Flows and Tutorial Detailed call flow diagrams