The GSM standards are defined by the 3GPP collaboration and implemented in hardware and software by equipment manufacturers and mobile phone operators.
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.
The key feature of a mobile phone is the ability to receive and make calls in any area where coverage is available.
So the first job of the Gateway MSC is to determine the current location of the mobile phone in order to connect the call.
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.
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.
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.
Mobile networks in Europe, Asia (except Hong Kong, Macau (Macao) and Singapore), Australia, and Argentina only charge their subscribers for outgoing calls.
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.
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).
In addition to general Internet access, other special services may be provided by the mobile phone operator, such as SMS.
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.
The General Packet Radio Service (GPRS) is a packet-switched data transmission protocol, which was incorporated into the GSM standard in 1997.
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.
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.