Services provided by GSM
From the beginning, the planners of GSM wanted ISDN compatibility in terms of the services offered and the control signalling used. However, radio transmission limitations, in terms of bandwidth and cost, do not allow the standard ISDN B-channel bit rate of 64 kbps to be practically achieved.
Isn't this a shame? What many wireless customers need most is a high speed data connection and this is what GSM provides least. Only 9.6kbs if everything works right. It is possible the GSM designers in the early 1980s never envisioned the need for such bandwidth. It may be true, too, that in most countries the radio spectrum needed to give every caller a 64kbs channel was never available. The add on technology EDGE (external link) promises higher data speed rates in the near to mid-term for GSM. Highest data rates will come in the long term when GSM changes into a radio service based on wide band code division multiple access, and not TDMA.
Using the ITU-T definitions (external link), telecommunication services can be divided into bearer services, teleservices, and supplementary services. The most basic teleservice supported by GSM is telephony. As with all other communications, speech is digitally encoded and transmitted through the GSM network as a digital stream. There is also an emergency service, where the nearest emergency-service provider is notified by dialing three digits (similar to 911).
* Bearer services: Typically data transmission instead of voice. Fax and SMS are examples.
* Teleservices: Voice oriented traffic.
* Supplementary services: Call forwarding, caller ID, call waiting and the like.
A variety of data services is offered. GSM users can send and receive data, at rates up to 9600 bps, to users on POTS (Plain Old Telephone Service), ISDN, Packet Switched Public Data Networks, and Circuit Switched Public Data Networks using a variety of access methods and protocols, such as X.25 or X.32. Since GSM is a digital network, a modem is not required between the user and GSM network, although an audio modem is required inside the GSM network to interwork with POTS.
GSM is an all digital network but many machines are still analog, as is most of the local loop. Thus, we need a modem, even though we are dealing with digital.
A FAX machine's digital signal processor converts an analog image into an instantaneous digital representation; a series of bits, all 0s and 1s. A modulator then turns these bits into audio tones representing the digital values. An analog FAX machine at the other end converts the tones received back into digital bits and then into an image.
This tedious process was required initially because local loops were and are primarily analog. In addition, digital services such as T1, fractional T1, or ISDN, where available, was and is extremely expensive. All digital equipment, such as Group 4 Fax machines, are far higher priced than their analog counterparts. The local loop will remain primarily analog for some time.
Other data services include Group 3 facsimile, as described in ITU-T recommendation T.30, which is supported by use of an appropriate fax adaptor. A unique feature of GSM, not found in older analog systems, is the Short Message Service (SMS). SMS is a bidirectional service for short alphanumeric (up to 160 bytes) messages. Messages are transported in a store-and-forward fashion. For point-to-point SMS, a message can be sent to another subscriber to the service, and an acknowledgement of receipt is provided to the sender. SMS can also be used in a cell-broadcast mode, for sending messages such as traffic updates or news updates. Messages can also be stored in the SIM card for later retrieval [2].
Supplementary services are provided on top of teleservices or bearer services. In the current (Phase I) specifications, they include several forms of call forward (such as call forwarding when the mobile subscriber is unreachable by the network), and call barring of outgoing or incoming calls, for example when roaming in another country. Many additional supplementary services will be provided in the Phase 2 specifications, such as caller identification, call waiting, multi-party conversations.
Excellent IEC tutorial on SMS is here: http://www.iec.org/online/tutorials/wire_sms/ (external link)