GSM / GPRS
GSM
Global System for Mobile Communications
earlier: Groupe Spéziale Mobile
GSM is an open system for mobile communication. One of its greatest strengths is its international expansion with uniform standardized connections in more than 200 countries.
GSM supports data and voice communication, which can be used worldwide with one standardized international telephone number (roaming).
The four frequencies 850 MHz, 900 MHz, 1800 MHz and 1900 MHz are used worldwide. Modern termination devices use the Quadband technology and can be thus used in all frequencies.
One city or one area has several base stations that use one frequency channel each. The mobile phone or the modem select the best receivable radio cell of the network and log in to the corresponding base station. If the location is not changed during conversation or data connection, the connection mostly stays on this same frequency channel.
Besides, the base station also transmits a table of alternative frequencies to the mobile phone or the modem; these frequencies are continuously scanned and one can also switch over to them if required.
A SIM card is required to identify the device. Various versions are available, depending on the application and the provider:
Prepaid card: Outgoing data connections, SMS and call
talk card: Outgoing data connections, SMS and call
data card: Incoming and outgoing data connections and SMS
Most of the network operators (providers) also provide combined talk/data contracts.
In such a case, two separate telephone numbers are provided on one SIM card for data and talk.
An additional GPRS option is mostly available on all cards.
A line (circuit switched data) GSM data connection is established by dialing a telephone number. The provider charges for data connections up to 14400 bps as per the connection period.
GPRS
(General Packet Radio Service)
GPRS is an additional service to exchange data in a packet through mobile GSM telephone network. In this technology, data is divided into small packets which are transferred separately and put together again at the receiver end. This enables optimum use of the entire serviceable bandwidth in the mobile radio cell.
In theory, up to eight time slots can be used, which would enable a maximum attainable transmission rate of 171,200 bps. In practice, a maximum of four time slots can be used in downlink and a maximum of two time slots can be used in uplink. The thus attainable data rate is up to 50,000 bps, depending on the keying used and on the number of allocated time slots depending on the network capacity.
The provider charges for the GPRS connections as per the data volume and not as per the connection time.
GPRS modems can be continuously connected so that a previous connection establishment is not required and data can be exchanged immediately.
GPRS is TCP/IP-based, which means that there is no need to dial a telephone number to establish the connection. Notifying the radio cell is sufficient to be able to use the service. Each mobile wireless device has a separate IP address in this case. This address is often dynamic, which means that the provider changes it every 24 hours. Some providers, such as Vodafone or T-Mobile provide special solutions for permanent IP addresses.