What is GPRS
GPRS is a technology of a cordless transfer of data, used in mobile communications, based on a package method. The main fundamental assumption of GPRS is a division of transmitted data into separate packages, instead of a used so far method of continuous transmission. This fact results in the following advantages both for a final user and an operator:
- Increase of the maximum connection bandwidth up to about 80 kb/s.
- Reducing the costs of using the connection working on the GPRS – data transmitted in the form of packages cause no necessity for maintaining constantly an open connection, regardless of the amount of data waiting for transmission, or their lack. The connection is made only when a subsequent package of data to transmit occurs. After the end of transmission the connection is stopped. The solution of this kind makes it possible to calculate payment for the amount of transmitted data, not for the time of connection duration.
- Reducing the requirements concerning the bandwidth of mobile communications networks. From the point of view of an operator, a person using network communications does not occupy all the time the connection of available band, but vacates it after sending or receiving another package. Vacated band can be this time allotted next user who waits for data transmission.
- The above-mentioned two features carry with themselves a subsequent advantage – the final user can all the time use the services based on communication which uses the GPRS technology, bearing slender costs. At the same time there is neither necessity for maintaining constant connection or nor for much comfortable stopping, and making new connections.
- The possibility of taking advantage of the already existing technical infrastructure of the GSM network, without the necessity for modernizing or creating a new network from the beginning.
From the technical point of view the functioning of package transmission is similar to the functioning of the computer LAN network which uses the access protocol to the CSMA/CD medium. When it comes to GPRS, radio waves, sending and receiving by a base station (BTS), are the transmission medium. Each of the stations supports several main frequencies. There are eight so-called time slots defined on every frequency. These are the periods of time enabling the transmission of a single data block. For the needs of the GPRS it is possible to allot simultaneously up to four slots when the communication between the base station and the terminal (telephone) is needed. Each of the time slots makes a separate communication channel of a strictly defined bandwidth conditioned by a negotiated channel coding algorithm. There are four, already mentioned algorithms which are used: CS1 of a bandwidth up to 9.05 kb/s, CS2 – 13.4 kb/s, CS3 – 15.6 kb/s and CS4 – 21.4 kb/s. The maximum total bandwidth of the line depends on the weakest applied encryption algorithm, and comes to its quadruple.
After connecting to network and making a connection, the telephone receives its unique identifier TFI (Temporary Flow Identity) which is used both for signing own packages and for comparing the signatures of packages sent by BTS. Conformity of identifiers means a package allotted for a particular device. If the device is going to send its own data, firstly they have to be divided into packages. Every package receives a token determining its order, thanks to which the receiving device will be able to reconstruct data. A transmitter sends the data in the moment of a free time slot’s appearance. This solution makes it easier to organize communication because there is no necessity here to negotiate each time the applied low-level transmission protocols, modem or addressing settings. Moreover, the communication channel is open only during transmission time vacating it only for other devices in idle time-out. A potential conflict between two or more devices will not cause a data loss but can only affect the efficient bandwidth of the line.
On the basis of the GPRS technology it is possible to create the transmission medium which enables connections using such protocols as TCP/IP, PPP or X.25. The TCP/IP protocol is used particularly often, thanks to which users can gain the access to the Internet or company networks, take advantage of such services as video-conference, access to a fax machine or e-mail, communicators, etc. The PPP protocol is usually used for the modem emulation by a cell phone. This function allows connecting the phone to the computer, and enables tunneling IP protocol in GSM network, providing the user with an access to the Internet from the computer. Another, less often used possibility, available thanks to the PPP protocol is taking advantage of such technical devices as cash dispensers, transcribers, gauges, etc. which use exactly this protocol. Using the X.25 protocol is practically zero due to taking over almost its whole functionality by the TCP/IP protocol, and partly – by the PPP protocol.
The GPRS technology underlies a solution called EDGE which introduces clearly higher speeds of data transmission, and streamlines some solutions used so far.