Info about Phone Service
Architecture of GPRS
Most existing 2G cellular systems are circuit-switched services in which a dedicated connection must be established in order for communications to take place. As long as the session lasts, the subscriber owns the channel and is billed for its usage even if no data is being sent. By contrast, packet-switching is a more efficient technology because it allows several users to share a channel. No actual connection needs to be established in order to send data-from the point of view of the subscriber the system is "always on" and ready to transmit. And instead of billing for connection time, subscribers are billed according to the amount of data they transmit and receive instead.
GPRS uses the same underlying TDMA time slot architecture as GSM, but instead of assigning only a single time slot to each user for transmission or reception, it can multiplex up to eight slots (the maximum for a TDMA frame) to give a maximum possible data transmission rate of 171 Kbps. In most implementations, however, three or four slots can are multiplexed for downstream transmission and only one for upstream, which makes practical transmission speeds much slower (but still better than GSM alone).
When a GPRS user wants to send data, the handset finds the first available time slot and sends the first chunk of data, sending more chunks as further slots become available. In this respect, GPRS operates like an Ethernet network-the more users sharing the system, the less bandwidth available for each user to transmit or receive data. Collisions do not occur, however, since time-division multiplexing (TDM) prevents contention from occurring.
GPRS was designed to transport both IP and X.25 packet data, but all current implementations use IP to provide connectivity with the Internet. To provide this connectivity, a GPRS mobile phone needs an IP address, which can be either statically or dynamically assigned. All existing application-layer Internet protocols can run over GPRS, including Hypertext Transfer Protocol (HTTP), File Transfer Protocol (FTP), Simple Mail Transfer Protocol (SMTP), and so on. Most GPRS systems use WAP as their application layer protocol, however, because it is more efficient for devices with limited memory and small displays.