Telecom ABC - G
The General Packet Radio Service (GPRS) is a technology for the support of packet switching traffic in a GSM network. GPRS enables high-speed wireless Internet and other data communications in GSM. The data speed of GPRS is more than four times greater speed than conventional GSM systems. Using a packet data service, subscribers are always connected and always on line so services will be easy and quick to access.
In GPRS, the GSM time slots are dynamically allocated to various users according to their instantaneous demand. Users can alternately transmit data in the same slot. All data is packetized and send instantaneously through available resources. This gives the user the experience of being permanently linked to the network.
In GSM the maximum data rate is 9.6 kbps per time slot. In GPRS the data is packetized which gives in principle an even lower data rate of 9.05 kbps of which 8 kbps is available for the user. However, in GPRS there are two technologies introduced to increase this data rate. Firstly, the error correction that is used can be adapted to the quality of the radio channel. Secondly, it is possible to use more than one time slot. In theory all 8 time slots can be used.
In GPRS there are four different channel codes introduced. These are given in the table below.
In theory GPRS will allow a maximum of 160 kbps netto. In practice, typical values are currently slightly over 40 kbit/s.
New network elements
GPRS requires the introducton of two new network elements in the GSM network:
The SGSN handles all packet switched data within the network and is respnosible for the authentication and tracking of the users. The SGSN performs the same functions as the MSC for voice traffic. The SGSN and the MSC are often co-located.
The GGSN is the interface from the GSM/GPRS network to external networks. The GGSN is also responsible for the allocation of IP-addresses.
Mobile terminal classes
In GPRS there are three different classes of mobile terminals defined: Class A: Capable of GPRS and circuit switched services (voice) simultaneously; Class B: Can operate on only GPRS services or voice service at a time. However it is capable of listening to incoming calls of both GPRS and voice; Claas C: Only capable of GPRS or circuit switched services. The terminal is either capable of only voice or GPRS or it can only listen to one of these services when it is on.
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