General Packet Radio Service (GPRS)

Lecturer: Dr. Hannu H. Kari, Helsinki University of Technology

Overview of GPRS service:

General Packet Radio Service (GPRS) is a new service designed for digital cellular networks (GSM, DCS, PCS). It utilises a packet radio principle and can be used for carrying end userís packet data protocol (such as IP and X.25) information from/to a GPRS terminals to/from other GPRS terminals and/or external packet data networks. GPRS is standardised in ETSI (European Telecommunications Standards Institute).

GPRS uses a packet-mode technique to transfer high-speed and low-speed data and signalling in an efficient manner over GSM radio networks. GPRS optimises the use of network resources and radio resources. Strict separation between the radio subsystem and network subsystem is maintained, allowing the network subsystem to be reused with other radio access technologies. GPRS does not mandate changes to an installed MSC base.

New GPRS radio channels are defined, and the allocation of these channels is flexible: from 1 to 8 radio interface timeslots can be allocated per TDMA frame, timeslots are shared by the active users, and up and downlink are allocated separately. The radio interface resources can be shared dynamically between speech and data services as a function of service load and operator preference. Various radio channel coding schemes are specified to allow bitrates from 9 to more than 150 kbit/s per user.

Applications based on standard data protocols are supported, and interworking is defined with IP networks and X.25 networks. Specific point-to-point and point-to-multipoint services are supported for applications such as traffic telematics and UIC train control. GPRS allows SMS transfer over GPRS radio channels.

GPRS is designed to support from intermittent and bursty data transfers through to occasional transmission of large volumes of data. Four different quality of service levels are supported. GPRS is designed for fast reservation to begin transmission of packets, typically 0,5 to 1 second. Charging will typically be based on the amount of data transferred.

Three classes of GPRS MSs are supported: A class-A MS can operate GPRS and other GSM services simultaneously. A class-B MS can monitor control channels for GPRS and other GSM services simultaneously, but can only operate one set of services at one time. A class-C GPRS MS can exclusively operate GPRS services.

GPRS introduces two new network nodes in the GSM PLMN: The Serving GPRS Support Node (SGSN), which is at the same hierarchical level as the MSC, keeps track of the individual MSs' location and performs security functions and access control. The SGSN is connected to the base station system with Frame Relay. The Gateway GSN (GGSN) provides interworking with external packet-switched networks, and is connected with SGSNs via an IP-based GPRS backbone network. The HLR is enhanced with GPRS subscriber information, and the SMS-MSCs are upgraded to support SMS transmission via the SGSN. Optionally, the MSC/VLR can be enhanced for more-efficient co-ordination of GPRS and non-GPRS services and functionality: e.g., paging for circuit-switched calls which can be performed more efficiently via the SGSN, and combined GPRS and non-GPRS location updates.

GPRS security functionality is equivalent to the existing GSM security. The SGSN performs authentication and cipher setting procedures based on the same algorithms, keys, and criteria as in existing GSM. GPRS uses a ciphering algorithm optimised for packet data transmission.

Cell selection may be performed autonomously by a MS, or the base station system instructs the MS to select a certain cell. The MS informs the network when it reselects another cell or group of cells known as a routeing area.

In order to access the GPRS services, a MS shall first make its presence known to the network by performing a GPRS attach. This operation establishes a logical link between the MS and the SGSN, and makes the MS available for SMS over GPRS, paging via SGSN, and notification of incoming GPRS data.

In order to send and receive GPRS data, the MS shall activate the packet data address that it wants to use. This operation makes the MS known in the corresponding GGSN, and interworking with external data networks can commence.

User data is transferred transparently between the MS and the external data networks with a method known as encapsulation and tunnelling: data packets are equipped with GPRS-specific protocol information and transferred between the MS and GGSN. This transparent transfer method lessens the requirement for the GPRS PLMN to interpret external data protocols, and it enables easy introduction of additional interworking protocols in the future. User data can be compressed and protected with retransmission protocols for efficiency and reliability.