What is GPRS ?


GPRS (General Packet Radio Service) is a step between GSM and 3G cellular networks. GPRS offers faster data transmission via a GSM network within a range 9.6Kbits to 115Kbits. This new technology makes it possible for users to make telephone calls and transmit data at the same time. (For example, if you have a mobile phone using GPRS, you will be able to simultaneously make calls and receive e-mail massages.) The main benefits of GPRS are that it reserves radio resources only when there is data to send and it reduces reliance on traditional circuit-switched network elements.

With GPRS, an IP data transmission protocol, which is characteristic of computer networks, is being introduced to GSM. IP is a data transmission protocol which is used in Internet, the largest computer network in the world today.

Main features of GPRS

Before introduction of GPRS, the radio capacity was used for calls and data transmission within the GSM network in a rather inefficient way. For data transmission the entire channel was occupied and was thus insufficiently used. With the GPRS technology, the channel is used more efficiently owing to the possibility of more than one user sharing the same channel. GPRS telephones user several channels for data transfer thus facilitating greater transfer speeds.

The GPRS infrastructure and mobile phones support a data transmission speed of up to 13.4Kbits per channel.

GPRS signaling and data traffic do not travel through the GSM network. The GSM network is only used for table look up, in the Location Register (HLR and VLR) data bases, to obtain GPRS user profile data.

GPRS Telephones

Owing to the fact that more than one channel is used for downlink, the GPRS mobile phones make possible greater data transmission speeds. There are several types of phones with regard to the number of channels they use for data transmission...

  • Type 2+1 – two downlink channels and one uplink data transmission channel

  • Type 3+1 – three downlink channels and one uplink data transmission channel

  • Type 4+1 – four downlink channels and one uplink data transmission channel

The GPRS mobile phones can be classified into the following three classes in terms of the possibility of simultaneous calls (via GSM) and data transmission (via GPRS)...

  • Class A – Simultaneous calls (via GSM) and data transmission (via GPRS)

  • Class B – Automatic switching between the GSM and the GPRS mode is possible according to telephone settings.

  • Class C – Hand operated switching between the GSM and the GPRS mode

Data Transmission Speeds

The supported data transmission speed per channel is 13.4Kbits. Depending on the type of phone, the following data transmission speeds are theoretically possible...

  • Type 2+1: Receive 26.8Kbits and send 13.4Kbits.

  • Type 3+1: Receive 40.2Kbits and send 13.4Kbits.

  • Type 4+1: Receive 53.6Kbits and send 13.4Kbits.

The Network

In the core network, the existing MSCs are based upon circuit-switched technology, and they cannot handle the GPRS style packet traffic. Thus two new components, called GPRS Support Nodes, are added:

  • Serving GPRS Support Node (SGSN)

  • Gateway GPRS Support Node (GGSN)

The SGSN can be viewed as a "packet-switched MSC;" it delivers packets to mobile stations (MSs) within its service area. SGSNs send queries to home location registers (HLRs) to obtain profile data of GPRS subscribers. SGSNs detect new GPRS MSs in a given service area, process registration of new mobile subscribers, and keep a record of their location inside a given area. Therefore, the SGSN performs mobility management functions such as mobile subscriber attach/detach and location management. The SGSN is connected to the base-station subsystem via a Frame Relay connection to the PCU in the BSC.

GGSNs are used as interfaces to external IP networks such as the public Internet, other mobile service providers' GPRS services, or enterprise intranets. GGSNs maintain routing information that is necessary to tunnel the protocol data units (PDUs) to the SGSNs that service particular MSs. Other functions include network and subscriber screening and address mapping. One (or more) GGSNs may be provided to support multiple SGSNs. More detailed technical descriptions of the SGSN and GGSN are provided in a later section.

Security

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.

Network Protocols Used

There are several protocols used in the network equipment. These protocols operate in both the data and signalling planes. The following is a brief description of each protocol layer:

  • Sub-Network Dependent Convergence Protocol (SNDCP): the protocol that maps a network-level protocol, such as IP or X.25, to the underlying logical link control. SNDCP also provides other functions such as compression, segmentation and multiplexing of network-layer messages to a single virtual connection.

  • Logical Link Control (LLC): a data link layer protocol for GPRS which functions similar to Link Access Protocol - D (LAPD). This layer assures the reliable transfer of user data across a wireless network.

  • Base Station System GPRS Protocol (BSSGP): BSSGP processes routing and quality of service (QoS) information for the BSS. BSSGP uses the Frame Relay Q.922 core protocol as its transport mechanism.

  • GPRS Tunnel Protocol (GTP): protocol that tunnels the protocol data units through the IP backbone by adding routing information. GTP operates on top of TCP/UDP over IP.

  • GPRS Mobility Management (GMM): protocol that operates in the signalling plane of GPRS and handles mobility issues such as roaming, authentication, and selection of encryption algorithms.

  • Network Service: protocol that manages the convergence sub-layer that operates between BSSGP and the Frame Relay Q.922 Core by mapping BSSGP's service requests to the appropriate Frame Relay services.

  • BSSAP+: protocol that manages paging for voice and data connections and optimizes paging for mobile subscribers. BSSAP+ is also responsible for location and routing updates as well as mobile station alerting.