The General Packet Radio Service (GPRS) allows an end user to send and receive data in packet transfer mode within a public land mobile network (PLMN) without using a permanent connection between the mobile station (MS) and the external network during data transfer.
This way, GPRS optimizes the use of network and radio resources (RRs) since, unlike circuitswitched mode, no connection between the MS and the external network is established when there is no data flow in progress. Thus, this RR optimization makes it possible for the operator to offer more attractive fees.
The principles defined for the Global System for Mobile Communications (GSM) radio interface were kept for GPRS, since the notions of time slot, frame, multiframe, and hyperframe have not changed for GPRS as compared with GSM. The GPRS standard proposes multislot allocations for data transmission; the network may allocate up to eight time slots per time division multiple access (TDMA) frame for a given mobile on uplink and downlink.
The GPRS standard proposes four channel coding types allowing throughput per slot ranging from 9.05 Kbps to 21.4 Kbps. This allows a theoretical throughput going up to 171.2 Kbps for data transmission when eight time slots are allocated to the MS.
A strict separation has been defined between the radio and network subsystems. The rationale for this is to reuse the network subsystem with other radio access technologies such as UMTS. The GPRS network subsystem is also called the GPRS core network or the GPRS backbone network.
The GSM network nodes such as mobile switching center/visitors location register (MSC/VLR), home location register (HLR), and base station subsystem (BSS) are reused in the GPRS network architecture. Two network nodes that are required for packet transfer in the GPRS core network are listed below, as follows:
- Gateway GPRS support node (GGSN). The GGSN is a packet router that works with external packet data networks (PDNs) and is interfaced with SGSNs via an IP-based GPRS backbone network. A PDN is an external fixed data network such as an Internet network connected to the GPRS network. Packets received from an MS via the SGSN are forwarded by the GGSN to the external PDN as well as the reverse.
- Serving GPRS support node (SGSN). The SGSN, which is at the same hierarchical level as the MSC, is the GPRS node serving the MS. It manages GPRS mobility and performs the access control functions so that a user may employ the services provided by a PDN. A SGSN is interfaced with HLR by the Signaling System No. 7 (SS7) network in order to keep track of the individual MSs’ location. It also ensures the routing of packets between the GPRS backbone network and the radio subsystem.
Other equipment used on existing GSM network has evolved to support data transmission in packet-switched mode. This equipment is listed as follows:
- BSS. The BSS is enhanced in order to handle the GPRS functions over the radio interface (e.g., new packet channels). A packet control unit (PCU) has been defined in the BSS to serve the GPRS functions.
- MSC/VLR. The MSC/VLR can be enhanced to coordinate the GPRS and non-GPRS services during the paging procedure for circuit- switched calls and during GPRS and non-GPRS location update procedures. This coordination takes place only when the Gs interface between the MSC/VLR and SGSN is present.
- HLR. The HLR has been updated in order to handle GPRS subscriber information and GPRS MS location information.
New interfaces are defined between the different network elements. These interfaces are standardized to allow interoperability between network nodes that are provided by different manufacturers in one network. These interfaces are as follows:
- Gb interface. The Gb interface is located between the SGSN and the BSS. It supports both signaling and data transfer. It is used for packet transfer, cell reselection, and such.
- Gn/Gp interface. The Gn/Gp interface is defined between GPRS support nodes in the GPRS core network. It is used for the transfer of packets and signaling between the GSNs. The Gn interface is defined between two GSNs (SGSN or GGSN) within the same PLMN, whereas the Gp interface is defined between two GSNs located in different PLMNs.
- Gs interface. The Gs interface is located between the MSC/VLR and the SGSN. Through this interface an association is created between the SGSN and the MSC/VLR to coordinate MSs that are both GPRS-attached and IMSI-attached for circuit-switched paging and for combined location procedures.
- Gr interface. The Gr interface between the SGSN and the HLR is used to retrieve or update the GPRS subscriber profile and location during GPRS mobility management (GMM) procedures.
- Gf interface. The Gf interface between the SGSN and the equipment identity register (EIR) allows verification of the terminal’s identity.
- Gc interface. The Gc interface is defined between the GGSN and the HLR. It is used to retrieve routing information needed to forward incoming packets from the PDN to the SGSN serving the mobile for which it is intended.
- Gi interface. The Gi interface is located between the GGSN and the external PDN. The protocols that are involved in this interface are dependent on the external PDN. The Internet Protocol (IP) is supported by this interface, but the Point-to-Point (PTP) Protocol may also be supported.