Chapter 3: Overview of GPRS

Chapter 3: Overview of GPRS


GPRS represents an evolution of the GSM standard, allowing data transmission in packet mode and providing higher throughputs as compared with the circuit-switched mode. This evolution is usually presented under the designation of 2.5G to point out that it is a transition technology between 2G and 3G.

The GPRS network architecture reuses the GSM network nodes such as MSC/VLR, HLR, and BSS. New network nodes have been introduced for the transport of packet data. These nodes are the gateway GPRS support nodes (GGSN) and serving GPRS support nodes (SGSN). The subnetwork formed by the SGSNs and the GGSNs is called the GPRS core network. In order to reuse the GSM nodes, new interfaces have been defined between the GSM network nodes and the different elements of the GPRS core network. The GPRS logical architecture is described in Section 3.1.

The protocol layer has been split into two planes. On one side there is the transmission plane, which is mainly used for the transfer of user data. The signaling plane is used for the control and support of the transmission plane functions. Section 3.2 deals with the transmission and signaling planes.

GPRS has kept such main principles of the GSM radio interface as the notions of time slot, frame, multiframe, and hyperframe structures. It was indeed chosen by the operators and manufacturers involved in the system design to provide high-data-rate packet-switched services with minimized impacts on the GSM standard. The principles of the physical layer are given in Section 3.3.1. The details related to the physical layer are presented in Chapter 4.

One of the main GPRS characteristics is that a physical connection is established in uplink only when the MS needs to send continuous data to the network, and in downlink only when the network needs to send continuous data to the MS. This physical connection is released in one direction as soon as the sending entity has no more data to send. Different allocation schemes for radio resource (RR) management have been defined in order to multiplex several MSs on the same physical channel. An overview of the principles related to RR management is presented in Section 3.3.2. A complete description of RR management can be found in Chapter 5.

A logical entity called PCU has been introduced within the BSS to manage the GPRS functions over the radio interface. Section 3.4 deals with the BSS architecture and discusses the several possible locations of the PCU.

In a GPRS network, an RA identifies one or several cells. As soon as an MS enters a new RA, the network must be notified of this change in order to update its location. The SGSN is in charge of GPRS mobility management (GMM). An overview of GPRS mobility is proposed in Section 3.5. The details related to GMM are given in Chapter 7.

If data transmission in packet mode does not require the establishment of an end-to-end connection, it is necessary to establish a context between the mobile and the network in order to exchange packets. This context allows the network to identify the IP address of the MS, identify the access point with the external network, and define the QoS associated with data transmission in packet mode. The concept of PDP context is explained in Section 3.6. The details related to PDP context management are given in Chapter 7.

The BSS and the GPRS backbone network are connected via the Gb interface in order to exchange user data and signaling information. The principles of the Gb interface are given in Section 3.8. The details related to the Gb interface are given in Chapter 6.

When a context is established between the MS and the network, IP packet exchange may start at any time between the mobile and the network without establishing a connection beforehand. The packets are conveyed in the GPRS backbone network. An overview of the general architecture of the GPRS backbone network is presented in Section 3.9. A complete description of the user plane between the MS and external data packet network is given in Chapter 8.