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Overview

GPRS provides a service from the data network end to the GPRS MS one. The GPRS protocol stack is depicted in Figures 1 and 2. The physical radio interface consists of an adaptable number of TDMA slots and provides a theoretical raw data transmition rate of 171 kbps. A Media Access Control (MAC) uses the resources of the physical radio interface and provides a service to the GPRS Logical Link Control (LLC) protocol between the MS and the serving GSN (SGSN). LLC is a modification of a High-Level Data Link Control (HDLC)-based Radio Link Protocol (RLP) with variable frame size. The two most important features offered by the LLC are the support of point-to-multipoint addressing and the control of data frame retransmission. From the standpoint of the application, GPRS provides a standard interface for the network layer.

Figure 1: GPRS layering according to the OSI model

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Figure 1 illustrates a global view of GPRS layers according to the OSI model. Nevertheless, our analysis of the GPRS protocol stack is from a different point of view, hence not all the units in the figure above are in the scope this analysis.

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The protocol architecture of the transmission Plane

Figure 2 – The GPRS protocol stack

Fig. 2 depicts the protocol architecture and layering of the GPRS transmission plane, providing transmission of user data and its associated signaling (i.e. - for flow control, error detection, and error correction)

GPRS backbone: SGSN­ and GGSN

 As mentioned before, user data packets are encapsulated within the GPRS backbone network. The GPRS Tunneling Protocol (GTP) tunnels the user data packets and related signaling information between the GPRS support-nodes (GSNs). The tunneling protocol is defined both between GSNs within one PLMN (Gn interface) and between GSNs of different PLMNs (Gp interface). In the transmission plane, the tunneling protocol employs a tunnel mechanism to transfer user data packets (that’s where it got its name from). In the signaling plane, the tunneling protocol specifies a tunnel control and management protocol. The signaling is used to maintain (i.e. -  create, modify, and delete) tunnels.

Below GTP (in the protocol stack), the standard protocols TCP or UDP are employed to transport the GTP packets within the backbone network. X.25 expects a reliable data link, thus TCP is used. UDP is used for access to IP-based packet data networks, which don’t require reliability in the network layer or below. IP is used in the network layer to route packets through the backbone. Ethernet, ISDN, or ATM-based protocols may be used below IP.

To sum up, in the GPRS backbone we have an IP/X.25-over-GTP-over-UDP/TCP-over-IP transport architecture.

Subnetwork Dependent Convergence Protocol

The Subnetwork Dependent Convergence Protocol (SNDCP) is used for data packets transfer between SGSN and MS. Its functionality includes:

Data Link Layer

The data link layer between the MS and the network is divided into two sub-layers: the LLC layer (between MS-SGSN) and the RLC/MAC layer (between MS-BSS).

The logical link control (LLC) layer provides a highly reliable logical link between an MS and its designated SGSN. Its functionality is based upon the HDLC protocol and includes: sequence control, in-order delivery, flow control, transmission errors detection, and retransmission. The data confidentiality is ensured by ciphering functions.

The RLC/MAC layer at the air interface includes two functions. The main purpose of the radio link control (RLC) layer is to establish a reliable link between the MS and the BSS. The Medium Access Control (MAC) layer controls the access attempts of an MS on the radio channel shared by several MSs. It employs multiuser multiplexing and scheduling and prioritizing based on the negotiated QoS. The GPRS MAC protocol is based on the principle of slotted Aloha.

Physical Layer                     

The physical layer between MS and BSS is divided into the two sublayers: the physical link layer (PLL) and the physical RF Layer (RFL).

The PLL provides a physical channel between the MS and the BSS. Its tasks include channel coding (transmission errors detection, forward error correction (FEC), indication of uncorrectable code words), interleaving, and detection of physical link congestion.

The RFL operates below the PLL. Among other things, it includes modulation and demodulation.

BSS­ SGSN Interface

The BSS GPRS Application Protocol (BSSGP) delivers routing and QoS-related information between BSS and SGSN. The underlying Network Service (NS) protocol is based on the Frame Relay protocol.

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Inter-working with IP Networks

We will now show how a GPRS based network can be interconnected with an IP-based (packet data) network, such as the Internet or corporate intranets. GPRS supports both IPv4 and IPv6. The Gi interface is the inter-working point with IP networks. From outside, i.e., from an external IP network's point of view, the GPRS network looks like any other IP subnetwork, and the GGSN looks like a usual IP router. Fig. 3 depicts the protocol stacks at the GGSN.

Figure 3 – The GGSN protocol stack

Fig. 4 shows an example of how a GPRS network might be connected to the Internet. Each registered user that wants to exchange data packets with the IP network gets an IP address. The IP address is taken from the address space of the GPRS operator (much like the way ISPs work today). In order to support a large number of mobile users, it is essential to use dynamic IP address allocation (in IPv4). Thus, a DHCP server (Dynamic Host Configuration Protocol) is installed. The address resolution between IP address and GSM address is performed by the GGSN, using the suitable PDP context.

Figure 4 – A connection between GPRS and an external network

Moreover, a domain name server (DNS) managed by the GPRS operator or the external IP network operator can be used to map between external IP addresses and host names. To protect the PLMN from unauthorized access, a firewall is installed between the private GPRS network and the external IP network.

With this configuration, GPRS can be seen as a wireless extension of the Internet all the way to a mobile station or mobile computer. The mobile user has direct connection to the Internet

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