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One of the important ideas of the GPRS network is that the added functionality of it does not interfere existing circuit-switched GSM services. The new packet data network is added as another layer in such a way that it is possible to reuse the existing infrastructure. The basis of GPRS architecture is the 2G GSM system which can be seen in figure 1:
Figure 1 – The GSM system architecture In figure 1 we can see the system architecture of a GSM public land mobile network (PLMN) including its main components. MS stands for a GSM mobile station. The radio area coverage of a base transceiver station (BTS) is what forms a cell. One base station controller (BSC) controls several BTSs together. A base station subsystem (BSS) is formed by The BTS and BSC. A switch called the mobile switching center (MSC) is responsible for the routing of the mobile stations' traffic in their respective cells. A dedicated gateway mobile switching center (GMSC) handles the connections that originate from and terminate in a fixed network (e.g. ISDN). GSM networks are structured hierarchically: They consist of one or more administrative regions, which are assigned to a MSC. The administrative regions are made up of one or more location areas (LA). Each location area consists of some cell groups each of which is assigned to a BSC. Call control and network management are available through some databases: the home location register (HLR), the visited location register (VLR), the authentication center (AUC), and the equipment identity register (EIR). For users who are registered with a network operator, permanent data (such as the user's profile) and temporary data as well (such as the user's current location) are stored in the HLR. In case of a call to a user, the first step is to query the HLR in order to determine the user's current location. VLRs are responsible for groups of location areas and for storing the data of those users who are currently in their areas of responsibility. In this data we can include parts of the permanent user data that have been transmitted from the HLR to the VLR for faster access. But the VLR might also assign and store local data such as a temporary identification. The AUC stores and generates security-related data such as keys used for authentication and encryption, whereas the EIR registers equipment data and not subscriber data.
Implementing GPRS along with the existing GSM architecture One of the main interests behind the standardization of GPRS is to make the transition as simple and cheaper as possible. We would like to modify the base stations’ hardware as little as possible. First of all, the base stations are the most expensive equipment in which the operators have invested, and it's not an option at all to replace them. Second, the base stations and their antennas enable the coverage which is so necessary to a cellular network; thus, they are usually located on high hills and on roofs and that makes it difficult and expensive to perform on site changes. A third aspect is that the cell sites are often rented from real estate owners. The owners sometimes lease the site to different and often competing operators. Therefore, the hardware must be compatible with all operators' technologies and that’s why GPRS can be introduced mainly as a software upgrade to existing stations, which often can be done remotely from a central maintenance point. This software enables voice and data users to share the base stations resource and the same air interface.
Figure 2: The integration of GPRS elements (at the bottom) with GSM elements All rights reserved www.rad.co.il In GSM, the interface is standardized to help connectivity between multiple base stations and a BSC. This interface can remain unchanged when GPRS is introduced in order to make the transition as smooth as possible. The data being transferred consists of both GPRS packet data and GSM data because these components share the same air interface. In order to achieve efficient packet data usage, different core networks are required: 1) The existing GSM core network for circuit-switched data. 2) New GPRS core network for packet data. This concept can be seen in figure 2 above. The BSC must divide the different data flows and direct them to the right network. This additional functionality requires new hardware at the BSC: the Packet Control Unit (PCU). The PCU separates packet data and circuit-switched data when it is received from the MS and multiplexes the different data streams from circuit-switched and packet-switched core networks into regular streams that go down to the cells. The PCU is an autonomous unit and could be physically separated from the BSC. The BSC also needs a software upgrade for GPRS in order to handle the new logical packet data. Therefore, most of the new functionalities added to the GPRS air interface are implemented in the BSC. One BSC is connected to several base stations, one MSC and one Serving GPRS Support Node (SGSN).
The GPRS core network consists of two main components (nodes). These two components are integrated in the existing GSM network: the Serving GPRS Support Node (SGSN) and the Gateway GPRS Network Node (GGSN), which together are called the GSN nodes. The integration of the GPRS core network and an existing GSM network can be seen in figure 3:
Figure 3: The GPRS core components integrated with GSM components All rights reserved www.rad.co.il To connect these new nodes to the radio network, a new open interface should be used. This new interface is called Gb. Gb is a high-speed Frame-Relay link that is based on E1 or T1 connections. The connection between different GSN nodes and other components of the core network is called the GPRS backbone (can be seen in figure 3). The backbone is a regular IP network that has access routers, firewalls and so on. The backbone can also be used to connect to other GPRS systems. Usually, the backbone is also connected to the operator's billing system via a billing gateway. The SGSN is the main component which enables the mobility of GPRS users. When connected to a GPRS network, The MS has a logical connection to its SGSN through which it can perform delivery between different cells without any change in the logical connection. The SGSN keeps track of which BSC to use when sending packets to a MS originating from outside networks. Its functionality is similar to a regular IP router, but it has an added functionality for handling mobile network issues (e.g. authentication of users, distribution of IP addresses, ciphering etc.). If the user moves from one SGSN service area to another SGSN service area, an inter-SGSN delivery can be performed. Most of the time, the user won't notice the delivery although the packets that were currently buffered in the old SGSN might be discarded and re-sent by using higher layers. The characteristics of a radio link are very different from those of a fixed link and bits over the air are more likely to be lost and as a result some additional functionality is needed. When a MS is connected to a site on the internet, for example, the majority of data loss occurs over the wireless link, and handling that with higher level protocols such as TCP would be wasteful. It is preferred, in our case, to have a quick retransmission protocol that only covers the wireless part and hides the loss from TCP, enabling it to fulfill its original task. For that goal we have the RLC protocol which operates within the MS and the base station and resends data that was lost over the air. The Logical Link Control (LLC) protocol which is located between the MS and the SGSN can be configured to perform similar functionality. The GGSN is actually a combined gateway, a firewall and an IP router. The GGSN handles interfaces to external IP networks, internet service providers (ISPs), routers, and other close nodes. From the external networks point of view, the GGSN appears as a gateway that can route packets to the users within its domain. The GGSN keeps track of the SGSN to which a specified MS is connected and forwards packets to it. The SGSN and GGSN can either be located together in a compact GSN (CGSN) solution or placed far from each other and connected via the backbone (as can be seen in figure 3). The backbone can be shared with other operators. Thus, the GPRS Tunneling Protocol (GTP) is used for management (will be elaborated later on). Packets that travel over the GPRS backbone have a stack with IP and TCP at two levels (as detailed in the protocol stack section). This is inefficient, but it makes communication secure and easier to implement.
The new interfaces The GPRS backbone will enable point-to-point calls, inter-working with the BSS, HLR, MSC, GMSC and the internet. New interfaces have been developed for GPRS. These interfaces are labeled with Gx in their names where x stands for a variety of interfaces as can be seen in figure 4.
Figure 4: GPRS new interfaces All rights reserved www.rad.co.il The Gn and Gp interfaces are defined between two SGSNs. This enables the SGSNs to exchange user profiles when a mobile station moves from one SGSN area to another. The Gi interface connects the PLMN with external private or public PDNs, such as the Internet or corporate intranets. There is a support for interfaces to IP (IPv4 and IPv6) and X.25 networks. The HLR stores the current SGSN address, the user profile and the PDP address for each GPRS user in the PLMN. The Gr interface is used to exchange this information between SGSN and HLR. For example, the SGSN informs the HLR about the current location of the MS. When the MS registers with a new SGSN, the HLR sends the user profile to the new SGSN. The signaling path between GGSN and HLR (Gc interface) might be used by the GGSN to query a user's location and profile in order to update its location register. Furthermore, the MSC/VLR may be extended with functions that allow efficient coordination between packet switched (GPRS) and circuit switched (conventional GSM) services. Paging requests of circuit switched GSM calls can be performed via the SGSN. For this purpose, the Gs interface connects the data bases of SGSN and MSC/VLR. To exchange SMS messages via GPRS, the Gd interface is used. It interconnects the SGSN with the SMS gateway MSC (SMS-GMSC).
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Last updated: 02-06-2003.
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