GPRS Architecture

Two new components have been added to the GSM network architecture for GPRS: serving GPRS support node (SGSN) and gateway GPRS support node (GGSN). Like MSCs and BSCs, a GSM network generally has many SGSNs and GGSNs, which constitute a GPRS core network. All other existing GSM components are also involved. Additionally, to support GPRS, the mobile station has to incorporate the GPRS terminal functionality. Ideally, for a BTS, only a software upgrade is necessary to enable voice and data communication via the same radio interface; and it can be done remotely. This way the large amount of BTS hardware and the number of antennas do not have to be changed.

On the BSC, a packet control unit (PCU) device is added to deal with data packets from BTSs. It has two functions: (1) separate packet-switched traffi c from circuit-switched traffic originating from mobile stations, and (2) multiplex circuit-switched traffic from the GSM core network and packet-switched traffic from the GPRS core network into an intermingled data frame to serving cells. Packet-switched traffic from a PCU on an MSC is sent to the corresponding SGSN, which keeps track of all serving mobile stations. GGSNs are directly connected to packet data networks (PDNs) such as X.25 or the Internet. To the external networks, a GGSN appears to be an IP router that takes responsibility for converting and forwarding data packets to destination mobile stations. It achieves this by performing translation between external network node addresses and GPRS mobile addresses and forwarding data to the SGSN responsible for the mobile station in question according to a GPRS tunneling protocol (GTP). Because GPRS data transmission is always initiated from a mobile station, for those mobile stations currently engaged in GPRS data transmission, a GGSN is able to maintain a list of records that map these mobile stations to their serving SGSNs. In reality, IP dominates both the external PDNs and the GPRS core network. GPRS supports both IPv4 and IPv6. The GTP is an IP-based protocol with Transmission Control Protocol (TCP) and User Datagram Protocol (UDP) as transport. As a consequence, the address translation conducted on a GGSN is actually IP network address translation (NAT) that allows mobile stations to use private IP addresses within the GPRS core network. For all mobile stations known to a GGSN, upon leaving the GPRS core network their private IP addresses will be translated into globally unique, routable IP addresses. The GPRS core network may also implement Dynamic Host Configuration Protocol (DHCP) servers and Domain Name System (DNS) servers for dynamic IP assignment and domain name mapping, respectively. The GGSN also performs authentication and collects traffic volume of each subscriber for billing.

Source of Information :  Elsevier Wireless Networking Complete 2010


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