System Overview

Terminal Equipment (TE)
The TE is the computer terminal that the end user uses. This is the component used for the GPRS system to transmit and receive end user packet data. For example, the TE could be a laptop computer. The GPRS system provides for IP connectivity between the TE and an Internet Service Provider (ISP), or a Corporate Local Area Network (LAN) connected to the GPRS system. From the users point of view the MT could be compared to a conventional modem.


Mobile Terminal (MT)
The MT communicates with a TE, and over the air with the BTS, the MT must be equipped with software for GPRS functionality when used in conjunction with the GPRS system. The MT is associated with a subscriber in the GSM system, the MT established SGSN. Channel reselection is provided at the radio link between the MT and the SGSN, the IP connection is static from the TE point of view, that is, the TE does not know it is mobile and retains its assigned IP address until the MT detached.


Base Station System (BSS)
The BSS consists of a Base Station Controller (BSC) and a Base Transceiver Station (BTS). The BTS is the radio equipment, that transmits and receives information over the air to let the BSC communicate with MSs in the BSCs service area. A group of BTSs is controlled by the BSC, however for GPRS to work on the BTS it must have the GPRS specific software.

The BSC provides all the radio related functions. The BSC can set up, supervise and disconnect circuit switched and packet switched calls, it has a high capacity switch, this provides function such as handover decisions, cell configuration data and channel assignment. The BSC must also be equipped with both the GPRS hardware and software when used for GPRS, one or several BSCs are served by an MSC, and a number of BSCs are served by an SGSN.
The BTS separates the MS originated circuit switched calls from the packet switched data communications, before the BSC forwards a circuit switched calls to the MSC/VLR, and packet switched data to the SGSN.
The standard GSM protocols are used with the BSC to achieve the desired compatibility.


Mobile Services Switching Center (MSC)
The MSC performs the telephony switching functions of the GSM circuit switched system, like the SGSN switches the GSM packet switched traffic, it controls calls to and from other telephony and data systems, such as the PSTN, ISDN, PLMN, Public Data Networks and possibility some private networks.


The SGSN Routing Area (RA)
The SGSN Routing Area (RA) is a subset of the MSC (CS) Location Area (LA). An MSC Location Area is a group of BSS cells, the system uses the Las to search for subscribers in the active state. An LA is the part of the network in which an MS may move around with out reporting its location to the network.
One MSC/VLR Service Area (SA) is made up of a number of LAs, the SA is the part of the network that is covered by one MSC. However there can be more than one MSC corresponding to one SGSN, one MSC can also be connected to several SGSNs.
Gateway Mobile Services Switching Center (GMSC)
The GMSC switches the circuit switched calls between GSM circuit switched network and the PSTN which is the fixed telephony network, hence it serves the function of routing incoming calls to the MSC where the mobile subscriber is currently registered, it is normally integrated in the same node as the MSC/VLR. The GMSC does not need any upgrading for GPRS.


The Home Location Register (HLR)
As stated in the section about the GSM Network the HLR is the database that holds all the subscription information for every person who has bought a from the GSM operator. The HLR stores information for the CS and PS communication, information stored the HLR includes, for example supplementary services, authentication parameters, Access Point Name (APN) such as subscribers ISP, and whether a static IP address is allocated to the MS. In addition, the HLR also includes information about the location of the MS. The main difference between this and the GSM system is that the information from the HLR is exchanged between the HLR and the SGSN.
The information that is exchanged between the HLR and the SGSN has been set up by the operator for the user, this information transfer is done when the operator changes the subscriber information, or when a new SGSN needs to have data for a subscriber after the MS has connected or in roaming, the old SGSN is also informed if the MS is roaming. The information that is going from the HLR to the SGSN is basically the routing information that is transferred upon an MS action, e.g. attach or roaming. For a roaming MS, the HLR may be in a different PLMN that the SGSN that is serving the MS.



Visitor Location Register (VLR)
The VLR database contains all the information about all MSs that are currently located in the MSC LA or the SGSN routing area respectively. The SGSN actually contains the VLR functionality for packet-switched communications, similarly, the circuit-switched VLR is an integrated component of the MSC. Another function of the VLR is that it contains the temporary subscriber information needed by the MSC or SGSN to provide services for visiting subscribers.
For MSs that support GPRS (PS) and GSM (CS), both the SGSN and the MSC will obtain location information from the HLR when the MS is combined-attached, i.e. both GPRS- and IMSI/CS-attached.
The GPRS VLR consists of software in a serving GRPS Support Node, the VLR contains information about the SGSN that is used.
The MSC/VLR is connected to the SGSN directly using the Gs interface, and indirectly via the BBS using the A and the Gb interfaces.

Serving GPRS Support Node (SGSN)
For the upgrading of the GSM network to cope with GPRS the SGSN is the primary component, and the SGSN is a new component in GSM. The SGSN forwards all incoming and outgoing IP packets addressed to/from an MS that is attached within the SGSN service area. The SGSN provides packet routing and transfer to and from the SGSN service area. SGSN serves all GPRS subscribers that are physically located within the geographical SGSN service area. A GPRS subscriber may be served by any SGSN in the network, all depending on the geographical location. The traffic is routed from the SGSN to the BSC, via the BTS to the MS. Also the SGSN provides:
 

• Ciphering and Authentication

• Session Management

• Mobility Management

• Logical Link Management toward the MS

• Connecting to HLR, MSC, BSC, GGSN and other nodes

• Output of billing data
   

Gateway GPRS Support Node (GGSN)
As with the SGSN the GGSN is a new primary component in the GSM network when using GPRS. The GGSN provides the following functions:

• The interface toward the external IP packet networks, the GGSN therefore contains access functionality that interfaces with an external ISP, functions such as, routers and RADIUS (Remote Dial-In User Services) servers. From and external IP networks point of view the GGSN is acting as a router for the IP addresses of all subscribers served by the GPRS network. So the GGSN exchanges routing information with the external network.

• GPRS session management, communication setup toward external network.

• Functionality for associating the subscribers with the right SGSN.

• Output billing data, the GGSN collects information for each MS, related to the external data network usage. Both the GGSN and the SGSN collect billing information on the usage of the GPRS network resources.

• Equipment Identity Register (EIR) See GSM section 2.1.2.3 (Page 25)
   

Authentication Center (AUC)
The AUC is a GSM component that provide triplets to the authentication and ciphering process used within GSM, the authentication for GPRS is the same as for GSM users, the only change is in the security for GPRS is related to ciphering, however this change does not require any change in software or hardware.