General Packet Radio Service

General Packet Radio Service (GPRS ) ( German: " General Packet Radio Service " ) is the name for the packet-switched service for data transmission in GSM networks.

• 3.1 Interfaces of the GSN (GPRS Support Node)
• 3.2 SGSN ( Serving GPRS Support Node )
• 3.3 GGSN (Gateway GPRS Support Node)

Data transmission

If GPRS is activated there is only virtually a permanent connection to the remote site (the so-called always-on mode). Only when real data is transmitted, the radio room is occupied, otherwise it is free for other users. Therefore, no radio channel needs to be reserved permanently (as in CSD) for a user. GPRS statements are, therefore, mainly depends on the quantity of data transferred and not the duration of the connection. However, this is also dependent on the respective contract terms with the operator.

In contrast to the circuit-switched (English circuit switched ) data service CSD GPRS is packet-oriented. That is, the data are converted at the transmitter into individual packets, transmitted as such, and reassembled at the receiver.

Channel bundling

GPRS technology allows theoretical data transfer rate of 171.2 kilobits per second in combining all eight GSM time slots of a channel. In practical operation, however, the number of usable time slots within a frame with the capability of the mobile station (English multislot capability ) and the limited networks. On the market there are (as of 2008 ) Devices of Multislot Class 12 with a maximum of four timeslots in the downlink and in the uplink ( but at the same time a maximum of 5 time slots ). The achievable data transfer rate is - depending on the coding scheme used ( depends on the signal and noise ratio) and dependent on the network load number of allocated time slots (time slots ) - up to 53.6 kbps (with 4 time slots and CS -2). This corresponds approximately to the speed of a V.90 modem telephone.

GPRS coding schemes

GPRS terminal classes

There are 3 different classes of devices to the fundamental skills of a terminal state. The terminal class is marked with a letter:

• Class A: Class A devices make it possible to simultaneously use packet and circuit -switched services. That is, additional data can be transmitted during a call.
• Class B: Class B devices can optionally use either a package or a circuit-switched service, but not both. That is, a call may be indicated, although during a data transmission; this is accepted, no data can be transferred during the call.
• Class C: Class C devices allow data transmission only, as long as they are registered with the GPRS service.

GPRS -enabled mobile phones are mostly for device class B. (2008 /2009) Some Nokia devices such as the N900, support class A.

Application

Very many new mobile phones support GPRS, such as data transmission service for viewing WAP pages. The Multimedia Messaging Service (MMS) is also based on GPRS. Often a computer or handheld device can be connected to a GPRS - enabled mobile phone to give this a full-fledged, albeit narrowband Internet access. The mobile phone then acts as a modem. And plug-in cards for laptops, which are small GPRS Modems and have a similar task are known. GPRS is also particularly suitable for telecontrol tasks. As a rule, only a small bandwidth is required, the transmission speed plays a minor role. Biggest advantages of GPRS in the Remote control range is the network coverage and availability of GSM, as well as lower capital expenditures compared to other transmission techniques. Another application is the detection of vehicles and objects, is used in the GPRS to transmit position and telemetry data. Furthermore, the GPRS data network to the mobile service push-to -talk is utilized.

Network architecture

The packet-switching service GPRS is based on the GSM mobile phone network, but requires additional network elements, the PCU, the SGSN and the GGSN.

See main article: GPRS Support Node, Packet Control Unit

Interfaces of the GSN (GPRS Support Node)

The GSM guidelines as well WPP3G ( the GSM successor to UMTS) define standardized interfaces between the various network elements. The following list outlines the most common interfaces ( signaling) and connections on ( signaling and user data). The largely English terms are taken from the "GSM Recommendations ," the rules. A translation is not always meaningful.

• Ga: the SGSN and GGSN interface to "Charging Control Function " that is, the fee collection
• Gd: Interface to the SGSN to the SMSC (Short Message Service Centre) is still rarely used
• Ge: Interface to the SGSN CAMEL ( intelligent network facilities )
• Gi: the GGSN connection to the ISP ( Internet Service Provider) and company networks ( corporate network ), but also to application servers such as MMSC (Multimedia Messaging Center ), WAP -GW (Wireless Application Gateway).
• Gb: compound of the SGSN to the BSC, Frame Relay or IP for user data and signaling
• Gn: connection between SGSN and GGSN, mostly ATM or IP ( Internet Protocol) for user data and signaling
• Gp: Connecting the SGSN to GGSN other PLMNs (eg for international roaming)
• Gr: Connecting the SGSN to the HLR ( this is where the subscriber data and QoS profiles)
• Gs: the SGSN interface to the MSC, is required for combined network applications in the GPRS network modes 1 and 3
• Gx and Gy: interfaces of the GGSN to the "Charging Control Function " so the billing, these two interfaces are intended more for intelligent services (different fee depending on the application ), while the SGSN usually settles only after transfer volume.
• Iu ( PS): Connecting the SGSN to the RNC, ATM, or IP for user data and signaling
• S3: interface to the MME in LTE networks ( user and signaling data over IP )
• LIC: Legal Interception Center ( interception of data connections by the State )

SGSN ( Serving GPRS Support Node )

The BSC couple of the GPRS packet data and pass them over Frame Relay or IP connections to the responsible serving GPRS support node (SGSN ) on. Each SGSN manages the GPRS data traffic for a greater number of BSCen. If the Gb interface over IP instead of Frame Relay is passed, a BSC with a group of SGSN communicate. This is called " SGSN in pool " or " Gb flex". Thus the network is failure and the load can be more evenly distributed between the SGSN.

An SGSN has a similar function as the MSC for voice communication for the GPRS data communication. The SGSN organized the posting of subscribers of GPRS services, organizes the exchange of radio cells and SGSN areas (GPRS routing area update or GPRS Cell reselection ) and organizes the routing of GPRS data. An SGSN uses therefor the following interfaces:

• Frame Relay or IP interfaces with BSC
• SS7 and SIGTRAN routes (SS7 over IP) to its own signaling network, for example to manage using the HLR subscriber data and locations and be able to address service nodes of the GSM network.
• IP link to another GPRS network elements, such as the GGSN to the data with the Internet, corporate networks, WAP servers, etc. to be able to exchange, for example.
• IP routes to the GGSNs foreign networks to allow participants from foreign networks that are located in its own network (roaming) to access GPRS services in their home network.

GGSN (Gateway GPRS Support Node)

The GGSN (Gateway GPRS Support Node ) represent the GPRS data connections to internal network services platforms or to data networks. A GGSN has the following interfaces, which are all based on IP:

• Interfaces to the SGSN of its own network
• Interfaces to the SGSN's foreign GPRS networks to enable their own students with the use of GPRS services from abroad.
• Interfaces to online billing systems (Traffic Analyser, Charging Gateway ) to settle GPRS usage in terms of quantity or target specific can.
• Interfaces to GPRS services platforms (eg WAP server, MMSC )
• Interfaces to data networks ( Internet, corporate networks)

Access to the GPRS network via an access point, the APN ( Access Point Name ) is called. The mapping of APN to a subscriber via the subscriber data which are stored in the HLR. A subscriber can use multiple APN.