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and also connects the GSM network to other types of networks, e.g. the public switched telephone network (PSTN), the old telephone network.

The SGSN, Sendng GPRS Support Node, is the MSC of the GPRS network. It has the same functionality as the MSC, but deals with packet-switched data.

HLR The HLR contains the information and data of the subscriber. In the HLR, checks are madę to see if the subscriber is online/offline, if there is some kind of barring activated, which may stop the subscriber from using the GSM service. Another important function in the HLR is to hołd information about which VLR the subscriber is using so that location management can be utilized.

Each HLR is responsible for a series of numbers, the MSCs knows which HLR each subscriber is related to. The HLR knows which VLR the subscriber is currently related to.

Because of the HLRs importance in this thesis we will explain it in morę detail in chapter 2.4.

VLR The VLR is a unit which, like the HLR, keeps subscriber information. But unlike the HLR, which holds information for a subset of all the operator’s subscribers, the VLR holds location information for the operator’s subscribers in the geographical areas it is responsible for. The VLR is updated when the subscribers move around and switches BSC. Compare this to the HLR which is updated when the subscriber changes VLR, keeping track of the same subset of subscribers, this subset only changed by operator control.

AuC The AuC is used by the HLR to authenticate the subscribers to pre-vent abuse of the network and to protect subscribers The AuC is some-times co-located with the HLR, in the case of the SMS_HLR, the AuC is part of the same node.

SM SC This node is responsible for sending and receiving short messages, the text messages that are commonly called “SMS”. The SMSC might storę text messages if the terminating subscriber is offline.

GMSC This node is responsible for routing calls in the network. The SMS-GSMC is responsible for routing Short Messages in the network.

GMLC The GMLC is a node used with positioning services to figurę out where a subscriber is. Depending on subscriber data, the GMLC may contact the HLR to get further information used in positioning or get an old location estimate directly from the HLR.