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2.2 SS7 Network

The SS7 network [15] is a collection of protocols which are used in telecom-munication. SS7 is short for Signaling System 7.

2.2.1 Network Components

The SS7 network consist of nodes which are responsible for different func-tions. Some of these nodes can be found in Figurę 2, but there are also some morę nodes used in this thesis that are not shown in this figurę. The nodes are identified by Point Codes (PC) and can be divided into three groups:

•    SSP - Service Switching Point is a voice switch with SS7 functionality. Can originate and terminate messages, but not transfer.

•    SCP - Service Control Point is a node which often ąueries a database and/or provides logie in the SS7 network.

•    STP - Signal Transfer Points, responsible for the transfer of SS7 messages b/w other SS7 nodes, much like a router in the IPA network. STP:s only route/transfer messages.

Mobile Station Mobile Stations (MS) are the units that are used to con-nect to the network. These can be handsets, laptops or really any physical device with ability to connect to the network. The MS has an IMEI number to uniąuely identify the hardware. Each subscriber has an IMSI to uniąuely identify, the IMSI is usually stored in a SIM-card on the device. Due to security reasons the IMSI is not sent in protocol messages if it can be avoided, instead a temporary number, TMSI, is used. The number which is used when calling or sending SMS to the MS is called MSISDN, this number can be used internationally. The MSs connect to the BSS in Figurę 2.

BTS and BSC These are the two major parts of the BSS. BTS stands for Base Transceiver Stations and BSC for Base Station Centrals. The BTS’s is the entry point to the network from the subscribers point of view, the radio transceiver we introduced in the section about cellular networks above. They can be placed on radio towers, on top of build-ings, in buildings and even on specific floors, depending on the size of celi. The BSC keeps track of which BTS the UE is using and which BTS’s that is close to it, so that if the UE moves into another celi, it can switch to the next BTS. The BTS connects the BSS to the NSS.

MSC and SGSN The MSC, short for Message Switching Centre, is a very important part of the GSM network. It is the node that connects the NSS and OSS together. It acts as a seryice router within the network

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