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Location Register (VLR). When a mobile station is switched on in a new location area,or itmoves to a new location area or different operator's PLMN, it must register with the network to indicate its current location. In the normal case, a location update message is sent to the new MSC/VLR,which records the location area information,and then sends the location information to the subscriber's HLR.The information sent to the HLR normally the SS7 address of the new VLR , although it may be a routing number .The reason a routing number is not normally assigned,even though it would reduce signaling , is that there is only a limited number of routing numbers available in the new MSC/VLR and they are allocated on demand for incoming calls.

If the subscriber is entitled to service , the HLR sends a subset of the subscriber information, needed for call control, to the new MSC/VLR, and sends a message to the old MSC/VLR to cancel the old registration.

For reliability reasons,GSM also has a periodic location updating procedure .If an HLR or MSC/VLR fails, to have each mobile register simultaneously to bring the data-

base up to date would cause overloading. Therefore, the database is updated as location updating events occur. The enabling of periodic updating, and the time period between periodic updates, is controlled by the operator, and is a trade-off between signalling traffic and speed of recovery. If a mobile does not register after the updating time period, it is deregistered.

A procedure related to location updating is the IMSI attach and detach.A detach lets the network know that the mobile station is unreachable , and avoids having to needlessly allocate channels and send paging messages. An attach is similar to a location update, and informs the system that the mobile is reachable

again.Theactivation of IMSI attach/detach is up to the operator on an individual cell basis.

Authentication and security

Since the radio medium can be accessed by anyone,authentication of users to prove that they are who they claim to be, is a very important element of a mobile network. Authentication involves two functional entities , the SIM card in the mobile, and the Authentication Center (AuC). Each subscriber is given a secret key , one copy of which is stored in the SIM card and the other in the AuC . During authentication, the AuC generates a random number that it sends to the mobile . Both the mobile and the AuC then use the random number,inconjuction with the subscriber's secret key and ciphering algorithm called A3, to generate a signed response (SRES) that is sent back to the AuC. If the number sent by the mobile is the same as the one calculated by the AuC, the subscriber is authenticated.

The same initial random number and subscriber key are also used to compute the ciphering key using an algorithm called A8. This ciphering key , together with the TDMA frame number, use the A5 algorithm to create a 1 1 4 bit sequence that is XORed with the 114 bits of a burst ( the two 57 bit blocks ) . Enciphering is an option for the fairly paranoid,since the signal is already coded, interleaved, and transmitted in a TDMA manner , thus providing protection from all but the most persistent and dedicated eavesdroppers.

Another level of security is performed on the mobile equipment itself, as opposed to the mobile subscriber . As mentioned earlier, each GSM terminal is identified by a unique International Mobile Equipment Identity ( IMEI ) number.A list of IMEIs in the network is stored in the Equipment Identity Register .The status returned in response to an IMEI query to the EIR is one of the following: ? White-listed

The terminal is allowed to connect to the network.

? Grey-listed

The terminal is under observation from the network for possible problems.

? Black-listed

The terminal has either been reported stolen,or is not type approved (the correct type of terminal for a GSM network). The terminal is not allowed to connect to the network.