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Call Process & Authentication Principle

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June 2, 2010 by Isybel Harto

How connection is established when the call is initiated by a mobile subscriber instead of a fixed one?

The mobile subscriber dials a number. In other words, the subscriber issues a service request to the network in which he is currently registered as a visitor. After receiving the request, the network analyses the data of the calling subscriber in order to do three things:
•Authorize or deny the use of the network.
•Activate the requested service.
•Route the call.

Location Update :

In practice, there are three types of location updates:
•Location Registration (power on)
•Generic
•Periodic

Location registration takes place when a mobile station is turned on. This is also known as IMSI Attach because as soon as the mobile station is switched on it informs the Visitor Location Register (VLR) that it is now back in service and is able to receive calls. As a result of a successful registration, the network sends the mobile station two numbers that are stored in the SIM (Subscriber Identity Module) card of the mobile station. These two numbers are the Location Area Identity (LAI) and the Temporary Mobile Subscriber Identity (TMSI).

Every time the mobile receives data through the control channels, it reads the LAI and Compares it with the LAI stored in its SIM card. A generic location update is performed if they are different. The mobile starts a Location Update process by accessing the MSC/VLR that sent the location data.

Periodic location update is carried out when the network does not receive any location update request from the mobile in a specified time. Such a situation is created when a mobile is switched on but no traffic is carried, in which case the mobile is only reading and measuring the information sent by the network. If the subscriber is moving within a single location area, there is no need to send a location update request.

Authentication Principle:

Authentication is a procedure used in checking the validity of subscriber data. With the help of the authentication procedure the operator prevents the use of false SIM modules in the network. The authentication procedure is based on an identity key, Ki, which is issued to each subscriber when his data are established in the HLR. The authentication procedure verifies that the Ki is exactly the same on the subscriber side as on the network side. Authentication is performed by the VLR at the beginning of every call

Establishment, location updates and calls termination (at the called subscriber side). In order to perform the authentication, the VLR needs the basic authentication information. If the mobile station was asked to broadcast its K i, this would undermine the principle of authentication, because identification data would be sent across the air. The trick is to compare the K i stored in the mobile with the one stored in the network without actually having to transmit it over the radio air interface. The Ki is processed by a random number with a “one way” algorithm called A3 and the result of this processing is sent to the network. Due to the type of the algorithm A3, it is easy to get the result on the basis of Ki and a random number, but it is virtually impossible to get the Ki on the basis of the result and random number (hence the name “one way” algorithm).

Security Algorithms:

GSM uses three algorithms A3, A5, A8. A3 and A8 are located in the SIM module and in the Authentication Centre (AC). A5 is located in the mobile station and in the BTS. The basic principle of GSM security functions is to compare the data stored by the network to the data stored in the subscriber’s SIM. The IMSI number is the unique identification of the mobile subscriber. Ki is an authentication key with a length of 32 hexadecimal digits. The algorithms A3 and A8 use these digits as a basic value in
Authentication.

The Authentication Centre generates information that can be used for all the security purposes during one transaction. This information is called an Authentication Triplet.
The authentication triplet consists of three numbers:
1 RAND
2 SRES
3 Kc

RAND is a Random number, SRES (Signed Response) is a result that the algorithm A3 produces on the basis of certain source information and Kc is a ciphering key that A8 generates on the basis of certain source information.

When the VLR has this kind of three-value combination and the Mobile Subscriber authentication procedure is initiated, the VLR sends the random number RAND through the BSS to the SIM in the mobile station. As the SIM has (or it should have) exactly the same algorithms as used in triplet generation on the network side, the RAND number that the SIM receives and inserts to the algorithm should produce exactly the same SRES value as the one generated on the network side. If the SRES value in the authentication triplet is the same as the SRES calculated and sent by the mobile station, the authentication procedure is successful.

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