gsm archtecture


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GSM Architecture : 

GSM Architecture

GSM System : 

GSM System Global System for Mobile Communication (GSM) is a digital wireless network standard designed by Standardization committees from major European telecommunications operators and manufactures. The GSM standard provides a common set of compatible services and capabilities to all mobile users across Europe and several million customers worldwide.

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MSC Mobile Switching Center VLR Visitor Location Register HLR Home Location Register Base Station VLR MSC VLR MSC HLR land link land link Radio link Base Station VLR MSC VLR MSC HLR land link land link Radio link GSM Architecture

GSM Architecture : 


Mobile Station : 

Mobile Station The MS consists two parts: the Subscriber Identification Module (SIM) and the Mobile Equipment (ME). In a broader definition, the MS also include a third part called Terminal Equipment (TE), which can be PDA or PC connected to the ME. In this case, the first two parts (ME and SIM) are called Mobile Terminal (MT) and SIM can be: A smart card, usually the size of a credit card. A smaller-card “plug-in SIM” A smart card that can be performed, which contains a plug-in SIM that can be broken out of it. The SIM is protected by a Personal Identification Number (PIN) between four to eight digits in the length. The PIN is initially loaded by the network operator at the subscription time. This PIN can be deactivated or changed by the user. To use the MS, the user is asked to enter the PIN.

Mobile Station : 

Mobile Station If the number is not correctly entered in the three connective attempts, the SIM is blocked and MS can’t be used. To unblock the SIM, the user is asked to enter the eight-digit PIN Unblocking Key (PUK). A SIM contains the subscriber-related information, including the PIN and PUK codes. The subscriber-related data also include a list of abbreviated and customized short dialing number, short message received when the subscriber is not present, and the names of the preferred networks to provide service and so on. The ME contains the noncustomer-related hardware and software specific to the radio interface. When the SIM is removed from an MS, the remaining ME cannot be used for reaching the service, except for emergency calls.

Base Station System : 

Base Station System The BSS connects Mobile Station (MS) and the Network and Switching Subsystem (NSS). The BSS consists of two parts: the Base Transceiver Station (BTS) and the Base Station Controller (BSC). The BTS contains transmitter, receiver, and signaling equipments to specific to the radio interface in order to contact the MSs. The BSC is responsible for the switching functions in BSS, and is in turn connected to an MSC in the NSS. The BSC supports radio channel allocation/relies. A BSC may connect to the several BTSs and maintain cell configuration data of these BTSs. The BSC communicates with BTSs using ISDN protocols via the A-bis interface. Capacity planning for BSC is very important. In busy hours, the processor load of the BSC is roughly distributed over call activates (around 20%-25%), SMS (around 10%-15%), location update (20%-25%), and hardware checking/network-triggered events (around 15%-20%). A BSC is typically engineered at 80% utilization.

Networking and Switching Subsystem : 

Networking and Switching Subsystem The NSS supports the switching functions, subscriber profiles, and mobility management. The NSS consists MSC, HLR, VLR, AuC. The basic switching function in the NSS is performed by the MSC. The current location of MS is usually maintained by the HLR and VLR. When an MS moves from the home system to a visited system, its location is registered at the VLR of the visited system. The VLR then informs the MS’s HLR of its current location. The Authentication Center (AuC) is used in the security data management for the authentication of subscriber. The AuC may be co-located with the HLR. The MSC is called the gateway MSC (GMSC). An MSC can functions, and by provisioning interface and the signaling link to the HLR. The GMSC obtains the location information and routes the calls to the visited MSC of the subscribers to receive the calls.

Location Tracking and Call Setup : 

Location Tracking and Call Setup The current location of an MS is maintained by a two-level hierarchical strategy with HLR and the VLRs. When an MS visits a new location, it must register in the VLR of the visited location. The HLR must also be informed about the registration. To access the MS, the HLR is queried to find the current VLR of the MS. The registration process of the MS moving from one VLR to another VLR is illustrated in figure and is described in the following steps.

The MS registration process : 

The MS registration process

HLR and VLR management steps : 

HLR and VLR management steps

The Mobile call delivery procedure : 

The Mobile call delivery procedure 3 1 MSC GMSC ( other switches 1 1 1 2 2 other switches 3 3 HLR VLR

Basic call delivery procedure steps : 

Basic call delivery procedure steps

GSM Security : 

GSM Security GSM security is addressed in two aspects: authentication and encryption. Authentication avoids fraudulent access by a cloned MS. Encryption avoids unauthorized listening. A secrete key, Ki is used to achieve authentication. Ki is stored in the AuC as well as in the SIM. The Ki value is unknown to the subscriber. To initiate the authentication process, the home system of the MS a 128 bit random number called RAND. This number is sent to the MS. By exercising an algorithm, A3, both the network (AuC) and the MS (SIM) use Ki and RAND to produce signed result (SRES) The SRES generated by the MS is sent to the home system and is compared with the SRES generated by AuC. If they are not identical, the access request is rejected.

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If the SREC and RAND generated by the the AuC are sent from the HLR to the visited VLR in advance, the SREC comparison can be done at the visited VLR. Since the visited system may not know the A3 algorithm of a roaming MS, authentication result SRES is generated at the home system. Algorithm A3 is dependent on the GSM service provider. If the MS is accepted for access, an encryption Kc is produced by an algorithm, A8, with Ki and RAND as inputs Like A3, A8 is specify to the home system. After the home system has generated Kc , the encryption key is sent to the visited. Kc and the TDMA frame number encoded in the data bits are used by an algorithm, A5, to cipher and deciphering the data stream between the MS and the visited system. The same A5 algorithm may be used in all systems participating in the GSM services.

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