GSM(2G-2.5G) Structure (NSS)
GSM Structure
Network Switching Subsystem (NSS)
NSS is core part of GSM system. NSS handle switching functions, mobility management function and manages the communications between mobile phone other telephone network. NSS includes with MSC, VLR, HLR, AUC and EIR that all are described in more detail below.
NSS is core part of GSM system. NSS handle switching functions, mobility management function and manages the communications between mobile phone other telephone network. NSS includes with MSC, VLR, HLR, AUC and EIR that all are described in more detail below.
Mobile Service Switching Center (MSC)
MSC is a one type of telephone exchange which provides circuit-switched calling (Ex. Call set-up, Routing control and Call Terminate) , mobility management (Hand over), GSM supplementary services (Ex. Call forwarding, Call Waiting, CLIP) and SMS terminating to the mobile phones roaming within the area that it serves. And some MSC has Gateway function and Charging and Accounting function. This means that it connects a call to other telephone network and generates a call detail record for charging and accounting.
Visitor Location Register (VLR)VLR is a temporary database of the subscribers who have roamed into the particular area which it serves. The data stored in the VLR has either been received from the HLR, or collected from the MS. These temporary data contain with IMSI, MSISDN, Subscriber services, Category, Triplets and LAI.In practice, for performance reasons, most vendors integrate the VLR directly to the MSC.
Home Location Register (HLR)
HLR is a central database that contains details of each mobile phone subscriber that is authorized to use the GSM core network. Examples of data stored in the HLR are Mobile Station ISDN Number (MSISDN), International Mobile Subscriber Identity (IMSI), Subscriber location (MSC/VLR, SGSN address), services (Ex. supplementary service, GPRS, SMS) and triplets for AUC. HLR will provide subscriber data to MSC/VLR/SMSC When MSC/VLR/SMSC have requested data of visited subscribers to HLR. MSISDN = CC + NDC + SN
CC: Country Code, NDC: National Destination Code, SN: Subscriber Number
Cell Global Identity (CGI) = LAI + CI, LAI :Location Area Identity, CI : Cell Identity
Authentication Center (AUC) AUC is a function to authenticate (A3 Algorithm) each SIM card that attempts to connect to the GSM core network (when mobile do mobile originated call, Location update Active or Deactivate services). If the authentication is successful, the HLR shall allow SIM to Location Update and use GSM services. Beyond above, AUC is a function to Ciphering (A5 and A8 Algorithm) for protecting voice interception
The AUC generates data known as triplets for the MSC to use during this procedure. The key of this process is Ki. Ki is shared secret between the AUC and the SIM. The Ki is securely burned into the SIM during manufacture and is also securely replicated onto the AUC.When the MSC asks the AUC for a new set of triplets for a particular IMSI, the AUC first generates a random number called as RAND. This RAND is combined with the Ki in A3 algorithm Signed RESponse or SRES. The Ki and RAND are fed into the A8 algorithm and a session key called Kc is calculated for ciphering process.
The Triplet (RAND, SRES, KC) sent back to the MSC. When a particular IMSI access to the GSM core network, the authentication process is started. The MSC sends the RAND value of the
triplet to the SIM. The SIM use this number and the Ki into the A3 algorithm as appropriate and an SRES is calculated and sent back to the MSC. If this SRES matches with the SRES in the triplet, the mobile is allowed to access GSM network.
After finished authentication, the MSC sends Kc to the Base Station Controller (BSC) so that all communications can be encrypted and decrypted. When mobile phone has a call, the mobile phone can generate the Kc itself by use the same RAND supplied during authentication and the Ki into the A8 algorithm for ciphering process.
For performance reasons, most vendors integrate the AUC directly to the HLR.
For performance reasons, most vendors integrate the AUC directly to the HLR.
Equipment Identity Register (EIR)
Mobile operator uses EIR to keeps a list of mobile phones IMSI (IMEI: International Mobile Equipment Identity), IMEI is burnt to memory of mobile phone by factory, and it is identity of each mobile phone terminal.
If mobile operators have active EIR function, only valid mobile phone is allowed to access their network.
If mobile operators have active EIR function, only valid mobile phone is allowed to access their network.
Operation Support System (OSS) OSS are computer systems used for operation and maintenance telecommunications system such as provisioning services, configuring network components, managing faults and traffic managing.