Location Update

Location Update(UL):

            When ever the MS tries to connect with the network location update happens. Here we gonna see the location update from MSC to the HLR.

             When your mobile station tries to latch with the network for the very first time, UL request will be sent respective operators MSC/VLR after that MSC will request HLR to provide the authentication   information to authenticate the mobile station(send authentication request). HLR will respond to the MSC/VLR with corresponding authentication vectors. Once MSC receives authentication vectors from HLR, it request the same to the MS(for detailed SAI please refer below) and MSC will compare both MS and HLR requests if both matches MSC will send the UL request to HLR else it will fail the UL.

                           Once the UL request reaches the HLR, HLR will send the Insert Subscriber Data request (ISD)to the MSC which carries what are all the services provided by the operators such as telephony services, SMS services, call waiting call forwarding, etc.
                       
                 Followed by the ISD request HLR will send the UL acknowledgment request to MSC to confirm that UL is done and the same will be notified to MS from MSC once it get done MS will be successfully latch with  that corresponding MSC/VLR.

First UL

UL

    The difference between first time UL and non-first UL is, when the HLR receives UL request from a new MSC/VLR, it will initiate cancel location to the previous MSC/VLR where the customer presently latched. So that customer datas will removed from existing MSC/VLR and the same will available on new MSC/VLR.

                         SAI: Send Authentication Information

Authentication Vectors:

                         There are two different types(majority) of authentication vectors used MSC/VLR to authenticate the MS. Those are Triplet or Quintet parameters together forms a authentication vector.

Quintet:

RAND - Random value

XRES – Expected Signed Response

CK - Ciphering Key

IK - Integrity Key

AUTN - Authentication Token

Triplet: 

RAND – Random Number

XRES- Signed Response

Kc - Ciphering Key

Send Authentication Information for GSM:

                         For GSM-SAI, triplet parameter together forms a authentication vector.

The new VLR sends a request to the HLR/AUC (Authentication Center) requesting the “authentication triplets” (RAND, SRES, and Kc) available for the specified IMSI.

The AUC, using the IMSI, extracts the subscriber's authentication key (Ki). The AUC then generates a random number (RAND), applies the Ki and RAND to both the authentication algorithm (A3) and the cipher key generation algorithm (A8) to produce an authentication Signed Response (SRES) and a Cipher Key (Kc). The AUC then returns to the new VLR an authentication triplet: RAND, SRES, and Kc.

The MSC/VLR keeps the two parameters Kc and SRES for later use and then sends a message to the MS. The MS reads its Authentication key (Ki) from the SIM, applies the received random number (RAND) and Ki to both its Authentication Algorithm (A3) and Cipher key generation Algorithm (A8) to produce an authentication Signed Response (SRES) and Cipher Key (Kc). The MS saves Kc for later, and will use Kc when it receives command to cipher the channel. 

The MS returns the generated SRES to the MSC/VLR. The VLR compares the SRES returned from the MS with the expected SRES received earlier from the AUC. If equal, the mobile passes authentication. If unequal, all signaling activities will be aborted.

Update Location for GSM