If the VLR is full and a new mobile user arrives, then the user is failed to register in the database, so it can't receive cellular services. This situation is called as VLR overflow. When VLR is full, a new user can't register using the registration procedure. To resolve this problem, overflow control algorithm 0-I, 0-II, 0-III and 0-IV are presented which allow a new user to receive services when the VLR is full. In the overflow control system an extra flag (1-bit) is required in HLR record. No changes are required in MS.
Algorithm 0-1: RegistrationWhen MS moves into particular VLR called V. If V is not full, then the normal registration procedure is carried out but if V is full, then the following steps are executed.
Step 1: Registration Request
Step 1.1: This is same as normal registration procedure.
Step 1.2: When the database is full V follows a replacement policy to select a record to be deleted as shown in Fig. (u3), is selected for deletion. The storage space of deleted record is used to store new information. The selected user (u3) is called as overflow user.
The replacement can be done on the various factor. For example, V may select a record randomly, select old record or select an inactive record (i.e. the user has not had call for a long time). We may select (u1) as a new overflow user (i.e. u3 = u1) and does not create any new VLR record for u1.
Step 1.3: We forward the registration request to HLR by indicating that u3's record is deleted due to database overflow.
Step 2: Registration Response
Step 2.1: The HLR updates the location of u1 and set overflow flag in front of u3. Note that u3 may be identical to u1 as described in 1.2.
Step 2.2: The HLR acknowledges the registration operation and send u1's profile to V. If the u1 is overflow user then the message does not include profile information.
Step 2.3: We send acknowledgement to the MS.
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