Section: C.4 Guidelines for interoperability in a multi-vendor environment
Content: The specification of a sequence number management scheme affects only the USIM and the AuC which are both under the control of one operator. Therefore, the specification of such a scheme is entirely at the discretion of an operator. Nevertheless, certain operators may not want to define a scheme of their own. Instead, they may want to rely on vendors implementing one of the schemes according to the profiles in C.3 or variants thereof. If these operators have multiple vendors for USIMs and/or AuCs, and the operators wish to move subscribers from the AuC of one vendor to that supplied by another one implementing a different scheme then this will work smoothly only when the following guidelines are adhered to by all the sequence number management schemes implemented in the operator’s domain. - The array mechanism specified in clauses C.1.2 and C.2 is used in the USIM to verify SQNs. The length of the used by the USIM to index the array shall be not less than the length of the used by the AuC when allocating index values. However, it is recommended that the same length of 5 bits is used in USIMs and AuCs. This is the same length as proposed for all profiles in clause C.3. - Relation to Annex F: if the AMF field is used to signal further parameters relevant to sequence number management (age limit L) then the formats of the AMF and its interpretation by the USIM must be the same for all implementations in the operator’s domain. - is larger than a specified minimum. This is necessary to accommodate schemes as in C.3.2 according to note 7 of C.2.3. We propose 228. - There are no requirements on the synchronicity of clocks in different AuCs for the time-based schemes. For the entirely time-based scheme, the following is recommended when moving users from one AuC to another one: The DIF value is updated in an appropriate manner when moving subscribers from an AuC to another AuC. More specifically, assume a user is moved from AuC1 to AuC2. If AuC1 is of profile 3 and AuC2 is of any profile then AuC1 sends GLC+DIF as SEQ_HE to AuC2. In the receiving end, if AuC2 is of profile 3 while AuC1 is of any profile then AuC2 sets DIF value for this user as DIF = SEQ_HE - GLC.