Section: 10.5.4.1 Extensions of codesets
Content: There is a certain number of possible information element identifier values using the formatting rules described in subclause 10.5: 128 from the type 3 & 4 information element format and at least 8 from the type 1 & 2 information element format. One value in the type 1 format is specified for shift operations described below. One other value in both the type 3 & 4 and type 1 format is reserved. This leaves 133 information element identifier values available for assignment. It is possible to expand this structure to eight codesets of 133 information element identifier values each. One common value in the type 1 format is employed in each codeset to facilitate shifting from one codeset to another. The contents of this shift information element identifies the codeset to be used for the next information element or elements. The codeset in use at any given time is referred to as the "active codeset". By convention, codeset 0 is the initially active codeset. Two codeset shifting procedures are supported: locking shift and non-locking shift. Codeset 5 is reserved for information elements reserved for national use. Codeset 6 is reserved for information elements specific to the local network (either public or private). Codeset 7 is reserved for user-specific information elements. The coding rules specified in subclause 10.5 shall apply for information elements belonging to any active codeset. The mobile station and the network shall not apply the "comprehension required" scheme (see 3GPP TS 24.007[ Mobile radio interface signalling layer 3; General Aspects ]  [20]) to information elements belonging to codesets different from codeset 0. IEIs with bits 5, 6, 7 and 8 all set to zero should not be allocated for new optional information elements in codesets different from codeset 0, because there are legacy mobile stations that apply the "comprehension required" scheme also to these information elements, e.g. if such a mobile station receives a SETUP message containing an unknown information element from codeset 5 with an IEI with bits 5, 6, 7 and 8 all set to zero, then the mobile station will release the call. Transitions from one active codeset to another (i.e. by means of the locking shift procedure) may only be made to a codeset with a higher numerical value than the codeset being left. An information element belonging to codeset 5, 6 or 7 may appear together with information elements belonging to codeset 0, by using the non-locking shift procedure (see subclause 10.5.4.3). A user or network equipment shall have the capability to recognize a shift information element and to determine the length of the following information element, although the equipment need not be able to interpret and act on the content of the information element. This enables the equipment to determine the start of the subsequent information element.