There is a certain number of possible information element identifier values using the formatting rules described in 4.5.1; 128 from the variable length information element format and at least 8from the single octet information element format.
One value in the single octet format is specified for shift operations described below. One other value in both the single octet and variable format is reserved. This leaves at least 133 information element identifier values available for assignment.
It is possible to expand this structure to eight codesets of at least 133 information element identifier values each. One common value in the single octet 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 4 is reserved for use by ISO/IEC standards.
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 4.5.1 shall apply for information elements belonging to any active codeset.
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 codesets4, 5, 6, or 7, may appear together with information elements belonging to codeset 0 (being the active codeset) by using the non-locking shift procedure (see 4.5.4).
A user of 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 upon the content of the information element. This enables the equipment to determine the start of a subsequent information element.
Codeset 7 information element shall be handled according to the procedures for unrecognized information elements (see 5.8.7.1) by the first exchange in the local network, unless allowed by a future service definition, bilateral agreement, or provision is made to support this across the local network for a specific user.
Codeset 6 is reserved for information elements specific to the local network (either public or private). As such they do not have significance across the boundaries between local networks, or across a national, or international boundary. Therefore, codeset 6 information elements shall be handled according to the procedures for unrecognized information elements (see 5.8.7.1) beyond local network boundary, unless allowed by bilateral agreement.
Codeset 5 is reserved for information elements reserved for national use. As such they do not have significance across an international boundary. Therefore, codeset 5 information elements shall be handled according to the procedures for unrecognized information elements (see 5.8.7.1) at the first exchange beyond the international boundary, unless there are bilateral agreements to the contrary.
Codeset 4 is reserved for information elements specified in ISO/IEC standards.