The non-locking shift procedure provides a temporary shift to the specified lower or higher codeset. The non-locking shift procedure uses a single octet information element to indicate the codeset to be used to interpret the next single information element. After the interpretation of the next single information element, the active codeset is again used for interpreting any following information elements. For example, codeset 0 is active at the beginning of message content analysis. If a non-locking shift to codeset 6 is encountered, only the next information element is interpreted according to the information element identifiers assigned in codeset 6. After this information element is interpreted, codeset 0 will again be used to interpret the following information elements. A non-locking Shift information element indicating the current codeset shall not be regarded as an error.
Bits |
|||||||||
8 |
7 |
6 |
5 |
4 |
3 |
2 |
1 |
Octet |
|
Shift identifier |
|||||||||
1 |
0 |
0 |
1 |
0* |
New codeset identification |
1 |
|||
*"0" in this position indicates locking shift |
|||||||||
Table 4-4/Q.931 Locking Shift information element
Codeset identification (bits 3 to 1): |
|
Bits |
|
3 2 1 |
|
0 0 0 |
Not applicable |
0 0 1 |
Reserved |
1 0 0 |
Codeset 4: information elements for ISO/IEC use |
1 0 1 |
Codeset 5: information elements for national use |
1 1 0 |
Codeset 6: information elements specific to the local network (either public or private) |
1 1 1 |
Codeset 7: user-specific information elements |
A locking Shift information element shall not follow directly on a non-locking Shift information element. If this combination is received, it shall be interpreted as though a locking Shift information element only had been received.
The non-locking Shift information element uses the single octet information element format and coding shown in Figure 4-10 and Table 4-5.
Bits |
||||||||
8 |
7 |
6 |
5 |
4 |
3 |
2 |
1 |
Octet |
Shift identifier |
Temporary codeset identification |
|||||||
1 |
0 |
0 |
1 |
1* |
1 |
|||
*"1" in this position indicates non-locking shift |
||||||||
Table 4-5/Q.931 Non-locking Shift information element
Codeset identification (bits 3 to 1): |
|
Bits |
|
3 2 1 |
|
0 0 0 |
Codeset 0 (initially active): Q.931 information elements |
0 0 1 |
Reserved |
1 0 0 |
Codeset 4: information elements for ISO/IEC use |
1 0 1 |
Codeset 5: information elements for national use |
1 1 0 |
Codeset 6: information elements specific to the local network (either public or private) |
1 1 1 |
Codeset 7: user-specific information elements |