DSS1 Message information elements (IEs) and their fields or components

The types of the IEs in the messages are (4.5/Q.931):

    Single octet information elements:

     bit:  8   7   6   5   4   3   2   1
          +------------------------------+
          |1 |Identifier |  Content      |
          +------------------------------+

     bit:  8   7   6   5   4   3   2   1
          +------------------------------+
          |1 |       Identifier          |
          +------------------------------+

Codeset change with shift:

l - bit 0 - locked, 1 - unlocked (on the followed IE)
ccc - bits codeset Id:

• 000 - unlocked only, initial, for Q.931 IEs
• 001 to 011 - reserved
• 100 Codeset 4: IEs for ISO/IEC use
• 101 Codeset 5: IEs for national use
• 110 Codeset 6: IEs specific to the local network (public or private)
• 111 Codeset 7: user specific IEs

    Variable length information elements:

     bit:  8   7   6   5   4   3   2   1
          +------------------------------+
          |0 |       Identifier          |
          +------------------------------+
          |     Length (octets)          |
          +------------------------------+
          |          Content             |
          +------------------------------+

          ext=1 last octet
          ext=0 the octet continues through the last octet