4.5.1 Coding rules

The coding of other information elements follows the coding rules described below. These rules are formulated to allow each equipment which processes a message to find information elements important to it, and yet remain ignorant of information elements not important to that equipment.

Two categories of information elements a redefined:

a) single octet information elements(see diagrams a) and b) of Figure 4-7);

b) variable length information elements(see diagram c) of Figure 4-7).

Figure 4-7/Q.931

Formats of information elements

For the information elements listed below,the coding of the information element identifier bits is summarized in Table 4-3.

Table 4-3/Q.931

Information element identifier coding

The descriptions of the information elements below are organized in alphabetical order. However, there is a particular order of appearance for each information element in a message within each codeset (see 4.5.2). The code values of the information element identifier for the variable length formats are assigned in ascending numerical order,according to the actual order of appearance of each information element in a message. This allows the receiving equipment to detect the presence or absence of a particular information element without scanning through an entire message.

Single octet information elements may appear at any point in the message. Two types of single octet information elements have been defined. Type 1 elements provide the information element identification in bit positions 7, 6, 5. The value "010" in these bit positions is reserved for Type 2 single octet elements.

Where the description of information elements in this Recommendation contains spare bits, these bits are indicated as being set to "0". In order to allow compatibility with future implementation, messages should not be rejected simply because a spare bit is set to "1".

The second octet of a variable length information element indicates the total length of the contents of that information element regardless of the coding of the first octet (i.e. the length starting with octet 3). It is the binary coding of the number of octets of the contents, with bit 1 as the least significant bit (2°).

An optional variable-length information element may be present, but empty. For example, a SETUP message may contain a called party number information element, the content of which is of zero length. This should be interpreted by the receiver as equivalent to that information element being absent. Similarly, an absent information element should be interpreted by the receiver as equivalent to that information element being empty.

The following rules apply for the coding of variable length information elements (octets 3, etc.):

a) The first digit in the octet number identifies one octet or a group of octets.

b) Each octet group is a self contained entity. The internal structure of an octet group may be defined in alternative ways.

c) An octet group is formed by using some extension mechanism. The preferred extension mechanism is to extend an octet (N) through the next octet(s) (Na, Nb, etc.) by using bit 8 in each octet as an extension bit. The bit value "0" indicates that the octet continues through the next octet. The bit value "1" indicates that this octet is the last octet. If one octet (Nb) is present, also the preceding octets (N and Na) must be present.

In the format descriptions appearing in 4.5.5 etc., bit 8 is marked "0/1 ext," if another octet follows. Bit 8 is marked "1 ext," if this is the last octet In the extension domain.

Additional octets may be defined later ("1 ext." changed to "0/1 ext.") and equipments shall be prepared to receive such additional octets although the equipment need not be able to interpret or act upon the content of these octets.

d) In addition to the extension mechanism defined above, an octet (N) may be extended through the next octet(s)(N1, N2 etc.) by indications in bits 7-1 (of octet N).

e) The mechanisms in c) and d) may be combined. Mechanism c) shall take priority in the ordering, such that all octets Na, Nb, etc. shall occur before octets N1, N2, etc. This rule shall apply even where the extension to octets N1, N2, etc. is indicated in one of octet Na, Nb, etc.

f) Similar conventions apply even when mechanism d) is being repeated, i.e. octets N.1 shall occur before octets N.1.1, N.1.2, etc.

g) Optional octets are marked with asterisks (*).

NOTES

1. It is not possible to use mechanism c)repeatedly, i.e. it is not possible to construct an octet 4a as this would become octet 4b.

2. Protocol designers should exercise care in using multiple extension mechanisms to ensure that a unique interpretation of the resultant coding is possible.

3. For a number of information elements there is a field that defines the coding standard. When the coding standard defines a national standard it is recommended that the national standard be structured similar to the information element defined in this Recommendation.

Figure 4-8/Q.931

Information element format using escape for extension