The following rules apply when Q.931messages are to be segmented for transmission:
a) The default maximum number of message segments is 8. If the message is too long to be segmented then a local maintenance activity shall be notified.
b) The first message segment shall begin with the Protocol discriminator information element immediately followed by the Call reference information element, the segment message type, the Segmented message information element, and octets starting with the first octet following the message type of the message being segmented, subject to the maximum length of the segment not exceeding the maximum size of the data link layer information field.
c) Each subsequent message segment shall begin with the Protocol discriminator information element immediately followed by the Call reference information element, the segment message type, the Segmented message information element, and one or more octets starting with the first octet following the message type of the message being segmented, subject to the maximum length of the segment not exceeding the maximum size of the data link layer information field.
d) The first segment indicator field of the Segmented message information element shall be set to indicate the first segment of a segmented message, and not set in any other segment.
e) The number of segments remaining field of the Segmented message information element shall be set to indicate how many more segments are to be sent, see Figure H.2.
f) The Message type information element shall be coded to indicate a segment message, and the Segmented message information element shall indicate the message type of the original message.
g) Once the first segment has been transmitted on a particular data link connection, then all remaining segments of that message shall be sent (in order) before any other message (segmented or not) for any other call reference is sent on that data link connection, i.e. a segmented message cannot be interleaved with any other messages.
h) In exceptional circumstances, the transmission of a segmented message may be aborted by sending a message or message segment containing a different call reference; sending a message with the message type not coded "segment message", or stopping the transmission of subsequent message segments pertaining to the same message.
i) The octet order for the Segmented message shall be preserved regardless of segment boundary.
Figure H.2/Q.931 – Relation between message and segments