Message for the transmission of the signalling information.
message flow first bit
---------> LSB bit1
+-- - - - - -+-----+-+---+-+---+-+---+
| | | | |F| |B| |
FCS--| SIF | SIO | |LI |I|FSN|I|BSN|- HDLC frame
| | | |>2 |B| |B| |
+- 8n (n>=2)-+--8--+2+-6-+1+-7-+1+-7-+
MTP Layer 2 header
|-----------------------|
max. 272octet
|------------------------------------|
Abbreviations:
BSN: Backward sequence number (the sequence of a SU being acknowledged)
BIB: Backward indicator bit
FSN: Forward sequence number (the sequence number of the SU which is carried)
FIB: Forward indicator bit
LI: Length indicator > 2 (the number of octets between LI and CK
If it is > 63, then the value is set to 63.)
SIO: Service information octet (the type of the SU)
The coding of the SIO:
|
+------+-----+
SSF SI
sub-service service -
indicator indicator
D C B A D C B A
0 0 0 0 - SNMM Signalling network management messages
0 0 0 1 - SNTM Signalling network testing and maintenance messages.
0 0 1 0 - SPARE
0 0 1 1 - SCCP (Signalling Connection Control Part)
0 1 0 0 - TUP (Telephone User Part)
0 1 0 1 - ISUP (ISDN User Part)
0 1 1 0 - DUP (call and circuit-related messages)
0 1 1 1 - DUP (facility registration and cancellation)
1 0 0 0 - Reserved for MTP Testing User Part
1 0 0 1 - Broadband ISDN User Part
1 0 1 0 - Satellite ISDN User Part
. . . . - spare
0 0 x x - international network
0 1 x x - spare
1 0 x x - national network
1 1 x x - reserved for national use
SIF: Signalling information field (the content of the SU) 2 <= size <= 272 octet