Like FDL where Meta Signalling has two variants, SDL can have three variants. These are UNI, NNI and internal to a Fairisle switch. Also, since the Meta Signalling may be present on the same wire as FDL Meta Signalling, it must be defined in a way that it can be distinguished from defined or experimental variants of FDL.
The exact VPI and VCI used for SDL Meta Signalling cells is yet to be chosen. This is because it is not yet clear what values we should use to avoid complications with the standards process.
The format adopted for SDL Meta Signalling information is displayed in table 2.
Table 2: SDL Meta Signalling Cell format
For connection management commands it is important to note that the only fields of this structure which are sent end to end are the OrigAddr, OrigPort, RespAddr, RespPort and appropriate parts of the argument. This structure is only sent on networks which use SDL.
The meta signalling commands used on SDL are identical to those used by all other instances of MSDL. They are therefore described in that generic document.