The Meta Signalling varies slightly between the two variants of FDL.
Since not all hardware can necessarily distinguish the source of an
FDL cell it is strictly forbidden to send a FDL Meta Signalling cell
of the incorrect variant.
Meta Signalling or connection management is an example of a
promiscuous association. It has therefore, as the first such
association, been allocated VCI 0x01,0x00. For the switch variant
care should be taken to ensure that the cell is sent to the local
management entity.
Meta Signalling cells, since on a promiscuous association, can arrive in any order and are therefore not considered as blocks. They shall therefore be sent with both the frame start and frame end bits set. The sequence field should contain the value 0x01 and the assmeble field many contain any legal value.
The format adopted for FDL Meta Signalling information is displayed in table 2.
Table 2: FDL 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 FDL.
Inside a fairisle switch the FDL Meta Signalling cell fields ``OrigVCI'' and ``RespVCI'' contain a full complement of four octets which are the fabric and port-controller route octets, followed by the two octet VCI as for the point to point variant.
A simple checksum has been implemented on FDL Meta Signalling cells.
This is generated on all transmitted cells and checked on received cells.
Cells which do not match the checksum are to be ignored.
The checksum is a four octet value stored in the last four octets of the cell.
It is generated in eleven stages each manipulating four octets of the cell and
the four octet checksum value. The checksum is believed to be the same
as that used by the XTP protocol (there is no other similarity with
that protocol).
Initialising with zero, each stage exclusive-ors the current value
with the next four octets. The value is then rotated so that the most
significant bit of octet 
becomes the least significant bit of
octet 
except that the least significant bit of octet 
becomes
the most significant bit of octet 
.
Finally the value is stored in the last four octets of the cell. Note that
the rotation occurs after the last exclusive-or - that is eleven times in
total.
The meta signalling commands used on FDL are identical to those used by all other current implementaions of MSDL. They are therefore described in that generic document.