Guralp Systems CD1.1 - Manual (Page 11)

Operator's Guide
One second later, at T
0
+ 15 seconds, the link is restored and
the third subframe is transmitted. It will arrive at T
0
+ 18
seconds. The multiplexer inspects the time-stamp and, as it
has no longer has an active collection labelled T
0
, it stores the
subframe in a new collection, also labelled T
0
, and sets an
alarm (specific to this collection) for the current time (T
0
+ 18
seconds) plus the time-out - i.e. T
0
+ 21 seconds. No more
packets labelled T
0
will arrive so, when the alarm fires, the lone
subframe will be wrapped up as a frame and transmitted on its
own.
If the time-out had been set to eight seconds, the delayed
subframe would have arrived in time to be sent along with the
other two: the packing efficiency would have been improved
but, as the completed frame would not have been transmitted
until T
0
+ 19 seconds, the overall latency would have
increased. The time-out should thus be adjusted to achieve the
desired trade-off between subframe packing efficiency and
overall data latency.
Where back-filled subframes are involved, it is assumed that
latency is of lesser concern - the data are already significantly
delayed - and packing efficiency is considered more important.
For this reason, although the algorithm is identical, a
separately configurable time-out is provided which only applies
to back-fill frames. It is expected that this time-out will
normally be set to a significantly higher value than the live
data time-out.
2.2 The GDI to CD1.1 converter (gdi2cd11)
This module is used to encode data from non-CD1.1 sources
into standards-compliant CD1.1 subframes. It should be used,
for example, when data from directly attached digitisers and
digital instruments is to be transmitted using CD1.1.
June 2010 11