Guralp Systems CD1.1 - Manual (Page 38)

CD1.1 Tools for Platinum
7 Using the CD1.1 Sender
This module is responsible for receiving subframes from the
multiplexor, assembling them into full CD1.1 frames,
transmitting them to a remote receiver (and logging the
transmissions), handling back-fill, satisfying retransmission
requests and digitally signing frames.
Note that the CD1.1 sender module does not require or verify
authentication data on frames it receives from the Data
Consumer (DC).
On start-up, the sender attempts to connect to the DC. The DC
responds to the connection request with a redirection
notification, which contains an I.P. address and port number, to
which the sender is required to re-connect. Progress in
connecting (including the redirected address) will be logged via
syslog (i.e. into /var/log/messages) or to the configured log-
file. Once connected, the sender waits for sets of subframes
from the CD1.1 multiplexor and sends them as a packaged
frame to the DC. After sending each frame, if the TCP output
buffer is empty, the sender will check for any outstanding back-
fill and, if required, transmit a back-filled frame.
7.1 Back-filling and retransmission
Back-fill occurs if the sender is disconnected from the DC for a
period of time. Retransmission occurs if the DC sends an
acknack frame detailing one or more gaps in its reception
record.
Whenever a real-time (i.e. not back-filled) data frame is
transmitted, its time-stamp is recorded in a database. If this
time-stamp does not match the previously- recorded time-
stamp plus the subframe duration, a back-fill gap is noted in
this database file. This allows the output module to be turned
off, or to crash, while ensuring that gaps are still recorded. It
also deals with the situation where a connection to the DC
becomes unavailable; once the connection is re-established,
the first frame to be transmitted will cause the output module
to realise there is a gap and it can then be back-filled correctly.
In this context, transmission counts as the frame being written
to the TCP socket correctly, and does not take into account
whether the DC has successfully received the frame. If the DC
does miss the frame due to, say a lost packet on the network
38 Issue C