Comtech EF Data CDM-625A - Manual (Page 606)

CDM-625A Advanced Satellite Modem MN-CDM625A
Appendix E Revision 3
E–16
E.12 Quad E1 Operation
Up to four complete or fractional E1 streams may be concatenated into a single Comtech
proprietary satellite frame structure using the Quad E1 feature. The “Balanced G.703” and “Aux
G.703” 9-pin D-sub female connectors on the CDM-625A rear panel each provide access to two
ports. See Sect. 3.2.2.2 G.703 Connectors for pinout details.
Selected timeslots for each active port are “dropped” onto the transmit satellite frame in order
of port number and drop channel number, with the reverse “insert” operation on the receive
side. For each port, if n=32 is selected, then it is not assumed that the E1 into the modem
contains framing. Data may be framed or unframed, and the modem will simply transmit the
entire 2048 kbps stream on that port’s portion of the satellite frame; this is why timeslot
selection is disabled for n=32. However, for any fractional value of “n”, the incoming E1 to that
port must have TS0 framing so that the selected timeslots to be dropped can be identified.
Recent improvements to Quad E1 operation allow the combining of asynchronous E1 streams.
Before this, it was important for all ports to be synchronous to one another in order to operate
without data loss – now, bit stuffing is performed on slower data streams to match them to
faster ones. Satellite overhead is used to identify extra data on each port that was sent over the
link. The extra data is then removed at the receiver, so that each port’s timing is maintained. As
a result, it is important that the ports used and the number of timeslots for each port match at
both ends of the link. The recommendation for G.703 requires that the actual bit rate be within
±50 ppm of 2048 kbps.
If asynchronous E1 streams are used at the transmit side of the link, it is important to keep the
receive buffer set to “Rx Satellite”. The “Tx Terrestrial” setting should only be used if all E1
streams are synchronous to one another (no bit stuffing over the satellite frame) and the
resulting E1 streams from the receiver need to be retimed to a local reference.