Comtech EF Data CDM-625 - Manual (Page 354)

CDM-625 Advanced Satellite Modem Revision 15
DoubleTalk Carrier-in-Carrier Option MN-CDM625
10–24
10.6.4.1 CnC-APC Framing and the Self-Locking Problem
Self-locking is a problem that has been previously reported to Comtech EF Data Product Support
on CnC links that have identical data rates, FEC and modulation.
In the case where both ends of the link are up and running, the CnC canceller performs as
intended. However, if the carrier at the remote side stops transmitting a carrier, the CnC
canceller will still cancel the interferer (its own outbound Tx signal), but the cancelation is not
perfect (it may be in the order -30 dB) and hence a weak signal appears at the input to the
demodulator that, because of the correct settings of data rate, FEC and modulation, is
sometimes acquired by the demod. This can sometimes be an issue because, although the Tx
carrier from the distant side is not transmitting, no demodulator alarm is generated and hence
the local end is not alerted – and data being transmitted is looped back to the receiver, and back
into the network.
The use of CnC-APC framing provides the means to eliminate the self-locking problem. Rather
than having an identical framing structure for the local and distant sides of the link, by
designating each side as Side A and Side B, each side has a different framing structure and is
therefore non-identical and incompatible. This makes it impossible for a modem in this CnC
mode to lock to itself. With this approach, there is no ‘master-slave’ relationship – both sides
are peers, and either side can be designated as A or B.
10.6.5 CnC-APC Response Time
After CnC-APC is activated, power may be modified at a rate that does exceed once every 1.75
seconds. This rate has been determined by taking into account the round trip satellite delay, the
time taken to accurately determine CnC Ratio and Eb/No following a power adjustment, and the
latency of the CnC-APC frame itself. The overall response time of the control system is therefore
limited by these factors, and deep fading events that exceed approximately 1 dB/second are
likely to be too fast for CnC-APC (or any other ‘classic’ scheme) to respond to. Users should bear
in mind that these limitations in response time are inherent in schemes involving round-trip
delays from geostationary satellites.
10.6.6 CnC-APC Setup
CAUTION – This procedure should be done only when there are CLEAR SKY conditions
at both ends of the link.
To set up CnC-APC, carefully follow and understand these steps:
1. Set up the CnC link in ‘normal’ mode according to the instructions earlier in this chapter, and
according to an appropriately calculated link budget.
The initial selection of CnC-APC normally requires that a person be present at both ends, to
coordinate the change of CnC from ‘normal’ to CnC-APC mode. Alternately, if an IP