Cnc-Apc Response Time; Cnc-Apc Setup - Comtech EF Data CDM-570A Installation And Operation Manual

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.
G.6.6

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.
G.6.7

CnC-APC Setup

To set up CnC-APC, carefully follow and understand the steps as presented here.
CAUTION – This procedure should be done only when there are CLEAR SKY
conditions at both ends of the link.
1. Set up the CnC link in 'normal' mode according to the instructions earlier in this appendix,
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
connection is available to the distant end that does not depend on the satellite link for
primary connectivity, the setup can be performed remotely.
Once the link is established, a further re-balance, activation or suspension does not
require another person to be present, or independent remote IP access.
2. Once the 'normal' CnC link has been established, determine that the link is stable and
that it has an acceptable link margin at both ends.
3. Decide, arbitrarily, which is Side A, and which is Side B.
DoubleTalk Carrier-in-Carrier Option
CDM-570A/570AL Satellite Modem with Optional Packet Processor
G–27
Revision 5
MN-CDM570A