G.2.2.1 Ecm Revert Cycle Timing; G.2.2.2 Ecmv2 Backoff Algorithm - Comtec CDM-840 Installation And Operation Manual

CDM-840 Remote Router
Appendix G

G.2.2.1 ECM Revert Cycle Timing

When a CDM-840 reverts from dSCPC to ECM, there is typically no wait because the CDM-840 is
always monitoring TAP cycles to synchronize. It then randomly selects a slot in the next cycle
and transmits. If the probe is not detected at the corresponding CDD-880 (as indicated by the
next TAP to the CDM-840), it backs off by doubling the cycle and selecting another random slot.
If the revert happened near the end of the cycle the CDM-840 wait for next TAP.
The ECM TAP cycle duration is designed to be approximately 3 seconds in order to correlate
with a recommended VMS message timeout of 5 seconds. This allows the CDM-840 to respond
to most VMS requests before the VMS times out and goes into "retry" mode. This optimizes for
normal conditions, but may result in slight delays in the event of a catastrophic recovery when a
large number of CDM-840s are all trying to transition from ECM to dSCPC mode.
The Slots in Frame setting is determined automatically by the HCC, based on the target cycle of
3 seconds and the assigned data rate of the ECM Channel. Essentially, the HCC determines the
required acquisition time based on the symbol rate and the data slot time based on the data
rate. (Note that only MODCOD 0 (BPSK .488) is supported for ECM so the data rate is
approximately half the symbol rate.) These are added to the Guard Band; the result (total time
per slot) is divided by 3 seconds, and that result is then rounded up to the next integer number
of slots. For example, for a typical Entry Channel at 64 Kbps with a Guard Band of 50 ms, this
results in a total of 8 slots per cycle.

G.2.2.2 ECMv2 Backoff Algorithm

The ECMv2 Backoff Algorithm optimizes between quick entry in a quiet network and minimal
collisions in an active network. Its key feature is the concept of fixed-length data slots within a
repeating frame (or cycle). Each total slot consists of three components:
•
Acquisition Preamble
• Data Segment for Management Messages
•
Guard Band to resolve timing uncertainty
An integral number of VersaFEC blocks are used for both the preamble and the data segment;
the Guard Band will vary due to system timing uncertainties and propagation delay due to the
geographic dispersion of the CDM-840s.
A Guard Band of 50ms is sufficient in most cases to accommodate geographical
latency differences and internal timing errors.
The preamble is pre-determined based on calibration tables for receiver acquisition at the
current data rate and MODCOD 0 (BPSK .488) (actual value is based on symbol rate; starting with
14 blocks and increasing by 1 block for every 556 ksps). The data segment is sized to support the
largest messages needed to complete the Entry Channel handshake protocol and maintain status
and CDRP (fixed at 6 VersaFEC blocks (765 bytes)). The actual size (in milliseconds) of each slot is
G–8
MN-CDM840
Revision 3