Versafec Acm (Ip-Acm; Versafec Acm Latency; Table P-1. Versafec Implementation Of Acm - 100 Ksymbols/Sec Example Case - Comtech EF Data CDM-570A Installation And Operation Manual

P.6

VersaFEC ACM (IP-ACM)

Appendix B.6. FEC (FORWARD ERROR CORRECTION) OPTIONS

VersaFEC ACM Latency

P.6.1
In an ACM system that has a number of ModCods, each having a different latency, what defines
the overall system latency? The answer is simple – the latency of the worst-case ModCod. (This
may not seem obvious to some, and it is beyond the scope of this chapter to provide a rigorous
defense of this statement. It is, however, a correct statement despite certain believers in non-
causal systems and encoders that possess the magical quality of negative latency...)
Examining the data in Table P-1, latency for each ModCod is shown for the example of
VersaFEC ACM at a fixed 100 ksymbols/second rate. Of particular note is that even though the
ModCods span a 7:1 variation in throughput, the latency is only varying between 25 and 34
milliseconds. A careful analysis will show that this is a consequence of using a constant number
of symbols per block. In the example shown the worst-case latency for this ACM scheme is 34
milliseconds, + WAN Buffer delay (which is configurable, with a minimum value of 20ms).
Table P-1. VersaFEC Implementation of ACM – 100 ksymbols/sec Example Case
ModCod Modulation
0
1
2
3
4
5
6
7
8 16-QAM
9 16-QAM
10 16-QAM
11 16-QAM
By way of comparison, consider the same 100 ksymbols/second rate, but this time using DVB-S2. It
becomes clear that there is an unintended penalty (besides demodulator complexity) to having a
constant number of bits per block. Each time the ModCod is lowered and the throughput is
reduced, the latency grows accordingly due to the block size being related to data rate, not symbol
rate.
Adaptive Coding and Modulation (ACM) Option
CDM-570A/570AL Satellite Modem with Optional Packet Processor
Code Rate
BPSK 0.488
QPSK 0.533
QPSK 0.631
QPSK 0.706
QPSK 0.803
8-QAM 0.642
8-QAM 0.711
8-QAM 0.780
0.731
0.780
0.829
0.853
OVERALL SYSTEM LATENCY = Worst-case ModCod (ModCod0)
Latency = 34 milliseconds + WAN Buffer delay
Spectral efficiency,
Bit rate (throughput)
bps/Hz
0.49
1.07
1.26
1.41
1.61
1.93
2.13
2.34
2.93
3.12
3.32
3.41
P–8
Minimum Latency,
In milliseconds,
for each ModCod
49 kbps 34 + WAN BUFFER
107 kbps 32 + WAN BUFFER
126 kbps 30 + WAN BUFFER
141 kbps 28 + WAN BUFFER
161 kbps 26 + WAN BUFFER
193 kbps 30 + WAN BUFFER
213 kbps 28 + WAN BUFFER
234 kbps 27 + WAN BUFFER
293 kbps 27 + WAN BUFFER
312 kbps 26 + WAN BUFFER
332 kbps 25 + WAN BUFFER
341 kbps 25 + WAN BUFFER
MN-CDM570A
Revision 5