Versafec Acm Latency - Comtech EF Data CDM-570A Installation And Operation Manual

CDM-570A/570A Satellite Modem with Optional Packet Processor
Appendix P

VersaFEC ACM Latency

P.7
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-2, 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-2. VersaFEC Implementation of ACM – 100 ksymbols/sec Example Case
ModCod Modulation
0
1
2
3
4
5
6
7
8
9
10
11
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.
Remembering that, for the ACM case, the system latency is equal to the latency of the worst-case
ModCod, DVB-S2 shows a severe penalty. For 16k short blocks, this calculates to be 329
milliseconds (+ WAN Buffer delay) versus 34 milliseconds (+ WAN Buffer delay) for VersaFEC ACM.
For 64k block DVB-S2, the core latency is 4 times higher. Assuming a WAN Buffer of 20
milliseconds:
•
Latency for 64k block DVB-S2 ACM at 100 ksps = 1336 milliseconds
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
16-QAM 0.731
16-QAM 0.780
16-QAM 0.829
16-QAM 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
MN-CDM570A
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
Revision 3