Versafec Acm Latency - Comtech EF Data CDM-625A Installation And Operation Manual

CDM-625A Advanced Satellite Modem
Appendix M

VersaFEC ACM Latency

M.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 appendix 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 M-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.
Table M-2. VersaFEC Implementation of ACM – 100 ksymbols/sec Example Case
ModCod Modulation
00
01
02
03
04
05
06
07
08 16-QAM
09 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.
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
•
Latency for 16k block DVB-S2 ACM at 100 ksps = 349 milliseconds
• Latency for VersaFEC ACM at 100 ksps = 54 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
0.731
0.780
0.829
0.853
OVERALL SYSTEM LATENCY = Worst-case ModCod (ModCod0)
Latency = 34 milliseconds, NOT INCLUDING WAN BUFFER OR SATELLITE PATH
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
M–8
MN-CDM625A
Minimum Latency,
In milliseconds
49 kbps 34
107 kbps 32
126 kbps 30
141 kbps 28
161 kbps 26
193 kbps 30
213 kbps 28
234 kbps 27
293 kbps 27
312 kbps 26
332 kbps 25
341 kbps 25
Revision 4