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[digitalnemesis]

Why do I still see FEC errors even though interleaving is off?

Is there still some very slight FEC on fast path?

My pings are 5 ms to bbc.co.uk whereas previously 14 ms.

[Chrysalis]

on the upstream?

FEC does not require interleaving, and BT do run FEC alongside fastpath on the US on the open profile.

I did once also see FEC alongside fastpath on my line on the DS when I had my fault a couple of years back.

[digitalnemesis]

FEC on ds on fast path

[Interceptor121]

You can have fec active together with retransmission and no delay. I have the same RS takes care of forward error correction

[digitalnemesis]

You can have fec active together with retransmission and no delay. I have the same RS takes care of forward error correction

I don't have G.INP. I think I have RS then on fastpath.

[WWWombat]

FEC and interleaving are two separate things.

Sure, FEC is more effective if it runs alongside interleaving, but it works on its own too. Not true the other way around - interleaving alone, without FEC, is pointless.

Having said that, FEC without interleaving (and without G.INP) is rare downstream, even if common upstream.

[digitalnemesis]

Yes, it's strange, I'm still seeing FEC on fastpath, no G.INP

Code: [Select]

                        VDSL2 framing
                        Bearer 0
MSGc:           18              30
B:              249             238
M:              1               1
T:              64              16
R:              4               16
S:              0.1579          0.8862
L:              12872           2302
D:              1               1
I:              254             255
N:              254             255

                        Counters
                        Bearer 0
OHF:            18897652                1014060
OHFErr:         5758            0
RS:             1209424551              1557613
RSCorr:         189897          0
RSUnCorr:       7460            0

                        Bearer 0
HEC:            6108            0
OCD:            39              0
LCD:            39              0
Total Cells:    349776833               0
Data Cells:     1134236159              0
Drop Cells:     0
Bit Errors:     0               0

ES:             1484            0
SES:            161             0
UAS:            226             58
AS:             47929

                        Bearer 0
INP:            0.00            0.00
INPRein:        0.00            0.00
delay:          0               0
PER:            2.53            14.23
OR:             75.71           20.23
AgR:            50479.97        8596.66

Bitswap:        896/897         0/0

Total time = 6 days 1 hours 51 min 7 sec
FEC:            189897          0
CRC:            5758            0
ES:             1484            0
SES:            161             0
UAS:            226             58
LOS:            1               0
LOF:            8               0
LOM:            39              0

[WWWombat]

Yup. 4 bytes out of every 254 used for FEC correction downstream, and 16 bytes out of 255 upstream. No interleaving, no G.INP.

[Chrysalis]

To be honest I would maybe accept light FEC interleaving on my connection DS, even without interleaving it has a noticeable affect on error counts.  Of course not as nice as g.inp but its a poor man's alternative when wanting to avoid horrible interleaving.

[digitalnemesis]

What do these letters represent exactly?

Code: [Select]

B:              249             238
M:              1               1
T:              64              16
R:              4               16
S:              0.1579          0.8862
L:              12872           2302
D:              1               1
I:              254             255
N:              254             255

[j0hn]

B (number of bytes in Mux Data Frame)
M (number of Mux Data Frames in FEC Data Frame)
T (Mux Data Frames over sync bytes)
R (number of check bytes in FEC Data Frame)
S (ratio of FEC over PMD Data Frame length)   
L (number of bits in PMD Data Frame)
D (interleave depth)

Alphabetical list of line parameters & error counters

[Interceptor121]

How do you calculate the FEC overhead? 4/64 is exactly my tested speed / sync rate indicating 6.25% overhead

[WWWombat]

I = Interleaving block size
N = RS block size

Which means interleaving latency is proportional to (I*D),
and user data per RS block = (N-R).

RS overhead = (R/N).

Note the sync speed is calculated from the accumulated "user data" only (ie N-R), so is calculated after the FEC overhead has been taken off.

The difference between "download speeds" (aka throughput, or goodput) measured by speedtesters, and sync speed comes from the size of the PTM, PPPoE and IP headers.

Remember that speed testers show roughly the same reduction for a line with fastpath and with G.INP (with its associated, rather low, levels of FEC).

[Interceptor121]

So in my case sync 40000 R=8 N=147 FEC overhead=5.44% delay=0 so no interleave. Overhead in kbps 2300. The line would sync at 42300 if FEC was off. So as my max is 43400 best case am getting FEC as result of capping to 40000

[ejs]

Not necessarily, because the FEC adds coding gain, which the modem will factor in when determining what speed it can connect at.