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[Weaver]

I need to convert from sync rate to IP PDU bit rate, the number of bits per second sent in IP packets including the IP headers.

My logic is 53 * 32 / 1492 (my MTU) = 32 ATM cells needed to transmit 1492 bytes should fit well as there's plenty of free space left in the last 40 byte cell payload to fit PPPoEoA overheads in.

Sound about right?

[Weaver]

The reason is that my Firebrick needs to be told how much us bit rate (IP PDUs) to reserve for each of my three outbound pipes. So I ought to convert us sync rates into a limit for the bit rate coming out from the Firebrick.

[Weaver]

Actually 2000 / 2272 comes out as ~0.88, support for my figure calculated by ruminating. The figure of 2272 bps is the ds sync rate for IP Profile 2000 in BTW 20CN ADSL1.

Have I got this right or should I reduce / fudge the factor down a bit to allow a safety margin for yet more unknown overheads and slowness?

It's upstream I'm calculating in this case, as I mentioned earlier.

[Weaver]

Huge amounts of small-hours experimentation would seem to indicate that for my Firebrick on a slow 20CN ADSL line, a suitable fudge factor for upstream is 0.888 (numbers in the range 0.880 - 0.895 all have their merits, values as high as 0.925 can push the download speeds slightly but really ruin the upstream speed which is the point of the entire tuning exercise). Take the upstream raw _sync rate_ given out by the modem, multiply by the above fudge factor and that is the number you out into the traffic shaper speed parameter in the Firebrick's config.

Are assuming that this converts from raw sync rate to bits per second of AAL5 SDUs. That is _everything that is sent to the modem_, eg. L3TPDU= IP header, IP payload and all.

[jelv]

The sync speed to IP Profile conversion is 88.2%...

[aesmith]

For what it's worth when I was testing upstream this weekend, the effective rate seemed to be 392,000 bits per second for an upstream synch rate of 448K.  That was over Ethernet so you'll need to remove the Ethernet overhead to get the raw L3 figure.   I'm on 20CN "IP Stream Max".

[Weaver]

Many thanks.