what does DSL sync rate actually mean?

It's the data rate rate on the line, like 54M is the over the air rate on 802.11g, expressed as the transmission fate of ATM cells in the case of ADSL and as the data rate of Ethernet frames in FTTC which uses packet transfer mode rather than ATM.

With the ATM cells (if applicable) are Ethernet frames, within those are PPP frames, within which are TCP/IP etc etc.

So the overheads mount up, esp where ATM is involved with a 5 byte overhead on a 48 byte packet.

In reply to a post by yarwell:

It's the data rate rate on the line... expressed as the transmission fate of ATM cells in the case of ADSL and as the data rate of Ethernet frames in FTTC...

So you're saying it's the data rate after ADSL or VDSL overheads have already been subtracted? ie it's not the data rate on the line

(sorry, I'm not trying to be difficult or pedantic, I'm trying to be clear)

In reply to a post by Bright:

So... Plusnet's IP Profile makes sense to me as a mechanism to help implement traffic prioritisation under congestion conditions - presumably when defined output buffer thresholds are exceeded, packets are discarded from lower priority queues first. And knowing the maximum output rate to the customer router helps to minimise dropped packets and optimise congestion control - if you know you can drain a buffer more quickly, you can afford to buffer more data. Thanks to Chris and MrSaffron for clarifying what's going on at the Plusnet end.

Bingo!

On ADSLx the sync speed is the raw bit-rate. All the wrappers, i.e. ATM; TCP/IP; framing or whatever use some of the bits, leaving less bits for data.

On VDSL2 where ethernet frames are being transmtted, not ATM packets, for the moment I pass .

My point is that the two are fundamentally different.

I would have thought that even on VDSL2 it would be the raw bit-rate, but have a feeling I'm wrong - from a vague memory of the relevant Openreach documentation. Which I'm too tired to find and re-understand at the moment. I expect yarwell has it right.

The data throughput is always going to be lower than the sync, whatever else is being done in terms of controlling the data flow.

no, it's inclusive of overheads. The ADSL syn is the ATM rate, so 2M is 2272 kbits/s of ATM cells out of which you get about 1900 kbits/s of useful data through a speedtest.

eta: Perhaps "data rate" wasn't the best phrase to use, "symbol rate" may be better - the rate at which information goes down the wires where "information" is anything digital, effectively the physical layer. The "goodput" of useful data arriving in your computer is 80-95% of that.

Edited by yarwell (Thu 14-Feb-13 19:35:22)