There may be some confusion behind this.

The line sync rate as reported by the modem will never be hit even if a 100% perfect computer and connection was present, because there are some inherent overheads in carrying IP traffic over the network to where it breaks out onto the Internet.

The 96.2% IP profile setting pretty much takes this into account.

We do not control the sync speed.

The traffic management on the Unlimited product is designed to allow all traffic through as fast as possible, it prioritises gaming/VoIP etc over other traffic if you are saturating your line though. There are no rate limits or speed caps on the Unlimited service.

In reply to a post by chrisparr:

We don't affect the sync of the line, we set the profile on our site to match the one reported on a BT Speedtest.

Probably been asked before then: what is the point in having a Plusnet IP profile in addition to the BT IP profile? More to go wrong when they are out of synch, surely?

My connection is 20CN and the reported throughput speeds as a percentage of the IP profile are around 96%-98.75% using the new BTw speedtester, whereas with the previous BT speedtester I would get around 88% or on a good day maybe 92% of the reported IP profile.

I don't think my connection has improved that much so why the improvement in throughput as a percentage of the IP profile?

It means we can control the QoS on the line rather than it being random. Having our own profile allows our traffic management platform to control which packets are dropped if the line is being utilised to it's full potential.

In reply to a post by RobertoS:

The Plusnet Current line speed seems to be set to 37(?) or 78, depending on the product.

Mine isn't it's 64.2, derived from my current IP profile of 64.23 Mbps.

In reply to a post by Bright:

Thanks for the explanations, Bald_Eagle1, RobertoS and others. That has filled several gaps in my understanding, but now lead on to several further questions I'm not clear about:

1. So what is the point of the BT IP Profile? If the DSL link syncs at a given speed and the IP Profile is then set to about 0.97 of that value, so what? What is it used for? Is it used for flow control? If so, what's the point? The actual DSL line rate could have dropped below the IP Profile value unnoticed, in which case it achieves nothing. And in any case, the actual DSL line rate limits the throughput on the line. Am I missing something obvious? I've never properly understood BT's DLM!!
   

Can anyone illuminate these issues for me, please?

Snipped the ones I can't answer.

BT use the IP profile internally, to throttle traffic from your ISP to you, so that even if your ISP doesn't have properly configured rate limiters, traffic sent to you cannot overwhelm the DSLAM and take out service for your neighbours as well.

This mostly matters in a DoS situation - if some bad guy is trying to send you 10GBit/s of traffic, it's clearly not going to all get through. If your ISP sent all 10GBit/s of traffic on to you, this would overwhelm the DSLAM's gigabit links to the rest of the BT network; by throttling elsewhere in the BT network (closer to the ISP), BT can arrange things such that the only people that suffer are other customers of your ISP, rather than your neighbours. That, in turn, gives your ISP an incentive to do things properly, and throttle at their end so that they don't hit the BT limiters.

The difference between IP profile and speedtest results will be

a) the problem of getting 100% accurate results from a test e.g. a 10 ms difference in timing can build up
b) the routing of the IP data that is the payload for the test, hence why lots of small packets can be bad if testing
c) Performance of firewall, CPE router, av software and operating system

The same theory as to why if your router at home has proper QoS you have to tell it the connection speed a lot of the time.

It would be dangerous to assume it, as plenty of packages around world where throughput is capped below the connection speed, and Ethernet WAN routers have no visibility of the DSL layer speeds.

Thanks to everyone who replied to my questions and apologies to the OP if I have slightly hijacked the original topic, but I think it all helps to clarify how line sync rate, IP Profiles and DLM actually work. My perception is that there are a lot of misconceptions around this whole subject.

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.

Also... I can see how BT's IP Profile helps them to manage congestion by flow controlling traffic where it enters their network, keeping excessive traffic flows out of their network in the first place. Which presumably also means that they need less capacity in the network to handle peaks. I guess they just randomly drop packets if the throughput of a given virtual circuit exceeds the IP Profile value...? Which would make Plusnet's IP Profile more important for traffic prioritisation and rate limiting before it's handed over to BTW.

And... one final question: what does DSL sync rate actually mean? Is it the raw line bit rate, or is it the data bearer channel throughput rate, after allowing for DSL frame, superframe and control overheads? Several posters referred to the overheads for TCP or UDP (or other protocols), and ethernet frames or ATM cells. But these are all on top of the DSL overheads as well. Even UDP over ethernet in full 1500 byte packets would have a slightly higher overhead than the 96.79% allowed on FTTC in calculating the IP Profile. So I'm wondering if that factor is actually just to allow for the DSL overheads...?

Any thoughts? Thanks again, folks!