DLM - The Big Question

Hello again,
I know it has been talked about before, and kitz.co.uk have some older details on it.
But my question is, Is there any published information available on DLM and does it really vary between ISP, i.e. BT Retail and Sky for example.

I'm mainly interested in DLM in relation to FTTC but any solid details on BT's / UK based implementations would be very useful!

Cheers,

flipdee

its managed by openreach so shouldnt vary across isp's unless they choose different DLM profiles. As there is 3 DLM profiles.

http://www.thinkbroadband.com/news/5929-reminder-abo...

Wow, Must have missed that article, thanks a lot MrSaffron!
Is there a definitive answer on vdsl modem reboots/power cycles?
Can the DSLAM definitely not distinguish between "issues" and modem reboots?
For example, if high latency and packet loss occur when upstream is maxed out, will DLM intervene?

Cheers,
flipdee

In reply to a post by flipdee:

For example, if high latency and packet loss occur when upstream is maxed out, will DLM intervene?

Thats an effect on TCP/IP caused by a saturated link. Its nothing to do with the physical layer (ADSL or VDSL) that your line is working at. DLM only looks at the physical layer.

Hi jchamier,
Thanks for your reply.
When the downstream is maxed out the ping doubles to around 120ms but no packet loss.
Is there a situation where DLM will not ensure a "robust" connection, is the packet loss described something the ISP is responsible for?
I'll have to check line stats during the packet loss.

Cheers,
flipdee

The DLM will not be worried about you saturating your line and delaying the TCP packets

Is there a definitive answer on vdsl modem reboots/power cycles? NO
Can the DSLAM definitely not distinguish between "issues" and modem reboots? Varied feedback from end-users
For example, if high latency and packet loss occur when upstream is maxed out, will DLM intervene? No, it will only intervene if it sees packet loss over the VDSL2 layer i.e. a packet it knew it had capacity to send did not reach the other end. TCP congestion is at a different layer.

Could that just be the processor in your router struggling to deal with the ping requests when your line is maxed out?

Assuming you are noticing the difference in ping by looking at the TTB broadband quality monitor?

In reply to a post by Ripley:

Could that just be the processor in your router struggling to deal with the ping requests when your line is maxed out?

Or just leaving them at the bottom of the list of things to worry about. I would assume any half-way decent router would prioritise its tasks and responding to ICMP requests (especially Echo Reply) ought to be one of the least important things on its list.

Hey there,
Thanks a lot for your replies.
It's actually a ping from a Sky Fibre connected network to the outside world (a known low latency host which is generally 35ms away from most ISP's.

I haven't tried pinging the Sky Hub from the outside world as I would consider it not a reliable test.
If the router is dropping packets to the outside world, I would suspect the TCP layer is saturated as suggested and some bandwidth throttling is in order.

Just a note, the upstream packet loss was being noticed when speed tests were carried out, obviously attempting to saturate the line, however this is the first time i've seen this packet loss on a "supposedly" good fttc line, which is why I was curious about DLM.
What other mechanisms would be used to prevent this from happening?
Cheers,
flipdee

In reply to a post by flipdee:

Hello again,
I know it has been talked about before, and kitz.co.uk have some older details on it.
But my question is, Is there any published information available on DLM and does it really vary between ISP, i.e. BT Retail and Sky for example.

For exchange-based ADSL, it does indeed work differently across the main providers.

In BT-Wholesale, 20CN equipment tends to use the older form of DLM, known as Automatic Margin Adjustment. 21CN equipment integrates this alongside banding, more properly known as Tiered Rate Adaption. Both systems incorporate the use of FEC for error correction, which makes use of interleaving to make the error correction work more effectively.

Someone posted a table of MTBE thresholds on the Plusnet forum that suggests, for BT's 21CN, what is being monitored - either the error rate (CRC's per 24 hours) or the rate of errored seconds per 24 hours (as MTBE, mean time between errors), plus the rate of resyncs/disconnections (as MTBR). Being red or green suggests that a line would be bumped up or down a list of profiles, which determines the next margin/band/FEC setting to use; a further post there expands upon this nicely.

I don't know what Sky's DLM techniques are, but they certainly seem to go for a softer startup, which looks like TRA in reverse - they seem to start you in a lower speed band (and presumably measure the error rate), then turn the speed up a little after a day. Repeated a few times, if your line can cope.

Openreach FTTC still uses FEC, but has a broader variety of settings. It doesn't use AMA, but does use TRA.

The NICC published a report on DSM techniques in the UK, dated 2010, but it doesn't name ISPs (DSM is slightly wider-reaching than, but includes, DLM). From the date, you can tell that this applies more to ADSL varieties than FTTC.

According to NICC, their 2014 to-do list includes more work on DSM, alongside exchange-based VDSL, plus use-cases for vectoring.

I'm mainly interested in DLM in relation to FTTC but any solid details on BT's / UK based implementations would be very useful!

The one thing I can add is that the VDSL2 specification includes the ability for the modem to signal loss-of-power to the DSLAM, aka "dying gasp".

Whether the DSLAM makes use of it is unknown, but if it does, then this implies it is better to power-down a modem rather than unplugging the RJ11 cable. In either case, the DLM for FTTC seems generally more immune to re-syncs caused by "user interference".

FEC (plus interleaving) seems to be the main tool used by DLM in FTTC - with the aim of making the line report very few errors. The theory seems to be that NGA-like speeds on internet access leads to users wanting more video services. Increased expectations imply that users will want a high-quality (broadcast-like) video service, and that implies very low packet loss, as they don't include a TCP layer for re-transmission.

The downside of FEC is that it steals 15-30% of your line's bandwidth (usually around 20%), while the associated interleaving adds a minimum of 8ms to latency.

There is an alternative to FEC that puts re-transmission of faulty packets down at the physical layer, rather than up at the TCP layer. This is known as PHYR. Will Openreach ever implement it? Unknown - but in the requirements for self-install FTTC modems, it is one of the requirements placed on modems.

_{Edit: Add FEC and PHYR for FTTC}

Edited by deleted (Tue 04-Feb-14 11:17:43)

Hi WWWombat,
Fantastic reply, thanks a lot.
This is the sort of information i've been looking for.

It's a pity "real" line stats aren't available on the sky hub.
However quick update on possible DLM activity.
Sky Hub first sync'd on a recently activated Sky Fibre Unlimited Line.
Traffic Type:PTM
Line Rate - Upstream (Kbps):1316
Line Rate - Downstream (Kbps):9896

Latest Sync Status as of 21:43 on 04/02/2014
Traffic Type:PTM
Line Rate - Upstream (Kbps):798
Line Rate - Downstream (Kbps):7959

I had expected the upstream to be reduced, I'll try to do a speed test again to see if it's helping with the packet loss under saturation conditions.

Does anyone know of a VDSL2 modem which can be used to be evaluate DLM activity?
I'm still waiting on a Fritz!Box 3390 to arrive, sounds like it's my best bet for decent line stats while maintaining firmware updates, with the current Huawei HG612 hacked firmware being out of date.

Cheers,
flipdee

Why's the HG612 firmware out of date? There are hacked versions from October which as far as I'm aware is the latest version.

http://www.skyuser.co.uk/forum/Skybroadband-fibre-he...
I believe telnet is still available though, so maybe not as big a problem after all.

Yes telnet is still available, and so is the web interface if you want it. Both the monitoring programs work as well. There is a large thread on the kitz forum with the various firmware versions. I'm on my phone, otherwise I'd find the thread and post a link.

Hi R0NSKI,
Did you let the remote update take place or is there a flash file available for the very latest?
Cheers,
flipdee

In reply to a post by flipdee:

Sky Hub first sync'd on a recently activated Sky Fibre Unlimited Line.
Traffic Type:PTM
Line Rate - Upstream (Kbps):1316
Line Rate - Downstream (Kbps):9896

Latest Sync Status as of 21:43 on 04/02/2014
Traffic Type:PTM
Line Rate - Upstream (Kbps):798
Line Rate - Downstream (Kbps):7959

They look more like ADSL sync rates than VDSL or am I getting the wrong end of the stick? What speed were you quoted when you signed up for fiber?

Thanks
Tim

Hi Tim,
Would you believe that's faster than the original speed estimated by sky (was 6Mbps), it's a long line and had been less than 1Mbps ADSL so 7/8Mbps is a great improvement.

Traffic Type:PTM is FTTC.

Thanks RobertoS, It's good to know for the future.

It's good to see that, even on longer lines, FTTC can give a better sync speed than ADSL in some circumstances.

Thanks
Tim

Hi timl,
I'm pushing FTTC to friends and customers on this basis, in nearly every case you will get a least a bit better speed for around the same money.
This is excluding one extreme scenario when BT Wholesale estimated 10Mbps DS FTTC but the line actually syncs at 1.9Mbps and can only sustain 0.6Mbps before latency becomes a major problem.

So for most people, yes, in the Sky Hub case a friend has no o2 signal and they really wanted an o2 Boostbox but o2 recommend at least 4Mbps broadband so this should cover this, along with actually being able to watch online streaming video etc!

cheers,
flipdee