120M : Looks like Virgin are determined to flush out old modems

[Kushan]

Well then, I obviously have misunderstood how the whole channel system works. I still maintain that 8 channels are better than 4 though, I can still see many scenarios whereby 4 would see congestion that 8 wouldn't.

[Chrysalis]

4 would see congestion maybe if an area is heavily utilised and eg. all 8 channels only have 5mbit/sec spare capacity, so then a 30mbit user would get 20mbit on 4 channels and full 30 on 8.

But if an area is that heavy utilised and given how spiky utilisation is on such small shared pipes then I think 8 channel modems would still see congestion as well just not so severe.

VM/ntl have for years supplied more channels than the modem can use, this was how they operated before bonding started, eg. back in the docsis1 days was 4 US channels load balancing modems that could only use 1 US channel. Of course it was configured in a flawed manner, as channel hopping often got results in dodging congestion. It still happens now with users connected to 1 or 2 US with 2 or 3 US in the pool.

On my vmng300 I did see channel rotation on the DS as well, but it wasnt much as was only 5 DS channels activated in my area, VM didnt bump it to 8 until after I started using my superhub.

I only changed to the superhub in my last month of service as I had FTTC installed and no longer cared as much for my VM connection, if I didnt have FTTC I likely would still be using the vmng300 now.

[qasdfdsaq]

Originally Posted by Kushan

Well then, I obviously have misunderstood how the whole channel system works. I still maintain that 8 channels are better than 4 though, I can still see many scenarios whereby 4 would see congestion that 8 wouldn't.

I still maintain the only time you'll ever see a difference is if the network is being run badly.

[Kushan]

Originally Posted by qasdfdsaq

I still maintain the only time you'll ever see a difference is if the network is being run badly.

This IS Virgin media...

[qasdfdsaq]

[Horizon]

Originally Posted by Kushan

Well then, I obviously have misunderstood how the whole channel system works. .

In my more tech savvie days, I used to be able to understand conversations such as in this thread, but these days such stuff goes over my head. So, like you, I am not sure what is being said here.

For those that understand such things, could someone please give an easy to understand summary of just how the internet is delivered to customers and where and why the congestion/problems happen?

I know there is/was about 750mhz of bandwidth/capacity on the cable network which is split into transport streams or channels. Each channel is about 8mhz each and there are about 75 channels in total, in theory, that could be used. Many of these channels are used for tv, currently 48 in my area. When I last looked at my modem's stats, I think there were 8 downstream channels and 2 upstream channels. So that makes 58 channels in total being delivered to my area.

I know in homes we have the cable modem. In the street we have the cabinet and eventually there is the headend. What next?

[Sephiroth]

What next? You ask:

In addition to Horizon's analysis, it more or less works like this (but can vary in different parts of the country according to who was the original cable supplier):

1/
A small street cabinet connects either 16 or 32 customers. A larger street cabinet connects 48 or 96. It is passive equipment.

2/
The cable from a cabinet passes along to another cabinet configured as above.

3/
5 or 6 cabinets go to a powered Launch Cabinet and 4 or 5 of these go to an optical node and it's fibre from there to the local hub. The optical node cabinet is active.

4/
In high density population areas, street cabinets may all be of the 48/96 capacity kind.

5/
The optical node originally would have supported only 2 upstream channels and these might have been split between two nodes at the VM end line card. So that's a heck of a lot of users who think they're getting, say 10 meg upstream sharing just 40 meg upstream capacity.

6/
A downstream service group comprises 16 channels, within which a user's bonding group currently has up to 8 downstream channels with c. 400 meg capacity to share across one or two optical nodes. VM are also bonding two upstream channels.

That explains how over utilisation can occur.

For high contention areas, VM should be adding more optical nodes and line cards and CMTS devices at their end to cope with the additional channels allocated. That's what they say they're doing.

7/
The optical node connects via fibre to a line card on a UBR/CMTS at the local hub/local head end. Many optical nodes terminate on a single line card. An individual's downstream and upstream (both shared with other users) terminate on the same line card andin the same Service Group.

[Horizon]

Thank you for very much for posting all that Seph. I'm digesting it all now, may have a few questions later.

---------- Post added at 16:49 ---------- Previous post was at 16:33 ----------

Ok, a few questions:

Would it be accurate to say that 16 channels or 800mb is the maximum download capacity available in any one area? And potentially using the most extreme example, that 800mb has to serve the following:

6 large street cabinets of 96 customers each = 576 customers in one launch cabinet.

5 launch cabinets of 576 customers each = 2880 customers at one node.

Thus one node at its maximum capacity has to share 800mb between 2880 customers.

So if all 2880 customers are downloading at the same time, the maximum speed they can get is 3.6mb each.

Is that correct so far?

[Sephiroth]

Originally Posted by Horizon

Ok, a few questions:

Would it be accurate to say that 16 channels or 800mb is the maximum download capacity available in any one area? And potentially using the most extreme example, that 800mb has to serve the following:

6 large street cabinets of 96 customers each = 576 customers in one launch cabinet.

5 launch cabinets of 576 customers each = 2880 customers at one node.

Thus one node at its maximum capacity has to share 800mb between 2880 customers. Is that correct so far?

These extrapolations have been attempted many times. In densely populated areas, cabs are usually fully subscribed and many a horror story exists about splitters in the cab.

So your 2880 customers per optical node is a maximum where no splitters are used. The node serves a service group of 16 downstream channels and the CMTS aims to load balance customers onto 8 channel bonding groups by some algorithm or other that can be tweaked by the admin.

In my area, I have examined street cabs and have assessed that an optical node probably services not more than 500 customers. I did a sensibility check on that. The Liberty Global web site says that VM passes 12 million homes and has 4 million cable broadband customers. That's 33%. In my area there are c. 3,000 homes passed. We have 2 optical nodes in Winnersh - so that's c. 500 customers per optical node.

Does that help?

So, make of that what you will.

[Horizon]

Ok, so you reckon in real terms, excluding inner city areas, its 500 customers sharing the 800mb capacity?

[Sephiroth]

That's what I think. All plus/minus of course.

[StevenNT]

Originally Posted by Sephiroth

These extrapolations have been attempted many times. In densely populated areas, cabs are usually fully subscribed and many a horror story exists about splitters in the cab.

I concur there with my personal experience, made my line almost unusable when it was added by a tech, and thankfully a tech came and removed it about 2 weeks later when I complained (armed with ping graphs and stats) via VM's Twitter team.

[MaverickJesus]

Out of interest, what do you see as a 'sensible' configuration? Keeping in mind reasonable financial constraints, of course.

Given LG have stepped back from ADSL/VDSL stuff, it seems likely they might concentrate on overhauling the core network ready for DOCSIS 3.1/IPv6/whatever.

[Kushan]

Originally Posted by MaverickJesus

Given LG have stepped back from ADSL/VDSL stuff, it seems likely they might concentrate on overhauling the core network ready for DOCSIS 3.1/IPv6/whatever.

If they plan to remain competitive, this is pretty much their only option. 3.1 is still a couple of years away but I dare say it's the most financially sound way to increase capacity on the network.

[qasdfdsaq]

Originally Posted by Horizon

Would it be accurate to say that 16 channels or 800mb is the maximum download capacity available in any one area? And potentially using the most extreme example, that 800mb has to serve the following:

Depends on the area, not all areas have 16 channels, some may have more, some may have less. They stared with as low as 3 per area and have been adding more as and when required. AFAIK most should have at least 8 by now but that's not necessarily the case.

There's no hard limit that says they cannot deliver more than 16 channels through one node either.

Should also point out your extreme example is indeed an extreme example, uptake, population density, node size, and cabinet size all vary widely, one cabinet does not always equal one node, and finally, 3.6Mb per customer isn't actually all that bad.

Not long ago ISPs were selling to a contention ratio of about 50:1, meaning a "Up to 24Mbps" ADSL customer could be allocated only 0.5Mbps (or less). I recall once hearing of a certain ISP only allocating sufficient bandwidth for 64Kbps per customer at peak time. Consumer ISPs have always been overselling to those sorts of levels, on the basis that most people will not be using most their speed most of the time. That's how consumer broadband ended up being so cheap.

---------- Post added at 18:57 ---------- Previous post was at 18:48 ----------

Originally Posted by MaverickJesus

Out of interest, what do you see as a 'sensible' configuration? Keeping in mind reasonable financial constraints, of course.

IMO 50:1 contention ratios are just fine, especially on faster lines. Back where I used to work, we had around 2,200 student residences sharing 1000Mbps of capacity. And that never went above 30% load. Course, that was 2006...

Last I recall the average consumer used something around 50GB a month or less. On an average connection speed of 12Mbps, that's about 1.2% the capacity of their line. Assuming usage was evenly distributed throughout the month (which it isn't) then you'd only need 144 kbps per customer for everyone to get full speed. Even on VM where average speeds are over double that, 3.6Mbps is still over ten times the bare minimum in worst case scenario, supposedly. Which really isn't that bad.