Virgin Media - how does it all work then?

digitalspace · 10-10-2009, 14:24

Hey,

I've been wondering how this Virgin Media technology works. My understanding is that you start off with a coax cable that goes from house to a green cabinet in the street. Inside this cabinet is an RF distribution box with a load of F-type taps. From this street cabinet/RF unit is a single fibre optic cable which runs to a "UBR (?)" located in some exchange-style building.

If my understanding is correct, then where does the bandwidth contention lie? In the green cab, or the UBR? During the day time my 20mb connection rarely achieves 10mb, which I can accept, but is it the UBR becoming busy or the local green RF amp? How many street cabinets will a UBR take, for example?

Many thanks in advance

Graham M · 10-10-2009, 15:28

Most street cabinets have a thicker Coax cable running between them and the fiber terminates at the larger cabinets which you can normally tell apart as theyre about 6ft tall and hum (like me

)

Most of the bandwidth constraint is likely to be at the UBR

10-10-2009, 17:09

Its all done with smoke and mirrors

This should give you a better understanding.
http://en.wikipedia.org/wiki/Hybrid_fibre-coaxial

digitalspace · 10-10-2009, 17:12

Yeah, I've seen that, I just wondered specifically how the VM network is put together

Graham M, smaller cabs are fed via coax from a larger one eh? Interesting stuff

I've never seen a 6ft green cab around here before!

Graham M · 10-10-2009, 17:28

Normally the ones I've seen are on little side roads, that may not always be the case mind

Sephiroth · 10-10-2009, 20:26

With digitalspace's question answered, the next question is how does VM load balance so that UBR's provide a roughly equal service?

Indeed this is a tough one because different locales may have different use profiles and I doubt that VM re-allocate individual users to different UBRs according to usage profile.

This, in turn, means there's pretty well a postocode lottery as per http://www.cableforum.co.uk/board/12...over-then.html.

Not good.

altis · 10-10-2009, 20:30

Try this:

http://homepage.ntlworld.com/draig.g...iles/frame.htm

Sephiroth · 10-10-2009, 20:44

Quote:

Originally Posted by altis

Try this:

http://homepage.ntlworld.com/draig.g...iles/frame.htm

I'm grateful for this link.

It is a good primer, but written in the 512Kbps days - 8 or 9 years ago, I guess. Now we're at 20 & 50 Mbps, it would be interesting to see an update; they haven't moved the UBR to the node so we're begining to unravel the postcode lottery issues, I'd have thought.

Ignitionnet · 10-10-2009, 22:03

Quote:

Originally Posted by Graham M

Most street cabinets have a thicker Coax cable running between them and the fiber terminates at the larger cabinets which you can normally tell apart as theyre about 6ft tall and hum (like me

)

Those aren't actually the nodes those are telco muxes and not related to the CATV side of the network. CATV fibre nodes are silent unless they need extra cooling and no bigger than your average cabinet.

---------- Post added at 23:03 ---------- Previous post was at 22:43 ----------

Quote:

Originally Posted by Sephiroth

I'm grateful for this link.

It is a good primer, but written in the 512Kbps days - 8 or 9 years ago, I guess. Now we're at 20 & 50 Mbps, it would be interesting to see an update; they haven't moved the UBR to the node so we're begining to unravel the postcode lottery issues, I'd have thought.

I'll write up some more but basically where previously there was 1 downstream port feeding 10 nodes of 500 homes passed for example there are now 1 or 2 downstream ports feeding each node of 500 homes passed.

Downstream ports feed multiple nodes via optical splitting in the hubsite on the downstream side, and optical combining on the upstream side as it runs the other way.

Where nodes are larger, such as say 5000 homes passed, these nodes have had fibre deployed deeper into them, replacing some of the coax with fibre and reducing the size of the node. Nodes usually feed up to 4 coaxial trunks, so simply have 4 bits of string to the node instead of 1 and you have split it into 4 nodal areas (areas able to be logically and physically separated from the rest of the network by virtue of their own fibre feed from hubsite) of 1250 homes passed.

Once each coaxial trunk has its' own fibre the only solution then is to go further down the chain to where that trunk has 'branches' and replace that run of copper with fibre by building a new node. Feed 4 bits of string down to the next 4-way junction on the coaxial network and you've just split a 1250 home node down into 4 nodal areas of just over 300 homes each.

That's pretty much it. The CATV network is like a tree, push the fibre deeper into the network and closer towards the end of the branches as the less customers or leaves to continue the analogy share the fibre.

Rather than using additional fibre to produce new nodal areas one can also use WDM / wave division multiplexing and digitise the analogue signals at for transport down the WDM'd fibre and decode back to analogue when they reach their destination for delivery to CPE or CMTS depending if downstream or upstream.

On the VM network downstreams are largely 38Mbps apart from most of the new DOCSIS 3 downstreams which deliver around 50Mbps. A few legacy network downstreams are 27Mbps.

Sephiroth · 10-10-2009, 22:26

Quote:

Originally Posted by Broadbandings

....On the VM network downstreams are largely 38Mbps apart from most of the new DOCSIS 3 downstreams which deliver around 50Mbps. A few legacy network downstreams are 27Mbps.

Is it the end-to-end downstream(s) you're referring to or to the hub? Or which segments?

Ignitionnet · 10-10-2009, 23:12

Quote:

Originally Posted by Sephiroth

Is it the end-to-end downstream(s) you're referring to or to the hub? Or which segments?

Downstream referring to a single DOCSIS bearing channel. These channels are between hubsite and home, via the optical and coaxial HFC network.

digitalspace · 10-10-2009, 23:13

Holy-thread-takeover batman! I'm mega confused now. Sephiroth is stealing my thunder!

Sephiroth · 10-10-2009, 23:31

Quote:

Originally Posted by digitalspace

Holy-thread-takeover batman! I'm mega confused now. Sephiroth is stealing my thunder!

Nah - Xocemp, altis and Broadbandings provided the intellectual material and I asked the necessary questions to square the circle FOR YOU.

What don't you understand now?

You asked a very good question of great interest to a lot of people. And the stuff discovered on this thread paints a first class picture of where bottlenecks could occur.

I feel a mega-diagram Visio-ing soon!

REM · 10-10-2009, 23:57

A diagram would be useful.

EDIT. Already mentioned above. Oops.

digitalspace · 11-10-2009, 00:30

Quote:

Originally Posted by Sephiroth

What don't you understand now?

Downstream / nodes / etc, they're all terms I understand but not in the context that Broadbandings is providing.

A nice simple diagram showing actual cables used between distribution / conversion points would be awesome. Take this for example, if the green cab is a simple RF amplifier and nothing intelligent, and all the fancy stuff lies in the UBR in some building somewhere, then why is it seemingly so difficult to upgrade an analogue cabled area to a digital service? Surely all it takes is to pull the fibre cable from analogue UBR and plug it in to a digital one?