Virgin Media - how does it all work then? [Archive]

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digitalspace

10-10-2009, 15:24

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, 16:28

xocemp

10-10-2009, 18:09

Its all done with smoke and mirrors :p:

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

digitalspace

10-10-2009, 18:12

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, 18:28

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

Sephiroth

10-10-2009, 21:26

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/12/33655753-manchester-the-honeymoon-period-over-then.html.

Not good.

altis

10-10-2009, 21:30

Try this:

http://homepage.ntlworld.com/draig.goch/Draig/cablemodem_files/frame.htm

Sephiroth

10-10-2009, 21:44

Try this:

http://homepage.ntlworld.com/draig.goch/Draig/cablemodem_files/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, 23:03

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 ----------

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, 23:26

....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

11-10-2009, 00:12

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

11-10-2009, 00:13

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

Sephiroth

11-10-2009, 00:31

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

11-10-2009, 00:57

A diagram would be useful.

EDIT. Already mentioned above. Oops.

digitalspace

11-10-2009, 01:30

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? ;)

browney

11-10-2009, 08:54

Sephiroth

11-10-2009, 09:27

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? ;)

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.....
You made me re-read Broadbandings' helpful update to the NTLWorld diagram and I came up with the above quote.

What's this about "analogue" in the broadband transport chain? Where is it analogue? Which equipments are analogue devices?

Impz2002

11-10-2009, 12:31

So what are those green boxes that are only about 20inches tall what are those?

also there is a building 1.6miles from me that has virgin media written all over it and it has about 6 satellite dishes (big ones) outside.

What is that building?

The tiny green boxes are BT AFAIK or are Telco cabinets of some sort.

The building you are refering to is a Franchise head end. This is where the UBR's are all connected to and where you will find the equipment that provides all the TV / VOD / Connection to VM's Backhaul for broadband connections. These regional centres are then either fed from Bromley or Langley Super-headends where all the content for TV comes from.

This is as far as i understand it and without any too technical terms :)

Impz

---------- Post added at 12:31 ---------- Previous post was at 12:29 ----------

I am sure Broadbandings will correct me if i am wrong as he is the man with the real technical knowledge :)

Impz

Ignitionnet

11-10-2009, 13:31

You made me re-read Broadbandings' helpful update to the NTLWorld diagram and I came up with the above quote.

What's this about "analogue" in the broadband transport chain? Where is it analogue? Which equipments are analogue devices?

The cable side of the CMTS / uBR outputs analogue signals, these are combined with the analogue outputs from the TV multiplexers and used to modulate the downstream laser.

Digital information is encoded in analogue signals using QAM (http://en.wikipedia.org/wiki/Quadrature_amplitude_modulation) these outputs are then used to modulate the downstream laser's output.

Lasers don't just have to be on or off :)

---------- Post added at 13:31 ---------- Previous post was at 13:24 ----------

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? ;)

The RF amplifiers have certain specifications.

For example, this (http://www.made-in-china.com/china-products/productviewtmEQkJGltxvP/CATV-Trunk-Amplifier-550MHz-CGGF5033-CG-B001-.html) kind of thing may well be what's used in analogue areas. If all that 550MHz is used up there's no space to put the digital signals.

Yes a straight switching over, unplug one input and plug in the other would be ideal but the people who receive analogue cable in the area might get a tad upset at their TV just going off like that ;)

The upgrades in areas with restrictions on amplifiers usually consist of replacing the amplifiers with higher rated ones that go to 750 or 860MHz, and depending on the quality of the coax in the network this may necessitate replacing that as well as higher frequency signals don't travel as well as lower frequency ones.

Sephiroth

11-10-2009, 14:12

The cable side of the CMTS / uBR outputs analogue signals, these are combined with the analogue outputs from the TV multiplexers and used to modulate the downstream laser.

Digital information is encoded in analogue signals using QAM (http://en.wikipedia.org/wiki/Quadrature_amplitude_modulation) these outputs are then used to modulate the downstream laser's output.

Lasers don't just have to be on or off .

Not being clued up on the insides of VM cable, I was surprised to see the word "analogue" there. Now, you're rarely wrong at this level of discussion. So any further knowledge tap from you would be appreciated.

Since analogue bandwidth is several times larger than digital channels and I'm told that RG (where I'm served by VM) is a digital solution, I would have expected to see digital bit streams (especially because of HDTV) modulated by QAM.

So what's this analogue stuff?

altis

11-10-2009, 15:02

All digital signals are analogue really if you look at them close enough.

Ignitionnet

11-10-2009, 16:02

Not being clued up on the insides of VM cable, I was surprised to see the word "analogue" there. Now, you're rarely wrong at this level of discussion. So any further knowledge tap from you would be appreciated.

Since analogue bandwidth is several times larger than digital channels and I'm told that RG (where I'm served by VM) is a digital solution, I would have expected to see digital bit streams (especially because of HDTV) modulated by QAM.

So what's this analogue stuff?

QAM is analogue, and the bit streams are encoded into QAM channels. Pure digital / baseband has a spectral efficiency of 1 bit per Hz, 256QAM has an efficiency of 8 bits per Hz. No real competition.

Analogue TV is 8MHz per channel, digital TV is digital streams encoded into analogue for transport. Once the QAM has been decoded it will produce 1s and 0s.

http://en.wikipedia.org/wiki/Quadrature_amplitude_modulation

This is a nice image.

https://www.cableforum.co.uk/images/local/2009/10/34.jpg

Each grouping in the lower right indicates 8 bits of data, there are 256 possible points in the constellation, 2 ^ 8 = 256. The signal that arrives at the decoder is actually 2 signals 90 degrees apart in phase, the decode splits them and decodes them, one into the horizontal axis on the constellation, one into the vertical axis, 16 possible values for each signal, that produces the 8 bits per Hz.

digitalspace

11-10-2009, 19:24

Confused again.... Does anyone have a pretty diagram of how VM supply my house with broadband? :D

Sephiroth

11-10-2009, 23:33

Confused again.... Does anyone have a pretty diagram of how VM supply my house with broadband? :D
You're right. I've taken this off to a separate thread.

I think, for now, we have a picture from the earlier parts of this thread as to how VM supply your house.

When Broadbandings and I have sorted out this crazy detail, I might get onto Visio and update what's been posted in this thread.

From my perspective (and I suspect yours) we really want to get to the bottom o whether or not the transport path from modem to centre is as efficient as possible (i.e. handles optimum bandwidth).

digitalspace

01-06-2011, 17:18

That Visio would be great ;)

I'm looking at my green box outside, and wondering where that connects to, and how. Hmmm. I live in Southampton, if that helps.

Dave9946

01-06-2011, 21:49

Hmm, interesting reading (dont blame me for this I never dug the thread up lol). But in theory should'nt virgin now be able to know down to a single street how much potential bandwidth capacity that street has based on houses in street, current customers in that street and potential customers in that street based on the size of the street?.

So if our street has reached a potential limit bandwidth capacity wise for both tv & broadband services and any current customers start to upgrade internet speeds or make more use of on demand\catch up services to go above capacity then is'nt it very wrong to singup new customers that they know there is no free capacity for knowing there is a potential risk of disruption to services to current and all customers on our street if they did?. There only options would therefor be to invest to freeup capacity (to expensive) or to restrict services dramatically to current customers but dont tell them there services are being restricted.

Sephiroth

01-06-2011, 22:10

Well, of course, they do know - exactly. Region, Area & Locality. I've said in many places what Dave9946 has said - before you buy, VM can tell you exactly what you can expect.

Now that would be honest!

What's more, they have the data necessary to tell each person who receives the dreaded letter what the usage parameters for the area should be.

They don't make apply the information they have in the interests of their customers and I find this questionable.

BTW, I've done Visio diagrams but I'm not satisfied that I've struck the right balance between detail and digestability. It needs something more than the Wikipedia diagram but lI'm struggling with where to draw the line. I might bbreak it up and put it into a PDF.

pip08456

01-06-2011, 22:13

Dave9946

01-06-2011, 22:37

pip08456

01-06-2011, 22:45

So if there are issues with the tv services, freezing channels and even the on-demand services not working at all is'nt that another indication of an over stretched capacity for a given area?. And do the tv & broadband use the exact same strand (for use of a better word lol) of fibre optic or cable?. If not I'd sure love to know how 1 affects the other in that it's always the broadband thats gets the blame for problems and never the tv or phone services that virgin complain is getting used to much even though I'll bet more people have issues with the tv and phone than the net.

In simple terms yes the same strand is used just as everything comes into your home (except phone) through the same piece of coax cable. Your STB receives exactly the same signals your Modem (hub) does but can only accept the TV signal it contains and vice versa.

Dave9946

01-06-2011, 23:07

Have I missed something in the other posts as I cant see how we can get problems with the tv channels (breaking up) on a regular basis, the catchup not working, the on demand not working yet get full trouble free speed on the internet at the same time near all the time if vis the same piece of line?. I'm getting an headache trying to understand it after reading it all again lol. But sure is a nice thread to have been dug up again.

essjay

01-06-2011, 23:07

These regional centres are then either fed from Bromley or Langley Super-headends where all the content for TV comes from.

Bromley headend hasn't existed for about 18 months / 2 years now everything (TV content) is fed from Langley with a disaster recovery headend in Knowsley.

VOD content is fed into Knowsley then "copied" to the 48 local headends and then distributed from there.

pip08456

01-06-2011, 23:16

Bromley headend hasn't existed for about 18 months / 2 years now everything (TV content) is fed from Langley with a disaster recovery headend in Knowsley.

VOD content is fed into Knowsley then "copied" to the 48 local headends and then distributed from there.

As Impz post was back in 2009 it was most likely correct then.

Nothing wrong with your update though.:D