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[c6em]

I ask as it’s pertinent as to why some houses seemingly get much better broadband than others on the same street.

Let’s take a simple situation: semi rural street in village.  D side from the PCP cabinet runs underground in verge.  Every now and then there is an underground joint box and the supply to a group of houses is split out, run up the pole to the distribution box at the top and hence via drop wires to the nearby individual houses – in several cases via intermediate pole(s) backtracking down the road……..and so on along the road up to the end.

Now this could be done by a single large umpteen core D side with split-offs at each of the 10 or so joint boxes.  However I’d guess it’s done by multiple D sides, with the first few joint boxes/distribution points being fed by their own dedicated D side from the PCP cabinet.  Now this makes installation/connection/maintenance easy, but at some point along the road the mathematics changes.  Instead of having multiple long length cables (expensive) carrying few lines filling the ducts (inconvenient) going to each joint box, instead it becomes more effective to use a single larger cable carrying many more lines feeding all those remaining joint boxes/distribution points further away en-mass.  So how are the lines for these next distribution points split out?  Is the umpteen core cable cut and then joined on to the next section with the required number of lines plus spares split off at this point, or is it done by managing to take out the required lines/spares without cutting the entire cable.  Even if I’ve misunderstood it there has to be a limit to the continuous length of a D side cable before a join is required regardless simply due to the physical impossibility of hauling say a mile of cable through a meandering cable duct.

You can see why those properties (eg me!) near to a cabinet seem to have MUCH better broadband (ADSL2+)  than those only a hundred yards or so further away:  a dedicated D side feeding the distribution point meaning hardly any cross talk from lots of other lines in the cable bundle and no joints between them and the cabinet  - always a source of “trouble” as well as the shorter distance.

[Black Sheep]

I'll be honest and say I can't quite get my head around what you're saying there, c6em ?? It may have something to do with the ammount of ale I supped yesterday, though ? 

D-side cabling is different dependant on various factors, but the bog-standard way it is structured, on a bog-standard urban route is as such.

Using overhead DP's (Telegraph Poles) as an example, and assuming each DP will require 20prs. There will be a 100pr cable leaving the 100pr terminating strip in the PCP, that will travel the required distance to the first DP. At the base of this DP (or nearby), will be an underground 'Joint Box' (JB). At this box, we will 'drop' 20pr's of the 100pr cable, and joint these to the 20pr cable leaving the JB that feeds the DP.
How this is done really depends on the cable gangs that installed the original 100pr cable ?? However, normal practice would be to pull a 100pr cable to the JB that feeds the 3'rd DP on the run. 'Slack' would then be pulled back into the previous 2 JB's, so that the cable jointer only has to remove the outer sheathing of the cable, and only has to 'pull out' (i.e. - cut into) the required 20prs for the DP. This means less jointing work, otherwise they would have to joint the full 100pr in each box !!

Now, the reason they would only pull the 100pr into the 3rd JB, is that todays D-side cable comes in 100,50,20,10,5 and 2pr formats. Previous 'Layered cables' came in all sorts of sizes. Ergo,by the time the 100pr cable gets to, and 'feeds' the 3rd DP, there will only be 40 more pairs left for the 2 remaining DP's. At this point, the cable gangs would run a 50pr cable to the next JB, and form here, a 20pr cable to the final JB.

There are of course, lots of anomalies other than the example above, and it's down to the 'Planner' to arrive at the most cost-effective solution. I know there are 'Bots' that do the workings out for them, ie- they input the parameters and the computer gives the solution.

Hope this helps, and I realise it's difficult to read and get your head round. 
 

[c6em]

Thanks for the reply – which yes I fully understand.
I was just curious and intrigued.  The last time I managed to see into the joint box feeding the group of houses around me, this one being the first JB on the road from the PCP (100 yards) the two things apparent to me:

1. There were something like 5 or 6 individual black sheathed cables running in/out the joint box – some curling round with what looked like a long sealed vulcanised type joint boots on and others simply running across from one side to the other.
So rather than 1off XXXpr cable it looked to me that they were using multiple smaller cables. Mind you this all dates from the 70’s and was a right old tangle so difficult to make out what was what - and there was water at the bottom!

2. That the D side cable feeding ‘my’ joint box was not one of the 5 or 6 continuing on along the road but was a separate dedicated 20pair (as a guess) feeding this joint box only and terminating at it.

I wondered whether this was the standard way of doing things and I could see reasons why BTOR (or rather the GPO in them days) might decide to do it like this.
I suspected this curious way of structuring the layout giving us in effect a dedicated D side clear of everyone else’s cross talk has a few decades later resulted in me and my immediate neighbours having a sync and download speed well in excess of what the standard ADSL2+ graphs vs attenuation suggest.

[Black Sheep]

Glad you understood. 

It's obviously impossible to say what cables are in 'your' joint box. There are different gauges, and whilst the obvious conclusion may be the thicker the cable, the higher the pair-count, this would be unwise to assume so.

Some of the earlier cables that have (for example) a 50pr count, have approximately the same outer gauge as a modern-day 20pr cable. A high poundage (0.9mm) 20pr modern-day cable, looks exactly the same as a 50pr 0.5mm modern-day cable. So in a nutshell, without tracking each individual node, judgement can't be made on what they are, or what they're 'feeding'.

It really does, as I say, come down to the planner. You may live in a brand new estate, with lets say 10 houses,  that is just 100mtrs from the PCP ?? It would obviously be frugal to cable using a 20pr direct from the PCP. If the same enclave was 1Km away from the PCP, it would more than likely be a new 20pr cabled back to the nearest 50/100prs cable with enough spares to give service to the properties. This could well mean feeding off a different PCP to other properties in the area ??

Each circumstance will be different, in a small or large way.

[burakkucat]

An interesting discussion. Thanks for starting it, c6em and thanks to Black Sheep for offering his professional observations.