Fibre cabinets and the way they have been installed.

Something has been puzzling me for a while now, and if anyone has a good explanation.. please post...

Now, we know the way that Openreach are installing cabs in the uk is.. (please correct me if im wrong here).

Additional Metal Cabinet installed somewhere close to existing PCP.
Trench Dug between the 2 cabinets.
Power supplied to fibre cab.
Fibre dragged to Fibre Cabinet
Copper connections between new cabinet and old cabinet.
Dslams installed
Testing etc etc
Conenctions when people order

What would have made faaaaar more sense to me: -

Install a much larger cabinet over the top of existing cab
Drag fibre to cab
Install DSLAMS in the top part of cabinet.
Test, and link up to copper bits as needed.

To me, this is far far more sensible, it has lots of other benefits as well.

Power is already there.
When it gets to the point when copper is defunct, you have all that space in the bottom of the cab.
And obviously Expansion becomes easier over the years.
No extra cab.
Less physical footprint.
Easier to job manage.
Less complaints from authorities about protecting the environment
Less /no ? planning permission.

I'm sure they spent thousands on expensive job managers to tell them how to install the fibre system into the UK, but with 5 minutes thought, i can see extensive benefits to doing it this way, rather than the existing way. especially with the long term view in mind.

#puzzled#

How is the power already there and do you relies the size that cabinet would have to be

-PCP cabs don't have power, they're passive.
-In many locations it wouldn't be practical to install a significantly larger cab in the same location, whereas finding a nearby location for a secondary cab is OK.
-Power trench to the FTTC cab doesn't have to be dug in very close proximity to the main telco ducts, only to the one for the tie cables. This reduces the risk of damage to D and E side cabling by power contractors who are often completely separate to the telco ducting contractors

There area few thoughts against in less than 5 mins

I'm in no condition to give a full reply to the OP, but reckon that's a decent one.

I would add a minor one, re:- "Conenctions when people order" is not a problem now. That is achieved by the current method.

A small top mounted cab has been on the list for some areas but never seen one in the wild

http://www.coolwebhome.co.uk/fibre-cornwall/ shows how full the fibre cabinet is, so to enlarge an existing cab to fit both in mean it would be a pretty large affair

Tiddly bit of power direct from the exchange. 50v at your master socket.

Wow, free power. You could run your house on that.

You might find rather a low amperage.

A further aspect is that the FTTC cabinet is now an active, managed, part of the network - it has battery backup, and is monitored remotely for doors opening etc.

Engineers who maintain the FTTC cabinets are a different group, with different skills, from the engineers who alter things in the PCP.

By keeping the cabinet separate, it also keeps the internals separate from the PCP, and keeps the engineers separate. Access to the PCP will be required regularly. Access to the FTTC cabinet will be rare.

A second aspect is this:
The FTTC cabinet can be pre-constructed, and tested, off-site. It makes for a "cleaner" install when it is actually moved on-site - which makes for a more controlled rollout, and therefore less surprises on the budget.

AFAIK PCPs are essentially just large junction boxes. No need for power in there. If there is a small amount of e.g. environmental / intruder monitoring equipment then, as has been mentioned, there is a limited amount of power availablt from a standard phone line.

In the UK, the supply is 50V DC (strictly, it is -50V relative to earth). It changes to AC when ringing (I think at 75V) - and it is the AC component that caused the bell-clapper to vibrate (and so ring) in old telephones.

The supply comes from the exchange in a single pair (the A and B wires, or earth & battery) - there is no concept of the "ringwire" in those wires.

Inside the house, one of the (old) jobs of the master socket was to use a capacitor to separate the AC and DC voltages, putting the AC down the separate ringwire. Phones from the 80's were designed to look on this wire for a ringing signal.

... an phones from before the 80s had the bells powered by this AC signal

Actually it should be -48v which was provided from a back of 12v lead acid batteries so could be -55v very close to the exchange. Power supplies in telephone switches are normally specified at -48v.

In reply to a post by WWWombat:

Inside the house, one of the (old) jobs of the master socket was to use a capacitor to separate the AC and DC voltages, putting the AC down the separate ringwire. Phones from the 80's were designed to look on this wire for a ringing signal.

There is a bit more to it really.

Prior to the advent of sockets multiple telephones in a house were wired in sequence - serially. The ringing signal was passed from phone to phone via separate wiring and could get complicated. This was why extension wiring was always done by BT.

When sockets started to be installed the change was made to a parallel system - all phones were connected across the one pair of wires. This made it much easier to add new phones (and had some other advantages), and meant BT could wipe their hands of such things.

The problem with the new system was that the majority of phones in use at that time had rotary dials and trembler bells. If a call was made from one phone then the dial pulses would invariable cause the bells in all the other phones to tinkle. This was seen as unacceptable.

The solution to this was to make all internal wiring three wire, with the ringing circuit extracted through a capacitor in the master socket. When the dial in any phone was rotated one of the dial off normal springs put a full short across this wire and one leg of the line. This short prevented the dial pulses tinkling the bells in the other phones.

Ian

Indeed, the GPO wiring plans pre-BT were something of a nightmare. That's why I very carefully chose to describe them as phones of the 80's! I very much meant the first batches of phones that were designed for the "ringers in parallel" configuration.

In reply to a post by MHC:

Actually it should be -48v which was provided from a back of 12v lead acid batteries so could be -55v very close to the exchange. Power supplies in telephone switches are normally specified at -48v.

My memory, from the training I did back when I worked on PABX's, was that standard public lines were -50V. Internet searches (now) confirm this memory - there are plenty that describe phone lines as -48V but highlight the UK as -50V.

However, I then went and worked on public phone systems, and I know that European standards were definitely -48V.

It isn't really a surprise that, looking at BT's standards *now*, they show a nominal voltage of -48V - inheriting the requirement from ETSI. The range of acceptable voltages is -40.5 through to -57.

I used to design telephone switching systems for BT and we would always, in accordance with specific design criteria, specify power supplies to be -48v

How far back does your design experience go?

The stuff that my memory relies on goes back to 1985, working at Plessey. Searching now, I can see that the current set of exchange power supplies are -48V, and were aimed at System X installations.

My experience in the 90's was of AXE-10 (but not destined for BT), so I know *they* worked at -48V as standard too.

It wouldn't surprise me to discover that -50V was a standard from GPO days, and that a change to -48V nominal would happen alongside the upgrade to System X and Y.

However, all my day-to-day stuff was software. The only time we worried about power was when we tried to use too much of it.

Late 80s and early 90s was my time designing within BT but some of the standards we were working with dated well back to the 60s and 70s and were GPO documents re-labelled BT. The 48v (nominal) does originate from lead-acid batteries and hence the wide spread of voltages referred to and measured.