BT "Green" Junction Boxes (Streetside) - What Happens Inside?

Question

What actually happens inside those green BT cabinets that you find on the street-corners?

I have heard that they are junction boxes but can somebody produce a detailed explanation of what they actually do?

What equipment (if any) is inside them? I would also appreciate links to pictures if somebody has them.

My current understanding is:

The telephone cable leaves my house from the BT Master Socket.

Then goes to the telegraph pole, then underground to one of these green cabinets. One last question, I noticed that there is a black box on the top of all of the telegraph poles, what are these?

Answer 1

They are giant connection boxes where the individual cables from each house are connected to terminals then these are then connected to a single v large many cored cable.

Answer 2

You can also find some other equipment bsides simple distribution cabinets and junction boxes.  

Some poles, for example, carry DACS units, which without going into too much technical detail, allow two telephones to operate simultaneously and independently over a single pair of wires.   They're common in rural areas at some distance from the exchange where there has been considerable demand for service, since it's cheaper and easier to install these systems than to run miles of new multi-pair cables.

Some cabinets can also contain line concentrators.  These are almost like small, remote extensions of the telephone switching system itself, allowing  a larger number of phone lines from a particular area to share a smaller number of lines back to the exchange.   They work on the principle that out of, any given number of phones only a small proportion are likely to be in use at the same time.  In fact that basic principle has formed the background of telephone switching systems for almost as long as the phone has been around.

Answer 3

as mentioned they are large junction boxes, and it runs like this:

the cable leaves yours to the pole with 2 pairs of wires in (so 4 individual conductors), you only use one set of 2 wire so they are refered to suprisingly as pairs by engineer and you use 1 pair per line.

this goes in to the black box on top of the pole, here its outer sheath is removed and you indivisual pair is terminated (uses either a s***w terminal or little connector that bites in to the outer sheath of the pair of wires and makes contect with the copper core) and these termminals are connected to a larger cable - usually containg 20 pairs - in the same way and provide a link (this is a great spot for testing etc and breaking down the circuit in to managable chunks for installation, faulting and anyting else you can think of.

that slightly larger cable then runs either underground to the big green juntion box or over head sometimes, especailly in the countryside

in the BGJB the outer covering of the cable is removed from that cable and the individual pairs are sorted out and put on blocks of connectors or strips where its terminated these are known as D-sides (distribution sides)

then they are connected using a single pair of wires, known as jumper wires, to another pair on another block or strip that goes back to the exchange, they are called e-sides

then these pairs are in a bigger cable that can in some cases contain up to 2400 pairs that then run back to the exchange.

there are about 5 different types of cab (short for cabinet) or PCP (primary cross-connect point) as they are known, heres a picture of one:

http://www.facebook.com/photo.php?pid=12...

hope that helps and feel free to drop me a line if you have any more questions

Answer 4

They are junction boxes, usually called cabinets. All that is in there are thousands of tiny wires. You have  cables from the exchange called E sides and cables that go to poles, underground feeds, internal points etc and these are called Distribution or D sides.

All we do inside them for example someone wants their line on, we will put on a piece of tracing equipment in your socket, climb the pole, indentify which is your wire, connect it to a spare pair, then go the cabinet.

We will then listen for your dial tone coming from the exchange and connect it to the pair coming from the pole, and that is what happens inside one.

There is no equipment inside 97% of our cabinets just wire, however we do have some with fibre optics, these do carry equipment but these are scarce.

The boxes at the top of the poles are called blocks, all they are is a way of terminating your overhead wire to the underground so it can go to a cabinet.

Each telephone number has a specified routing so we know what goes where, the line starts in the exchange with a termination number, so a full route could be-- ab123-- cabinet6 - E side 123 to D side 649, pole or DP231 pair 9, everything is numbered and all ( I hope ) engineers can count in a cabinet. Hope this helps.

For a picture of them try either a BT or the Openreach website.