communication over telephone lines

[Anonymous]

I have always been intrigued about how you can transmit and receive data over a cable at the same time.
Let's take a simple example; if I'am talking over the phone(A) and someone is hearing (B), what I talk reaches (B)and he hears it. Then (B) responds to me (A)
and vice versa. What I speak goes over the telephone line and what the other person responds goes back over the same telephone line. We don't always speak to each other at the same time, but, and this is my question, if we talk at the same time to each other
how is it that the voice signals transmited to and from as waves at the same time over the telephone line do not collide with each other if they are being transmited "up and down" at the same time over the same cable. I don't know if I made myself understood but since I'm not a telecommunication professional I try to ask using my own words.
Thank you for your answer.
Manny

Archived topic from Iceteks, old topic ID:3262, old post ID:26471

[Red Squirrel]

As far as I know DS0 lines (basically, a Phone line) are full duplex, which means they can send and receive at the same time. I believe 1 wire is for sending and 1 is for receiving, and to complete the circuit it simply uses the ground at each end. (I may be wrong though).

Archived topic from Iceteks, old topic ID:3262, old post ID:26472

[richardj]

  I believe 1 wire is for sending and 1 is for receiving, and to complete the circuit it simply uses the ground at each end.  (I may be wrong though).

Both wire are needed to either talk or receive.

Maybe it would answer the question better by saying that the signals do not 'collide'

for the same reasons radio channels do not collide.

The are of different frequencys.

In order to allow more long-distance calls to be transmitted, the frequencies transmitted are limited to a bandwidth of about 3,000 hertz. All the frequencies in your voice below 400 hertz and above 3,400 hertz are eliminated. That's why someone's voice on a phone has a distinctive sound .

Archived topic from Iceteks, old topic ID:3262, old post ID:29962

[Red Squirrel]

Hmm that could be too, that it multiplexes across both lines so for 1/8000th(or whatever number) of a second the lines are receiving, then sending, etc... It's so fast it's not noticable. But I was always sure it was 1 wire to send and one to receive. I'd have to do some research for myself to see for sure.

Archived topic from Iceteks, old topic ID:3262, old post ID:29963

[richardj]

Hmm that could be too, that it multiplexes across both lines so for 1/8000th(or whatever number) of a second the lines are receiving, then sending, etc...  It's so fast it's not noticable.  But I was always sure it was 1 wire to send and one to receive.  I'd have to do some research for myself to see for sure.

Actually

It works like any other DC electrical appliance-------------

You need a neg & a positive.

If either side is broken or open niether talk or receive will work cus you have an incomplete circuit.

The only cavieot to this is that if the negative side is open in the cable & you ground the negative at the house you can pull dialtone --with a really loud hum.

Otherwise the only grounding done at the house is for lightning protection.


Archived topic from Iceteks, old topic ID:3262, old post ID:30216

[gd253]

Now my knowledge of phone networks is getting a bit long in the tooth but they use or used to use, PCM (Pulse Code Modulation).

When you talk, there are gaps in your speech. PCM fills those gaps with other people's conversations and it all gets unscrambled at the other end. This is how they can send thousands of conversations down the one phone line. QED.

--gd

Archived topic from Iceteks, old topic ID:3262, old post ID:33797

[richardj]

They send thousands of conversations by having thousands of pairs of wires.

It's called cable.

Archived topic from Iceteks, old topic ID:3262, old post ID:33800