Here is a question that has been nagging me lately. What is the difference (besides the obvious) between a two wire phone circuit and a four wire phone circuit? I cannot imagine four wires going from the subscriber to the CO.

Just showing my relative ignorance on the subject of telephony.

Best,

Joe

Special circuits, such at T1s used to be two separate pairs,one for transmit and one for receive. This is not usually the case anymore as newer HDSL technology allows them to be combined on one pair. At the customer's premises, they are still broken out into separate transmit and receive circuits.

Four wire circuits were also used for voice in foreign exchange environments where a telephone number may be coming from a distant CO in another city or even another state. Again, with today's digital technology, this isn't a requirement anymore. Back in the day, transmit and receive were kept separate in order to overcome longer distances than possible with a traditional two wire loop.

The telephone set is actually a 4 wire device (look at your handset) 2 wires for transmit and 2 for receive. To save cable in the Outside Plant there is a hybrid circuit in the phone to convert 4 wire to 2 wire.

Four wire provides better transmission and reception. Like Ed said, standard T-1s are 4 wire. Tie lines (connections between two switches) used to come in two flavors in the Analog days - 2 wire and 4 wire (E&M).

T-3s, and all fiber connections, while only 2 wire/strand are still transmit & receive and NOT Tip & Ring.

The old US Military Autovon system used 4 wire telephone sets for better (and more secure) communications.

To sum up, 4 wire is better then 2 wire, but it's more expensive and as a result is used much less often.

Sam

It's important to know from a technical side as hybrid circuits (4-wire to 2-wire conversions) can introduce echo.

And I agree with Sam that 4-wire is better than 2-wire...if nothing more than troubleshooting purposes.

There's still a lot of stuff riding on 4-wire designs, and I, for one am happy that that's the case.

While most of T-1s and 56/64K circuits are designed as a single pair nowadays (distance permitting)it never seizes to amaze me how many old and not-so-old analog and tie lines are still around, most of them with separate T and R pairs.

A lot of folks love converting 4-wire circuits to 2-wire ones, whenever possible. I never do. Truth be told, I've done the exact opposite on many occasions.

While I don't have the mileage required to confidently state that two-pair designs are better, I will state that I do like them better.

My $0.02 only...