PHONE FAQs (Frequently Asked Questions)


			Fig 1. The Phone Line

               
                  RELAY
                200 Ohms   Telephone    . Subscriber
                -------    Exchange     .
                -------                 .  TIP + (red)
          o-----~~~~~~~--o---------------------------------------o
          |       5 H    |              .                        |
          |              |              .                        |
         +|              |              .                        |
         ---             |              . No 22 AWG wire         |
          -  48V DC      |              . up to 10 Miles Long    |
         ---             |              .                        |
          -       RELAY  |              .                        |
          |     200 Ohms |              .                        |
          |     -------  |              .                        |
          |     -------  |              . RING - (green)         |
          o-----~~~~~~~--|---------o---------------------o       |
                  5 H    |         |    .                |       |
          Audio      2uF |     2uF |    .                |       |
          coupling 250V ---  250V ---                    o-phone-o
          Capacitors    ---       ---                   R=200 Ohms
                         |         |
 o-------------- \--------         |
 |                                 |
Line Card        RELAY Contacts    |
 |                                 |
 o-------------- \------------------    


How your phone system works?

Your phone is connected to the local exchange (which we will call a wire center) via a twisted pair of 22 Gauge copper wires. The phone company provides a DC voltage to the other end of the wires (normally 48 volts, but can be as high as 52 volts). This voltage is supplied by battery, which explains why your phone still works in a power failure. When your phone is "on hook", there is an open circuit and the voltage can be measured directly at the phone. When you lift the receiver, the phone goes "off hook" and completes the circuit. The phone equipment at the other end senses this drop in voltage, and connects your pair of wires through capacitors to a line card. The voltage will drop to between 3 and 9 volts DC, depending on the resistance built into your phone and the length of copper line between you and the wire center. The old pulse dial phones dialed the number by making and breaking this circuit, but these days it is all done by tones.

Dial tones are generated by your phone, and all line sounds are generated by the local exchange. All communication between you and the line card are AC or analogue signals riding on the DC current (the DC powers your phone). The line card digitizes your analogue signals for any further processing. If it is a local call, the switching is done locally. Anything beyond what is classified as local is routed to a DLC (Digital Loop Carrier) for switching at a higher level regional exchange. A DLC is basically a T1 which is broken down into 24 individual channels, each of which can carry a single digitized voice channel (64 Kbps). ALL SWITCHING IS DONE IN DIGITAL FORM. Edson is classified as a level 5 exchange, and serves as a regional center for the surrounding area (Robb, Marlboro, Niton Junction etc).

The manner in which your signals are handled explains why your phone company can provide many of the services that it does, such as call forwarding, call waiting, etc. It also explains why some consumers have better quality connections than others. The longer the length of copper between you and the wire center, the more likely the line is to pick up interference. Interference is also be more likely to occur if the impedance (resistance to AC) at your end of the copper pair is not matched to the impedance at the telephone companies end. Devices that you connect to the same line as your modem should be of very high impedance (close to an open circuit) when not in use, and be coupled closely to the phone jack.

			Fig 2. What is a bridged tap?

from   -------------------o-------------------------------------------
Phone  -------------------|-o-----------------------------------------
Company                   | |
                          | |
                          | |
                          o-|------------------o telecom
                          | o------------------o Device 1
                          | |
                          | |
                          o-|------------------o telecom
                          | o------------------o Device 2
                          | |
                          | |
                          o o
                        telecom
                        Device 3

When the phone company taps into a phone cable to service your location, they should, but don't always, disconnect it from the remaining cable. When the technician wired your location, he/she should not splice into a pair of wires to add a new phone jack. Each piece of wire that does not carry current is a potential antenna that can pick up interference.

			Fig 3. The proper way?

from   -------------------o--x-----------------------------------------
Phone  -------------------|-o--x---------------------------------------
Company                   | |
                          o-|-----------------o    telecom
                            o-----------------|--o Device 1
                                              |  |
                                              |  |
                                              |  |
                          o-------------------|--o telecom
                          | o-----------------o    Device 2
                          | |
                          | |
                          o o
                        telecom
                        Device 3

In order to provide 56K Modem service, your provider must have a digital connection to the local telephone exchange. This reduces the total length of copper wire used to carry the analogue signal, but only by the distance from the provider to the local exchange (Yellowhead-Dot-Com is located about 1000 ft from the Edson Exchange). If you have a poor connection using 28.8/33.6K, you will likely get a poor connection with 56K.

xDSL uses the same pair of copper wires, but it is a "dry pair" or "unconditioned" (no supplied voltage), and is limited to somewhere between 12,000 and 20,000 feet, depending on the system used. That pair of wires is between you and your provider, so the closer your provider is to the local exchange, the better. DSL stands for Digital Subscriber Line, and the "x" is a substitute for the various brands of DSL. An "A" would stand for asymmetric, "S" for symetric etc. Asymetric means that the data traveling in one direction is going at a different rate than the data going the other direction.

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