DDD NetworkDirect Distance Dialing (DDD) or direct dial is a telecommunications term for a network-provided service feature in which a call originator may, without operator assistance, call any other user outside the local calling area. DDD requires more digits in the number dialed than are required for calling within the local area or area code. DDD also extends beyond the boundaries of national public telephone network, in which case it is called or International Direct Distance Dialing (IDDD).

Private Line ServiceIn wired telephony, a private line or tie line is a service that involves dedicated circuits, private switching arrangements, and/or predefined transmission paths, whether virtual or physical, which provide communications between specific locations.

Telephone CircuitA telephone circuit is a single-user circuit on a telephone communication system. Typically this refers to the physical wire or other signaling medium connecting the user's telephone apparatus to the telecommunications network, and usually also implies a single telephone number for billing purposes reserved for that user.

These wires were typically copper, although aluminum has also been used, and were carried in balanced pairs separated by about 25 cm (10") on poles above the ground, and later as twisted pair cables. Modern lines may run underground, and may carry analog or digital signals to the exchange, or may have a device that converts the analog signal to digital for transmission on a carrier system. A RJ-14 jack is used to connect the device to the telephone line.

Data ModemsA modem (modulator-demodulator) is a device that modulates an analog carrier signal to encode digital information, and also demodulates such a carrier signal to decode the transmitted information. The goal is to produce a signal that can be transmitted easily and decoded to reproduce the original digital data. Modems can be used over any means of transmitting analog signals, from driven diodes to radio.

The most familiar example is a voice-band modem that turns the digital data of a personal computer into modulated electrical signals in the voice-frequency range of a telephone channel. These signals can be transmitted over telephone lines and demodulated by another modem at the receiver side to recover the digital data.

Synchronous ModemsSynchronous - Synchronous modems can be faster than asynchronous. They depend on timing to communicate. Data is transmitted in frames with synchronization bits which are used to be sure the timing of the transmission and reception of data is accurate. Synchronous modems are normally used on dedicated leased lines. Synchronous modems are one of binary synchronous communications protocol (bisync), high level data link control (HDLC), or synchronous data link control (SLDC).

Asynchronous ModemsThe common modem used today. Each byte is placed between a stop and a start bit. Each modem must operate with the same start and stop bit sequence, operate at the same baud rate and have the same parity settings for the data checking in order to communicate correctly. Define parity checking.

Modem SynchronizationSynchronization is the way the device is timed to be in step with the clock. There are two different possible way to set up synchronization for a synchronous transmission. First way is set up a separate clock line between transmitters and receiver but this only works for short range transmission since in long range transmissions the clock pulses can receive timing errors. The better choice for long distance is to embed the clocking information in the data signal. You can do this with Manchester or differential Manchester encoding for digital signals.