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Two-Wire Circuits

Two-wire circuits are those that carry information signals in both directions over the same physical link or path. Typically, such a circuit is provisioned through the use of a single twisted pair, copper wire connection. Within such a circuit, two wires are required to complete the electrical circuit and both wires carry the information. A typical example is a local loop connection between the Central Office or Central Exchange switching center and the individual single line or multiline residence or business terminal equipment, as depicted in Figure 2.3.


Figure 2.3.  Two-wire, twisted pair, local loop connection between a Central Office Exchange and a single-line set or Key Telephone System (KTS).

Two-wire circuits typically cover short distance; for example, most two-wire local loops are shorter than 20,000 feet [2-2]. Additionally, the bandwidth or capacity of such a circuit is relatively low, and only a single conversation is accommodated. Finally, two-wire circuits, generally speaking, are analog in nature; therefore, quality is poor.

Four-Wire Circuits

According to the most basic definition, four-wire circuits are those that carry information signals in both directions over separate physical links or paths, and in support of simultaneous, two-way transmission. Traditionally, such a circuit was provisioned through the use of two copper-pairs, one for transmission (forward path) and one for reception (reverse path); such a circuit is known as physical four-wire. However, current technology accommodates four-wire transmission over a single physical link or path such as coaxial cable, microwave, satellite, or fiber optic cable. In other words, the circuit may be physical two-wire (or even physical one-wire) and logical four-wire, performing as a four-wire circuit but employing fewer than four wires.

Although the absolute cost of four-wire circuits is higher than that of two-wire circuits, they offer considerably improved performance. Four-wire circuits accommodate multiple, simultaneous communications in a two-way, or conversational mode. Additionally, such circuits offer greater bandwidth, or capacity, and are typically digital, rather than analog in nature—as a result, error performance is generally improved. Long haul circuits (traditionally defined as [ge]50 miles or 80 km) usually are four-wire [2-2]. Figure 2.4 illustrates typical examples of cost-effective applications of four-wire circuits, specifically to interconnect PBX, CO, and Tandem switches in a voice environment.


Figure 2.4  Four-wire, twisted pair trunk connections between a Central Office Exchange and a PBX and between a Tandem Office Exchange and Central Office Exchange.

Bandwidth

Bandwidth is a measure of the capacity of a circuit or channel. More specifically, bandwidth refers to the total frequency that is available on the carrier for the transmission of data; as opposed to overhead signaling and control information. There is a direct relationship between the bandwidth of a circuit or a channel and both its frequency and the difference between the minimum and maximum frequencies supported. While the information (data) signal does not occupy the total capacity of a circuit, it generally and ideally occupies most of it. The more information to be sent in a given period of time, the more bandwidth required.

Carrier

Carrier is a constant signal on a circuit that is at a certain frequency, or within a certain frequency range. The carrier can accommodate both an information-bearing signal and signaling and control information, which is used to coordinate and manage network operation.

Hertz (Hz)

Hertz, named after Heinrich Rudolf Hertz, the physicist who discovered radio waves, is the measurement of bandwidth over analog circuits. Hertz refers to the number of electromagnetic wave forms (signals or signal changes) transmitted per second. Although some applications operate in very low capacity environments, measured in Hz or hundreds of Hz, the frequencies generally are much higher. Hence, analog bandwidth typically is measured in kHz or kiloHertz (thousands of Hz), MHz or MegaHertz (millions of Hz), or GHz or GigaHertz (billions of Hz).

Baud

Baud is an old term that refers to the number of signal events (signals or signal changes) occurring per second over an analog circuit. Generally baud is used to describe the signaling rate of a modem for data transmission over an analog circuit, with the baud rate being roughly equivalent to Hertz. Baud rate and bps, often and incorrectly, are used interchangeably. The distinction will be discussed in more detail in Chapter 7.

Bits per Second (bps)

Bps is the measurement of bandwidth over digital circuits. It refers to the number of binary data bits that can be transmitted per second. Over an analog circuit, the sine wave can be manipulated to allow multiple bits to be transmitted at a given baud rate, even without the application of special compression techniques. A thousand (1,000) bps is a kilobit per second or Kbps, a million (1,000,000) bps is a megabit per second or Mbps, a billion (1,000,000,000) bps is a gigabit per second or Gbps, and a trillion (1,000,000,000,000) bps is a terabit per second or Tbps.

Transmission Facilities

In terms of bandwidth, and in contemporary digital context, transmission facilities can be categorized as narrowband, wideband or broadband.

Narrowband
A single channel ([le]64 Kbps) or some number of 64 Kbps channels (N × 64 Kbps), but less than wideband.
Wideband
Wideband is multichannel capacity that is between 1.544 Mbps and 45 Mbps according to U.S. standards (2.048 Mbps-34 Mbps according to European/international standards.)
Broadband
Broadband is multichannel capacity [ge]45 Mbps according to U.S. standards ([ge]34 Mbps according to European/international standards.)

Analog versus Digital

Along one dimension, communications can be classified in two categories, analog and digital. In the analog form of electronic communications, information is represented as a continuous electromagnetic wave form. Digital communications represents information in binary form (1s and 0s) through a series of discrete blips or pulses.


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