Characteristics of Cables

Specific cable configurations vary enormously in their ability to carry a signal, in their usefulness for a particular situation and in their cost. Nevertheless they all share a common set of characteristics:

Resistance

All wire has resistance to the flow of electrical current and this is one factor in limiting the distance over which a signal may travel. The thicker the wire, the less resistance. While it reduces the signal (and also the

noise) resistance does not cause distortion of the signal.

A typical resistance value for 20 gauge (.8118 mm) wire at 70 deg F is 10 ohms per thousand feet. For 26 gauge (.4094 mm) wire this figure is 40 ohms per thousand feet.

16 Coaxial cables used for long line telephone transmission in the past did not have a continuous plastiC insulator but rather small plastiC insulators spaced at regular intervals. This type of cable has many better characteristics than the ones with continuous plastiC but is very hard to bend around corners and to join.

Leakage

This is caused by conduction between the wires through the "insulator".

This is another source of signal loss.

Inductance

Current flowing in a wire always produces a magnetic field around the wire. This field changes with the flow of current. These changes in magnetic field absorb power at some times and replace it back into the circuit at other times. This causes distortion of the digital signal.

(Inductance is not all bad, since it tends to compensate for distortion caused by capacitance effects.)

Capacitance

Capacitance is the effect caused by electrostatic attraction between opposite charges in conductors which are physically close to one another. Like inductance, capacitance takes power from the circuit at some times and discharges it back into the circuit at other times

-causing distortion. However, it is extremely important to understand that capacitance and inductance work in different (in simple terms, opposite) ways.

Capacitance is most influenced by the distance between conductors and the material between them. This material is called the "dielectric".

Other things being equal, if wires are separated by air the capacitance is about half of what it would be if the wires were separated by PVC. This is the reason that many cables use plastic foam for an insulator.

In all cable situations except the "open wire" case, capacitance is

"large" and is considered the major factor limiting a cable's ability to carry a signal.

The above factors can be lumped together into a single characteristic called

"impedance". Impedance is defined as the ratio of voltage over current at any pOint on the line and is a constant for a particular cable.

A normal transmission channel will change any digital pulse sent along it (see Figure 14).

Original Pulse

OR

.As Received

Figure 14. Distortion of a Digital Pulse by a Transmission Channel. Both of the

illustrated alternatives are perfectly normal pulse shapes after transmission down a good transmission channel.

Chapter 2. A Review of Digital Transmission Technology 33

2.3.2.1 Impairments in Transmission Channels

Thermal Noise

Heat (at any temperature) causes electrons to "bounce around" at random. This causes small random variations in the signal and is usually called background noise.

Physical Environment

Cable characteristics can vary significantly with temperature sometimes in the space of an hour or so. Other environmental factors such as the presence of moisture can also change the transmission characteristics of the cable.

In some telephone environments the major source of impulse noise is the relatively high voltage pulses (up to 50 volts) used for dialing in old style (step-by-step) telephone exchanges. Some telephone environments use as many as two thousand twisted pairs in the same cable.

In other environments, impulse noise can arise if an (unshielded) twisted pair passes close (in the same duct) to normal electrical power wiring.

Whenever a high current device (such as a motor) is switched on there is a current surge on the wire. Through stray coupling, a power surge can cause impulse noise on nearby communications cabling.

Reflected Signals

If a signal traveling on a wire encounters a change in the impedance of the circuit, part of the signal is reflected back towards the sender. Of course, as it travels back to the sender, it could be reflected back in the original direction of travel if it encounters another change in impedance.

Reflected signals (sometimes called echoes) can cause a problem in a unidirectional environment because they can be reflected back from the transmitter end, but this is not always very serious.

The real problem comes when a two-wire line is used for full-duplex communication (such as at the BJSDN "U" interface). Here reflections can cause serious interference with a signal going in the other direction.

Impedance changes over the route of a circuit can have a number of causes such as a change in the gauge or type of wire used in the circuit or even a badly made connection. However, the worst impedance mismatches are commonly caused by the presence of a "bridged tap".

A bridged tap is simply where another wire (pair of wires) is connected to the link but its other end is not connected to anything. Bridged taps are quite common in some telephone environments.

Intersymbol Interference Interference.

Intersymbol interference takes place when a particular line state being transmitted is influenced by a previous line state. This is usually a result of the characteristics of the circuit causing the signal to be "smeared" so that the end of one symbol (bit or group of bits) overlaps with the start of the next.

Crosstalk

Crosstalk is when a signal from one pair of wires appears on another pair of wires in the same cable - through reactive (capacitive or inductive) coupling effects. There are two kinds of crosstalk which can have different amounts of significance in different situations. These are called "Near End Crosstalk" (NEXT) and "Far End Crosstalk" (FEXT).

The most common type of NEXT occurs when the Signal transmitted from a device interferes with the signal in the other direction being received by that device. This can be a problem in the LAN environment using unshielded cable since the transmit pair and the receive pair are bound closely together in the cable.

Another type of NEXT is interference from any transmitter at the same end of the cable as the receiver being interfered with. (This is common in the telephone situation where there can be hundreds of wire pairs in the same cable.)

FEXT is caused by interference from transmissions by other devices at the far end of the cable. In most (though not all) digital signaling situations, FEXT is not a significant problem (because it happens at the transmitter end of a cable where the signal is strong relative to the interference).