# Communication Systems/Communication Mediums

Signals need a channel to follow, so that they can move from place to
place. These **Communication Mediums**, or \"channels\" are things like
wires and antennae that transmit the signal from one location to
another. Some of the most common channels are listed below:

## Twisted Pair Wire

**Twisted Pair** is a transmission medium that uses two conductors that
are twisted together to form a pair. The concept for the twist of the
conductors is to prevent interference. Ideally, each conductor of the
pair basically receives the same amount of interference, positive and
negative, effectively cancelling the effect of the interference.
Typically, most inside cabling has four pairs with each pair having a
different twist rate. The different twist rates help to further reduce
the chance of crosstalk by making the pairs appear electrically
different in reference to each other. If the pairs all had the same
twist rate, they would be electrically identical in reference to each
other causing crosstalk, which is also referred to as capacitive
coupling. Twisted pair wire is commonly used in telephone and data
cables with variations of categories and twist rates.

Other variants of Twisted Pair are the **Shielded Twisted Pair** cables.
The shielded types operate very similar to the non-shielded variety,
except that Shielded Twisted Pair also has a layer of metal foil or mesh
shielding around all the pairs or each individual pair to further shield
the pairs from electromagnetic interference. Shielded twisted pair is
typically deployed in situations where the cabling is subjected to
higher than normal levels of interference.

## Coaxial Cable

Another common type of wire is **Coaxial Cable**. Coaxial cable (or
simply, \"coax\") is a type of cable with a single data line, surrounded
by various layers of padding and shielding. The most common coax cable,
common television cable, has a layer of wire mesh surrounding the padded
core, that absorbs a large amount of EM interference, and helps to
ensure a relatively clean signal is transmitted and received. Coax cable
has a much higher bandwidth than a twisted pair, but coax is also
significantly more expensive than an equal length of twisted pair wire.
Coax cable frequently has an available bandwidth in excess of hundreds
of megahertz (in comparison with the hundreds of kilohertz available on
twisted pair wires).

Originally, Coax cable was used as the backbone of the telephone network
because a single coaxial cable could hold hundreds of simultaneous phone
calls by a method known as \"Frequency Division Multiplexing\"
(discussed in a later chapter). Recently however, Fiber Optic cables
have replaced Coaxial Cable as the backbone of the telephone network
because Fiber Optic channels can hold many more simultaneous phone
conversations (thousands at a time), and are less susceptible to
interference, crosstalk, and noise then Coaxial Cable.

## Fiber Optics

**Fiber Optic** cables are thin strands of glass that carry pulses of
light (frequently infrared light) across long distances. Fiber Optic
channels are usually immune to common RF interference, and can transmit
incredibly high amounts of data very quickly. There are 2 general types
of fiber optic cable: single frequency cable, and multi-frequency cable.
single frequency cable carries only a single frequency of laser light,
and because of this there is no self-interference on the line.
Single-frequency fiber optic cables can attain incredible bandwidths of
many gigahertz. Multi-Frequency fiber optics cables allow a
Frequency-Division Multiplexed series of signals to each inhabit a given
frequency range. However, interference between the different signals can
decrease the range over which reliable data can be transmitted.

## Wireless Transmission

In **wireless transmission systems**, signals are propagated as
Electro-Magnetic waves through free space. Wireless signals are
transmitted by a transmitter, and received by a receiver. Wireless
systems are inexpensive because no wires need to be installed to
transmit the signal, but wireless transmissions are susceptible not only
to EM interference, but also to physical interference. A large building
in a city, for instance can interfere with cell-phone reception, and a
large mountain could block AM radio transmissions. Also, WiFi internet
users may have noticed that their wireless internet signals don\'t
travel through walls very well.

There are 2 types of antennas that are used in wireless communications,
**isotropic**, and **directional**.

### Isotropic

People should be familiar with isotropic antennas because they are
everywhere: in your car, on your radio, etc\... Isotropic antennas are
omni-directional in the sense that they transmit data out equally (or
nearly equally) in all directions. These antennas are excellent for
systems (such as FM radio transmission) that need to transmit data to
multiple receivers in multiple directions. Also, Isotropic antennas are
good for systems in which the direction of the receiver, relative to the
transmitter is not known (such as cellular phone systems).

### Directional

Directional antennas focus their transmission power in a single narrow
direction range. Some examples of directional antennas are satellite
dishes, and wave-guides. The downfall of the directional antennas is
that they need to be pointed directly at the receiver all the time to
maintain transmission power. This is useful when the receiver and the
transmitter are not moving (such as in communicating with a
geo-synchronous satellite).
