Transmission: Media and Systems

The media needs to carry information, and to represent this information requires
a variation in the electrical or optical signal. This variation can take many different
forms, but is generally referred to as modulation, and can use analog or digital
techniques. Modulation is used to vary the electrical or optical signal to represent
information on transmission media.

A fundamental aspect of the transmission medium is the frequency at which it
is designed to work. This will differ from telecommunications system to telecommunications
system and can include multiple frequencies to provide for multiple
communication channels.

Radio systems such as GSM can have hundreds of frequencies specified for
use within a specified part of the frequency spectrum, whereas copper-based telephony
systems might only have a single frequency band specified for use. GSM
needs multiple frequencies to allow different frequencies to be used in different
geographical parts of the network (to avoid radio interference within the communication
channels), whereas copper wire physically separates the communication
channels. This illustrates the difference between unbounded and bounded media
(respectively).

Transmission systems are designed to organize the information in a way that
allows the equipment at either end of the media to work in unison. They provide a
scheme for coding and decoding the information such that one or more communication
channels can be identified (on the media, and at the specified frequency), and
include extra information so that the equipment can be effectively synchronized
and managed. If problems are experienced within the system, this can be notified
via specific alarm channels, allowing remedial action to commence.

Copper Twisted-Pair Cable
Although they are the oldest of the media types used in telecommunications, copper
cables remain the foundation of most national fixed telecommunications systems,
especially within the local loop between the customer’s premises and the local
telephone exchange.

Copper was chosen for its good conductivity, together with its price and flexibility.
There are metals that have better electrical conductivity properties, but most
are more expensive than copper. The requirement for early telephone circuits was
that the media should be capable of carrying low-bandwidth audio signals. The first
copper cables were paper insulated. Paper worked well as an insulator, but as the
number of cable pairs increased, the cables became extremely stiff due to internal
friction. In the 1950s, plastics were introduced as the insulating material, with lowdensity
polyethylene, high-density polyethylene, and polypropylene all being used.

Crosstalk is a phenomenon where signals intended for transmission on a circuit
are electrically induced into adjacent circuits, causing interference. This was a particular
problem of early cables. To reduce this, twists were introduced along each
pair of cables, with up to 25 unique twists being used within a 25-pair cable group,
each spaced at a different distance. It was assumed in the 1980s and 1990s that copper
was an old technology with a limited future. Telecommunications companies
planned ahead to install fiber and coaxial cable systems in the local loop. Eventually,
the cost of doing so was judged to be too high in most cases.

Today, new techniques have been developed to transmit higher data rates than
had previously been thought possible using copper wires with technologies such as
Asymmetric Digital Subscriber Line (ADSL). This means that telecommunications
companies have the opportunity to bring in revenue from high-speed data services
using the existing cable.

Copper Coaxial Cable
A variation of the use of copper is its application in coaxial cables (Figure 2.14).
Here, an inner conductor is first covered in an insulating material and then surrounded
by a wire mesh or metallic screen. The cable is so named because both the
inner conductor and the outer screen share the same axis. Coaxial cables are far
more tolerant of electrical noise than traditional copper pairs and are able to transmit
higher data rates. The use of coaxial cabling dramatically reduces crosstalk. The
main application for coaxial cables was to serve inter-exchange trunk connections
where a pair of cables is used for carrying multiple voice channels, one each for
the transmit and receive paths. Coaxial cabling is also used for so-called thin-wire
Ethernet connections. However, this system has the disadvantage that any failure
along the cable route will cause the entire network to fail.

Radio
Radio systems can be used to transmit signals from a few meters to several thousand
kilometers and can be used for both point-to-point and broadcasting applications.
Radio signals are a type of electromagnetic radiation, with similar properties
to light but with a much shorter wavelength. As with all electromagnetic radiation,
radio waves have both an electric and a magnetic field that travel at right angles
to each other. As the signal travels outward, it can be compared to a stone being
thrown into a pond. Much like the waves on a pond, radio waves get weaker the
further from the transmitter they are. In radio, this progressive weakening of the
signal is referred to as attenuation or path loss.
The frequency of a radio signal will determine how it can be transmitted. Lowerfrequency
signals, such as those in the very low frequency (VLF), low frequency (LF),
and medium frequency (MF) bands that cover frequencies up to about 2 MHz, can
propagate using surface or ground waves. As currents are induced in the Earth, this
has the effect of slowing down the part of the wave that is closest to the ground,
causing a “bending” of the wavefront around the surface of the Earth. Hence, these
lower frequencies can be transmitted either by line of sight or by surface waves. The
combined effect is termed “ground wave” .

Another method of radio propagation is by the use of sky waves.
Certain frequencies, including those in the high frequency (HF) band have the
property of being refracted by layers in the atmosphere known as the ionosphere
and troposphere. Essentially, when the signal reaches a heavily ionized layer, this
layer reflects the signal toward an area of lower ionization. This makes it possible to
send HF transmissions many thousands of miles around the globe. It is possible for
sky waves to propagate lower frequencies but in most cases these work only under
certain atmospheric conditions or at night.

At yet higher frequencies, such as those in the very high frequency (VHF), ultra
high frequency (UHF), and super high frequency (SHF) bands, most of the radio
signal is transmitted by direct waves. Although there is still some component of
ground- and sky-wave propagation, most of the signal relies on line-of-sight transmission.

As frequency increases, so path loss increases. This necessitates the use of highly
focused, directional antennas for microwave transmission such as that used in satellite
communications. One of the main advantages of radio transmission is that
it removes the need for expensive cable-laying activities, which can account for 50
percent of telecommunications infrastructure costs. Coupled with the recent progress
using the radio spectrum more efficiently has led to radio being heralded as one of the
most promising media types for the next generation of telecommunications services.

Optical Fiber
Although first proposed in 1966 by Kao and Hockham, it was only in 1970 that Maurer,
Keck, and Schultz designed and produced the first optical fiber that had characteristics
that made it suitable for use in telecommunications. Their work enabled the
production of fibers that had very little attenuation (loss of signal strength or intensity
in the cable), and which kept most of the light traveling through the cable.
An optical fiber has a very thin core of glass or silica surrounded by an outer
cladding made of a similar material, but with a lower refractive index.

Transmission Systems
Transmission systems are complex and involve many different aspects of information
transfer and managing that information. They refer fundamentally to the way
in which channels can be identified on the transmission medium, rather than the
way the application data is coded or any higher layer transport protocols/systems
(such as IP technology used to code Internet-type traffic).

Just about in all cases, the application data or transport protocols for any network
will require the final coding and synchronization that will allow the information
to be carried and identified within one or more specific channels on the
transmission medium (often at or around a specified “carrier” frequency). It is this
final coding process (and the additional features provided within the coding) that
defines the transmission system used.