Page 37 - DCAP406_DCAP_207_Computer Networks

Computer Networks/Networks

Notes medium. The actual range of frequencies supporting a given communication is known as
a pass band. These are given in Table 3.1.

Bandwidth: In a very general way, we may say that bandwidth is the difference, expressed
in Hertz, between the highest and the lowest frequencies of a band. In general, the higher
the bandwidth, the more will be the data transmission rate or throughput. It should be
noted that bandwidth and data transmission rate are very closely interrelated to each
other. Clearly, any transmission system becomes more attractive if the available bandwidth
is greater, introduced errors are fewer, and the maximum distance between various network
elements (amplifiers, repeaters, and antennae) is greater.

Distances: The higher frequency signals offer greater bandwidth; they also generally
suffer to a greater extent from signal attenuation than lower frequencies. This fact results
in more errors in transmission, unless the amplifiers/repeaters are spaced more closely
together. It clearly demonstrates the close and direct relationship between bandwidth,
distance, and error performance.
Bandwidth, in this context, refers to the raw amount of bandwidth the medium supports.
Error performance refers to the number or percentage of errors, which are introduced in
the process of transmission. Distance refers to the minimum and maximum spatial
separation between devices over a link, in the context of a complete, end-to-end circuit.

Propagation delay: Propagation delay refers to the length of time required for a signal to
travel from transmitter to receiver across a transmission system. While electromagnetic
energy travels at roughly the speed of light (30,000 Kms per second) in free space. In
contrast, the speed of propagation for twisted pair or coaxial cable is a fraction of this
figure. The nature of the transmission system will have considerable impact on the level
of propagation delay. In other words, the total length of the circuit directly influences the
length of time it takes for the signal to reach the receiver.
Security: Security, in the context of transmission systems, addresses the protection of data
from interception as it transverses the network. Particularly in the case of data networking,
it also is important that access to a remote system and the data resident on it be limited to
authorized users; therefore, some method of authentication must be employed in order to
verify that the access request is legitimate and authentic.
Resistance to environmental conditions: Resistance to environmental conditions applies
most especially to wired systems. Twisted pair, coaxial, and fibre optic cables are
manipulated physically as they are deployed and reconfigured. Clearly, each has certain
physical limits to the amount of bending and twisting (flex strength) it can tolerate, as
well as the amount of weight or longitudinal stress it can support (tensile strength),
without breaking (break strength). Fibre optic cables are notoriously susceptible in this
regard. Cables hung from poles expand and contract with changes in ambient temperature;
while glass fibre optic cables expand and contract relatively little, twisted pair copper
wire is more expansive.
The issue of resistance to environmental conditions also applies to airwave systems, as
reflective dishes, antennae, and other devices used in microwave, satellite, and infrared
technologies must be mounted securely to deal with wind and other forces of nature.
Additionally, the towers, walls and roofs on which they are mounted must be constructed
and braced properly in order to withstand such forces.

Physical dimensions: The physical dimensions of a transmission system must be considered
as well. This is especially true, once again, in the case of wired systems. Certainly, the
sheer weight of a cable system must be considered as one attempts to deploy it effectively.
Additionally, the bulk (diameter) of the cable is of importance, as conduit and raceway

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