1.3 DATA TRANSMISSION CHANNELS
The data transmission has to be done over a transmission channel or media. It can be
classified as:
a) Guided Channels
b) Unguided Channels
1.3.1 Guided Media
Guided media provide a physical connection between two devices. A signal traveling
through guided media is directed and contained within the physical limits of the
medium. There are several different Guided media, however we define only the most
popular as given below:
a) Twisted pair cable
b) Optic Fiber cable
a) Twisted Pair Cable
Twisted pair cable is still the most common transmission media. A twisted pair cable
consists of two conductors which are normally made of copper. Each conductor has its
own plastic insulation typically 1 mm thick. These cables are twisted together. The
wires are twisted in a helical form, similar to a DNA molecule. Twisting is done to
reduce crosstalk. Twisted Pairs (Figure 1.3) are very effective for relatively short
distances (a few hundred feet), but can be used for up to a few kilometers. A twisted
pair has a bandwidth to distance ratio of about 1 MHz per kilometer. The performance
of the twisted pair can be substantially improved by adding a metallic shield around
the wires. Shielded wires are much more resistant to thermal noise and crosstalk
effects. Twisted pairs are used for long distance connections e.g. telephone lines
which are usually organized as larger cable containing numerous twisted pairs.
Twisted pair cabling comes in several varieties, two of which are very important:
Category 3 and Category 5. Category 5 has more twists per centimeter resulting in less
crosstalk and a better quality signal.
b) Optical Fiber
An optical fiber consists of two concentric cylinders: an inner core surrounded by a cladding. Both the core and the cladding are made of transparent plastic or glass material as shown in the Figure 1.4, which transmit signals in the form of light. Optical fiber use reflections to guide light through a channel. The density of the core and cladding must differ sufficiently to reflect the beam of light instead of refracting. The core is used for guiding a light beam, whereas the cladding (which has a different refractive index) acts as a reflector to prevent the light signal instead of electrons, it does not suffer from the various noise problems associated with electromagnetic signals. The signal is usually generated by a laser or Light Emitting Diode (LED). Optical fibers can provide bandwidth to distance ratios in order of 100s of MHz per kilometer. Like other cables, hundreds of optical fibers are usually housed within one cable They are being increasingly used as telecommunication carriers for long distance digital trunk lines. Current trends promise that they will replace twisted pair residential loops in the near future.
Advantages
1) Higher Band width – it can support higher band width and hence can transfer data at a higher rate.
2) Less signal attenuation – its transmission distance is greater than the twisted pair and it can run for 50Kms without regeneration.
3) Immunity to electromagnetic interface
4) These cables are much lighter than the copper cables
5) These cables are more immune to tapping than the copper cables.
Disadvantages
1) Installation or maintenance – it needs expertise which is not available everywhere.
2) Unidirectional – Propagation of light is unidirectional and we need two fibers for bidirectional communication.
3) Costly – the cables and interfaces used are relatively expensive.
1.3.2 Unguided Media
Unguided media is used for transmitting the signal without any physical media. It
transports electromagnetic waves and is often called wireless communication. Signals
are broadcast through air and received by all who have devices to receive them. It can
be categorized as follows:
a) Radio waves
b) Micro waves
c) Infrared
a) Radio Waves
Electromagnetic waves ranging in frequencies between 3 Kilo-Hertz and 1 Giga-Hertz
are normally called radio waves. Radio waves are easy to generate and can travel long
distances and can penetrate buildings easily, therefore widely used for
communication. These are omni-directional which implies that these travel in all
directions from the source, so the transmitter and receiver do not have to be carefully
aligned physically.
Radio signals have been used for a long time to transmit analog information. They are
particularly attractive for long distance communication over difficult terrain or across
the oceans, where the cost of installing cables can be too prohibitive.
An increasingly-popular form of radio is cellular radio, which is currently being used
by carriers for providing mobile telephone networks. These operate in the VHF (Very
High Frequency) band and subdivide their coverage area into conceptual cells, where
each cell represents a limited area which is served by a low-power transmitter and
receiver station. As the mobile user moves from one cell area to another, its
communication is handed over from one station to another. Radio waves transmitted
by one antenna are susceptible to interference by another antenna due to its
Omni-directional property. Radio waves can be received both inside and outside the
building.
Radio waves are very useful in multicasting and hence used in AM and FM radios,
cordless phones and paging. You may be wondering about the term multicasting. If
the communication is between single source and destination then it is called unicast;
on the other hand, if one source is transmitting signal and any destination that is in the
range may be able to reach it then it is called broadcast. Multicast is when a source
transmits a signal for some specific group of destinations which may be more than
one.
Bluetooth: Bluetooth is a very popular application of short wave length radio
transmission in the frequency band of 2400 to 2480 MHz. It is a proprietary wireless
technology standard used for exchanging data over short distances in mobile phones
and other related devices. It allows wireless devices to be connected to wireless host
which may be a computer over short distances. You may have it for transferring data
between a mobile phone and a computer provided both have Bluetooth technology.
b) Microwaves
Electromagnetic waves ranging from 1 to 300 Gigahertz are called microwaves.
Microwaves are unidirectional that is the sending and receiving antennas need to be
aligned
Microwave is by far the most widely used form of radio transmission. It operates in
the GHz range with data rates in order of hundreds of Mbps per channel.
Telecommunication carriers and TV stations are the primary users of microwave
transmission.
An important form of microwave system is a satellite system, which is essentially a
microwave system plus a large repeater in the sky as shown in Figure 1.5. The signals
transmitted by earth stations are received, amplified, and retransmitted to other earth
stations by the satellite. Like other microwave systems, the bandwidth is subdivided
into channels of 10s of MHz each, providing data rates in order of 100s of mbps.
Because of their high bandwidths, satellites are capable of supporting an enormous
number and variety of channels, including TV, telephone, and data. The satellite itself,
however, is a major investment and typically has a limited lifetime (at most a few
decades).
Unidirectional property of microwave helps in avoiding interference by a pair of
aligned antenna to another. High frequency micro waves cannot be received inside the
building.
c) Infrared
Infrared signals range between 300 Giga-Hertz to 400 Tera-Hertz. These can be used
for short range communication. High range infrared rays cannot be used for long
range communication as it cannot penetrate walls. This also helps in avoiding
interference Infrared signals are generated and received using optical transceivers. Infrared
systems represent a cheap alternative to most other methods, because there is no
cabling involved and the necessary equipment is relatively cheap. Data rates similar to
those of twisted pairs are easily possible. However, applications are limited because of
distance limitations (of about one kilometer). One recent use of infra-red has been for
interfacing hand-held and portable computing devices to Local Area Networks as
shown in Figure 1.6.
It cannot be used outside building as rays of sun contain infrared which leads to
interference in communication. Infrared having wide bandwidth can be used to
transmit digital data with a very high data rate. Infrared signals can be used for
communication between keyboards, mouse and printers.
Check Your Progress 1 👈
1. What is the need of computer networks?
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2. In the context of communication system, what does a MODEM do?
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3. How can you improve the performance of twisted pair cables?
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4. Describe the principal of optical fiber and its advantages and disadvantages?
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