The Physical Layer

Transmission Media- Twisted Pair, Coaxial Cable, and Fiber

Optics.
Wireless Transmission:
Radio Transmission, Microwave Transmission, Infrared, Light
Transmission. Multiplexing-Frequency division, time division,
code division, Switching.
• Data is represented by computers and other telecommunication devices
using signals.
• Signals are transmitted in the form of electromagnetic energy from one
device to another.
• Electromagnetic signals travel through vacuum, air or other transmission
mediums to move from one point to another(from sender to receiver).
• Electromagnetic energy (includes electrical and magnetic fields) consists of
power, voice, visible light, radio waves, ultraviolet light, gamma rays etc.
• Transmission medium is the means through which we send our
data from one place to another.
Transmission media can be divided into two parts
1.Guided Media and
2.Unguided Media.
Bounded or Guided Transmission Media

In guided media, the signal is contained within the physical limits of the
transmission medium and is directed along with it.

Guided media, which are those that provide a channel from one
device to another, include
Twisted-Pair Cable,
Coaxial Cable, and
Fibre-Optic Cable.

•Twisted-pair and coaxial cable use metallic (copper) conductors that

accept and transport signals in the form of electric current.
•Optical fibre is a cable that accepts and transports signals in the form of
light.
Twisted Pair Cables :
These are a type of guided media. It was invented by Alexander Graham Bell. Twisted pair cables
have two conductors that are generally made up of copper and each conductor has insulation.
These two conductors are twisted together, thus giving the name twisted pair cables.
One of the conductors is used to carry the signal and the other is used as a ground reference only.
The receiver uses the difference of signals between these two conductors. The noise or crosstalk in
the two parallel conductors is high but this is greatly reduced in twisted pair cables due to the
twisting characteristic.
Twisted Pair Cables are further of two types :
1.Unshielded Twisted Pair Cables (UTP) :
•These are a pair of two insulated copper wires twisted together without any
other insulation or shielding and hence are called unshielded twisted pair
cables.
•They reduce the external interference due to the presence of insulation.
•Unshielded twisted pair cables are arranged in pairs so that we can add a
new connection whenever required.
Advantages –
•These cables are cost-effective and easy to install owing to their compact size.
•They are generally used for short-distance transmission of both voice and data.
•It is less costly as compared to other types of cables.
Disadvantages –
•The connection established using UTP is not secure.
•They are efficient only for a distance up to 100 meters and have to be installed in
pieces of up to 100 meters.
•These cables have limited bandwidth.
2. Shielded Twisted Pair Cables (STP) :

These types of cables have extra insulation or protective covering over

the conductors in the form of a copper braid covering.
This covering provides strength to the overall structure of the cable. It
also reduces noise and signal interference in the cable.
The STP cables are also color-coded like the UTP cables as different
color pairs are required for analog and digital transmission. These
cables are costly and difficult to install.
Advantages –
•They are generally used for long-distance communication and transmission and are
installed underground.
•The protective shield prevents external electromagnetic noise penetration into the cable.
•They have a higher bandwidth as compared to UTP.

Disadvantages –
•These cables are very expensive.
•They require a lot of maintenance which increases the cost more.
•These can be installed underground only.
•The length of the segment is similar to UTP for these cables.
Applications of Twisted pair cables :
1.Twisted Pair cables are used in telephone lines to provide data and
voice channels.
2.The DSL lines make use of these cables.
3.Local Area Networks (LAN) also make use of twisted pair cables.
4.They can be used for both analog and digital transmission.
RJ-45 is a very common application of twisted pair cables.
•DSL-A technology for high-speed network or Internet access over
voice lines.
• The following categories of UTP cables are defined:
1.Category 1(cat-1):
supports analog voice data and is commonly used for telephone
communication.
It is used in Integrated Services Digital Network(ISDN), a type of WAN
connectivity option.

2. Category 2(cat-2):
supports digital voice communication up to a speed of 4 Mbps.
They are commonly used in IBM token ring networks.

3.Category 3 (cat 3) or 10baseT:

The maximum data transfer rate supported by the Cat-3 is 16 Mbps.
It is used to transmit data at 10Mbps. It is also represented as 10baseT.
4. Category 4(cat 4):
supports a maximum data transfer rate of 20 Mbps, which is only
slightly higher than that offered by Cat-3.
Therefore, Cat-4 is rarely used.

5. Category 5(cat 5):

is used for networks that require data transfer rates higher than that
supported by cat-3.
It supports data transfer rates upto 100Mbps and is used with Fast
Ethernet.
An enhancement of Cat 5 called cat-5E has also been developed.
Cat-5E uses a higher-grade copper wire than that used by Cat-5 and can
support data transfer rates upto 1000Mbps.
It is used to implement Gigabit Ethernet.
• Cat-6 and Cat-7 support higher bandwidth and data transfer rates have also
been developed.

• Cat-6 works at a different frequency range than Cat-5E.

• In a network, a cable may sometimes pickup a signal intended for another
cable. This is called Crosstalk.

• Cat-7 is even more resistant to signal attenuation and crosstalk than cat-5E and
cat-6 cables.

• Cat-7 uses a different connector to interface with the NIC.

• Returning to twisted pair, Cat 5 replaced earlier Category 3 cables with
a similar cable that uses the same connector, but has more twists per
meter.
• More twists result in less crosstalk and a better-quality signal over
longer distances, making the cables more suitable for high-speed
computer communication, especially 100-Mbps and 1-Gbps Ethernet
LANs.
• New wiring is more likely to be Category 6 or even Category 7. These
categories has more stringent specifications to handle signals with
greater bandwidths.
• Some cables in Category 6 and above are rated for signals of 500 MHz
and can support the 10-Gbps links that will soon be deployed.
• Crosstalk is a disturbance caused by the electric or magnetic fields
2. Coaxial cables:
1. A coaxial cable consists of two concentric conductors separated by insulation.
2. The inner conductor transmits electric signals and
3. The outer conductor acts as a ground.
4. The entire assembly is wrapped in a sheath of Teflon or PVC.
5. The conductor used in coaxial cable is copper wire. It is used for both the
inner and outer conductors.
6. A thicker coaxial cable can transmit more data than a thinner one.

7. Coaxial cables were the original transmission medium specified by IEEE for
use with Ethernet.
Coaxial cable, or coax (pronounced /ˈkoʊ.æks/) is a type of electrical
cable consisting of an inner conductor surrounded by a concentric
conducting shield, with the two separated by a dielectric (insulating material);
many coaxial cables also have a protective outer sheath or jacket.
Advantages
•Coaxial cables support high bandwidth.
•It is easy to install coaxial cables.
•coaxial cables have better cut-through resistance so they are
more reliable and durable.
•Less affected by noise or cross-talk or electromagnetic
inference.
•Coaxial cables support multiple channels
•They can transmit signals up to 10 kilometers.
Disadvantages
•Coaxial cables are expensive.
•The coaxial cable must be grounded in order to prevent any
crosstalk.
•As a Coaxial cable has multiple layers it is very bulky.
•There is a chance of breaking the coaxial cable and attaching a “t-
joint” by hackers, this compromises the security of the data.
•They support only the bus topology.
•Coaxial cables do not support star topology, which is the most
common topology used in LANs.
• The commonly used coaxial cables in Ethernet LANs are:

1.10base2 or thinnet:
support data transfer rate of 10 Mbps and can transmit signals without
attenuation over a distance of 185meters.

2.10base5 or thicknet:
supports a data transfer rate of 10 Mbps over a distance 500meters.
Fiber Optics
•A fibre-optic cable is made of glass or plastic and transmits signals in the form of
light.
•Fiber optics, or optical fiber, refers to the technology that transmits information as
light pulses along a glass or plastic fiber.
•Fiber optics are used for long-haul transmission in network backbones, highspeed
LANs and high-speed Internet access such as FttH (Fiber to the Home).
•An optical transmission system has three key components: the light source, the
transmission medium, and the detector.
•Conventionally, a pulse of light indicates a 1 bit and the absence of light indicates a 0
bit. The transmission medium is an ultra-thin fiber of glass
•A fiber-optic cable, also known as an optical-fiber cable, is an assembly similar to
an electrical cable, but containing one or more optical fibers that are used to carry
light. The optical fiber elements are typically individually coated with plastic layers
and contained in a protective tube suitable for the environment where the cable is
used.
• Wireless Transmission- Radio Transmission, Microwave Transmission,
Infrared, Light Transmission. Multiplexing-Frequency division, time
division, code division, Switching.
• Radio Transmission
• Radio frequency (RF) waves are easy to generate, can travel long
distances, and can penetrate buildings easily, so they are widely
used for communication, both indoors and outdoors.
• Radio waves also are omnidirectional, meaning that they travel in all
directions from the source, so the transmitter and receiver do not
have to be carefully aligned physically.
• The properties of radio waves are frequency dependent.
• Radio waves can travel long distances, and interference between
users is a problem. With radio, the signal drops by the same fraction
as the distance doubles
• In the electromagnetic spectrum, all omnidirectional waves in the
frequencies 3KHz to 1GHz are called radio waves. They are widely
used for communications since they are easy to generate, can travel
long distances and can penetrate buildings.
• Radio waves have omnidirectional antennas, i.e. antennas that can
send signals in all directions.
• The properties of radios waves vary according to their frequencies.
However, radio waves at all frequencies are prone to interference
from electrical equipments like motors etc.
Some of the areas of applications of radio waves are −
•Broadcasting and multicasting
•Fixed and mobile radio communications
•AM and FM radio
•Television
•Marine communication
•Wireless computer networks
•Cordless phones
Microwave Transmission
•Microwaves travel in a straight line, so if the towers are too far apart, the
earth will get in the way . Thus, repeaters are needed periodically.
•Unlike radio waves at lower frequencies, microwaves do not pass through
buildings well. It is weather and frequency dependent.
•Microwave communication is so widely used for long-distance telephone
communication, mobile phones, television distribution, and other purposes
that a severe shortage of spectrum has developed.
•It has several key advantages over fiber. The main one is that no right of way is
needed to lay down cables. By buying a small plot of ground every 50 km and
putting a microwave tower on it, one can bypass the telephone system entirely.
•Microwave is also relatively inexpensive. Putting up two simple towers
and putting antennas on each one may be cheaper than burying 50 km of fiber
through a congested urban area or up over a mountain
• In the electromagnetic spectrum, waves within the frequencies 1GHz to
300GHz are called microwaves.
• Microwaves travel in straight lines, and so the transmitter and receiver
stations should be accurately aligned to each other.
• Microwave propagation is line – of – sight propagation. So, towers
hoisting the stations should be placed so that the curvature of the earth or
any other obstacle does not interfere with the communication.
• Since it is unidirectional, it allows multiple receivers in a row to receive
the signals without interference.
• Microwaves do not pass through buildings. So, indoor receivers cannot
be used effectively.
• Microwaves are often refracted by the atmospheric layers. The refracted
rays take longer time to reach the destination than the direct rays. This
causes out of phase transmission, called multipath fading.
Microwave Transmission

Applications
• Long distance telephone communication
• Cellular phones
• Television networks
• Satellites
• Wireless LANs
Infrared Transmission
•Unguided infrared waves are widely used for short-range communication.
•The remote controls used for televisions, VCRs, and stereos all use infrared
communication.
•They are relatively directional, cheap, and easy to build but have a major
drawback: they do not pass through solid objects. In general, as we go from long-
wave radio toward visible light, the waves behave more and more like light and
less and less like radio. On the other hand, the fact that infrared waves do not
pass through solid walls. It means that an infrared system in one room of a
building will not interfere with a similar system in adjacent rooms or buildings:
you cannot control your neighbor’s television with your remote control.
Furthermore, security of infrared systems against eavesdropping(over hearing)
is better than that of radio systems precisely for this reason. Infrared
communication has a limited use on the desktop.
• Infrared waves are those between the frequencies 300GHz and
400THz in the electromagnetic spectrum. Their wavelengths are
shorter than microwaves but longer than visible light. Infrared
propagation is line of sight.
• They cannot penetrate walls and sun’s infrared rays interfere with
these rays. So cannot be used for long – range communication. As
their usage is confined within closed space, they do not need any
government permissions for their applications.
Applications of Infrared Waves in Communications
•Remote controls for television, stereos and other home appliances.
•Wireless LANs
•Wireless modem, keyboard, mouse, printer etc
•Fire detectors
•Night vision systems
•Intrusion detection systems
•Motion detectors
Light Transmission
•Unguided optical signaling or free-space optics has been in use for
centuries. A more modern application is to connect the LANs in two
buildings via lasers mounted on their rooftops.
•Optical signaling using lasers is inherently unidirectional, so each end
needs its own laser and its own photodetector.
•This scheme offers very high bandwidth at very low cost and is relatively
secure because it is difficult to tap a narrow laser beam. It is also
relatively easy to install and, unlike microwave transmission, does not
require an FCC license. The laser’s strength, a very narrow beam, is also
its weakness here.
• Electromagnetic waves within the frequency range of 400 THz – 790
THz are detected by the human eye. Light transmission is line of sight
propagation and is blocked by obstacles.
• Optical signaling can be obtained by laser signals.
• Communication using light signals is safe for eyes and cost effective.
Also, it does not require any license for usage. So, it is increasingly
being used for ubiquitous computing and IoT systems.
• Channels are often shared by multiple signals.
• After all, it is much more convenient to use a single wire to
carry several signals than to install a wire for every signal.
This kind of sharing is called multiplexing. It can be
accomplished in several different ways.
• Multiplexing is the process of combining multiple signals
into one signal, over a shared medium.
• We will present methods for time, frequency, and code
division multiplexing.
• Fibre optic is categorized into 2 main groups:
1. Multimode (MM)
2. Single Mode (SM) fibre
• MM is typically less expensive, for relatively shorter distances.
• SM is often more expensive but can support very long distances.
• Multi mode fiber • Single Mode Fiber
1.Core size can be 50 or 1.Core size is 9 Micro meter
62.5Micro-meter 2.Maximum distance: 40Km
2.Maximum distance: 600 meters 3.Data Rates: Up to 40 and
3.Typical Data Rate:1Gbps 400Gbps
4.Transceivers are based on LEDs 4.Transceivers are based on
LASERS
• Multiplexing is a technique used to combine and send the multiple data
streams over a single medium.
• The process of combining the data streams is known as Multiplexing and
hardware used for multiplexing is called as a Multiplexer.
• Multiplexing technique is widely used in telecommunications in which
several telephone calls are carried through a single wire.
• Multiplexing is achieved by using a device called Multiplexer(MUX) that
combines n input lines to generate a single output line.
• Multiplexing follows many-to-one, i.e., n input lines and one output line.
• Demultiplexing is achieved by using a device called Demultiplexer (DEMUX)
available at the receiving end.
• DEMUX separates a signal into its component signals (one input n outputs).
• Frequency is a measure of the number of cycles that are
done per unit of time and is generally measured in hertz
(cycles per second).
• Data cabling is normally rated in kilohertz (kHz) or megahertz
(MHz).
• Frequency is the number times something occurs in a
specific amount of time.
• Bandwidth is the maximum amount of data transmitted
over an internet connection in a given amount of time
1. FDM is a technique in which the available bandwidth of a
single transmission medium is subdivided into several
channels.
2. The main aim of FDM is to subdivide the available bandwidth
into frequency channel is given to different devices.
3. The input signals are translated into frequency bands by
using modulation techniques and they are combined by a
multiplexer to form a composite signal.
4. Using the modulation technique, the input signals are transmitted
into frequency bands and then combined to form a composite
signal.
5. FDM is mainly used in radio broadcasts and TV networks.
1. Frequency-division multiplexing (FDM) is a technique by which the
total bandwidth available in a communication medium is divided into a series of non-
overlapping frequency bands, each of which is used to carry a separate signal.
 This allows a single transmission medium such as a cable or optical fiber to be
shared by multiple independent signals.
 Another use is to carry separate serial bits or segments of a higher rate signal
in parallel.
• Frequency division multiplexing (FDM) is a technique of multiplexing which means
combining more than one signal over a shared medium. In FDM, signals of different
frequencies are combined for concurrent transmission.
• This technique uses various frequencies to combine streams of data, for sending
them on a communication medium, as a single signal.
Frequency Division Multiplexing
•Multiplexing schemes have been developed to share lines among many signals.
FDM (Frequency Division Multiplexing) is used to share a channel. It divides the
spectrum into frequency bands, with each user having exclusive possession of
some band in which to send their signal.
•This scheme has been used to multiplex calls in the telephone system for
many years, but multiplexing in time is now preferred instead. However, FDM
continues to be used in telephone networks, as well as cellular, terrestrial
wireless, and satellite networks at a higher level of granularity.
•In the first generation of mobile phones, FDM was used.
•The use of FDM in television broadcasting
•FDM is used to broadcast FM and AM radio frequencies.
TDM-Time division Multiplexing
In TDM, the total time available in the channel is distributed among
different users.
Therefore, each user is allocated with different time interval known
as a Time slot at which data is to be transmitted by the sender.
In TDM technique, data is not transmitted simultaneously rather
the data is transmitted one-by-one.
In TDM, the signal is transmitted in the form of frames.
Frames contain a cycle of time slots in which each frame contains
one or more slots dedicated to each user.
It can be used to multiplex both digital and analog signals but mainly
used to multiplex digital signals.
Time Division Multiplexing (TDM):
•This happens when the data transmission rate of media is
greater than that of the source, and each signal is allotted a
definite amount of time. These slots are so small that all
transmissions appear to be parallel.
•In frequency division multiplexing all the signals operate at the
same time with different frequencies, but in time-division
multiplexing, all the signals operate with the same frequency at
different times.
•Time Division Multiplexing (TDM)
•In TDM, the time frame is divided into slots. This technique is
used to transmit a signal over a single communication channel,
with allotting one slot for each message.
Time Division Multiplexing
Time Division Multiplexing
•An alternative to FDM is TDM (Time Division Multiplexing). Here, the
users take turns (in a round-robin fashion), each one periodically getting the
entire bandwidth for a little burst of time. An example of three streams being
multiplexed with TDM is shown in Fig. 2-27.
•Bits from each input stream are taken in a fixed time slot and output to the
aggregate stream. This stream runs at the sum rate of the individual streams. For
this to work, the streams must be synchronized in time. Small intervals of guard
time analogous to a frequency guard band may be added to accommodate small
timing variations.
•TDM is used widely as part of the telephone and cellular networks. To avoid one
point of confusion, let us be clear that it is quite different from the alternative
STDM (Statistical Time Division Multiplexing). The prefix ‘‘statistical’’ is added to
indicate that the individual streams contribute to the multiplexed stream not on a
fixed schedule, but according to the statistics of their demand. STDM is packet
switching by another name.
3. Code Division Multiplexing
•There is a third kind of multiplexing that works in a completely different way
than FDM and TDM.
•CDM (Code Division Multiplexing) is a form of spread spectrum
communication in which a narrowband signal is spread out over a wider
frequency band. This can make it more tolerant of interference, as well as
allowing multiple signals from different users to share the same frequency
band.
•Because code division multiplexing is mostly used for the latter purpose it is
commonly called CDMA (Code Division Multiple Access).
•Code division multiplexing (CDM) is a networking technique in which
multiple data signals are combined for simultaneous transmission over a
common frequency band.
•When CDM is used to allow multiple users to share a single communications
channel, the technology is called code division multiple access (CDMA)
• We know that switching is the common terminology used in
networking.
• It is the process of exchanging information between two
communication devices. There are two types of
switching: circuit switching and packet switching.
• What is switching vs routing?
• Just as a switch connects multiple devices to create a
network, a router connects multiple switches, and their
respective networks, to form an even larger network. These
networks may be in a single location or across multiple
locations.
Switching
•From the point of view of the average telephone engineer, the phone system is
divided into two principal parts: outside plant (the local loops and trunks, since
they are physically outside the switching offices) and inside plant (the switches,
which are inside the switching offices).
• Two different switching techniques are used by the network nowadays: circuit
switching and packet switching.
•The traditional telephone system is based on circuit switching, but packet
switching is beginning to make inroads with the rise of voice over IP technology.
•We will go into circuit switching in some detail and contrast it with packet
switching. Both kinds of switching are important enough that we will come back to
them when we get to the network layer.
•In simple terms, multiplexing is done to maximize the use of a communications
channel. Whereas, the switching is the manipulation of the ends of the
communications channel and is used to make the connections.
Circuit switching is not convenient for Packet switching is suitable for handling bilateral traffic.
handling bilateral traffic.

In-Circuit switching, the charge

In Packet switching, the charge is based on the number of bytes and
depends on time and distance, not on connection time.
Circuit Switching
•Conceptually, When you or your computer places a telephone call, the
switching equipment within the telephone system seeks out a physical path
all the way from your telephone to the receiver’s telephone. This technique
is called circuit switching.
•It is shown schematically in Fig. 2-42(a). Each of the six rectangles represents
a carrier switching office (end office, toll office, etc.). In this example, each
office has three incoming lines and three outgoing lines. When a call passes
through a switching office, a physical connection is (conceptually) established
between the line on which the call came in and one of the output lines, as
shown by the dotted lines.
•An important property of circuit switching is the need to set up an end-to-
end path before any data can be sent.
Packet Switching
•The alternative to circuit switching is packet switching, shown in Fig. 2-42(b) and
described in Chap. 1.
•With this technology, packets are sent as soon as they are available. There is no
need to set up a dedicated path in advance, unlike with circuit switching. It is up
to routers to use store-and-forward transmission to send each packet on its way
to the destination on its own.
•This procedure is unlike circuit switching, in which the result of the connection
setup is the reservation of bandwidth all the way from the sender to the receiver.
• All data on the circuit follows this path. Among other properties, having all the
data follow the same path means that it cannot arrive out of order.
•With packet switching there is no fixed path, so different packets can follow
different paths, depending on network conditions at the time they are sent, and
they may arrive out of order.
• Packet-switching networks place a tight upper limit on the size of
packets. packet-switched networks can handle interactive traffic.
• It also reduces delay since the first packet of a long message can be
forwarded before the second one has fully arrived. However, the
store-and-forward delay of accumulating a packet in the router’s
memory before it is sent on to the next router exceeds that of circuit
switching. With circuit switching, the bits just flow through the wire
continuously.
• Because no bandwidth is reserved with packet switching, packets may have to
wait to be forwarded.
• This introduces queuing delay and congestion if many packets are sent at the
same time. On the other hand, there is no danger of getting a busy signal and
being unable to use the network. Thus, congestion occurs at different times
with circuit switching (at setup time) and packet switching (when packets are
sent).
• If a circuit has been reserved for a particular user and there is no traffic, its
bandwidth is wasted. It cannot be used for other traffic. Packet switching
does not waste bandwidth and thus is more efficient from a system
perspective. Understanding this trade-off is crucial for comprehending the
difference between circuit switching and packet switching. The trade-off is
between guaranteed service and wasting resources versus not guaranteeing
service and not wasting resources.
• Packet switching is more fault tolerant than circuit switching. In fact, that is
why it was invented. If a switch goes down, all of the circuits using it are
terminated and no more traffic can be sent on any of them. With packet
switching,
• packets can be routed around dead switches.
Twisted-pair cables:
One of the oldest and still most common transmission media is twisted
pair.
A twisted pair consists of two insulated copper wires, typically about 1 mm
thick. The wires are twisted together in a helical form, just like a DNA
molecule. Twisting is done because two parallel wires constitute a fine
antenna. When the wires are twisted, the waves from different twists cancel
out, so the wire radiates less effectively.
The twisted pair cables generally used in LANs are of the following types:

1.Shielded Twisted Pair(STP)

2. Unshielded Twisted Pair(UTP)

• The most common application of the twisted pair is the telephone system.
• Nearly all telephones are connected to the telephone company (telco) office by
a twisted pair.
• Both telephone calls and ADSL Internet access run over these lines.
• Twisted pairs can run several kilometers without amplification, but for longer
distances the signal becomes too attenuated and repeaters are needed.
• Twisted pairs can be used for transmitting either analog or digital information.
The bandwidth depends on the thickness of the wire and the distance traveled.
A signal is usually carried as the difference in voltage between the two wires in
the pair.
This provides better immunity to external noise because the noise tends to affect
both wires the same, leaving the differential unchanged.
1. Shielded Twisted Pair(STP):
• In STP, an extra layer of metal foil is present between the twisted pairs of
copper wires and the outer sheath. The purpose of this layer is to provide
additional protection(shielding) from EMI and RFI. STP is more expensive
than UTP and is generally used in networks where cables pass closer to
device that cause high EMI.

2. Unshielded Twisted Pair(UTP):

• UTP cables are most commonly used communication medium used in LANs.
• Of the 4 pairs in a UTP cable, only two pairs are actually used for communication in
LANs and provide speed of upto 100 Mbps.
• All 4 pairs are used only for higher transmission rates such as 1000Mbps.
• UTP cables are capable of supporting different bandwidths and transmission speeds.
• Twisted-pair cabling comes in several varieties.
• The garden variety deployed in many office buildings is called Category 5
cabling, or ‘‘Cat 5.’’
• A category 5 twisted pair consists of two insulated wires gently twisted
together.
• Four such pairs are typically grouped in a plastic sheath to protect the
wires and keep them together
1.Links that can be used in both directions at the same time, like a two-
lane road, are called full-duplex links.
2.In contrast, links that can be used in either direction, but only one way
at a time, like a single-track railroad line. are called half-duplex links.
3.A third category consists of links that allow traffic in only one direction,
like a one-way street. They are called simplex links.
Advantages of Unshielded Twisted Pair Cable
1.Installation is easy
2.Flexible
3.Cheap-UTP cables are inexpensive and are resistant to physical
stress.
4.It has high speed capacity,
5.100 meter limit
6.Higher grades of UTP are used in LAN technologies like Ethernet.
7.They also offer the flexibility to select a particular category of cable
depending on the network requirements and resist attenuation over
distances that are adequate for most LANs.
Disadvantages of Unshielded Twisted Pair Cable
1.Bandwidth is low when compared with Coaxial Cable
2.Provides less protection from interference.
3.UTP cables should not be used if the network is located in an
environment with high EMI.
• ADSL (Asymmetric Digital Subscriber Line) is a technology that
facilitates fast data transmission at a high bandwidth on existing
copper wire telephone lines to homes and businesses
Coaxial Cable
•Another common transmission medium is the coaxial cable (known to its
many friends as just ‘‘coax’’ and pronounced ‘‘co-ax’’).
• It has better shielding and greater bandwidth than unshielded twisted
pairs, so it can span longer distances at higher speeds.
•Two kinds of coaxial cable are widely used.
1.One kind, 50-ohm cable, is commonly used when it is intended for
digital transmission from the start.
2.The other kind, 75-ohm cable, is commonly used for analog transmission
and cable television.
•A coaxial cable consists of a stiff copper wire as the core, surrounded by
an insulating material. The insulator is encased by a cylindrical conductor,
often as a closely woven braided mesh. The outer conductor is covered in
a protective plastic sheath.
• Coaxial is called by this name because it contains two conductors
that are parallel to each other.
• Copper is used in this as centre conductor which can be a solid wire
or a standard one. It is surrounded by PVC installation, a sheath
which is encased in an outer conductor of metal foil, barid or both.
• Outer metallic wrapping is used as a shield against noise and as the
second conductor which completes the circuit. The outer conductor is
also encased in an insulating sheath. The outermost part is the
plastic cover which protects the whole cable.
• The construction and shielding of the coaxial cable give it a
good combination of high bandwidth and excellent noise
immunity.
• The bandwidth possible depends on the cable quality and
length. Modern cables have a bandwidth of up to a few
GHz.
• Coaxial cables used to be widely used within the telephone
system for long-distance lines but have now largely been
replaced by fiber optics on longhaul routes.
• Coax is still widely used for cable television and
metropolitan area networks, however.