Chapter-2-Physical-Layer-v1.pptx hai dami

The Physical Layer
⮚ Physical layer is the first layer that physically connects the two systems that need to communicate. It
transmits data in bits and manages simplex or duplex transmission by modem. It also manages Network
Interface Card’s hardware interface to the network, like cabling, cable terminators, topography, voltage
levels, etc.
⮚ One of the major functions of the physical layer is to move data in the form of electromagnetic signals
across a transmission medium.
⮚ The data usable to a person or an application are not in a form that can be transmitted over a network.
⮚ For example, an image must first be changed to a form that transmission media can accept.
⮚ Physical layer provides its services to Data-link layer. Data-link layer hands over frames to physical layer.
Physical layer converts them to electrical pulses, which represent binary data. The binary data is then sent
over the wired or wireless media.
⮚ Physical layer in the OSI model plays the role of interacting with actual hardware and signalling
mechanism. Physical layer is the only layer of OSI network model which actually deals with the physical
connectivity of two different stations.
⮚ The rules and procedures that are defined for interaction between the physical layers are known as
physical layer protocols.
⮚ The quality of data is maintained by the physical layer by applying the protocols and maintaining the data
bit rate.

The main functions of the Physical layer include:
⮚ Encoding and signaling: It defines the method of encoding digital data into electrical,
optical, or radio signals for transmission over the physical medium.
⮚ Physical media: It specifies the characteristics of the physical media used for data
transmission, such as cables, fiber optics, or wireless channels.
⮚ Data transmission: It establishes the rules for transmitting raw data bits over the
physical medium, including aspects like bit synchronization, line coding, and error
detection.
⮚ Physical topologies: It defines the physical arrangement or layout of network devices
and how they are connected, such as bus, star, ring, or mesh topologies.
⮚ Transmission modes: It describes the ways in which data is transmitted between
devices, such as simplex (one-way), half-duplex (two-way but not simultaneously), or
full-duplex (two-way simultaneously).
⮚ Switching mechanism: It also provides a Switching mechanism wherein data packets
can be forwarded from one port (sender port) to the leading destination port.

Signals
⮚ A signal is an electromagnetic or electrical current that carries data from one system or
network to another. In electronics, a signal is often a time-varying voltage that is also an
electromagnetic wave carrying information, though it can take on other forms, such as
current.
⮚ When data is sent over physical medium, it needs to be first converted into electromagnetic
signals. Data itself can be analog such as human voice, or digital such as file on the disk. Both
analog and digital data can be represented in digital or analog signals.
⮚ There are two main types of signals used in electronics: analog and digital signals.

Analog Signals
⮚ Analog signals are in continuous wave form in nature and represented by continuous
electromagnetic waves.
⮚ An analog signal is time-varying and generally bound to a range (e.g. +12V to -12V), but there
is an infinite number of values within that continuous range. An analog signal uses a given
property of the medium to convey the signal’s information, such as electricity moving through a
wire. In an electrical signal, the voltage, current, or frequency of the signal may be varied to
represent the information. Analog signals are often calculated responses to changes in light,
sound, temperature, position, pressure, or other physical phenomena.
⮚ When plotted on a voltage vs. time graph, an analog signal should produce a smooth and
continuous curve.
Digital Signals
⮚ A digital signal is a signal that represents data as a sequence of discrete values. A digital signal
can only take on one value from a finite set of possible values at a given time. With digital
signals, the physical quantity representing the information can be many things: Variable electric
current or voltage.
⮚ Digital signals are discrete in nature and represent sequence of voltage pulses. Digital signals
are used within the circuitry of a computer system.

Difference between Analog and Digital Signals
Analog Signals Digital Signals
Analog signal is continuous and time varying. Digital signal have two or more states and in binary form.
Troubleshooting of analog signals are difficult. Troubleshooting of digital signals are easy.
An analog signal is usually in the form of sine
wave.
An digital signal is usually in the form of square wave.
Easily affected by the noise. These are stable and less prone to noise.
Analog signals use continous values to represent
the data.
Digital signals use discrete values to represent the data.
Accuracy of the analog signals may be affected
by noise.
Accuracy of the digital signals are immune from the noise.
Analog signals may be affected during data
transmission.
Digital signals are not affacted during data transmission.
Analog signals use more power. Digital signals use less power.
Examples: Temperature, Pressure, Flow
measurements, etc.
Examples: Valve Feedback, Motor Start, Trip, etc.
Components like resistors, Capacitors, Inductors,
Diodes are used in analog circuits.
Components like transistors, logic gates, and micro-
controllers are used in Digital circuits.

Transmission Impairment
⮚ Signals travel through transmission media, which are not perfect. The imperfection causes signal
impairment. This means that the signal at the beginning of the medium is not the same as the
signal at the end of the medium. What is sent is not what is received.
The three different causes of impairment are attenuation, distortion, and noise.
⮚ Attenuation: The strength of a signal decrease with the increase in distance travelled over a
medium. Attenuation means loss of energy. When any signal travels over a medium or channel, it
loses some of its energy in the form of heat in the resistance of the medium. Attenuation decides
the signal to noise ratio hence the quality of received signal. For the receiver to interpret the data
accurately, the signal must be sufficiently strong. When the signal passes through the medium, it
tends to get weaker. As it covers distance, it loses strength.
⮚ Distortion: Signals are sent over media with pre-defined speed and frequency. If the signal speed
and frequency do not match, there are possibilities that signal reaches destination in arbitrary
fashion. In digital media, this is very critical that some bits reach earlier than the previously sent
ones.
⮚ Noise: The random or unwanted signal that mixes up with the original signal is called noise. There
are several types of noise such as induced noise, crosstalk noise, thermal noise and impulse noise
which may corrupt the signal.

Thermal Noise
⮚ Thermal noise is the random motion of electrons in a wire which creates an extra signal not
originally sent by the transmitter. Heat agitates the electronic conductors of a medium which
may introduce noise in the media. Up to a certain level, thermal noise is unavoidable.
Intermodulation
⮚ When multiple frequencies share a medium, their interference can cause noise in the medium.
Intermodulation noise occurs if two different frequencies are sharing a medium and one of
them has excessive strength or the component itself is not functioning properly, then the
resultant frequency may not be delivered as expected.
Crosstalk
⮚ This sort of noise happens when a foreign signal enters into the media. This is because signal
in one medium affects the signal of second medium.
Impulse
⮚ This noise is introduced because of irregular disturbances such as lightening, electricity,
short-circuit, or faulty components. Digital data is mostly affected by this sort of noise.

Data rate
⮚ Data rate can be defined as how fast can we send the data, in bits per second, over a
channel. Maximum Data Rate (Channel Capacity) is the tight upper bound on the rate at
which information can be reliably transmitted over a communication channel.
Data rate depends on three factors:
⮚ The bandwidth available
⮚ The level of the signals we use
⮚ The quality of the channel (the level of noise)
There are two theoretical formulas to calculate the data rate:
⮚ Nyquist for a noiseless channel
⮚ Shannon for a noisy channel

Noiseless Channel: A noiseless channel is a communication system in which the signal transmitted from the sender
to the receiver is free from any unwanted background noise or interference. In other words, a noiseless channel is a
theoretical ideal where the signal quality is perfect and the information transmitted is not degraded by any
extraneous factors.
Advantages:
⮚ Maximum data rate is high
⮚ Error-free transmission
⮚ Low latency: Since there is no noise in the channel, the transmission delay is very low. This means that data
can be transmitted quickly and in real-time.
⮚ High signal quality: A noiseless channel provides high signal quality, which means that the data is transmitted
with high accuracy and without any distortion.
⮚ Suitable for critical applications: A noiseless channel is well-suited for applications that require high
reliability and precision, such as in medical equipment, military communication, and aerospace systems.
⮚ Easy to design and implement: Since a noiseless channel is a theoretical concept, it is easy to design and
simulate in a controlled environment. This allows researchers to study the theoretical limits of communication
systems without having to worry about practical limitations.
⮚ Useful for benchmarking: A noiseless channel is a useful benchmark for evaluating the performance of
communication systems. By comparing the performance of real-world systems to the theoretical limits of a
noiseless channel, researchers can identify areas where improvements can be made.

Disadvantages:
⮚ Not realistic as most channels have some degree of noise.
⮚ Cost: Implementing a noiseless channel requires expensive equipment and resources,
making it impractical for many applications.
⮚ Limited range: A noiseless channel has a limited range, meaning that it cannot be used
for long-distance communication.
⮚ Vulnerability to interference: Although a noiseless channel is free from external noise,
it is still vulnerable to interference from other sources such as electromagnetic radiation,
which can cause errors in transmission.
⮚ Lack of error correction: Since a noiseless channel is error-free, it does not provide any
error correction mechanism. This means that any errors that do occur in transmission
cannot be detected or corrected, making the communication less reliable.
⮚ Incompatibility with existing systems: Most existing communication systems are
designed to operate in noisy channels. A noiseless channel may not be compatible with
these systems, which would require significant changes to be made to the infrastructure.

Noiseless Channel:
⮚ Nyquist Bit Rate: For a noiseless channel, the Nyquist bit rate formula defines the
theoretical maximum bit rate.
⮚ Bit Rate = 2 * Bandwidth * log2(L) bits/sec
⮚ In the above equation, bandwidth is the bandwidth of the channel, L is the number of signal
levels used to represent data, and Bit Rate is the bit rate in bits per second.
⮚ Bandwidth is a fixed quantity, so it cannot be changed. Hence, the data rate is directly
proportional to the number of signal levels.
⮚ Note: Increasing the levels of a signal may reduce the reliability of the system.

Examples:
⮚ Input1: Consider a noiseless channel with a bandwidth of 3000 Hz transmitting a signal
with two signal levels. What can be the maximum bit rate?
⮚ Output1 : Bit Rate = 2 * 3000 * log2(2) = 6000bps
⮚ Input2: We need to send 265 kbps over a noiseless channel with a bandwidth of 20 kHz.
How many signal levels do we need?
⮚ Output2 : 265000 = 2 * 20000 * log2(L)
⮚ log2(L) = 6.625
⮚ L = 26.625
= 98.7 levels
⮚ The amount of thermal noise present is measured by the ratio of the signal power to the
noise power, called the SNR (Signal-to-Noise Ratio).

Noisy Channel: A noisy channel is a communication system in which the transmission of data is impaired
by external interference or internal degradation of the signal. This interference, known as noise, can take
many forms, including thermal noise, electromagnetic interference, and crosstalk.
Advantages:
⮚ More realistic as most channels have some degree of noise
⮚ Techniques like error correction can be used to improve transmission reliability.
⮚ Longer range: Unlike a noiseless channel, a noisy channel can be used for long-distance communication
as it can propagate signals over large distances.
⮚ Greater flexibility: A noisy channel can be used for a wide range of applications, from simple voice
communication to high-speed data transfer.
⮚ Lower cost: Since most communication channels are noisy, using a noisy channel is generally more cost-
effective than implementing a noiseless channel.
⮚ Higher capacity: A noisy channel can support higher data rates than a noiseless channel by using
advanced modulation schemes and error correction techniques. This makes it possible to transmit more
data over the same channel bandwidth.
⮚ Adaptable: Communication systems using a noisy channel can be designed to adapt to changing
conditions, such as variations in signal strength or interference levels. This makes them more reliable and
adaptable in dynamic environments.

Disadvantages:
⮚ Maximum data rate is lower than in noiseless channels
⮚ Higher probability of errors in transmission
⮚ Increased complexity: In a noisy channel, additional techniques such as error correction and signal
processing are required to ensure reliable transmission. This adds complexity to the system design and can
increase the cost of implementation.
⮚ Limited range: The presence of noise in a channel can limit the range of the communication, particularly in
wireless systems, where interference from other sources can also affect the quality of the signal.
⮚ Interference: Noise can come from many sources, including other electronic devices and environmental
factors such as weather conditions, which can interfere with the transmission and degrade the quality of the
signal.
⮚ Degraded signal quality: The presence of noise in a channel can cause distortion in the signal, resulting in a
loss of signal quality and clarity. This can make it difficult to distinguish between different data values,
leading to errors in transmission.
⮚ Security issues: The presence of noise can make it easier for unauthorized users to intercept and decode the
signal, leading to potential security issues such as data theft or unauthorized access to sensitive information.
⮚ Advances in digital signal processing and error correction techniques have allowed for the development of
more sophisticated modulation and encoding schemes that can increase the maximum data rate of noisy
channels. However, these techniques can also increase the complexity and cost of the communication
system.

Noisy Channel Shannon Capacity: In reality, we cannot have a noiseless channel; the channel is always
noisy. Shannon capacity is used, to determine the theoretical highest data rate for a noisy channel:
⮚ Capacity (Maximum Data Rate) = bandwidth * log2(1 + SNR) bits/sec
⮚ In the above equation, bandwidth is the bandwidth of the channel, SNR is the signal-to-noise ratio, and
capacity is the capacity of the channel in bits per second. Bandwidth is a fixed quantity, so it cannot
be changed. Hence, the channel capacity is directly proportional to the power of the signal, as SNR =
(Power of signal) / (power of noise).
⮚ The signal-to-noise ratio (S/N) is usually expressed in decibels (dB) given by the formula:
⮚ Decibels (dB) = 10 * log10(S/N)
⮚ So for example a signal-to-noise ratio of 1000 is commonly expressed as:
⮚ 10 * log10(1000) = 30 dB.
⮚ This tells us the best capacities that real channels can have. For example, ADSL (Asymmetric Digital
Subscriber Line), which provides Internet access over normal telephonic lines, uses a bandwidth of
around 1 MHz. the SNR depends strongly on the distance of the home from the telephone exchange,
and an SNR of around 40 dB for short lines of 1 to 2km is very good. with these characteristics, the
channel can never transmit much more than 13Mbps, no matter how many or how few signals level
are used and no matter how often or how infrequently samples are taken.

Examples:
⮚ Input1 : A telephone line normally has a bandwidth of 3000 Hz (300 to 3300 Hz) assigned
for data communication. The SNR is usually 3162. What will be the capacity for this
channel?
Output1 : C = 3000 * log2(1 + SNR) = 3000 * 11.62 = 34860 bps
⮚ Input2 : The SNR is often given in decibels. Assume that SNR(dB) is 36 and the channel
bandwidth is 2 MHz. Calculate the theoretical channel capacity.
Output2 : SNR(dB) = 10 * log10(SNR)
SNR = 10(SNR(dB)/10)
SNR = 103.6
= 3981
⮚ Hence, C = 2 * 106
* log2(3982) = 24 MHz
⮚ The maximum data rate, also known as the channel capacity, is the theoretical limit of the
amount of information that can be transmitted over a communication channel. The
maximum data rate for noiseless and noisy channels can be calculated using Shannon’s
theorem.

Performance
⮚ One important issue in networking is the performance of the network-how good it is?
It can be referred as Quality of Service (QoS) that is an overall measurement of the
network performance.
There are four factors of determining network performance.
▪ Bandwidth
▪ Throughput
▪ Delay
▪ Jitter Bandwidth

Bandwidth
⮚ Bandwidth, or precisely network bandwidth, is the maximum rate at which data transfer occurs
across any particular path of the network.
⮚ Bandwidth is basically a measure of the amount of data that can be sent and received at any
instance of time. That simply means that the higher the bandwidth of a network, the larger the
amount of data the network can be sending to and from across its path. Be careful not to
confuse bandwidth with closely related terms such as the data rate and the throughput.
⮚ Bandwidth is only one component of what an individual sees as the speed of a network. People
frequently mistake bandwidth with internet speed in light of the fact that Internet Service
Providers (ISPs) tend to claim that they have a fast “40Mbps connection” in their advertising
campaigns. True internet speed is actually the amount of data you receive every second and that
has a lot to do with latency too. “Bandwidth” means “Capacity” and “Speed” means “Transfer
rate”.
⮚ Bandwidth is characterized as the measure of data or information that can be transmitted in a
fixed measure of time. The term can be used in two different contexts with two distinctive
estimating values. In the case of digital devices, the bandwidth is measured in bits per
second(bps) or bytes per second. In the case of analog devices, the bandwidth is measured in
cycles per second, or Hertz (Hz).

⮚ More bandwidth does not mean more speed. Let us take a case where we have double the
width of the tap pipe, but the water rate is still the same as it was when the tap pipe was half
the width. Hence, there will be no improvement in speed. When we consider WAN links, we
mostly mean bandwidth but when we consider LAN, we mostly mean speed. This is on the
grounds that we are generally constrained by expensive cable bandwidth over WAN rather
than hardware and interface data transfer rates (or speed) over LAN.
⮚ Bandwidth in Hertz: It is the range of frequencies contained in a composite signal or the
range of frequencies a channel can pass. For example, let us consider the bandwidth of a
subscriber telephone line as 4 kHz.
⮚ Bandwidth in Bits per Seconds: It refers to the number of bits per second that a channel, a
link, or rather a network can transmit. For example, we can say the bandwidth of a Fast
Ethernet network is a maximum of 100 Mbps, which means that the network can send 100
Mbps of data.
[Note: There exists an explicit relationship between the bandwidth in hertz and the bandwidth in
bits per second. An increase in bandwidth in hertz means an increase in bandwidth in bits per
second. The relationship depends upon whether we have baseband transmission or transmission
with modulation. ]

Latency
⮚ In a network, during the process of data communication, latency(also known as delay) is
defined as the total time taken for a complete message to arrive at the destination, starting
with the time when the first bit of the message is sent out from the source and ending with the
time when the last bit of the message is delivered at the destination. The network connections
where small delays occur are called “Low-Latency-Networks” and the network connections
which suffer from long delays are known as “High-Latency-Networks”.
⮚ High latency leads to the creation of bottlenecks in any network communication. It stops the
data from taking full advantage of the network pipe and conclusively decreases the
bandwidth of the communicating network. The effect of the latency on a network’s
bandwidth can be temporary or never-ending depending on the source of the delays.
⮚ Latency is also known as a ping rate and is measured in milliseconds.
⮚ In simpler terms latency may be defined as the time required to successfully send a packet
across a network. It is measured in many ways like a round trip, one-way, etc.
⮚ It might be affected by any component in the chain utilized to vehiculate data, like
workstations, WAN links, routers, LAN, and servers, and eventually may be limited for large
networks, by the speed of light.
⮚ Latency = Propagation Time + Transmission Time + Queuing Time + Processing Delay

Propagation Time
⮚ It is the time required for a bit to travel from the source to the destination. Propagation
time can be calculated as the ratio between the link length (distance) and the propagation
speed over the communicating medium. For example, for an electric signal, propagation
time is the time taken for the signal to travel through a wire.
⮚ Propagation time = Distance (m) / Propagation speed (ms)
Example:
⮚ Input: What will be the propagation time when the distance between two points is 12,
000 km? Assuming the propagation speed to be 2.4 * 10^8 m/s in cable.
⮚ Output: We can calculate the propagation time as-
⮚ Propagation time = (12000 * 10000) / (2.4 * 10^8) = 50 ms

Transmission Time
⮚ Transmission Time is a time based on how long it takes to send the signal down the
transmission line. It consists of time costs for an electromagnetic (EM) signal to propagate
from one side to the other, or costs like the training signals that are usually put on the front of a
packet by the sender, which helps the receiver synchronize clocks. The transmission time of a
message relies upon the size of the message and the bandwidth of the channel.
⮚ Transmission time = Message size / Bandwidth
Example:
⮚ Input: What will be the propagation time and the transmission time for a 2.5-kbyte message
when the bandwidth of the network is 1 Gbps? Assuming the distance between sender and
receiver is 24, 000 km and speed of light is 2.4 * 10^8 m/s.
⮚ Output: We can calculate the propagation and transmission time as:
⮚ Propagation time = (24000 * 10000) / (2.4 * 10^8) = 100 ms
⮚ Transmission time = (2560 * 8) / 10^9 = 0.020 ms
⮚ Note: Since the message is short and the bandwidth is high, the dominant factor is the
propagation time and not the transmission time(which can be ignored).

Queuing Time
⮚ Queuing time is a time based on how long the packet has to sit around in the router. Quite
frequently the wire is busy, so we are not able to transmit a packet immediately. The queuing
time is usually not a fixed factor, hence it changes with the load thrust in the network. In cases
like these, the packet sits waiting, ready to go, in a queue. These delays are predominantly
characterized by the measure of traffic on the system. The more the traffic, the more likely a
packet is stuck in the queue, just sitting in the memory, waiting.
Processing Delay
⮚ Processing delay is the delay based on how long it takes the router to figure out where to send
the packet. As soon as the router finds it out, it will queue the packet for transmission. These
costs are predominantly based on the complexity of the protocol. The router must decipher
enough of the packet to make sense of which queue to put the packet in. Typically the lower-
level layers of the stack have simpler protocols. If a router does not know which physical port
to send the packet to, it will send it to all the ports, queuing the packet in many queues
immediately. Differently, at a higher level, like in IP protocols, the processing may include
making an ARP request to find out the physical address of the destination before queuing the
packet for transmission. This situation may also be considered as a processing delay.

Throughput
⮚ Throughput is the number of messages successfully transmitted per unit time. It is controlled
by available bandwidth, the available signal-to-noise ratio, and hardware limitations.
⮚ The maximum throughput of a network may be consequently higher than the actual
throughput achieved in everyday consumption.
⮚ The terms ‘throughput’ and ‘bandwidth’ are often thought of as the same, yet they are
different. Bandwidth is the potential measurement of a link, whereas throughput is an actual
measurement of how fast we can send data.
⮚ Throughput is measured by tabulating the amount of data transferred between multiple
locations during a specific period of time, usually resulting in the unit of bits per
second(bps), which has evolved to bytes per second(Bps), kilobytes per second(KBps),
megabytes per second(MBps) and gigabytes per second(GBps).
⮚ Throughput may be affected by numerous factors, such as the hindrance of the underlying
analog physical medium, the available processing power of the system components, and end-
user behavior. When numerous protocol expenses are taken into account, the use rate of the
transferred data can be significantly lower than the maximum achievable throughput.

Let us consider: A highway that has a capacity of moving 200 vehicles at a time. But at a
random time, someone notices only 150 vehicles moving through it due to some congestion
on the road. As a result, the capacity is likely to be 200 vehicles per unit time and the
throughput is 150 vehicles at a time.
Example:
⮚ Input: A network with bandwidth of 10 Mbps can pass only an average of 12, 000
frames per minute where each frame carries an average of 10, 000 bits. What will be the
throughput for this network?
⮚ Output: We can calculate the throughput as-
⮚ Throughput = (12, 000 x 10, 000) / 60 = 2 Mbps
⮚ The throughput is nearly equal to one-fifth of the bandwidth in this case.
⮚ [Note: Throughput= Size of transmitted data (in bits) / Time duration (in seconds) ]

Jitter
⮚ Jitter is another performance issue related to the delay. In technical terms, jitter is a “packet
delay variance”.
⮚ It can simply mean that jitter is considered a problem when different packets of data face
different delays in a network and the data at the receiver application is time-sensitive, i.e.
audio or video data. Jitter is measured in milliseconds(ms). It is defined as an interference in
the normal order of sending data packets.
⮚ For example: if the delay for the first packet is 10 ms, for the second is 35 ms, and for the
third is 50 ms, then the real-time destination application that uses the packets experiences
jitter.
⮚ Simply, a jitter is any deviation in or displacement of the signal pulses in a high-frequency
digital signal.
⮚ The deviation can be in connection with the amplitude, the width of the signal pulse, or the
phase timing.
⮚ The major causes of jitter are electromagnetic interference(EMI) and crosstalk between
signals. Jitter can lead to the flickering of a display screen, affects the capability of a processor
in a desktop or server to proceed as expected, introduce clicks or other undesired impacts in
audio signals, and loss of transmitted data between network devices.

Factors Affecting Network Performance
Below mentioned are the factors that affect the network performance.
⮚ Network Infrastructure: Network Infrastructure is one of the factors that affect network
performance. Network Infrastructure consists of routers, switches services of a network like
IP Addressing, wireless protocols, etc., and these factors directly affect the performance of
the network.
⮚ Applications Used in the Network: Applications that are used in the Network can also
have an impact on the performance of the network as some applications that have poor
performance can take large bandwidth, for more complicated applications, its maintenance
is also important and therefore it impacts the performance of the network.
⮚ Network Issues: Network Issue is a factor in Network Performance as the flaws or
loopholes in these issues can lead to many systemic issues. Hardware issues can also impact
the performance of the network.
⮚ Network Security: Network Security provides privacy, data integrity, etc. Performance can
be influenced by taking network bandwidth which has the work of managing the scanning
of devices, encryption of data, etc. But these cases negatively influence the network.

Transmission media
⮚ Transmission media is a communication channel that carries the information from the sender to the
receiver. Data is transmitted through the electromagnetic signals. The main functionality of the
transmission media is to carry the information in the form of bits through LAN(Local Area
Network).
⮚ It is a physical path between transmitter and receiver in data communication. In a copper-based
network, the bits in the form of electrical signals. In a fiber based network, the bits in the form of
light pulses.
⮚ The electrical signals can be sent through the copper wire, fiber optics, atmosphere, water, and
vacuum.
⮚ The characteristics and quality of data transmission are determined by the characteristics of
medium and signal.
⮚ Transmission media comes in two forms: Guided and Unguided media.
⮚ Guided Media: All communication wires/cables are guided media, such as UTP, coaxial cables,
and fiber optics. In this media, the sender and receiver are directly connected and the information is
send (guided) through it.
⮚ Unguided Media: Wireless or open air space is said to be unguided media, because there is no
connectivity between the sender and receiver. Information is spread over the air, and anyone
including the actual recipient may collect the information.

Classification Of Transmission Media:

Guided Media vs Unguided Media
Feature Guided Media Unguided Media
Definition
Signals transmitted along a
physical medium.
Signals transmitted through the air or vacuum.
Examples
Twisted Pair, Coaxial Cable,
Optical Fiber
Radio Waves, Microwaves, Infrared Waves
Installation/
Maintenance
Generally more complex and
expensive.
Often simpler and less expensive.
Susceptibility
to Interference
Less susceptible to interference.
Susceptible to interference from other devices and
environmental factors.
Data Transfer
Rates
Often higher data transfer rates.
Data transfer rates can be affected by distance and
environmental conditions.
Flexibility
Typically less flexible in terms of
mobility.
More flexible, supports mobility and accessibility.
Signal
Degradation
Limited signal degradation over
distance.
Signal strength and quality may degrade over
distance and due to obstacles.
Examples of
Use
Wired networks, LANs, MANs.
Wireless networks, Wi-Fi, Bluetooth, cellular
communication.

Guided Transmission Media: Guided media are also known as wired or bounded media. These
media consist of wires through which the data is transferred. Guided media is a physical link
between transmitter and recipient devices. Signals are directed in a narrow pathway using
physical links. These media types are used for shorter distances since physical limitation limits
the signal that flows through these transmission media.

⮚ Twisted Pair Cable: In this type of transmission media, two insulated conductors of a single
circuit are twisted together to improve electromagnetic compatibility. These are the most widely
used transmission medium cables. These are packed together in protective sheaths. They reduce
electromagnetic radiation from pairs and crosstalk between the neighboring pair. Overall, it
improves the rejection of external electromagnetic interference. These are further subdivided into
unshielded and shielded twisted pair cables.
⮚ Unshielded Twisted Pair Cable(UTP): These consist of two insulated copper wires that are
coiled around one another. These types of transmission media block interference without
depending on any physical shield. The unshielded twisted pair are very affordable and are
simple to set up. These provide a high-speed link.
⮚ Shielded Twisted Pair (STP): This twisted cable consisted of a foil shield to block external
interference. The insulation within these types of the twisted cable allow greater data
transmission rate. These are used in fast-data-rate Ethernet and in data and voice channels of
telephone lines.
⮚ Coaxial cable: These guided transmission media contain an insulation layer that transmits
information in baseband mode and broadband mode. Coaxial cables are made of PVC/Teflon and
two parallel conductors that are separately insulated. Such cables carry high frequency electrical
signals without any big loss. The dimension of cable and connectors are controlled to give them
constant conductor spacing for efficient functioning as a transmission line.

⮚ Optical Fiber Cable: Also known as fiber optic cable, these are thin strands of glass that guide
light along their length. These contain multiple optical fibers and are very often used for long-
distance communications. Compared to other materials, these cables can carry huge amounts of
data and run for miles without using signal repeaters. Due to lesser requirements, they have less
maintenance costs and it improves the reliability of the communication system. These can be
unidirectional as well as bidirectional in nature.
⮚ Stripline: This is a transverse electromagnetic (TEM) transmission media that is built on the
inner layers of multi-layer printed circuit boards. These are used in high or low-level RF signals
that require isolation from surrounding circuitry. It is a type of printed circuit transmission line
in which a signal trace is sandwiched between the upper and lower ground place. Stripline
minimizes emissions electromagnetic radiation is completely enclosed within homogeneous
dielectric. Along with the reduced emissions, it also shields against incoming spurious signals.
⮚ Microstripline: While Microstripline is simiar to stripline, it is not sandwiched and are present
above the ground plane. These can be fabricated with any technology where the conductor is
separated from the ground plane by a dielectric layer called subtrated. These transmission media
convert microwave frequency signals. Microstrip is also used for building microwave
components such as couplers, filters, power dividers, antennas, etc. In comparison with the
traditional waveguide technology, it is less expensive.

Unguided Transmission Media: Also known as unbounded or wireless media, they help in
transmitting electromagnetic signals without using a physical medium. Here, air is the
medium. There is no physical connectivity between transmitter and receiver. These types of
transmission media are used for longer distances however they are less secure than guided
media. There are three main types of wireless transmission media.
⮚ Radio Waves: Radio waves are transmitted in every direction throughout free space.
Since these are omnidirectional, sent waves can be received by any antenna. These waves
are useful when the data is to multicasted from one sender to multiple receivers. Radio
waves can cover large areas and even penetrate obstacles such as buildings and walls. The
frequency of these waves ranges between 3 kHz to 1GHz. Due to its omnidirectional
nature, issues such as interference might arise when another signal with the same
bandwidth or frequency is sent.
⮚ Infrared: These waves are useful for only very short distance communication. Unlike
radio waves, they do not have the ability to penetrate barriers. Their range varies between
300GHz – 400THz. Since they have larger bandwidth, the data rate is very high for
infrared waves. These have less interference and are more secure.

⮚ Microwaves: For these waves, it is important for the transmitter and receiver antenna to
be aligned. This is why it is known as line-of-sight transmission. Due to this, they are
suitable for shorter distances. They comprise of electromagnetic waves with frequencies
ranging between 1-400 GHz. Microwaves provide bandwidth between the range of 1 to 10
Mbps. Distance covered by the signal is proportional to the height of the antenna. For
travelling to longer distances, the height of the tower should be increased. These are
further sub categorized as terrestrial and satellite type microwave transmission.
⮚ Terrestrial type microwave transmission: In this type, high directional antennas are
used for line of sight propagation paths that use frequency between 4-12 GHz. These
are parabolic antennas having diameters that range from 12 inches to feet depending
on their spacing.
⮚ Satellite type microwave transmission: Signals are transmitted to those spaces where
satellites are positioned and they retransmit the signal to appropriate locations. Since
they only receive and retransmit the signal, they act as repeaters. It is a much more
flexible and reliable method of communication in comparison with cables and fiber
systems.

Pros and Cons of Transmission Media in Computer Networks
Type Advantages Disadvantages
Unshielded
Twisted Pair
Less expensive,
Easy to install,
High speed
Attenuation leads to short-distance communication,
Susceptible to external interference
Shielded Twisted
Pair
Reduced crosstalk,
Faster than UTP
Bulky and expensive,
Difficult to install
Optical Fiber
Cable
Increased bandwidth,
Immunity to interference
High-cost, Fragile
Coaxial Cable
High bandwidth,
Noise immunity
Complete disruption due to single cable failure
Stripline
Better isolation,
Less loss of radiation
Complex troubleshooting,
Expensive
Microstripline
Easy interconnection and adjustments,
Major fabrication advantage over
stripline due to its open structure
Only for a short distance
Radio
Easy to generate,
Can penetrate obstacles
More interference
Infrared Less interference Cannot penetrate obstacles

Applications of Transmission Media in Computer Networks
Type Uses
Unshielded Twisted Pair Telephonic applications
Shielded Twisted Pair Fast data rate ethernet
Optical Fibre Cable For transferring large volume of data
Coaxial Cable Cable TVs, Analog TV
Stripline Solid-state microwave systems
Microstripline Solid-state microwave systems
Radio Cordless phones, AM/FM radios
Infrared Wireless mouse, printers, keyboards
Microwave Mobile phones, televisions

Multiplexing and Demultiplexing
⮚ Multiplexing is the process of combining multiple signals into one signal, over a shared
medium. If analog signals are multiplexed, it is Analog Multiplexing and if digital signals are
multiplexed, that process is Digital Multiplexing.
⮚ The process of multiplexing divides a communication channel into several number of logical
channels, allotting each one for a different message signal or a data stream to be transferred.
The device that does multiplexing can be simply called as a MUX while the one that reverses
the process which is demultiplexing, is called as DEMUX.
⮚ In Demultiplexing, at the receiver's side to obtain the data coming from various processes. It
receives the segments of data from the network layer and delivers it to the appropriate
process running on the receiver's machine.

Analog Multiplexing:
⮚ The analog multiplexing techniques involve signals which are analog in nature. The analog
signals are multiplexed according to their frequency (FDM) or wavelength (WDM).
Frequency Division Multiplexing (FDM):
⮚ In analog multiplexing, the most used technique is Frequency Division Multiplexing FDM.
This technique uses various frequencies to combine streams of data, for sending them on a
communication medium, as a single signal.
⮚ Example: A traditional television transmitter, which sends a number of channels through a
single cable, uses FDM.
Wavelength Division Multiplexing (WDM)
⮚ Wavelength Division Multiplexing is an analog technique, in which many data streams of
different wavelengths are transmitted in the light spectrum. If the wavelength increases, the
frequency of the signal decreases.
⮚ Example: Optical fibre Communications use the WDM technique, to merge different
wavelengths into a single light for the communication.

Digital Multiplexing
⮚ The term digital represents the discrete bits of information. Hence the available data is in
the form of frames or packets, which are discrete.
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. Of all the
types of TDM, the main ones are Synchronous and Asynchronous TDM.
⮚ Synchronous TDM : In Synchronous TDM, the input is connected to a frame. If there
are ‘n’ number of connections, then the frame is divided into ‘n’ time slots. One slot is
allocated for each input line. In this technique, the sampling rate is common to all
signals and hence same clock input is given. The mux allocates the same slot to each
device at all times.
⮚ Asynchronous TDM: In Asynchronous TDM, the sampling rate is different for each
of the signals and the clock signal is also not in common. If the allotted device, for a
time-slot, transmits nothing and sits idle, then that slot is allotted to another device,
unlike synchronous.

Switching
Switching is process to forward packets coming in from one port to a port leading towards
the destination. When data comes on a port it is called ingress, and when data leaves a port
or goes out it is called egress. A communication system may include number of switches and
nodes. At broad level, switching can be divided into two major categories:
⮚ Connection Oriented: Before switching data to be forwarded to destination, there is
a need to pre-establish circuit along the path between both endpoints. Data is then
forwarded on that circuit. After the transfer is completed, circuits can be kept for
future use or can be turned down immediately.
⮚ Connectionless: The data is forwarded on behalf of forwarding tables. No previous
handshaking is required and acknowledgements are optional.
⮚ Homework: Describe the procedures for connection-oriented communication.

Circuit Switching
⮚ Circuit switching is a switching technique that establishes a dedicated path between
sender and receiver.
⮚ In the Circuit Switching Technique, once the connection is established then the
dedicated path will remain to exist until the connection is terminated.
⮚ Circuit switching in a network operates in a similar way as the telephone works.
⮚ A complete end-to-end path must exist before the communication takes place.
⮚ In case of circuit switching technique, when any user wants to send the data, voice,
video, a request signal is sent to the receiver then the receiver sends back the
acknowledgment to ensure the availability of the dedicated path. After receiving the
acknowledgment, dedicated path transfers the data.
⮚ Circuit switching is used in public telephone network. It is used for voice
transmission.
Communication through circuit switching has 3 phases:
⮚ Circuit establishment
⮚ Data transfer
⮚ Circuit Disconnect

Chapter-2-Physical-Layer-v1.pptx hai dami

Message Switching
⮚ Message switching was a technique developed as an alternate to circuit switching,
before packet switching was introduced. In message switching, end users communicate
by sending and receiving messages that included the entire data to be shared. Messages
are the smallest individual unit.
⮚ Message Switching is a switching technique in which a message is transferred as a
complete unit and routed through intermediate nodes at which it is stored and
forwarded.
⮚ In Message Switching technique, there is no establishment of a dedicated path between
the sender and receiver.
⮚ The destination address is appended to the message. Message Switching provides a
dynamic routing as the message is routed through the intermediate nodes based on the
information available in the message.
⮚ Message switches are programmed in such a way so that they can provide the most
efficient routes.
⮚ Each and every node stores the entire message and then forward it to the next node. This
type of network is known as store and forward network.
⮚ Message switching treats each message as an independent entity.

Packet Switching
⮚ The packet switching is a switching technique in which the message is sent in
one go, but it is divided into smaller pieces, and they are sent individually.
⮚ The message splits into smaller pieces known as packets and packets are given
a unique number to identify their order at the receiving end.
⮚ Every packet contains some information in its headers such as source address,
destination address and sequence number.
⮚ Packets will travel across the network, taking the shortest path as possible.
⮚ All the packets are reassembled at the receiving end in correct order.
⮚ If any packet is missing or corrupted, then the message will be sent to resend
the message.
⮚ If the correct order of the packets is reached, then the acknowledgment
message will be sent.

Approaches of Packet Switching:
⮚ There are two approaches to Packet Switching:
Datagram Packet switching:
⮚ It is a packet switching technology in which packet is known as a datagram, is considered as an
independent entity. Each packet contains the information about the destination and switch uses
this information to forward the packet to the correct destination.
⮚ The packets are reassembled at the receiving end in correct order.
⮚ In Datagram Packet Switching technique, the path is not fixed.
⮚ Intermediate nodes take the routing decisions to forward the packets.
⮚ Datagram Packet Switching is also known as connectionless switching.
Virtual Circuit Switching
⮚ Virtual Circuit Switching is also known as connection-oriented switching.
⮚ In the case of Virtual circuit switching, a pre-planned route is established before the messages are
sent.
⮚ Call request and call accept packets are used to establish the connection between sender and
receiver.
⮚ In this case, the path is fixed for the duration of a logical connection.

Circuit Switching vs Message Switching
S.N
.
Circuit Switching Message Switching
1
Circuit Switching is done by setting a
physical path between two systems.
In message Switching, data is first stored by one
node then forward to another node to transfer the
data to another system.
2 In circuit switching, data is not stored.
In message Switching, data is first stored, then
forwarded to the next node.
3
Circuit Switching needs dedicated physical
path.
Message switching does not need dedicated
physical path.
4
Circuit Switching is costlier than message
Switching.
The cost of message switching is less than circuit
switching.
5
Circuit switching routing is manual type
routing.
Message Switching routing is not manual type
routing.
6
Circuit switching reserves the full bandwidth
in advance.
Message Switching does not reserve the entire
bandwidth in advance.
7
In circuit switching, charge depend on time
and distance.
In message switching, charge is based on the
number of bytes and distance.
8
Congestion occurs for per minute in circuit
switching.
In message switching, no congestion or very less
congestion occurs.

Circuit Switching vs Packet Switching
S.N. Circuit Switching Packet Switching
1
In circuit switching, each data unit know the
entire path address which is provided by the
source.
In Packet switching, each data unit just know the final
destination address intermediate path is decided by the
routers.
2
In Circuit switching, data is processed at source
system only
In Packet switching, data is processed at all
intermediate node including source system.
3
Resource reservation is the feature of circuit
switching because path is fixed for data
transmission.
There is no resource reservation because bandwidth is
shared among users.
4 Circuit switching is more reliable. Packet switching is less reliable.
5
Wastage of resources are more in Circuit
Switching
Less wastage of resources as compared to Circuit
Switching
6 It is not a store and forward technique. It is a store and forward technique.
7
Congestion can occur during connection
establishment time, there might be a case will
requesting for channel the channel is already
occupied.
Congestion can occur during data transfer phase, large
number of packets comes in no time.
8
In Circuit switching, charge depend on time and
distance, not on traffic in the network.
In Packet switching, charge is based on the number of
bytes and connection time.
9
Recording of packet is never possible in circuit
switching.
While recording of packet is possible in packet
switching.

Message Switching vs Packet Switching
S.N. Message Switching Packet Switching
1 A complete message is passed across a
network.
Message is broken into smaller units known as
Packets.
2 In this, computer language used is
ASCII, baudot, morse.
In packet switching, binary type is used.
3 In message switching there is no limit
on block size.
Packet switching places a tight upper limit on
block size.
4 Message exist only in one location in
the network.
Parts i.e. packets of the message exist in many
places in the network.
5 Example: Hop-by-hop Telex
forwarding and UUCP(UNIX-to-
UNIX Copy Protocol)
Example: Frame Relay, IP, and X.25
6 Physical links are allocated
dynamically.
Virtual links are made simultaneously.
7 Access time is reduced due to increase
in performance as packets are stored in
disk.
Packets are stored in main memory.

Telephone Network
⮚ Telephone Network is used to provide voice communication. Telephone Network uses Circuit
Switching. Originally, the entire network was referred to as a plain old telephone system (POTS)
which uses analog signals. With the advancement of technology, i.e. in the computer era, there
comes a feature to carry data in addition to voice. Today’s network is both analogous and digital.
Major Components of Telephone Network: There are three major components of the telephone
network:
⮚ Local loops
⮚ Trunks
⮚ Switching Offices
⮚ There are various levels of switching offices such as end offices, tandem offices, and regional
offices. The entire telephone network is as shown in the following figure:

⮚ Local Loops: Local Loops are the twisted pair cables that are used to connect a subscriber
telephone to the nearest end office or local central office. For voice purposes, its bandwidth
is 4000 Hz. It is very interesting to examine the telephone number that is associated with
each local loop. The office is defined by the first three digits and the local loop number is
defined by the next four digits defines.
⮚ Trunks: It is a type of transmission medium used to handle the communication between
offices. Through multiplexing, trunks can handle hundreds or thousands of connections.
Mainly transmission is performed through optical fibers or satellite links.
⮚ Switching Offices: As there is a permanent physical link between any two subscribers. To
avoid this, the telephone company uses switches that are located in switching offices. A
switch is able to connect various loops or trunks and allows a connection between different
subscribes.

Mobile Networks
⮚ Mobile Networks or Cellular networks are high-speed, high-capacity voice and data communication
networks with enhanced multimedia and seamless roaming capabilities for supporting cellular devices
(wireless end devices).
⮚ With the increase in popularity of cellular devices, these networks are used for more than just
entertainment and phone calls.
⮚ Cellular telephony is designed to provide communications between two moving units, called mobile
stations (MSs), or between one mobile unit and one stationary unit, often called a land unit.
⮚ A service provider must be able to locate and track a caller, assign a channel to the call, and transfer the
channel from base station to base station as the caller moves out of range.
⮚ To make this tracking possible, each cellular service area is divided into small regions called cells.
⮚ Each cell contains an antenna and is controlled by a solar or AC powered network station, called the base
station (BS).
⮚ Each base station, in turn, is controlled by a switching office, called a mobile switching center (MSC).
⮚ The MSC coordinates communication between all the base stations and the telephone central office.
⮚ It is a computerized center that is responsible for connecting calls, recording call information, and billing.

⮚ Cable Networks: The cable TV network started as a video service provider, but it has
moved to the business of Internet access

Homework
1. Define unguided media and write a difference between radio waves and microwaves.
2. What are the different types of multiplexing? Explain each with its advantages?
3. Explain the concept of transmission media. Illustrate through an example.
4. What is the source of different type of errors that may crop up during data transmission?
5. Briefly explain and distinguish between circuit switching, packet switching and message
switching.
6. Write a short note:
I. Message switching.
II. Shannon capacity
III. Noise and throughput with an example

Differentiate between the following:
⮚ Guided and Unguided Media
⮚ Packet Switching and Message Switching
⮚ Bridge and Gateway
⮚ Propagation Speed and Time
⮚ Distortion and Noise in Signal Transmission
⮚ Circuit Switching and Packet Switching
⮚ Virtual Circuit and Datagram
⮚ WDM and FDM
⮚ Analog and Digital Signal

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