Computer

Networks:
Unit 2
Prof. Swarnalata Bollavarapu
Contact: swarnalata.b@nmims.edu
Contents
Communication Signals
Purpose of the physical layer
Physical layer operation
Physical layer standards
Fundamental principles
Signaling and encoding
Physical media types
Transmission impairment
Performance
Circuit and Packet Switching
Ref Book: Mark Dye et.al, “Network Fundamentals”, CCNA
Exploration Companion Guide, Cisco Press
Purpose
The role of the OSI physical layer is to encode the binary digits that represent
data link layer frames into signals and to transmit and receive these signals
across the physical media—copper wires, optical fiber, and wireless—that
connect network devices

These bits must travel over a physical medium

The physical medium is capable of conducting a signal in the form of

voltage, light, or radio waves from one device to another.

the physical layer encodes the logical frame with patterns of data that will
make it recognizable to the device that will pick it up on the other end of the
medium.
Purpose
The delivery of frames across the local media requires the following physical
layer elements:
■ The physical media and associated connectors
■ A representation of bits on the media
■ Encoding of data and control information
■ Transmitter and receiver circuitry on the network devices
After the signals traverse the medium, they are decoded to their original bit
representations of data and given to the data link layer as a complete frame.
Operation
Each medium has unique signaling used to represent the bits
Copper cable: Patterns of electrical pulses
Fiber-optic cable: Patterns of light pulses
Wireless: Patterns of radio transmissions

To mark the beginning and end of frames, the transmitting device uses a bit
pattern that is unique and is only used to identify the start or end of frames.
Standards
Unlike the other layers, the physical layer, along with some similar
technologies in the data link layer, defines hardware specifications, including
electronic circuitry, media, and connectors.
the physical layer specifications were defined by electrical and
communications engineering organizations like ISO, IEEE, ITU etc
Design considerations:
Physical and electrical properties of the media
■ Mechanical properties (materials, dimensions, and pinouts) of the
connectors
■ Bit representation by the signals (encoding)
■ Definition of control information signals
Fundamental principles
Physical components carry the message in a reliable and consistent manner
so that the receiver gets the message as it was sent.
Encoding : The bits in the encapsulated data link layer frame need to be
grouped, or encoded, into patterns recognized by Layer 1 devices. After
transmission, the receiving Layer 1 device decodes patterns and hands the
frame up to the data link layer. It also does control information to indicate the
start and end of frames
The process of signaling involves determining how to represent the binary
bit on a specific medium. For example, if the medium is copper, the signal
will be in the form of positive and negative patterns of voltage.
Recap
Fundamental layer underlying any network
Consists of hardware
Provides basic communication channel
Main role: transmit bits over a physical link between devices
Main operation: encoding (bits to signal) and decoding (signal to bits)
Standards at physical layer: specifications in terms of type of connections,
cables and connectors
Recap
Analog and Digital Data
Analog and digital correspond to continuous and discrete.
Analog data:- voice and video are continuously
varying patterns of intensity so they can be
said as analog data.
Digital data:- it is a data stored in computer in
form of 0 and 1.
Analog and digital signals
Signals: electric or electromagnetic encoding of data.
• Analog signal – a continuously varying electromagnetic
wave that may be propagated over a variety of medium
depending on the spectrum (e.g., wire, twisted pair, coaxial
cable, fiber optic cable and atmosphere or space
propagation).
• Digital signal – a sequence of voltage pulses that may be
transmitted over a wire medium.
Analog signal characteristics
Peak amplitude: It represents the absolute value of its highest intensity,
proportional to energy it carries.
Period and frequency: period refers to as amount of time, In seconds, a
signal needs to complete one cycle.
frequency refers to no of the periods or cycles in one second.
F=1/T and T=1/f
Phase:
The term phase describes the positions of the waveform relative to time
zero.
Phase is measured in degree or radians.

Bandwidth:-
the range of frequency a medium can pass is said to be as bandwidth.
The bandwidth is property of medium.
Digital Signal
Data can be represented by digital signals. 1 can be represented
by positive voltage and 0 can be represented by 0 voltage.
Two terms bit rate and bit interval are used to describe digital
signals.
Bit rate:- number of bit intervals per second, usually expressed
in bits per second(bps).
Bit interval:- time required to send one single bit
Bit interval=1/bit rate
Transmission Impairment
Impairment Types:
Attenuation
Means loss of energy. When signal travels through medium it loses
some of its energy.
Distortion
Means signal changes its form or shape.
Noise
Unwanted or random signal that mixes with the original signal. Several
type of noise such as thermal noise, induced noise, cross-talk and
impulse noise may corrupt the signal
Signaling and Encoding
Wires and wireless channels carry analog signals such as continuously
varying voltage, light intensity, or sound intensity.
To send digital information, we must devise analog signals to represent bits.
The process of converting between bits and signals that represent them is
called digital modulation/ encoding

there must be synchronization between the sending and the receiving units.
Synchronization means that both sending and receiving units agree on the
timing of the signals.
Synchronization of the signals assures that the bits will be in order and can be
properly interpreted by the receiving unit.
Signaling and Encoding
There are several different methods of representing these binary digits on
physical media as a signal.
Each method finds a way to convert a pulse of energy into a defined amount
of time known as a bit time.
Bit time is the time it takes for a NIC at OSI Layer 2 to generate 1 bit of data
and send it out to the media as a signal.

Different signaling methods vary in the way they represent bits in the bit
time.
Three possible variations of a signal that can represent encoded bits are
Amplitude
Frequency
Phase
Signaling and Encoding
Signal methods:
Signaling and Encoding
Signal methods:
Signaling and Encoding
Different signal methods have different advantages and performance standards, but it
is essential that all devices on the network use the same method so that the messages
from sending devices can be read by the receiving devices.

two methods—nonreturn to zero (NRZ) and Manchester encoding—provides a

fundamental understanding of their function in the physical layer.
Signaling and Encoding
Nonreturn to zero (NRZ):
It samples the voltage level on the medium during a bit time. The method defines
which voltage levels represent 1 and 0, with a low voltage being 0 and a higher
voltage representing a 1
Signaling and Encoding
Manchester Encoding:

It is a signaling method that looks for a change in voltage in the middle of a

bit time.
A voltage change from low to high within the bit time represents a 1.
Conversely, a voltage drop within the bit time from a high to a low voltage
represents a 0.
When there are repeating bit values, meaning consecutive 1s or 0s, a
transition will occur at the edge of the bit time, so a repeated rise or fall will
occur in the middle of the bit time.
Signaling and Encoding
Manchester Encoding:
Signaling and Encoding
Control information is in the form of zeros and ones that indicate where a
frame start and where the frame ends.
Circuit and Packet Switching
Switched communication networks are those in which data transferred from
source to destination is routed between various intermediate nodes.
Switching is the technique by which nodes control or switch data to transmit
it between specific points on a network.
Three types:
Circuit switching
Message switching
Packet switching
Physical media types
Comparison of physical media

Media- physical components- signal

Reflection question 1
What is the purpose of encoding?
A. To identify the start and stop bits in a frame
B. To denote the physical layer’s connectors of computers in
relation to the way they connect to network media
C. To control the way frames are placed on the media at the data
link layer
D. To represent the data bits by using different voltages, light
patterns, or electromagnetic waves as they are placed onto the
physical media
Reflection question 1
What is the purpose of encoding?
A. To identify the start and stop bits in a frame
B. To denote the physical layer’s connectors of computers in
relation to the way they connect to network media
C. To control the way frames are placed on the media at the data
link layer
D. To represent the data bits by using different voltages, light
patterns, or electromagnetic waves as they are placed onto
the physical media
Reflection question 2
What best describes the purpose of the physical layer?
A. Ensures reliable transmission of data across a physical link
B. Determines connectivity and path selection between two end
systems
C. Establishes the physical addressing, networking topology, and
media access
D. Defines the functional specifications for links between end
systems and the electrical, optical, and radio signals
Reflection question 2
What best describes the purpose of the physical layer?
A. Ensures reliable transmission of data across a physical
link
B. Determines connectivity and path selection between two end
systems
C. Establishes the physical addressing, networking topology, and
media access
D. Defines the functional specifications for links between end
systems and the electrical, optical, and radio signals
Reflection question 3
What is synchronization?
A. Keeping the correct time of day on all network machines
B. The timing mechanism devices use when transmitting data
C. Devices processing bits to the data link layer at the same
speed
D. Constant bit times throughout the network
Reflection question 3
What is synchronization?
A. Keeping the correct time of day on all network machines
B. The timing mechanism devices use when transmitting
data
C. Devices processing bits to the data link layer at the same
speed
D. Constant bit times throughout the network
Physical media and its types
The physical layer is concerned with network media and signaling. This layer
produces the representation and groupings of bits as voltages, radio frequencies, or
light pulses.
Various standards organizations have contributed to the definition of the physical,
electrical, and mechanical properties of the media available for different data
communications
Types:
Copper
Fiber
Wireles
Physical media and its types
Copper Media:
Copper is the most common medium for connecting network devices. Copper
connects hosts to devices such as routers, switches, and hubs within a LAN.

Copper media has standards defined for

each of the following
■ Type of copper cabling used
■ Bandwidth of the communication
■ Type of connectors used
■ Pinout and color codes of connections to the media
■ Maximum distance of the media
Physical media and its types
Copper Media:
Copper is an effective medium because it conducts electrical signals very
well, but it has its limitations like attenuation and noise
There are different types of copper cable designed to meet the specific needs
of different networks.
The most common ones are:
unshielded twisted-pair (UTP) cabling (used in Ethernet LANs)
coaxial cable
shielded twisted-pair cables.
Physical media and its types
Unshielded twisted-pair (UTP):
UTP in Ethernet consists of eight wires twisted into four color-coded pairs
and then wound inside a cable jacket.
The colored pairs identify the wires for proper connection at the terminals.
This twisting helps in dealing with magnetic interference from wires within
the cable is called crosstalk.
There are several categories of UTP cable. Each category indicates a level of
bandwidth performance as defined by the IEEE
Types:
Cat 3

Cat 5,Cat 5e

Cat 6
Physical media and its types
Unshielded twisted-pair (UTP):
The most common UTP cable connector in LAN devices is an RJ-45 connector.
Most computers accessing a network through cable use an RJ-45 connector plugged
into the computer network interface card at one end and a hub or switch device at the
other.
Physical media and its types
Coaxial Cable (Coax):
has a single, coated copper wire center and an outer metal mesh that acts as
both a grounding circuit and an electromagnetic shield to reduce interference
carries signal of higher frequency ranges
Usage: Cable TV
Physical media and its types
Fiber :
fiber-optic cable uses light pulses conducted through special glass conductors
to carry data.
The cable is engineered to be as pure as possible and to allow reliable light
signals to traverse the medium.
fiber has greater bandwidth and can run much farther than cable without
needing a signal enhanced, but the higher cost of fiber-optic cable and
connectors, along with special training required for installing fiber, limits its
feasibility to special uses.
Physical media and its types
Fiber :
The light carried on fiber cables is generated by either a laser or a light
emitting diode
(LED) that converts the data to light pulses.
At the receiving end, devices called photodiodes interpret the light signal,
decode the bit pattern, and send it up to the data link layer.
There are two basic types of fiber-optic cable: single-mode and multimode
Physical media and its types
Wireless Media:
Wireless media carry electromagnetic radio signals that represent the binary
data of the data-link frame.
Wireless technologies transmit and receive signals through the medium of the
open atmosphere
Although wireless has advantages, there are some disadvantages to its use

Wireless LAN:
Wireless access point (AP): Concentrates the wireless signals from users
and connects, usually through a copper cable, to the existing copper-based
network infrastructure such as Ethernet
■ Wireless NIC adapter: Provides wireless communication capability to
each network host
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.
•Fixed data can be transferred at a time in circuit switching technology.
Packet Switching
•the message is divided into smaller pieces and 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.
Performance
•Bandwidth: It is characterized as the measure of data or information that can
be transmitted in a fixed measure of time
•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 the hardware limitations
•Latency (Delay): It 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. 
Link Performance:
Data Rate: how many bits per second can be transferred on the link,in Mbps,
Gbps

Loss rate: what is the probability of packet error rate on the link?

Delay: how much time does it take for the bits to reach other end
Reflection questions
1. Copper cable and fiber-optic cable are examples of ____________, which
is used to carry the communication signal.

2. What is the most common UTP connector type?

3. Which of the following measures the actual data transfer rate over a medium?
A. Bandwidth
B. Output
C. Throughput