The fundamental purpose of a communications system is the
exchange of data between two parties. This section
introduces a simple model of communication.
• Source - generates data to be transmitted.
• Transmitter - converts data into transmittable signals.
• Transmission System - carries data from source to
• Receiver - converts received signal into data.
• Destination - takes incoming data.
This device generates the data to be transmitted; examples
are telephones and personal computers.
Usually, the data generated by a source system are not
transmitted directly in the form in which they were
generated. Rather, a transmitter transforms and encodes the
information in such a way as to produce electromagnetic
signals that can be transmitted across some sort of
For example, a modem takes a digital bit stream from an
attached device such as a personal computer and transforms
that bit stream into an analog signal that can be handled by
the telephone network.
• Transmission system:
This can be a single transmission line or a complex
network connecting source and destination.
The receiver accepts the signal from the transmission
system and converts it into a form that can be handled by the
Takes the incoming data from the receiver.
Data communications are the exchange of data between two
devices via some form of transmission medium such as a
For data communications to occur, the
communicating devices must be part of a communication
system made up of a combination of hardware (physical
equipment) and software (programs).
The effectiveness of a data communications system depends on
four fundamental characteristics:
The system must deliver data to the correct destination.
The system must deliver the data accurately. Data that have
been altered in transmission and left uncorrected are
The system must deliver data in a timely manner. Data
delivered late are useless.
In the case of video and audio, timely delivery means
delivering data as they are produced, in the same order that
they are produced, and without significant delay. This kind of
delivery is called real-time transmission.
Jitter refers to the variation in the packet arrival time. It is the
uneven delay in the delivery of audio or video packets.
Five components of data communication
The message is the information (data) to be
communicated. Popular forms of information include text,
numbers, pictures, audio, and video.
The sender is the device that sends the data message.
It can be a computer, workstation, telephone handset, video
camera, and so on.
The receiver is the device that receives the message.
It can be a computer, workstation, telephone handset,
television, and so on.
4) Transmission medium:
The transmission medium is the physical path by which a
message travels from sender to receiver. Some examples of
transmission media include twisted-pair wire, coaxial cable,
fiber-optic cable, and radio waves.
A protocol is a set of rules that govern data
communications. It represents an agreement between the
communicating devices. Without a protocol, two devices may
be connected but not communicating, just as a person
speaking French cannot be understood by a person who
speaks only Japanese.
Data Representation Forms
Data representation is defined as the methods used to
represent information in computers. Different types of data
can be stored in the computer system. Information today
comes in different forms such as text, numbers, images,
audio, and video.
1) Text: Text includes combination of alphabets in small case as
well as upper case. It is stored as a pattern of bits. Prevalent
encoding system : ASCII, Unicode(universal character
2) Numbers: Numbers include combination of digits from 0 to 9.
It is stored as a pattern of bits. Prevalent encoding system :
3) Images: Images are also represented by bit patterns. In its
simplest form, an image is composed of a matrix of pixels
(picture elements), where each pixel is a small dot. The size of
the pixel depends on the resolution.
4) Audio: Audio refers to the recording or broadcasting of sound
or music. Audio is by nature different from text, numbers, or
images. It is continuous, not discrete. Even when we use a
microphone to change voice or music to an electric signal, we
create a continuous signal.
5) Video : Video refers to the recording or broadcasting of a
picture or movie. Video can either be produced as a continuous
entity (e.g., by a TV camera), or it can be a combination of
images, each a discrete entity, arranged to convey the idea of
• In simplex mode, the communication is unidirectional, as on a
• Only one of the two devices on a link can transmit; the other
can only receive.
• Keyboards and traditional monitors are examples of simplex
• The keyboard can only introduce input; the monitor can only
• The simplex mode can use the entire capacity of the channel
to send data in one direction.
• In half-duplex mode, each station can both transmit and
receive, but not at the same time.
• When one device is sending, the other can only receive, and
• The half-duplex mode is like a one-lane road with traffic
allowed in both directions.
• When cars are traveling in one direction, cars going the
other way must wait.
• In a half-duplex transmission, the entire capacity of a channel
is taken over by whichever of the two devices is transmitting
at the time.
• In full-duplex mode(also called duplex), both stations can
transmit and receive simultaneously.
• The full-duplex mode is like a two-way street with traffic
flowing in both directions at the same time.
• In full-duplex mode, signals going in one direction share the
capacity of the link with signals going in the other direction.
• This sharing can occur in two ways: Either the link must
contain two physically separate transmission paths, one for
sending and the other for receiving; or the capacity of the
channel is divided between signals traveling in both
• One common example of full-duplex communication is the
telephone network. When two people are communicating by a
telephone line, both can talk and listen at the same time.
• The full-duplex mode is used when communication in both
directions is required all the time.
• The capacity of the channel, however, must be divided between the
A network is a set of devices (often referred to as nodes)
connected by communication links. A node can be a computer,
printer, or any other device capable of sending and/or
receiving data generated by other nodes on the network.
A network must be able to meet a certain number of criteria. The
most important of these are performance, reliability, and security.
• Performance can be measured in many ways, including transit
time and response time.
• Transit time is the amount of time required for a message to
travel from one device to another.
• Response time is the elapsed time between an inquiry and a
• The performance of a network depends on a number of factors,
including the number of users , the type of transmission medium,
the capabilities of the connected hardware, and the efficiency of
In addition to accuracy of delivery, network reliability is
measured by the frequency of failure, the time it takes a link to
recover from a failure, and the network's robustness in a
Network security issues include protecting data from
unauthorized access, protecting data from damage and
development, and implementing policies and procedures for
recovery from breaches and data losses.
Physical Structures- network attributes
Type of Connection:
A network is two or more devices connected through
links. A link is a communications pathway that transfers data
from one device to another. For visualization purposes, it is
simplest to imagine any link as a line drawn between two points.
There are two possible types of connections: point-to-
point and multipoint.
• A point-to-point connection provides a dedicated link
between two devices.
• The entire capacity of the link is reserved for transmission
between those two devices.
• Most point-to-point connections use an actual length of wire
or cable to connect the two ends, but other options, such as
microwave or satellite links, are also possible.
• When you change television channels by infrared remote
control, you are establishing a point-to-point connection
between the remote control and the television's control
• A multipoint (also called multidrop) connection is one in
which more than two specific devices share a single link.
• In a multipoint environment, the capacity of the channel is
shared, either spatially or temporally.
• If several devices can use the link simultaneously, it is a
connection. If users must take turns, it is a timeshared
The physical structure of network is called topology. The
topology of a network is the geometric representation of the
relationship of all links and linking devices to one other. Two
or more devices connect to a link; two or more links form a
There are five basic topologies:
In a mesh topology, every device has a dedicated point-to-
point link to every other device . The term dedicated means
that the link carries traffic only between the two devices it
• The use of a dedicated links guarantees that each connection
can carry its own load, thus eliminating the traffic problems
that can occur when links must be shared by multiple devices.
• A mesh topology is robust. If one link becomes unusable, it
does not incapacitate the entire system.
• Privacy. When every message sent travels along a dedicated
link, only the intended recipient sees it.
• Point-to Point links make fault identification and fault isolation
• A big amount of cabling and a big number of I/O ports
required, i.e, the hardware required can be very expensive.
• Because every device must be connected to every other
device, installation and reconfiguration are difficult.
• For these reasons a mesh topology is usually implemented in
a limited fashion.
• In a star topology, each device has a dedicated point-to-point
link only to a central controller, usually called a hub.
• A star topology does not allow direct traffic between devices.
If one device wants to send data to another, it sends the data
to the controller, which then relays the data to the other
• In a star topology, the central hub acts like a server and the
connecting nodes act like clients.
• It is easy to install and reconfigure
• The star topology is robust. If one link fails, only that link is
affected. This factor also lends itself to easy fault
identification and fault isolation.
• A big amount of cabling and a big number of I/O ports
required, i.e, the hardware required can be very expensive.
• Note: Star topology requires far less cable than Mesh
• A tree topology is a variation of a star. As in the star, nodes in
a tree topology are linked to a central hub that controls the
traffic to the network. The majority of devices connect to a
secondary hub that in turn is connected to a central hub.
• The central hub in a tree is an active hub.
• It incorporates elements of both a bus topology and a star
• This topology is the combination of bus and star topology.
• This topology provides a hierarchical as well as central data
arrangement of the nodes.
• As the leaf nodes can add one or more nodes in the
hierarchical chain, this topology provides high scalability.
• The other nodes in a network are not affected, if one of their
nodes get damaged or not working.
• Tree topology provides easy maintenance and easy fault
identification can be done
• Supported by several hardware and software vendors.
• This network is very difficult to configure as compared to the
other network topologies.
• Due to the presence of large number of nodes, the network
performance of tree topology becomes a bit slowly.
• Requires large number of cables compared to star and ring
• The Backbone appears as the failure point of the entire
segment of the network.
• The establishment cost increases as well.
• If the bulk of nodes are added in this network, then the
maintenance will become complicated.
• In a bus topology one long cable acts as a backbone to link all
the devices in the network.
• In a bus topology all devices in the network are connected by
one central RJ-45 network cable or coaxial cable. .
• The single cable, where all data is transmitted between
devices, is referred to as the bus, backbone, or trunk.
• It is the easiest network topology for connecting peripherals
or computers in a linear fashion.
• It works very efficient well when there is a small network.
• Very cost-effective as compared to other network topology
i.e. mesh and star.
• Easy to install.
• Bus topology uses less cabling than mesh, star, or tree
• Bus topology is not great for large networks.
• Identification of problem becomes difficult if whole network
• If a main cable is damaged, whole network fails or splits into
• This network topology is very slow as compared to other
• In ring network topology ,each node connects to exactly two
other nodes, forming a single continuous pathway for signals
through each node – a ring.
• Data travels from node to node, with each node along the
way handling every packet.
• Easy to install and reconfigure. To add or delete a device
require only two connections.
• Easy fault isolation.
• Good communication over long distance
• Handle high volume traffic
• Fault identification is easy
• No data collision
• A break in the ring (such as a disabled station) can disable the
• This topology is ordered for all computer to communicate
with each other, all computers must be turned on.
• Data packets must pass through every computer between the
sender and also a recipient side, therefore, this makes it
Local Area Networks
• Local area networks, generally called LANs, are privately-owned
networks within a single building or campus of up to a few
kilometers in size.
• They are widely used to connect personal computers and
workstations in company offices and factories to share resources
(e.g., printers) and exchange information.
• LANs are distinguished from other kinds of networks by three
characteristics: (1) their size, (2) their transmission technology, and
(3) their topology.
• LANs may use a transmission technology consisting of a cable to
which all the machines are attached, like the telephone company
party lines once used in rural areas.
• Traditional LANs run at speeds of 10 Mbps to 100 Mbps, have low
delay (microseconds or nanoseconds), and make very few errors.
• There are three topologies of LAN network , they are the bus, ring
Metropolitan Area Networks
• A metropolitan area network (MAN) is a network with a size
between a LAN and a WAN.
• It normally covers the area inside a town or a city.
• It is designed for customers who need a high-speed
connectivity, normally to the Internet, and have endpoints
spread over a city or part of city.
• A good example of a MAN is the part of the telephone
company network that can provide a high-speed DSL line to
• Another example is the cable TV network that originally was
designed for cable TV, but today can also be used for high-
speed data connection to the Internet.
Wide Area Network
• A wide area network (WAN) provides long-distance
transmission of data, image, audio, and video information over
large geographic areas that may comprise a country, a
continent, or even the whole world.
• WAN can be as complex as the backbones that connect the
Internet or as simple as a dial-up line that connects a home
computer to the Internet.
The OSI Reference Model
• The OSI reference model is an abstract description for layered
communications and computer network protocol design . The
model is called the ISO OSI (Open Systems Interconnection)
Reference Model because it deals with connecting open
• The OSI model is conceptual blue print of how communication
should take place between two dissimilar network elements
in a given network.
• All the process required for this effective communication are
logically grouped in to layers. Computer networks are
implemented using the concept of layered protocol.
• It is responsible for transmitting raw bit stream between two
• That is it may convert the sequence of binary digits in to
electrical signals, light signals or electromagnetic signals
depending on whether the two nodes are on a cable circuit,
fiber optic circuit or microwave/ radio circuit respectively.
• This includes the layout of pins, voltages, cable specifications,
hubs, repeaters, network adapters etc.
The physical layer is also concerned with the following:
a) Representation of bits: Data in this layer consists of stream
of bits. The bits must be encoded into signals for
transmission. It defines the type of encoding i.e. how 0’s and
1’s are changed to signal.
b) Data rate: The transmission rate-the number of bits sent
each second-is also defined by the physical layer. In other
words, the physical layer defines the duration of a bit.
c) Line Configuration: This layer connects devices with the
medium: Point to Point configuration and Multipoint
d) Topologies: Devices must be connected using the following
topologies: Mesh, Star, Ring and Bus.
e) Transmission Modes: Physical Layer defines the direction of
transmission between two devices: Simplex, Half Duplex, Full
2.Data Link Layer
• The physical layer simply transmits the data from the sender
node to the receiver node as raw bits.
• The responsibility of the data link layer to detect and correct
any errors in the transmitted data.
• Since the physical layer is concerned with a raw bit stream,
the data link layer partitions it in to frames, so that error
detection and correction can be performed independently for
• Data link layer is most reliable node to node delivery of data.
• It forms frames from the packets that are received from
network layer and gives it to physical layer.
Other responsibilities of the data link layer include the following:
a) Framing: The data link layer divides the stream of bits
received from the network layer into manageable data units
b) Physical addressing: The Data Link layer adds a header to the
frame in order to define physical address of the sender or
receiver of the frame, if the frames are to be distributed to
different systems on the network.
c) Flow control: If the rate at which the data are absorbed by the
receiver is less than the rate at which data are produced in the
sender. This prevents traffic jam at the receiver side.
d) Error Control: Error control is achieved by adding a trailer at
the end of the frame. Duplication of frames are also prevented
by using this mechanism. Data Link Layers adds mechanism to
prevent duplication of frames.
e) Access Control: Protocols of this layer determine which of the
devices has control over the link at any given time, when two or
more devices are connected to the same link.
• The main responsibility of the network layer is determining
how packets are routed from source to destination.
• It encapsulates frames in to packets, which can be transmitted
from one node to another node by using high level addressing
and routing scheme.
• The main aim of this layer is to deliver packets from source to
destination across multiple links (networks).
• If two computers (system) are connected on the same link
then there is no need for a network layer.
• The two popular network layer protocols are X.25 and IP.
Routers operate at this layer.
Other responsibilities of the network layer include
• Logical addressing: The physical addressing implemented by
the data link layer handles the addressing problem locally. If a
packet passes the network boundary, we need another
addressing system to help distinguish the source and
destination systems. The network layer adds a header to the
packet coming from the upper layer that, among other things,
includes the logical addresses of the sender and receiver.
• Routing: When independent networks or links are connected
to create internetworks
(network of networks) or a large network, the connecting
devices (called routers or switches) route or switch the packets
to their final destination. One of the functions of the network
layer is to provide this mechanism
• The main responsibility of this layer is manages end –to-end
message delivery in network. The basic function of the
transport layer is to accept data from the session layer, split
up in to smaller units if need be, passes these to the network
– The most popular transport layer protocols are the TCP(
transmission control protocol) & UDP(user datagram
• TCP is reliable connection oriented protocol. The main
functionality of connection oriented protocol is to provide
reliable end-to end data transport service. connection
oriented service is similar to phone calls.
• UDP is unreliable connectionless protocol. The main
functionality of connectionless protocol is to provide
unreliable end –to-end data transport service. Connectionless
services are similar to a post card.
• Flow control and error control mechanism is also specified in
• The services provided by the first three layers (physical, data
link, and network) are not sufficient for some processes. The
session layer is the network dialog controller. It establishes,
maintains, and synchronizes the interaction among
• Dialog control. The session layer allows two systems to enter
into a dialog. It allows the communication between two
processes to take place in either half duplex (one way at a
time) or full-duplex (two ways at a time) mode.
• The presentation layer is concerned with the syntax and
semantics of the information exchanged between two
• Translation: The processes (running programs) in two systems
are usually exchanging information in the form of character
strings, numbers, and so on. The information must be
changed to bit streams before being transmitted.
• Encryption: To carry sensitive information, a system must be
able to ensure privacy. Encryption means that the sender
transforms the original information to another form and
sends the resulting message out over the network. Decryption
reverses the original process to transform the message back
to its original form
The application layer enables the user, whether human or
software, to access the network. It provides user interfaces and
support for services such as electronic mail, remote file access
and transfer, shared database management, and other types of
distributed information services.