data communications introduction

1. PRINCIPLES OF DATA COMMUNICATIONS
What is data communication?
i.
ii.
iii.
iv.
The transfer of data from one machine to another machine such that the sender
and receiver both interpret the data correctly is called data communication.
The actual generation of the information is not part of data communications. Data
communication is interested in the transfer of data, the method of transfer and the
preservation of the data during the transfer process.
Data communications provides rules and regulations that allow computers with
different operating systems to share resources. The rules and regulations are
called protocols in data communications.
Jitter: Jitter refers to the variation in the packet arrival time. It is an uneven delay
in the delivery of audio or video packets. E.g., suppose video packets are sent
every 30 ms. If some of the packets arrive with 30-ms delay and others with40-ms
delay, it results in an uneven quality in the video.
Effectiveness of data communication:
The effectiveness of a data communication system depends on the following four
characteristics:
1. Delivery: Data must be delivered to the correct destination. Data must be received
only by the intended user.
2. Timeliness: The system must deliver data in a timely manner. Data is useful when it is
delivered at the right time. Real-time delivery of data means that it is delivered as it
is produced, in the same order, and without any significant delay.
3. Accuracy: The system must deliver data accurately. Data must not be altered in the
course of transmission. If any errors take place while data is being transmitted, these
errors must be corrected before data can be used.
Channel
The term channel refers to a communications path between two computers or devices. It
may refer to the physical medium, such as coaxial cable, satellite transmission, or an
ordinary phone line. In computer networks, coaxial cable and twisted–pair cable are used.
Bandwidth:
Bandwidth is defined as the capacity of a transmission medium to transmit data.
Bandwidth is also defined as the frequency range of a channel, measured as the
difference between the highest and lowest frequencies that the channel supports. The
maximum transmission speed depends on the available bandwidth. The larger the
bandwidth, the higher the transmission speed.
Analog Signals:
An analog signal is a continuously varying signal and can take on any value between any
two values. Sine waves are examples of analog signals. Speech is another example of
analog signal. It varies continuously in amplitude (volume) and frequency (pitch).
Analog signal can be propagated over a variety of transmission media.
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2. Prof. Mukesh N. Tekwani [mukeshtekwani@hotmail.com]
The major characteristics of analog signals are:
Frequency (n): It is the rate at which the waveform
changes. It is also the number of waves passing
through a given point in one second. Frequency is
measured in units of Hertz (Hz). E.g., a 50 Hz signal
changes 50 times in one second.
Wavelength (λ): It is the distance between two successive points on a wave which are in
the same phase of motion. It can also be defined as the distance between two successive
crests or two successive troughs. It is measured in units of meters. Frequency and
wavelength are related by the equation v = n λ, where v is the wave velocity.
Amplitude(A): The maximum displacement of a wave from its mean position is its
amplitude. The amplitude of a wave gives an indication of the strength of the wave.
Amplitude is measured in units of meters or decibels (dB)
•
•
•
The intensity (or strength) of a wave is directly proportional to the square of its
amplitude. That is, I α A 2 , where I is the intensity of the wave and A is the
amplitude.
The variations in amplitude and frequency are used to encode data.
Digital Signals:
Digital signals have two discrete states, 0 and 1. 0 corresponds to OFF and 1 corresponds
to ON.
High
Low
0
1
0
0
1
0
1
1
0
Digital Signal
Components of Data Communication System:
A data communication system is made up of five components. These are:
1. Message: The message is the information or data to be communicated. A
message may consist of text, pictures, audio, video, or a combination of these.
2. Sender: The device that sends the message is called the sender. This may be a
computer, a video camera, a Web camera, or a telephone instrument.
3. Medium: The physical path through which a message travels from the sender
to the receiver is called the medium. E.g., coaxial cable wire, twisted pair
wire, fiber-optic cable, radio waves, etc. The word “channel” is also used to
refer to the medium between the sender and the receiver.
4. Receiver: The device that receives the message is called the receiver. This can
be a computer, a television or radio set, or a mobile (telephone) instrument.
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5. Protocol: A protocol is a set of rules that control the process of data
communication. When two or more devices exchange data, there must be
agreements on how this exchange will take place. E.g., at what speed will the
sender transmit data, in which language, what if errors take place during the
process of data transmission, and how these errors will be corrected, how to
know that all the data has been transmitted. All these issues are handled by a
set of rules called the protocol.
Computer Networks:
A computer network is an interconnection of two or more computers such that they can
share resources and information. These computers can be linked together using cables,
telephone lines, or through satellites.
All networks must have the following:
•
•
•
A resource to share such as printer, modem, database, etc. Resource may be
software or hardware.
A medium for transmission of data ( i.e., a communications link),
A set of rules governing how to communicate (protocols).
Distributed Processing
(i)
(ii)
(iii)
(iv)
Distributed processing means that processing will occur on more than one
processor.
These multiple computers communicate with each other through a
computer network.
In distributed processing, a task is divided among many computers.
Thus, a single computer is not responsible for all the processing, but each
separate computer handles a part of the processing job.
Applications of distributed processing:
1. Internet, World Wide Web
2. Scientific computing, including grid computing
3. Real-time process control, e.g., nuclear power plant control system, aircraft
control system
4. Banking system
5. Airline and railway ticket reservation system
6. Telecommunications, cellular networks
Advantages of distributed processing:
1. Distributed databases: One computer system cannot have the capacity to
store all the data of an organization. In such a case, the database is
distributed among multiple computer systems.
2. Security: The system can be designed so that a user can perform a limited set
of operations and on a small set of the database. E.g., a bank may allow
access to its database through an automated teller machine (ATM) but these
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transactions are limited only to the user’s account, and not on the entire
database.
3. Faster problem solving: When many computers are working at the same time
on different parts of a problem, the problem can be solved faster than by a
single machine working alone. Typical applications are breaking encryption
codes, complicated scientific calculations, weather forecasting, etc.
4. Collaborative processing: Multiple computers and multiple users may
interact on a single task.
5. Security through redundancy: Multiple computers running the same
program at the same time provide security through redundancy. If one
computer fails, the others can carry on the job.
TYPES OF COMPUTER NETWORKS
All computer networks can be classified into three categories based on the
following factors:
a) size (number of computers )of the network,
b) geographical range of the network i.e., the distance it covers,
c) ownership i.e., owned by a single organization, a public body like the
municipal corporation, a national organization like the India Railways or
airlines, or a global organization
Local Area Network (LAN):
A local area network is a network of computers in a limited geographical region,
e.g., within a room or a building.
Features of LAN:
The main features of a local area network (LAN) are:
1. A local area network (LAN) is owned by a single organization.
2. A LAN can consist of as few as 2 computers, or as many as 100 computers
and peripherals such as printer, modem, etc.
3. All the computers and peripherals are in a single room or within a building or
a campus.
4. All computers and peripherals are within a range of a few kilometers.
5. One of the computers may be a file server (or server); others are called
workstations.
6. LANs use a shared media such as a cable which is spread throughout the area
to cover the various computers.
7. LANs have a speed of about 100 Mbps.
8. There is very little delay in transmission of data.
9. Very few errors take place in the transmission process.
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10.
The arrangement of computers in a network is called the “network
topology”. A LAN may use any one of the following topologies: the star
topology, ring topology, and bus topology.
Metropolitan Area Network (MAN):
The main features of a metropolitan area network (MAN) are:
1. A metropolitan area network is spread out over a bigger area compared to a
LAN. E.g., it may occupy an entire metropolis area (i.e. a city) or a large
campus.
2. A MAN is owned by a single organization or may be shared by many
organizations within a city.
3. A MAN is used to provide interlinking between various LANs. E.g., a company
can use a MAN to interconnect all its local area networks in various parts of a
city.
4. A metropolitan area network may span a distance of about 50-70 kms.
5. MAN supports real-time transaction backup system.
6. The data transfer speed is very high in a MAN.
7. These are usually based on ring topology, but start topology is not used.
8. The medium used for transmission of data is fibre optic cable.
Wide Area Network (WAN):
A wide area network covers many cities, states, countries or even continents.
Features of WAN:
The main features of a wide area network (WAN) are:
1. A wide area network covers many cities, states, countries or even continents.
2. A WAN interconnects many smaller LANs or MANs together to form a larger
network.
3. WAN will use various communication technologies such as telephone network,
satellite communications, and under–sea cables.
4. Communication channels between computers are provided by a third party
e.g., a telephone company, or a satellite service carrier, or a public data
network.
5. Wide Area Networks operate at lower speed of about 1-10 Mbps.
6. There are more chances of data being lost or corrupted during the course of
transmission.
7. Different types of computers, using different operating systems are connected
together.
8. A WAN owned by a single organization is called an enterprise network.
9. Examples of WAN are: banking networks to support core-banking system,
airline reservation systems, Indian Railways reservation system, credit card
companies, international cargo and courier companies.
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6. Prof. Mukesh N. Tekwani [mukeshtekwani@hotmail.com]
NETWORK TOPOLOGIES
The geometric and physical arrangement of a computer network is called Network
he
topology. A computer network consists of nodes and links. A node may be a
computer, while a link is the communication path between the nodes. The word
“channel” is also used for the link.
There are four main topologies: (a) star, (b) bus, (c) ring, and (d) mesh t
topology.
Star Topology:
This is the most common arrangement of computer
systems and links between them. All devices are
connected to central hub. The function of the hub is
to deliver the data to-and from the computers. The
and
hub may be a server computer.
Features of Star Topology:
1. All nodes (clients) are connected directly to a
central system (the hub).
2. Each node can communicate only with the central
hub and not directly with another computer.
3. If it is desired to send data from one node to anoth er, it can be done only by
another,
sending data to the central hub, which in turn will deliver it to the destination
computer.
4. This topology is useful if it is required to keep a centralized database.
Advantages of Star Topology:
1. It is easy to add and remove node (clients).
nodes
2. If one of the nodes fails, the network continues to work.
3. It is easier to diagnose network problems through the central hub.
Disadvantages of Star Topology:
1. If the central hub fails, the entire network will fail.
2. More cable is required in thi s topology as each computer must be connected
this
to the central hub.
Bus Topology:
Drop
Bus
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7. Prof. Mukesh N. Tekwani [mukeshtekwani@hotmail.com]
This topology is very popular with local area networks. We can imagine a bus
picking up various people from one bus stop and dropping off people as it travels,
and then picking up a few more.
Features of Bus Topology:
1. All nodes (clients) are connected to the cable by drop lines and taps. A drop
line is a connection running between the device and the main cable. A tap is a
connector that punctures the sheathing of the cable to create a contact with
the metal core.
2. There is a limit on the number of devices that can be connected onto the main
cable.
3. Only one device is allowed to transmit at a time.
4. There is a terminator at each end of the cable. The purpose of this terminator
is to absorb the signal that travels to the end. If the terminator does not
absorb the signal, then the same signal is reflected back to the bus confusing
the data flow.
5. The bus is considered to be a passive network because the computers are
dependent on the signal that is being transmitted.
6. There is a collision handling system which ensures that data travels without
errors and is delivered correctly at the destination.
Advantages of Bus Topology:
1. Bus topology is very easy to setup.
2. If one computer fails on the network, then the others are not affected.
3. A repeater can be used to extend the bus configuration.
4. Bus topology uses the least amount of cabling and is therefore very cost
effective.
Disadvantages of Bus Topology:
1. It is difficult to isolate a fault in this topology.
2. If new nodes are added, the data transfer rate on the network may reduce.
3. This topology is not suitable for large networks.
4. Signal reflection at the nodes can cause deterioration in the quality of signal.
5. If there is a fault or a break in the cable, all transmission on the network will
stop.
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8. Prof. Mukesh N. Tekwani [mukeshtekwani@hotmail.com]
Ring Topology:
Ring topology is also used for local area networks. In this arrangement, a
network cable passes from one node to another until all nodes are connected in
the form of a ring or loop. There is a direct point
point-to-point link between
point
neighbouring nodes.
Features of Ring Topology:
s
1. Each device is connected to the two adjacent devices on either side, thus
forming a ring or a loop like structure.
2. A “token” is circulating in the network. Any device that wants to transmit data
must first capture the free token and rep lace the data with its own message.
replace
3. The signal is passed along the ring in one direction, from device to device,
until it reaches its destination.
4. The address of the sending device and the receiving device are sent along with
the message to be transmitted. The message travels along the ring till it
reaches the destination computer.
5. Each device handles every message that flows in the network.
6. The links between computers are unidirectional. Any data transmitted by a
node comes back to the same nod
node.
Advantages of Ring Topology:
1. Ring networks can extend over longer distances than other types of networks.
2. Ring topology offers high performance for a small network.
3. Since each device is connected only to its two neighbours, adding or removing
nodes in the network is easy.
Disadvantages of Ring Topology:
1. If any one computer fails, then the entire network will stop working.
2. It is relatively expensive and difficult to install.
3. Troubleshooting is difficult in this type of network.
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PROTOCOLS
A protocol is a standard set of rules that determines how computers
communicate with each other across networks. When computers communicate
with each other they exchange a series of messages. To understand and act on
these messages, computers must agree on what a message means.
A protocol defines the following: (i) what is communicated (syntax), (ii) how it is
communicated (semantics), and (iii) when it is communicated (timing).
Elements of a protocol:
The main elements of a protocol are:
(i)
Syntax: The term syntax refers to the structure or format of the data. E.g.,
a simple protocol might expect the first 8 bits of data to be the address of
the sender, the second 8 bits to be the address of the receiver, and the rest
of the stream to be the message itself.
(ii)
Semantics. The word semantics refers to the meaning of each section of
bits. How is a particular pattern to be interpreted, and what action is to be
taken based on that interpretation? For example, does an address identify
the route to be taken or the final destination of the message?
(iii)
Timing. The term timing refers to two characteristics: when data should be
sent and how fast they can be sent. For example, if a sender produces data
at 100 Mbps but the receiver can process data at only 1 Mbps, the
transmission will overload the receiver and some data will be lost.
Protocols enable different types of computers (such as Mac, PC, Unix machine,
etc) to communicate inspite of their differences.
STANDARDS
Standards are required for creating and maintaining an open and competitive
market for equipment manufacturers and in guaranteeing national and
international interoperability of data and telecommunications technology and
processes. Standards provide guidelines to manufacturers, vendors, government
agencies, and other service providers to ensure the kind of interconnectivity
necessary in today's marketplace and in international communications.
Data communication standards fall into two categories: de facto (meaning "by
fact" or "by convention") and de jure (meaning "by law" or "by regulation").
De facto. Standards that have not been approved by an organized body but have
been adopted as standards through widespread use are de facto standards. De
facto standards are often established originally by manufacturers who seek to
define the functionality of a new product or technology.
De jure. Those standards that have been legislated by an officially recognized
body are de jure standards.
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