Need for Communication
A communication service enables the exchange of information between users at different locations.
Communication services & applications are everywhere. Some examples are given below
Need for Computer Communication over Distances
Computer communication has become essential for the following reasons:
(a) Computers can send data at a very fast speed over long distances using satellite and microwave links. Therefore, the cost of transfer of documents using computers is cheaper than
other conventional means like telegrams.
(b) Computers can have fax system integrated with them. This allows them to send pictures along with the text. Thus the newspaper reports can be prepared and sent all over the world at a very high speed by composing and publishing them from different centers.
1. The time taken to transmit the messages is very small. Hence different computers can be
2. +connected together and the users can work together as a group. Software packages have been developed for group working in Data Base Management (DBMS) and graphic works.
(c) Different departments of an organization may be separated physically being at distant places but their data could be stored on a central computer. This data is accessed by computers located in different departments. The data at the central computer may be updated from time to time and accessed by all users. This prevents any bottlenecks in the smooth functioning of the organization. The latest data (say for inventory) will be easily available at all times to all the users.
(d) Fluctuations of prices in foreign exchange and shares/equities can be communicated instantaneously using the medium of computer communications only. The transfer can be accelerated and verified at any instant of time.
Data Communication:
Data Communication is defined as the exchange of data between two devices via some form of transmission medium such as a wire cable. The communicating devices must be a part of a communication system made up of a combination of hardware ( physical equipment) and software (programs).
Characteristics of data Communication :
The effectiveness of a data communication depends on three characteristics
1.Delivery
2.Accuracy
3.Timeliness
Delivery : The system must deliver data to correct destination.
Accuracy: The system must deliver data accurately.
Timeliness: The system must deliver data in a timely manner. Data delivered late are useless. 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.
Components:
The components of a data communication are
Message
Sender
Receiver
Medium
Protocol
Sender: The sender is the device that sends the data message. It can be a computer or workstation telephone handset, video camera and so on..
Receiver: The receiver is the device that receives the message. It can be a computer or workstation telephone handset, video camera and so on..
Medium: The transmission medium is the physical path by which a message travels from sender to receiver. It could be a twisted pair wire , coaxial cable, fiber optic cable, or radio waves.
Protocol: A protocol is a set of rules that governs data communications. It represents an agreement between the communicating devices.
Data representation.
Information comes in different forms such as text, numbers, images, audio and video.
Text.
Text is represented as a bit pattern ,
The number of bits in a pattern depends on the number of symbols in the language.
Different sets of bit patterns have been designed to represent text symbols. Each set is called a code. The process of representing the symbols is called coding.
ASCII
The American National Standards Institute developed a code called the American Standard code for Information Interchange .This code uses 7 bits for each symbol.
Extended ASCII
To make the size of each pattern 1 byte(8 bits),the ASCII bit patterns are augmented with an extra 0 at the left.
Unicode
To represent symbols belonging to languages other than English,a code with much greater capacity is needed. Unicode uses 16 bits and can represent up to 65,536 symbols.
ISO
The international organization for standardization known as ISO has designed a code using a 32 – bit pattern. This code can represent up to 4,294,967,296 symbols.
Numbers
Numbers are also represented by using bit patterns. ASCII is not used to represent numbers.The number is directly converted to a binary number.
Images
Images are also represented by bit patterns.An image is divided into a matrix of pixels,where each pixel is a small dot. Each pixel is assigned a bit pattern.The size and value of the pattern depends on the image.The size of the pixel depends on what is called the resolution.
Audio
Audio is a representation of sound. Audio is by nature different from text, numbers or images. It is continuous not discrete
Video
Video can be produced either a continuous entity or it can be a combination of images.
Direction of data flow
Communication between two devices can be simplex, half-duplex or full-duplex
Simplex
In simplex mode ,the communication is unidirectional. Only one of the devices on a link can transmit; the other can only receive.
computer network:
Definition :
An interconnected collection of autonomous computers” interconnected = able to exchange information
A set of nodes connected by communication links .A node can be any device capable of sending &/or receiving data to &/or from other nodes in the network
A connected collection of hardware and software that permits information exchange and resource sharing.
information = data, text, audio, video, images, …
resources = printers, memory, link bandwidth
Uses of networks
1 •companies & organizations
2 •resource sharing: programs, equipment, data…
3 •high reliability: multiple processors/links/file copies/...
4 •scalability: gradually improve system performance
5 •rapid communications & remote cooperation
6 •saving money
7 •private individuals:
8 •access to remote & diverse information sources
9 •communicating with other people
10 •entertainment
11 •education, healthcare, access to government...
Distributed Processing
Networks use distributed processing which is termed as a task divided among multiple computers. Instead of a single machine responsible for all aspects of a process, separate computers handle a subset.
Performance
Performance can be measured by means of transit time, response time, number of users, type of transmission medium, capabilities of the connected hardware and the efficiency of the software.
Transit time
The amount of time required for a message to travel from one device to another.
Response time
The elapsed time between an inquiry and a response.
Reliability
Reliability is measured by the frequency of failure ,the time it takes a link to recover from a failure.
Security
Network security is protecting data from unauthorized access.
Type of connection
There are two possible type of connections
Point-to-point
Multipoint
Point-to-point:
A point-to-point connection provides a dedicated link between two devices. The entire link is reserved for transmission between those two devices.
Ex. Change of television channel by infrared remote control. A point-to-point connection is established between the remote control and the televisions control system.
Multipoint:
A multipoint (also called multidrop) connection is one in which more than two specific devices share a single link. The capacity of the channel is shared either spatially or temporally.
Physical Topology:
Physical Topology refers to the way in which network is laid out physically. Two or more links form a topology. The topology of a network is the geometric representation of the relationship of all the links and the linking devices tone another.
The basic topologies are
Mesh
Star
Bus and
Ring
Mesh:
In a mesh topology each 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 connects.
A fully connected mash network therefore has n(n-1)/2 physical channels to link n devices. To accommodate that many links every device on the network has (n-1) I/O ports.Merits.
· Dedicated link guarantees that each connection can carry its own data load. This eliminates the traffic problems that occur when links shared by multiple devices.
· If one link becomes unusable ,it does not incapacitate the entire system.
· Privacy or security: When every message travels along a dedicated line only the intended recipient
Demerits
· The amount of cabling and the I/O ports required
· Installation and reconnection are difficult
· The sheer bulk of the wire accommodate more space than available.
The hardware required to connect each link can be prohibitively expensive.
Star topology:
Each device has a dedicated point to point link only to a central controller usually called a hub. If one device has to send data to another it sends the data to the controller, which then relays the data to the other connected device.
Merits
· Less expensive than a mesh topology. Each device needs only one link and I/O port to connect it to any number of others.
· Installation and reconfigure is easy.
· Robustness. If one link fails only that link is affected.
· Requires less cable than a mesh.
Demerits
· Require more cable compared to bus and ring topologies.
One long cable acts as a backbone to link all the devices in a network Nodes are connected to the bus 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 either splices into the main cable or punctures the sheathing of a cable to create a contact with a metallic core. As the signal travels farther and farther ,it becomes weaker .So there is limitation in the number of taps a bus can support and on the distance between those taps.(In this diagram taps and connectors are
Merits
· Ease of installation.
· Bus use less cabling than mesh or star topologies.
Demerits
· Difficult reconnection and isolation.
· Signal reflection at the taps can cause degradation in quality.
· A fault or break in the bus cable stops all transmission. It also reflects signals back in the direction of origin creating noise in both directions.
Ring
Each device has a dedicated point to point connection only with the two devices on either side of it.A signal is passed along the ring in one direction from device to device until it reaches the destination.Each device in the ring incorporates a repeater. It regenerates the bits and passes them along ,when it receives the signal intended for another device.
Merits:
· Easy to install and reconfigure.
· To add or delete a device requires changing only two connections.
· The constraints are maximum ring length and the number of devices.
· If one device does not receive the signal within a specified period, it issue an alarm that alerts the network operator to the problem and its location
Demerits
· A break in the ring disables the entire network. It can be solved by using a dual ring or a switch capable of closing off the break.
Categories of Network
The three primary categories are of network are Local Area Network (LAN), Metropolitan Area Network (MAN), and Wide Area Network(WAN). The category into which a network fall is determined by its size, ownership, the distance it covers and its physical architecture.
LAN
· A LAN is usually privately owned and links the devices in a single office, building or campus.
· A LAN can be as simple as two PCs or it can extend throughout a company. LAN size is limited to a few kilometers. The most widely used LAN system is the Ethernet system developed by the Xerox Corporation.
· It is designed to allow resources (hardware , software or data) to be shared between PC’s or workstations. It may be used to provide a (shared) access to remote organizations through a router connected to a Metropolitan Area Network (MAN) or a Wide Area Network (WAN).
· One of the computers may be given a large capacity disk drive and may become a server to other clients. Software can be stored on this server and used by the whole group.
· The size of the LAN may be determined by the licensing restrictions on the numbers per copy of software. or the number of users licensed to access the operating system.
· Also differentiated from other types of network by transmission media and topology.
· LAN use only one type of transmission medium
· The common LAN topologies are bus, ring and star.
· LANs have data rates in the 4 to 10 megabits per second. Can also reach 100 Mbps with gigabit systems in development.
· Intermediate nodes (i.e. repeaters, bridges and switches) allow LANs to be connected together to form larger LANs. A LAN may also be connected to another LAN or to WANs and MAN’s using a "router
Metropolitan Area Network( MAN)
A MAN is designed to extend over an entire city.
· May be a single network such as cable TV network
· May be a means of connecting a number of LANs into a larger network
· Resources may be shared LAN to LAN as well as device to device
Example A company can use a MAN to connect the LANs in all its offices throughout a city.
· A MAN can be owned by a private company or it may be a service provided by a public company ,such as local telephone company
· Telephone companies provide a popular MAN service called (SMDS) Switched Multi-megabit Data Services.
Wide Area Network (WAN)
· A WAN provides long distance transmission of data .,voice ,image and video information over large geographic areas.
· It may comprise a country ,continent or even the whole world. Transmission rates are typically 2 Mbps, 34 Mbps, 45 Mbps, 155 Mbps, 625 Mbps (or sometimes considerably more).
· WAN utilize public, leased, or private communication equipment usually in combinations and therefore span an unlimited number of miles.
· A WAN that is wholly owned and used by a single company is referred to as an Enterprise Network. The figure represents the comparison of the different types of networks
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