Computer Science

Basic Definations Subnetting concepts Algorithm and Time Set Theory Basics DBMS Basics Data Communications Computer Graphics Basics
History Creations

Shared Ethernet Networks implementing technical standard access to physical media by the IEEE 802.3 working group under the name MAC access (Medium Access Control). MAC uses a general technique called random access.


The physical layer represents the first level of the hierarchy of the reference model. This level is responsible for transporting the bits on physical media varied. To access a support, use an access interface.


The Optical Switches are used to connect optical links between them. At incoming optical fibers correspond outgoing optical fibers. If the switch uses an electrical part, the switch is said optoelectronics and not only optical. These switches are based on the interconnection of switching elements, that is to say, switches which have two input gates and two output ports as shown in Figure. Connected in series, these elementary switches allow making great switches.  The design of these devices, however, poses many problems.


The idea at the basis of the wavelength routing network consists of maximum reuse the same wavelengths. The figure illustrates a node of a routing network wherein wavelengths of the same wavelength are used repeatedly.


VDSL modems (Very high bit rate DSL) achieve speeds much higher than ADSL modems, but a few blocks away. Their capacity is several tens of megabits per second. VDSL modems can make the output of a PON (Passive Optical Network) to extend their connection to the user. PON being in ATM technology, VDSL modems must in this case accept the ATM frame.


As in any other walk of life a protocol is just an agreed set of rules to be followed by all concerned. In this case all concerned means Modems which, by following recognized rules of procedure, can communicate with each other. At least, they can if they all follow the same set of rules. This being the software industry there is no such thing as a single standard. Instead many such protocols have been designed, each one just different enough so that there can be no communication between them.


A null modem cable is a cable that plugs into the communication port (the serial port, the one where you would plug in the cable for a modem) of two computers that are pretty close to each other. Then the two computers can share information back and forth, even faster than if they were connected by a modem and phone lines. If you want to be able to connect your portable computer directly to your bigger computer without modems, you need to hook them together with a null modem cable. Or you can use a standard serial cable and get a null modem adaptor, which is just a plug with a female jack on one side and a male jack on the other.


If you don't quite know what a fax is, first read the definition on the previous page. And if you're shaky on what a modem is, it might be smart to read that definition too. Anyway, a fax modem is simply a modem designed for sending and (usually) receiving faxes via your computer. Using a fax modem, you can send a document to someone else's fax machine without having to print out the document and then stand there and make sure the pages don't jam in the fax machine. In fact, if the other person has a fax modem attached to their computer, they can receive the document directly into their computer and read it on their screen. Of course, they can print it to their own printer and then have a copy on real paper instead of that disgusting fax paper.


Distributed computing is a quick but kind of stuffy way to describe the situation when a business relies on lots of small computers located throughout the organization, rather than a few big machines at some central location. They may still have a few big machines, but many important duties are assigned to the personal computers and workstations, too. All the computers are tied together in a network, communicating with each other so that different portions of an application run on different computers.


Broadcast: Any form of communication in which a single sender transmits messages to many receivers at once, the most familiar examples being the television and public radio systems. The opposite of broadcast is POINT-TO-POINT or narrowcast communication, between just a single transmitter and a single receiver - a telephone conversation for example. When such a multiple connection is made via a network cable as opposed to wireless, such communication is often called MULTIPOINT, as opposed to a point-to-point or UNICAST.


The acronym bps stands for bits per second, which is a measurement of how fast information (data) travels between two devices. When two modems converse via the telephone line, the speed of modem communications is measured in bps. (When people talk about modems, they often use the word "baud" to mean the same thing as bps, though technically the two terms are not synonymous.


baud, baud rate A baud is the number of signaling elements per second sent by a communications device such as a modem, In theory, a modem with a high baud rate means fast transmission. The baud rate is therefore equal to the bit rate only if each signal element represents one bit of information.



Application Server: A network SERVER that enables users to run application programs such as word processors or spreadsheets on the server itself, rather than downloading the application's code and running it on their local workstation. Application servers are often used in conjunction with diskless workstations or so called THIN CLIENTS.


Availability of different operating systems, hardware platforms and the geographical dispersion of the computing resources necessitated the need of networking in such a manner that computers of all sizes can communicate with each other, regardless of the vendor, the operating system, the hardware platform, or geographical proximity. Therefore, we may say that internetworking is a scheme for interconnecting multiple networks of dissimilar technologies. To interconnect multiple networks of dissimilar technologies use both additional hardware and software. This additional hardware is positioned between networks and software on each attached computer. Thus, system of interconnected networks is called an inter network or an Internet.


TCP/IP encapsulate upper layers using headers for the purpose of exchanging control and status information about the progress of the communication because its protocols also engage in peer talk by encapsulating data with protocol headers before submitting it to the underlying layer for subsequent delivery to the network. The data communication using TCP/IP and Ethernet as it is passed down the layers by an application on node X to node Y across the network.


A LAN Operating System, or Network Operating System (NOS), is software that provides the network with multi-user, multitasking capabilities. The operating system facilitates communications and resource sharing, thereby providing the basic framework for the operation of the LAN. The operating system consists of modules that are distributed throughout the LAN environment. Some NOS modules reside in servers, while other modules reside in the clients.


In addition to the attached devices also referred to as nodes or station, LANs may make use of other devices to control physical access to the shared medium to extend the maximum reach of the LAN, and to switch traffic. Such hardware is in the form of NIC/NIU, transceiver, MAU, hubs, bridges, routers, and gateway.


A protocol is a set of mutually accepted and implemented rules at both ends of the communications channel for the proper exchange of information. Protocols comprise standards which, at a basic level, include the dimensions of line setup, transmission mode, code set, and non-data exchanges of information such as error control (detection and correction).


The distinguishing feature of star topology is that all nodes are joined at a single point, as shown in Figure. This single point is called as a central node, hub, or switch, to which all other devices are attached directly, generally via UTP or STP. This topology is frequently used for networks in which control of the network is located in the central node. This method is optimal when the bulk of communication is between the central and outlying nodes. If traffic is high between outlying nodes, an undue switching burden is placed on the central node.


The ring architecture is a distributed architecture, with minimal connectivity and a topology of two links connected to every node as shown in Figure and forms unbroken circular configuration. Figure shows a network laid out in a physical ring, or closed loop, configuration. Transmitted messages travel from node to node around the ring. Each node must be able to recognize its own address in order to accept messages.


                                        Bus Topology

Bus topologies are multipoint electrical circuits that can be implemented using coaxial cable, UTP, or STP. Data transmission is bidirectional, with the attached devices transmitting in both directions. While generally operating at a raw data rate of 10 Mbps, actual throughput is much less.


                   Point-to-point Connections

The point-to-point scheme provides separate communication channels for each pair of computers. When more than two computers need to communicate with one another, the number of connections grows very quickly as number of computer increases. Above figure illustrates that two computers need only one connection, three computers need three connections and four computers need six connections.


                         Metropolitan Area Network

A Metropolitan Area Network (MAN) covers larger geographic area such as cities or districts. By interconnecting smaller networks within a large geographic area, information is easily disseminated throughout the network. Local libraries and government agencies often use a MAN to connect to citizens and private industries. It may also connect MANs together within a larger area than LAN. The geographical limit of a MAN may span a city.


                        Computer System Model

The term computer networks resulted from the 'combination of two major areas, namely computers and communications.


                                            Need For Computer Networks

The term network is defined as a set of computers of different types, terminals, telephones, and other communication equipments, connected by data communication links, which allow the network components to work together. The network components may be located within a small area or spread over many remote locations. In any case, data communications hold the network together.


The modems can be classified according to their characteristics as


Range - Short Haul, Voice Grade (VG), Wide band

Line Type - Dial-up, Leased, Private

Operation Mode - Half Duplex, Full Duplex, Simplex

Synchronization - Asynchronous, Synchronous

Modulation - AM, FM/FSK,PM

Transmission Media - Radio, Optical and Dial up.


In "frequency shift keying (FSK)", the frequency of a sinusoidal carrier is shifted between two discrete values. One of these frequencies (f1) represents a binary "1" and the other value (f0) represents a binary "0". The representation of digital data using FSK is as shown in Fig. Note that there is no change in the amplitude of the carrier.


Amplitude Shift Keying (ASK) is the simplest type of digital CW modulation. Here the carrier is a sinewave of frequency fc. We can represent the carrier signal mathematically as follows:


• The SMDS is designed to connect the multiple LANs together. This is the first high speed broadband service offered to the public.

• Fig.(a) shows the interconnection of four LANs using six high speed leased lines. But this interconnection is expensive.


                          Wide Area Network

 WAN is the acronym for, Wide Area Network and refers to a network used to connect different equipments from remote areas. This technology connects sites that are in diverse locations. Wide Area Networks (WANs) connect larger geographic area, such as New York, Canada, or the world. The geographical limit of WAN is unlimited. Dedicated transoceanic cabling or satellite uplinks may be used to connect this type of network. Hence, a WAN may be defined as a data communications network that covers a relatively broad geographic area to connect LANs together between different cities with the help of transmission facilities provided by common carriers, such as telephone companies. WAN technologies function at the lower three layers.


LAN looks like an acronym that a board of directors spent a lot of money and time trying to create, but it actually stands for any generic local area network. A network is a group of computers and other devices connected together so they can pass information back and forth.


The transmission technology can be categorized broadly into two types:

1. Broadcast networks and

2. Point-to-point networks.


Connectionless service is a self-contained action and does not include establishment, maintenance and releasing a connection.

• Each message carries the full destination address and is treated and routed independently of all other messages.

  1. In connection oriented service a connection is established between the communicating n+1entity using the service provided by the n-entity.
  2. The service users of connection oriented service undergo three different phases:
  1. Hybrid networks are the networks that are based on both peer-to-peer & client-server relationship.
  2. Hybrid networks incorporate the best features of workgroups in peer-to-peer networks with the performance, security and reliability of server-based networks.
  3. Hybrid networks still provide all of the centralized services of servers, but they also allow users to share and manage their own resources within the workgroup.

Router: Routers are devices (computers) containing software that help in determining the best path out of the available paths, for a particular transmission. They consist of a combination of hardware and software. The hardware includes the physical interfaces to the various networks in the internet work. The two main pieces of software in a router are the operating system and the routing protocol.


Switching: Every time in computer network you access the internet or another  computer network outside your immediate location, your messages are sent through a maze of transmission media and connection devices. The mechanism for moving information between different computer network and network segment is called switching in computer network.


In computer network Switches and Bridges are the layer 2 computer network devices. Once the number of users began to push the limits of a single computer network segment, there was a need to create a new segment to link two computer networks together a device called bridges accomplished this. basically the bridges have 2 ports, one for each computer network, bridges actually inspect the data that passes through them and make decisions about whether to send it to the other computer network or not.


To access Internet Through a Cable TV. Computer Network requires a cable Modem. It has two interfaces on it one for computer  and other for Cable Network The Computer side interface is usually is a USB Interface or 10Mbps Ethernet And The Cable Network is common cable wire interface.


Modem is abbreviation for Modulator – Demodulator. Modems are used for data transfer from one computer network to another computer network through telephone lines. The computer network works in digital mode, while analog technology is used for carrying massages across phone lines.


A packet is the unit of data that is routed between an origin of  computer  network and a destination of computer network on the Internet or any other packet-switched computer network. When any file (e-mail message, HTML file, Graphics Interchange Format file, Uniform Resource Locator request, and so forth) is sent from one place of computer network to another place of computer network on the Internet, the Transmission Control Protocol (TCP) layer of TCP/IP of computer network.


A computer in computer network that connects to the server and uses the network services to perform user's tasks is a client computer.


The IEEE 802.3 standard is popularly called as Ethernet. It is a bus based broadcast network with decentralized control. It can operate at 10 Mbps or 100 Mbps or above. Computers on an Ethernet can transmit whenever they want to do so. If two or more machines transmit simultaneously, then their packets collide. Then the transmitting computers just wait for an arbitrary time and retransmit their signal. There are various technologies available in the LAN market but the most popular one of them is Ethernet.


IEEE 802.4 Token Bus : In token bus Computer network station must have possession of a token before it can transmit on the computer network. The IEEE 802.4 Committee has defined token bus standards as broadband computer networks, as opposed to Ethernet's baseband transmission technique. Physically, the token bus is a linear or tree-shape cable to which the stations are attached


Ethernet : IEEE 802.3 Local Area Network (LAN) Protocols : Ethernet protocols refer to the family of local-area network (LAN) technology covered by the IEEE 802.3. It is working examplc of the more general carrier sense multiple access with collision detect (CSMA/CD). In the Ethernet Computer Network standard, there are two modes of operation: half-duplex and full-duplex modes. In the half duplex mode, data are transmitted using the popular Carrier-SenseMultiple Access/Collision Detection (CSMA/CD) protocol on as hared medium.


The term Network Topology defines the geographic Physical or logical arrangement of computer networking devices. The term Topology refers to the way in which the various nodes or computers of a network are linked together. It describes the actual layout of the computer network hardware. Two or more devices connect to a link; two or more links form a topology. Topology determines the data paths that may be used between any pair of devices of the network.

  1. In the peer to peer computer network model we simply use the same Workgroup for all the computers and a unique name for each computer in a computer network.
  2. There is no master or controller or central server in this computer network and computers join hands to share files, printers and Internet access.
  3. It is practical for workgroups of a dozen or less computers making it common environments, where each PC acts as an independent workstation and maintaining its own security that stores data on its own disk but which can share it with all other PCs on the network.

  4. Software for peer-to-peer network is included with most modern desktop operating systems such as Windows and Mac OS.


The two different computer network connection types are

(A) Point-to-Point Connection
(B) Multipoint Connection.

A point-to-point connection is a direct link between two devices such as a computer and a printer. It uses dedicated link between the devices. The entire capacity of the link is used for the transmission between those two devices. Most of today's point-to-point connections are associated with modems and PSTN (Public Switched Telephone Network) communications. In point to point networks, there exist many connections between individual pairs of machines.


A computer network consists of two or more computers that are linked in order to share resources such as printers and CD-ROMs, exchange files, or allow electronic communications. The computers on a computer network may be linked through cables, telephone lines, radio waves, satellites, or infrared light beams.


Computer Networks fall into three classes regarding the size, distance and the structure namely: LAN (Local Area Network), MAN (Metropolitan Area Network), WAN (Wide Area Network).


Internetworking started as a way to connect disparate types of computer networking technology. Computer network term is used to describe two or more computers that are linked to each other. When two or more computer networks or computer network segments are connected using devices such as a router then it is called as computer internetworking.

  1. A Computer networking model where one or more powerful computers (servers) provide the different computer network services and all other user'of computer  network (clients) access those services to perform user's tasks is known as client/server computer networking model.


File servers. -With a file server, the Computer Network client passes requests for computer network files or file records over a computer network to the file server. This form of computer network data service requires large bandwidth and can slow a computer network with many users down considerably. Traditional LAN (Local area Network) computing allows users to share resources, such as data files and peripheral devices, by moving them from standalone PCUs onto a Networked File Server (NFS).


Types of Computer Networks can be classified on various properties. The Computer networks can also be classified on the basis of Computer network technology used by them. There are two types of Computer networks in this category.


During 20th century the most important technology has been the information gathering, its processing and distribution. The computers and communications have been merged together and their merger has had a profound effect on the manner in which computer systems are organized.