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· Introduction
· Communication media
· Modems
· Data flow
· Communication Process
· Baseband versus Broadband
· Processors
· Components of network
· Network architectures
· Media access control
· Error control in network
· Network errors
· Cause of errors
· Error presentation
· Error detection
· Retransmission Error Correction
· Forward error correction
· Data link protocols
· Network protocols
· Addressing
· Routing
· Networking
· LAN commitments
· Ethernet
· Token Ring
· High speed LAN and backbone Network
· Components of Backbone Network
· Network Design

Communication means to an exchange of information between two or more parties and it can be exchange in a varity of way, that might be in the form of word, letters, messages drawing, body movement or any other symbols that represents the idea which make to other understandable. Data communication entails electronically exchanging data or information. It is the movement of computer information from one point to another by means of electrical or optical transmission system. This system often is called data communication networks.  In today's computing world, data refers to facts, statistics, pictures , voice and other information that is digitally coded and intelligible to variety of electronic machines.
The key technology of the information age is computer communications.  The value of high-speed data communication network is that it brings the message sender and receiver closer together in time.  Data communication and networking is a truly global area of study.  It facilitates more efficient use of computers and improves the day to day control of business by providing faster information flow. The information society where information and intelligence are the key drivers of personal, business, and national success has arrived. Data communication is the principle enabler of the rapid information exchange and will become more important than the use of computer themselves in the future.
To be more familiar that what is data communication and its capabilities along with its limitation, we have to be familiar with the data communication media, components of data communication

Communication media
It is necessary to have a pathway or the media to be transmitted data from one point to another. The term media means the device that transmit the voice or the data form one point to another.  Many different types of transmission media are in use today for example: copper wire, glass or plastic cables, which is called fiber optic cables, or air transmission (radio, infrared, microwave, or satellite). There are two types of media one is guided media and another is radiated. In Guided media the messages flow through the physical media like twisted pair-wire, coaxial cable fiber optic cables, the media guided is the signal. In Radiated media messages is broadcast through the air such as infrared, microwave, or satellite.

Guided media
Twisted-pair cable: Twisted- pair wire is the most commonly used wire.  Twisted pair wires usually are twisted and used to minimize the electromagnetic interference (EMI) between one or any other pair in the bundle. There are two types of two types of twisted pair cable, shielded and unshielded twisted pair. Shielded twisted pair (STP) cable consists of pairs of insulated solid wire surrounded by a braided or corrugated shielding.  Shielded twisted pair reduces the interruption of the data communication signals caused by the Radio frequency interference (RFI) or Electromagnetic interference (EMI).  Another type of twisted pair is unshielded twisted pair which is most frequently used network cabling because of It is relatively inexpensive to purchase, widely available and easy to work with; and it can be made relatively unobtrusive by running it inside walls, floors, and ceilings. Unshielded twisted pair is telephone wire consisting of  wire pairs within an insulated outside covering. Unshielded twisted pair cable is known as 10BASET cable. I had include the most frequently used twisted pair cable types and its data transmission rate as specified by the IEEE.

Twisted pair Type Shielding Transmission Rate
Type 1 Shielded 4 Mbps
Type 2 Combination 4 Mbps
Ttpe 3 Unshielded 10 Mbps
Type 4 Unshielded 16 Mbps
Type 5 Unshielded 100 Mbps


Coaxial cable: Coaxial cable is another type of guided media.  Coaxial cable is similar to television. Network coaxial cable must meet the specification established by Institute of Electrical and Electronic Engineers (IEEE). These specifications require 50 ohms of resistance. Network coaxial cable is also known 10BASE2 cable.   Coaxial cable has a copper core (inner conductor) with an outer cylindrical shell for insulation. Coaxial cable is much less susceptible to electromagnetic interference and can carry much more data than the twisted pair wire.  Because of this it is used to carry high speed data traffic as well television signals but it is more expensive and difficult to work with and relatively inflexible and also it cost 10-20 times more then twisted pair wire.

Fiber-optic cables: Fiber optics consist of thousands of very thin filaments of glass fibers which can conduct lights pulse generated by laser at transmission frequencies that approach the speed of lights. Fiber optics has greater data transmission rate as well as the great security interference tapping.  A single size of hair fiber optic cable can carry up 30000simulaneous telephone calls, compared to about 5500 calls on a standard metallic coaxial cable. The earlier fiber optic system were multimode, which means the light could reflect inside the cable at many different angles where as single-mode fiber optic cables transmit a single direct beam of light through a cable that ensures the light only reflects in one pattern, in part because the core diameter has been reduced from 50 microns to about 8 to 10 microns.   Fiber optic cables can carry huge amounts of information at extremely fast data rates.

Radiated media
Radio: one of the most commonly used radiated media is wireless or radio media. Radio transmission uses the same basic principles as standard radio transmission.  On the network, every device has a radio receiver/ transmitter on a specific frequency range that does not interfere with commercial radio stations. Radio data communication does not have depend on microwave or satellite links, especially for short ranges such as within an office setting.  Radio is being used increasingly to connect computers and peripheral equipment or computers and local area networks. No needed metallic wire for radio data transmission; equipment connected by radio can be highly mobile without being leashed to wall.  Radio communication device are fairly inexpensive and easy to install (especially with out cabling).  Radio allows high data rates, typically in the lower megabit per second range.

Infrared: Another increasingly popular data communications media is infrared radiation. Infrared transmission uses low frequency light waves to carry the data through the air in a direct line-of-sight path between two points. Infrared lights is red light below what is commonly visible to human eyes, but light nonetheless that can be modulated or pulsed for conveying information. As radio data transmission, infrared doesn't need metallic wire, equipment can be highly mobile without being tethered to a wall and it doesn't have electronic interference problems. A TV remote control can be a example of infrared.

Microwave: Microwave system were first used extensively to transmit extremely high frequency radio signals in a line-of-sight path between relay stations spaced approximately 30 mile apart. Microwave radio transmission performs the same function as cables do. Microwaves signals approach the frequency of visible light waves, they exhibit the same characteristics as light waves, such as reflection, focusing, or refraction. To minimize the line-of-sight problems microwave antennas were usually placed on the top of the building, tall towers, or in the mountain peaks.  Microwave system were adopted by the long distance telephone carrier because it does not requires the laying of any cables and microwave communication can be set up quickly with the less cost. Microwave transmission are susceptible environmental interference during sever weather such as heavy rain or snowstorm.   Although microwave data communication is fairly widely used but in a recent year it is replaced by satellite communication system.

Satellites:  A satellite is a specialized wireless receiver/transmitter that is lunched by a rocket and placed in orbit around the earth.  Modern satellite can receive and re-transmit thousands of signals simultaneously.  Transmission through the satellite is much more like transmission through microwave, in microwave information are transmit through the microwave dish antenna where as in satellites transmission, information are transmit through the satellite 500 to 22300 miles in space.

Data transmission and communication device
Data is the specific individual facts or the facts, from which decision can be made, it is the raw facts that can be processed in to accurate and relevant information. Digital and analog is the two types of fundamental data, computers produce the digital data where as telephone produces analog data. Digital data that are binary either on or off and the analog data is the sound wave. Analog signals are continuous waves that carry information by the characteristics of the waves. Analog transmission is the transmission of a continuously variable signal as opposed to a direct on/off signal. The traditional way of transmits telephone signal is an example of analog transmission. Digital signal is a discontinuous signal whose various states are discrete intervals apart, such as +15 volts and -15 volts. Most computers transmit the data in a digital form over the digital circuit to the attached device for example to the  "printer" same as analog voice data also can transmit to the analog form through the telephone networks. Data can be transmitted from one specific form to another. For example; as we know most of computer produce the digital data; we can convert digital data to analog voice data by using the special device called modem through the analog telephone circuit.
Data transmission may be either asynchronous or synchronous. Asynchronous transmit only one character at a time and during transmission the character is preceded by a start bit and followed stop bit that lets the receiving device known where a character begins ends. Asynchronous transmission is inherently inefficient due to the additional overhead required for start and stop bits, and the idle time between transmission.  It is therefore generally used for only low speed data transmission.  Synchronous transmission is a form of transmission in which data is sent as a block using frames or packets. In Synchronous transmission a group of character is sent to communication link in a continuos bit stream while data transfer is controlled by a timing signal initialed by the sending device.

A modem is a device that modulates and demodulates the signals. Modulation means the conversions of data from digital signal to analog signal and demodulates means the conversion of data signal from analog to digital signal. Modems are always use in pairs. The unit at the sending end converts the digital signal to analog signals for transmission through the analog circuit; at the receiving end, another modem converts analog to digital for the receiving computer. Modem transmission rate is measured in bits-per-second and it usually expresses bps. The amount of data actually transferred for one system to another is depends upon the speed of the modem. We can find many different types of modem in market today. To transmit the information or data between two computers using modem, both computer must need to use same standard and type of modem. Usually most of the modem support the various standards so that they can be able to communicate with several different type of modem. When these modems connect to each other, they first try to connect in a high-speed standard and if its unsuccessful than keep try to connect in lower speed until it connects.  Some modems can changes the data rate during the transmission, so if a circuit is noisy, this type of modem can slow down to reduce the effects of errors and this change of data rare during transmission is called fast retrain. High speed modem takes less time to transmit the data than the lower speed modem and it also the save the on-line charge.  You might want to calculate the transmission time when you are downloading the files on the Internet. File transfer time can be estimate easily by using the following formula:
File transfer time = Numbers of time *number of bits per byte
                                  Bits per second transmission speed

Data flows
There is three way to transmiyt ths data: simplex, half-duplex, and full-duples. Simplex is one way transmission, it uses one circuit in one direction only such as in radio or TV transmission. It is simple and relatively inexpensive.
Half-duplex (HDX) is two-way transmission and it also use only one circuit as simplex does but it is used in both directions, walkie-talkie is the good example of the half-duplex processor. Half duplex is a circuit, which can transmit the signal in two directions but only once at a time.  In Half-duplex users can transmit and receive signals or data but can not do both simultaneously.
Full duplex (FDX) Full duplex transmission is also the two-way transmission but it uses two circuits for communication. Full duplex allows users to communicate in both way simultaneously (i.e. a common telephone) with no turnaround time.  This mode is clearly easier to use then the half duplex but the cost may be significant, especially over long distance.

Communication Process
Different media and hardware are used in data communication
Interfaces: an interface is a physical connection between two communication device. There are two type of interface on is parallel mode and the next is serial mode. Parallel mode is the way the internal transfer of binary data takes place inside a computer. In parallel mode, bits are transmitted simultaneously on 8/32 separate connection.  A typical parallel communication cable will have control and data wires equal to the number of bits that can be transmitted at one time, in Parallel.
A serial data transfer, most often used for long distance communication, it transfer one bit at a time. Serial mode is the predominant method of transferring message in data communication. It means that a streams of data is sent via communication circuit one bit than second bit, and so on until all the bits are transmitted. Serial transmission is slower than the parallel transmission mode.
Bandwidth: Bandwidth refers to the range of frequencies available in any communication channels. Bandwidth is a very important concept in communications because the transmission capacity is largely depend on its bandwidth. In general, the greater the bandwidth, the greater communication capacity. The sounds refer to the three characteristic amplitude, frequency, and phase. The height of the wave is called amplitude, the length of the wave is frequency and phase refers to the direction in which the wave begins. Bandwidth refers to a range of frequencies. The bandwidth of the human voice is from 20 Hz to 14000 Hz or 13880 Hz. For many communication applications, a small bandwidth is usually adequate. Linking a student PC to university main frame is typically done at 2400 to 140400 bps. Graphical displayed on a screen requires the greater bandwidth then the text data does.

Baseband Versus Broadband
Baseband communication uses one signal at a time on a single communication channel. Digital transmission is the transmission of electrical pulse.  Digital information is sent over a Baseband circuit in serial fashion one bit at a time. Since it has a single channel, it is impossible to integrate with parallel signals across the Baseband cable. However it is easy to tape in to this cable in order to connect or disconnect workstation. In contrast broadband channel can carry more then one signal at a time, and also carry the different frequency in the same time. Signals on broadband network are always analog, represented by various in the strength or frequency of a carrier signals.  Using broadband cabling LAN can share a cable with other analog signals like cable television channel. It is an old, well established and mature technology and the total capacity of the broadband is greater although a single broadband channels transmission rate is lower than Baseband channel.

There are many types of communication hardware device specially designed to process data communications rapidly and to manage circuits. Here I'm going to cover front-end processor and multiplexers.
Front-end processor (FEP): A front-end processor is the special purpose device, this specialized computer manages all routing communications with peripheral devices. For large mainframe installations, a front-end processor is typically a special purpose minicomputer. The primary application of the FEP is to serve as the interface between the host/client computer and the data communication network with its thousands of terminals or microcomputers. An FEP perform all the function as associated with data link layer and some of with the network layer, it can handle coding and decoding data, error diction, retrieve the message recording, interpreting, and processing of the control information, addressing, routing message to the right computer, and breaking the long message in to small packets. Front-end processor can provide up to 30 percent additional processing time for mainframe.
Intelligent controller are scaled-down FEPs. This device perform the same perform to the FEP as the FEP does to the host computer. Remote intelligent controller that is also called intelligent terminal controller is a microprocessor based intelligent device that controls a group of network. This device is used to reduce the transmission cost between the terminals and the host computers, and because it reduce the processing on the host or FEP.

Multiplexors is an electronic device that allows a single communications channel to carry data transmissions simultaneously from many sources. Multiplexing a data communication means that two or more information is sent simultaneously via the communication circuit. Multiplexors is transparent, when the circuits are multiplexed at one end and demultiplexed at the other, each user's terminal or microcomputer thinks it has its own separate connection to the host computer.  The objective of multiplexor is to reduce communication costs by allowing the more efficient use of circuits through sharing. In general multiplexed circuit must have equal high-speed transmission capacity as the sum of the circuit combines (i.e. one 57600 bps circuit could multiplex maximum of four 14400 bps circuits). Typically, a multiplexor merges the transmissions of several terminals at one end of communication channels, while a similar unit separates the individual transmission at the receiving end. There are three type of multiplexers:
1. FDM
2. TDM
FDM stand frequency division multiplexing.  In FDM circuits are Shares by dividing it horizontally, so that many signals can travel a single communication circuit simultaneously. FDM's are inflexible because once you determine how many channels are required, it is difficult to add new channels with out buying a new multiplexers, and must use guardbands to separate the frequency (*guardbands are unused space to separate frequencies).
TDM stands for Time Division Multiplexing which shares a communication circuit among two or more terminals by taking turns, dividing the circuit vertically. Time division multiplexing doesn't need guardbands so it is more efficient and less costly.,
STDM stands for statistical Time Division Multiplexing, is the exception to the rule that the capacity of the multiplexed circuit must equal the sum of the circuits it combines. By using STDM you can connect more computers or terminals to a circuit than using FDM and TDM. Selecting the transmission speed for the multiplexed circuit is based on a statistical analysis of the usage requirements of the circuits to be multiplexed. STDM provides more efficient use of circuit and save money.  But generally it has two problems.
1. It cause time delay
2. All data must be identified by an address that specifies the device to which it belongs

Components of Network
There are three basic hardware components for a data communication network:
Server or host computer: A server is a machine that provides clients with service.  Servers share a specific resource with other computers. Examples of the servers are the database server, that provides the large database and the communication server that provides connection to another network, to commercial database, or to a powerful processor. In larger LANs, the server is dedicated to being a server.  In a peer-to-peer LAN, the server may be both a server and a client computer.  The server stores data or software that can be accessed by the client. There are file, database, network, access, modem, facsimile, printer, and geteway server.  Servers are usually microcomputers (often more powerful than the other microcomputers on the network), but they may be minicomputers or mainframes.
Client:  A client is a computer such as a PC or a workstation attached to the network, which is used to access shared network resources. Client computer uses the resources shared by server computer. The client is the input/output hardware device at the other end of communication circuit. It typically provides users with access to the network and data and software on the server. There are three major categories of clients: terminals, minicomputers/workstation, and special purpose terminals.
Circuit:  The circuit is the pathway through which the message passes.  Circuits can be twisted pair cable, coaxial cable, fiber optic cable, microwave transmission, and so forth.  There are many devices in the circuit that perform special functions, such as hubs, switches, routers and gateways.
Actually, a network does not need a server or host computer.  Some networks are designed to connect a set of similar computer that share their data and software with each other.  This type of networks is called peer to peer network.  But this type of net work is not suitable for the big organization and peer to peer network is only good for 10 PCs. If you have to network with more than 10 PCs then I don't recommend to choose peer to peer network.

Figure1.1 shows a small network that has four microcomputers (clients) connected by a hub and cables (circuit).  In this network, message moves through the hub to and from the computers.  All computers share the same circuit and must take turns sending messages.  The router is a special device that connects two or more networks.  The router enables computers on this network to communicate with computers on the other network (e.g. Internet).  This network has three servers.  Although one server can perform many functions, networks are often designed so that a separate computer is used to provide different servers.  The file server stores data and software that can be used by computers on the network.  The print server, which is connected to the printer, manages all printing requests from the client computer on the network.  The web server stores documents and graphics that can be accessed with web browsers (e.g. Netscape Navigator, Internet explorer).  The web server can respond to requests from computers on this network or any other computer on the Internet.

Network Architectures
Network architecture guides a network builder to make a plan or structure by which the component parts are related.  Network architecture is a framework of rules to ease the operation, maintenance, and growth of a communication network by isolating the user and the application programs from the details of the network. The work done by any application program can be separated in to four general functions. The first one is data storage, almost all application programs requires data to be stored and retrieved. The section function is data access logic, when data is stored than it need to be accessed. The third one is application and the fourth function is presentation logic. These four functions are the basic building block of any application. There are three fundamental network architecture:
1. Host Based Architecture
2. Clint Based Architecture
3. Clint-Server Architecture
Host-based architecture: Host-based architecture performs virtually all the network. It was the very first data communication network architecture and the host computer performs all four function of application program. This is the simple architecture and works well. In host based network all data are flow from the one central host. Client computers enable users to send and receive the message from the host computer. Client computers send the message to the host computer for processing and accepted the instruction from the host on what to display. In host based network all work must be done by the host computer because of these host computers become overloaded and can not process quickly for the all user's demand. Response time become slower and network managers are required to spend more money to upgrade the host computers.
Client based architecture: The clients are the microcomputer on a LAN and host computers are servers on the same network. In client based architecture all process must be done by the client computer or the application software on the client computer is responsible for the all four application program functions; the server simply stores the data. As host based architecture, this architecture also have overload problem, since all data are process from the client so if more users are in active in the same network than logically the network circuit can not perform well and be overloaded. The fundamental problem of this architecture is all the data from the server must travel to the clients for processing which makes slow the entire network.
Client server architecture: In client-server computing, several servers may work together over the network to support the business application. Client-server architecture makes a balance between client computer and host computer.  In client server architecture, they split the application program function in to two groups, client handles the presentation logic, and server handle the data access logic and data storage while application logic may be on both or can reside either on client or on the server.  Because of its nature, the network is not overloaded with entire files being transferred back and forth through the network circuit for processing at each remote terminal.  For example; if the insurance company wants to request the list of all the customer and their insurance police, the client would accept the request than format the request to be understood by the server and transmit it to the server. On the other side, upon receiving the request form the client, the server search the database for all the requested and than transmit only the matching request to the client.

Media access control
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