· 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
· LAN commitments
· Ethernet
· Token Ring
· High speed LAN and backbone
Network
· Components of Backbone
Network
· Network Design
Introduction
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 |
Figure:
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.
Figure:
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.
Figure:
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.
Modems
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
Figure:
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.
Figure:
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.
Figure:
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.
Processors
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.
Figure:
Multiplexors
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
3. STDM
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