Transmitting and Receiving by suchenfz


									             Transmitting and Receiving
Transmitting and receiving is the transfer of data and information within and
between information systems.
The hardware includes
                  Modems
                  Networks
                  internal components of the computer.

                                                      A modem allows data to be
                                                      transferred from one
                                                      computer to another.

                                                             Types of Modems

                                                      Acoustic coupler modems
                                                      These have cradles into which
telephone handset is placed. They are prone to errors and have limited transmission

Direct connect modems or dial-up modems.
 These are more common, much reliable and quite inexpensive.
The external modem is more common and can be taken from computer to computer
with ease.
Internal modem slots inside your computer with ease and is safer from a security

Optical Modems transmit data over optical fibre lines by converting electric signals
from computer into pulses of light.

Null Modem cables
The computers are linked by a special
cable through their serial ports.

       Features in modems
      Auto-answer, where the modem
       will answer an incoming call.
      Auto-disconnect, where the modem will disconnect automatically at the end of
       a session.
      Auto-dial, where the modem will take care of dialing.

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      Auto-baud, where the modem will automatically adjust to the same speed as
       the called or calling modem.
      Auto-redial, where the modem will re-dial every couple of minutes if necessary.
      Software is used to direct the use of the hardware.

                    Mail
                    Phone
                    Fax
                    Radio
                    television.

             Types of Transmission
Packet switching
Is the transfer of data in small “packets”, each with a controlled format and made of
a maximum size, ranging from a few hundred bits to several thousand bits.
The data packets allow data to be checked and correctly reassembled and may take
different routes to their destinations.

Circuit switching
When a telephone call is made, a circuit-switching network is used. For the duration
of the call, one circuit or telephone line is set aside for that call. No one else can use
that telephone line during the duration of that call.
                     Types of data transmissions

Asynchronous data transmission
Data is sent one character at a time. Also referred
to as start/stop transmission. Data transfer is
controlled by a start bit (0 bit) and a stop bit (bit),
for each character.
When the start bit is detected, the answering device sets itself to receive the group
of eight bits that follows it. (7 bits are used for ASCII representation of the
character and 1 bit for parity). The stop bit tells the receiver to stop wait for
another start bit before receiving. The use of start and stop bits allows the receiver
to synchronise itself with the incoming transmission, character by character.

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Asynchronous transmission is used mainly when the sending device is operated
manually or it operates intermittently and generally at a slower transmission speeds.
    The speeds are less than 1200bps or about 600 baud.
    Is inexpensive
    Slow
    has limited error checking capabilities.

Synchronous data transmission.
                               This is termed as block transmission. It sends data as
                               blocks of hundreds or thousands of characters in
                               timed sequences. Clocks in the receiving and
                               transmitting terminals are used to make sure that
                               data are processed at the same rate.
When a block of data needs to be sent, no start or stop bits are used but one or
more synchronisation characters, called Sync bytes, precede each block of data. The
start and end of each block is marked by special characters.
Data is accepted until either the end character or the predetermined number of
characters in a block, have been received.

Synchronous transmissions are used when transmission speeds are very high above
1200 bps. This is used in direct communication between computers or between a
computer and a peripheral device. For example many laser printers use synchronous
transmission at 9600 baud.
    It is efficient
    has sophisticated error checking.
    It is fast but expensive.

Baud Rate
The baud rate is a measure of times per second that a data communications signal
changes: with each change one or more bits of data can be transmitted. The time

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required sending a character depends both on the encoding method and the signaling
speed (the number of times per second that the signal changes its value or voltage).

                    Signal changes

             1 0 0 0 01 1 10 1

Bits per second
Is the measure of the speed of data transmission: the number of bits transmitted in
one second.

Parity Bits
In both, Asynchronous and synchronous methods of data transfer, a parity bit may
be used for error checking. When parity checking is used, only seven bits from the
character byte are used to represent the character and the eighth bit is used for
parity checking. Parity can be set to odd, even or none. Parity can be odd, even or

                              Modes of transmission
The Transmission of data between devices via a communication channel may be in one
of the three modes:
                      Simplex transmission
                      Half duplex transmission
                      Full duplex transmission

  Simplex Transmission
This allows communication between devices in only one direction. For example
television signals.

Half Duplex Transmission
The communication can travel in both directions between stations, but in only one
direction at a time; for example, CB radios.

Full Duplex Transmission
The communication can travel in both directions, simultaneously, an example is the
telephone when it is used for voice communication.

Parallel Transmission
In parallel transmission, each bit has its own piece of wire along which it will travel.

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     Is often used for sending data to printer.
     Limited to short distance (less than 16 meters).
     It is dependent on cable type.
Over the length of 16 meters, radio frequency interference can cause problems, due
to low level of signal or signal distortion.

Serial Transmission
In serial transmission, the bits move down the wire one at a time in single file.

    Effective for transmitting over short and long distances.
    It is slower than parallel transmission.

Hardware for transmitting and receiving includes modems and networks as well as the
internal components of the computer
Communication within the computer
Communication within a computer involves transferring data between peripheral
devices, the hard disk and the CPU. Every part of the computer is linked to the CPU
either directly or indirectly by a bus.

A bus consists of electrical circuits between input, storage, process and output
devices. The greater the width, the more data that can be transmitted at the one
The speed of the bus is also an important factor in the performance of the
computer. A faster bus will make the applications run faster. There are two
different types of buses.
An internal bus is located on the motherboard and links the CPU and memory..
An expansion bus connects peripheral devices to the CPU and memory. If the
expansion bus connects directly to the CPU and bypasses RAM it is called a local bus.
There are different types of expansion buses, such as ISA, EISA, VESA and PCI.

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A port is a socket used to connect peripheral devices. Ports are either parallel or

Parallel ports transmit eight bits (one byte) at a time along eight parallel lines. They
are used to connect devices that send and receive large amounts of data such as
printers and disk drives.
Serial ports transmit data one bit at a time using only one communication line. Serial
ports are used to connect almost any peripheral device, such as a mouse, keyboard,
modern or plotter. The most common type of serial port is the RS-232.

The process of sending pre-arranged signals to define protocols between two
computers. It establishes compatibility, correct modulation and speed. When both
devices are switched on they perform two steps:
    The modem performs a self-check.
    Turns on “data set ready” to signal that it is ready for work.

Communications Protocol
Protocols are strict procedures for establishing and maintaining communication. Some
rules concern who will transmit first and how fast they will transmit data, error
checking, parity type to use and the number of bits that will make up a character.

              Types and Purpose of Protocols

Error Checking
When using telephone line and a modem the error can be one bit per 100,000
Optic fibre, the error rate will be less than one in one hundred million bits

Errors may be attributed to:
    External electromagnetic interference from devices close to the computer.
    Poor insulation of the cables.
    Bad atmospheric conditions.
    Crossed connections

Methods used for error detection
Parity Check
When a character code is transmitted, and the type of parity used is known, it can be
determined whether the character bit sent is correct or not by checking the number
of 1‟s against odd or even parity. Parity checking can only show if there is an error in

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a given character: it cannot tell when any character, or any block of characters, is
totally missing.

 2. Checksum
In this method, the number of „0‟ and „1‟ bits in a file or block of data is added
together and the sum (a redundancy value) is transmitted along with the file or data

 The receiving computer also adds the number of bits in the message and if its total
is different from the sum sent with the transmission, then an error is detected but
the position of error is not identified.

3. Block Check Character (BCC)
This is one of the most widely used error detecting codes used in synchronous
Such codes attach extra digits to each block of data to be transmitted. This extra
digit is called the Block Check Character (BCC).
BCC is calculated using the binary numerical value of the block.
    If the transmitted BCC matches the one at the receiving end, a positive
        Acknowledgement (ACK) is sent.
    If the BCC does not match the one on the receiving end, a Negative
        Acknowledgement (NAK) is sent, which informs the transmitting station to re-
        send the block.
This use of control characters is called the Acknowledgement- negative
acknowledgement sequence (ACK-NAK).

4. Cyclic Redundancy Check
The sender calculates vertical and longitudinal parity according to a polynomial code
and sends this parity character along with the data. The receiver calculates the same
and compares the calculated values to determine errors. CRC is considered quite
reliable and is becoming a standard for error detection.

Error correction
Retransmission of damaged block is the only solution. If errors occur, only the
damaged block needs to be transmitted. Backward correction.

Forward error correction. This method is used for data correction at the receiving
end of transmission. It requires no request or reverse message to re-send a single or
block of data.

A network is three or more devices (nodes) connected in such a way that they can
communicate with each other.

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Other features of networks:
   Physical connection between devices (cables and hardware couplers).
   Set of common protocols for devices to function.
   Access method that allows reasonable access to nodes.
   System of identifying each node with a number or name.

 In a network, each device is called a node. Node in a network can be a device such as
terminal, PC, desktop workstation or a larger computer.

Some networks have a series of nodes linked to a central computer called the host.

File Server
is a dedicated computer with a large hard disk that holds files and software to be
accessed by other nodes or workstations in a network.
Network Server
is a device, which provides file management, printing, and communication facilities to
the other devices attached on the network.

                           Network Topologies
Topology refers to the arrangement or layout of nodes on a network.
Types of Topologies:

Star network
                                                      The central station is very
                                                      important because if it fails,
                                                      the entire network goes down.
                                                      The signals are sent through
                                                      the central station and circuit
                                                      switched to the correct
                                                      receiving station over a
                                                      physical path.
                                                           Because all messages go
                                                             through the central
                                                             station, there can be
      The failure of any outer station does not affect the network.

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      It is possible to expand the network by adding a separate cable from server or
       node to the computer.
      The centre of a star topology network is called the hub.
      The hub accommodates the network software and directs communications
       within the network.
      The hub may also act as a gateway to another LAN.
      The star network is an example of a point-to-point channel configuration,
       where a computer is connected directly to another computer or a device.

Ring Network
Traffic can be one way or two
It does not require a main
computer to control the
It can be very fast if two-way
traffic is allowed.
In this form of network the
transmission is broadcast and
the destination station is able
to recognise its address on a
signal and receive that signal.

Bus network

This network is very reliable and provides for expansion.
If one station fails, the rest of the network can continue to operate.
Bus networks have their size extended by the use of repeaters that boost the
signal‟s strength.
Bus network is an example of a multipoint channel configuration, where multiple
                                                         devices are connected to a
                                                         single line.
                                                         With this topology it is easy
                                                         to add or remove a node
                                                         without affecting the
                                                         The cable system must be
                                                         terminated at both ends.

A major concern in any network is to avoid having two or more devices attempting to
transmit a signal at the same time. When this occurs it is termed as collision. With
collision, data loss or network crash can happen.

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             Methods used to avoid collisions:
Carrier sense multiple access/collision detection.
Carrier sense multiple access/collision avoidance.
Token passing.

Carrier sense multiple access collision detection (CSMA/CD):
The device monitors the channel to determine whether it is in use or whether it is
available for transmission. In this method the transmitting node listens to the
response from receiving node. If the response is absent, or indicates an error in the
signal, the message is retransmitted.

2. Carrier sense multiple access collision avoidance (CSMA/CA):
Any node waiting to send a signal listens to the network and when there is no traffic,
pauses for a period of time before transmitting its message.
After the message is transmitted, the sending node listens for the response from
the receiving node and if it is incorrect or absent, the message is retransmitted.

Token passing
Token passing is another method of controlling collision.
A token is only one, unique string of bits and on a network.
The token moves continuously around the network when no transmissions are
When a node needs to transmit a signal, it must wait for the token and when it
appears, it is captured by the device and held while the message is transmitted.
                    Types of networks
In computer communications, there are two types of networks:
    LAN: Local Area Network.
    WAN: Wide Area Network

                    Local Area network
In a local area network, the computers and printers for example are close together
(such as in different rooms in a building or in two buildings close together).
The distance involved in LAN is short so no modems used.
What does a LAN consist of?
    Cabling or transmission medium
    Devices/nodes ( terminals, printers, servers, modems)
    Interface between devices and transmission medium:
    buffers
    packet assembly and disassembly facilities(LAN interface card)
    gateways (allows LAN‟s using different protocols to communicate.
    Bridges ( allow LANs with similar protocols to communicate)
    Servers ( print server, disk server)

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Advantages of LAN
   Allows data sharing between computers.
   Allows sharing of expensive devices such as printers.
   Allows people to access electronic mail.
   Provides access to other LAN‟s and WAN‟s.
   Have good security features.
   Allows for centralised management.

Types of LAN‟s
There are two types of LAN‟s:
    Peer to Peer.
    Server client.

                    Types of LAN protocols:
Ethernet Protocol.
Ethernet is a very popular LAN protocol. It is a 10 million bit per second network.
The data is divided into packets surrounded by LAN protocol.
More than 1000 devices can be attached.
Ethernet is easy to install, has low error rates and is cheap.

Token ring
Token ring network was designed by IBM to overcome problems in ring networks.
In a ring network, a station has to wait for a lull before transmitting its message.
Uses twisted pair wire, and allow speed of up to 16million bps.
Up to 260 stations can be attached and many rings can be connected via bridges.

                    Wide Area Networks (WAN’S).
Wide area networks spread over a wide area such as state, a country, or the world. A
WAN is generally long distance and consists of data terminals controlled by users
and some computer communications equipment owned by for example Telecom. They
can use various communication links like telephone lines, microwave, fibre optic
cables, coaxial cables and satellites.

Packet switching is used in WAN‟s.
The devices used to send data to a computer system or receive data from a
processor are called terminals. Terminals can be
                 Dumb
                 Smart
                 intelligent.

Dumb terminals do not contain a processor and usually consist of a keyboard and

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Smart terminals can do some low-level processing such as text editing.
Intelligent terminals have both memory and processing capabilities. Most personal
computers are classified as intelligent terminals and are called workstations on a


Compression reduces the number of bits required to represent data. It compacts the
data so that it takes up less space on a disk and requires less time to download.
Compression is either
                               Lossy
                               lossless.

Electronic mail (email) allows communication with other users by sending and
receiving electronic messages using a computer. Email messages are often short and
are typed quickly. Messages are written offline and sent when the user logs onto the
LAN or the Internet. Attachments are computer files sent with email messages, such
as image, audio, video or text files.
Data encryption is used to prevent data from being intercepted and read during
transmission. The data is 'scrambled' so that it is meaningless to anyone other than
the recipient, who uses the same encryption software to convert it back.

                    Social ands Ethical Issues

Thee accuracy of information received from the Internet is not guaranteed. Must
compare data from a number of sources and determine which data is accurate

One method to protect data from deliberate or accidental damage is the use of
certificates. A certificate o digital signature is an encrypted code that identifies a
particular personal Web site. It aims to ensure that unauthorised persons cannot
access data.

To cite an Internet source it is important to include the author's name, title of the
completed work, URL of the page and download date.

This can be an issue when receiving information from the Internet. Some Web sites
use cookies to record information about visitors to their site. Some people argue

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that cookies store personal information about the user, and that this is an invasion of
Nature of work
Changing nature may mean learning new skills (upskilling), requiring fewer
skills(deskilling) or learning additional skills (multiskilling)

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