Communications systems by lonyoo


									                           Communications Systems

                      3.1 Characteristics of communications systems

A communication system enables people to send and receive data and information. All
communication systems have five basic components:
   1. The data source produces the data to be sent
   2. The transmitter encodes the data into a signal suitable for a transmission medium
   3. The transmission medium is a channel, such as a cable, in which the signal is
      transmitted to the destination. The signal may be changed or distorted during
   4. The receiver decodes the signal back into the original data or an approximation of the
   5. The destination is the receiver of the information.

These components can be applied to any communication system, such as the radio. In radio,
the data source is the person speaking into the microphone. The transmitter is the microphone
and associated electronics that change the sound into a signal. The transmission medium, or
channel, is the space between the transmitting and receiving antennas. The receiver is the
radio that converts the signal into the original sounds, and the destination is the person
listening to the radio

Good communication systems have an accurate, reliable and secure transmission medium.
Good communication depends on protocols, handshaking, speed of transmission and error

A protocol is a set of rules that governs the transfer of data between computers. It defines how
the information is transmitted and how the errors are detected. Two computers must use the
same protocols when they are communicating; otherwise, the data transfer may be
unsuccessful. Protocols are written into internationally accepted standards, such as the OSI
reference model.

The OSI reference model divides data communication into seven layers. Each layer expresses
the standard, using a protocol. The bottom layers are responsible for transfer of data from one
place to another. They include protocols that specify the type of plugs, the format of data, the
method of transmission and error checking. The top layers examine the exchange of data
between application programs. They include protocols that specify file transfer, passwords
and network management

Handshaking is an agreement about which protocol to use to accomplish the exchange of
information. It is a series of signals that flow between devices during data transmission.
Handshaking involves sending signals to indicate the type of protocol to be used. The
transmitting device will send a signal and wait for an appropriate response. When two devices
successfully handshake; a connection is made. When a handshake is not successful, then the
devices ‘hang up’ and try again.

There are two methods of handshaking to control the flow of data:
    Hardware flow control uses a dedicated connection, such as a wire. It is only practical
        when devices are close enough to be linked with a cable. A common hardware
        protocol is RTS/CTS (request to send/clear to send)

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       Software flow control uses a special code sent with the data. It is used for long
        distance communication. A common software protocol is XON/XOFF (X stands for
        transmit). If a break in transmission is needed, then the XOFF command is sent.
        When transmission is to start again, then the XON command is sent

Speed of Transmission
The speed of data transmission is determined by the transmitting device and the bandwidth.
The Bandwidth is the capacity of the channel, or transmission medium. A transmission
medium with a high bandwidth can transfer more data.

The speed of data transfer is measured by the number of bits per second or by the baud rate:
    Bits per second (bps) are the maximum number of bits that can be transmitted in one
       second. This measure of speed includes special bits used in asynchronous
       transmission and any error checking bits. Bps is also called the bit rate.
    Baud rate is the maximum number of data symbols or electrical signals that can be
       transmitted in one second. Because a data symbol can contain more than one bit of
       data, the baud rate and the bit rate may be different. For example, 1200 baud might
       transmit at 4800 bps

Error Checking
When data arrives at its destination, it may contain errors. These errors may be caused by
interference with the signal or simply by errors in encoding and decoding the data
     Parity checking is a method of checking for errors in data transmission using an
         additional bit called a parity bit. This bit is used only for the purpose of identifying
         whether the bits being moved have arrived successfully
     Checksum is a method of checking for errors in data transmission by counting the
         number of bits in a data packet. A data packet is created by dividing the total data into
         smaller groups. The count of the bits in a data packet is attached to the data packet. It
         is used by the receiver to check whether all the bits have arrived successfully.
     Cyclic redundancy check (CRC) is a method of checking for errors in data
         transmission using a division process. The data is divided into predetermined lengths
         and divided by a fixed divisor. The remainder of the calculation is attached and sent
         with the data. When the data is received, the remainder is recalculated. If the
         remainders do not match, an error in transmission has occurred.

Communication Settings
Communication settings can be changed by the user to ensure a connection between two
devices. The settings are often a parameter. A parameter is a variable that is given a constant
value for a particular application. Some common parameters:
     Bits per second is the speed of transmission, such as 56 000.
     Data bits are the number of bits in each group of data. Each data group is usually sent
        as a byte, such as 7-bit ASCII or an 8-bit ASCII.
     Parity is whether the data contains a parity bit for error detection. Parity is odd, even
        or none.
     Stop/start bits are the number of stop and start bits used in asynchronous
        transmission. This parameter is used to identify each byte. The normal range is
        between 0 and 2. Some systems only use a stop bit.
     Flow control is the software handshaking protocol, such as XON/XOFF

                          3.2 Examples of Communication systems

Teleconferencing is the use of an electronic transmission to allow a meeting to occur at
the same time in different locations.

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       An audio conference is a single telephone call involving three or more people at
        different locations. It is a service provided by a telephone company. Audio data is
        transmitted and received using the existing telephone lines
       A video conference is a meeting that allows people in different locations to see video
        images of each other on a screen, as well as hear speech. This may require special
        communication arrangements because of the high bandwidth required to transmit

Teleconferencing simulates a face-to-face meeting and reduces costs. Money is saved by not
buying airfares, hotel rooms and meals. Teleconferencing also saves people the time and
energy involved in travelling. However, tele-conferencing does remove the inter-personal
relationship achieved through a face-to-face meeting. Physical contact and informal
discussions are often needed to clarify ideas and develop partnerships

Messaging systems
A messaging system is used to send messages to people in different locations who may
receive the message at a later time. Messaging systems involve the creation, storage,
exchange and management of messages.

Traditional systems
    A telephone is a system for transmitting sounds or speech between distant locations
        along telephone lines. It is a convenient method of communicating with people
        around the world. A telephone answering system is a messaging system. It stores
        messages and allows a person to hear the message at a later time
    A fax, or facsimile, machine is a system of transmitting and reproducing documents
        by means of signals sent over telephone lines. The fax machine scans a document and
        converts it into a bit-mapped image. This image is compressed and transmitted along
        the telephone network to a destination fax machine. This machine decompresses the
        image and reconstructs the original document.

Voice mail
Allows communication with other people by storing and forwarding spoken messages. The
sender dials a voice-mail number and records a message. The message is digitally stored on a
computer system and can only be retrieved by the intended receiver of the message. To
retrieve a message, you dial into the voice-mail system using any telephone and enter an
account number.

Voice mail combines the features of a telephone answering system and some of the concepts
of email. It provides some advantages over email. More people have access to a telephone
than to a computer equipped for email. People also often express their feelings more clearly
with the spoken word. However, email is much better at communicating complex information
and sending different data types.

Electronic mail
Allows communication with other email users by sending and receiving electronic messages
using a computer. It is fast, economical and a convenient way to send messages to people all
over the world. Email can be written to anyone who has an email address. Most email
messages contain two main parts: the header and the body of the message. The header is
similar to an envelope used in traditional letters. The body of the message is typed using the
email software or is imported or copied from a word processor.

Electronic commerce
Electronic commerce, or e-commerce, is the buying and selling of goods and services via the
Internet. E-commerce provides 24-hour availability, global reach, the ability to interact and

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provide customer information, and a multimedia environment. The most popular e-commerce
sites sell computer products, books, gardening products, and music or office supplies.
However, e-commerce is expected to expand into most retail areas.

EFTPOS (electronic funds transfer at point-of-sale) is a system that allows people to purchase
goods and services using a credit or debit card. It is the electronic transfer of money from the
customer’s bank account to the retailer’s bank account. Each point-of-sale terminal is linked
to the computer of the customer’s bank using the account number on the card.

Electronic banking
Electronic banking allows customers to view their account balances and transaction histories,
transfer money between accounts and pay bills using Bpay. Electronic banking raises the
issue of security. All banks are determined to make their online banking services safe from
inference and to secure customer details. Data encryption is used to secure the data transfer
between the customer’s computer and the bank’s computer.

                              3.3 Transmitting and Receiving

Transmission of Data
Parallel transmission is the transmission of data simultaneously using separate channels.
Serial transmission is the transmission of data one after the other. Serial transmission is used
to transmit data to peripheral devices, such as modems and printers, and is used on networks.

Serial transmission can be either synchronous or asynchronous. Asynchronous transmission is
the sending of data by identifying each byte with special start and stop bits. Synchronous
transmission requires all the data to be sent at the same rate. The same number of bytes is sent
each second. This is synchronised by each device using a clock. Synchronous transmission is
faster and more efficient than asynchronous transmission as there are no extra bits. It is used
on larger computer systems.

The direction of data flow can be simplex, half-duplex or full-duplex mode.
    Simplex mode allows transmission in one direction only, from the sender to the
        receiver. An example of simplex mode is the radio, a telegram or television.
    Halfduplex mode allows transmission in both directions but not at the same time.
        This means the sender and the receiver take turns. An example of half-duplex mode is
        an intercom, walkie-talkie or disk drive.
    Full-duplex mode allows transmission in both directions at the same time

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A network is a number of computers and their peripheral devices connected together in some
way. Each device in a network is called a node. Terminals are devices that send data to and
receive data from another computer system. If the terminal has both memory and processing
capabilities, it is called an intelligent terminal. Most personal computers are classified as
intelligent terminals and are called workstations on a network.

Local area networks (LANs) connect computers within a building or group of buildings on
one site. LANs cover a small geographical area, and the computers are linked together by
coaxial cable or fibre-optic cable. There are three main advantages in using a LAN:
    Sharing limited hardware resources, such as printers, hard disks and modems
    Sharing application software, such as word processing, database, spreadsheet and
        graphics programs
    Improved communication among users by sending electronic messages.

Wide area networks (WANs) connect computers over hundreds or thousands of kilometres.
WANs often consist of a mainframe computer called the host and a number of terminals.

Most data sent over a network uses packet switching. Packet switching is a technique that
divides messages into small data packets, transmits the packets and later joins the packets to
form the original message. It allows multiple users to use the same transmission line by
interspersing the data packets from different users. Data packets may not be sent along the
same path and could arrive at the destination at different times and in the wrong order. Each
data packet contains an address and control instruction to reassemble the message in the
correct order.

Network access methods
Network topologies
Network topology is the physical arrangement of the devices in a network. There are many
possible network topologies, such as star, bus and ring.

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A star topology has a central computer with each device connected directly to it. The central
computer serves as a switch. It receives messages and sends them to the destination device.
Star topology requires extra cabling because each device needs a cable to the central
computer rather than to the nearest device. If one device or cable is broken, the network can
still operate. If the central computer fails, then the network fails. Limited by the processing
power of the central computer. Star networks use a time-sharing system that allocates a
certain amount of CPU time for each user. Most common topology for a mainframe.

A bus topology is an arrangement where all the devices are attached to a direct line called the
bus. Each device has a unique identity and can only recognise those signals intended for it.
Devices check the bus and retrieve their messages as data travels along the bus. Each device
is considered to be connected to every other device and can communicate directly along the
network to any other device

A ring topology is an arrangement where all devices are attached so that the path is in the
shape of a continuous circle. Each device in the ring has a unique address. Data flow is in one
direction, moving from device to device until the data arrives at its destination

Network access methods
Ethernet—the first industry-standard LAN access method, or protocol, based on a bus
topology. Ethernet allows data to be transmitted simultaneously to all nodes on the network in
both directions. With data packets travelling simultaneously, collisions will occur and will
cause errors. To overcome this problem, Ethernet uses a method called Carrier Sense Multiple
Access and Collision Detection (CSMA/CD). In CSMA/CD, all nodes have the ability to
sense signals on the network. When a node wishes to transmit, it ‘listens’ to the bus for
signals. When there are no signals on the bus, it transmits. Occasionally a collision will occur
if two nodes sense a clear bus at the same. When a collision is detected, each device stops
transmitting and then retransmits at another time.

Token ring—a LAN access method, or protocol, based on a ring topology. The token ring
operates by continually passing special data packets called tokens between nodes on the
network. Workstations with data to send capture a free token and attach data along with
addressing information. A busy token with data cannot be used by other nodes. When the data
arrives at the destination, the data is replaced with an acknowledgment and sent back to
original sending node

                                      Network Hardware
Network interface card
Each computer connected to the network requires a special network interface card. A network
interface card (NIC) is an expansion card that fits into an expansion slot of a computer or
other device, so that the device can be connected to a network.

A server is a computer that provides services to other computers on the network Individual
computers log on to the server, which gives them access to files, applications or peripheral
devices. There are different types of servers:
         A file server is a controlling computer in a network that stores the programs and
            data shared by users. The files stored on this server can be retrieved by any node
            provided it has access rights.
         A print server is a computer in a network that controls one or more printers and
            stores data to be printed. A print server can be used with or without a file server
         A mail server is a computer in a network that provides email facilities. It stores
            incoming mail for distribution to users and forwards outgoing mail to appropriate

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           A Web server is a computer in a network that provides a connection to the
            Internet. All the Internet traffic is directed through this server.

Routers and switches
Bridge: A combination of hardware and software to link two similar networks. It often
connects LANs that use the same protocol, such as Ethernet. A bridge examines each data
packet on a LAN and forwards any data packets addressed to a connected LAN. Bridges are
faster than routers because they connect networks that are using the same protocol.

Gateway: A combination of hardware and software to link two different types of networks.
This usually involves converting different protocols. For example, a gateway could be used to
convert a TCP/IP packet to a NetWare IPX packet.

A hub is a central connecting device in a network. Data arrives at the hub from one or more
devices and is forwarded out using just one cable. Hub can also include a router. Most hubs
were originally passive. The data simply passed through the hub without any change.
Intelligent hubs are more frequently used in today’s networks. They often contain a CPU and
network operating system. This allows them to perform some of the functions of a server.

Transmission Media
Wire transmission transfers the data through wires and cables. These cables must be protected
from damage, they take up space, and they can be difficult to install. However, wire
transmission can carry large amounts of data with little interference from other signals
     Twisted-pair cable consists of two thin insulated copper wires, twisted to form a
        spiral. Twisting reduces the amount of interference from other cabling. The two main
        types of twisted-pair cables are unshielded twisted-pair (UTP) and shielded twisted-
        pair (STP). UTP is the most common; STP is used in ‘noisy’ environments where its
        shield protects against electromagnetic interference.
     Coaxial cable consists of a single copper wire surrounded by an insulator, grounded
        shielding and an outer insulator. The shielding allows data to be transmitted with little
        distortion. It is commonly used over distances of less than a few kilometres. The
        bandwidth for a coaxial cable is 10 Mbps
     Fibre-optic cable uses a laser of light to carry data in small glass fibres, diameter of a
        human hair. It is free from electromagnetic and radio interference, is very secure and
        can transmit data at high speeds without errors. The bandwidth for fibre-optic cables
        is in excess of 400 Mbps

Most LANs use either twisted-pair cable or coaxial cable. Fibre-optic cable is usually too
expensive and difficult to install. There are two types of transmission used: baseband and
broadband. Baseband networks use the entire capacity of the cable to transmit only one signal
at a time. Most LANs are baseband. Broadband networks divide the cable so that several
signals can be transmitted at the same time.

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Wireless transmission moves the data through air and space. It does not need a fixed physical
connection between the source and the destination. Radio and television are examples of
wireless transmission. Wireless transmission involves microwaves, satellites, wireless
networks and mobile phones.

                                      Network Software

Each computer in a network must have appropriate network software. This software contains
the ‘rules’ for communication. Network software is organised by a network administrator. A
network administrator is a person who manages a network within an organisation. His or her
responsibilities include network security, installing new applications, distributing software
upgrades, monitoring daily activity, enforcing licensing agreements, etc. These
responsibilities are completed using the network operating system.

Network operating systems
A network operating system (NOS) is an operating system that is designed primarily to
support computers connected on a LAN. One part of the network operating system resides in
each node and another resides in the server. The network operating system performs a range
of different tasks.

Network operating system tasks
The network operating system controls the flow of data between the devices on the network
and controls the requests for data. It organises messages from nodes until the network is ready
to process each message. The tasks performed by a network operating system include:
     administration—adds, removes and organises users; installs hardware devices and
        software applications; and carries out maintenance operations, such as backup
     file management—gives users access to the remote hard disks on the server and
        provides a file system and the ability to manage a network directory
     applications—handles requests from users to share data and applications
     resource management—allows network devices, such as printers and modems, to be
        shared; assigns users to printers; and orders print jobs
     security—monitors and restricts access to network resources

Logon and logoff procedures
Logon is the procedure used to get access to the network. The user is identified by means of a
user ID and a password. The password is an important security measure and must be not be
readily available or an easily guessed word. The password file should be encrypted and
protected from unauthorised access. The correct procedure for logging off should always be
carried out. It clears the communication line for another user.

An intranet is a private network that uses a similar interface to the Web. The main purpose of
an intranet is to share information and computing resources among the employees of an
organisation. When the intranet provides access to the Internet, it is through firewalls.
Firewalls monitor the flow of data in both directions to maintain the security of the
organisation. An extranet is an intranet that is accessible to customers, suppliers or others
outside the organisation. It provides such information as product descriptions, answers to
frequently asked questions, warranties and how to contact customer service.
                                3.4 Other information processes

Collecting data for a communication system involves generating the data to be transmitted.
Example is when a salesperson in a shop scans the barcodes of a product. The product’s
details are collected and transmitted to a central computer. Collecting data involves a range of

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collection devices to gather different types of data. The choice of device depends on the
application and the type of data to be transmitted.

Encoding and Decoding
Encoding involves converting data from its original form into another form for transmission.
Decoding is the reverse process. It converts data from the form used for transmission back
into the original form. The type of encoding and decoding depends on whether the original
data is in analogue or digital form. Analogue data is represented by using continuous variable
physical quantities, such as voltages. Digital data is represented in the form of digits or

There are four encoding and decoding possibilities in transmission
    Analogue data to analogy signal. The wave shape of the data is encoded into the
        signal. A telephone encodes analogue data in the form of sounds into analogue
        signals suitable for the telephone line. If the signal is corrupted, there is no way of
        restoring the original analogue data.
    Digital data to analogue signal. A series of 0s and 1s is encoded into a continuous
        wave. A modem encodes digital data from a computer into analogue signals for the
        telephone line. When the analogue signal is received by another modem, it decodes
        the analogue signal into digital data
    Digital data to digital signal. A series of 0s and 1s is transmitted by sending it through
        a channel as a series of on and off pulses. Data transmitted in a LAN is digital data
        using a digital signal. There is a low error rate for this type of transmission. Digital
        data is encoded into a digital signal by the computer or a specific peripheral device
    Analogue data to digital signal—the wave shape of the data is encoded into a series of
        0s and 1s. This process of generating digits or numbers is called digitising. Images
        are digitised using such devices as scanners, and sounds are digitised using a process
        called sampling. The transmission of television using a cable is an example of
        analogue data to digital signal

Attachments are computer files, such as text, video, sound, pictures or programs, sent with an
email message. All email programs need to ‘encode’ file attachments into characters since the
Internet TCP/IP protocol does not allow transmission of binary code. Many email programs
use the Multipurpose Internet Mail Extension (MIME) protocol to do this. It is an Internet
protocol that is able to negotiate many different operating systems and types of software

Client server architecture
Client-server architecture describes the software relationship between the client (user) and the
server. A client sends a request to a server according to an agreed protocol, and the server
responds. It is similar to a customer (client) sending an order (request) on an order form
(protocol) to a supplier (server) for particular goods (data). Most business applications today
use client-server architecture, as does the Internet.

Displaying is the presentation of information in the form of text, numbers, images, audio or
video. A range of hardware and software combinations can be used to display different types
of information in a communication system.

                       3.5 Issues related to communications systems

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Messaging systems
Messaging systems have improved communication between people; however, they have
raised a number of issues:
     Social context. Ideas delivered by messaging systems appear less forceful and caring
         than ideas delivered personally. Messaging systems have difficulty when
         communication depends on users expressing their feelings
     Danger of misinterpretation. Communication often depends on the context, inflection
         in the speaker’s voice, and body language. For example, ‘this has been a great day’
         could have a negative or a positive meaning depending on the way the words are
     Power relationships. Messaging systems may change the relationship between people
         in an organisation. For example, email could provide an easy avenue for the lowest
         paid worker of an organisation to provide information to the senior manager
     Privacy and confidentiality. A characteristic of messaging systems is that the
         messages are stored. Email and voice mail store messages on servers, and these can
         be accessed by the people who are providing the service. Hackers may also break the
         security of these servers. Telephone conversations can be intercepted, and fax
         messages can be read by anybody near the machine
     Electronic junk mail. Unwanted mail is a problem for messaging systems. People can
         send an email message to one person or thousands of people very easily.
     Information overload. This term refers to the enormous amount of information that
         people have to absorb. Messaging systems are a source of information. The large
         amount of email and voice mail received by some people has increased their
         workload and caused stress

Internet trading
Many businesses are establishing a Web site to promote their goods and services. The Internet
provides significant advantages for consumers, such as more information about products and
services, shopping globally and increased competition that has resulted in lower prices. Some
of the implications of Internet trading include:
      Taxation. Present governments have been unable to tax transactions on the Internet.
         The increased business on the Internet will reduce the money governments receive
         from their goods and services taxes
      Employment ramifications. The increase in Internet trading requires more people to
         be employed in the information technology industry. It may result in fewer shop
         fronts and fewer people employed to provide this type of service
      Nature of business. Traditional businesses that provide opportunities for human
         interaction are being challenged. For example, people are choosing to buy their
         groceries using the Internet instead of visiting the store
      Trade barriers. The developments in communication technology have made trade
         barriers between countries irrelevant. The whole issue of where one country ends and
         another one begins is open to question. People are buying and selling goods on the
         Internet with little thought given to the countries involved or to trade barriers

The Internet provides access to a large amount of offensive material, such as
pornography, racism and violence, and the information is not hard to find. The Internet
allows children to access any material they wish, either deliberately or unintentionally.
Some people believe that offensive material should be banned, while others argue that
banning any material compromises our free society.

There have been many unsuccessful attempts to censor material on the Internet by
governments and law enforcement bodies all around the world. The Internet Industry

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Association (IIA) represents Australian ISPs It has released a code of conduct that deals
with censorship of online content. The code requires ISPs to remove offensive content
from their servers and to block access to classified material hosted on overseas sites.
The federal government has passed a law requiring ISPs to subscribe to the IIA’s code.

The difficulty with enforcing censorship is the enormous number of Web sites and the
fact that thousands of new sites are published daily. Monitoring Web sites on a global
basis is impossible. Clearly multinational agreement is needed on offensive material.
However, this agreement may be difficult to obtain and regulate. The prime responsibility
for preventing children from accessing offensive material rests with parents and teachers

Internet banking
Internet banking allows customers to view their account balances and transaction histories,
transfer money between accounts, and pay bills using Bpay. It provides banking services 24
hours a day but cannot cater for cash withdrawals or for cash or cheque deposits. Issues
arising from Internet banking include
      Security. All banks are determined to make their online banking services safe from
         inference and secure for customer details. Data encryption is used to secure the data
         transfer between the customer’s computer and the bank’s computer
      Changing nature of work. People working for the bank are not carrying out the
         services provided by Internet banking. Banks require more people with information
         technology skills and fewer people with banking skills
      Branch closures and job loss. With many customers using Internet banking, EFTPOS
         and ATMs, there is less need to access the facilities provided by a bank branch. This
         has resulted in branch closures and job losses

Radio and video
There are currently thousands of Web sites broadcasting radio from around the world. These
radio stations cater for a variety of tastes. For example, it is possible to listen to a major
sporting event live on the Internet. In addition to the Internet providing radio services, there
are many Web sites with video.

Video on the Internet is replacing videotapes and other media. It allows organisations to
create unlimited video channels. These video channels are used for sales, training,
communication and a host of other purposes. Video on the Internet saves time, reduces costs
and provides the ability to view the video globally. The size and quality of the video is
currently less than that of a normal television broadcast; however, it will improve with
developments in technology, such as increasing bandwidth

Working from home
Telecommuting is working at home and electronically communicating with the office. Even
though working at the office is not likely to disappear, advances in telecommunication are
likely to make telecommuting more common in the future. Factors that will affect the future
of telecommuting include the availability of bandwidth, the perceived value in
telecommuting, and the opportunities to work collaboratively across large distances

The main advantages of telecommuting are greater flexibility in work hours; saving money on
transport, clothing and food; and saving time. In particular, it benefits people who are
physically impaired or required to look after small children. The employer saves on
overheads, such as office space and furniture. Working from home can have its
disadvantages. People can miss the social and professional contact offered by an external
place of work. They experience feelings of loneliness and isolation. Telecommuting can also
blur the distinction between work and home life. The home is no longer a place where the
pressures of work can be forgotten

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