Investigate Data Communications.
Communications & Networking
Investigate Data Communications
Unit 3 Element 3.1
Assignment 1
By Nicky Wilson
GNVQ Advanced IT
Action Plan
25-4-95
27-4-95
Received assignment which is to cover element 3.1 all PCs. My initial task is
to gather all relevant information on telecommunication authorities, the types
of connectivity and network services. I will research through the books A
level BTEC First degree computing , the book Data and Computer
Communications as well as to search through the CD ROM on Groliers
Encyclopedia. As well as lecture notes. Take notes on any relevant
information
27-4-95
-5-95
Research information on the devices and modes of data communications.
research through above books and CD ROM's and take relevant notes.
-5- 95
4-5-95
Find out about devices and modes of data communication, look through
magazines in resource center and various pc magazines for relevant
Information, as well as above literature. Take notes on relevant information.
4-5-95
6-5-95
6-5-95
9-5-95
9-5-95
0-5-95
Research information on data communications protocol parameters, by above
literature make notes.
Make notes on the use of data communications and network services,
analysing them to determining devices, modes of data communications,
transmission rate and protocol parameters.
Word process first draft, and take to tutor for first review
0-5-95 -
2-5-95
After outcome of first review take tutors advice accordingly.
3-4-95 -
7-4-95
Make sure to check work to see if any important facts have been omitted,
ask Tutor for a second review. After outcome of second review finalise any
missing facts. Word process final draft, check the work for mistakes and
hand in finished report for the 19-5-95
Bibliography
KNOTT, G (1993) A level, BTEC & First Degree Computing
WAITES, N Business Education Publisher ltd 1990
STALLINGS, W (1993) Data & Computer Communications, Third Edition
Macmillan Publishing Company 1991
PC USER February & March 1995
GROLIERS CD ROM
ENCARTA 95
LECTURE NOTES
S.M.H. COLLINS (1988, 1994) Dictionary of computing, 2nd Edition
Nicky Wilson
Advanced GNVQ
Communications
Unit 3 Assignment 1
Introduction
In this report i will outline telecommunications authorities, types of connectivity, network services, the devices
used and modes of data communications. The report will also describe transmission media, standards for data
communications and data communications protocol parameters. It will also show an analysis of uses of data
communications and network services, determining the devices, mode, transmission rate and protocol
parameters.
For communicating today you have to have certain standards these are the telecommunications standard setting
authorities, and public telecommunications authorities
ISO
The ISO / OSI model is the International standards Organisation/ Open system Interconnection layered
architecture that defines how computers and networks should interact. This group was founded in 1946 and has
issue more than 5000 standards on a wide range of areas. Its purpose is to promote the development of
standardisation and related activities to facilitate international exchange of goods and services. Standards have
been made to cover items from screw threads to solar energy's
ANSI
This is the AMERICAN NATIONAL STANDARDS INSTITUTE organisation which specifies computer and
software standards, including those of high level programming languages. They are a non profit government
federation of standards - making and standards using organisations. It has a wide range of members which
include professional societies, trade associations, government and regulatory bodies, industrial companies and
consumer groups. ANSI publishes national standards but does not develop them, standards are developed by
other groups that are accredited to develop standards for ANSI consideration. A lot of this work is done by the
Institute of Electric and Electronics Engineers (IEEE), who developed the IEEE 802 local area networks standards.
CCITT
The COMITE` CONSULTATIF INTERNATIONALE DE TELEGRAPHIE ET TELEPHONIE, These are an
International committee that defines out communications protocols and standards. This body standards are set
mainly to do with modems, faxes, data communication formats ( The way that data is passed between the
system).
Public Telecommunications Authorities
British Telecom
Under the British Telecommunications act 1981 the functions of the post office were divided between two
separate organizations. The post office retained control of postal services and BT formally British Telecom was
created to provide a telecommunication service. BTs obligations under its operating license include the
provision of universal telecommunications serviced, a service in rural areas, and essential services, e.g. public
call boxes and emergency services. This is a star network, it is Public switched Data Network, who provide a
service for its customers throughout the UK .
Mercury
Mercury communications is licensed to provide national and international telecommunications services for
residential and business customers. These services utilize the digital network created by Mercury. Mercury can
also provide other services. They provide public and private telephone services, international packet services,
national and international telex, electronic messaging, electronic mail and access to telex via a PC. Data network
services and customer equipment
Types of Connectivity
In computer network communications a group of devices is interconnected so that devices can communicate and
share resources. for example the branch office computers of a company might be interconnected so that they can
pass information to one another quickly, a company may want there computers to be interconnected so that they
can all share the same hard disk.
local are network LAN & Wide Area Network (WAN)
A network combined to one building with work stations which are usually microcomputers distributed in different
rooms is called a LAN, the systems are connected by wires and which is restricted to a limited geographical
area. An example of the use of local area network is in the computing department in Yorkshire Coast college,
where approx. 16 386PC are all linked to a file server.
A computer network where the systems are connected ...
This is a preview of the whole essay
can all share the same hard disk.
local are network LAN & Wide Area Network (WAN)
A network combined to one building with work stations which are usually microcomputers distributed in different
rooms is called a LAN, the systems are connected by wires and which is restricted to a limited geographical
area. An example of the use of local area network is in the computing department in Yorkshire Coast college,
where approx. 16 386PC are all linked to a file server.
A computer network where the systems are connected by wires or satellite and where the systems are remote
from each other, for example in different towns is known as a wide area network (WAN). In organisations with
many branches it is becoming popular to maintain a LAN at each branch for localised processing and to link each
LAN into a WAN covering the whole organisation. This way enables branches of an organization to have
control over there own processing and yet have access to the organisations main database at headquarters.
Public Switched Telephone Network (PSTN) & Public switched Data Network (PSDN)
The public switch data network is a communication that has been setup and controlled by a public
telecommunications authority for the exchange of data.(works on top of phone lines. The PSTN is the term used
to describe the Analogue telephone network, it lacks bandwidth it cannot carry all information to reproduce the
voice properly.
Normal hearing is 50hz - 20 khz
phone line is 50hz - 8khz
Integrated services Digital network (ISDN):
The integrated services digital network (ISDN) is a projected worldwide public telecommunications network that
will service a wide variety of user needs. The ISDN will be defined by the standardisation of user interfaces, and
will be implemented as a set of digital switches and paths supporting a broad range of traffic types. The ISDN
run 16 times normal phone lines 64 or 128kbs, for major lines it may be 10 times faster. Video is faster, usually
shared lines, ideal for computer communications.
Broadcast
Television and radio broadcast a signal nationwide, anybody with a TV or radio will be able to pick up the signal.
Cellular telephone network broadcasts a signal, the receivers will pick up the signal. Cellular Radio, mobile
radiotelephone system, in use in several major cities of the world, that was given its first U.S. test in Chicago in
978 and has since been adopted in other urban areas. The system, a miniature version of large radio networks, is
named for the unit "cells" into which it divides an area. Each cell has a radius of about 1.5 to 2.4 km (1 to 2.5 mi)
and is equipped with a radio transmitter that employs its own range of radio frequencies. The same range can be
repeated several times across a large city as long as the cells using them are not neighbors. As a mobile
radiophone moves through this pattern of cells, its user's calls made as on regular telephones are switched from
one cell to the next by a computerized system.
Network services are explained
Electronic mail services
Electronic mail refers to messages that are transmitted from computer to computer over ordinary telephone lines
under the direction of an intermediate service. This service is a host computer that receives messages, holds
them, and sends them to the proper destination. With this system, it is not necessary for the sender and receiver
of a message to be connected simultaneously. To be able to use electronic mail you need to have a computer, a
Modem, a telephone line, and an electronic mail service.
EDI Electronic Data Interchange.
This is a system of sending orders, paying invoices or transferring information over a telephone line or network,
or telephone line using an electronic mail service. It is quite commonly used to send instructions to pay money
from one company to another, or from one bank to another.
Bulletin Boards
There are many bulletin board systems operating, these are information and message database that is accessible
by the use of a modem and computer link. Users of these services simply exchange information on hobbies, buy
and sell goods and services and exchange computer programs.
Public network service e.g. Teletext
Systems such as Ceefax and Oracle which use spare bandwidth on television to send pages of information from a
computer database to television sets fitted with special adapters. The database provides information on a variety
of subjects of general and special interest.
Private network systems
CompuServe is one of the largest US based on line services, it provides an access to a database, it has over two
million members around the world, 60,000 in the UK. It has 2,000 phone lines and access is by local number in
most of the country. Membership to join this service costs around £6 a month and on line charges are about
£3.20 per hour. (The Guardian 15th December 1994 pg 24.)
Janet is a network that was set up to link universities together. JOINT ACADEMIC NETWORK, they can use E
mail to transmit data and Communications
Devices and modes of data communications are explained.
Two different types of equipment are used in Communications
Data terminal Equipment (DTE) This is a device which has the ability to transmit data in a digital form, over a
cable or communications link, for example a computer or computer terminal
Data communications equipment or Data circuit Terminating equipment (DCE)
This takes input from a DTE and then converts into a form that is suitable for transmission to another DCE, which
will in turn then convert it back into a form for transmission to another DTE. For example Modem (modulator/
Demodulator)
DTE FAX DCE DCE DTE
PSDN
COMPUTER MODEM MODEM COMPUTER
Connectors standards
By far the most common interface standard in the United states is the 232 standard issued by the Electronics
Industries association. The RS - 232C allows various connectors to be used . It was designed for connecting
DCE to DTE.
Original Pcs had a 25 pin connector similar to the diagram
later Pcs had a 9 pin D connector, but do.
not have all specs of a 25 pin D RS 232 C connector
Modes of Data communications are explained
A transmission may be simplex, half duplex or full duplex..
Simplex transmission In simplex transmission signals are transmitted in only one direction, one station is the
transmitter and the other is the receiver, e.g. television, radio
Sender Receiver
Half duplex transmission, In half duplex operation both stations may transmit, but only one at a time, e.g. walky
talky.
Sender Receiver
Receiver Sender
Duplex Transmission, This allows communication in both directions at the same time. In interactive systems,
when on demand inquiries are needed this mode is appropriate.
Sender/
Receiver Receiver/
Sender
Asymmetric duplex
Serial
In reference to data transfer, serial transmission is the sending of information one bit at a time over a single wire,
as through a serial port.
Parallel
Side by side, neither converging nor diverging. In graphics, parallel refer to lines that run evenly, side by side. In
parallel data transfer, information is sent simultaneously in groups; for example, 8 bits of a byte of data are
transmitted at the same time over eight separate wires in a cable.
Asynchronous transmission
This is a transmission method on which the time between transmission characters is arbitrary. Transmission is
controlled by start and stop bits and no additional synchronizing or timing information's required.
Synchronous transmission.
This is the method of transmission in which data is transmitted at a fixed rate and which the transmitter and
receiver are both synchronized.
Transmission Media
The transmission media is the physical path between transmitter and receiver In data transmission systems, the
characteristics and quality of data transmission are determined by the nature of the signal and the nature of the
medium. In the case of guided medium, the medium itself is more important in determining the limitations of
transmission. Shown below is typical characteristics for guided media.
Point to point characteristics of guided media.
Transmission media Total data rate Bandwidth Repeater Spacing
Twisted pair 4mbps 250khz 2 - 10km
Coaxial cable 500mbps 350mhz 1 -10km
Optical fiber 29bss 2Ghz 10 -100km
Two Wire
Signal 1 = -12v RS232C
Ground (av) 0 = +12v
The cheapest type of cabling is Two Wire, this is typically used for DCE - DTE, maximum distance is 50 meters,
with a speed of 20kbps, if any faster you may get cross talk, the interference may be on one side
Twisted Pair
A twisted pair consists of two insulated copper wires arranged in a regular spiral pattern, a wire pair acts as a
single communications link, a number of these pairs are bundled together wrapping them in a tough protective
sheath, over long distances they may contain hundreds of pairs. The wires in a pair have thickness from 0.016 to
0.036 inches. Speed will go up to 1mbps - 200ms. It is by far the most common transmission medium for both
analog and digital data, it is the backbone of the telephone system as well as the workhorse for interbuilding
communications
Coaxial Cable
Coaxial cable like twisted pair consists of two conductors, but is made differently to allow it to operate over a
wider range of frequencies. It consists of a hollow outer cylinder conductor that surrounds a single inner wire
conductor. The inner conductor can be solid or stranded, the outer either solid or braided. The inner conductor
is held in place by either regulated spaced insulating rings or a solid dielectric material. The outer conductor is
covered with a plastic shield. There are two main sizes 1/4 and 1/2 inch. Coaxial cable transmits at 10 - 20 mbps.
Coaxial cable perhaps has the most versatile transmission medium and is enjoying increasing utilization in many
applications, the main ones are
* long distance telephone and television transmission
* Television distribution.
Fiber optic cable
This is a relatively new technology with vast potential importance, fiber optics is the channeled transmission of
light through hair thin glass fibers. The way that it works is that the light is prevented from escaping the fiber by
total internal reflections, what this means is that a process takes place when a light ray travels through a medium
with an index or refraction higher than that of the medium surrounding it. In this case the fiber core has a higher
refractive index than the material that is around the core, so that the light that hits that material is reflected back
into the core, where it continues to travel down the fiber.
Fiber optic technology has been applied in many areas but its greatest impact has come in the field of
telecommunications, where optical fibers offers the ability to transmit audio, video and data information as coded
light pulses. Fiber optics is becoming the preferred mode of transmitting communications of all kinds. Its
advantages over older methods all of which involve the movement of electrons through metallic cables are many,
they include vastly increased carrying capacity, lower transmission losses, much smaller cable size and no
electrical interference because it is glass. Fiber optic can carry hundreds of TV Video channels and thousands of
telephone channels. The fiber optic cable can carry about 100 million bits of information per second, at the end of
the century that figure could be 2,000 million bits per second. It is also harder to tap into so it is more secure.
You can't cut and join fiber optic cable because it will leak. The problems with fiber optic cabling is that it is very
hard to work with.
Standards for Data Communication
ASCII
American Standard Code for Information Interchange. In computing, a coding scheme that assigns numeric
values to letters, numbers, punctuation marks, and certain other characters. By standardizing the values used for
these characters, ASCII enables computers and computer programs to exchange information. ASCII provides for
256 codes divided into two sets standard and extended of 128 each. These sets represent the total possible
combinations of either 7 or 8 bits, the latter being the number of bits in 1 byte. The basic, or standard, ASCII set
uses 7 bits for each code, yielding 128 character codes from 0 through 127 (hexadecimal 00H through 7FH). The
extended ASCII set uses 8 bits for each code, yielding an additional 128 codes numbered 128 through 255
(hexadecimal 80H through FFH).
RS-232-C Standard, in computer science, an accepted industry standard for serial communications connections.
Adopted by the Electrical Industries Association, this Recommended Standard (RS) defines the specific lines and
signal characteristics used by serial communications controllers to standardize the transmission of serial data
between devices. The letter C denotes that the current version of the standard is the third in a series.
The CCITT V24 covers data transmission over the telephone network. The V24 is a list of definitions for
interchange circuits between data terminal equipment CRS 232C
RS232 signals fall into one of the following 3 categories.
. Data (e.g. TXD, RXD)
RS232 provide two independent serial channels, which can be described as primary and secondary.
Both of these channels provide for full duplex operation, e.g. simultaneous transmission and
reception.
2. Handshake control RTS CTS
Handshake signals provide the way that flow of serial data is controlled allowing foe example a DTE
to open a dialogue with DCE before actually transmitting data over the serial data path.
3. Timing e.g. TC RC
For synchronous rather than the more usual asynchronous mode of operation, is necessary to pass
clock signals between the devices. These timing signals provide a means of synchronising the
received signal to allow successful decoding.
Data communications protocol and parameters are described
Protocol parameters are various aspects that make communicating easy, a protocol is a set of exceptional
standards, agreed and accepted way of doing things. A parameter is a various aspect of a particular area that
govern how it works.
Handshaking
This is the procedure obeyed where a user wishing to start a dialogue with another computer signals using either
hardware or software. An example of a situation is shown below
Hardware Handshaking
DTE DCE DTR = Data Terminal Ready
DTR DTR DSR = Data Set Ready
DSR DSR DTE = Computer
DCE = Modem
Handshake is used to Initiate conservation
Flow control
Is required in data communications systems in order to ensure that transmission rates match the processing
capabilities at each end of the link, it should also ensure that the size of buffer storage is not exceeded by the
amount of incoming data. Various protocols are in common use including Xmodem, Ymodem and kermit, many
communication software packages assist several of these protocols and allow users to select that which is
employed.
Encoding
When entering data into a computer via a keyboard, each selected element is either a alphabetical or numerical
character, for example is encoded by the electronics within the keyboard into an equivalent binary coded pattern
using one of the standard coding schemes that are used for the interchange of information. In order to represent
all the characters on a keyboard with a pattern, either 7 or 8 binary digits are used. By using 7 bits it means that
28 different elements can be represented, while 8 bits can represent 256 elements. A similar procedure is
followed on output except in the case of the printer it will decode each received binary coded pattern to print the
matching character. The coded bit patterns for each character are often refereed to as passwords.
Error control
Techniques are needed to guard against loss of damage of data and control Information. Most techniques
involve error detection, based on a frame check sequence and Protocol Data Unit (PDU) transmission. If a
sending entity fails to receive or acknowledgment to a PDU within a specified period of time it will retransmit.
There are two basic strategies in common use to enable a sending DTE to determine whether the receiving DTE
has correctly received the transmitted element. Echo checking and automatic repeat request.
Echo checking for example if you type login command to a remote system. It is sent to the other end and sent
back to you displaying the character you sent, manual checking.
ARQ Automatic repeat request
Two types, Idle RQ with this a transmitter sends data, receiver checks, then either sends message back OK or to
redo. Continuous RQ the transmitter continuos to send messages, in the background it sends it back saying
wrong, the transmitter will then send the bad block back later
Teletext Is the generic name for electronic systems that transmit data via broadcast or cable TV signals to
specifically equipped television receivers. Teletext services generally provide about 100 "pages," or frames, of
text and graphic information, usually timely material such as news headlines, sports scores, weather, and traffic or
financial reports. Information is grouped by menus, and viewers can request that frames be displayed on demand,
usually by keying numbers into a hand-held, remote-control device. Entertainment information e.g. TV program
listings and TV program captions can also be transmitted via teletext.
Teletext systems use the vertical blanking interval (VBI) of the TV signal. The VBI is a thick black bar that
may be seen when the video signal rolls on a TV screen; it carries 21 scanning lines, 8 of which have been
allocated by the Federal Communications Commission (FCC) for teletext and data transmission. Teletext data is
encoded within a standard TV signal, but users must have a special decoder to retrieve and view the text and
graphic content. Such decoders may be built into a TV set or cable TV decoder, or may be installed as an external
device. Its drawback is that it only able to communicate one way it uses simplex transmission
Database Computer Telecommunications User TV
Videotext
Videotex is an interactive electronic technology that delivers information and transactional services such as
banking and shopping through a computer/telephone communications system. Users can retrieve information or
conduct transactions (such as paying bills, buying airline tickets, or relaying electronic messages) using a special
terminal device hooked up to a TV set, or to a personal computer, and connected usually via telephone lines to a
host computer. Customers pay a per-use charge or a monthly subscription fee to use the service. It uses duplex
transmission.
Viewdata
This is an International term for services like prestel. Viewdata is a 2 way communication system that works over
the phone network. It tends to be used for services like
* Teleshopping
* Telebanking
* Banking services
Prestel uses a split baud rate ( sent to the user at 1200 Baud
sent back 75 baud to computer.
Facsimile
This system can utilize the telephone or telex networks to allow users to transmit an accurate copy of a
document. The information text or picture is digitized by a facsimile machine which scans the page automatically
to produce the required signals. With digital facsimile standards CCITT Group 3 now available can be used to
transmit a page of data at 64kbps in 5 s.
File transfer
In the file transfer communications, two devices are connected, either two computers, two terminals, or a
computer and a terminal. One device then transmits an entire data or program file to the other device, for example
a person who works at home might connect a home computer to an office computer and then transmit a document
stored on a diskette to the office computer.
Network services
CompuServe If you a PC or Macintosh a modem, phone line and suitable communications software you can sign
up to CompuServe and download the. it has over two million members around the world, 60,000 in the UK. It has
2,000 phone lines and access is by local number in most of the country. Membership to join this service costs
around £6 a month and on line charges are about £3.20 per hour. (The Guardian 15th December 1994 pg 24.)
To join the CompuServe Information Manager on line just follow the instructions below set you comms software
to use 7 bits one stop bit, and even parity (7 - 1 - E.) Then dial the CompuServe node in London on 071 - 490
8881 (all speeds including 9600 up to 14,400) and respond to the prompts as shown below.
HOST NAME CIS USER ID 177000, 5606 PASSWORD EXPLORE/ WORLD AGREEMENT NO GUARDIAN
SERIAL NUMBER 93006 After competing sign up, log on with your new ID and GO CISSOFT to download the
CompuServe Information manager communications software or GO LOGON for alternative access numbers
(outside London).
Bulletin boards There are many bulletin board systems operating, these are information and message database
that is accessible by the use of data terminal equipment (modem and computer link.) Microland Bulletin Board
system is an example I have picked out of the PC Mart issue 43, There are numbers to contact to download 0891
990 505, or to view 04832 725 905. Microland is connected to the Internet, and provide full global mail, news,
software archive and interactive services. There are many more bulletin boards about which offer these services.
Evaluation
I am satisfied with the outcome of the assignment, I feel that I have covered the criteria and the range
that was required. The way I approached the assignment was as such, first of all I researched
information from the books, A level BTEC and first degree computing. The magazines PC User Feb
& March Issues 1995, Groliers Encyclopedia, Encarta 95. I also used Lecture notes and other books
the Dictionary of computing and Data Communications.
The next process was to decide which way, was the best way to try and cover the Pc and ranges for
the unit. Eventually I decided that it would be easier to work through the Pcs in order thus starting at
Pc 1.
The other way I thought of approaching the assignment was to gather information on the analysis
stage which was the last Pc. The reason why I didn't do it this way was because I felt that it would
be better for me to do the other Pcs first because when they were completed, I could spend more time
on the final Pc.
The way that I tried to checked the validity of the Information was by, trying to compare the
information that I had it with the different books. magazines and CD ROM's to see if it was correct.
I also asked my wife to read through my work and make any constructive remarks, which she did, by
saying rather sarcastically "What an interesting piece of work "
I have done the work as instructed on my action plan, and kept to the dates that were stated for
reviews, so all in all I feel that I have completed this assignment in accordance with my original action
plan.
If any criticism is to be applied to my work, I could have explained a little more on the analysis stage,
but overall I feel happy with my report.
