Chapter 60 Review
Data Communications and Standards
Communications Software
Communication software enables computers to communicate with each other, controlling transmission by specifying:
- Speed of transmission
- Direction of data flow
- Method of transmission
- Which computer code is used (ASCII or EBCDIC)
- Type of parity used( Odd, Even, None)
Rules and conventions which allow computers to communicate with each other is known as the communications protocol.
Speed of Transmission
Transmission speed is related to bandwidth of the communications channel between the two computers. Examples of this could be a telephone line, or a fibre optic / coaxial cable or satellite link.
BASEBAND – carries one signal at a time. A bit value of 0 or 1 is sent by the presence or absence of a voltage in the cable. These signals are very fast but are usually sent over short distances.
BROADBAND – can carry multiple signals on a fixed carrier wave, with the signals for 0 and 1 sent as variations on the wave. This means that data, audio and video can take place simultaneously.
The speed at which data is sent is expressed as the baud rate and is measured in BPS (bits per second). The transmission of a single character is made up of:
- 7 or 8 bits for the character
- Start bit
- Stop bit(s)(1 or 2 bits)
- Parity bit .
Modems
Telephone lines were originally designed for speech, which is transmitted in analogue or wave form.
In order for digital data to be sent along a telephone line, it must first be converted to analogue form and then converted back at the other end. The equipment takes its name from the process of converting the data which is called MOdulation and DEModulation. MODEMS typically transmit data at rates between 9,600 and 56,600bps.
Direction of Transmission
Simplex – Data can flow in one direction only.
Half-Duplex – Data can flow in both directions but not at the same time.
Full Duplex –Data can flow in both directions at the same time.
Serial and Parallel Transmission
Serial transmission: each character is transmitted one bit at a time over a single channel. There are two types of serial mode:
- Synchronous transmission enables whole blocks of data to be sent in timed sequence, and is much faster.
- Asynchronous transmission one character at a time is transmitted, with a start, stop and parity(if used)bit sent with each character. This is generally used for lower speed transmission.
Parallel transmission: A character is transmitted , together with a parity bit (if used) simultaneously over multiple wires . Parallel transmission is limited to communications over short distances of a few metres because of the risk of data corruption due electro magnetic fields and the physical characteristics of the transmission medium. .
Telecommunications Standards and Protocols
In order to allow machines from different manufacturers to communicate with each other over LAN’s or WAN’s, standards covering all aspects of communication need to be defined. These standards range from, from the number of pins on connectors, the transmission of data, to how a particular software package displays a menu at a users terminal..
Standards can be divided into:.
DE FACTO STANDARDS – are established by the fact that a particular product dominates the markets, like Intel microprocessors or Microsoft products.
DE JURE STANDARDS – are identified by industry groups or by government.
THE OSI MODEL (Open System Interface) – created by the International Standards Organisation to guide the development of standards for all aspects of network operations and management, regardless of type of technology, vendor or country of origin.
Internet Protocol (IP)
Communications on the Internet is governed by the protocol TCP/IP (Transmission Control Protocol / Internet Protocol) which has become a de facto standard. By following this standard set of communications rules, Internet computers can efficiently control and route data to your computer, through the computers at your Internet Service Provider and Network Service Provider, from the server holding the information you are looking for..
Addressing Mechanisms on the World Wide Web
Every host computer connected to the Internet has a unique IP address. This is a collection of four numbers separated by full stops (e.g. 212.207.139.129).
The DOMAIN NAME SYTSTEM translates these numbers into a domain name which easily identifies the organisation and often the country in which the computer is located. (e.g. amazon.com)
A URL (Uniform Resource Locator) is the standard address used to find a page, web server or other device on the Internet. Each resource (page, graphic, file etc) has its own unique URL.
An example of a URL is:
The First Part of the Address (
Specifies the protocol used for connection. HTTP is Hypertext Transfer Protocol which refers to websites. Other protocols can be used such as:
which is a secure version of HTTP and is used within credit card sites for example. (Hypertext Transfer Protocol Secure)
FTP:// which is file transfer protocol, which allows you to transfer files from your computer to a host on the internet.
TELNET:// is a protocol which allows your computer to establish a connection with a computer or device on the Internet and act as a remote terminal .. NB
The Second Part of the Address (WWW)
Indicated which part of the Internet being used, in this case it is the world wide web.
The Third Part of the Address (yahoo)
Is the first part of the domain name called the sub-domain, which refers to the company or people running the site.
The Fourth Part of the Address ( . ac)
Is known as the top level domain. It tells you what sort of institution is behind the site, for example:
.ac or .edu or sch – refers to an educational institute
.co – refers to a company that trades in one single country.
.gov or .org – refers to a government or charity organisation
.
The Fifth Part of the Address (e.g. UK) – refers to the country in which the site is located
An exception is the domain of .com refers to a country that trades internationally and as such does not have country part of the address..