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1.2.1 Serial and parallel data transmission

Transmission of data

Data transmission, digital transmission, or digital communications is the physical

transfer of data over a point-to-point or point-to-multipoint communication channel.

Examples of such channels are copper wires, optical fibres, wireless communication

channels, storage media and computer buses. The data are represented as an

electromagnetic signal, such as an electrical voltage, radiowave, microwave, or

infrared signal.

While analog transmission is the transfer of a continuously varying analog signal

over an analog channel, digital communications is the transfer of discrete messages

over a digital or an analog channel.

Serial Transmission

In telecommunication and computer science, serial communication is the process of

sending data one bit at a time, sequentially, over a communication channel or

computer bus. Serial communication is used for all long-haul communication and

most computer networks, where the cost of cable and synchronization difficulties

make parallel communication impractical.

Serial transmission has the advantage of being simple and reliable because other

next it is not transmitted until the current one has been received. However,

because only one bit can be transmitted at a time, the transmission is slow.all

peripheral devices that connect through a universal serial bus(USB) use serial data

transmission.

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Parallel transmission

If the devices are connected by more than one wire, then more bits can be sent

simultaneously. A sensible number of wires would be eight, because then whole

byte can be sent at the same time.

Parallel transmission of data is faster than serial transmission because all the bits

are travelling at the same time. It is less reliable as the bits can become muddled

up. The bits might reach out of sequence at the destination. This problem is call

‘skew’. Parallel transmission is suitable for short distances.

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Error detection and correction

A parity bit, or check bit is a bit added to the end of a string of binary code that

indicates whether the number of bits in the string with the

value one is even or odd. Parity bits are used as the simplest form of code. There

are two variants of parity bits: even parity bit and odd parity bit. In the case of

even parity, the number of bits whose value is 1 in a given set are counted. If that

total is odd, the parity bit value is set to 1, making the total count of 1's in the set

an even number. If the count of ones in a given set of bits is already even, the

parity bit's value remains 0.

In the case of odd parity, the situation is reversed. Instead, if the sum of bits with

a value of 1 is odd, the parity bit's value is set to zero. And if the sum of bits with a

value of 1 is even, the parity bit value is set to 1, making the total count of 1's in

the set an odd number.

7 bits of data (count of 1 bits)

8 bits including parity

even odd

0000000 0 00000000 00000001

1010001 3 10100011 10100010

1101001 4 11010010 11010011

1111111 7 11111111 11111110

Error detection

If an odd number of bits (including the parity bit) are transmitted incorrectly, the

parity bit will be incorrect, thus indicating that a parity error occurred in the

transmission. The parity bit is only suitable for detecting errors; it

cannot correct any errors, as there is no way to determine which particular bit is

corrupted. The data must be discarded entirely, and re-transmitted from scratch.

On a noisy transmission medium, successful transmission can therefore take a long

time, or even never occur. However, parity has the advantage that it uses only a

single bit and requires only a number of XOR gates to generate.Parity bit checking

is used occasionally for transmitting ASCII characters, which have 7 bits, leaving

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For example, the parity bit can be computed as follows, assuming we are sending

simple 4-bit values 1001.

Type of bit parity Successful transmission scenario

Even parity

A wants to transmit: 1001

A computes parity bit value: 1+0+0+1 (mod 2) = 0

A adds parity bit and sends: 10010

B receives: 10010

B computes parity: 1+0+0+1+0 (mod 2) = 0

B reports correct transmission after observing expected even

result.

Odd parity

A wants to transmit: 1001

A computes parity bit value: 1+0+0+1 + 1 (mod 2) = 1

A adds parity bit and sends: 10011

B receives: 10011

B computes overall parity: 1+0+0+1+1 (mod 2) = 1

B reports correct transmission after observing expected result.

Integrated circuits are more expensive when they have more pins. To reduce the

number of pins in a package, many ICs use a serial bus to transfer data when

speed is not important.

As suggested by the name Universal Serial Bus, data transmission in the bus

occurs in a serial form. Bytes of data are broken up and sent along the bus one bit

at a time, with the least significant bit first as illustrated by following figure 3.

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1.2.2 Security aspects

Internet security Internet use poses security challenges from spyware, as well as hacking and

malware. Spyware is malware that secretly collects various types of personal

information from a user’s computer. Users are also vulnerable to Internet-based

deception (or computer-based ‘social engineering’) Such as:

Phishing

Pharming

Advance fee fraud.

Some other Internet security problems include:

Spam Browser cookie files Adware

Hacking and other computer crime

Threats to security and measures to combat them include:

The crime of gaining unauthorized access to computer data, known as hacking

Other forms of computer crime including:

Data theft

Identity (ID) data theft, including phishing and Pharming

Fraudulent trading

Banking fraud

Espionage

Infringement of the copyright of software or music

Distributing or acquiring child pornography

Harassment (cyberstalking)

Child grooming

Distribution of malware

Denial-of-service attacks

Malware (which can be detected and removed by anti-virus software) including:

Virus

Worm

Trojan horse

Scareware

Spyware

Crimeware

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Computer Security

Firewall

A firewall is a utility program (or it can also be a hardware device) that filters the

information coming through the Internet connection into your personal computer or

into a company's network. Its main aim is to try to stop any attempts by intruders

to enter your system through the internet connection.

It is set up to allow mainly one way access, i.e. you can go out onto the Internet

and access pages, but it checks every data packet that either enters or leaves the

computer against a set of rules. If the data coming back is from an unauthorized

source then it is blocked. More.

You may have heard people saying, 'I can't get on that site at school because it's

been blocked'; that is the firewall in action.

All networks and indeed all home PCs should have an active firewall.

Spyware protection

Spyware is a software program that once installed on your computer can monitor

and collect personal information about your web surfing habits and the web pages

that you visit.

You are usually unaware that your machine has been infected by spyware and it

can sit in the background, collecting information about you and then transmitting it

back to the author of the spyware - often advertisers who want to know what you

are interested in and what adverts you click on.

Spyware may be installed without your knowledge by downloading some shareware

or other software that seems to do something useful e.g. a free game or utility.

Whilst spyware is normally harmless, it can interfere with your control of the

computer in other ways. It can install additional software which redirects the web

browser to sites that it wants you to visit. It can also change the computer settings

which can result in slow connect speeds and it can prevent you from accessing

secure internet services.

Key loggers are another form of spyware, but they are not so harmless. The

purpose of a key logger is to monitor and store a record of every keystroke made

on the computer. This data may then be sent to a remote server without your

knowledge. It is specifically looking for user names and passwords used for things

such as bank accounts, credit card numbers etc.

Computer security software normally includes an anti-spyware section to help

detect these malicious programs. You need regularly scan your system for spyware

and key loggers and remove them if they are found.

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Anti-virus

A computer virus is a software program that is designed to copy itself over and over

again and to attach itself to other programs.

Viruses do not damage the computer hardware. They affect the data and programs

stored on the computer.

They can cause an immense amount of time wasting and financial loss for people

and businesses. For example, the "Melissa" virus was so effective in spreading itself

that many huge corporations had to shut down their email systems until the virus

could be contained.

The operating system tries to make sure that data is not corrupted by viruses. In

order to do this, antivirus software is used.

There are a number of anti-virus programs available for wiping out viruses for

example Norton, McAfee, Sophos and AVG.

The anti-virus software contains a database of known viruses. Every time a file is

opened, the anti-virus utility checks the content, looking for hidden viruses against

its database.

When the utility recognises a virus pattern, it will alert the user, and ask them

whether they want to remove the virus and clean the system, or whether they

simply want to 'quarantine' the virus.

New and more damaging viruses are being written all the time and so to keep up

with these new threats, the anti-virus companies provide constant updates to their

software. Normally, you log on to the internet and the software will update itself

either automatically or after a certain period set by you.

There is usually a small annual fee involved for this service, but it is a small price to

pay for peace of mind.

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1.2.3 Internet principles of operations

Web browser and internet server

The most common use of the Internet is to view web pages on the World Wide Web

and for this you need a software application called a 'web browser'.

A web browser will take in all the data coming down from the web site and convert

it into a format you can see (or listen to). After all, a web page is made up of many

pieces of data - text, images, video, and sound. The job of the browser is to

'render' the page into the correct format.

As a client/server model, the browser is the client run on a computer that contacts

the Web server and requests information. The Web server sends the information

back to the Web browser which displays the results on the computer or other

Internet-enabled device that supports a browser.

An information resource is identified by a Uniform Resource Identifier (URI/URL)

which may be a web page, image, video or other piece of content. Hyperlinks

present in resources enable users easily to navigate their browsers to related

resources.

Web servers are computers that deliver (serves up) Web pages. Every Web server

has an IP address and possibly a domain name. For example, if you enter the URL

http://www.maxpapers.com/index.html in your browser, this sends a request to the

Web server whose domain name is maxpapers.com. The server then fetches the

page named index.html and sends it to your browser. Any computer can be turned

into a Web server by installing server software and connecting the machine to the

Internet.

What is hypertext transfer protocol (http)?

HTTP (Hypertext Transfer Protocol)

is the set of rules for transferring

files (text, graphic, images, sound,

video, and other multimedia files

on the World Wide Web

That's why all Web site addresses

begin with "http://". Whenever you

type a URL into your browser and

hit Enter, your computer sends an

HTTP request to the appropriate

Web server. The Web server, which

is designed to handle HTTP

requests, then sends to you the

requested HTML page.

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HTML (hypertext markup language)

HTML or HyperText Markup Language is the standard markup language used to

create web pages. It is written in the form of HTML elements consisting of tags

enclosed in angle brackets (like <html>). HTML tags most commonly come in pairs

like <h1> and </h1>, although some tags represent empty elements and so are

unpaired, for example <img>. The first tag in a pair is the start tag, and the second

tag is the end tag (they are also called opening tags and closing tags).

Web browsers can read HTML files and compose them into visible or audible web

pages. Browsers do not display the HTML tags and scripts, but use them to

interpret the content of the page. HTML elements form the building blocks of all

websites. HTML allows images and objects to be embedded and can be used to

create interactive forms.

HTML Structure

HTML structure Vs Presentation

Understanding the difference between structure and presentation can be difficult at first, especially if you’re used to not thinking about the semantic

structure of a document. However, it’s very important to understand this, since controlling the presentation of a document with CSS becomes much

easier if structure and presentation are separated.

Structure consists of the mandatory parts of an HTML document plus the

semantic and structured markup of its contents.

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Presentation is the style you give the content. In most cases presentation is about

the way a document looks, but it can also affect how a document sounds – not everybody uses a graphical web browser.

Separate structure from presentation as much as possible. Ideally you should end

up with an HTML document which contains the structure and content, and a

separate CSS file which contains everything that controls presentation.

MAC addresses A media access control address (MAC address) is a unique identifier assigned to

network interfaces for communications on the physical network segment. MAC

addresses are used as a network address for most IEEE 802 network technologies,

including Ethernet and WiFi. Logically, MAC addresses are used in the media access

control protocol sublayer of the OSI reference model.

MAC addresses are most often assigned by the manufacturer of a network interface

controller (NIC) and are stored in its hardware, such as the card's read-only

memory or some other firmware mechanism. If assigned by the manufacturer, a

MAC address usually encodes the manufacturer's registered identification number

and may be referred to as the burned-in address (BIA). It may also be known as an

Ethernet hardware address (EHA), hardware address or physical address.A network

node may have multiple NICs and each NIC must have a unique MAC address.

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IP address

An Internet Protocol address (IP address) is a numerical label assigned to

each device (e.g., computer, printer) participating in a computer

network that uses the Internet Protocol for communication. An IP address

serves two principal functions: host or network interface identification and

location addressing. Its role has been characterized as follows:

"A name indicates what we seek. An address indicates where it is. A route

indicates how to get there.

Why was it developed?

If two machines are networked together, then it is pretty obvious that when

you send data from one machine it can only go to the other machine. So no

need for complicated addressing. But as soon as more and more machines

were added to networks, they needed to be identified by a unique address.

So IP addressing was invented. And it looks something like this

198.168.000.001

There are four sets of numbers because it is a 4 byte scheme - 32 bits. The

first three numbers tend to be used to identify the network while the last

number tends to pinpoint a particular computer on that network.

IP addresses are binary numbers, but they are usually stored in text files

and displayed in human-readable notations, such as 172.16.254.1 (for Pv4),

and 2001:db8:0:1234:0:567:8:1 (for IPv6).

An IP address can be static or dynamic. A static IP address will never

change and it is a permanent Internet address. A dynamic IP address is a

temporary address that is assigned each time a computer or device accesses

the Internet.

Cookies

A cookie is a small text file that is stored on your computer when you visit

some websites. A cookie is used to remember settings from your last visit to

a website. For example Amazon.com uses cookies to help identify you when

you revisit the site. The website reads the cookie and can then look up

details such as you name, address, credit card number from their database.

This saves you from having to re-enter your data every time you visit.

Note: First, and contrary to popular belief, cookies are NOT programs. They don’t do anything at all.

They are simple ’text files’ which you can read using the Notebook program on your own PC

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