1.2.1 data transmission

7/24/2019 1.2.1 Data Transmission

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AHMED THAKUR

COMPUTER SCIENCE2210

https://www.facebook.com/groups/OAComputers/

[email protected], 0300-8268885Page 1

1.2 COMMUNICATION AND INTERNET TECHNOLOGIES

Understanding of what is meant by transmission of data

Bandwidth

Bandwidth is a fundamental measure of performance within any communication network, whether

it is an analogue system such as radio or a digital system such as a network.

After reading this you should be able to

Explain what is bandwidth

Describe the main types of network cable

Be able to compare their performance

Any communication channel has to be able to transmit information from one location to another.

The method might be analogue such as radio or it might be digital such as a computer network.

Bandwidth is a measure that quantifies the capability of a communication channel to transmit

information.

In the analogue domain, it is measured in 'Hertz' or Cycles per Second. For instance a typical PAL

television channel has a bandwidth of 8 Megahertz. Which means all the video and audio signals

for a TV channel resides in this set of frequencies.

In theory you could define the bandwidth of a digital system in terms of frequency, but it is far more

useful to describe bandwidth in terms of bits per second. After all, you are not particularly interested

in the shape of the digital signal but rather how much information can the network handle.

So, digital networks are measured in Bits per Second or bps.

Generally the wider the bandwidth the faster it is.

It is important to note that it is 'bits' per second not 'byte' which of course is 8 bits.

1.2.1 Data Transmission


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Bandwidth Bottleneck

Let's consider an everyday situation - viewing a web page. What is the bandwidth of this

communication path?

The illustration below shows the various communication channels between your computer and the

web server

You can see that the fastest link is within the telecom network itself at 10 Giga bits per second.

However, the slowest link is the low bandwidth link between the local ISP and the home router. This

has typically a 13Mbps bandwidth for data downstream (fiber to box link) and an even lower 3 Mbps

for upstream data. If you have a standard copper wire link to the exchange, then it is probably even

lower.

Therefore the bandwidth of this network is effectively only 13 Mbps and the bottleneck is the linkbetween the home router and the ISP.

Bandwidth and uses

The reason that bandwidth is so important is that it determines what you can do in terms of real-time

services.

If you only want to send text-only email then a very slow link is fine. For example a 56 Kbps dial-up

modem is more than adequate. Indeed, before broadband became prevalent that is all people

had available. The internet was very much a text-based experience.

If you want to receive a large file such as a high quality 10 Megabyte image, then 56kbps is far too

slow. It would take 1428 seconds or 23 minutes to download (10 million x 8) / 56000

A copper-wire based broadband link is typically around 7 Mbps therefore the same picture file would

only take about 20 seconds to download. Having a high bandwidth link makes it possible to use

multimedia services such as


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Video: viewing streaming video in real time without hesitation

Audio: such as using VoIP telephone or listening to streamed music.

If the bandwidth is too low for such services, then the only option is to download the complete audio

or video file then play it back offline.

Baud RateThe rate that voltage changes is called the baud. In the simple case described above, if the voltage

changes 10 times every second the baud is said to be 10.

Bit Rate

The bit rate is the term given to the rate that bits are transmitted. In the simple case described above

the bit rate is the same as the baud. If we could generate four voltages, instead of two, we could

use each change in signal to represent two bits.

TYPES OF NETWORK CABLES

Copper Cable

One of the key things that determines bandwidth is the physical nature of the cable being used.

A signal becomes weaker the longer it travels along a cable, eventually becoming so weak that it is

no longer detectable above natural noise. Therefore the length of cable determines the bandwidth

of the link.

For instance the bandwidth of broadband to the home is determined by the length of copper cable

between the house and the nearest telephone exchange. This is the so called 'last-mile' bottleneck.

Coaxial cable

This consists of a solid copper core surrounded by insulation which is then surrounded by a copper

shielding and finally covered with a plastic sheath. Coaxial cable is widely used for television wiring

as it has enough bandwidth to handle a television signal over a typical run from antenna totelevision.

Early computer networks also used coaxial cable with a bandwidth of 10Mbps. But for high speed

networks (100 Mbps and above) coax cable is no longer sufficient

Twisted Pair

In order to gain enough bandwidth another form of copper cable is used. Namely twisted pair cable

There are 8 colour-coded wires with each related pair twisted around one another. Twisting it in this

way reduces signal loss over any given length of cable.

Twisted pair cable is widely used in 100 Mbps and 1 Gbps networks. In order to guarantee the

performance of the cable, standards have been created such as CAT 5e and CAT 6. A 'Cat 5e' UTP


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cable is sufficient for bandwidths up to 1 Gbps for reasonable run lengths. For networks needing to

run up to 10 Gbps then a Cat 6 cable should be used. Of course, this is more expensive so cable

selection should be based on what bandwidth is actually required.

Shielded Twisted Pair Cable

In order to improve performance even more, shielded twisted pair cable (STP) has copper shielding

wrapped around each twisted pair and another shield wrapped around the whole cable.

This reduces electrical interference and so allows the bandwidth to be higher for any given length.

Fibre-optic Cable

Copper cable is adequate for network cable runs for up to a 100 metres, but above that the signal

becomes too weak, therefore an alternative technology is needed.

Fibre-optic cable has an astounding bandwidth, it is limited more by the electronics either side of

the cable than the bandwidth of the cable itself. For instance in recent experiment, a 160 km length

of high performance fibre-optic cable carried up to 14 Tera bits per second!

Fibre optic cable uses light to transmit information rather than electrical signals. Unlike copper cable

it is not prone to electrical interference.

Fibre optic cable works by a light signal being 'launched' at one end of the glass thread core. The

light is reflected internally down the fibre until it reaches the other end. Light sensitive electronics

then pick up the signal.

The downside of fibre is the cost - it is more expensive that ordinary UTP network cable therefore it is

only cost-effective if there is a very high bandwidth requirement or if the network has very long cable

runs.

If fibre-optic could be laid from the telephone exchange right up to the house then broadband

bandwidth of 100 Mbps is quite possible. A number of countries have now invested to make this a

reality. Perhaps the UK may one day have fibre to every house as well. At the moment fibre-to-

cabinet is the norm in the UK, where it is fibre from the exchange to the nearest junction box, then

normal copper cable to the home.


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Wireless Network

An alternative to setting up a network with copper or fibre cable is Wireless. Connection between

computer and router is achieved using radio waves.

This has the strong advantage of not requiring cables to be laid through a building. On the otherhand radio is very prone to being weakened by walls and other objects.

The bandwidth of a wireless network is lower than a physical network.

For example the 802.11g wireless standard states a theoretical bandwidth of 54 Mbits/s. In practice

the available bandwidth is much lower than this because of a weakened signal - expect about 12

Mbits/s in a typical installation covering up to 50 yards away from the wireless access point.

As I write this, my wireless connection is 13 Mbit/s and the WAP is only 20 yards away but there are

walls in the way.

However, 13 Mbit/s is fine for most purposes except for high bandwidth applications such as


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Data Transmission

Data transmission refers to the movement of data in form of bits between two or more digital devices.

This transfer of data takes place via some form of transmission media (for example, coaxial cable,

fiber optics etc.)

Distinguish between serial and parallel data transmission Reasons for choosing serial or parallel data transmission

Types of Data Transmission

1. Parallel transmission

Definition: Within a computing or communication device, the distances between different

subunits are too short. Thus, it is normal practice to transfer data between subunits using a

separate wire to carry each bit of data. There are multiple wires connecting each sub-unit and

data is exchanged using a parallel transfer mode. This mode of operation results in minimal

delays in transferring each word.

In parallel transmission, all the bits of data are transmitted simultaneously on separate

communication lines.

In order to transmit n bits, n wires or lines are used. Thus each bit has its own line.

All n bits of one group are transmitted with each clock pulse from one device to

another i.e. multiple bits are sent with each clock pulse.

Parallel transmission is used for short distance communication.

As shown in the fig, eight separate wires are used to transmit 8 bit data from sender to

receiver.


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Advantage of parallel transmission

It is speedy way of transmitting data as multiple bits are transmitted simultaneously with a single

clock pulse.

Disadvantage of parallel transmissionIt is costly method of data transmission as it requires n lines to transmit n bits at the same time.

2. Serial Transmission

Definition:When transferring data between two physically separate devices, especially if the

separation is more than a few kilometers, for reasons of cost, it is more economical to use a single

pair of lines. Data is transmitted as a single bit at a time using a fixed time interval for each bit.

This mode of transmission is known as bit-serial transmission.

In serial transmission, the various bits of data are transmitted serially one after the other.

It requires only one communication line rather than n lines to transmit data from sender to

receiver. Thus all the bits of data are transmitted on single line in serial fashion.

In serial transmission, only single bit is sent with each clock pulse.

As shown in fig., suppose an 8-bit data 11001010 is to be sent from source to destination. Then

least significant bit (LSB) i,e. 0 will be transmitted first followed by other bits. The most significant

bit (MSB) i.e. 1 will be transmitted in the end via single communication line.

The internal circuitry of computer transmits data in parallel fashion. So in order to change this

parallel data into serial data, conversion devices are used.

These conversion devices convert the parallel data into serial data at the sender side so that

it can be transmitted over single line.

On receiver side, serial data received is again converted to parallel form so that the interval

circuitry of computer can accept it

Serial transmission is used for long distance communication.

Advantage of Serial transmission

Use of single communication line reduces the transmission line cost by the factor of n as

compared to parallel transmission.

Disadvantages of Serial transmission

1. Use of conversion devices at source and destination end may lead to increase in overall

transmission cost.

2. This method is slower as compared to parallel transmission as bits are transmitted serially oneafter the other.


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Types of Serial Transmission

There are two types of serial transmission-synchronous and asynchronous both these transmissions

use 'Bit synchronization'

Bit Synchronization is a function that is required to determine when the beginning and end of the

data transmission occurs.

Bit synchronization helps the receiving computer to know when data begin and end during atransmission. Therefore bit synchronization provides timing control.

Asynchronous Transmission

Asynchronous transmission sends only one character at a time where a character is either a

letter of the alphabet or number or control character i.e. it sends one byte of data at a time.

Bit synchronization between two devices is made possible using start bit and stop bit.

Start bit indicates the beginning of data i.e. alerts the receiver to the arrival of new group of

bits. A start bit usually 0 is added to the beginning of each byte.

Stop bit indicates the end of data i.e. to let the receiver know that byte is finished, one or

more additional bits are appended to the end of the byte. These bits, usually 1s are called

stop bits.

Addition of start and stop increase the number of data bits. Hence more bandwidth is

consumed in asynchronous transmission.

There is idle time between the transmissions of different data bytes. This idle time is also known

as Gap The gap or idle time can be of varying intervals. This mechanism is called Asynchronous,

because at byte level sender and receiver need not to be synchronized. But within each

byte, receiver must be synchronized with the incoming bit stream.

Application of Asynchronous Transmission

1. Asynchronous transmission is well suited for keyboard type-terminals and paper tape devices.

The advantage of this method is that it does not require any local storage at the terminal or

the computer as transmission takes place character by character.

2. Asynchronous transmission is best suited to Internet traffic in which information is transmitted

in short bursts. This type of transmission is used by modems.


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Advantages of Asynchronous transmission

1. This method of data transmission is cheaper in cost as compared to synchronous e.g. If lines

are short, asynchronous transmission is better, because line cost would be low and idle time

will not be expensive.

2. In this approach each individual character is complete in itself, therefore if character is

corrupted during transmission, its successor and predecessor character will not be affected.

3. It is possible to transmit signals from sources having different bit rates.4. The transmission can start as soon as data byte to be transmitted becomes available.

5. Moreover, this mode of data transmission in easy to implement.

Disadvantages of asynchronous transmission

1. This method is less efficient and slower than synchronous transmission due to the overhead of

extra bits and insertion of gaps into bit stream.

2. Successful transmission inevitably depends on the recognition of the start bits. These bits can

be missed or corrupted.

Synchronous Transmission

Synchronous transmission does not use start and stop bits.

In this method bit stream is combined into longer frames that may contain multiple bytes. There is no gap between the various bytes in the data stream.

In the absence of start & stop bits, bit synchronization is established between sender &

receiver by 'timing' the transmission of each bit.

Since the various bytes are placed on the link without any gap, it is the responsibility of

receiver to separate the bit stream into bytes so as to reconstruct the original information.

In order to receive the data error free, the receiver and sender operates at the same clock

frequency.

Application of Synchronous transmission

Synchronous transmission is used for high speed communication between computers.

Advantage of Synchronous transmission

This method is faster as compared to asynchronous as there are no extra bits (start bit & stop bit)

and also there is no gap between the individual data bytes.

Disadvantages of Synchronous transmission

1. It is costly as compared to asynchronous method. It requires local buffer storage at the two

ends of line to assemble blocks and it also requires accurately synchronized clocks at both

ends. This lead to increase in the cost.

2. The sender and receiver have to operate at the same clock frequency. This requires proper

synchronization which makes the system complicated.


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Comparison between Serial and Parallel transmission

Comparison between Asynchronous and Synchronous.


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Distinguish between simplex, duplex and half-duplex data transmission

Simplex

Simplex is one direction. A good example would be your keyboard to your CPU. The CPU never needs

to send characters to the keyboard but the keyboard always sends characters to the CPU. In many

cases, Computers almost always send characters to printers, but printers usually never send

characters to computers (there are exceptions, some printers do talk back). Simplex requires only

one lane (in the case of serial).

Half-Duplex

Half-Duplex is like the dreaded "one lane" road you may have run into at construction sites. Only one

direction will be allowed through at a time. Railroads have to deal with this scenario more often since

it's cheaper to lay a single track. A dispatcher will hold a train up at one end of the single track until

a train going the other direction goes through. The only example I could think of for Half-Duplex is

actually a Parallel interface. Even though parallel is eight lanes, data travels through the lanes in the

same direction at the same time but never in both directions at the same time. The IEEE-1284 allows

printers to send messages to the computer. The printer cannot send these messages while the

computer is sending characters but when the computer stops sending characters, then the printer

can send messages back. It's kind of like some roads that head into downtown. In the morning,

they're one way roads, allowing traffic to go into downtown. In the evening their one way roads,

allowing traffic to head out of downtown. The only advantage that Half-Duplex would have is the

single lane or single track is cheaper than the double lane or double track.

Full-Duplex

Full-Duplex is like the ordinary two-lane highway. In some cases, where traffic is heavy enough, a

railroad will decide to lay a double track to allow trains to pass in both directions. In communications,

this is most common with networking. Our fiber optic hubs have two connectors on each port, one

for each lane of a two-lane roadway. Full-Duplex fiber is two cables bundled or tied together to form

the two-lane roadway. In 100Base-TX, the two lanes are housed in the same jacket. RS232 was alsodesigned to handle Full-Duplex but some of our short haul modems and converters give the user the

option to go Half-Duplex or Simplex to reduce the number of conductors needed to connect

between them.

Understanding of the need to check for errors

Error detection

It is the detection of errors caused by noise or other impairments during transmission from the

transmitter to the receiver.

Error correction

It is the detection of errors and reconstruction of the original, error-free data.

Waiting for its turn


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Transmission Errors

Transposition Errors:Errors occurred due to the misplacement of characters of a data item.

Transcription Errors:Errors occurred while typing the data / data entry / copying data from source

document into computer.

Omission Errors:Errors occurred due to the loss of character(s) or data item while transferring datafrom source to computer.

Addition Errors: Errors occurred when some character(s) that is not in the actual data/source

document is added by mistake during transferring data into the computer.

Random Errors:When the characters/units of the data item are misplaced from their actual place.

Causes of Transmission Errors

thermal noise

Impulse noise (e.g., arcing relays)

signal distortion during transmission (attenuation) crosstalk

voice amplitude signal compression (companding)

quantization noise (PCM)

jitter (variations in signal timings)

Receiver and transmitter out of synch.

Another secure-computing need is to ensure that the data has not been corrupted during

transmission or encryption. There are a couple of popular ways to do this:

Error detection

Error detection is most commonly realized using a suitable hash function (or checksum algorithm). Ahash function adds a fixed-length tagto a message, which enables receivers to verify the delivered

message by recomputing the tag and comparing it with the one provided.

There exists a vast variety of different hash function designs. However, some are of particularly

widespread use because of either their simplicity or their suitability for detecting certain kinds of errors

(e.g., the cyclic redundancy check's performance in detecting burst errors).

Checksum

Probably one of the oldest methods of ensuring that data is correct, checksums also provide a form

of authentication because an invalid checksum suggests that the data has been compromised in

some fashion. A checksum is determined in one of two ways. Let's say the checksum of a packet is

1 bytelong. A byte is made up of 8 bits, and each bit can be in one of two states, leading to a totalof 256 (28) possible combinations. Since the first combination equals zero, a byte can have a

maximum value of 255.

1. If the sum of the other bytes in the packet is 255 or less, then the checksum contains that exact

value.

2. If the sum of the other bytes is more than 255, then the checksum is the remainder of the total

value after it has been divided by 256.

Let's look at a checksum example:

Bytes total 1,151

1,151 / 256 = 4.496 (round to 4)

4 x 256 = 1,024 1,151 - 1,024 = 127 checksum


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Cyclic Redundancy Check(CRC)

CRCs are similar in concept to checksums, but they use polynomial division to determine the value

of the CRC, which is usually 16 or 32 bits in length. The good thing about CRC is that it is very accurate.

If a single bit is incorrect, the CRC value will not match up. Both checksum and CRC are good for

preventing random errors in transmission but provide little protection from an intentional attack on

your data. Symmetric- and public-key encryption techniques are much more secure.

All of these various processes combine to provide you with the tools you need to ensure that theinformation you send or receive over the Internet is secure. In fact, sending information over a

computer network is often much more secure than sending it any other way. Phones,

especially cordless phones, are susceptible to eavesdropping, particularly by unscrupulous people

with radio scanners. Traditional mail and other physical mediums often pass through numerous hands

on the way to their destination, increasing the possibility of corruption. Understanding encryption,

and simply making sure that any sensitive information you send over the Internet is secure (remember

the "https" and padlock symbol), can provide you with greater peace of mind.

How parity bits are used for error detection

Parity bits

A parity bitis a bit that is added to a group of source bits to ensure that the number of set bits (i.e.,bits with value 1) in the outcome is even or odd. It is a very simple scheme that can be used to detect

single or any other odd number (i.e., three, five, etc.) of errors in the output. An even number of

flipped bits will make the parity bit appear correct even though the data is erroneous.

Extensions and variations on the parity bit mechanism are horizontal redundancy checks, vertical

redundancy checks, and "double," "dual," or "diagonal" parity (used in RAID-DP).

Example

In order to give a better chance of errors being discovered, a block of data can have vertical as

well as horizontal checks. Complete the table by inserting the bits that should be on the bottom row.

The answer is:

1 0 0 1 1


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Error correction

Automatic repeat request (ARQ)

Automatic Repeat reQuest (ARQ) is an error control method for data transmission that makes use of

error-detection codes, acknowledgment and/or negative acknowledgment messages, and

timeouts to achieve reliable data transmission. An acknowledgment is a message sent by the

receiver to indicate that it has correctly received a data frame.

Usually, when the transmitter does not receive the acknowledgment before the timeout occurs (i.e.,

within a reasonable amount of time after sending the data frame), it retransmits the frame until it is

either correctly received or the error persists beyond a predetermined number of retransmissions.

Three types of ARQ protocols are:

Stop-and-wait ARQ,

Go-Back-N ARQ,

Selective Repeat ARQ.

ARQ is appropriate if the communication channel has varying or unknown capacity, such as is the

case on the Internet. However, ARQ requires the availability of a back channel, results in possibly

increased latency due to retransmissions, and requires the maintenance of buffers and timers forretransmissions, which in the case of network congestion can put a strain on the server and overall

network capacity.

For example, ARQ is used on shortwave radio data links in the form of ARQ-E, or combined with

multiplexing as ARQ-M.

Error-correcting code

An error-correcting code (ECC) or forward error correction (FEC) code is a system of adding

redundant data, or parity data, to a message, such that it can be recovered by a receiver even

when a number of errors (up to the capability of the code being used) were introduced, either

during the process of transmission, or on storage. Since the receiver does not have to ask the sender

for retransmission of the data, a back-channel is not required in forward error correction, and it istherefore suitable for simplex communication such as broadcasting. Error-correcting codes are

frequently used in lower-layer communication, as well as for reliable storage in media such as CDs,

DVDs, hard disks, and RAM.

Understanding of the use of serial and parallel data transmission, in Universal Serial Bus (USB)

and Integrated Circuit (IC)

I / O ports are ports and INPUT / OUTPUT devices that enable computers to connect with various

external devices through the purpose-built port-connector interfaces or devices within the casing

system are complimented on the motherboard or on a card that is inserted the slot.

Most newer motherboards have extra connectors that allow them to connect to external connectorsmounted on sheet metal brackets on the back of the computer or with special performances of

facades physical size FDD or CD player, which is installed in front of the computer case, which include

connectors for connecting USB devices , microphones, headphones, slots for inserting memory cards

and similar devices, or for an entirely different purpose, such as the regulation of rotation of the CPU

fan heatsink.

Each device communicates with the computer via electronic logic circuits that connect the device

to one of the bus system, and about the way of communication taking care program routines

incorporated as an integral part of the BIOS and the available options chipsets and motherboards

that uses the records in the BIOS , and in the end, the possibilities of your operating system.


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There are three basic ways to transfer data through the I/O ports:

1. PARALLELPIO

(Programmed Input/Output)

2. SERIALUART

(Universal Asynchronous Receiver Transmitter)

3. SERIALUSB

(Universal Serial Bus)

USB (Universal Serial Bus)

It is an advanced technological solution for connecting external devices to your computer. USB

technology is the goal of streamlining the bus master computer of the special expansion cards, as

well as to facilitate insertion and disconnecting external devices, and their automatic recognition

(plug-and-play) without the need to restart (reboot). Serial data is exchanged, and during the

development of the next version accepted:

USB is asymmetric design, and consists of a multi-server and multiple units that include the server as

a branch through special devices (hub) and thus generate the trees are rooted form. When USB ispossible to have 5 levels of branching for each controller server, you can connect 127 devices less

the number of hub-ins that are attached to the same USB server. Currently there are two standard

USB connectors:

Standard USB connector

Mini USB connector

The theoretical maximum data rate in USB 2.0 is 480 Mbit/s (60 MB/s)per controller and is shared

amongst all attached devices.

For USB 3.0, typical write speed is 7090 MB/s, while read speed is 90110 MB/s.

Integrated Circuit

Communications between devices such as computers and printers and across networks is

performed adequately with serial lines. Data transmission over much shorter distances inside

integrated circuits (ICs) is best done using parallel channels. ICs employ three buses for

communication:

Data bus: carries data between memory and processor and also between ports and other parts e.g.

sound card, graphics card, have own processors and memory and some data are moved to

processor and main memory; bi-directional, data flow to processor and back to memory.

Address bus: carries address from processor to location in main memory (some ports may be

memory-mapped so have addresses too); unidirectional, no need for an address to flow back toprocessor.

Control bus: carries signals from processor to parts of CPU and system to activate something e.g. to

read or write data to a memory location or I/O device.

These buses use parallel channelsrather than single, serial ones. Speed and throughput of data are

of prime concern in IC design so it makes sense to maximise this with as many lines as possible.


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How Parallel Ports Work

If you have a printer connected to your computer, there is a good chance that it uses the parallel

port. While USB is becoming increasingly popular, the parallel port is still a commonly used interface

for printers.

Parallel ports can be used to connect a host of popular computer peripherals:

Printers

Scanners CD burners

External hard drives

Iomega Zip removable drives

Network adapters

Tape backup drives

How Serial Ports Work

The external connector for a serial port can be either

9 pins or 25 pins. Originally, the primary use of a serialport was to connect a modem to your computer.

The pin assignments reflect that. Let's take a closer

look at what happens at each pin when a modem is

connected.

9-pin connector:

1. Carrier Detect - Determines if the modem is

connected to a working phone line.

2. Receive Data - Computer receives information

sent from the modem.

3. Transmit Data - Computer sends information to

the modem.4. Data Terminal Ready - Computer tells the

modem that it is ready to talk.

5. Signal Ground- Pin is grounded.

6. Data Set Ready- Modem tells the computer that

it is ready to talk.

7. Request To Send- Computer asks the modem if it can send information.

8. Clear To Send- Modem tells the computer that it can send information.

9. Ring Indicator - Once a call has been placed, computer acknowledges signal (sent from

modem) that a ring is detected.

25-pin connector:

1. Not Used2. Transmit Data- Computer sends information to the modem.

3. Receive Data- Computer receives information sent from the modem.

4. Request To Send- Computer asks the modem if it can send information.

5. Clear To Send- Modem tells the computer that it can send information.

6. Data Set Ready- Modem tells the computer that it is ready to talk.

7. Signal Ground- Pin is grounded.

8. Received Line Signal Detector- Determines if the modem is connected to a working phone

line.

9. Not Used: Transmit Current Loop Return (+)

10. Not Used

11. Not Used: Transmit Current Loop Data (-)

12. Not Used13. Not Used

14. Not Used

15. Not Used

An example of a parallel port on the back of a desktop computer.


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16. Not Used

17. Not Used

18. Not Used: Receive Current Loop Data (+)

19. Not Used

20. Data Terminal Ready- Computer tells the modem that it is ready to talk.

21. Not Used

22. Ring Indicator - Once a call has been placed, computer acknowledges signal (sent from

modem) that a ring is detected.23. Not Used

24. Not Used

25. Not Used: Receive Current Loop Return (-)

Voltage sent over the pins can be in one of two states, Onor Off. On (binary value "1") means that

the pin is transmitting a signal between -3 and -25 volts, while Off (binary value "0") means that it is

transmitting a signal between +3 and +25 volts...

How USB Ports Work

The goal of USB is to end all of these headaches. The Universal Serial Bus gives you a single,

standardized, easy-to-use way to connect up to 127 devices to a computer.

Just about every peripheral made now comes in a USB version. A sample l ist of USB devices that you

can buy today includes:

Printers

Scanners

Mice

Joysticks

Flight yokes

Digital cameras

Webcams

Scientific data acquisition devices

Modems

Speakers Telephones

Video phones

Storage devices

Network connections

USB Cables and Connectors

Connecting a USB device to a computer is simple -- you find the USB connector on the back of your

machine and plug the USB connector into it.

If it's a new device, the operating system auto-detects it and asks for the driver disk. If the device has

already been installed, the computer activates it and starts talking to it. USB devices can be

connected and disconnected at any time.

A typical USB connector, called an "A" connection

Many USB devices come with their own built-in cable, and the cable has an "A" connection on it. If

not, then the device has a socket on it that accepts a USB "B" connector.

A typical USB connector, called an "A" connection


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AHMED THAKUR1.2 COMMUNICATION AND INTERNET TECHNOLOGIES

A typical "B" connection

The USB standard uses "A" and "B" connectors to avoid confusion:

"A" connectors head "upstream" toward the computer.

"B" connectors head "downstream" and connect to individual devices.

By using different connectors on the upstream and downstream end, it's impossible to ever get

confused -- if you connect any USB cable's "B" connector into a device, you know that it'll work.

Similarly, you can plug any "A" connector into any "A" socket and know that it'll work.

A typical "B" connection

1.2.1 data transmission

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