Data Transmission

Data Transmission

Outline

• Types and methods of data transmission

• Methods of error detection

• Encryption

Why this topic?

Data is frequently transferred from one device to another. The two

devices could be in the same building or thousands of kilometres away. Irrespective of the distance travelled, the transmission of data needs to be considered with respect to:

• how the data is transmitted

• How can errors following transmission be detected and can the data be recovered

• the role of encryption to make sure data that falls into the wrong hands can’t be used

Types and methods of data transmission

Poll

Is it possible to send a big file like 100GiB within a second?

• Yes

• No

Open Ended

Why do you think is impossible

Packet

• A small unit of data

• A small part of a message/data that is transmitted over a network; after transmission all the data packets are reassembled to form the original message/data.

• The packets of data are usually quite small, typically 64 KiB,

  which are much easier to control than a long continuous stream of data.

Packet Structure

A typical packet is split up into:

• header

• Payload

• Trailer

​Packet header

This consists of

• the IP address of the sending device

• the IP address of the receiving device

• the sequence number of the packet (this is to ensure that all the packets can be reassembled into the correct order once they reach the destination)

• packet size (In bytes)
  

​Payload

This is the actual data being sent in the packet.

Word Cloud

Describe one or two words that come to mind when you think about the word 'trailer' in relation to the way you watch movies.

Multiple Choice

What is the purpose of dividing data into packets during transmission?

• To make the data transmission slower

• To reduce the chances of successful transmission

• To improve the reliability and efficiency of data transmission

• To increase the cost of network infrastructure

Multiple Choice

In a network, what is the purpose of the header in a data packet?

• It contains the actual data being transmitted

• It provides information about the destination and source of the packet

• It is a security feature to encrypt the data

• It is unnecessary and can be omitted for faster transmission

Multiple Choice

What is a data packet in the context of data transmission?

• A physical cable used to connect devices

• A unit of data that is transmitted over a network

• A type of encryption algorithm

• A software application for data storage

Multiple Choice

What happens if a data packet is lost during transmission

• The entire message is lost

• The destination automatically requests retransmission

• The missing packet is ignored, and the rest are delivered

• The network crashes

Multiple Choice

Which part of a packet is also known as a footer?

• Packet header

• Trailer

• Payload

Multiple Choice

Which part of a packet has information about the IP address of the sending and receiving device?

• The packet header

• The Payload

• Trailer

Multiple Choice

Which part of the packet check for errors?

• Packet header

• Payload

• Trailer

Packet Switching

Packet Switching

• A method of transmitting data packets accross the network.

• Each data packet is able to take an individual pathway across the network.

• Router : A device that controls which pathway will be used to transmit each packet.

• There is no need to tie up a single communication line

• It is relatively easy to expand usage

• A high transmission rate is possible

• It is possible to overcome failed/busy lines by re-routing packets

Benefits of packet switching

Drawbacks of packet switching

• Packets lost need to be re-sent

• Prone to errors with real-time streaming

• Delay when packets are re-ordered.

Multiple Choice

What is the role of routers in a packet-switched network?

• Transmit voice data

• Establish dedicated circuits

• Forward packets between networks

• Provide encryption for data

Multiple Choice

What is packet switching?

• Method of grouping data transmitted over a network into packets
• Converting data into sound waves for transmission
• Sending data through physical mail
• Using Morse code to transmit information

Multiple Choice

Which of the following is a characteristic of packet switching?

• Data is sent as a whole without breaking into packets
• Data is broken into packets and sent independently
• Packets are reassembled at the destination before being processed
• Packet switching is slower than circuit switching

Multiple Choice

In packet switching, what happens if a network node receives more packets than it can transmit at once?

• It discards the excess packets immediately
• It sends an error message to the sender and drops the excess packets
• It slows down the transmission speed to accommodate the excess packets
• It stores the excess packets in a buffer until it can transmit them.

Multiple Choice

What is a packet in the context of packet switching?

• A unit of data transmitted over a network
• A type of computer virus
• A form of currency used in online transactions
• A type of hardware used in networking

Data Transmission

Data transmission can be either over a short distance (for example, computer to printer) or over longer distances (for example, from one computer to another in a global network).
Essentially, three factors need to be considered when transmitting data:
1. The direction of data transmission (for example, can data transmit in one direction only, or in both directions)
2. the method of transmission
3. how will data be synchronised - To make sure that the data arrives in a correct order

Methods of Data Transmission

​Simplex data transmission

Simplex mode occurs when data can be sent in ONE DIRECTION ONLY (for example, from sender to receiver). An example of this would be sending data from a computer to a printer.

Half-duplex data transmission

Half-duplex mode occurs when data is sent in BOTH DIRECTIONS but NOT AT THE SAME TIME
An example of this would be a walkie-talkie where a message can be sent in one direction only at a time; but messages can be both received and sent.

Full-duplex data transmission

Full-duplex mode occurs when data can be sent in BOTH DIRECTIONS AT THE SAME TIME
An example of this would be a broadband internet connection.

Serial data transmission

Serial data transmission occurs when data is sent ONE BIT AT A TIME over a SINGLE WIRE/CHANNEL.
Examples are like USB and Bluetooth

Parallel data transmission

Parallel data transmission occurs when SEVERAL BITS OF DATA (usually one byte) are sent down SEVERAL CHANNELS/WIRES all at the same time. Each channel/wire transmits one bit

Word Cloud

Which method of data transmission sends data along a single wire?

Word Cloud

Which method of data transmission sends data multiple bits at a time?

Word Cloud

If a data transmission connection sends data one bit at a time, in both directions, but not at the same time, What kind of data transmission methods are used?

Pair Work (5 Minutes)

Attempt Question number 11 from your course book.

Individual Work

1. Explain the followings

• USB port

• USB Cable

• USB Connection

• USB Device

2. Give two advantages and two disadvantages of USB Interface

Homework

Question 12 - 16 from your course book

Multiple Choice

Packets have a header which contains the IP address of the sender and

the receiver

• TRUE

• FALSE

Multiple Choice

Packets don’t require any form of error checking

• TRUE

• FALSE

Multiple Choice

USBs use a protocol that allows for error-free data transmission between

device and computer

• TRUE

• FALSE

Multiple Choice

Serial data transmission suffers from data skewing

• TRUE

• FALSE

Multiple Choice

The longest cable length supported by USB is 5 metres or less

• TRUE

• FALSE

Multiple Choice

Simplex data transmission occurs when data is transmitted one bit at a time

• TRUE

• FALSE

Multiple Choice

USB uses serial data transfer

• TRUE

• FALSE

Multiple Choice

USB uses serial data transfer

• TRUE

• FALSE

Can you raed tihs?

“I cnduo’t bvleiee taht I culod aulaclty uesdtannrd waht I was rdnaieg.

Unisg the icndeblire pweor of the hmuan mnid, aocdcrnig to rseecrah at

Cmabridge Uinervtisy, it dseno’t mttaer in waht oderr the lterets in a

wrod are, the olny irpoamtnt tihng is taht the frsit and lsat ltteer be in the

rhgit pclae. The rset can be a taotl mses and you can sitll raed it whoutit a

pboerlm.

Tihs is bucseae the huamn mnid deos not raed ervey ltteer by istlef, but

the wrod as a wlohe.

Aaznmig, huh? Yeah and I awlyas tghhuot slelinpg was ipmorantt! See if

yuor fdreins can raed tihs too”

(from an unknown source at Cambridge University)

Methods of error detection

The need to check for errors

Errors can occur during data transmission due to:

1. Interference

2. problems during packet switching

Methods of checking errors

1. parity checks

2. checksum

3. echo check.

1. Parity checks

• Parity checking is one method used to check whether data has been changed or corrupted following data transmission. This method is based on the number of 1-bits in a byte of data.

• The parity can be either called EVEN (that is, an even number of 1-bits in the byte) or ODD (that is, an odd number of 1-bits in the byte).

• The left most bit is reserved for a parity bit.

Consider this example

Byte

Odd

Even

Since there are 4 ones odd parity is 1 and even parity is 0

​	​	​	​	​	​	​	​
​	​	​	​	​	​	​	​
​0	1​	​1	​0	​1	1​	​0	0

Find the parity bits for each of the following bytes:
The parity bit is denoted using letter x

Identifying errors

If odd parity is used there is no error if and only if the received packet has odd parity, otherwise there is an error.
The same applies to even

Which of the following received bytes indicate an error has occurred following data transmission?

Multiple Choice

1 1 1 0 1 1 0 1

even parity being used

• No error

• Error

Multiple Choice

0 1 0 0 1 1 1 1

even parity being used

• No error

• Error

Multiple Choice

0 0 1 1 1 0 0 0

even parity being used

• No error

• Error

Multiple Choice

1 1 1 1 0 1 0 0

odd parity being used

• No error

• Error

Multiple Choice

1 1 0 1 1 0 1 1

odd parity being used

• No error

• Error

Multiple Choice

1 1 1 1 1 1 1 1

odd parity being used

• No error

• Error

Multiple Choice

0 0 0 0 0 0 0 0 even parity being used

• No error

• Error

Multiple Choice

1 1 1 0 0 0 0 0

odd parity being used

• No error

• Error

Multiple Choice

0 1 0 1 0 1 0 1

even parity being used

• No error

• Error

Multiple Choice

1 1 1 0 0 0 1 1

odd parity being used

• No error

• Error

2. Check sum

A checksum is a method used to check if data has been changed or corrupted following data transmission. Data is sent in blocks, and an additional value, called the checksum, is sent at the end of the block of data.
The checksum is then compared from the receiving and the sending side. if they are the same indicates that there is no error. otherwise there is an error in sending the files.

3. Echo check

With echo check, when data is sent to another device, this data is sent back again to the sender. The sender’s computer compares the two sets of data to check if any errors occurred during the transmission process.

4.Automatic Repeat Requests (ARQs)

• A type of error detection method that uses acknoledgement and timeout to see if data has arrived correctly after transmission.

• Acknowledgement: A message that is sent from one device to another to indicate whether the data is received correctly.

• timeout: A period of time that is set and used to wait for an acknowledgement to be received.
  

Check digits

A check digit is the final digit included in a code; it is calculated from all the other digits in the code. Check digits are used for barcodes on products, such as International Standard Book Numbers (ISBN) and Vehicle Identification Numbers (VIN)

Homework

1. Explain what is the difference between positive and negative acknowledgement method in ARQ

2. Exam style questions Question number six (Page 51)

Multiple Choice

Some error detection methods use a calculated value to check for errors.

Select one box to show which error detection method does not use a calculated value to

check for errors.

• Check digit

• Checksum

• Echo check

• Parity check

Multiple Choice

The data is transmitted one bit at a time down a single wire. The computer can transmit data

to the printer and the printer can transmit data to the computer, using the same connection.

Select two data transmission methods that will transmit data in this way.

• parallel full-duplex

• parallel half-duplex

• parallel simplex

• serial full-duplex

• serial half-duplex

Encryption

A method of securing data for storage or transmission that scrambles it and make it meaningless.
encryption helps to minimize the risk of loosing data from hackers.

Plaintext and ciphertext

• The original data being sent is known as plaintext. Once it has gone through an encryption algorithm, it produces ciphertext:

Symmetric encryption

• Symmetric encryption uses an encryption key; the same key is used to encrypt and decrypt the encoded message.ciphertext:

• The real difficulty is keeping the encryption key a secret (for example, it needs to be sent in an email or a text message which can be intercepted). Therefore, the issue of security is always the main drawback of symmetrical encryption, since a single encryption key is required for both sender and recipient.

Asymmetric encryption

• Asymmetric encryption was developed to overcome the security problems associated with symmetric encryption. It makes use of two keys called the public key and the private key:

• public key (made available to everybody)

• private key (only known to the computer user).

Multiple Choice

What is meant by the term ciphertext when used in encryption?

• an encryption or decryption algorithm

• an encrypted message

• a type of session key

• another name for plaintext

• text following an encryption algorithm

Multiple Choice

When carrying out asymmetric encryption, which of the following users would

keep the private key?

• the sender

• the receiver

• both sender and receiver

• all recipients of the message

Multiple Choice

In encryption, which of the following is the term used to describe the message

before it is encrypted?

• simpletext

• plaintext

• notext

• firsttext

• ciphertext

Multiple Choice

Which of the following is the biggest disadvantage of using symmetric encryption?

• it is very complex and time consuming

• it is rarely used anymore

• the value of the key reads the same in both directions

• it only works on computers with older operating systems

• there is a security problem when transmitting the encryption key

Multiple Choice

Which of the following is the correct name for a form of encryption in which

both the sender and the recipient use the same key to encrypt and decrypt?

• symmetric key encryption

• asymmetric key encryption

• public key encryption

• same key encryption

Multiple Choice

What of the following is the final number in a code, which is calculated from all

the numbers in the code; its purpose is to find errors in data entry?

• parity check

• checksum

• check digit

• parity bit

Multiple Choice

Which of the following is a form of error detection that makes use of a system

of acknowledgements and timeouts?

• automatic repeat request

• parity bit

• echo check

• parity bit

Multiple Choice

Which of the following methods uses an extra bit added to a byte to ensure it

contains an even number of 1s or odd number of 1s?

• cyclic redundancy check

• check digit

• parity check

• echo check

Multiple Choice

Which of the following uses a calculated value which is sent after a block of

data; the receiving computer also calculates the value from the block of data

and compares the values?

• parity check

• check digit

• automatic repeat request

• checksum

Multiple Choice

Which of the following describes the check where the receiving computer

sends back a copy of the data to the sending computer to allow it to compare

the data?

• echo check

• automatic repeat request

• checksum

• check digit