Cho 2

Question 1

Data sent over long distances usually is broken up into

Answer 1

Data packets

Question 2

Advantages of data packets

Answer 2

Small which is easier to control than a long continuous stream of data
It’s also a benefit if the route is busy

Question 3

Packet structure

Answer 3

Packet header
Payload
Trailer

Question 4

Header

Answer 4

IP address of the sending and receiving device
The sequence number
Packet size

Question 5

Payload

Answer 5

Consists of the actual data in the packets

Question 6

Trailer

Answer 6

Some way of identifying the end of packer this is the essential to allow each packet to be separated from each other as they travel from sending to receiving station
And an error checking method to ensure the packet arrived error free

Question 7

Cyclic redundancy checks (CRCs)

Answer 7

This involves the sending computer adding up all the 1 bit in the payload and storing this as a hex value in the trailer before it is sent
Once the packet arrives the receiving computer recalculates the number of 1 bits in the payload
The computer then checks this value against the one sent in the trailer
If the two values match then no transmission errors have occurred otherwise the packet needs to be re sent

Question 8

Packet switching example for a photograph going from two computers

Answer 8

I sent for. Computer A to computer B. The photograph will be split up into a number of packets before it’s sent. There will be several possible routes for the packets between computer a sender and computer B receiver . Each stage in the route contains a routers. A router receives a data packet and based on the information in the header decided where to send it next

Question 9

Job of a router

Answer 9

Decides where to send it next

Question 10

Packet switching

Answer 10

Method of data transmission in which a message is broken up into a number of pakcets. Each packet can then be sent independently from start point to end point. At the destination the packets will need to be reassembled into their correct order. At each stage there are nodes that contain a router. Each router will determine which route the packet needs to take, in order to reach its destination

Question 11

Order of packet switching

Answer 11

Each packet will follow its own path or route
Routers will determine the route of each packet
Routing selection depends on the number of packets waiting to be processed at each node
The shortest possible path available is always selected this may not always be the shifters path that could be taken since certain parts of the route may be too busy or not suitable
Unfortunately packets can reach the destination in a different order to that in which they were sent

Question 12

Benefits of packet switching are

Answer 12

There is no need to tie up a single communication line
It is possible to overcome failed, busy or faulty lines by sim plying re routing packets
It is relatively easy to expand package usage
A high data transmission rate is possiblem

Question 13

Drawbacks of packet switching

Answer 13

Packets can be lost and need to be resent
The method is more prone to error with real time streaming
There is a delay at the destination whilst the packets are being re ordered

Question 14

Why can packets get lost

Answer 14

Because they keep bouncing around from router to router and never actually reach their destination. Eventually the network would just grind to a halt as the number of lost packets mount up clogging the system

Question 15

How can you overcome hopping

Answer 15

A Hop number is added to the header of each packet and this number is reduced every time it reaches a router each packet has. A max hop number to start with. Once the hoo number i]recahes 0nand the packet hasn’t reached their destination the packet is deleted

Question 16

Data transmission

Answer 16

Can be either over a short distance or over longer distances

Question 17

Three factors of data transmission

Answer 17

The direction of data transmission
The method of transmission
How will data be synchronised
These factors are considered by a communication protocol

Question 18

Simplex data transmission

Answer 18

Simplex mode occurs when data can be sent in ONE DIRECTION ONLY ex sending data from a computer to a printer

Question 19

Half duplex

Answer 19

Data is sent in BOTH DIRECTIONS BUT NOT AT THE SAME TIME ex walkie talkie where a message can be sent in one direction only at a time but messages can be both received and send

Question 20

Full duplex

Answer 20

BOTH DIRECTUONS AT THE SAME TIME
Ex board band internet connection

Question 21

Serial data transmission

Answer 21

Occurs when data is sent One bit at a time over a single wire / channel

Question 22

Serial data transmission feuaturee

Answer 22

Works well over long distances
Data is transmitted slower than parallel data transmission
Because only one channel wire is used data will arrive at its destination fully sync ex connecting a computer to printer via usb

Question 23

Parallel data transmission

Answer 23

Occurs when several bits of data are sent down several channels wires all at the same time

Question 24

Parallel data features

Answer 24

Works well over short distances
Over long distances data can become skewed and hits can arrive out to order
The longer the wure the worse it can become
Faster method of data transmission than serial.
T

Question 25

Components of serial

Answer 25

Less risk of external interference than with parallel due to fewer wires More reliable transmission over longer distances Transmitted bits with have the risk of being skewed Used if the amount of data being sent is relatively small since transmission rate is slower than parallel Used to send data over long distance Less expensive

Question 26

Parallel features compared to serial

Answer 26

Faster rate of data transmission than serial Works well over shorter distances Since several channels wired used to transmit data, the bits can arrive out of synchronisation Preferred method when speed is important If data is time sensitive parallel is the most appropriate transmission method Parallel ports require more hardware making then more expensive Easier to program in out or output overwritten

Question 27

Universal Serial Bus USB

Answer 27

Form of serial data transmission. USB is the most common type of input output port found on computers and has led to a standardisation method for transfer of data between devices and a computer.

Question 28

USB allows both

Answer 28

Half duplex Full duplex

Question 29

How does it work when a device is plugged into a computer using ine of the USB PORTS

Answer 29

The computer automatically detects that a device is present this is due to a small change in the voltage on the data signal wires in the USB cable. The device is automatically recognised and the appropriate device driver software is loaded up so that the computer and device can communicate effectively If a nee device is detected the computer will look for th4 device driver matches the device if this is not available the user is promoted to download the appropriate driver software

Question 30

Benefits of USB systems

Answer 30

Devices plugged into the computer are automatically detected and device drivers are automatically loaded up Connections can only fit one way preventing incorrect connections being made It has become an industry standard which means considerable support is available Can supportmdifferent data transmission rates No need to external power source since cable supplies 5v power USB protocols notifies the transmitter to re transmit data if any error are detected this leads to error free data transmission It is relatively easy to add more USB ports if necessary by using USB hubs USB is backward compatible

Question 31

Drawbacks of USB systems

Answer 31

Standard usb only supports a maximum cable length of 5m beyond that usb hubs are needed to extend the cable length Even though USB is backwards compatible very early USB standards may not always be supported by the latest computers Even the latest version have a data transfer rate which is slow compared to eternal connections

Question 32

The need to check for errors

Answer 32

Errors can occur during data transmissions due to Interference Problems during packet switching Skewing of data If words are not recognised - data corruption

Question 33

Parity checks

Answer 33

Parity checking is one method used to check whether data has been changed or corrupted following data transmission. The parity can be either odd or even. The number of 1s one of the buts in the byte is reserved for a parity bit. The parity bit is set according to whether the parity being used is even or odd.

Question 34

Parity blocks

Answer 34

This method of a block of data is sent and the number of q bits are horizontally and vertically identical Ed’s where the error is. Agreement has to be made

Question 35

Checksum

Answer 35

Method used to check if data has been changed or corrupted following data transmission. Data is sent in blocks and an additional value

Question 36

Checksum

Answer 36

Method used to check if data has been changed or corrupted following data transmission. Data is sent in blocks and an additional value

Question 37

Checksum process

Answer 37

When a block of data is about to be transmitted the checksum is calculated from the block of data The calculation is done using an agreed algorithm which has been agreed by a sender and receiver The checksum is then transmitted with the block of data At the receiving end, the checksum is recalculated by the computer using the block of data The recalculated checksum is then compared to the checksum dent with the data block if the two checksums match then no transmission errors have occurred otherwise a request is made to resend a block of data

Question 38

Echo check

Answer 38

When data is sent to another device this data is sent back again to the sender. The senders computer check the two sets of data to check is any errors occurred during the transmission process Data is sent a copy of the data sent is returned to the reader and it’s resent if any errors

Question 39

Check digit

Answer 39

Final digit included in a code it is calculated from all the other digits in the code. Check digits bare used for barcodes on products such as international standard book numbers and vehicle identification numbers. They are used to identify Nero’s in data entry caused by mid type ing or mis scanning a barcode an incorrect digit entered for ed 5327 instead of 5306ntwo numbers changed in order or phonetic errors

Question 40

Method for check digit

Answer 40

Add all the odd numbered digits tigether Add all the even numbered digits tigether and multiply by the result by 3 Add all the results from 1 and 2 together and divide by 10 Take the remainder if it is zero then use this value otherwise subtract the remainder from 1h to find the check digit

Question 41

2nd check digit method

Answer 41

Each digit in the number is given a weight Multiply the number and weight Divide by 11 Number is correct if there are no reminders

Question 42

Automatic repeat request ARQ

Answer 42

ARQ uses positive and negative acknowledgements (messages sent to the receiver indicating that data has/has not been received correctly) and timeout (this is the time interval allowed to elapse before an acknowledgement is received) » the receiving device receives an error detection code as part of the data transmission (this is typically a Cyclic Redundancy Check - refer to Section 2 this is used to detect whether the received data contains any transmission e » if no error is detected, a positive acknowledgement is sent back to the sending device » however, if an error is detected, the receiving device now sends a negative acknowledgement to the sending device and requests re-transmission of the » a time-out is used by the sending device by waiting a pre-determined are of time ... " .. and if no acknowledgement of any type has been received by the sendi device within this time limit, it automatically re-sends the data until a positive acknowledgement is received .... » ... or until a pre-determined number of re-transmissions has taken place » ARQ is often used by mobile phone networks to guarantee data integrity-ARQ uses positive and negative acknowledgements (messages sent to the receiver indicating that data has/has not been received correctly) and timeout (this is the time interval allowed to elapse before an acknowledgement is received) » the receiving device receives an error detection code as part of the data transmission (this is typically a Cyclic Redundancy Check - refer to Section 2 this is used to detect whether the received data contains any transmission e » if no error is detected, a positive acknowledgement is sent back to the sending device » however, if an error is detected, the receiving device now sends a negative acknowledgement to the sending device and requests re-transmission of the » a time-out is used by the sending device by waiting a pre-determined are of time ... " .. and if no acknowledgement of any type has been received by the sendi device within this time limit, it automatically re-sends the data until a positive acknowledgement is received .... » ... or until a pre-determined number of re-transmissions has taken place » ARQ is often used by mobile phone networks to guarantee data integrity-

Question 43

Purpose of encryption

Answer 43

When data is transmitted over any public network (wired or wireless), there is always a risk of it being intercepted by, for example, a hacker. Under these circumstances, a hacker is often referred to as an eavesdropper. Using encryption helps to minimise this risk. Encryption alters data into a form that is unreadable by anybody for whom the data is not intended. It cannot prevent the data being intercepted, but it stops it from making any sense to the eavesdropper. This is particularly important if the data is sensitive or confidential (for example, credit card/bank details, medical history or legal documents).

Question 44

Purpose of encryption

Answer 44

When data is transmitted over any public network (wired or wireless), there is always a risk of it being intercepted by, for example, a hacker. Under these circumstances, a hacker is often referred to as an eavesdropper. Using encryption helps to minimise this risk. Encryption alters data into a form that is unreadable by anybody for whom the data is not intended. It cannot prevent the data being intercepted, but it stops it from making any sense to the eavesdropper. This is particularly important if the data is sensitive or confidential (for example, credit card/bank details, medical history or legal documents).

Question 45

The original data being sent is known

Answer 45

Plaintext

Question 46

The original data being sent is known

Answer 46

Plaintext

Question 47

Once plain text has gonna through encryption algorithm it produces

Answer 47

Cipher text

Question 48

Symmetric encryption

Answer 48

uses an encryption key; the same key is used to encrypt and decrypt the encoded message. First of all, consider a simple system that uses a 10-digit denary encryption key (this gives 1 × 1010 possible codes); and a decryption key.

Question 49

Ex of symmetric encryption Suppose our encryption key is: 4291362856

Answer 49

Suppose our encryption key is: 4291362856 which means every letter in a word is shifted across the alphabet +4, +2, +9, +1, and so on, places. For example, here is the message COMPUTER SCIENCE IS EXCITING (plaintext on the top line of Figure 2.19) before and after applying the encryption key (for I

Question 50

To get back to the original message, from encryption

Answer 50

it will be necessary t decryption key; that is, 42 9 1 3 6 2 8 5 6. But in this ca process would be the reverse of encryption and each lette pply the same the decryptio ould be shifte -2, -9, -1, and so on.

Question 51

Modern computers can

Answer 51

crack this encryption 1 e seconds. To try to combat this, we now use 256-bit binary in a matter of give 2' (approximately, 1.2 x 10") possible combinations acryption keys enough as we head towards quantum computers.)

Question 52

Why is difficult keeping encryption a secret

Answer 52

Security issues it can be sent which can be intercepted

Question 53

Asymmetric encryption

Answer 53

Develop to be more secure

Question 54

Asymmetric

Answer 54

Asocited inen symetic encryption. It makes use of i key ale i asymmetric encryption was developea to overcome th ecurity problems key and the private key: » public key (made available to everybody) private key (only known to the computer user).

Question 55

How does asymmetric work

Answer 55

Both types of key are needed to encrypt and decrypt messages. We will use an example to explain how this works; suppose Tom and Jane me the same company and Tom wishes to send a confidential document to die 1 Jane uses an algorithm to generate a their computer of keys (private ar public) that they must keep stored on their computers; the matching pt keys are mathematically linked but can't be derived from each other. 2 Jane now sends her public key to Tom. Jane sends Tom Tom Jane her public key —0 Public key - Public key -0 Private key Figure 2.20 Jane sends Tom her public key 3 Tom now uses Jane's public key (~o) to encrypt the document he wishes send to her. He then sends his encrypted document (ciphertext) back to i Tom Tom sends Jane +-0 Public key an encrypted document Jane #-0 Public key -0 Private key A Figure 2.21 Encrypted document sent from Tom to Jane 4 Jane uses her matching private key (-o) to unlock Tom's document and decrypt it; this works because the public key used to encrypt the docume and the private key used to decrypt it are a matching pair generated on i computer. (Jane can't use the public key to decrypt the message.)