Data transmission

Why do we need to check for errors during data transmission?

As errors can occur during data transmission due to interference which can cause data loss, data gain and data change.

Parity Check

The numbers of 1s are counted in each byte

The parity bit is added to each byte before transmission to make the sum of the bits in each byte odd or even

After the transmission if it turns changes from even or odd to odd or even an error is detected

Otherwise no error is detected

Checksum

Value calculated using the other elements in the data stream

Value transmitted at the end of a block of data

Value recalculated after transmission

Values compared after transmission to check for error

Echo check

Data sent is transmitted back

Both sets of data compared

If they are identical there is no error

If not then error is detected

ARQ

Error checking protocol
Timer is started when sending device transmits a data packet to receiver
Receiving device checks packet data for errors
If error detected then request sent to resend data and negative acknowledgement is used
Resend request is repeated until data is sent correctly or it reaches its limit
Sends acknowledgement that data is received using positive acknowledgement
If acknowledgement not received in time timeout occurs data is resent until acknowledgement received or maximum number of attempts reached

Check digit

Digit is calculated from the data

It is appended to the data

Digit is recalculated when data is entered

Digits compared to check for errors

Examples of where check digit is used

International standard book numbers

Bar codes

Positive ARQ only

Error checking protocol
Timer is started when sending device transmits a data packet to receiver
Receiving device checks packet data for errors
If there are no error positive acknowledgement is sent

If error detected then no acknowledgement sent
If acknowledgement not received in time timeout occurs data is resent until acknowledgement received or maximum number of attempts reached

Parity block check

Parity byte is added to the end of the data so if the data was arranged in a grid the number on ones in each column and row would be odd/even,

This allows the exact location of the error to be pinpointed as the location in which the error has occurred’s row and column will be detected

Need and purpose of encryption

Protects from unautorised access by making it meaningless

Makes it unable to be deciphered even if intercepted

Encryption

Jumbling up or scrambling data so that the message makes no sense

Plain text

data that is not encrypted

Cypher text

Data that has been encrypted

How is data encrypted symmetrically

Encryption key is used

Encryption algorithm is used

Encryption key/ algorithm is applied to plain text to convert it so cypher text

Same key is used to encrypt and decrypt data

Key

Binary string of certain length that when applied to an encryption algorithm can encrypt plain text and decrypt cipher text

Asymmetric encryption

Public and private key

Public key is known to everyone and used to encrypt messages

Private key is known only to receiver and decrypts messages

Sender sends data encrypted by the public key

Receiver receives data decrypted by the private key only known to them

Why are data packets used?

Faster to route

Packet switching stages

1. Data is broken down into packets 2. Packets are assigned a header 3. Each packet makes its way to the destination 4. Routers control the route for each packet 5. Packets arrive and are reordered accordingly

Full duplex

Data can travel in both directions

USB is serial or parallel?

Serial

Advantages of packet switching

Minimal interference and corruption as individual packets can be resent if lost or damaged

Saves time and bandwidth as whole file doesn't have to be resent in case of corruption

Quicker as each packet finds the fastest way around the network

Harder to hack as each packet travels across the network separately with minimal data

USB drawbacks

Only around 5 meters cable length

Older USB have limited transfer rates

Very old USB may not be supported in the future

USB benefits

Devices are automatically detected and drivers are automatically loaded for communication

Cable connectors fit only one way, preventing incorrect connection ensuring compatible data transmission

USB usage is standardized, a lot of support from online and retailers

High speed transmission

USB

Widely used standard for data transmission

Trailer contains

Additional security information

End of packet notification

What are the three main components of a packet?

Header, payload, and trailer

Data transmission

The process of sending data from one device to the other both with wires and wirelessly

Disadvantages of Parallel: Skew

Skew is the difference in arrival times of parallel data bits, causing timing misalignment

Disadvantages of Parallel: Crosstalk

Crosstalk is the unwanted coupling of a signal from one transmission line into another, causing interference and signal degradation

Examples of full Duplex

Local and wide area networks

Advantages of Half Duplex

Cheaper than full duplex for 2 way communication

Disadvantages of Parallel

Skew (difference in arrival times of parallel data bits, causing timing misalignment)

Crosstalk (unwanted coupling of a signal from one line to another, causing interference)

Expensive

Bandwidth

Maximum data which can be sent over a network connection over a period of time

Header contains

Source IP address

Destination IP address

Packet number

What does TCP stand for?

Transmission Control Protocol

Examples of Simplex

From computer to monitor

Wired data transmission

Serial (stream of bits sent along a single wire)

Parallel (multiple bits sent simultaneously, each in its own wire)

Payload contains

The actual data being transferred

Advantages of Simplex

Low cost

More simple

Example of serial transmission

USB

Examples of parallel transmission

Traditional printer cable

Examples of Half duplex

Printer cable which waits for data to be received before sending low ink message

Corruption

Data is changed or lost in some way, or data is gained that originally was not in the packet

Simplex

Data travels in one direction

Advantages of Full Duplex

Faster, 2 way simultaneous data transmission

What are packets?

Small chunks of data that make up a larger piece of data that has been broken down by the TCP protocol so that it can be transmitted over the internet

Wired data transmission: Serial

Stream of bits is sent along a single wire

Half Duplex

Data can travel in both directions, but only one at a time

Wired data transmission: Parallel

Multiple bits sent simultaneously, each in its own wire

Routing

Finding the most optimal path over a network

Advantages of Serial

Reliable over long distances

Cheaper

Less interference

Packet switching

The process in which data is broken down into packets and sent across the internet

Examples of USB usages

Keyboards

Mice

Mobiles