2 data transmission

DATA TRANSMISSION

the process of sending information from one device to another over a network

how is data sent?

data is broken down into smaller units called “packets”

packets contain…

1. header

2. source

3. destination addresses

4. sequence number of the packet

how are packets transmitted and reassembled in a network?

they are sent out individually and can take different routes to reach the destination. at the end, the packets are reassembled into the original data

HEADER

contains the destination address, originating address (source address) and packet number so that the correct order is restored once it arrives

PAYLOAD

the contents that need to be sent

TRAILER

has data that indicated the end of the packet and a checksum for the destination to check it transmitted correctly

PACKET SWITCHING

the process of breaking down data into packets and transmitting over a network

process of packet switching

1. break down data into packets

2. send the packets through a network

3. routers control the route taken

4. reorder packets into the correct order

1. data is broken down into packets

divided into smaller units each containing a potion of the data and a header with information about the packet

2. each packet can take a different route

when the packets are now transmitted, they make different routes to reach their destination. each packet are individually moved based on network conditions such as traffic levels, to optimize efficiency and reliability

3. a router controls the route a packet takes

a router is a device that connects different networks and routes packets to their destination

4. packets are reordered and reassembled into the original data

put back together

SERIAL TRANSMISSION

bits are sent one signal at a time over a single wire

• one bit is sent at the same time

• once the bit has been received, the next bit sent

PARALLEL TRANSMISSION

several wires are used so that many bits can be sent at the same time. mainly used within computer systems and components on a circuit board level

multiplexing

the process of managing various signals between devices, three common methods of doing this are, ‘simplex’, ‘half-duplex’, and ‘duplex’

SIMPLEX

data can only be sent from its source in one direction

HALF-DUPLEX

data can travel in both directions alongside a single connection, but cannot travel (receive and send) simultaneously

FULL DUPLEX

data can be sent in both directions simultaneously, over separate communication lines

• computers between two computers is LAN, where both computers receive and send data at the same time

SERIAL TRANSMISSION advantages

• simple hardware and cabling

• low cost

• low power consumption

• suitable for long-distance communication

SERIAL TRANSMISSION disadvantages

• slower data transfer rate

• limited bandwidth

• not suitable for large amounts of data

PARALLEL TRANSMISSION advantages

• high data transfer rate

• suitable for large amounts of data

• more efficient than serial transmission

PARALLEL TRANSMISSION disadvantages

• complex hardware and cabling

• higher cost

• higher power consumption

• limited maximum cable length

SIMPLEX TRANSMISSION advantages

• simple hardware and cabling

• suitable for one-way transmission

• no need for a return channel

SIMPLEX TRANSMISSION disadvantages

• data can only be transmitted in one direction

• limited to simple communication scenarios

• not suitable for bidirectional communication

HALF-SIMPLEX TRANSMISSION advantages

• data can be transmitted in both directions

• suitable for limited bandwidth scenarios

HALF-SIMPLEX TRANSMISSION disadvantages

• data can only be transmitted in one direction at a time

• not suitable for high-bandwidth communication

• more complex than simplex transmission

FULL-SIMPLEX TRANSMISSION advantages

• data can be transmitted in both directions simultaneously, over separate communication lines

• suitable for high bandwidth communication

• more efficient than duplex transmission

FULL-SIMPLEX TRANSMISSION disadvantages

• more complex hardware and cabling

• higher cost

• higher power consumption

ERROR CHECKING

when data is sent from one device or component to another, there is always a chance that the data might become corrupt/damaged during transfer

data integrity

refers to ensuring the data is correct and as it was intended to be

possible transmission errors:

• electrical interference

• power surges

• synchronisation issues

• wear and tear on the cable or connectors

transmission errors may cause:

• lost data

• gained data

• changed data

PARITY BITS

2 types; even and odd. when sending a byte of data, one bit is used as a parity bit

EVEN PARITY BIT

the parity bit is set to be 1 or 0 so the total number of 1s transmitted are an even number

ODD PARITY BIT

all 1s must add up to an odd number

using parity for error detection:

• if the wrong number of bits are found an error has occurred

• it isn’t possible with parity checks to know which bit was incorrectly transmitted

PARITY BLOCK CHECK

in a parity block check, each byte has a parity but whilst an additional parity byte is sent at the end of the block (7 bits, 1 parity bit = 8 bits = 1 byte)

CHECKSUMS

a value that represents the number of bits in a transmission message

• run through an algorithm that produces a checksum which it also sends

• when the file arrives, the receiving computer generates the checksum and compares it with the checksum that was sent

• if the checksums don’t match, the file may have been corrupted or altered during transmission

CHECKSUMS steps

1. a checksum is created and sent with the file

2. when received the checksum is again generated

3. if it doesn’t match the sent checksum, an error has occurred

ECHO CHECK

an echo check acts in the same way as an echo from a wall

ECHO CHECK steps

1. the data is transmitted to the receiver

2. the same data then echoed back to the transmitter

3. the transmitter compares the data with the original. if it isn’t the same, the data is transmitted again

CHECK DIGIT

an additional digit the end of a set of other numbers designed to check for mistakes in input or transmission

ISBN

printed books have a unique barcode with an ISBN (International Standard Book Number). contains 10 digits, with the last digit being the check digit

UPC

UPC (Universal Product Code) contains series of bars and spaces of different widths, representing different digits

calculating the check digit:

check digits are calculated by multiplying each number by a weight of 1 or 3 as shown below

describe how an automatic repeat query (ARQ) can be used to establish that data is received without error

• the sender first sends a block of data

• the receiving computer then sends back an acknowledgement

• when the sender receives the acknowledgement, it sends the next block of data

• if the acknowledgement is not received after a period of time, a timeout occurs and the data is resent

• this process continues until the acknowledgement is received

ARQ flowchart

if the sender does not receive an acknowledgement within a certain time period, known as TIMEOUT, it assumes that the data packet was lost or corrupted during transmission. the sender then retransmits the data packet, continuing until the sender receives a positive acknowledgement from the receiver

negative acknowledgements

when a data block is correctly received, an acknowledgement is sent

• when the receiver detects an error in a data block that has been sent, a negative acknowledgement (NACK) is sent

• by sending a NACK, the sender doesn’t need to wait so long in a timeout - it can immediately send the data block again

ENCRYPTION

is a process used to secure data during transmission by converting the original data into a coded form that cannot easily be understood by unauthorised users. encryption is used to encode a file or data into something that cannot be read or understood.

why does encryption need to occur?

1. Confidentiality

2. Integrity

3. Authentication

4. Compliance

problems when encryption is not done

• security breaches - data could be intercepted

• privacy violations - personal or sensitive information could be exposed

• miscommunication - could be misunderstood or misinterpreted

• compliance violations

SYMMETRIC ENCRYPTION

Uses a single shared key to both encrypt and decrypt data. A key is shared between the sender and receiver. the receiver uses the key to then decrypt the data

ASSYMETRIC ENCYRPTION

uses a pair of keys, one for encryption (public key) and one for decryption (private key). 

• the public key is accessible by anyone and uses to encrypt data

• the private key is kept secret by the receiver and used to decrypt data

anyone can encrypt the data but only the receiver can decrypt it.

what is USB (Universal Serial Bus)?

USB is a serial cable commonly used to connect external devices (e.g., keyboards, mice, flash drives) to computers, laptops, tablets, and smartphones.

what are the benefits of USB?

✅ USB 1 operates at speeds between 1.5 and 12 Mbps
✅ easier connection compared to older parallel and serial cables
✅ can connect various devices by installing necessary drivers

what are the drawbacks of USB?

❌ USB cables should be ≤5 meters before needing a USB hub
❌ not fast enough for internal devices like CPUs or hard drives

how is USB used to transmit data?

USB transmits data serially, meaning one bit at a time over a single data line. It follows a master-slave architecture, where the host device (e.g., a computer) controls data flow.