Computer Science IGCSE Unit 2

Data Packet

Data Packet

1. A small part of a message/data that is transmitted over a network

2. After transmission all the data packets are reassembled to form the original message/data

Packet Header

Part of data packet which has

1. IP address of the sender/receiver

2. Packet number (to allow reassmebly of data packets)

Packet trailer

Part of data packet that indicates the end of the data packet and contains the Cyclic Redundancy Check error check

CRC

Cyclic Redundacy Check — error checking method in which all the 1bits in the data packet payload are added and the total is stored in the packet trailer at sending point
This calculation is repeated at receiving station and the two CRC result numbers calculated.

Payload

The actual data being carried in a data packet

Node

• Devices that are connected to other devices in a network

• Stages in a network that can receive and transmit data packets (routers are nodes in communication networks)

Packet Switching

• Method of transmitting data

• Break down message into many data packets

• Send along different pathways independently of each other

Router

Device that enables data packets to be moved between differente networks — joining a LAN to a WAN (one router connected to different devices make a LAN, network of LANS = WANS)

Real time streaming

• Transmission of data over a network for live events

• Data is sent as soon as received or generated

Hopping/hop number

• Number in a data packet header used to stop data packets that never reach their destination from clogging up data paths/routes

• Hop number reduces by 1 every time it moves from one router to another, when hop number reaches 0, packet gets deleted.

Simplex

Data that can be sent on one direction only

Half-duplex

Data that can be sent in both directions but not at the same time

Full-duplex

Data that can be sent in both directions at the same time

Serial data transmission

Sending data down one channel/wire one bit a time

Parallel data transmission

Sending data down several channels/wires several bits at a time (usually 1 byte)

Skewed data

Data that arrives at the destination with the bits no longer synchronised

USB

Universal Serial Bus

1. Serial data transmission

2. Industry standard

3. Connecting computers to devices via USB port

Parity check

Method used to check if data has been transferred correctly

• Parity protocol (either even or odd partity) is agreed upon

• Parity bit will show 0 if the number of 1 bits are even or odd denpdning on type of parity used (even number of 1s in even parity will have 0 in partiy bit and so on)

• Partiy bit will show 1 if the number of 1 bits are not even or odd depeding on type of partiy used. If odd partiy is used and there is an even number of 1bits, the partiy bit shows 1.

Parity bit

A bit (either 0 or 1) added to a bype of data in the most significant bit position to ensure the byte follows the correct even or odd parity protocol.

Parity block

A horizontal and vertical partiy check on block of data being transmitted

Parity byte

Extra byte of data sent at the end of a parity block. Composed of partiy bits generated from vertical partiy check of the data block.

Checksum

• Verification method used to check if data transferred has been altered or corrupted

• Calculated from the block of data being sent

• Checksum value sent after each data block

ARQ

Automatic Repeat Request

• Method of checking data transmission errors

• Makes sure of acknowledge and timeout to auto request re-sending data if time interval before postive acknowledgment is too long

Acknowledgement

Message sent to the receiver indicating that data has been received correctly (used in the ARQ error detection method)

Timeout

Describes the amount of time allowed to elapse before an acknowledgement is received to re-sent data (in the ARQ error detection method)

Echo check

• Data transmission error checking method

• Data is sent to a receiver and then immediately sent back to the sender

• Sender then checks if the received data matches the sent data

Check Digit

• Data entry check (NOT data transmission check)

• Additional digit added to a number to check if the entered number is error-free

Eavesdropper

• Hacker

• Intercepts (steal) data being transmitted on a wired or wireless network

Encryption

Making data meaningless using encryption keys; without the correct decryption key the data cannot be decoded (unscrambled)

Plaintext

Messages that are in readable form before putting in encryption algorhythm

Cypertext

Encrypted data that is the result of putting a plaintext message through an encryption algorithm

Encryption algorithm

• Complex piece of software

• Takes plaintext

• Generates an encrypted string known as ciphertext

Symmetric Encryption

Type of encryption is whcih the same encryption key is used both to encrypt and decrypt a message

Asymmetric encryption

A type of encryption that uses public keys and private keys to ensure data is secure

Public key

type of encryption key that is known to all users

Private Key

Type of encryption key which is known only to the single computer/user