Computer Science Chapter 2 - Data Transmission

Data packets

Header

Payload

Trailer

Header

contains IP address of sender and receiver

contains sequence number of packet

contains size of packet

payload

contains actual data

trailer

contains method of identifying the end of the packet

contains error-checking methods

packet switching

• Data is broken down into packets

• Each packet could take a different route

• Router controls route a packet takes

• Packets may arrive out of order

• Packets are reordered once last packet arrives

serial data transmission

• data sent one bit at a time over a single wire

• better for longer distances

• Slower but cheaper

• Used when transmitting small sized datas

• data does not become skewed

Parallel data transmission

• data of several bits are sent down several wires at the same time

• better for short distances

• expensive but faster than serial

• data could be skewed

universal serial bus(USB) advantages and disadvantages

-only fits one way, prevents incorrect connections

-Industry standard

-limited cable length

reasons errors occur after data transmission

can occur due to interference (data loss, data gain or data change)

odd or even parity check

uses number of 1-bits in a byte, left most bit is parity bit, amount of ones in a byte is equal to if it’s odd or even parity check.

if two bits change during transmission, no error is detected

doesn’t identify changed bits

checksum

value calculated using specific algorithm by sender, receiver would calculate checksum value with same set of data and same algorithm. If values don’t match, request is sent to re-send data

echo check

data send back from receiver to sender for verification, sender compare received and original data for errors

doesn’t know if error occured during or after transmission

automatic repeat query

• uses acknowledgements and timeouts to make sure user received data

• receiver checks data for errors, positive acknowledgement is sent if no error

• negative acknowledgement will be sent if errors were found and data will be sent again

• sender uses timeouts to wait a pre-determined amount, if no acknowledgement after timeout data will be sent again

purpose of encryption

making data non-sensical and incomprehensible to hackers and other attackers

symmetric encryption

uses same key for encrypting and drcypting data

asymmetric encryption

uses public key(accessible to everyone) and private key(available to user)

receiver has private key, they send public key to sender. Sender encrypts message with public key and data can be decrypted using private key