2. data transmission (copy)

Packets

data is broken down into packets to be transmitted

Header

• IP address of sender

• IP address of receiver

• sequence number of packets

• size of packet

Payload

contains the actual data

Trailer

• method of identifying end of the packet

• error-checking method

Packet switching

• data is broken down into packets

• each packet could take a different route

• router controls route packet takes

• packets may arrive out of order

• packets are reordered at the end

Advantages of packet switching

• high transmission speed

• easy to expand package usage

• possible to overcome busy or failed nodes

• no need to create a single line of communication

Disadvantages of packet switching

• packets may be lost

• prone to errors in real-time streaming

• delay the the receiver while packets are being reordered

Simplex

one direction only

Half-duplex

both directions, one at a time

Full-duplex

both directions, at the same time

Serial data transmission

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

• e.g. telephone lines

Parallel data transmission

data is sent several bits at the same time over several wires

• e.g. computer to printer

Serial vs. parallel

Serial:

• better for long distances

• cheaper

• used when a small size of data is transmitted

• slower

Parallel:

• better for short distances

• expensive

• faster

Universal Serial Bus (USB)

asynchronous serial data transmission method

Advantages of USB

• automatically detected

• only fits one way, prevents incorrect connections

• backwards compatible

• industry-standard

Disadvantages of USB

• transmission rate less than 120MB/sec

• maximum cable length is about 5m

Parity checks

• uses number of 1-bits in a byte

• LSB is the parity bit

• two bits may change during transmission so error is not found

• cannot locate the changed bits (use parity blocks to overcome this)

Checksum

• sender calculates checksum value using a specific algorithm

• receiver calculates checksum value using the same algorithm

• compares checksum value for errors

Echo check

• receiver sends data back to sender for verification

• sender compares received and original data for errors

• cannot tell if error occurred during which transmission

Types of errors during data transmission

Check digits

• occurs during data entry

• calculated from all other digits in the data

• e.g. ISBN 13, Modulo-11

Types of errors during data entry

• incorrect digit entered

• transposition errors

• omitted/extra digits

• phonetic errors

Automatic Repeat Request (ARQ)

• uses acknowledgements timeouts to make sure user received the data

• receiver checks data for any errors, if none is found, a positive acknowledgement is sent

• if a negative acknowledgement is sent/timeout, data will be re-sent

Encryption

process of converting readable data into unreadable characters to prevent unauthorised access

Plaintext

original text being sent

Ciphertext

text produced after encryption

Symmetric encryption

• uses an encryption key

• key used to encrypt and decrypt the encoded message

Asymmetric encryption

• developed to overcome security problem

• two keys, public (everyone) and private (sender) key

• receiver uses public key to encrypt message

• sender uses private key to decrypt message