Studied by 3 people
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
