unit 2: data transmission Flashcards
data packets
small part of a data that is transmitted over a network
data packet structure
- packet header
- payload
- trailer
packet header
- IP address of source device
- IP address of destination device
- sequence no. of packet
- size of packet (in bytes)
payload
actual data in the packet
trailer
*method of identifying end of packet
*some form of error checking
Cyclic redundancy checks (CRCs)
- sending comp. adds up all 1-bits in payload and storing as hexa in trailer before sending
- receiving comp. recalculates no. of 1-bits in payload
- comp. checks this value against hexa in trailer
- if matching, no transmission errors
process of packet switching
- each packet follows own path (route)
- routers determine the route
- routing selection depends on no. of packets waiting to be processed in a node
- shortest possible path available is always selected
- unfortunately, packets may reach destination in diff. order when sent
benefits of packet switching
- no need to tie up a single communication line
- possible to overcome faulty lines: rerouting
- easy to expand package usage
- high data transmission rate
drawbacks of packet switching
- lost: need to be resent
- more prone to errors with real-time streaming
- delay at destination
hopping
method where a hop no. is added to the header and it reduces by 1 each time the packet leaves a router
there’s a max. no. of hop numbers and if the hop reaches 0, it will be deleted at next router
the receiving computer notices missing packet and then requests for the packet to be resent
factors to consider in data transmission
- direction
- method
- how data will be synchronised
they’re considered by communication protocol
simplex
data is sent in ONE DIRECTION ONLY
half- duplex
data is sent in BOTH DIRECTIONS but NOT AT SAME TIME
full duplex
BOTH DIRECTIONS AT THE SAME TIME
serial data transmission
data is sent ONE BIT AT A TIME over A SINGLE WIRE/ CHANNEL
