Data packets and compression Flashcards
why would you split up the data into different smaller packets
each data can be sent on a different route to its destination - good if a particular route was very busy / out of action
what is a drawback of splitting up data into different packets
have to reassemble the data back when it reaches its destination
3 sections that a packet is typically split into
packet header
payload
trailer
what does the packet header consist of
IP address of sending / receiving device
sequence number of packets (so that they can be reassembled in the correct order)
packet size (ensure that the receiving station can check if all of the packets have arrived)
what does the payload consist of
actual data being sent
what does the trailer consist of
a way of identifying the end of the packet - to allow each packet to be separated from eachother as they travel
an error checking method
what is cyclic redundancy check (how does it work)
when the sending computer adds up all the 1 bits in the payload and then stores this as a hex value in the trailer before it is sent
the receiving computer recalculates the number of bits in the payload
computer checks this value against the value in the trailer
if the two values match, no transmission errors have occurred - if it doesn’t match, data needs to be resent
what is packet switching
method of data transmission in which a message is broken up into number of packets
each packet can be sent independently from start to end point
at the destination, the packet needs to be reassembled
what happens at each stage of transmission in packet switching
there are nodes that contain a router
each router determines which route the packet needs to take to reach its destination using destination IP address
benefits of packet switching
possible to overcome failed / busy lines by re-routing packets
high data transmission rate
no need to tie up a single communication line
drawbacks of packet switching
packets can be lost and need to be resent
more prone to errors than real-time streaming
delay at the destination as the packets need to be reordered
why would you compress a file?
to save storage space on devices such as a hard disk drive / solid state drive
reduce time taken to stream a music / video file
reduce time taken to upload / download / transfer files across a network
can reduce costs by reducing file size
what is lossless compression
all the data bits from the original file are reconstructed when the file is uncompressed
why might lossless compression be important
important for files where data loss is disastrous - eg. spreadsheet file
features of lossless compression
code must be exact same as the original file or else it won’t work
a compression algorithm is used
no data is removed in the process
