Ethernet Flashcards
What problem do access control protocols solve?
coping with packet collissions
what are packets referred to at the ethernet level?
frames
what is first part of ethernet frames?
Preamble
how many bits in preamble?
56 bits of alternating 1s and 0s
whats followed after preamble?
SFD (Start frame delimiter)
Whats after SFD?
Destination and source addresses
what is special MAC address used to designate?
broadcast packet (sent to all machines)
what does frame type identify?
what is in the packet
What comes after frame type?
data section
what does it mean if the ethernet line is silent?
nothing is being sent
how do you know its the end of the packet?
if the carrier disappears
What is CRC?
cyclic redundancy check
number created from the packet
receiver then regenerates the same packet and if they don’t match packet must have been corrupted
what else does CRC do?
enables error detection
What does error detection do?
enables us to detect if a packet has been corrupted in transit
can we correct the error?
No CRC does not allow to correct the error
Whats one type of error detection?
Checksum
What is checksum?
we run an algorithm before we send the packet that generates the number
the same algorithm is run after it has been received
What if the numbers don’t match?
the packet has most likely been corrupted
how does checksum work?
set the checksum to 0
for each byte add the value of that byte to the checksum
the end result of all those additions is the checksum
what kind of errors are in error transmission??
bust errors
what are bust errors?
small set of bits that get corrupted (changed) and are very likely on a network
What is CRC built around?
exclusive-OR and
shift register
which CRC version does ethernet use?
32 bit wide version
what does the 16bit CRC use?
3 shift registers, 3 exclusive-ORs
