mid 1 Flashcards
(48 cards)
circuit switched (pros, cons, ex)
- dedicated comunication path
pros:
- no path sharing
- ensures reliable connection
cons:
- inefficient for short data bursts
- lots of idle time
ex: landline
packet switched (pros, cons, ex)
msg is broken into packets
packets travel indep accross network
pros:
- efficient use of bandwidth
- more reliable: can choose a diff path
- supports multiple users at once
ex: Internet
FDM (what, cons, ex)
- multiplexing strategy
- mult. signals can be sent at once using diff frequencies
cons:
- can have interference
- less efficient if a frequency is not used
ex: radio
TDM (what, cons, ex)
- multiplexing strategy
- divid time slots for each user
- trans 1 signal at a time
pros:
- efficient, unused time slots are reasigned
- less interference
ex: ethernet
statistical multiplexing
- in a switch
- packets buffered until sent
- diff techniques for queueing.
- if packet arrives and buff is full, packet is dropped
what is encapsulation
- when a datagram (packet, frame…) is passed to the next layer it get “encapsulated” by headers and footers by that next layer.
what is bandwidth
max amount of data that can be transmitted per second
measured in bps
what is throughput
actual amount of data that can be transmitted per second
what is latency
the total time it takes for a message to completely arrive at the destination
latency = Pd + Td + Qd + (Procc d)
transmission delay
how long it takes for host to push entire message onto link
data size / bandwidth
propagation delay
amount of time for 1 bit to travel from one end of link to other end of link.
Pd = distance / signal speed
RTT
round trip delay
time for a bit to be sent + time for ack to be received.
RTT = 2*Pd
Services of the data link layer
- encoding
- framing
- reliable delivery between adjacent nodes
- flow control
- error detection
- error correction
what is encoding
converting bits to signals to be sent over a link (high / low)
NRZ pros, cons
1 =>high
0 => low
cons:
- long consecutive 1 or 0 cause baseline to wonder and clock sync problems
NRZI pros, cons
1 => transition
0 =>maintain
pros:
- solves long consecutive 1s
cons:
- does not solve 0s
manchester encoding pros, cons
1 = -|_
0 = _|-
pros:
- solves 1s and 0s
cons:
- only 50% efficient since it needs to at most 50% more transitions than bits
4B / 5B pros
-insert extra bit to break 1s and 0s
- use table to convert 4bits to 5bit codes
- then encode with NRZI
pros:
- 4B 5B solves 0s then NRZI solves 1s
- 80% efficinet
why framing (objective, challenges)
- obj: need to transit a stream of bits
- chal: how to mark start and end of a stream?
sentinel framing cons
- mark end of fram with a special bit pattern
con:
- the bit pattern might exist in data
length-based framing
- frame length is a field in the header
- end of frame calculated from length sent at start of frame
cons:
- length field could be corupted
clock based framing
- continous stream of fixed length frames
- clocks must be synchronized
error detection: what is it
add edc to determine if errors are present in data
2D parity
- split data into 7 bit blocks
- add 1 parity bit to end of blocks
- add 1 or 0 and make sure resulting 8 bits have even # of 1s
- do same for columns
- can detect 1, 2, or 3 bit errors
- receiver can correct 1-bit errors without retransmission.
