lecture 7 Flashcards
3 types of multiple-access protocols
- random access protocols
- controlled access protocols
- channelization protocols
what protocols does wifi use
CSMA
about contention (random access) protocols (2)
- no station is superior to another station
- no station permits/does not permit another station to send
two types of aloha protocols
- pure aloha
- slotted aloha
throughput in pure aloha =
G * e^-2G
the maximum throughput in pure aloha is when G =
- 0.5
- S = 0.184
how does pure aloha protocol work (3)
- transmit at any time
- detect collision after sending
- if collision occurs, wait a random time and retry
checksum error in pure aloha
- if the first bit of a frame collides with the last bit of another frame
- both will have to be retransmitted
how does slotted aloha protocol work
- divide time into intervals (slots), one for each frame
- stations agree upon time intervals
- users transmit only at beginning of a time slot
throughput in slotted aloha =
G * e^-G
maximum throughput in slotted aloha is when G =
- 1
- S = 0.368
graph for aloha protocols
- throughput per time frame on the y-axis
- G (attempts per packet time) on the x-axis
how do CSMA protocols improve channel utilization
listen to the medium/stations before sending
1-presistent
the station transmits with a probability of 1 whenever it finds the channel idle
p-presistent
- if empty, sends with probability p
- defers with probability q = 1 - p
what is channelization
a multiple access method in which the available bandwidth of a link is shared in time, frequency or through code, between diff stations
in frequency division multiple access (FDMA)
the available bandwidth of the common channel is divided into bands that are separated by guard bands
in time division multiple access (TDMA)
the bandwidth is just one channel that is timeshared between different stations
in code division multiple access (CDMA)
- one channel carries all transmissions simultaneously
- code attached to each station
- code is decrypted when it reaches destination
IEEE 802.3 =
ethernet
IEEE 802.11 =
internet
a network interface card (NIC) can be in one of three states
- transmitting
- receiving
- cycling between transmitting and receiving
what is the period of silence a NIC waits before it can transmit
9.6 us (microsec)
why is the period of silence necessary
- to allow the receiver to cycle from transmitting mode to receiving mode
- this prevents the frame from being lost
in CSMA/CD, to detect the collision, the sender must still be
in transmitting mode
which layer does the NIC work under
data link
to guarantee that sender is still in transmission state when a collision happens
the transmission delay should be twice the propagation delay
- T = 2 * P + extra delays if any
data rate of basic/standard ethernet
10Mbps
how to find min frame size
- RTT = 2 * propagation time
- propagation time = distance/speed + (n)delay of each repeater
- transmission delay = packet size/ 10Mbps = RTT
what is the minimum frame size according to IEE standards
- 46 bytes/368 bits in pure data portion
- 64 bytes/512 bits including overheads (because we are in the data link layer)
- 18 bits of added overhead
to find how many bytes of padding is needed
46 - present byte value = amount of padding in BYTES
binary exponential backoff
- wait 0 to (2^i) - 1
- i = number of collisions
the more collisions we have
the lower the probability of sending
what is the maximum frame size according to IEE standards
1518 bytes/12,144 bits
why do we need maximum and minimum frame size (2 pts each)
minimum:
- to ensure sender is still in transmitting mode
- for the transmission delay to equal twice the propagation delay, otherwise it will equal less and RTT wont be achieved
maximum:
- to restrict large chunk of data being sent without any fragmentation
- retransmission of the frame in the case of errors (even 1 bit) would take less time
