MultipleAccessControl

Question 1

what is distributed MAC?

Answer 1

no single host is responsible for access

Question 2

coordinated access

Answer 2

each host is “scheduled” to transmit in certain intervals of time(token ring), schedule chosen to avoid interference between simultaneous transmissions

Question 3

random access

Answer 3

each host randomly decides to transmit

Question 4

Pure Aloha

Answer 4

transmit the packet immediately, if not already transmitting another packet

Question 5

Slotted Aloha

Answer 5

time is divided into slots, and a station can only transmit at the beginning of a slot, window of vulnerability lowered to one time slot

Question 6

what is the access probability?

Answer 6

probability that a host will independently transmit in a given slot

Question 7

Carrier Sense Multiple Access

Answer 7

only transmit if the channel is idle, which is measured with energy detection

Question 8

Carrier Sense Threshold(Pcs)

Answer 8

if the received power(Pr) < Pcs, channel is idle, else channel is busy

Question 9

what are the tradeoffs between large and small Pcs

Answer 9

with a large Pcs you have greater spatial reuse, but more interference

Question 10

what is the upper bound of the interference of a transmitter at a receiver?

Answer 10

<= Pcs * (gain of interrupting sender and receiver / gain of first sender and interrupter), with gain = power received / power transmitted

Question 11

when will a server transmit while another sever is transmitting?

Answer 11

when the transmitted power of the first server times the gain between them <= the Pcs

Question 12

what is the hidden terminal problem?

Answer 12

when two nodes are out of range/blocked from each other, and they simultaneously transmit to the same node, resulting in a collision neither of them detect so they may continue transmitting and creating collisions, can try to avoid by lowering the Pcs.

Question 13

what is the exposed terminal problem

Answer 13

when two nodes within range of each other want to transmit to two nodes that are not within range of each other, but due to a low Pcs the sending nodes do not transmit despite there being no risk of interference

Question 14

how should the optimal Pcs be found?

Answer 14

optimal Pcs is a function of network topology and traffic characteristics

Question 15

what are simultaneous transmissions?

Answer 15

transmissions that occur at the same time on the same frequency

Question 16

what are concurrent transmissions?

Answer 16

transmissions that occur at the same time on different frequencies

Question 17

RTS/CTS(or virtual carrier sensing)

Answer 17

a node sends a request-to-send packet to a node it wants to send data to that contains the duration of the proposed transmission, and the other node sends a clear-to-send packet that acknowledges the RTS, and the first node sends the data shortly after. other hosts that hear the rts/cts know to keep quiet for the duration.

Question 18

when is it good to use rts/cts?

Answer 18

when the data packets are large and collisions frequent, not beneficial if data packets are too small(above a threshold)

Question 19

can control packets(rts/cts) have collisions?

Answer 19

yes, but they are short so the collision cost is low

Question 20

what is busy-tone mechanism?

Answer 20

when a host is receiving data it transmits a busy-tone, which means the interference bound only relies on the Pcs

Question 21

what are some issues with busy tone?

Answer 21

can consume a significant amount of energy if the busy tone is sent continuously

Question 22

can you use both physical and virtual CS?

Answer 22

yes, the channel is assumed idle if both PCS and VCS indicate it is idle

Question 23

p-Persistence

Answer 23

used to reduce collision probability, if the channel is deemed idle the node transmits data with a probability of p, often a backoff interval is added so the nodes do not waste too many valid opportunities before transmitting a packet