MultipleAccessControl Flashcards
what is distributed MAC?
no single host is responsible for access
coordinated access
each host is “scheduled” to transmit in certain intervals of time(token ring), schedule chosen to avoid interference between simultaneous transmissions
random access
each host randomly decides to transmit
Pure Aloha
transmit the packet immediately, if not already transmitting another packet
Slotted Aloha
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
what is the access probability?
probability that a host will independently transmit in a given slot
Carrier Sense Multiple Access
only transmit if the channel is idle, which is measured with energy detection
Carrier Sense Threshold(Pcs)
if the received power(Pr) < Pcs, channel is idle, else channel is busy
what are the tradeoffs between large and small Pcs
with a large Pcs you have greater spatial reuse, but more interference
what is the upper bound of the interference of a transmitter at a receiver?
<= Pcs * (gain of interrupting sender and receiver / gain of first sender and interrupter), with gain = power received / power transmitted
when will a server transmit while another sever is transmitting?
when the transmitted power of the first server times the gain between them <= the Pcs
what is the hidden terminal problem?
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.
what is the exposed terminal problem
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
how should the optimal Pcs be found?
optimal Pcs is a function of network topology and traffic characteristics
what are simultaneous transmissions?
transmissions that occur at the same time on the same frequency
what are concurrent transmissions?
transmissions that occur at the same time on different frequencies
RTS/CTS(or virtual carrier sensing)
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.
when is it good to use rts/cts?
when the data packets are large and collisions frequent, not beneficial if data packets are too small(above a threshold)
can control packets(rts/cts) have collisions?
yes, but they are short so the collision cost is low
what is busy-tone mechanism?
when a host is receiving data it transmits a busy-tone, which means the interference bound only relies on the Pcs
what are some issues with busy tone?
can consume a significant amount of energy if the busy tone is sent continuously
can you use both physical and virtual CS?
yes, the channel is assumed idle if both PCS and VCS indicate it is idle
p-Persistence
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
