ECE361 Midterm 1: e. Medium Access Control (MAC) protocols

what are medium access control (mac) protocols used for?

sharing the use of transmission media that can be accessed concurrently by multiple users

what is transmission media

physical pathway that data travels on

what are the 2 sublayers of the link layer

logical link control and medium access control

who are the senders and receivers during MAC protocol

logical link sublayer packet → encapsulates into frame → sends to physical layer

physical layer bits → encapsulates into frame → sends to logical link sublayer as a frame

what does it mean by contention based MAC protocols

multiple nodes compete or “contend“ for access to a shared communication medium in an uncoordinated manner

what are 5 different aspects considered during contention based mac protocols

1. no coordination: stations transmit whenever they have data to send (eg. ALOHA)

2. carrier sensing (listening before transmitting): stations sense the channel before transmitting a data packet (eg. CSMA)

3. listen before and during transmission: stations listen before transmitting and stop if noise is heard while transmitting (CSMA/CD)

4. collision avoidance (floor acquisition): stations carry out a handshake to determine which one can send a packet (eg. IEEE 802.11)

   1. handshake: automated process where 2 stations exchange signaling messages, a connection, and rules

5. collision resolution: stations determine which one should try again after a collision

what are aspects of conflict free protocols

• fixed assignment (eg. TDMA, FDMA)

• reservations

• polling

• token passing

• dynamic transmission scheduling

what is ALOHA

• first MAC protocol using packet switching

• transmits at will — poor performance

• has 2 frequency bands: up link (sending data) and downlink (ack)

what is the pure ALOHA protocol

what is the equation for throughput of pure aloha

(only consider uplink)

S = G * e^(-2G)

G = average channel load

how does the channel use work with pure ALOHA

• packets have the same length

• system is in equilibrium

• after a success, channel can go back to being successful at some time

• system cannot be silent indefinitely

what is the vulnerability of a frame in pure ALOHA

2 * packet time period

what is slotted aloha

• improving the throughput of ALOHA by reducing the time a packet is vulnerable by using a slotted channel — discrete time slots

• stations can start transmitting only at the beginning of time slots

• physical layer takes care of time synchronization for slotting

what is the slotted aloha protocol

what is the throughput of slotted ALOHA

S = Ge^(-G)

what are the limitations in ALOHA results

real throughput is uplink and downlink channels, not just uplink

how do priority ACKs work

all stations can transmit at well unless hearing an ACK

what does CSMA stand for and how does it work

• carrier sense multiple access

• reduces the vulnerability period to one propagation delay by listening before transmitting

what is the carrier sense multiple access protocol

what is non persistent carrier sensing

• station senses shared channel before deciding to transmit

• when busy: back offf for a random period of time before attempting to sense and transmit again

• max propagation delay much smaller than packet lengths

throughput for CSMA

S = Ge^(-aG)/G(1 + 2a) + e^(-aG)

S = Normalized throughput (the average number of successful packet transmissions per packet transmission time)

G = Normalized offered traffic load (total packet arrival rate

a = normalized propagation delay a = max propagation delay/packet transmission time

vulnerability of non persistent CSMA

one propagation delay

what is the limitation of CSMA that CSMA/CD addresses

once a packet is sent, feedback to the sender occurs a round trip time since it waits for the ACKK

what is the CSMA/CD protocol

(assume)

• all stations hear each other

• propagation delay is much smaller than packets

what problem does collision avoidance address

hidden terminal problem where terminals cannot hear each other’s transmissions to a receiver, causing interference

what is half duplex

type of communication that allows data to travel in both directions but only one direction at a time

how does collision avoidance work

• stations exchange small control packets to determine which sender can transmit to a receiver

how does sender initiated collision avoidance work

RTS (s to r) → CTS (r to s) → data (s to r) → ACK (r to s)

RTS = request to send

CTS = clear to send

RTR = ready to receive

how does receiver initiated collision avoidance work

RTR (r to s) → data (s to r) → ACK (r to s)

how does floor acquisition multiple access work

• no acks, stations use carrier sending to send any packet

1. S sends RTS packet to R

2. if R receives packet successfully, reply with a CTS packet back to S

   1. CTS is much longer than an RTS

3. if another sender tries to send an RTS, it will cause interference with ongoing CTS, backs off

what is non persistent CSMA/CA protocol?