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Lecture 2
MAC II: Collision Avoidance
and Controlled Random Access
Reading: •
V. Bharghavan, A. Demers, S. Shenker, and L. Zhang, "MACAW: A Media Access Protocol for Wireless LAN's," Proc. SIGCOMM '94 , September 1994, pp. 212-225.
D. Goodman, R. Valenzuela, K. Gayliard, and B. Ramamurthi, "Packet Reservation Multiple Access for Local Wireless Communications," IEEE Transactions on Communications, Vol. 37, No. 8, August 1989, pp. 885-890. - J.-C. Chen, K. Sivalingam, P. Agrawal, and S. Kishore, "A Comparison of MAC Protocols for Wireless Local Networks Based on Battery Power Consumption,"Proceedings of IEEE INFOCOM '98, April 1998.
Location-Dependent CarrierSensing
CS depends on location of the node
Transmitter performs CS
Know the state of the channel at transmitter
Cannot determine the state of the channel at the receiver
Causes three problems
Hidden Nodes
Exposed Nodes
Capture
4
Exposed Nodes
A node that is within range of the sender but out of range of thedestination
An exposed node will sense a busy channel and not transmit
Suppose node B is transmitting to node A Node C will sense a busy channel However, node C could transmit to node D D is outside the range of node B No collision Node C (exposed node) backs-off
Causes underutilization of the channel
A
B
C
D
Capture Effect
When receiver can cleanly receive a transmission fromone of two simultaneous transmissions
Collisions may not cause both packets to be “lost”
Strongest user can successfully capture receiver
Packet from strongest user may survive collisions
Often closest user captures receiver due to smallpropagation path loss
A
B
C
Node C capturesthe channel fromNode A
Capture Effect (cont.)
If capture is included in throughput analysis, MAC protocols canachieve higher throughput
Throughput analysis typically assumes no capture and is a lowerbound on throughput in practice Upper bound on throughput can approach one if some station alwayscaptures the colliding packets True throughput lies somewhere between two bounds but closer tolower bound
Capture results in unfair sharing of the bandwidth
Near-far effect: Nodes close to the receiver can easily capture thereceiver and prevent nodes further away from communicating withthe receiver Wireless MACs aim to ensure fairness in the presence of capture
CSMA/CA: CSMA with CollisionAvoidance (CA)
Collision avoidance implemented using ACKs, backoffs, packetreservation
CA implemented using
Out-of-band signaling Receiving nodes transmit a signal to let nodes in their rangeknow that the channel is busy Eliminates hidden nodes Increases the number of exposed nodes BTMA, DSMA, etc. Control handshaking Each stage of the handshake lets nodes know if they are in rangeof the transmitter, receiver, or both Tradeoff in overhead for handshaking and the number of hiddennodes eliminated MACA, MACAW, etc.
Data Sense Multiple Access(DSMA)
Forward control channel used to inform nodes if reversechannel is idle or busy Nodes detect a busy-idle msg on forward controlchannel If busy-idle message indicates no users transmitting onreverse channel, a user can send its packet Does not eliminate collisions
Multiple nodes might note free channel and send datasimultaneously
Æ
collision
Retransmission strategy as in CSMA
Similar to BTMA but busy-tone information piggy-backedon control information being sent from BS to mobiles
MACA
Problem with CS is that transmitter cannot determine state ofcarrier at receiver
Hidden terminal problem Exposed terminal problem
MACA implements hand-shaking collision avoidance
Precede data transmission with request-to-send (RTS) packet RTS contains length of expected data transmission (all phases) All nodes in vicinity of Tx node enter backoff for duration ofmessage delivery If Rx node successfully receives RTS, replies with a clear-to-send(CTS) packet CTS packet contains length of expected data transmission All nodes in vicinity of Rx node enter backoff for duration ofmessage delivery Upon receipt of CTS, Tx node sends data
MACA (cont.)
Backoff
If a node does not receive a CTS in response to an RTS,it will eventually assume a collision occurred
Other nodes in vicinity also trying to use the channel
Node backs off before retransmitting RTS to reduceprobability of another collision
Node chooses a random number x between 1 and BO
Node must wait x time slots before retransmitting
MACA uses binary exponential backoff (BEB)
If an RTS is unsuccessful (i.e., it does not elicit a CTS), theBO is doubled (up to a maximum value)
Otherwise, wait time set to minimum value
MACA (cont.)
Collisions usually occur in RTS/CTS packets
If RTS/CTS packets successful, most nodes know that channel isbusy and will defer their own transmissions Some nodes may not receiver RTS or CTS due to othertransmissions in their vicinity Data has a high probability of no collision
RTS/CTS packets short compared to data packets
Packets destined to collide do not use much of the channel resources Packets that do require large amounts of channel resources have ahigh probability of being successfully transmitted Good policy!
MACAW (cont.)
If node A does not receive ACK after a certain time, nodeA retransmits RTS
If node B had received the data
ACK message was lost, not data message
Node B sends ACK in response to RTS
If node B did not receive the data
Node B sends CTS in response to RTS
Data is retransmitted
Node A’s backoff counter increases if initial data transferwas unsuccessful to minimize future collisions
DS Messages in MACAW
Nodes in range of transmitter (A) but not receiver (B) (e.g.,exposed nodes) may not hear CTS and thus not know if RTS-CTSexchange successful
If these nodes backoff until end of expected data transfer, mightbe a waste if CTS not received by A
Nodes can employ CS when expect node A to be tx data
If channel sensed idle, nodes can assume RTS-CTS unsuccessful andnode A is currently backed-off If channel busy, nodes must defer their own transmission
Instead of CS, MACAW uses Data Send (DS) msgs
Transmitter (A) sends DS message after receiving CTS to informexposed nodes that A will now send data packet Nodes receiving DS must defer their tx for the allotted time
DS messages ensure fairness
RRTS Messages (cont.)
D must be lucky enough that the random time chosento transmit an RTS falls during the time between anACK and RTS from the A-B stream Use request-RTS (RRTS) messages
When A-B data transfer complete, C sends an RRTSpacket to D
When D receives RRTS packet, D sends RTS
D-C stream can now commence
Ensures fairness
Backoff in MACAW
BEB can result in one stream starving another
With every unsuccessful RTS, node will BO Node with successful RTS will have BO at minimum value Node with unsuccessful RTS will have BO at maximum value If no max BO, one node would eventually have infinite BO Other node would permanently capture channel High throughput but not fair allocation
All pkt headers modified to include node’s BO value
Nodes receiving packet copy this BO value All nodes will have equal BO values
BO algorithm modified to mult inc, linear dec
Upon collision, BO increased to min(1.5*BO, BO max
Upon success, BO decreased to max(BO-1, BO min
