multi-rate medium access control
TRANSCRIPT
What is Multi-Rate?Ability of a wireless card to automatically operate at several different bit-rates
(e.g. 1, 2, 5.5, and 11 Mbps for 802.11b)
Part of many existing wireless standards(802.11b, 802.11a, 802.11g, HiperLAN2…)
Virtually every wireless card in use today employs multi-rate
Example Carrier ModulationsBinary Phase Shift Keying
One bit per symbolMade by the carrier and its inverse
Quadrature Phase Shift KeyingTwo bits per symbolUses quadrature carrier in addition to normal carrier(90° phase shift of carrier)4 permutations for the inverse or not of the two carriers
Example Carrier Modulations (cont.)16 - Quadrature Amplitude Modulation
4 bits per symbolAlso uses quadrature carrierEach carrier is multiplied by +3, +1, -1, or -3(amplitude modulation)16 possible combinations of the two multiplied carriers
Example Carrier Modulations (cont.)64 - Quadrature Amplitude Modulation
6 bits per symbolAlso uses quadrature carrierEach carrier is multiplied by +7, +5, +3, +1, -1, -3, -5, or -7 (amplitude modulation)64 possible combinations of the two multiplied carriers
802.11a Rates resulting fromCarrier Modulation and Coding
Advantage of Multi-Rate?Direct relationship between communication rate and the channel quality required for that rateAs distance increases, channel quality decreasesTherefore: tradeoff between communication range and link speedMulti-rate provides flexibility to meet both consumer demands
1 Mbps2 Mbps5.5 Mbps
11 Mbps
Lucent Orinoco 802.11b card ranges usingNS2 two-ray ground propagation model
Throughput vs. Distance for 802.11a
802.11 Frame Exchange OverheadExchange means not all time is spend sending actual data
RTS
CTS
DATA
ACK
Sender
Receiver
cw
Medium time used for transmission
Actual time sending application data
Multi-rate Frame in 802.11b
802.11b Frame Exchange Duration
0 1 2 3 4 5 6 7 8 9 10 11 12 13 14
1.0
2.0
5.5
11.0
Rat
e (M
bps)
Medium Time (milliseconds)
MAC Overhead Data4.55 Mbps
3.17 Mbps
1.54 Mbps0.85 Mbps
Medium Time consumed to transmit 1500 byte packet
Multi-rate Frame in 802.11a
52 us
Hops vs. ThroughputSince the medium is shared, adjacent transmissions compete for medium timeEffective end-to-end throughput decreases when sending across multiple hops
1 2 3
Effect of Transmission
Source Destination
Request to Send (RTS)Clear to Send (CTS)DATAACK
1 2 3 4 5 6 7 8X X X X X X X
Multi-Hop Throughput Loss (TCP)
1 2 3 4 5 6 7 8 9
1.0 Mbps2.0 Mbps
5.5 Mbps11.0 Mbps0.0
0.5
1.0
1.5
2.0
2.5
3.0
3.5
4.0
Thro
ughp
ut (M
bps)
Hops
1.0 Mbps
2.0 Mbps
5.5 Mbps
11.0 Mbps
Auto Rate ProtocolsSelects the rate to use for a packetARF
Adaptive based on success/failure of previous packetsSimple to implementDoesn’t require the use of RTS CTS or changes to 802.11 spec
Receiver Based Auto Rate (RBAR)Uses SNR measurement of RTS to select rateFaster & more accurate in changing channelRequires some tweaks to the header fields
Opportunistic Auto Rate (OAR)Adds packet bursting to RBARAllows nodes to send more when channel conditions are goodImplements temporal fairness instead of packet fairness
MAC Layer Fairness ModelsPer Packet Fairness: If two adjacent senders continuously are attempting to send packets, they should each send the same number of packets.Temporal Fairness: If two adjacent senders are continuously attempting to send packets, they should each be able to send for the same amount of medium time.In single rate networks these are the SAME!
Temporal Fairness Example
3.9831.609Total Throughput
0.4500.7131 Mbps Link
3.5330.89611 Mbps Link
OARTemporal Fairness
802.11Packet
Fairness1 Mbps
11 Mbps
1 Mbps
11 Mbps
Per Packet Fairness
Temporal Fairness