fine-grained channel access in wireless lan
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Fine-grained Channel Access in Wireless LAN. SIGCOMM 2010 Kun Tan, Ji Fang, Yuanyang Zhang,Shouyuan Chen, Lixin Shi, Jiansong Zhang, Yongguang Zhang. Trends in 802.11 WLANs. PHY data rate increases 802.11n up to 600Mbps 802.11ac/ad up to >1Gbps - PowerPoint PPT PresentationTRANSCRIPT
Fine-grained Channel Access in Wireless LAN
SIGCOMM 2010
Kun Tan, Ji Fang, Yuanyang Zhang,Shouyuan Chen, Lixin Shi, Jiansong
Zhang, Yongguang Zhang
Trends in 802.11 WLANs• PHY data rate increases– 802.11n up to 600Mbps– 802.11ac/ad up to >1Gbps
• Data throughput efficiency degrades with PHY data rate
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Reasons for Low Throughput Efficiency• Contention resolution overhead due to CSMA• Coarse-grained channel allocation– Whole channel allocated to a single station
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Possible solutions
• Reduce overhead– Infeasible, physical laws/technology
• Increase useful channel time – frame aggregation– OK, used in 802.11n but– Practical limitations: 80% efficiency at 300Mbps
requires frame size of 23KB!
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An Alternative ApproachFine-Grained channel Access
• Divide channel into smaller subchannels
• Multiple users contend for and use subchannels simultaneously – Based on traffic demands
• Amortize MAC coordination, increase channel efficiency
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Challenges• Need to avoid interference between
neighbor subchannels
• Traditional approach: guard bands– High overhead
• OFDM – Orthogonal Frequency Division Multiplexing– “Eliminates” need for guard bands– Requires tight synchronization (100s of nsec)
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OFDM – High Level Overview• Divides spectrum into
many small, partially overlapping subcarriers
• Subcarrier frequencies “orthogonal” to each other
• OFDM system with FFT size N– N subcarriers, each with
bandwidth B/N
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OFDM as multi-access technology• Different stations assigned different subcarriers
in the same channel–WiMAX, LTE
• Symbol timing alignment is critical
• Requires tight synch with cellular BS– Use of guard times, CP (cyclic prefic)
– 802.11: CP-to-symbol length ratio 1:4 (0.8μs to 3.2μs)
OFDM-based Channel Access in WLANs• Challenge 1: Coordinate random
access among multiple stations– Cannot use cellular-type synchronization– Need a new OFDM architecure for
distributed coordination
• Challenge 2: Longer symbol length to maintain 1:4 CP-to-symbol length ratio–Makes backoff mechanism inefficient– Need new MAC contention mechanism,
new backoff scheme
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Paper Contributions• Design and implementation of FICA– Cross-layer architecture based on OFDM– Enables fine-grained subchannel random
access in WLANs
• Two key techniques– New PHY architecture based on OFDM– Novel frequency domain contention
method
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FICA Overview• Uplink transmission
• Downlink transmission similar
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• Using carrier sensing
• Using reference broadcast
Symbol Time Misalignement
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PHY Architecture
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• Each 802.11 channel (20Mhz) divided into 1.33Mhz subchannels– 14 + guardband
• Each subchannel divided into 17 subcarriers– 16 + pilot
• Data is transmitted over all 16 subcarriers
Frequency Domain Contention
• Allocate K subcarriers per subchannel– Contention band
• Each node contending for a subchannel picks randomly a subcarrier and sends a ‘1’ in M-RTS
• AP arbitrates contention and sends winning subcarriers in M-CTS
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Issues in Frequency Domain Contention
• What if 2 nodes choose the same subcarrier?– Collision– No transmission
• How large should K be?– K=16 (initial backoff value in 802.11)
• Who is returning M-CTS?– Only potential receivers– Allocate 40 subcarriers, hash receiver’s ID into
0..39, set appropriate subcarrier
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M-RTS, M-CTS
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Frequency Domain Backoff• How many subchannels can a node contend
for?– n=min(Cmax, lqueue)
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Downlink Transmission• AP can transmit simultaneously to many clients
– Different subchannels per client, has to contend for each subchannel
• Two-way traffic– FICA uses no backoff, AP and station can send M-RTS
simultaneously
• Solution: use different DIFS to prioritize transmissions– Fixed DIFS to all stations, 2 DIFS to AP– If AP uses short DIFS, use long DIFS next time– If AP receives M-RTS, use short DIFS next time– Fair interleaving of uplink-downlink, not among all
stations!
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Multiple Domains – Hidden Terminals
• Hidden terminals– Collisions may cause M-RTS/M-CTS loss– Random backoff after M-CTS loss
• Multiple domains– Nodes may receive inconsistent M-CTS from
different nodes– Node only allowed to transmit if wins contention
in all domains it participates.
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Evaluation• Simulation
• Implementation
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Simulation Setup• Event-based simulator• Only uplink traffic• Packet loss only due to collisions• Compare against 802.11n– No aggregation– Full aggregation–Mixed traffic
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Simulation Results No Aggregation
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Simulation Results Full Aggregation
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• All nodes saturated, frame size 18KB!
Simulation Results Mixed Traffic
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Implementation• Sora platform [NSDI ‘09]– Fully programmable software radio
platform
• Implementation cannot run in real time– Takes too long to transfer PHY frames
from CPU to RCB (Even though Sora is the fastest platform available)
– Have to prestore all PHY frames in RCB
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Evaluation – Time Misalignment
With Broadcasting With Carrier Sensing
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Reliability of PHY Signaling
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Demodulation Performance
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Conclusion• Trend in 802.11 WLANs– Throughput efficiency decreases as data rate
increases
• Fundamental reason– Entire wide-band channel allocated to one node
• FICA– Cross-layer design to enable fine-grained
subchannel random access – New PHY arhitecture based on OFDM– New frequency domain backoff scheme
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