1 disruption-tolerant link-level mechanisms for extreme wireless network environments vijay...
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Disruption-Tolerant Link-level Mechanisms for Extreme Wireless Network Environments
Vijay Subramanian1, K. K. Ramakrishnan2 and Shiv Kalyanaraman1
1-(Rensselaer Polytechnic Institute) , 2-(AT&T Labs Research)
Status
Reports
Packets,FEC
Repairs
TCP Sender TCP Receiver
We gratefully acknowledge support from AFOSR ESC Hanscom and
MIT Lincoln Laboratory, Letter No. 14-S-06-0206
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Growth of Wireless NLOS Mesh Deployments
Taipei
Philadelphia
San Francisco
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Wireless Mesh Disruptions: Time-Scales & Reslience Strategies
1-10s
Bit errors
10ms-1s
Interference, Capture Effects, Jamming(Short-Medium term)
1-100sec
Longer-term path/link disruptions
Packet Erasures, Burst losses
10 s-1ms
Basic FEC Adaptive Hybrid ARQ/FEC(link/transport)LT-TCP, LL-HARQ
Cross-layer functions(transport/link/MAC)
Cross-layer functions+ multi-path transport(transport/routing/link/MAC)
In this paper, we look at link- and cross-layer protocol design under medium time-scale disruption conditions (10ms-1s).
Wireless links of NLOS meshed networks susceptible to poor performance &
outage due to path loss, shadowing, fast and multi-path fading, and interference
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Recent Work: LT-TCP (Small Time-Scale)
• SACK: Exponential falloff in performance with PER
• 5%+ PER • 100 ms+ RTT
• LT-TCP: Linear falloff in performance with PER
• Scales well for even up to 50% PER • Scales well with RTT (100ms)
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LT-TCP, LL-HARQ Scheme Features
• Initial transmission consists of data + PFEC packets.
• Feedback from the receiver indicates the number of units still needed for recovery.
• RFEC packets are sent in response to the feedback.
• If k out of n units reach the receiver, the data packets can be recovered.
• LT-TCP at the transport layer and LL-HARQ at the link layer.
• LL-HARQ operates with a strict limit of 1 ARQ attempt to bound latency.
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Medium Time-Scale: Issues
• Cross-system Interference and Co-channel interference are the main causes of packet loss.– Residual loss rate on these wireless links can be non-
trivial.– Over multiple hops, the end-end loss rate can be
significant.
• End-End residual loss leads to timeouts at TCP-SACK, low throughput/goodput and poor utilization on an end-end basis.
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Medium Time-Scale: Cross-Layer Issues
• PHY/MAC protocols can confuse interference with noise and perform rate-adaptation (a.k.a. adaptive modulation/coding). – With rate-adaptation, packets can be on the air longer. – In CSMA-based MAC, the same diversity modes (time-, frequency-, code-)
are being competed for by all users to achieve high bit-rate– The “collision” periods will now be longer and reducing goodput & increasing
latency.– We experiment with turning off rate adaptation (in practice, when interference
is sensed)
• MAC backoff also increases ARQ delays at the link layer• Variable packet transmission time => large and variable queuing delays,
confusing the AQM scheme.• Disruptions more than several RTTs => LL-HARQ cannot overcome &
we propose a mode-switching algorithm to complement it.
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Disruptions Cause:Interference Capture Effects in 802.11 Links
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Link-Layer Interactions and Interference
• 802.11b WLANs operate in unlicensed ISM band– 2.412 - 2.480 GHz– Can operate at 11 , 5.5 , 2 or 1 Mb/s raw data rate with rate
adaptation algorithms that are typically proprietary.
• Smaller Cells without fine-grained adaptive power-control: Higher probability of interference.– Rate adaptation counter-productive with CSMA/CA MAC.
• Cross-System Interference– Other systems that use the unlicensed ISM band include
Bluetooth , cordless phones, microwaves etc..
• Co-Channel Interference– Other WLAN systems using the same frequency in close
proximity.– Hidden Nodes in Remote Cells
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Co-Channel Interference: Simulation Setup
• Node 1 is uploading data through BS-1• Node 2 is downloading a large file from BS-2
– Capturing the channel
• Node 1 is effectively experiencing capture for 250 ms every 2 seconds.
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Results for Co-Channel Interference
• When RTT > 100 ms, the residual loss of even one WLAN hop
(subject to capture effects) can lead to low TCP-SACK throughput:
• LT-TCP + modest parameter changes at the link/MAC restores performance.
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Disruptions (100% loss) With High Loss (0-50%) when no disruptions
Possible Causes: Outage due to path loss/shadowing/interference (cell edges), mobility
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Simulation Setup: 1-hop and 4 hops
ON Period: 100% Loss (Disruption)
OFF Period: 0-50% Loss (High Loss!)
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Performance of LL-HARQ with Longer Disruptions
To mitigate disruptions, link enhancements are needed even with LL-HARQ at the link-layer.
Disruption ON/OFF Model: 100ms in ON State (100% loss).
100ms in OFF state (p% loss).
LL-HARQ at links. Average PER shown in x-axis.
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LL-Mode-Switching with Disruption Detection
The link operates in either pipelined mode (no outage)or in stop-wait “probing” mode (outage detected). “Outage” = all fragments of a packets lost. [ HARQ limit of 1 applies to pipelined mode only. Separate ARQ limit for stop-wait probing mode.]
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TCP-SACK and LT-TCP Performance under Disruptions with LL-Mode-Switching
Disruption-tolerance (Mode-switching) enhancements to LL-HARQ: Low per-hop residual loss rate (even for 50%-100% ON-OFF case)!LT-TCP still useful for multiple hops to deal with accumulated loss rate
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Trade-off between ARQ and Performance (with Disruptions)
During disruptions, having unbounded ARQ attempts (w/ stop-and-wait mode) is counter-productive due to spurious timeouts (despite having lower residual loss) !
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TCP-SACK and LT-TCP Performance under Disruptions with LL-Disruption Enhancements (I)
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Link and Transport Throughput and Goodput over 1 hop and 4 hp scenarios.
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Summary
• LT-TCP provides robustness even under conditions of large and bursty loss rates.– Avoids timeouts– High Goodput– Increased Dynamic Range
• TCP performance over wireless with residual erasure rates 0-50% (short- or long-term).
• Outage and Disruptions at link-level impact the performance of transport layer protocols.
• Outage detection and protection at link layer can improve performance even under severe conditions (with support at the transport layer).
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Thanks!
Researchers:
•Vijay Subramanian: •[email protected] (Rensselaer Polytechnic Institute)
•K.K. Ramakrishnan, •[email protected] (AT&T Labs Research)
•Shivkumar Kalyanaraman: •[email protected] (Rensselaer Polytechnic Institute)
Thanks also for the support from AFOSR ESC Hanscom and MIT Lincoln Laboratory, Letter No. 14-S-06-0206
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Extra Slides
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Basic Link-level Scheme in more Detail
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Link-level Insights
• Link Level Recommendations include:– Moderating the rate adaptation technique as a
function of interference detection– Larger buffers with flexible AQM/ ECN markings– Enhance TCP with LT-TCP mechanisms to be
robust in high loss rate scenarios.– Link-level can provide persistence beyond
disruption time-scales
Link-level ARQ is not a panacea even with LANs since end-end latency (spurious timeouts), and residual loss matter.
But LL can provide persistence across a short-term disruption period using mode-switching.
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Future Research Directions
• Test and validate our approaches using real-world traces and data sets.
• Compare our proposed approach with other existing schemes such as SCTCP, WTCP etc..
• Move towards a real-world implementation to study the impact of practical constraints.– Proposed platform is Linux (2.6+)– This will help quantify processing costs (memory
and time) and ensure backward compatibility.
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Co-Channel Interference
• Future cells could be small and compact to provide high data rate to users.
• Users may be able to connect to multiple base stations (some using the same frequency)
• RTS/CTS may also not be able to help if interfering node is far.
• Assume rate-adaptation is turned off and cells operate at 11Mb/s with only the MAC transmission rate at 1 Mb/s.
How can end-end mechanisms, such as TCP enhancements
help to make
communication robust under these high loss scenarios?
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Interference & Capture: Negative Effects
• Co-channel interference (many users, unplanned environments)
• Cross-system interference (eg: Wifi vs Bluetooth, microwave ovens, jamming)
• MAC protocols like CSMA/CA => some users can consistently “capture” the channel– Unfairness, temporary outage (100s of ms)
• Link-level disruptions can also be caused due to mobility, temporarily misaligned directional antennas etc
Rate-adaptation with CSMA/CA and high load: harmful if the source of packet loss
is interference rather than fading => we turn off rate-adaptation if interference.