1 Disruption-Tolerant Link-level Mechanisms for Extreme Wireless Network Environments Vijay Subramanian 1, K. K. Ramakrishnan 2 and Shiv Kalyanaraman 1 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

2 Growth of Wireless NLOS Mesh Deployments Taipei Philadelphia San Francisco

3 Wireless Mesh Disruptions: Time-Scales & Reslience Strategies 1-10  s 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

4 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)

5 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.

6 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.

7 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.

8 Disruptions Cause: Interference Capture Effects in Links

9 Link-Layer Interactions and Interference b WLANs operate in unlicensed ISM band – 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

10 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.

11 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.

12 Disruptions (100% loss) With High Loss (0-50%) when no disruptions Possible Causes: Outage due to path loss/shadowing/interference (cell edges), mobility

13 Simulation Setup: 1-hop and 4 hops ON Period: 100% Loss (Disruption) OFF Period: 0-50% Loss (High Loss!)

14 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.

15 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.]

16 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

17 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) !

18 TCP-SACK and LT-TCP Performance under Disruptions with LL-Disruption Enhancements (I)

19 Link and Transport Throughput and Goodput over 1 hop and 4 hp scenarios.

20 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).

21 Thanks! Researchers: Vijay Subramanian: (Rensselaer Polytechnic K.K. Ramakrishnan, (AT&T Labs Shivkumar Kalyanaraman: (Rensselaer Polytechnic Thanks also for the support from AFOSR ESC Hanscom and MIT Lincoln Laboratory, Letter No. 14-S

22 Extra Slides

23 Basic Link-level Scheme in more Detail

24 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.

25 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.

26 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?

27 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.