WinlabMadwifi → Pipeline MAC paper Discussion

Paper Title:

Cooperative Scheduling Via Pipelining in 802.11 Wireless Networks

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Ramana Rao Kompella, Sriram Ramabhadran, Ishwar Ramani, Alex C. Snoeren
University of California, San Diego

Motivation

• Per packet Micro-scheduling need firmware/hardware(MAC) upgrade
• Macro-scheduling could be done in multiple packet basis periodically, e.g 10ms.
• Cooperative scheduling extensions can be supported using a new layer on top of the existing MAC layer.

Features of Scheme

• each node conducts "global scheduling" based on "load information" of all other nodes.
  • the global scheduling determines a TDMA-like slot/bandwidth allocation
• 3 stages, 2 ½ pipelining:
  • Estimation — load exchange — scheduling
  • equivalent to predict — signaling — access
  • "load excange" phase overlapping with "estimation"
  • First cycle's scheduling overlap with second cycle's "load exchange" phase
• Kernel code modification
  • packet_tx
  • packet_rx
  • stop_queue
  • wake_queue

Diagrams

Figure 1: 802.11 extension layer architecture

Figure 2:2.5-stage pipeline architecture. Each pipeline stage is equal in duration, but estimation stage and load exchange stage for a given cycle overlap with each other.

Experiment Evaluation

using 2 nodes and 1 AP, configure 1 node to have twice bandwidth as another node (interferer/competitor). All use NETGEAR cards (Atheros-chip).

Weakness

• a solution for IEEE 802.11 Wireless LAN, not a solution for multi-hop
• no solution for signalling loss, wrong scheduling automatically fades out in next cycle.
• rely on AP's beacon to sync all nodes.
• very simple experiment scenario and incomplete results.

Disucussion

1. Is a kernel patch a good approach? The kernel & driver are difficult to develop. As long as the software can be stablized, it is a feasible approach.
2. 10ms scheduling resolution, good enough? No. Schedule too many packets in a batch. It's better to use HW timer, a 64-bit timing counter in Atheros chip.