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Wireless Network
Coding
Martin Xu
Outline
• Introduction
• New Solutions
– COPE
– ANC
• Conclusions
Introduction
• Wireless: mobility
• Problem: severe throughput limitation
• Solutions
– COPE
– ANC
–…
• Let’s take a look at traditional wireless…
Traditional Wireless
Router
Traditional Wireless
Router
4 transmission required
Traditional Wireless
Router
4 time slots required
COPE
• Katti, S., Rahul, H., Hu, W., Katabi, D., Médard, M., and Crowcroft, J. 2006.
XORs in the air: practical wireless network coding. In Proceedings of the
2006 Conference on Applications, Technologies, Architectures, and
Protocols For Computer Communications (Pisa, Italy, September 11 - 15,
2006). SIGCOMM '06. ACM Press, New York, NY, 243-254.
• Forward multiple packets in a single
transmission
• Let’s take a look at how COPE deals with
the aforementioned example
COPE
Router
XOR =
COPE
Router
COPE
Router
XOR
=
XOR
Higher throughput (3 transmissions required)
=
COPE
• Transparent coding layer between IP and
MAC
• Forward multiple packets in a single
transmission
• Never delay packets
• HOW?
– Opportunistic Listening
– Opportunistic Coding
– Learning neighbor state
COPE - Listening
• “Broadcast” in a small neighbor
• Each node stores overheard packets for a
limited time
• Pseudo broadcast
– Broadcast results in poor reliability and lack
of backoff
– Pseudo broadcast unicast packets that are
meant for broadcast
COPE - Coding
• To transmit n packets, p1, ..., pn, to n nexthops,
r1, ..., rn, a node can XOR the n packets
together only if each next-hop ri has all n − 1
packets pj for j = i.
COPE - data structure
• Reception report
– Reports are piggybacked on packets
– If no packets to send, periodically send
reports
• Output queue (FIFO)
• Two per-neighbor virtual queue
– packet-size distribution in the Internet is
bimodal with peaks at 40 and 1500 bytes
• Packet info
COPE - Performance
• With no hidden terminals, TCP’s
throughput can increase by 38%
• flows, COPE increases UDP throughput by
3-4x
ANC
• Katti, S., Gollakota, S., and Katabi, D. 2007. Embracing wireless
interference: analog network coding. In Proceedings of the 2007
Conference on Applications, Technologies, Architectures, and Protocols For
Computer Communications (Kyoto, Japan, August 27 - 31, 2007).
SIGCOMM '07. ACM Press, New York, NY, 397-408.
• Strategically exploit interference instead of
avoiding it
– Interfered signal is not exactly the sum
– Channel distorts signals
– Two signals are never synchronized
ANC - Algorithm
• Decode small part of uninterfered signal
(MSK)
• Decode interfered signal
– Decomposition using amplitudes of the
original ones and the interfered
– Four possible angles
– Choose the 90 degree one
• Decode the rest of the uninterfered signal
ANC - Performance
• For the example used at the beginning,
median throughput gain of ANC over
routing 70%, COPE 30%
• For X-topology, median throughput gain
over routing is 65%, over COPE 28%
• For chain topology, median throughput
gain over routing is 37%
Conclusion
• Both implementation that yields large
throughput gains
• COPE
– Simple and practical
• ANC
– Embrace Broadcast and Interference