1. Xors in the air
Sachin Katti, Hariharan Rahul, Wenjun Hu, Dina Katabi, Muriel Medard, Jon Crowcroft

2. Definition: Network coding
Allowing the routers to mix (code) packets from different sources in order to increase the information content of each transmission
increasing the network throughput

3. Motivation
Wireless implementation suffer from a sever throughput limitation.

4. Current approach
This process requires 4 transmissions

5. Network coding approach
This process requires 3 transmissions instead of 4

6. Conclusions from the example
Network coding Leads to large
bandwidth savings.
It exploits the shared nature of the
wireless medium which, for free,
broadcasts each packet in a small
neighborhood around its path.

7. The butterfly example Without network coding All links have a capacity
of one message per unit
of time.

8. XOR  is one of the simplest form of data coding
With Network Coding
With network coding
XOR  is one of the simplest form of data coding
By sending the XOR of b1 and b2 on the middle link , we can deliver two messages per unit of time to both receivers.

9. Conclusion
Allowing the routers to mix the bits in forwarded messages can increase network throughput.

10. COPE A new forwarding architecture for wireless mesh networks.
It inserts a coding layer between the IP and MAC layers.
The new layer detects coding opportunities and exploits them to forward multiple packets in a single transmission.

11. Cope incorporates 3 main techniques :
Opportunistic listening
Opportunistic Coding
Learning neighbor state

12. Opportunistic Listening
The nodes snoop on all communications over the wireless medium
The nodes store the overheard packets for a limited period T (default=0.5s)
Each node broadcast reception reports telling
its neighbors which packets it has stored

13. Opportunistic Coding
The key question : What packets to code together to maximize throughput?

14. Node may have multiple options
Node may have multiple options. The aim: Maximizing the number of packets delivered in a single transmission Constraints: Ensuring that each intended next hop has enough information to decode the packet.

15. Example

16. Important Results The coding algorithm should ensure that all
next hops of an encoded packet can decode
their corresponding packets.
If not, unneeded data will be forwarded to areas where there is no interested receiver, wasting much capacity.

17. Simple Rule
To transmit n packets :p1,…pn to n next hops 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.

18. Learning Neighbor State
The key question :
How does a node know what packets its neighbors have?

19. Each node announces to its neighbors the packets it stores in reception reports. Problem: what happens at times of severe congestion or at times of light traffic?

20. Severe Congestions
Reception reports may get lost in collisions
Light traffic
The reception reports may arrive too late, after the node has already made a suboptimal coding decision.

21. Conclusion
A node cannot rely solely on reception reports, and may need to guess whether a neighbor has a particular packet.

22. Node may make an incorrect guess Causing to the coded packet to be undecodable at some next hop. The relevant packet is retransmitted, encoded with a new set of packets.
חיסרון של COPE.

23. Cope in ad Hoc Network
The researchers run experiments on 20-node testbed to calculate the throughput increase provided by COPE in an ad hoc network.

24. TCP TCP flows arriving according to a Poisson process.
Pick sender and receiver randomly for transferring files.
TCP does not show any significant improvement with coding.

25. GUILTY

26. GUILTY TCP’s reaction to collision related losses.
There are a number of nodes sending packets, but they are not within the carrier sense range.
HIDDEN TERMINAL PROBLEM
Creating many collision related losses that
cannot be masked even with the maximum
number of MAC retries.

27. Would TCP be able to do better with COPE if
we eliminated collision related losses?

28. Hypothesis performing
Compressing the topology of the testbed by bringing the nodes closer together, so that they are within carrier sense range.
Results:
Reduce the loss rate
Enable TCP make better use of the medium

29. Cope in UDP

30. COPE greatly improves the throughput of
these wireless networks.
At low demands (below 2Mb/s), coding
performs similarly to no coding.
As demands increase, both network
congestion and the number of coding
opportunities increase.

31. how much of the coding is due to guessing, as opposed to reception reports?

32. Low demands increasing dependence on guessing.
As demands increase the importance of guessing
decreases.
As demands surge even higher, the network
becomes significantly congested, leading to high loss
rates for reception reports. Hence, decisions is again
made based on guessing.

33. THE END !