Interview talk at various universities and labs

1
The Importance of Being Opportunistic
Sachin Katti Dina Katabi, Wenjun Hu, Hariharan
Rahul, and Muriel Medard
2
Bandwidth is scarce in wireless

• Can we send more while consuming less bandwidth?

3
Current Approach
Relay
Bob

• Requires 4 transmissions
• Can we do it in 3 transmissions?

4
A Network Coding Approach
Relay
Bob
3 transmissions instead of 4 ? Save bandwidth
5
Network Coding

• Routers mix bits in packets, potentially from
  different flows
• Theoretically shown to achieve capacity for
  multicast
• No concrete results for unicast case

6
How to apply network coding?
State-of-the Art
R

• Multicast

S
R

• Given Sender Receivers

• Given Flow Rate Capacities

R
7
How to apply network coding?
State-of-the Art
In Practice

• Unicast

• Multicast

• Given Sender Receivers

• Many Unknown Changing Sender Receivers

• Given flow rate capacities

• Unknown and bursty flow rate

Min-Cost Flow Optimization
?
Find the routing, which dictates the encoding
8
How to apply network coding?
State-of-the Art
In Practice

• Unicast

• Multicast

• Given Sender Receivers

• Many Unknown Changing Sender Receivers

• Given flow rate capacities

• Unknown and bursty flow rate

Min-Cost Flow Optimization
Opportunism
Find the routing, which dictates the encoding
9
Opportunism (1)

• Opportunistic Listening
• Every node listens to all packets
• It stores all heard packets for a limited time

10
Opportunism (1)

• Opportunistic Listening
• Every node listens to all packets
• It stores all heard packets for a limited time

A
D
B
11
Opportunism (1)

• Opportunistic Listening
• Every node listens to all packets
• It stores all heard packets for a limited time
• Node sends Reception Reports to tell its
  neighbors what packets it heard
• Reports are annotations to packets
• If no packets to send, periodically send reports

12
Opportunism (2)

• Opportunistic Coding
• Each node uses only local information
• Use your favorite routing protocol
• To send packet p to neighbor A, XOR p with
  packets already known to A
• Thus, A can decode
• But how to benefit multiple neighbors from a
  single transmission?

13
Efficient Coding
A
D
C
B

• Arrows show next-hop

14
Efficient Coding
Bad Coding C will get RED pkt but A cant get
BLUE pkt
A
D
C
B

• Arrows show next-hop

15
Efficient Coding
Better Coding Both A C get a packet
A
D
C
B

• Arrows show next-hop

16
Efficient Coding
Best Coding A, B, and C, each gets a packet
A
D
C
B
To XOR n packets, each next-hop should have the
n-1 packets encoded with the packet it wants

• Arrows show next-hop

17
But how does a node know what packets a neighbor
has?

• Reception Reports
• But reception reports may get lost or arrive too
  late
• Use Guessing
• If I receive a packet I assume all nodes closer
  to sender have received it

18
Putting It Together
A
C
D
B
19
Putting It Together
A
C
D
B
20
Putting It Together
A
C
D
B

• Dont reorder packets in a flow ? Keeps TCP
  happy
• No scheduling ? No packet is delayed

21
Beyond Fixed Routes
D
A
B
S
No need for A transmission
S transmits
Route Chosen by Routing Protocol
Opportunistic Routing BM05

• Piggyback on reception report to learn whether
  next-hop has the packet
• cancel unnecessary transmissions

22
Opportunism

• Unicast
• Flows arrive and leave at any time
• No knowledge of rate
• No assumption of smooth traffic

C
K
23
Performance
24
Emulation Environment

• We use Emstar
• Real code simulator
• 802.11 radios
• Power Level 200mW
• 11Mbps bit rate
• Simulated radio channel

25
Recall Our Simple Experiment
XOR
Relay
Bob
3 transmissions instead of 4 ? 25 throughput
increase
26
Practical artifacts make network coding perform
poorly
Throughput (KB/s)
330 KB/s
270 KB/s

• Network coding requires broadcast
• But 802.11 broadcast has no backoff ? more
  collisions

27
Pseudo Broadcast

• Ideally, design a back-off scheme for broadcast
  channels
• In practice, we want a solution that works with
  off-the-shelf 802.11 drivers/cards

Our Solution

• Piggyback on 802.11 unicast which has
  synchronous Acks and backoff
• Each XOR-ed packet is sent to the MAC address of
  one of the intended receivers

28
XOR with Pseudo-Broadcast Improves Throughput
XOR with Pseudo-Broadcast
Throughput (KB/s)
495 KB/s
No Coding
XOR
330 KB/s
270 KB/s

• Improvement is more than 25 because 802.11 MAC
  gives nodes equal bandwidth shares
• Without coding, relay needs twice as much
  bandwidth
• With coding, all nodes need equal bandwidth

29
Larger experiment

• 100 nodes
• 800mx800m
• Senders and receivers are chosen randomly
• Metric
• Total Throughput of the Network

30
Opportunistic Coding vs. Current
Network Throughput (KB/s)
Opp. Coding
Our Scheme
Opp. Coding (no guessing)
No Coding
No Coding
A Unicast Network Coding scheme that works well
in realistic situations
No. of flows
31
Preliminary Implementation Results

• Linux Kernel
• 802.11 MAC
• Click Elements (part of Roofnet)
• Only Opportunistic Coding Pseudo Broadcast (No
  opportunistic Listening)

Relay
Bob
32
Implementation Results
Ratio of throughput with coding to without coding
Network Coding Doubles the Throughput
33
Conclusion

• First implementation of network coding in a
  wireless network
• Learned Lessons
• Be opportunistic (greed is good!)
• Can do a good job with multiple unicast
• 5x higher throughput in congested networks
• Preliminary implementation results show
  throughput doubles

34
The Wireless Environment

• Multi-hop wireless networks (e.g., Roofnet)

35
Opportunistic Coding vs. Current
Network Throughput (KB/s)
Our Scheme
No Coding
A Unicast Network Coding scheme that works well
in realistic situations
No. of flows