Cool Topics in Networking CS144 Review Session 8 November 20, 2009 Samir Selman.

Cool Topics in Networking

CS144 Review Session 8November 20, 2009

Samir Selman

Announcements

• Lab 5 : Due Thursday Dec 3• Final Exam: Wednesday, December 9

12:15pm - 3:15pm• For those of you submitting late, contact us

before your deadline if you need an additional extension. Tell us:– Where you are– How much more time you need

Today’s Cool Topics

1. Network Coding

2. Wireless Sensor Networks

Current Wireless

Router

C

Router

Current Wireless

Traditional Routing requires 4 time slots

C

Router

Current Wireless


C

XOR

=

Router


Network Coding

C

Network Coding

Router


C

Network Coding

XOR

= XOR

=

Router


With Network Coding need only 3 time slots Higher throughput

C

I - COPE

Sachin Katti, Hariharan Rahul, Wenjun Hu, Dina Katabi, Muriel Medard, and Jon Crowcroft, "XORs In The Air: Practical Wireless Network Coding," ACM SIGCOMM, 2006.

http://nms.lcs.mit.edu/~dina/pub/xor.pdf

COPE Coding Opportunities

COPE Coding Opportunities

II- Opportunistic Routing• Traditional routing chooses next hop before transmitting a packet.

• Poor Link Quality => Probability of chosen next hop receiving packet is low

• Solution: Opportunistic Routing allows any node that overhears the transmission and is closer to the destination to participate in forwarding the packet

• Challenge: Multiple nodes might hear a packet broadcast and unnecessarily forward the same packet.

EXOR• EXOR solves this issue by tying the MAC to the Routing and imposing a strict schedule on the routers.

•The scheduler goes in rounds. Forwarders transmit in order, and only one forwarder is allowed to transmit at a given time.

•Other nodes listen to learn which packets were overheard by other nodes.

•Problem: This strict scheduling prevents forwarders from exploiting spatial reuse (even when multiple packets can be received by their respective receivers).

MORE

Szymon Chachulski, Michael Jennings, Sachin Katti, and Dina Katabi, "Trading Structure for Randomness in Wireless Opportunistic Routing," ACM SIGCOMM, 2007

http://nms.lcs.mit.edu/~dina/pub/MORE.pdf




MORE

• MORE solves the problem with Opportunistic Coding without tying Routing to the MAC.

• Instead it uses Network Coding + Randomness.

• Basically nodes randomly mix packets before forwarding them.

• This ensures the routers hearing the same transmission do not forward the same packet.

MORE - Examples

Unicast Case

•Src sends P1,P2•Dest luckily overhears P1.•Router doesn’t know what dest received (P1).•In any case R can forward P1 + 2P2•Dest now has two received packets

• P1•P1 + 2P2

•Dest can solve 2 eqns with 2 unknowns to retrieve P2.•Conclusion: R only forwarded one packet instead of two =>Higher throughput

MORE - Examples

Multicast Case

•Without Network Coding, src has to retransmit the union of the lost packets ( 4 pkts ).

•With Network coding can retransmit only 2 randomly coded pkts and allow all destinations to decode their respective packets.

• Src retransmits pa = p1+ p2 + p3 + p4, and pb= p1 + 2p2 + 3p3 + 4p4.

Analog Network Coding

Sachin Katti, Shyamnath Gollakota, and Dina Katabi, "Embracing Wireless Interference: Analog Network Coding," ACM SIGCOMM, 2007.

http://nms.lcs.mit.edu/~dina/pub/anc.pdf



Analog Network Coding (ANC)

Instead of router mixing packets…

Exploit that the wireless channel naturally mixes signals



Router

C

Router


1) Phil and David transmit simultaneously

Interference

C

Router


1) Phil and David transmit simultaneously2) Router amplifies and broadcasts interfered signal

C

Router


1) Phil and David transmit simultaneously2) Router amplifies and broadcasts interfered signal3) Phil subtracts known signal from interfered signal

C

Router


1) Dina and Robert transmit simultaneously2) Router amplifies and broadcasts interfered

signal3) Dina subtracts known signal from interfered

signal

Analog Network Coding requires 2 time slots

Higher throughput

Analog Network Coding requires 2 time slots

Higher throughput

C

It Is More Than Going From 3 To 2!

• Philosophical shift in dealing with interference• Strategically exploit interference instead of

avoiding it

• Promises new ways of dealing with hidden terminals

C C CC

Hidden Terminal Scenario

R1 R2Src Dst

P1


C C CCR1 R2Src Dst

P2


P1

1) Src and R2 transmit simultaneously

C C CCR1 R2Src Dst


1) Src and R2 transmit simultaneously2) R1 subtracts P1, which he relayed earlier to

recover P2 that he wants

P1 P2

C C CCR1 R2Src Dst


R2 and Src are hidden terminals Today : Simultaneous transmission

Collision ANC : Simultaneous transmission

Success!

P1 P2

C C CCR1 R2Src Dst


Other Benefits of ANC: First step toward addressing hidden

terminals ANC extends network coding to new

scenarios

Other Benefits of ANC: First step toward addressing hidden

terminals ANC extends network coding to new

scenarios

C C CCR1 R2Src Dst

Wireless Sensor Networks

1. A sensor network is an Ad-hoc network composed of densely populated tiny electronic sensing devices.

2. Basic function of the network is to observe some phenomenon.3. Characteristics:

a. Low cost, Low power, Light weightb. Densely deployedc. Prone to failuresd. Two ways of deployment: randomly, pre-determined

4. Objectives:1. Monitor Activities2. Gather and fuse information3. Communicate it to special node “Base Station”.

Computer Revolution

0.5 oz, 2.25 x 1.25 x 0.25 inch25 lb, 19.5 x 5.5 x 16 inch

~14 mW~ 64 W

~ $35~ $6K (today)

512 KB Flash160 KB Floppies

128 KB RAM16-256 KB RAM

4 MHz4.77 MHz

MICAZ Mote (2005)Original IBM PC (1981)

Sensor Node Hardware Platform

Software Platform

WSN Applications

WSN Applications

WSN Applications

WSN Applications

WSN Applications

WSN protocols

Protocol Requirements:1. Energy Efficient (Maximize node lifetime)2. Self Configuring3. Scalable4. Redundant5. Efficient (less computation, less memory

requirements, less energy consumption…)6. Robust

Energy Efficiency

I. Sources of Energy Consumption:1. Communications (Transmitting & Receiving)2. Computations3. Sensing

II. Sources of Energy Wastage in Communications:1. Collisions2. Overhearing3. Idle Listening4. Control Packets overhead5. Over emitting

WSN Protocol Research

Questions?

Cool Topics in Networking CS144 Review Session 8 November 20, 2009 Samir Selman.

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