Cool Topics in Networking CS144 Review Session 8 November 20, 2009 Samir Selman.
Post on 19-Dec-2015
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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
Network Coding
XOR
= XOR
=
Router
Traditional Routing requires 4 time slots
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.
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
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.
Analog Network Coding (ANC)
Instead of router mixing packets…
Exploit that the wireless channel naturally mixes signals
Analog Network Coding
Router
Analog Network Coding
1) Phil and David transmit simultaneously2) Router amplifies and broadcasts interfered signal
C
Router
Analog Network Coding
1) Phil and David transmit simultaneously2) Router amplifies and broadcasts interfered signal3) Phil subtracts known signal from interfered signal
C
Router
Analog Network Coding
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
Hidden Terminal Scenario
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
Hidden Terminal Scenario
R2 and Src are hidden terminals Today : Simultaneous transmission
Collision ANC : Simultaneous transmission
Success!
P1 P2
C C CCR1 R2Src Dst
Hidden Terminal Scenario
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)
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
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