Network Coding for Wireless Networks Wireless Communication Project by Group 2 1

Network Coding for Wireless Networks

Wireless Communication Project by Group 2

1

Shang Shang Ma

Rong

Yang Yang Wang Chen

Network Coding for Wireless Networks

INTRODUCTION1

COPE DESCRIPTION2

COPE IMPLEMENTATION 3

ANALOG NETWORK CODING4

2

Network CodingWhat?

Why?

Definitions

Introduction

3

1

Max-flow

Min-cut Theory

2

Unicast, Multicast

3

Through-put

Definitions

4

Max-Flow Min-Cut Theorem

• (From Wiki) The max-flow min-cut theorem is a statement in optimization theory about maximal flows in flow networks

• The maximal amount of flow is equal to the capacity of a minimal cut.

• The maximum flow in a network is dictated by its bottleneck.

5[1] S.-Y. R. Li, R. W. Yeung, and N. Cai, “Linear network Coding”, IEEE Trans.

http://en.wikipedia.org/wiki/Optimization_(mathematics)

http://en.wikipedia.org/wiki/Maximum_flow_problem

http://en.wikipedia.org/wiki/Flow_network

Graph

• Graph G(V,E): consists of a set and a set – V consists of sources, sinks, and other nodes– A member e(u,v) of E has a to

send information from u to v A

D

S

B

C

T

V of verticesE of edges.

S

A B

D C

T

3 3

3 3

2

2

2

4

capacity c(u,v)

6

Min-Cuts and Max-Flows

• Cuts: Partition of vertices into two sets• Size of a Cut = Total Capacity Crossing the Cut• Min-Cut: Minimum size of Cuts = 5• Max-Flows from S to T• Min-Cut = Max-Flow

S

A B

D C

T

3 3

3 3

2

2

2

4S

A

D

3 3

3

2

2

B

C

T

3

3

2

2

43

2

3

2

3

21

7

Unicast | Multicast | Broadcast

Multicast

Broadcast

Unicast

Unicast communication is one-to-one.

Multicast communication is one-to-many.

Broadcast communication is one-to-all.

8

Throughput

• The amount of data transferred from one place to another or processed in a specified amount of time.

• Data transfer rates for disk drives and networks are measured in terms of throughput. Typically, throughputs are measured in kbps, Mbps and Gbps.

9

Wire vs Wireless

An edge between two nodes means that the two nodes are physically connected.

Multicast communication is studied while network coding

of multiple unicast flows remains a largely unknown territory.

The traffic rate (or distribution) is predetermined and

do not change.

Wirevs

Wireless

The channel of one particular edge is actually shared by other neighboring edge.

Unicast communication is the dominate traffic pattern.

Traffic rates are varies over time rather than constant.

Network Modeling

Traffic Pattern

Traffic Rate

WIRE WIRELESS

10

Network CodingWhy?

Definitions

What?

Introduction

11

What is NETWORK CODING

Network Coding is a field of information theory and coding theory and is a method of attaining maximum information flow in a network.

Network Coding Theory points out that it is necessary to consider encoding/decoding data on nodes in network in order to achieve optimal throughput.

12

Multicast Problem

• Butterfly Networks: Each edge’s capacity is 1.

• Max-Flow from A to D = 2

• Max-Flow from A to E = 2

• Multicast Max-Flow from A to D and E = 1.5

• Max-Flow for each individual connection is not achieved.

A

B C

F

G

D E

1

1

1

1

1 1

1

1

10.5 0.5

0.5 0.5

13

• Ahlswede et al. (2000)– With network

coding, every sink obtains the maximum flow.

A

B C

F

G

D E

b1 b2

b1

b1

b2

b2b1+b2

b1+b2 b1+b2

[2] Rudolf Ahlswede, Ning Cai, Shuo-Yen Robert Li, and Raymond W. Yeung. Network information flow. 14

COPE

COPE is an opportunistic approach to network coding to increase the throughput of wireless mesh networks.

COPE inserts a coding layer between the IP and MAC layers, which identifies coding opportunities and benefits from them by forwarding multiple packets in a single transmission.

15[3] S. Katti, D. Katabi, W. Hu, and R. Hariharan, “The importance of being opportunistic: Practical network coding for wireless environments

An information exchange scenario

BobAlice

Relay

Alice’s packetBob’s packet

Bob’s packetAlice’s packet

• Multi-hop unicast requires 4 transmissions• Can we do better?

16

Can Network Coding help - An idea

BobAlice

Relay

Alice’s packet Bob’s packet

Bob’s packetAlice’s packet

3 transmissions instead of 4 Saves bandwidth & power 33% throughput increase

3 transmissions instead of 4 Saves bandwidth & power 33% throughput increase

XOR =

17

Analog Network Coding

Analog network coding mixes signals instead of bits.

Wireless routers forward signals instead of packets. It achieves significantly higher throughput than both traditional wireless routing and prior work on wireless network coding(COPE).

18

Network CodingWhat?

Definitions

Why?

Introduction

19

1Improve network throughput

2Superior performance in reducing the number of retransmissions in lossy networks

3

Is able to smoothly handle extreme situations where the server and nodes leave the system

Why is NETWORK CODING

20

COPE


Contents

General idea1

Opportunistic listening2

Opportunistic coding3

Opportunistic routing4

General idea of cope

Cope is an opportunistic approach to network coding to increase throughput of wireless mesh networks.

The main characteristic of COPE is opportunism.

What is cope?

What is opportunism

?


Opportunistic listening

Opportunistic coding

Opportunistic routing

A

B CE

D


Opportunistic Listening

1.Nodes have opportunities to hear packets even when they are not the intended receiver.

2.Nodes store all the packets they hear within a limited time slot T.

3.Nodes send reception reports to their neighbors, helping to create more coding opportunities.

A

B CE

D

Output E1 E2 E3

Output A1 A2 A3

pool

pool

Que

A

B

E

D

E1:B to EA1:D to A

A

B CE

D

Output E2 E3

Output A1 A2 A3

pool

pool

E1

E1

E1

Que

A

B

E

D

E1:B to EA1:D to A

A

B CE

D

Output E2 E3

Output A1 A2 A3

Que

A

B

E

D

pool

pool

E1

E1

E1 E1:B to EA1:D to A

A

B CE

D

Output E2 E3

Output A1 A2 A3

Que E1

A

B

E

D

pool E1

pool

I have E1

E1:B to EA1:D to A

A

B CE

D

Output E2 E3

Output A1 A2 A3

Que E1

A E1

B

E

D

pool E1

pool

E1:B to EA1:D to A

A

B CE

D

Output E2 E3

Output A2 A3

Que E1

A E1

B

E

D

pool E1

pool

A1

A1

A1E1:B to EA1:D to A

A

B CE

D

Output E2 E3

Output A2 A3

Que E1

A E1

B

E

D

pool E1

pool

A1A1

A1

E1:B to EA1:D to A

A

B CE

D

Output E2 E3

Output A2 A3

Que E1 A1

A E1

B

E

D

pool E1

pool A1

I have A1

E1:B to EA1:D to A

A

B CE

D

Output E2 E3

Output A2 A3

Que E1 A1

A E1

B

E A1

D

pool E1

pool A1

Node A have packet E1Node A want packet A1So I can give Node A

E1+A1

Node E have packet A1Node E want packet E1So I can give Node E

E1+A1

Coding opportunity

E1:B to EA1:D to A

A

B CE

D

Output E2 E3

Output A2 A3

Que E1 A1

A E1

B

E A1

D

pool E1

pool A1

Coding opportunity

E1 A1E1+A1

E1:B to EA1:D to A

A

B CE

D

Output E2 E3

Output A2 A3

Que

A E1

B

E A1

D

pool E1

pool A1

Coding opportunity

E1+A1

E1:B to EA1:D to A

A

B CE

D

Output E2 E3

Output A2 A3

Que

A E1

B

E A1

D

pool E1

pool A1

E1+A1E1+A1

E1:B to EA1:D to A

A

B CE

D

Output E2 E3

Output A2 A3

Que

A E1

B

E A1

D

pool E1

pool A1

E1+A1

E1+A1

E1+(E1+A1)=A1 A1 received!

A1+(E1+A1)=E1 E1 received!

E1:B to EA1:D to A

Opportunistic Coding

Each node should answer this question based on local information and without consulting with other nodes.

Each node has several options to decide which packets to XOR together to gain the maximum throughput.

How can the node decide which packets needed to XOR together?


A B

C

D


P1

P2

P4

P3

Packets Next Hops

P1P2P3P4

Node

B’s queue

B

A

B

C

D


P1P2P3P4

P3P4

P1P4

P1P3

= P1 P2+

=P1 P2+

A B

C

D

Bad coding decision

Packet node

P1-----AP2-----BP3-----CP4-----D


P1P2P3P4

P3P4

P1P4

P1P3

= P1 P3+

=P1 P3+

=P3 P1+

A B

C

D

Better coding decision

Packet node

P1-----AP2-----BP3-----CP4-----D


P1P2P3P4

P3P4

P1P4

P1P3

= P1 P3+

=P1 P3+

=P3 P1+P4+

P4 +

P4 +

=P3 P4+P1 +

A B

C

D

Best coding decision

Packet node

P1-----AP2-----BP3-----CP4-----D


Theory:

To transmit n packets, p1,…,pn, to n receivers, r1,…,rn, a node can XOR the n packets together only if each intended receiver ri has all n-1 packets pj for j not equals to i.


rn-2

r1 rn

rn-1r2

r3

riri-1

P1…Pi-1,Pi+1…PnP1…Pi-2,Pi…Pn

:XOR P1 to Pn

PiPi-1

P3

P2

P1

Pn-2

Pn-1

Pn


Whenever a node has a chance to transmit a packet, it tries to find the largest n that satisfies the above rule.

The node tries to maximize the number of packets delivered in a single transmission.

Opportunistic Routing

1.The path is stored in the packet itself.

2.Check the path, find the opportunities to routing.

Can we further reduce the

number of transmissions?

Opportunistic Routing

A S B D

Path: SABD

source Destination

Receptionreport

Conclusion

• Main idea of cope:

1.Overhear on the medium.

2.Learn the status of its neighbors.

3.Detect coding opportunities.

4.Code as long as the receivers can decode.

COPE IMPLEMENTATION

&PERFORMANCE

52

Agenda

Data Structure for each node

Pseudo broadcast mechanism

COPE layer

COPE header

Gain

53

DATA STRUCTURE

54

A SCENARIO

AB

C

D

E

F

G

RR

Reception report

P

IP packet

Ack

IP acknowledge

P

P

P

PPP

P

P

P

Packet Delivery

55

A SCENARIO(II)

AB

C

D

E

F

G

RR

Reception report

P

IP packet

Ack

IP acknowledge

Ack

Ack

Ack

Ack

Packet acknowledge

56

A SCENARIO(III)

AB

C

D

E

F

G

RR

Reception report

P

IP packet

Ack

IP acknowledge RRRR

RRRR

RRRRRR

RRRR

RR

RRRRRRRR

RR

Reception Report

57

DATA STRUCTURE

AB

C

D

E

F

G

RR

Reception report

P

IP packet

Ack

IP acknowledge

58

DATA STRUCTURE

P1

CIn Node C

Output Queue：P2P3P4

Retransmission pool：

Upper Layer

P5P6 P1

Packet Transmitted

59

DATA STRUCTUREC

In Node C



Upper Layer

P5P6

P1

Ack ReceivedP1 can be deleted

If ack is not received after a long time

P1

P1 returns to Output Queue to waitanother transmission opportunity

60

DATA STRUCTURE

RR： a,b,c,d,x

P1

CIn Node C



Upper Layer

P5P6

Opportunity Listening Queue：

If node C receive a neighbor’s packet whose destination is not C, Node C just put the packet into the above queue.

Px is head for node DNot C.

Pa Pb Pc Pd

Periodical,Each node inform its neighbor about what they’ve got in its Opportunity Listenning Queue. That is , Reception Report

61

DATA STRUCTURE

P1

CIn Node C



Upper Layer

P5P6

Neighbors’ virtual Queue：

A queue

B queue

D queue…… …………………………

Opportunity Listening Queue：Pa Pb Pc Pd Px

62

DATA STRUCTURE

P1

CIn Node C



Upper Layer

P5P6

Neighbors’ virtual Queue：

A queue

B queue

D queue…… …………………………


RR from A

Pa Pb Pc Pd Px

RR from B

RR from D

Used for decoding

Used for Coding

63

• Packet headed to the same destination can never be coded together

• It is not necessary to code a packet in opportunity listening queue

• It is not necessary to transmit a packet when it is received again from its destination

• A node can decode a packet which is coded with one of the packet he once send

DATA STRUCTURE

LOGIC

64

DATA STRUCTURE

Output Queue：


Upper Layer


P1P2P3P4P5P6 P1P1

65

DATA STRUCTURE

4 PARTS:

Output Queue

Retransmission Pool

Opportunity Listening Queue

Neighbor’s Virtual Queue

66

PSEUDO BROADCASTMECHANISM

Laptop1

Laptop2

Laptop3

Both laptop2 and laptop3 are the intendedReceiver of packet from laptop1

[4] Katti S, Rahul H, Hu WJ, Katabi D, Muriel M, Crowcroft J. XORs in the air: Practical wireless networking. 67

PSEUDO BROADCAST

802.11 MAC:Current WLAN medium access mechanism

1. DCF CSMA/CA(mandatory) -Distributed Foundation Wireless MAC

-Collision avoidance via randomized back-off-ACK packet for acknowledgement(not for broad cast)

2. DCF RTS/CTS(optional)-avoids hidden terminal and exposed terminal

3. Point Coordination Function(infrastructure mode)

How to apply COPE? Opportunistic Listenning?Multiple intended receiver?All neighbors can receive?

BROADCAST

68

PSEUDO BROADCAST

802.11 MAC:802.11 Broadcast mode:

1. DCF CSMA/CA? Yes

2. Back off? No

3. DCF RTS/CTS? No

4. ACK? No

POOR RELIABILITY

69

PSEUDO BROADCAST

802.11 MAC:

SOLUTION?

1. Develop a brand-new MAC access protocol which is suitable for COPE broadcast

2. Add a new layer on top the current 802.11 MAC layer to make link-to-link broadcast

reliable.

Hard to implement…..Unrealistic….802.11 WLAN MAC already pervasive

Feasible!

2. PSEUDO BROADCAST

70

PSEUDO BROADCASTReliability

1. ACK2. Retransmission(ACK timeout)3. Multiple Intended Receiver

Packet

SIFS

Packet

ACK

DIFS& backoff

SIFS

ACK

One receiverMulti receiver

ACK ACK

ACK ACK

Collision!

Synchronous Acknowledgement does not work!Receiver can send ACK packet asynchronously

1. We can treat Ack packet as a normal packet, ACK packets from different Receivers also go through the DIFS and back off procedure to avoid collision

2. Or we can piggy back on packet travelling in the reverse direction

71

PSEUDO BROADCASTWhat should be contained in COPE

packet?

1. XORed(coded) packet with multiple receivers

2. uncoded packet with multiple receivers(all neighbors)

3. Reception Report with multiple receivers (all neighbors)

4. Packet Acknowledge with certain receiver

Later on we will give a detailed description about the COPE packet format

72

COPE LAYERWhere does cope layer lies in?

Network Layer

COPE layer

MAC layer

It is the COPE layer where the four data structure: Output Queue, Retransmission pool, Opportunistic listening pool And neighbors’ virtual queue lies in.

COPE layer is a very slim layer lies between Network layer and MAC layer.

COPE layer process the network layer packets before they are send to MAC layerCOPE layer process the MAC layer frames before theyAre send to Network layer

73

COPE LAYER

Network Layer

COPE layer

MAC layer

Sender side

Get a packet from network layer

Add the packet into Output Queue

Encode if possible

74

COPE LAYER

Network Layer

COPE layer

MAC layer

Sender side

Get a packet from network layer

Add the packet into Output Queue

Encode if possible

Encoded?

Add packet to Retransmission

poolPiggy back Reception Report

Piggy back asynchronous acknowledge

Yes

No

To MAC layer 75

COPE LAYER

Network Layer

COPE layer

MAC layer

receiver side

Get a packet from MAC

Extract acks for me

Delete correponding

packets in retransmission pool

If there is any

Extract Reception ReportRecord RR in

neighbors’ virtual queueIf there is any

Am I the next hop?Add to opportunity

listening poolNo

Yes

76

COPE LAYER

Network Layer

COPE layer

MAC layer

receiver side

I am the next hop

This packet is encoded? Decodable?

Decode and schedule Ack

Am I destination?

Deliver to Network layer

Send into Output Queue

Yes

No

YesNo

Yes

77

COPE HEADER

COPE header

IP header

MAC header

Data Frame

Encoded Packet ID

ReceptionReport(Packet ID)

Asynchronous ACK

1. Encoded packet numberEnocded packet IDsIntended receiver s

2. Number of packets receivedPackets ID

3. Received packets’ IDACK receiver

78

Coding Gain

79[3] S. Katti, D. Katabi, W. Hu, and R. Hariharan, “The importance of being opportunistic: Practical network coding for wireless environments

Coding Gain

80[4] Katti S, Rahul H, Hu WJ, Katabi D, Muriel M, Crowcroft J. XORs in the air: Practical wireless networking.

Coding Gain

81[4] Katti S, Rahul H, Hu WJ, Katabi D, Muriel M, Crowcroft J. XORs in the air: Practical wireless networking.

ANALOGNETWORK

CODING

82

Alice-Bob topology

83[5] Katti S, Gollakota S, Katabi D: Embracing Wireless Interference: Analog Network Coding

Traditional Approach

Alice

Router

Bob

Alice transmits

Router forwards

Bob transmits

Router forwards

Time slot 1 Time slot 2 Time slot 3 Time slot 484

Digital Network Coding

Alice

Router

Bob

Alice transmits Bob transmits Router forwards

Time slot 1 Time slot 2 Time slot 385

Analog Network Coding

Alice

Router

Bob

Alice & Bobtransmits Router forwards

Time slot 1 Time slot 286

How can we do it?

Alice

Bob87

How can we do it?(Cont.)

AliceREC

AliceSND

88


• What will Alice do?Demodulate and do some signal

processingNote the starting bit for interferenced

signalDo XOR for interferenced signal

89


• Smart Alice!• She must learn the characters of the

wireless channel• She must store the packets which already

sent by herself

• She may do the XOR job effectively

90

Easier?

• Choose the right modulation method

•MSK• Some experiments are already done by

using Software Defined Radios (SDR).• Successful results showed that we got

significant throughput gains compared to COPE and traditional ways.

91

Drawbacks

• Vulnerable to noises• Difficult to use in more complex topology

networks

Future investigation is still needed ! ! !

92

WHAT WE MAY LEARN FROM ANC?

• Think differently• Electrical engineer & computer

scientist• PHY layer & MAC layer &

Network layer93


94


95

Documents

Network Coding for Wireless Networks Wireless Communication Project by Group 2 1