Network Coding for Wireless Networks
Wireless Communication Project
by Group 2
1
Network Coding for Wireless Networks
1
INTRODUCTION
2
COPE DESCRIPTION
3
COPE IMPLEMENTATION
4
ANALOG NETWORK CODING
2
Introduction
Why?
What?
Network
Coding
Definitions
3
Definitions
1
2
Max-flow
Min-cut
Theory
Unicast,
Multicast
3
Throughput
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.
[1] S.-Y. R. Li, R. W. Yeung, and N. Cai, “Linear network Coding”, IEEE Trans.
5
Graph
• Graph G(V,E): consists of a set V of vertices
and a set E of edges.
– V consists of sources, sinks, and other nodes
– A member e(u,v) of E has a capacity c(u,v) to
send information from u to v
3
A
2
S
3
D
B
3
2
4
2
T
C
3
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
3
S
3
A 3 3
3
S2 2
D
3 2
A
3
3
4 1
2
D
B
2
2
C
3
2
2
3
2 3
B
2
4T
C
T
3
7
Unicast | Multicast | Broadcast
Broadcast
Multicast
Unicast
Unicast
communicat
ion is oneto-one.
Multicast
communicat
ion is oneto-many.
Broadcast
communicat
ion is oneto-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
Wire
vs
Wireless
WIRE
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.
WIRELESS
Network Modeling
The channel of one
particular edge is actually
shared by other neighboring
edge.
Traffic Pattern
 Unicast communication is
the dominate traffic pattern.
Traffic rates are varies over
time rather than constant.
Traffic Rate
10
Introduction
Definitions
Why?
Network
Coding
What?
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
1
1
B
C
0.5
1
0.5
1
F
1
1
G
0.5
1
D
1
10.5
E
13
b1
• Ahlswede et al.
(2000)
– With network
coding, every sink
obtains the
maximum flow.
B
A
b1
b2
b2
C
F
b1
b1+b2
b1+b2
D
G
b2
b1+b2
E
14
[2] Rudolf Ahlswede, Ning Cai, Shuo-Yen Robert Li, and Raymond W. Yeung. Network information flow.
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.
[3] S. Katti, D. Katabi, W. Hu, and R. Hariharan, “The importance of being opportunistic: Practical network coding
15
for wireless environments
An information exchange
scenario
Relay
Alice
Alice’s packet
Bob’s packet
Bob
Bob’s packet
Alice’s packet
• Multi-hop unicast requires 4 transmissions
• Can we do better?
16
Can Network Coding help - An idea
XOR
=
Relay
Alice
Alice’s packet
Bob’s packet
Bob
Bob’s packet
Alice’s packet
3 transmissions instead of 4
 Saves bandwidth & power
 33% throughput increase
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
Introduction
Definitions
What?
Network
Coding
Why?
19
Why is NETWORK CODING
1
Improve network throughput
2
Superior 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
20
COPE
Wireless Communication Project
by Group 2
Contents
1
General idea
2
Opportunistic listening
3
Opportunistic coding
4
Opportunistic routing
General idea of cope
What is 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
opportunism?
General idea of cope
Opportunistic listening
Opportunistic coding
Opportunistic routing
General idea of cope
A
B
C
D
E
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.
pool
A
Que
A
B
E
D
C
B
Output
E1
E2
E
E3
pool
D
Output
A1
A2
A3
E1:B to E
A1:D to A
pool
A
Que
A
B
E
E1
D
B
Output
C
E
E1
E2
E3
pool
E1
D
Output
A1
A2
A3
E1:B to E
A1:D to A
pool
E1
A
Que
A
B
E
D
C
B
E
E1
Output
E2
E3
pool
E1
Output
A1
A2
A3
D
E1:B to E
A1:D to A
pool E1
I have
E1
A
Que
E1
A
B
E
D
C
B
Output
E2
E
E3
pool
D
Output
A1
A2
A3
E1:B to E
A1:D to A
pool E1
A
Que
E1
A
E1
B
E
D
C
B
Output
E2
E
E3
pool
D
Output
A1
A2
A3
E1:B to E
A1:D to A
pool E1
A
Que
E1
A
E1
B
E
D
C
B
Output
E2
E
E3
pool
A1
A1
D
Output
A2
A3
A1
E1:B to E
A1:D to A
pool E1
A
Que
E1
A
E1
B
E
D
A1
B
Output
E2
C
E
A1
A1
E3
pool
D
Output
A2
A3
E1:B to E
A1:D to A
pool E1
A
Que
E1
A
E1
A1
B
E
D
C
B
Output
E2
E
I have A1
E3
pool A1
D
Output
A2
A3
E1:B to E
A1:D to A
pool E1
A
Que
A
Node A have packet E1
Node E haveCoding
packet A1
Node A want packet
A1
Node E want
packet
E1
opportunity
So I can give
Node A
So I can give Node E
E1
A1 E1+A1
E1+A1
E1
B
E
A1
D
C
B
Output
E2
E
E3
pool A1
D
Output
A2
A3
E1:B to E
A1:D to A
pool E1
Coding
opportunity
A
Que
E1
E1
E1+A1
A1
A1
A
E1
B
E
A1
D
C
B
Output
E2
E
E3
pool A1
D
Output
A2
A3
E1:B to E
A1:D to A
pool E1
Coding
opportunity
A
Que
A
E1+A1
E1
B
E
A1
D
C
B
Output
E2
E
E3
pool A1
D
Output
A2
A3
E1:B to E
A1:D to A
pool E1
A
Que
A
E1
B
E
A1
D
E1+A1
C
B
Output
E2
E
E3
pool A1
D
Output
A2
A3
E1:B to E
A1:D to A
E1+(E1+A1)=A1 A1 received!
pool E1
E1+A1
A
Que
A
E1
B
E
A1
D
C
B
Output
E2
E E1+A1
E3
pool A1
A1+(E1+A1)=E1 E1 received!
D
Output
A2
A3
E1:B to E
A1:D to A
Opportunistic Coding
How can the node decide which packets
needed to XOR together?
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.
Opportunistic Coding
C
A
B
D
Opportunistic Coding
Packets
P1
P2
Next Hops
A
B
P4
P3
P2
B’s queue
P3
P4
C
D
B
Node
P1
Opportunistic Coding
Packet
node
P1----- A
P2-----B
P3----- C
P4----- D
P1
=
P2
P4
P4
P3
P2
A
P4
+
P1
C
P1
B
P3
D
=
P1
+
P2
P3
Bad coding decision
P1
Opportunistic Coding
Packet
node
+
P1
P1----- A
P2-----B
P3----- C
P4----- D
P3
P4
P4
P3
P2
A
P4
=
P1
C
P1
B
P3
D
P3
+
=
P1
=
P1
+
P3
P3
Better coding decision
P1
Opportunistic Coding
Packet
node
P4
P1----- A
P2-----B
P3----- C
P4----- D
+
P1
+
=
P4
P4
P3
P2
A
P1
P4
+
P3
+
P1
C
B
P1
P4
P3
+
P3
+
= P4
P3
D
= P1
=
P1
+
P3
+
P4
P3
Best coding decision
P1
Opportunistic Coding
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.
Opportunistic Coding
Pi-1
Pi
ri-1
ri
P3
P2
Pn-2
r3
P1…Pi-1,Pi+1…Pn
P1…Pi-2,Pi…Pn
r2
P1
rn-2
Pn-1
rn-1
Pn
r1
rn
:XOR P1 to Pn
Opportunistic Coding
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
Can we further reduce the
number of transmissions?
1.The path is stored in the packet
itself.
2.Check the path, find the
opportunities to routing.
Opportunistic Routing
Path: SABD
Reception
report
source
A
S
B
Destination
D
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
Packet Delivery
A
B
P
RR
Reception report
PC
DP
P
IP packet
Ack
IP acknowledge
G
E
PF
55
A SCENARIO(II)
Packet acknowledge
A
B
Ack
RR
Reception report
C
Ack
D
P
IP packet
Ack
IP acknowledge
G
Ack
E
Ack
F
56
A SCENARIO(III)
Reception Report
A
RR
RR
B
RR
Reception report
RR
C
RR
D
P
IP packet
Ack
IP acknowledge
RR
G
RR
E
RR
F
57
DATA STRUCTURE
A
B
RR
Reception report
C
D
P
IP packet
Ack
IP acknowledge
G
E
F
58
DATA STRUCTURE
In Node C
C
Upper Layer
P6
Output Queue：
P5
P4
P3
P2
P1
Retransmission pool：
Packet Transmitted
59
DATA STRUCTURE
In Node C
C
Upper Layer
P6
Output Queue：
P5
P4
P3
P2
Retransmission pool：
P1 can be deleted
Ack Received
P1
P1ack
If
returns
is nottoreceived
Output Queue
after a long
to wait
time
another transmission opportunity
60
DATA STRUCTURE
In Node C
C
Upper Layer
P6
Output Queue：
P5
P4
P3
P2
P1
Retransmission pool：
Px is head for node D
Not C.
Opportunity Listening Queue：
RR：a,b,c,d,x
Pa
Pb
Pc
Pd
If
node C receivenode
a neighbor’s
whose
destination
is not
Periodical,Each
inform itspacket
neighbor
about
what they’ve
gotC,
Node
C just put the
packet into
the above
in its Opportunity
Listenning
Queue.
That queue.
is , Reception Report
61
DATA STRUCTURE
In Node C
C
Upper Layer
P6
Output Queue：
P5
P4
P3
P2
P1
Retransmission pool：
Opportunity Listening Queue：
Pa
Pb
Pc
Pd
Px
Neighbors’ virtual Queue：
A queue
B queue
D queue
……
…………………………
62
DATA STRUCTURE
In Node C
C
Upper Layer
P6
Output Queue：
P5
P4
P3
P2
P1
Retransmission pool：
Used for
decoding
Opportunity Listening Queue：
Pa
Pb
Pc
Pd
Px
Neighbors’ virtual Queue：
A queue
RR from A
B queue
RR from B
D queue
RR from D
……
…………………………
Used for
Coding
63
DATA STRUCTURE
LOGIC
• 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
64
DATA STRUCTURE
Upper Layer
P6
Output Queue：
P5
P4
P3
P2
P1
Retransmission pool：
Opportunity Listening Queue：
65
DATA STRUCTURE
4 PARTS:
Output Queue
Retransmission Pool
Opportunity Listening Queue
Neighbor’s Virtual Queue
66
PSEUDO BROADCAST
MECHANISM
Laptop2
Both laptop2 and laptop3 are the intended
Receiver of packet from laptop1
Laptop3
Laptop1
67
[4] Katti S, Rahul H, Hu WJ, Katabi D, Muriel M, Crowcroft J. XORs in the air: Practical wireless networking.
PSEUDO BROADCAST
Current WLAN medium access mechanism
802.11 MAC:
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 Broadcast mode:
802.11 MAC:
1. DCF CSMA/CA? Yes
2. Back off?
No
3. DCF RTS/CTS?
No
4. ACK?
No
POOR RELIABILITY
69
PSEUDO BROADCAST
SOLUTION?
802.11 MAC:
1. Develop a brand-new MAC access protocol
which is suitable for COPE broadcast
Hard to implement…..Unrealistic….
802.11 WLAN MAC already pervasive
2.
a new layer on
top the current 802.11
2. Add
PSEUDO
BROADCAST
MAC layer to make link-to-link broadcast
reliable.
Feasible!
70
PSEUDO BROADCAST
Reliability
1. ACK
2. Retransmission(ACK timeout)
3. Multiple Intended Receiver
Multi
receiver
One receiver
DIFS& backoff
SIFS
SIFS
Synchronous Acknowledgement does not work!
Packet
Packet
Receiver can send ACK packet asynchronously
ACK
ACK
1. We can treat Ack packet as a normal packet, ACK packets
ACK also go through the DIFS ACK
from different Receivers
and back off
procedure to avoidACK
collision
ACK
2. Or we can piggy back on packet travelling in the reverse direction
Collision!
71
PSEUDO BROADCAST
What 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 LAYER
Where does cope layer lies in?
 COPE layer is a very slim layer lies between
Network layer and MAC layer.
Network Layer
COPE layer
MAC layer
COPE layer process the network layer packets before
they are send to MAC layer
COPE layer process the MAC layer frames before they
Are send to Network layer
It is the COPE layer where the
four data structure:
Output Queue,
Retransmission pool,
Opportunistic listening pool
And neighbors’ virtual queue lies in.
73
COPE LAYER
Sender side
Get a packet from network layer
Network Layer
Add the packet into Output Queue
COPE layer
Encode if possible
MAC layer
74
Get a packet from network layer
COPE LAYER
Sender side
Add the packet into Output Queue
Encode if possible
Yes
Network Layer
Encoded?
No
COPE layer
Piggy back Reception
Report
Add packet to
Retransmission
pool
MAC layer
Piggy back asynchronous
acknowledge
To MAC layer
75
COPE LAYER
receiver side
Yes
Am I the next hop?
No
Add to opportunity
listening pool
Network Layer
COPE layer
Record RR in
Extract Reception Report
neighbors’ virtual
queue
If there is any
MAC layer
Extract acks for me
Delete
correponding
packets in
retransmission pool
If there is any
Get a packet from MAC
76
COPE LAYER
receiver side
Deliver to
Network layer
Send into Output Queue
No
Network Layer
Yes
Am I destination?
COPE layer
Decode and
schedule Ack
MAC layer
No
Yes
This packet is encoded?
Decodable?
Yes
I am the next hop
77
COPE HEADER
MAC header
Encoded Packet
ID
COPE header
Reception
Report(Packet ID)
IP header
Asynchronous
ACK
1. Encoded packet number
Enocded packet IDs
Intended receiver s
2. Number of packets received
Packets ID
3. Received packets’ ID
ACK receiver
Data Frame
78
Coding Gain
[3] S. Katti, D. Katabi, W. Hu, and R. Hariharan, “The importance of being opportunistic: Practical network coding
79
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.
ANALOG
NETWORK
CODING
82
Alice-Bob topology
[5] Katti S, Gollakota S, Katabi D: Embracing Wireless Interference: Analog Network Coding
83
Traditional Approach
Alice transmits
Bob transmits
Router forwards
Router forwards
Alice
Router
Bob
Time slot 1
Time slot 2
Time slot 3
Time slot 4
84
Digital Network Coding
Alice transmits
Bob transmits
Router forwards
Time slot 1
Time slot 2
Time slot 3
Alice
Router
Bob
85
Analog Network Coding
Alice & Bob
transmits
Router forwards
Time slot 1
Time slot 2
Alice
Router
Bob
86
How can we do it?
Alice
Bob
87
How can we do it?(Cont.)
Alice
REC
Alice
SND
88
How can we do it?(Cont.)
• What will Alice do?
Demodulate and do some signal
processing
Note the starting bit for interferenced
signal
Do XOR for interferenced signal
89
How can we do it?(Cont.)
• 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
92
93
Wireless Communication Project
by Group 2
94
Wireless Communication Project
by Group 2
95