Network-Coding based Scheduling
And Routing Schemes for SerivceOriented Wireless Mesh Networks
Jian-Liang Pan
Nguyen Thi Mai Phuong
Yu-Chen Chang
Tai Yang Wu
Tai Long Chen
1
Talk Outline

Introduction & Background
- Wireless Mesh Networks

Mesh vs. Ad-Hoc Networks

Wireless Network Coding

Routing Strategies

Link Scheduling & Channel Assignment

Performance Evaluation
2
Wireless Mesh Networks



WMN - Wireless Mesh
Network:
 Ad-hoc network with a
core which has limited
mobility
Mesh Router:
 A wireless base station
with limited or no mobility
 Infrastructure of the
network
Mesh Clients:
 A wireless node which is
fully mobile, may also act
as a router in some WMNs
Image: Indigo Systems – WMN for
Environmental Monitoring
3
Hybrid WMN
4
Overview
Node Types
Wireless routers
Gateways
Printers, servers
Link Types
Intra-mesh wireless links
Stationary client access
Mobile client access
Mobile clients
Stationary clients
Internet access links
5
Gateways




Multiple interfaces (wired &
wireless)
Mobility

Stationary (e.g. rooftop) –
most common case

Mobile (e.g., airplane,
busses/subway)
Serve as (multi-hop)
“access points” to user
nodes
GW
Relatively few are needed,
(can be expensive)
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Wireless Router

At least one wireless interface.

Mobility




Stationary (e.g. rooftop)

Mobile (e.g., airplane,
busses/subway).
Provide coverage (acts as a
mini-cell-tower).
Do not originate/terminate data
flows
Many needed for wide areas,
hence, cost can be an issue.
7
Users
• Typically one interface.
• Mobility
– Stationary
– Mobile
• Connected to the mesh
network through wireless
routers (or directly to
gateways)
• The only sources/destinations
for data traffic flows in the
network.
8
User – Wireless Router Links
• Wired
– Bus (PCI, PCMCIA,
USB)
– Ethernet, Firewire, etc.
• Wireless
– 802.11x
– Bluetooth
– Proprietary
• Point-to-Point or Point-toMultipoint
• If properly designed is not a
bottleneck.
• If different from router-torouter links we’ll call them
access links
9
Router to Router Links
• Wireless
– 802.11x
– Proprietary
• Usually multipoint to
multipoint
– Sometimes a collection
of point to point
• Often the bottleneck
• If different from router-touser links we’ll call them
backbone links
10
Gateway to Internet Links
• Wired
– Ethernet, TV Cable,
Power Lines
• Wireless
– 802.16
– Proprietary
• Point to Point or Point-toMultipoint
• We’ll call them backhaul
links
• If properly designed, not
the bottleneck
11
How it Works
• User-Internet Data
Flows
– In most applications the
main data flows
• User-User Data Flows
– In most applications a
small percentage of data
flows
12
Mesh vs. Ad-Hoc Networks
Ad-Hoc Networks




Multihop
Nodes are wireless,
possibly mobile
May rely on
infrastructure
Most traffic is user-touser
Wireless Mesh Networks




Multihop
Nodes are wireless,
some mobile, some
fixed
It relies on
infrastructure
Most traffic is user-togateway
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Abstract

One of critical problem :
improve the network throughput
- Channel assignment problem

Key technology: Network Coding

Step1:
analyze the throughput improvement obtained by wireless
network coding schemes in wireless mesh networks.

Step2:
develop a heuristic joint link scheduling, channel
assignment, routing algorithm
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Wireless Network Coding
15
Wireless Network Coding


Network coding is a particular in – network data
processing technique that exploits the characteristics of
the wireless medium (in particular, the broadcast
communication channel) in order to increase the capacity
or the throughput of the network
(Network Information Flow – Rudolf Ahlswede, Ning
Cai, Shou – Yen Robert Li, Senior Member, IEEE, and
Raymond W.Yeung, Senior Member, IEEE)
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Wireless Network Coding

Let G=(V,E) be a graph (G is the set of nodes of a point –
to – point communication, E is the set of edges) with
source s and sinks t1,t2, …tL.

R=[Rij,(i,j) E], Rij is the capacity of an edge (i,j) E.

h is the rate of information source

F=[Fij,(i,j) E] is a flow in G from s to tl (l=1,…L) if for
all (i,j) E : 0  Fij  Rij and for all i  V except s and tl:

i ':( i ',i )E
Fi 'i 
F
j:( i , j )E
ij
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Wireless Network Coding


F is a max-flow from s to tl in G if F is a flow from s to tl
whose value is greater than equal to any other flow from
s to tl
Max – Flow Min – Cut theorem:
Let G = (V,E) be a graph with source s and sinks
t1,…,tL, and the capacity of an edge (i,j) be denoted by
Rij. (R,h,G) is admissible if and only if the values of a
max – flow from s to tl are greater than or equal to h
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Wireless Network Coding
19
Wireless Network Coding
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Wireless Network Coding


A total of 9 bits are sent (network coding)
If network coding is not allowed, at least one more bit
has to be sent
=> Saving 10% bandwidth
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Wireless Network Coding


Using the scheme in fig.8(b), if 2 bits are sent in each
edge, then 4 bits can be multicast to all the sinks
If network coding is not allowed:

Let B = {b1,…,bk} be the set of bits to be multicast to all the
sinks.

Let the set of bits sent in the edge (s,i) be Bi, where |Bi| = 2,
i = 1, 2, 3

B = Bi  Bj for any 1  i  j  3

We have: B3 ( B1  B2 )  ( B3  B1 )  ( B3  B2 )  B
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Wireless Network Coding

Therefore:
k | B3  ( B1  B2 ) |
| B3 |  | B1  B2 |
| B3 |  | B1 |  | B2 |  | B1  B2 |
 6k

k  3 or if network coding is not allowed, only 3 bits can
be multicast to all sinks (if 2 bits are sent in each edge)
=> Throughput of network can be increased by one-third
using a very simple network code
23
Wireless Network Coding
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Wireless Network Coding
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Butterfly Network

When two or more signal transmission path of
partial overlap ，Encoded to improve the signal
transmission rate of the technology can be called
「 common path coding」



Each arrow indicates assumed to
send a signal ， the signal value is 0
or 1，A point given by the two
signals x and y，should be sent to
the B point and C point.
Let M points x and y send a
representative to the similarities and
differences of the signal ， when
the B point to receive this signal and
x, can be solved y; Similarly, Cpoint can also be solved x.
Graph signal x ⊕ y ，x ⊕ y is
called 「binary sum」 ，it not only
represents a coded form, is also a
mathematical "linear" code.
Use network coding，the encoding method
has three restrictions:
1. Each encoded packet must be the linear
independence.
2. The node after the packet encoded into the nodes
need to encode packets with each other linearly
independent .
3. Enter the node will be re-encoded packets and
transmitted.
Oct. 23, 2007
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Linear Network Coding
P1，P2 ， ‧ ‧ ‧ ‧ ‧ ‧ Pn the packets for the node S
e = {e1, e2, e3 ‧ ‧ ‧ ‧ ‧ ‧ en} is coded coefficients
Figure (2), (3) is linear combination of matrix encoding.
Linear Network Decoding

The coefficient matrix is converted into an antimatrix, the re-encoded packet multiplied by the
value of the packet can be restored.
Network coding of fault tolerance
反矩陣的算法(一) :高斯消去法


步驟:

(步驟1) 將單位矩陣In連接於所給矩陣A之後

(步驟2) 利用高斯消去法將步驟1中的A矩陣化成單位矩陣In
求A-1的教學範例:

求出矩陣A的反矩陣，
[A In]
反矩陣的算法(二)
伴隨矩陣(adj A)
餘因子(cofactor) Aij:
伴隨矩陣(adj A)的算法
Routing Strategies
Two types ：

Shortest single-path routing

Optimized single-path routing
41
Shortest single-path routing
● In particular, single-path routing can be obtained by
Dijkstra’s algorithm.
42
Dijkstra’s Algorithm
Initial
43
Dijkstra’s Algorithm
44
Dijkstra’s Algorithm
45
Dijkstra’s Algorithm
46
Dijkstra’s Algorithm
47
Dijkstra’s Algorithm
48
Dijkstra’s Algorithm
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Dijkstra’s Algorithm
Finish
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Optimized single-path routing

Select the path that provides the maximum flow in the
multipath routing for each session and following two
steps：
 Solve the LP with multipath routing. For each session, we
select the path that achieves the highest flow.
 Input these obtained optimized single-path routes for each
session to the LP and re-solve the LP.
51
Link Scheduling

Let Ts (Ts ≥ 1) be this period of time in the unit of time
slots. Thus, we can update the channel assignment every
Ts time slots.
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Channel Assignment

Note that the problem of optimal channel assignments is
NP-hard.

Therefore design a heuristic greedy algorithm to obtain
the schedule, which approximates the optimal solution
derived by the LP.
53
Channel Assignment (Cont.)

The NC links combinations with larger unassigned flows
have higher priority to select the channels.

After going through all the NC traffic and all the links for
the unicast traffic, we check if there are still unassigned
flows. If so, we move on to the next Ts time slots and
repeat the scheduling process until all unassigned flows
become zero.
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PERFORMANCE EVALUATION



We solve the linear program to obtain the
theoretically optimized network throughputs and the
corresponding flows for the non-NC scheme ,
conventional NC scheme, NC3 scheme, and NC5
scheme.
multipath routing can significantly increase the
network throughput gain as compared to singlepath routing.
NC5 and NC3 can increase network throughput by
24 and18 percent
55
The network throughput gain of wireless NC schemes over
over the non-NC scheme in the random graph network with
34 nodes the non-NC scheme in the random graph network
with 34 nodes.
56

The impact of routing strategies on network throughput.
57
The performance impact of multichannel and multiradio in
the random graph network where there are 20 unicast
sessions.
58
CONCLUSIONS


the analog NC scheme increases by 24 and 34
percent on average as compared to the
conventional NC scheme and non-NC scheme,
respectively.
We modeled the throughput gains of both the
conventional NC and analog NC schemes over
the traditional non-NC scheme in multichannel
multiradio wireless mesh networks.
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Thank you!
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