Transcript Document

Topology Control, Interference,
and Throughput for
Wireless Mesh Networks
presented by Qin LIU
Outline
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Introduction
Network Model
Interference Model
Power Adjustment
Channel Assignment
Future Work
Introduction
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A wireless mesh network (WMN) is a multi-hop
wireless network that consists of mesh clients and
mesh routers.
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Mesh routers form the backbone of WMNs.
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Some of mesh routers are called gateway nodes and
connected with a wired network.
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provide Internet access
Architecture
Benefits
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Reduction of installation costs
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Large-scale deployment
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WLAN: One hop communication has limited coverage.
WMN: Multihop communication offers long distance communication
through intermediate nodes.
Reliability
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Only a few mesh router have cabled connections to the wired network.
Redundant paths between a pair of nodes in a WMN increases
communication reliability.
Self-Management
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A WMN is a special ad hoc network.
Applications
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broadband home networking
community and neighborhood networking
enterprise networking
metropolitan area networks
transportation systems
building automation
health and medical systems
security surveillance systems
…
Features
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Support for ad hoc networking, and capability of selfforming, self-healing, and self-organization
Mobility dependence on the type of mesh nodes
Multiple types of network access
Dependence of power-consumption constraints on the
type of mesh nodes
Compatibility and interoperability with existing
wireless networks
Multi-channel multi-radio system
Multi-channel Multi-Radio System
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There are multiple non-overlapping channels
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IEEE 802.11b/a standards offer 3 and 12 non-overlapping
channels, respectively.
Each node is equipped with multiple radios
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interference reduction
communicate with more than one neighbor at the same time
full duplex operation
throughput improvement
Topology Control in WMNs
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A topology consists of a set of nodes and links, and it
describes the connectivity information of the network.
Links in topology are the result of some controlled
parameters, such as transmission power and channel
assigned.
A good topology is critical to network performance.
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too dense  energy consumption & interference throughput
too sparse  long path, disconnected network
Reducing energy consumption and interference may be
conflicting goals. [Burkhart 2004]
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We focus on topology control for interference reduction.
Topology Control in WMNs
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Topology control in WMNs includes two steps:
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Power adjustment
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Power adjustment
Channel assignment
Define the physical topology of network
A link between two nodes if they are reachable via transmission power.
Channel assignment
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Define the logical topology on the top of the physical topology
A link between two nodes if they are reachable and use a common channel.
Network Model
V : A set of nodes, representing the wireless devices in
the Euclidean plane.
 pmax (v): the maximum transmission power of node v
 p(u, v): the least required energy to transmit a message
from u to v
 G(V, E): network graph, any link e = (u, v)  E if
pmax (u )  p(u, v)
 GP(V, EP): physical topology, EP  E
GP is a subgraph of G
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Network Model
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C: # of channels
Q(v): # of radios on node v, and typically Q(v) < C
A(v): the set of channels assigned on v, |A(v)|=Q(v)
GL(V, EL): logical topology, any logical link e = (u, v; k)
 EL iff (u, v)  EP and k  A(u)  A(v)
There may be multiple logical links between a pair of
nodes in GL, and it is a multi-graph.
Example
C
C
A
A
E
E
D
D
B
B
physical topology
network graph
{2, 3}
C
{1, 2}
A
3
2
1
E
1
2
{1, 3}
1
1
B
{1, 2}
D
{1, 2}
logical topology
Interference Model
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Interference model specifies conditions where a signal
can be successfully received.
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Physical Model
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transmission from u to v (SNR: signal-to-noise ratio, SS: signal strength)
SNRuv 
SSuv
N   kkisVtransmitting SS kv
k u
 SNRthreshold
Interference Model
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Protocol Model (transmission from u to v)
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Other Interference Models
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p(u)  p(u, v), and
no other interfering transmitter w, d(w, v)  (1 + )∙ d(u, v) ( > 0)
Transmitter Model (Tx-model)
Transmitter-receiver Model (Tx-Rx model)
IEEE 802.11 MAC protocol
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RTS-CTS
Symmetrical communication: Both the sender and the receiver should be free
from interference for a successful transmission.
Classification of
Interference Reduction Methods
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Interference reduction based on network topology only
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network planning
MIN interference while keeping certain network properties, such as kconnectivity and spanner
Interference reduction based on network topology and
traffic flows between nodes
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network planning and routing
MAX network throughput
Network Properties
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K-connectivity
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The k-connected graph contains at least k independent paths between any pair of
nodes.
 Two or more paths are independent if they none of them contains an inner
node of another.
The deletion of any set of less than k nodes in the k-connected graph still leaves a
connected graph.
Spanner
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stretch factor: distance stretch factor, energy stretch factor, hop stretch factor
d GP (u , v)
distance stretch factor
max
u ,vV d (u , v )
G
dG(u, v) (resp. dGP (u, v) ) denotes the minimum distance between u and v in G
(resp. GP)
GP is a spanner of G if the stretch factor is within a constant.
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Power Adjustment
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Reduce interference of all transmitting signals
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Link-based Interference Reduction
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define the interference of a link
Node-based Interference Reduction
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define the interference of a node
Link-based Interference Reduction
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Minimize the node coverage interference
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Cov(e) = |{wV| d(u, w)  d(u, v)}} { wV| d(v, w)  d(v, u)}|
# of nodes that are affected when the link (u, v) is active.
The network interference is defined as the maximum (or total, average) node
coverage in the physical topology.
MST is the optimal solution when minimizing the maximum node coverage
in a connected physical topology.
,v)
d(u
u
,u)
d(v
node coverage
v
Link-based Interference Reduction
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Minimize the link interference
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# of links interfered by the link (u, v) in GP
This definition of interference has been proposed, but no work on
minimizing such interference in physical topology control has been
reported.
,v)
d(u
u
v
,u)
d(v
link interference
Node-based Interference Reduction
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Minimize the sender-based interference
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p(u, v)
the transmission power of u: p(u )  (umax
,v )EP
the interference of node u: ISGP (u) |{v | p(u, v)  p(u)}|
# of nodes that receive signals transmitted by u
Minimize the maximum sender-based interference while keeping the
network k-connected or spanner.
Mnimize the average sender-based interference in a connected topology
(NP-hard?)
v
w
x
y
u
IS(v) = 4
IS(u) = 1
Node-based Interference Reduction
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Minimize the receiver-based interference
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the interference of node v: IRGP (v) |{u | p(u, v)  p(u)}|
# of nodes that affects node v
It is more realistic because interference occurs at the receiver instead of
the sender.
A 4  -approximation algorithm has been proposed to MIN the
maximum receiver-based interference while keeping the topology
connected in a highway model.
v
IR(v) = 2
IR(u) = 2
w
x
y
u
Channel Assignment
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Efficient channel assignment can greatly reduce the
interference effect of close-by transmissions.
Categories of channel assignments
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static assignment
dynamic assignment
hybrid assignment
Channel assignment only
Combine channel assignment and routing
Channel Assignment Only
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Minimum Interference Survivable Topology Control
assumption: same transmission range r, same interference range R,
interference disk Du a disk centered at u with radius R
 link interference: node x, y, u and v
such that d (u, v)  r and d(x, y)  r and
k  A(u)  A(v)  A(x)  A(y) and
R
k
u
v
x DuDv or y DuDv
k
e1 = (x, y; k) interferes with e2 = (u, v; k)
y
x
 link co-channel interference
I(e): # of links in GL that interfere with e
 topology interference: I (GL )  max I (e)
eGL
 objective: Minimize I(GL) while keeping the network k-connected. (Np-hard)
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A Heuristic Algorithm
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Before a channel assignment is known, the actual interference
of links are unknown.
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potential interference  Do not consider channel.
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First get a k-connected structure with minimum potential
interference from the physical topology.
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Then assign the least used channels nearby to links in the nonincreasing order of potential interference.
Combine Channel Assignment & Routing
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Given traffic demand, there is a circular dependency
between channel assignment and routing
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Routing link capacity  channel assignment  link’s expected load 
routing
LP-based Routing and Channel Assignment
M. Alicherry, R. Bhatia, and L. Li, “Joint Channel Assignment and Routing for
Throughput Optimization in Multi-radio Wireless Mesh Networks,” MOBICOM 2005.
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constrained maximum network flow problem
LP-based
Channel Assignment & Routing
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Problem: Given one destination u0, and the traffic demand du of each node u,
find the optimal channel assignment, routing and scheduling scheme that
achieves the maximum throughput.
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Notations:
 Nu: set of nodes with the transmission range of u
 Nu: set of nodes that within the interference range of node u, and u  Nu
 The system works in a periodical synchronized mode where each cycle
contains T time slots.
yuvkt is the binary variable, yuvkt  1 only if link (u, v) is active on channel k at
time slot t
LP-based
Channel Assignment & Routing
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Radio Constraint: at any time, a node can use at most Q(u)
different channels to send packets.
C
y
vNu k 1
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kt
uv
 Q(u ), u  N , t  1,
,T.
Interference Constraint (Schedulable Constraint): at any time,
two interference links can not be active at the same channel.
Sufficient condition:
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 
uNu  Nv vNu
C
D
RI
A
B
E
F
yuktv  1, (u, v)  E, k  1,..., C, t  1,...T
AB interferes with CD and EF. When AB is
active, CD and EF should keep silent.
But CD and EF do interfere with each other,
and they can be activated at the same time.
LP Relaxation
1 T kt the percentage usage of link (u, v) on channel k
y

t 1 uv
T
the available bandwidth of (u, v) on channel k,
c T
xuvk   t 1 yuvkt
where c is the bandwidth of each channel
T
Basic structure of LP
opt. obj. max  du
uN
C
node constraint:   xuvk  cQ(u ), u  N ,
vNu k 1
interference constraint:
 x
uNu  N v vNu 
k
uv
 1, (u, v)  E , k  1,..., C
flow constraint: 1. the traffic sent out from node u is equal to du
2. the total incoming traffic is equal to the total outcoming traffic
in other intermediate nodes
capacity constraint: the total traffic on a link (u,v;k) is no more than its bandwidth xuvk
traffic demand constrait: 0  du  du , u  N , u  u0
LP Relaxation
• Due to relaxation in LP, the channel assignment may
not be feasible. Post-processing is needed to make
channel assignment feasible.
Future Work
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Which interference criterion is more proper?
What is the appropriate optimizing objective?
Many optimization problems of topology control are NPhard so that efficient algorithms are valuable.
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especially for channel assignment
Distributed algorithms for practical networks.
Consider power adjustment and channel assignment
together.
Interference-aware routing
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QoS call admission
QoS multicast call admission
Thanks!
Q&A