Improving Capacity and
Flexibility of Wireless Mesh
Networks by Interface Switching
Yunxia Feng, Minglu Li and Min-You Wu
Presented by: Yunxia Feng
Dept. of Computer Science & Engineering
Shanghai Jiao Tong University
Shanghai, China
Outlines

Problem Description

Hybrid Channel Assignment Protocol (HCAP)

The Interface Assignment Strategies

Coordination Policy of HCAP

The Channel Assignment Algorithm

Evaluation Results

Conclusions and Future Works
Problem Description
Fig. 1 The I-WMN Architecture
Gateways (GAs): The routers that connect to wired networks via wired links;
SNs: The mesh routers within the transmission range of GAs;
GNs: All of the mesh routers except GAs and SNs.
Problem Description (cont.)

Interference in I-WMNs is serious




Heavy traffic loads;
Multiple hops between the source and destination nodes;
One interface can only fix on one channel at the same time.
Possible Resolutions

Equipping each node with multiple interfaces;



Fix interfaces on different channels.
However, it is difficult to equip each node with the same
number of interfaces as available channels.
Dynamic channel assignment (DCA)



Co-channel interference is minimized;
Improves the network capacity;
Enhances the scalability of WMNs.
Components of Multi-interface DCA

Interface assignment strategy:


Decides the interface switching mode of nodes;
Coordination scheme:

Provides methods to implement communications
between nodes under given interface assignment
strategy.
Problems Induced by DCA

Switching Overheads


switching delay
Dependence Problems

Connection dependence


Nodes cannot communicate with each other due to switching
to different channels.
Channel dependence

Changing the channel of a particular link may result in
changing allocated channels of many other links in the
network.
Hybrid Channel Assignment Protocol
(HCAP)

Premises




GAs distribute across the network, and any two GAs do not
interfere with each other even if they are assigned the same
channel;
One SN node can only establish one routing path to the GA nodes
at a time;
HCAP adopts one of current synchronization protocols;
If each node is equipped with only two interfaces, then,


A SN is within the transmission range of only one GA.
If each node is equipped with more than two interfaces, then

A SN is within the transmission range of the same number of GAs as
the number of its interface that adopt static strategy.
The Interface Assignment Strategies

GA Nodes:


Static interface assignment strategy to avoid frequent
interface switching.
All the Other Nodes (SN and GN nodes):

Hybrid strategy to keep flexible


fixed interface (FI): the interface that adopts static strategy;
switching interface (SI): the interface that dynamically switch
channels.
The Interface Assignment Strategies
(cont.)

Declaration & Definitions





The channel assigned to every FI is different from that
of nodes within its interference range as possible.
The channel is referred to the node’s receiving channel
(CRec);
The corresponding interface is referred to its receiving
interface (IRec).
Dynamic links: links between hybrid nodes
Static links: links between GA nodes and SN nodes
within their transmission range.
Questions in Implementing HCAP

Which interface should a CRec be assigned to?



CRec is assigned to the FI of GN nodes;
CRec is assigned to the SI of SN nodes.
Reasons



The FI of every SN node should fix on one channel of the GA
within its transmission range.
Every SN node should fix its FI on one channel of the GA within
its transmission range to communicate with the GA since none
interface on GA nodes can switch interfaces dynamically.
The assigned channels of GAs should be shared among the
fewest nodes to minimize the interference level on them due to
the heaviest loads.
The sketch of HCAP
(a) Network topology
(b) Sketch of HCAP
Fig. 2 A simple example of HCAP
Coordination Policy of HCAP


Divide the time of hybrid nodes into slots, whereas
several slots compose one Cycle.
Connection dependency problem in HCAP


Suppose that B is transmitting packets to node G on link at
present. If node C wants to send some packets to B, there is
connection dependency problem.
Resolution

Designate several slots in every Cycle for the SI of every SN
node to fix on its CRec. We denote these slots by Receiving Slots
(RS) to differentiate with other slots. The number and positions
of RSs in a Cycle are determined by the settings of each specific
network.
Broadcast Problem

Description:

There is no permanent links between hybrid nodes


The FIs of hybrid nodes are assigned different channels,
whereas their SIs switch to different channels dynamically.
Resolution:

Take several slots from the left slots of every Cycle as
Broadcast Slots (BS), during which the SI of all hybrid
nodes switch to a predefined channel to only transfer
broadcast packets. The number and positions of RSs in
a Cycle are determined by the settings of each specific
network.
Channel Assignment Algorithm

Three Phases

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In the first phase, the algorithm assigns channels to both
interfaces of GA nodes.
In the second phase, the algorithm assigns channels to the FI of
SN nodes.
At last, the algorithm assigns CRec to hybrid nodes.
Channel Assignment Sequence

The algorithm always assigns channel to the node that has the
biggest η. If multiple nodes have the biggest η, they are assigned
channels in a random order. This sorting gives higher priority for
nodes that have more restrictions.
Default Simulations Settings

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A 36-node I-WMN, where each node is equipped with
two half-duplex interfaces.
The 36 nodes randomly distribute in a 6×6 square grid
network, and the distance between two neighboring nodes
equals to transmission range.
Four nodes are designated as GA nodes and connected to
the wired network.
The ratio between interference range and transmission
range is set to 2.
The simulation period is between 24-58 seconds.
Utilize the DSR protocol for route selection.
Evaluation Results
Throughput (Mbps)
500
400
300
HCAP
HYBRID
STATIC
200
100
8
12
16
20
24
Number of traffic flows
Fig.3 Throughput Comparison
Fig. 4 Adaptation to Flow changes
Evaluation Results
525
Aggregate Throughput (Mbps)
550
90
Successful traffic flows ratio(%)
100
80
70
60
50
40
30
HCAP
HYBRID
STATIC
20
10
0
0
50
100
150
200
250
300
350
400
Throughput (Mbps)
Fig. 5 Throughputs vs. traffic fairness
500
475
450
425
400
375
350
325
300
1
2
3
4
5
6
7
8
9
10
Broadcast Slot Time (ms)
Fig. 6 Impact of broadcast slot
Conclusions & Future Works




We have presented a hybrid channel assignment protocol
to improve the capacity and flexibility of WMNs.
The protocol does not depend on prior information on the
traffic loads, whereas it can adapt automatically to the
changes in the network.
Extensive simulations show that the protocol improves
the network capacity and flexibility. It also achieves
better per-flow fairness, whereas it keeps both interface
switching overheads and coordination complexity under
tolerable range.
As future work, we will provide analyses results and
describe how to the algorithm in details.
Thank You
Q&A?