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Mesh Networks

Prof. W.S. Hwang

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Central Control of Star Topology of Wireless IEEE Networks

Figure 4.1 provides examples for IEEE 802.11 WLAN and IEEE 802.15 WPAN. In the figure, STA or DEV use a single central node to relay traffic.

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IEEE 802.11 Basic Service Sets Interconnected by IEEE 802.3

Figure 4.2 The current IEEE 802.11 standard makes use of bridging capability to interconnect several APs and their BSSs.

Presently, all existing wireless standards need bridging or routing to connect with other networks that may be based on wire or wireless.

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Outline

Introduction

Classification of Wireless Mesh Networks

General Problem Statement

Path Selection

Medium Access Control

Exploiting the Capacity of the Radio Channel by Spatial Reuse

Hidden Devices – Potential Interferers

Exposed Devices - Unused Capacity

Fairness and Congestion Avoidance

Routing

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4.1 Introduction

Figure 4.3 Wireless mesh networks extend the reach of an AP/gateway, provide easy deployment and redundant paths. Nodes in the mesh may be referred to as vertex, while links form edges of a graph representing the mesh topology.

obstacle

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IP Bridging

Figure 4.4 Devices use routers as default gateway for any device they cannot communicate directlywith by means of layer-2 functionality. This hierarchy enables the Internet. Using layer-2 addressesonly would be impossible to handle.

For destinations outside the broadcast domain, direct frame exchange is impossible, devices rely on routers to forward their frames. “Path selection” is called to distinguish the term “Routing”.

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4.2 Classification of Wireless Mesh Networks

Devices in flat hierarchy network needs path selection functionality

to forward frames in multi-hop.

In hierarchical mesh networks

Mesh-able devices provide the mesh networking service to other

non-mesh-able device that don’t have relaying capabilities.

Sufficient for static mesh networks

Mesh-able devices need extra resource such as memory,

computing power and multiple transceivers.

BSS support function may operate in band or out of band.

Wireless mesh networks may operate on single or multiple

frequency channels.

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Classification of Wireless Mesh Networks

Figure 4.5 Characteristics of wireless mesh networks vary depending on the available resources. Resources are the wireless medium and hardware capabilities. Protocols for wireless mesh networks need to consider these resources.

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4.3 General Problem of Relaying

reduce end-to-end throughput and increase overall latency/delay. Path Selection

Topology and link speed of wireless networks change quickly. Path metrics to provide path selection decision: (change within a

short duration, so estimates them is required)• Packet error probability

• Congestion status of receiving relay node

• Availability of relay node on a certain frequency channel.

• Bandwidth needed for transmission.

Frame forwarding (IP layer), Cross-layer can improve it. Medium Access Control

Sum of continuously overlapping single-hop networks. Hidden and exposed node problems.

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4.4 Exploiting the Capacity of the Radio Channel by Spatial Reuse

Figure 4.6 Spatial frequency reuse is an important aspect for wireless mesh networks. Multiple devices in mutual receive range aim at a highly efficient spectrum usage. Spatial frequency reuse, even when applied to a multi-hop link, increases spectrum capacity.

A string topology ofequidistantnodes

Interference rangeassumed to be lessthan twice the distancebetween two neighboringdevices

Minimum spatialfrequency reusedistance is 3 hops.

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Hidden Devices – Potential Interferers

Figure 4.7 In wireless mesh networks, each device has more indirect than direct neighbors. Hiddendevices have high potential of interference, therefore.

Hidden station (STA): an STA whose transmissions cannot be detected using carrier sense by a secondSTA, but whose transmissions interfere with transmissions from the second STA to a third STA.

To inhibit hidden devices is a handshake (RTS/CTS).

In large area, the reservation frames are not received by all devices. Collisions still occur.

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Busy Tone

Figure 4.8 With busy tone, a receiving device can signal an ongoing frame reception to its neighborhood.

Other methods to reserve medium: using dedicated signaling channels.

An additional narrowband transceiver to transmit a busy tone.Whenever a device is receiving, it sends a non-modulated busy tone in the narrow band channel.

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Exposed Devices – Unused Capacity

Device decides channel isn’t available although its transmission would not cause harmful interference.

Capacity of wireless medium is wasted. For example: B transmitting to A, and C senses the transmission

and falsely conclude that it may not send to D. In fact such a transmission would cause bad reception only in the zone between B and C.

A B C D

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4.5 Fairness and Congestion Avoidance

Fairness: a specific means of resource sharing. In IEEE 802.11, all frames have equal chance to access medium, no

matter what size the payload and which PHY mode is used. In IEEE 802.11e, fairness among frames is based on TXOP. Different

PHY modes achieve different throughput.

Flow Admission Control to prevent capacity sharing by too large number of devices, and fulfill the QoS requirement. Unfairness is applied to newly arriving flows. Need to establish fairness and support QoS.

TCP Congestion algorithm draws wrong conclusions and throttles down the window size making wireless links appear weak capacity. Fluctuation in channel conditions and device mobility result high frame

error ratio. Two different ARQ protocols: TCP congestion avoidance and IEEE

802.11 back-off, resulting in non-predictable performance behavior.

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Traffic Aggregation

Figure 4.9 A relaying device carries the traffic aggregated from three other devices. Prioritization of the forwarding device is necessary to ensure sufficient performance.

An early congestion detectionand avoidance is necessaryin wireless mesh networks.

Border has more capacity than center nodes, since fewer neighbors at border.

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4.6.1 Routing Algorithms

Figure 4.10 Ad hoc routing protocols can be classified by adaptation strategy.

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4.6.1.1 Ad-hoc On-demand Distance Vector Routing (AODV)

Figure 4.11

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4.6.2 Common Link Layer Behavior (Link Adaptation)

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4.6.3 Link Breakage Prediction

Figure 4.14

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4.6.5 Early Route Rearrangement (ERRA)

Figure 4.15

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4.6.6 Early Route Update (ERU)

Figure 4.16

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4.6.7 Simulation Results

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