Chap4 - Mesh Networks - Basics

7/28/2019 Chap4 - Mesh Networks - Basics

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Mesh Networks - Basics


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Contents

Introduction

Classification of Wireless Mesh Networks

General Problem Statement

Exploiting the Capacity of the Radio Channelby Spatial Reuse

Fairness and Congestion Avoidance

Routing


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INTRODUCTION


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Introduction

The wireless mesh network (WMN) isa communication network made up ofradio nodes in which there are at least

two pathways of communication toeach node.

In full mesh topology, each node isconnected directly to each of theothers. In partial mesh topology,

some nodes are connected to all theothers, but some are connected onlyto those other nodes with which theyexchange the most data.


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Introduction Characteristic of the WMN:

the capability to relay frames from one device toanother


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Introduction

Characteristic of the WMN: Mesh networks potentially have no hierarchy

Most of the traffic is directed and received from acentral device

No such a central device in WMN Everybody relays every others


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Introduction

Mesh supporting MAC layer needs toconsider Multi-hop path exists bet/ the nodes in the WMN

Different from the situation where a central neighbor

(AP) control all the transmissions. Routing needs to be handled by each relaydevices in the broadcast medium

Path selection: routing function in the mesh-able MAC layer Distinguished from the routing: identification

of possible hops from source to destination inthe IP layer


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Introduction

Key elements in designing a wireless meshnetwork Security: the WMN may consist of devices

mutually unknown to each other Path selection MAC adaptation


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CLASSIFICATION OF WIRELESS

MESH NETWORKS


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Classification of Wireless

Mesh Networks

WMN may operate with or without a hierarchical

structure:

Flat mesh networks

Hierarchical mesh networks

In flat hierarchy,

Any device is able to forward frames

Any device operates as a sink or source, also as a relay.

The devices needs path selection functionality and the

capability to support multi-hop traffic


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Mesh Networks

In hierarchy mesh networks,

only mesh-able device provide the meshnetworking service to othernon-mesh-able

devices that do not have relaying capabilities Typically, mesh-able devices are APs (Access

Point)

Only mesh-able devices need extra resourcessuch as memory, computing power and multipletranceivers to be able to operate the WMN


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Mesh Networks

With regard to the frequency channels used,WMNs with respect to the mesh function incomparison to the BSS support function may

operate in band orout of band WMNs may operate on single or multiple

frequency channels


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Classification of Wireless Mesh

Networks


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GENERAL PROBLEM STATEMENT


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General Problem

Statement

New phenomena emerging from the WMN:

Multi-hop transmission reduces the end-to-endthroughput and overall latency/delay increases

Self-interference of relayed frames andunpredictable path metrics


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General Problem

Statement Path Selection

Routing algorithm in wired networks Routing metric: hop count, link speed, cost for transiting traffic, etc.

NOTtake into account frequent change of topology and link speed

How about wireless networks?

Topology (connectivity of nodes) and link speed changes Path metrics of wired networks appear insufficient for WMNs

Path metrics for WMNs may need to consider as an addition: Packet error probability that depends on SINR

Congestion status of receiving relay node

Availability of relay node on a certain frequency channel

Bandwidth needed for transmission


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General Problem

Statement Path Selection

Path metrics for WMNs are all time variant andmay essentially change within a short duration.

This information may be available in the MAC

layer only. NOTin the routing layer. The WMN developed at the IEEE cover only

layers 1 and 2 and must provide transparency tohigher layers.

The MANET routing protocols cant be usedsince frame forwarding is performed in the IPlayer.


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General Problem

Statement - MAC

Medium Access Control (MAC) Problem in relaying in multi-hop networks cant be

solved by just applying single-hop MAC protocolmultiple times.

The WMN may be seen as A sum of a number of continuously overlapping

neighboring single-hop networks

Coordination of their channel access in an area

larger than that of a single-hop network isneeded. Hidden and exposed terminal problem To be

discussed later


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EXPLOITING THE CAPACITY OF THE

RADIO CHANNEL BY SPATIAL REUSE


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Exploiting the Capacity of theRadio Channel by Spatial Reuse

Relaying the frame in the WMN

Each relay node operates as both receiver andtransmitter

The wireless medium in the vicinity of a relay node isoccupied once for frame reception and a second timefor frame transmission.

Assuming a string topology of equidistant nodes, thespatial reusedistancecan be easily explained. See thefollowing Fig.


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Exploiting the Capacity of the RadioChannel by Spatial Reuse

Min spatial reusedistance = 3 hops


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Hidden Devices Potential Interferers

The receiving device(B) is inreception range of two otherdevices(A,C).

However, the latter ones (A,C)

are out of mutual receptionrange. Hence, transmissionsto the RX device (B) cannot bedetected by a possibleinterferer (C).

In WMNs, each device hasmore indirect than directneighbors. Therefore, hiddendevices have high potential ofinterference.

RxTx

Sensing/Rx

Hidden Devices in Wireless Networks

A

B

C


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Hidden Devices Potential Interferers

B transmits a busy toneon a different channel.Device C, which isoutside reception of the

transmitting device A,receives the busy toneand defers from channelaccess.

With busy tone, a

receiving device cansignal an ongoing framereception to itsneighborhood.

A BC

f1

f2

A

B

Busy tone

Busy Tone Concept to OvercomeHidden Device Threat


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Exposed Devices Unused

Capacity

A device is called exposed if, according to theprotocol applied, the device decides that thechannel is not available, so that it refrains fromchannel access, although its transmissionsimultaneously to another ongoing transmission

would not cause harmful interference.

TxRx

What the node C is doing:1. Perform carrier sense (CS)2. Sense busy. So, would not trxWhat if it trx? Its ok. No harm

AB CD

d d


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Exposed Devices Unused

Capacity

Since exposed devices are not harmful toothers, most wireless standards do not takeinto account

Obstacles, walls and buildings provide sufficientshadowing that may allow interference-freesimultaneous transmission in the same channel.

Detection and identification of opportunitiesfor simultaneous transmission are important forthe design of MAC protocols of dense WMNs.


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FAIRNESS AND CONGESTION

AVOIDANCE

d C


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Fairness and Congestion

Avoidance

Fairness denotes a specific means ofresource sharing Equally share the link (or bandwidth)

Different kinds of fairness IEEE 802.11 is based onframes: No matter whatsize the payload and which PHY mode is used fortrx, all frames have equal chance.

IEEE 802.11e is now based on The capacity (trx rate) of the WMN Transmission Opportunities (TXOP)

F i d C i


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Avoidance

When the number of flows is large and thecapacity is given, none of the traffic flowsmay be able to fulfill its QoS requirements. Flow Admission Control (FAC) Traffic flow prioritization

Both FAC and prioritization are needed inthe WMNs to establish fairness and support

QoS traffic flows.

F i d C i


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Avoidance

The capacity of a wireless mesh network isgiven by The relaying capacity of the highest loaded relay

node (bottleneck) The bottleneck is typically the portal device

connected to the Internet TCP over wireless

TCP was designed for wired communicationnetworks

Congestion avoidance algorithm operates on theassumption that frame losses are due to insufficient

capacity of congested routers High fluctuation of wireless links TCP draws thewrong conclusion and throttles down the windowsize

F i d C i


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Avoidance

With multiple devicesinterconnected, bottleneckdevices will result that limit thecapacity of the mesh network.

Relaying devices need to take

into account that subsequentdevices may carry traffic ofother routes, too. How?

A relaying device carries thetraffic aggregated from three

other devices. Prioritization of the forwarding

device is necessary to ensuresufficient performance.

Traffic Aggregation in WMNs


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ROUTING


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Routing Algorithms

Proactive protocols constantly maintain and detect paths

to all possible destinations. Reactive protocols save overhead and set-up paths only

when needed. Hybrid protocols combine both aspects

MANETAd-hocRouting

Protocols


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Routing Algorithms

Further classification may distinguish

between link state and distance vectorbasedalgorithms,

hierarchy of path selection entities and the usage of location information for forwarding

decisions.


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Routing Algorithms

Proactive routing protocols use periodic floodingto broadcast information

to devices about routes, known neighbors andothers.

This enables short path set-up times and ensuresthat the latest parameter values of the routingmetrics are always present in the transmit range.

Increased overhead is a drawback

Examples: DSDV, OLSR


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Routing Algorithms

Reactive routing protocols establish a route on request only, reducing path

selection overhead but introducing high delayfor the first frame to be transmitted

Path selection procedure must be executedbefore data frames can be exchanged.

Reactive routing protocols avoid maintaining

unused routes, but pay for this by a higher routediscovery and packet transmission delay.

Examples: DSR, AODV

Ad h O d d Di


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Ad-hoc On-demand Distance

Vector Routing (AODV)

AODV is a serious candidate for WMNs. Owing to its reactive nature, the protocol

avoids maintaining unused routesintroducing a higher delay than proactiveprotocols when establishing a route

The algorithm is divided into three parts: Route Discovery

Route Maintenance Local Repair


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Ad-hoc On-demand Distance Vector

Routing (AODV) Route Discovery

Route discovery:1. Checks routing table to determine whether it

has a valid route available2. If a route is known, then forwards to the next

hop, otherwise initiates a route discoveryprocess.3. Broadcasts a Route Request (RREQ) message

and floods the network (See the Fig in the next

page)


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Routing (AODV) Route Discovery

RREQ contains: source IP address destination IP address a sequence number from the source device the last known sequence number from the

destination device a broadcast ID, which is incremented with each

broadcast sent by the source

: a unique identifier forthe RREQ


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Ad-hoc On-demand Distance Vector Routing(AODV) Route Discovery

After receiving the RREQ, dev D2:

Checks whether it is the dst or if its routing tablecontains a valid route to the dst.

If dst is unknown: D2 builds a new reverse route entry for src D1. Reverse route

contains:

Source IP address of the RREQ

According sequence number

Hop count towards the source D1 IP address of the neighbor dev where the RREQ was received from

Then increments the RREQ hop count and re-broadcasts theRREQ.


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After receiving the RREQ, dev D2 (cont.): If D2 is the dst or the dst is known:

D2 compares the received sequence number (A) from theRREQ and the last received one (B) stored in routing table.

If A


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Each dev receiving the RREP: Builds a forward path entry containing:

The dst. IP addess Neighbor IP address of the last RREP sender Hop count towards the dst. Each routing entry is associated with a certain lifetime (updated

each time the entry is used)

To limit overhead, an extending-ring searchmechanism is proposed:

Send repeated RREQ with an increase Time-To-Live (TTL) Advantage: avoids flooding the whole network if dst. is

near Disadvantage: delays route discovery if dst. is far away.


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Routing (AODV)

A route request is flooded, and establishes the reverse path Destination unicasts the route reply and establish the forward path Route breaks are repaired locally

Source

RREQDst

Source

RREP Dst

Src

Dst

Upstream Node

Downstream Node

Src

Dst

Local Repair

(a) Route Discovery Process (b) Route Repair shown for Unicasting

Ad hoc On demand Distance


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Route Maintenance If the source moves during an active session, it

reinitiates the route discovery process. When either the dst. or an intermediate device moves

and the routes breaks, a Route Error (RERR) packet is

sent to the source. This RERR is sent by the device onthe source side of the break (upstream)

When the neighbors receive the RERR, they mark theaffected route entries as invalid and send RERR to allneighbors that are affected by the broken link.

The source recovers the route when it receives the RERR.



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Local Repair A method to repair broken route locally When a route breaks, the upstream device decides

either to repair the route or to send an RERR message. A device using local repair sends an RREQ searching for a

new route to the destination device. Note that therepairRREQ will not reach the source thus preventingcreation of loops.

The initiator of the local repair updates its routing entryand compares the stored hop count with the recentlyreceived one.

If the new hop count to the destination is larger than the formerhop count, the device creates an RERR message for the sourcedevice

If the repair attempt fails, an RERR is sent back to thesource.



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Based on link adaptation related informationavailable from the PHY, a device may be ableto predict from the history of MCSs used the

current link state so that a device is able torearrange a route before it breaks

Two link state prediction based routerearrangement algorithms: Early Route Rearrangement (ERRA) Early Route Update (ERU)

Common Link Layer Behavior


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Common Link Layer Behavior

(Link Adaptation)

Most wireless communication standard leave LA opento vendor-specific implementation. In general, most LA algorithms react to degrading

channel conditions by switching to a lower data rate(more robust) PHY mode.

If the SINR of the wireless medium improves again,a higher data rate (lest robust) mode is chosen. The following LA algorithm is based on

IEEE802.11a PHY/MAC (Weiss, 2004) An enhanced version of auto rate fallback (ARF)

Quick reaction to fast channel condition changeswhile taking into account slow channel conditionchanges

Common Link Layer


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Common Link Layer

Behavior (Link Adaptation)

Common Link Layer


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Common Link Layer

Behavior (Link Adaptation)


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Link Breakage Prediction

Link quality degrades when devices depart from each other. Sequential step-down of LA may serve to hint to the

network layer that a link will break soon, triggering routeadaptation to prepare for an alternate route.

During link operation, step-up and step-down might happenmultiple times before a link will break. By summing allweights related to LA steps within a certain time period,where step-down is counted as negative, step-up aspositive, an indicator for link reliability is gained.


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Link Breakage Prediction

Actions for Expected Link


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Actions for Expected Link

Break

Assuming LA provides useful information about the linkstate, it may trigger actions such as trying to rescue a link orguaranteeing a certain required link quality by establishinga new route.

A device may distinguish 3 cases:

Outgoing link on a route is switched to lower PHY mode but theincoming links remain unchanged. The link to a next device fadesaway since it moves away

Both incoming as well as outgoing links are switched to lower PHYmode, indicating movement of the observing device itself.

Incoming link is switched to a lower PHY mode, but the outgoing

links remain unchanged. This indicates that the device at the otherend is moving away.

Early Route Rearrangement


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(ERRA)

The ERRA protocol is derived from the local repairidea that is part of the AODV routing protocol.

ERRA does not wait until the link is broken but

prior to breakage rearranges the route to avoiddisruption.

ERRA proactively, by rearrangement, prepares foran alternate route to avoid interruption of service.


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Example

The initial route starts from source device 2to dst. 7, intermediate device 5 is movingaway.

Device 4 detects the movement, triggers theERRA. It locally broadcasts a route to device7 rearrangement request (ERRA_REQ).

Device 6 responds (ERRA_REP) and providesvia device 8 an alternative route



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(ERRA)


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Early Route Update (ERU)

Like ERRA, the ERU protocol proactivelyupdates the routing table, and takes MCSstep-down to the lowest PHY mode as a

trigger. The current stepped-down link is used to

establish alternate routes.


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Dev 5 is assumed to leave a route. Dev 4 is trigged by its LA procedure

when switching to BPSK1/2 and trxits neighborhood table(ERU_PATCH_INFO) piggy-backedto some data pkt sent to dev 5.ERU_PATCH_INFO: Breakage Hop Counter (BHC): to

count the number of hops trxted ona route, representing the size of theunstable part of the route.

Dev 5 forwards the info to dev 6that has a steady outgoing link todev 7.

Dev 6 searches but does not find anintersection between theneighborhood received and its owndev 6 and its neighbor further re-broadcast the list.


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Dev 11 has dev 8 in itsneighbor list and dev 8 isincluded in neighbor list ofdev 4 A route 6-11-8-4bypassing the weak link 4-5is found.

Dev 11 responds withERU_REP to dev 4 via dev8.

The number of broadcastscorrespond to the BHC (inexample here BHC=2),greatly reducing trafficcompared to flooding thevicinity dev 4.

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Chap4 - Mesh Networks - Basics

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