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Ad Hoc and Mesh Networks:Architecture and Technology Overview

Rutgers, The State University of New JerseyD. Raychaudhuri

ray@winlab.rutgers.eduwww.winlab.rutgers.edu

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Introduction: The Mesh Network Opportunity ….

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Ad Hoc and Mesh Networks: 1st Gen Products

From Firetide

Peer-to-peer network that allows groups of nearby users to communicate,

exchange files, stream media, work collaboratively, …

Ad Hoc Mesh

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Ad Hoc and Mesh Networks: BackgroundSeveral distinct motivations for ad hoc and mesh

Connecting ad hoc cluster of mobile users (tactical, vehicular, P2P)Networks involving embedded low-power devices (sensor nets)Access without wired infrastructure (rural, developing countries)Short-range radio cost-performance wide area

1st generation products were for specialized marketsTactical, specialized ad hoc applicationsSensing applications with power constraints

2nd gen products are for existing telecom markets, exploiting exceptional cost-performance of commodity radios…

Initially rural telecom, hobbyists metro mesh todayNow migrating to mainstream broadband accessIs cellular next?

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Ad Hoc and Mesh Networks: The PC Analogy

Wired High-Speed Network(Ethernet Switch or Internet)

Wired High-Speed Network(Ethernet Switch or Internet)

Mainframe Computer

Distributed PC’s

Cellular BTS TowerNetworked Low-Cost Radios

~$10K/GIPS

~$0.5K/GIPS – cheap but uncoordinated CPU cycles

Distributed PC solution dominatesfor most regimes except supercomputing

Technical issues: communication latency, overhead,parallel computation issues, execution control, unreliable networks, etc. mostly solved!

Lower cost, higher capacity, more robust??

~$1M/Mbps (long-range)

~1K/Mbps – cheap short-range but uncoordinatedbasic transmission

?? Distributed PC solution dominatesfor most regimes except supercomputing

Lower cost, higher capacity, more robust??

Technical issues: communication latency, overhead,Concurrent transmission issues, network control, unreliable channels, etc. not solved yet!

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Ad Hoc and Mesh Networks: The PC Analogy (contd.)

The $49 Mesh Node from Meraki Networks*!

1000 node metro mesh would cost just ~$50K in capital to cover a ~10 sq-Km area…!!

*Stanford and MIT student startup

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Ad Hoc and Mesh Networks: Product Space

Radio Range

End-UserServiceBit-Rate

1 m 10 m 100 m 1Km 10 Km

10 Kbps

100 Kbps

100 Mbps

1 Mbps

10 Mbps

2G Cellular

3G Cellular

AccessNetworks(WiMax)

MetroMesh

802.11b

802.11a,g

IndoorMesh LAN

Wide-area access

WLAN office/home and campus access

Dense office

& home access

UWB

DenseAP

Mesh Regime

TacticalAd hoc net

Mesh extends 802.11x radios to cover:

Metro mesh (medium range, high capacity)Access networks (extended range, lower capacity)Indoor WLAN (higher capacity, coverage)

Region of use can be even greater with new non 802.11 radios

Cellular wide-area equivalentSwitched Ethernet equivalent indoors

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Ad Hoc and Mesh Networks: 2nd Gen Products

Dual-radio ad-hoc router(includes wired interface for AP sites)

(above photo shows WINLAB’s ORBIT node)

RadioNodes

~50-100 mspacing

Ad-hocRadiolinks

Access Point (wired)

Ad-Hoc Radio Node

Office WLAN (faster, more scalable) than current 802.11 Metro Area Mesh Network (dense, high capacity, low cost)

Commercial vendors: Tropos, Motorola, Nortel, Nokia, …

Commercial vendors: Firetide, Cisco, …

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Ad Hoc and Mesh Networks: Problems with Current Technology

1st gen, and to some extent, 2nd gen solutions suffer from several technical problems:

Poor scalability – too many hops!CSMA/CA MAC implies “exposed nodes” which cannot tx in parallelMAC protocols never designed for multi-hop wireless to begin with!Topology changes rapidly – increases routing overheadOverall control overhead can be very highRouting unaware of changes in PHY speed/quality“Self-interference” effect for TCP flows

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Ad Hoc and Mesh Networks: Technology IssuesProducts are being released, but…

Mobile ad hoc networks don’t really work well for tactical & vehicularMesh network performance is marginal, OK for low-cost scenarios only

Major technical challenges areScalability (overcoming Gupta & Kumar) – hierarchies, multi-channelPHY capacity improvements – collaboration, MIMO, network codingTopology discovery and self-organization in mobile scenariosReducing control overheads in existing 802.11 MAC and MANET routingMitigating MAC “exposed node” problem for parallel transmissionsIntegrated or cross-layer MAC & routing approaches without the performance problems of conventional layered protocolsIntroducing service features such as QoS, multicast, …Network Security!

WINLAB research covers several of the above topics...

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Key Technologies

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Key Technologies: Hierarchical ArchitectureHierarchical structure essential for scalability

Classical “Gupta & Kumar” result shows mesh throughput per node does down as sqrt(n)System can scale with multiple frequencies and proper ratio of MN, FN and APE.g, if MN~100, ~10 FN’s & ~3 AP’s needed (…note significant reduction in # wired nodes)

Wired Internet Infrastructure

Gateway node

Potential bottleneck

“Flat” mesh network with ad-hoc routing: does not scale!

Wired Internet InfrastructureMulti-tiered

Interfaces towired network

Hierarchical architecture with multi-radio forwarding nodes and AP’s

Ad-hoc associations

Ad-hoc associations

Throughput per node scales ~ 1/sqrt(n)

Throughput per node scales with right ratio of FN’s, AP’s

Grid Portals/Access Points

Multi-radioForwarding Node

15 20 25 30 35 40 45 50 55 60 6510

15

20

25

30

35

40

45

50

System offered load (Mbps)

Sys

tem

Thr

ough

put (

Mbp

s)

Total System Throughput for flat and hierarchical topologies

FlatHierarchical

Sample experimental result on ORBIT showing linear scaling & ~2.5X capacity (for a mesh network with ~20 MN, 4FN, 2AP)

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Transmit Power Required: 1mW

AssocReq

Transmit Power Required: 10mW

FN

AP

• Scan all channels, record neighbors• Decide neighbor based on objective• Associate with neighbor

Key Technologies: Discovery and Self-OrganizationOnly a subset of available links made available to routing – achieves balance between routing overhead and route availabilityDynamic topology formation based on different such as max throughput, min delay or power

Send beaconsFN

AssocReq AssocReq

Logical topology

FN

Interface TwoSend beaconsAccept AssociationsForward client Data

Interface OneScan all channelsFind minimum delay links to APAssociate with AP

Wired Internet Infrastructure

PHY

MACDISCOVERY

ROUTING

Sample Result showingsignificant reduction in routing

overhead

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0 200 400 600 800 1000 12000

100

200

300

400

500

600

700

800

900

1000

O ffe red Load (k bps )

Syst

em th

roug

hput

(kbp

s)

S y s tem th roughpu t (S c ena rio I)

M H M e tricPA R M A

0 500 10000.5

0.6

0.7

0.8

0.9

1

Offered Load (kbps)

Pac

ket D

eliv

ery

Rat

io

Packet Delivery Ratio (Scenario I)

MH MetricPARMA

0 500 1000

100

200

300

400

500

600

700

Offered Load (kbps)

End

-to-E

nd D

elay

(ms)

End-to-end Delay (Scenario I)

MH MetricPARMA

Improved performance with PARMA compared to MH metric.PARMA has the same behavior as MTM when no congestion.

Routing Metric = Σ pkt size/link speed + MAC congestion

Key Technologies: Cross Layer Routing

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Key Technologies: Multi-Channel MeshMulti-channel mesh (>>1 radio per node) can improve performance significantly by supporting concurrent transmissions & reducing/eliminating 802.11 MAC overheads

Many 2nd gen mesh products use 5-6 radios per nodeAlgorithms for optimizing throughput given constraints on # radios, # channelsPossible to use 802.11a hardware and avoid MAC effects entirely

f1

f2

f3

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Key Technologies: Clean-Slate PHY/MAC for Mesh

Wideband, agile short-range OFDM radio design optimized for speedFor example, ~2 x 20 Mhz bandwidth, max bit-rate ~250 MbpsAdditional low-bit rate PHY for control, flexible TDMA based MACRadios can switch channels and bit-rates on a slot-by-slot basis (~ms) –allows for unconstrained FD/TDMA allocations

2x20 Mhz

Agile RF Front End D/A

OFDMBaseband

MAC controlInterface

~25-200 MbpsService data

Programmable radio board at WINLAB

2.4 GhzRF

1 Mbps802.11b

PHY

ControlPlane data

PHY Module

#1

PHY Module

#2Control

PHY

ComputingModule

Grid Node Platform

Note: 2 x 200 Mhz agile radios with TD capabilityshould be sufficient for ~50 mbps duplex per node

Assuming ~3-4 hops to a wired AP

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Key Technologies: Global Control PlaneImportant architectural idea – clean separation of control & data planesReduces control overhead and enables contention-free global MAC/routing algorithmsCan use a single low rate channel (e.g. 1 mbps 802.11b) for control

http://www.uninett.no/wlan/throughput.html

Example of WLAN throughput breakdown

Control penaltyIn current WLANs

(increases with bit-rate)

Low Bit Rate PHY

Bootstrapping/Discovery

Link stats

Flow stats

Control Plane Data Plane

Fast/Agile PHY

Lower MAC

Integrated MAC/Routing

Frequency assignments,TDMA schedule and route

selection

Distributed or Global ControlIntegrated MAC/Routing Algorithm

Broadcast MAC & Routing

Control Plane

Data Plane

Ethernet and 802.11 drivers

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Key Technologies: Integrated Routing and MAC

Global allocation of routes and MAC time slots at the same time to completely eliminate contentionAllocation algorithm works on both frequency (FD) and time (TD)Algorithm checks for compatible time slot and freq at each receiverAllows for more parallel transmissions (fewer “exposed nodes”) and eliminates packet contentionSignificant performance improvement over conventional layered 802.11 + AODV etc.Requires GCP-type capability for distribution of control

Comparison of Individual and Aggregate Throughput

0200000

400000600000800000

1000000

12000001400000

flow 1 flow 2 flow 3 flow 4 flow 5 Total

Global Scheduling

802.11

Aloha

Slot Aloha

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Key Technologies: Cognitive Radio

AA

BB

D

C

D

E

F

Cognitive radios provide PHY, MAC flexibility needed to implement cooperative multi-channel ad hoc networks with better performance

Cognitive radio can achieve multi-channel + flexible MAC performance PHY can be optimized between neighboring nodesHigh spectrum efficiency possible via dynamic spectrum algorithmsNetworks may utilize a control channel similar to GCP, etc.

Bootstrapped PHY &control link

End-to-end routed pathFrom A to F

PHY A

PHY BPHY C

Control(e.g. CSCC)

Multi-mode radio PHYAd-Hoc Discovery

& Routing Capability

Adaptive WirelessNetwork Node

(…functionality can be quitechallenging!)

~250 Mbps

Control Plane

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A

B

C

D E

R1 R2 R3

R4 R5

rejected

rejected

flow 1 (t1)

flow 2 (t2)

flow 3 (t3)

request

Responsebw resv

:::

W

Hops=0

:::ForwardingSet a timer

Check bandwidth availability

WNhopsB contupconsume ×++= )1

2,1min(

drop

:::

W

Hops=0

:::Resv BwSet a timerforwarding

Check bandwidth availability

upconsume

contconsume BWNhopsB +×= )

2,min(

drop

Establishes QoS routes with reserved bandwidth on a per-flow basisMonitors interference from adjacent nodesPerforms admission control to maintain network utilization below the congestion point

Key Technologies: QoS-Aware Routing

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Location is a more natural addressing mechanism

Location becomes more important than a network address

Opportunistic message forwarding within geographic perimeterRetransmissions from different vehicles Delay-tolerant networking

Desired message delivery zone

(Idealized) Broadcast range

Irrelevant vehicles in radio range for few seconds

Passing vehicle,in radio range for tens of seconds

Following vehicle,in radio range for minutes

Key Technologies: Geocasting in Ad Hoc Vehicular Networks

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Experimental Results

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Access PointUS Robotics 2450 AP

AMD Elan SC400 processor

1 MB Flash, 4 MB RAM

Prism-2 based PCMCIA card

Forwarding nodeCompulab 586 CORE

AMD Elan SC520 CPU

2 MB NOR flash + 64 MB NAND Flash on board

Dual PCMCIA slots

SensorsIntrinsyc Cerfcube

Intel PXA 250 (XScale processor)

CF-based wireless support

HA

RD

WA

RE

PLA

TFO

RM

SOFT

WA

RE

802.11b ad-hoc mode

Experimental Results: WINLAB Prototype circa 2002

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Experimental Results: ORBIT Radio Grid

ORBIT: 400 nodes in 20m x 20m– two 802.11 radios each (atheros and intel-based)Intended for ad hoc and mesh network studies

Antennas

Mini ITX-based SSF PC

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Urban

300 meters

500 meters

Suburban

20 meters

ORBIT Testbed

20 meters

HallwayOffice

30 meters

Experimental Results: Radio Mapping

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Experimental Results : ORBIT Proof-of-Concept for Metro Mesh Scenario

Flat Hierarchical

System Parameters: 0.9 sq. km, 20 mobiles/sensors, 4 FNs, 2 APs802.11a with multiple frequencies

15 20 25 30 35 40 45 50 55 60 6510

15

20

25

30

35

40

45

50

System offered load (Mbps)

Sys

tem

Thr

ough

put (

Mbp

s)

Total System Throughput for flat and hierarchical topologies

FlatHierarchical

Flat

Hierarchical

• “SOHAN” system evaluated for realistic deployment scenario with ~25 nodes

• Results show that system scales well and significantly outperforms flat ad-hoc routing (AODV)

APFN

MN

Mapping on to ORBITRadio grid emulator

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Mapping of the actual path onto ORBIT.

Emulated path.Emulated path.Actual path.Actual path.A B

C

D

E

F

H

G

Experimental Results: Mobility Emulation for MANET Studies

Goal: Emulate mobility for MAC and higher layers for larger number of nodes

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ObjectivesDemonstrate a vehicular 802.11a experiment.Actors : Sender, ReceiverDetails : Receiver is stationary. Sender moves around the parking lot. Sender transmits ICMP packets addressed to the receiver. Both nodes use 802.11a, channel 36. Receiver logs per-packet RSSI using Libmac.

Results (Snapshot of RSSI)

Experimental setup

Experimental Results: ORBIT Vehicular Setup

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Experimental Results: Ongoing Work

IRMA – Integrated Routing and MAC (..requires software MAC capability under development)

Includes a global control plane (GCP)Centralized and distributed control algorithms to be compared

Cross-layer routing with DCMA cut-through switching MAC

Switched multi-radio mesh scenarioBased on DCMA (Acharya) MAC, distributed cross-layer routing

Vehicular ad hoc network scenariosDense MAC experimentsGeocasting protocol evaluation