4/12/2009

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Cognet: An Experimental Protocol Stack for

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Cognitive Radio Networks

P t St ki t C i M ll U i itPeter Steenkiste, Carnegie Mellon UniversityDipankar Raychaudhuri, RutgersJoe Evans, University of Kansas

Cognitive Wireless Networks

• Network that adapts globally to diverse external factors

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Lots of themApplicationsto diverse external factors▫ Nature, density, mobility,

… of nodes▫ Channel conditions, …▫ Nature, QoS, … of traffic

• Lots of activity in the design f iti di

Error, flow control

Group and topology formation, routing, …

Timing, collisions,

Transport

Network

MACof cognitive radios▫ Software defined radios as

an enabling technology• What about cognitive

networks: Where are they?

g, ,priorities, …

Modulation, coding, spectrum/DSA, …

MAC

PHY

Dynamic Spectrum Access

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Why A Global Control Plane?

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MAC PHY

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Today’s Wireless Cognitive Network

Cognet: Protocol Stack for Cognitive Networks

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• A network-wide control plane:▫ Bootstrapping of communication

App/Prot tr

GlobalInternet

Control Plane Protocol

▫ Bootstrapping of communication▫ Runtime coordination of cognitive behavior

• Set of basic cognitive protocols

MAC/PHY

FE

App/Prot

MAC/PHY Ctr

MAC/PHY

FE

MAC/PHY Ct

MAC/PHY

FE

App/Prot

MAC/PHY Ctr

MAC/PHY

FE

App/Prot

MAC/PHY Ctr

MAC/PHY

FE

App/Prot

MAC/PHY Ctr

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U S

Implementing a Cognitive MAC and PHY

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Host • Easy to program• Traditional environmentUser Space

Kernel

BusCPU

• Hard to program

Traditional environment

• Relative open• E.g. device driver, user, ..

• Far from radio

120us

25us

25ms

SDR

CPU• FPGA, RT, μengine, …

• Historically a closed box • Similar to today’s NICs

• Close to radioFE

FPGA

negligible

20us

USRP +Gnu Radio

Overcoming the SDR Limitations • Split-functionality

approach:

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Data Plane Control Plane

▫ High performance mechanisms on NIC

▫ Flexible control on host▫ Coordination using meta-

data, control channel

• Core MAC functions:

MAC

PHY

Control Prot.

MAC

PHY

Prot.

Appl.

Host

BusCore MAC functions:▫ Fine-grained Radio Control▫ Access to PHY information▫ Precision Scheduling in Time▫ Backoff, carrier Sense▫ Fast-Packet Recognition▫ Dependent Packets

FE

PHY

MAC

SDR

APIs

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TDMA Protocol Evaluation

• TDMA-based protocol similar to Bluetooth:C i i i f & l

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▫ Construct piconet consisting of master & slaves▫ Uses precision scheduling▫ 650us slot times

• Perform ten 100KB file xfers100KB file xfers

• Vary number ofslaves

CSMA Protocol Evaluation• Uses the following core

functions:

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▫ Carrier sense, backoff, fast-packet recognition, and dependent packets

• Ten transfers of 1MB files between pairs of nodes

Emulatorresults

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“CogNet” Control Plane Protocol

• Common framework for spectrum allocation, PHY/MAC bootstrap topology discovery etc

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bootstrap, topology discovery, etc.• Data plane uses parameters as specified by control plane

protocol▫ Controls PHY, MAC, Network modules, etc.

• GCP runs over separate “network”

▫ WiFi USRP Gl b l C t l Pl

Data Plane

▫ WiFi, USRP, ….

Control PHY

Control MAC

SpectrumMgmt

- BootstrapDiscovery

PHY

MAC

Network

Transport

Application

Control Plane Data Plane

Global Control Plane

Control API

Data Path

Establishment

Naming&

Addressing

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CogNet GCP: Beacon Format

• Beacon: short message for low-level functions1 8 16 24 32

• Focus: discovery, maintaining control channel▫ Can learn who is around

MSG Type Flags Sequence NumberSource...

...Identifier Max PHY RateMax Transmit Power Beacon Transmit Power

Num of Reach MAC Type MAC Busy Indicator

NA CF FD 0 0 0 0 08 10 12 14 16

Flags:

▫ Basic information on signal propagation environment

• Information for specific control tasks can be added as additional headers▫ What functions should use the GCP?

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Interface to Global Internet

• Generally need to exchange information about “ h bilit ” “di ”

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“reachability” or “discovery”▫ Wireless nodes may be on multiple “networks”▫ Association may be dynamic, traffic specific▫ Need to consider link capabilities, load, services,

etc. … more diverse than in, e.g. WiFi▫ Includes services contents Includes services, contents, …

• Work in progress▫ Will build on GCP, information collected through

control framework▫ Look at Internet control plane efforts – help!

CogNet Experiments: Spectrum Coordination

Data Radio Service

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PHY Type IEEE 802.11g(Atheros AR5212)

Bluetooth(USB Dongle)

Frequency 2427-2447MHz 2402-2480MHz

Modulation OFDM (256 FFT) QAM

FHSS

Transmit Power 18dBm 4dBm (~10m) (class 2)20dBm (~100m) (class 1)

PHY Rate 1M 54Mbps AutoRate Upto 1Mbps (class 2)

BT

PHY Rate 1M-54Mbps AutoRate Upto 1Mbps (class 2)Upto 4Mbps (class 1)

Data session Pareto ON/OFF variable rate CBR: 5 sec

random session

Constant audio streaming (64, 128,320,512,

1024kbps)

WiFi

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CogNet Experiments: Baseline

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UDP throughput results with and without interference from other BT/WiFi users

Wifi Performance

0 00

20.00

40.00

60.00

80.00

100.00

120.00

Perc

enta

ge T

hrou

ghpu

t

802.11g Throughput

BT-Performance

20

40

60

80

100

120

BT-Throughput

0.00No-Interf With-Interf

Coexistence Effect on Wifi

0No-Int erf Wit h- Int er f

Coe x i st e nc e Ef f e c t on BT

Throughput Drops by ~3-4x in the case of 802.11g nodes and by ~1.5-2x for bluetooth nodes in densetopologies with 4 wifi and 4 Bt links. Results Averaged over 5 different topologies & load conditions.indicates the need for spectrum coordination

How can we avoid or reduce interference?Can I control bandwidth allocation?

CogNet Experiments: GCP-based Coordination

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WifiBluetooth

Alt ti Wifi (BT-Rate) Wifi (BT-BO)BT (BT-Rate)Alternatives :

• No coordination• BT rate adaptation: BT

adapts rate based on amount of WiFi traffic nearby

• BT backoff: BT defers when nearby WiFi device is transmitting

1M 5M 10M 15M

-50

0

50

100

WiFi offered load (bps)BT load 1Mbps

BT (BT-Rate) BT (BT-BO) Total (BT-Rate) Total (BT-BO)

Thro

ughp

ut Im

prov

emen

t (%

)

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Adaptation of the MAC Protocol• Use the most efficient protocol for the given channel

conditions▫ Switching of the MAC “on-the-fly”▫ Minimize disruption of the network▫ Maintain a fully distributed protocol

• Use Cognet GCP for information exchange and coordination of adaptation▫ Exchange information on observed loss rates▫ Propagate request for MAC switch if needed▫ Use MAC/PHY framework to switch

• Example: CSMA versus TDMA▫ Based on loss rates: low -> TDMA, high -> CSMA▫ Other policies possible: utilization, traffic type, …

MAC Adaptation Architecture• Implemented using GR, USRP, & new architecture

Data Control

One MACconnectedControl &

Status

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What is Next?• Interface global Internet• More powerful platform

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• Richer set of control plane mechanismsMore powerful platform

• Expand MAC/PHY, GCP controls

Global Cognet Control PlaneControl Prot.

Prot.Appl.

Data Plane Control Plane

p• Gain experience with

more challenging tasks, e.g. cooperative communication

MAC/PHYFE

App/ProtMAC/PHY C

trMAC/PHY

FE

App/ProtMAC/PHY C

tr

MAC/PHYFE

App/ProtMAC/PHY C

tr

MAC/PHYFE

App/ProtMAC/PHY C

tr

MAC/PHYFE

App/ProtMAC/PHY C

tr

Internet

FE

MACPHY

PHYMAC

MACPHY

Host

SDR

Bus

Conclusion• Cognitive networks allow global control over all

networks and functions

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▫ Including physical layer (spectrum) and MAC• Global control plane supports coordination▫ Bootstrapping, neighbor discovery, etc.

• Control framework places control on host while maintaining high performance and flexibility▫ Approach is not specific to SDRs!

• Implemented on GNU Radio & USRP and evaluated on open testbed, wireless emulator, Orbit

• Open source: watch https://www.cgran.org/

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Extra Slides

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Prototype Implementation• Control Plane & Channel (CSMA-based):▫ Controls currently active MACy▫ Monitors loss rates▫ Periodically broadcasts loss rates▫ Transmits request for MAC switch based on loss

environment

d i h i• Adaptation technique:▫ Low loss rates: TDMA▫ High loss rates: CSMA▫ Other policies possible: utilization, traffic type, …

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Limitation: Delays and Jitter

• Delay affects carrier sense, fast packet-detection and dependent-packets

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detection, and dependent packets• Jitter affects precision scheduling and backoff

Medium

Ker

nel

Userspace

negligible

15ns20us

120us

25us 25ms

Jitter!

+

ADC

DACAntenna

FPGA

Fro

nt

En

d Bus (USB)+

K

MACModulation,

FramingUSRP +

Gnu Radio

Precision Scheduling Evaluation• Test is the

accuracy of

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yinter-frame spacing for TDM-style protocol

• Measured

Timestamp: < 125ns

with 125ns precision

• Results averaged over 50 runs

Host:~1ms error

Kernel: ~35us error

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Carrier Sense Evaluation

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• Test is the length of gthe blind spot

• Test set up is the same as for precisions scheduling

CSMA Protocol Evaluation• 802.11 like using the following core functions:▫ Carrier sense, backoff, fast-packet recognition,

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and dependent packets• GNU Radio & USRPs configuration:▫ Target bitrate of 0.5Mbps▫ Use 2.485GHz, avoid 802.11 interference▫ Ten transfers of 1MB files between pairs of nodes

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Signal Propagation Emulation

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Real hardware high degree of realism

Digital emulation of channels full control

Isolated from environment fully repeatability

Programmable very diverse experiments

New Features: Metadata• Per-block metadata allows for fine-grained radio

control and can be used for PHY information

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f

MAC

Bus (USB)

MAC

FPGA

+MAC

Building BlocksMAC

Control

Radio Hardware Host

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Enabling Performance• A control channel and metadata are not novel…▫ Novelty is in how we use them to split MAC

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functionality

• Core MAC protocol functions:▫ Fine-grained Radio Control▫ Access to PHY information▫ Precision Scheduling in Time▫ Precision Scheduling in Time▫ Backoff▫ Carrier Sense▫ Fast-Packet Recognition▫ Dependent Packets

Putting it all together…

• Split framework and core components i l t d GNU R di & USRP

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implemented on GNU Radio & USRP▫ Being released on https://www.cgran.org/

• End-to-end evaluation by implement two common styles of MACs

Show feasibility of design in keeping flexibility▫ Show feasibility of design in keeping flexibility

• Evaluation in both an open testbed and on the CMU wireless emulator testbed▫ Used to evaluate total performance gain

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Implementing a Cognitive MAC and PHY

• Hard to program• Historically a closed

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• Easy to program• Relative open y

box as in today’s NICs• Close to radio

(Software) Radio Hardware

Medium Host

Ker

nel

Userspace

Relative open (device driver)

• Far from radio

+

ADC

DACAntenna

FPGA

Fro

nt

En

d Bus (USB)+

K

MACModulation,

Framing

CPU