MAC, Physical Layer, Energy Consumpion and IEEE 802.15.4

Lecture 8 September 28, 2004

EENG 460a / CPSC 436 / ENAS 960 Networked Embedded Systems &

Sensor Networks

Andreas Savvidesandreas.savvides@yale.edu

Office: AKW 212Tel 432-1275

Course Websitehttp://www.eng.yale.edu/enalab/courses/eeng460a

Announcements

Appointment schedule for projects Student presenter for Oct 12 – Diffusion routing Project proposal

• 1 page description of your project (including references)• Should include:

o What is the problem your solving and what is the new feature that you are adding to the problem

– Narrow down the problem you will be working on, be very precise with what you are going to do

o Give an initial list of paper references on which your paper will be basedo A list of resources that you will need for the project (any additional HW, SW

and sensors)• Do not exceed 1-page!!!!• Email to andreas.savvides@yale.edu

o Filename: name1_and_name2_proposalo Email Subject: EENG460 Project Proposal

Frequency Bands and Data Rates

In 2.4GHz band 62.5 ksymbols/second• 1 symbol is 4 bits• 1 symbol is encoded into a 32-bit pseudorandom sequence the chip

chip rate = 62.5 x 32 = 2000 kchips/sRaw data rate = Symbol rate * chips per symbol = 62.5 * 4 = 250kb/s• In 868/915 MHz bands

1 bit symbol (0 or 1) is represented by a 15-chip sequence

Physical Layer Transmission Process

Binary Data fromPPDU

Bit to Symbol Conversion

O-QPSKModulator

Symbol to Chip Conversion

RF Signal

Radio Characteristics

Power output• The standard does not specify a power output limit.

• Devices should be able to transmit -3dBmo In US 1Watt limit in Europe 10mW for 2.4GHz band

Receiver should be able to decode a packet with receive power of• -85dBm in 2.4GHz and -92dBm in the lower frequency

bands

What does that mean in terms of range?

Going from Watts to dBm

1mW

mW)P(in 10logdBm)P(in

+20dBm=100mW

+10dBm=10mW

+7dBm=5mW

+6dBm = 4mW

+4dBm=2.5mW

+3dBm=2mW

0dBm=1mW

-3dBm=.5mW

-10dBm=.1mW

Friss Free Space Propagation Model22

44

d

cGG

dGG

P

PRTRT

T

R

er transmittandreceiver between distance -

light of speed -

metersin h wavelengt-

antenna receiving and ing transmittfor the gainspower theare and

(in watts) antennas ing transmittand receiving at the espower valu - and

d

c

GG

PP

RT

RT

Same formula in dB path loss form (with Gain constants filled in):

kmMHzB dfdBL 1010 log20log2044.32)( How much is the range for a 0dBm transmitter 2.4 GHz band transmitterand pathloss of 92dBm?

Friss Free Space Propagation Model22

44

d

cGG

dGG

P

PRTRT

T

R

er transmittandreceiver between distance -

light of speed -

metersin h wavelengt-

antenna receiving and ing transmittfor the gainspower theare and

(in watts) antennas ing transmittand receiving at the espower valu - and

d

c

GG

PP

RT

RT

Same formula in dB path loss form:

kmMHzB dfdBL 1010 log20log2044.32)( How much is the range for a 0dBm transmitter 2.4 GHz band transmitterand pathloss of 92dBm?

Highly idealized model. It assumes:• Free space, Isotropic antennas• Perfect power match & no interference• Represent the theoretical max transmission range

Propagation Mechanisms in Space with Objects

Reflection • Radio wave impinges on an object >> λ (30 cm @1 GHz)• Earth surface, walls, buildings, atmospheric layers

Diffraction• Radio path is obstructed by an impenetrable surface with sharp

irregularities (edges)• Secondary waves “bend” arounf the obstacle• Explains how RF energy can travel without LOS

Scattering• When medium has large number of objects < λ (30cm @1 GHz)• Similar principles as diffraction, energy reradiated in many directions• Rough surfaces, small objects (e.g foliage, lamp posts, street signs)

Other: Fading and multipath

A more realistic model: Log-Normal Shadowing Model

XdnfndBL kmMHzB 1010 log10log1044.32)(

• Model typically derived from measurements

dB)(in deviation

standard with dB)(in r.vGaussian mean -zero is

X

• Statistically describes random shadowing effects• values of n and σ are computed from measured data using linear regression

• Log normal model found to be valid in indoor environments!!!

Transmit Power Levels in Chipcon CC2420

= 1mW = 43.5mW

Radio supply voltage= 2.5VAnd Power = I*V

Budgeting Battery Power

Assuming power drain is the same for Transmitting and Receiving = 43.5mW

We need to power the device from a 750mAh battery for 1 year

What is the duty cycle we need to operate at?

Budgeting Battery Power

Assuming power drain is the same for Transmitting and Receiving = 43.5mW

We need to power the device from a 750mAh battery for 1 year What is the duty cycle we need to operate at?

1 year has 365 x 24 = 8760 hours

The average current drain from the battery should be

Average power drain

AhmAhIavg 868760/750

AV 86*5.2Pavg

Computing Duty Cycle

off arereceiver andmitter both transen battery wh thefromdrain Current I

on istter or transmireceiver either theen battery wh thefromdrain Current I

on istter or transmireceiver either timeofFraction T

Where

I*)T-(1 I*TI

stby

on

on

stbyonon onavg

0.38% 0038.02017400

2086

I I

IIT

mA86I ,20I mA,5.17I Assuming

stbyon

stbyavgon

avgstbyon

A

Energy Implication

Active transceiver power consumption more related to symbol rate rather than raw data rate

To minimize power consumption:• Minimize Ton - maximize data rate• Also minimize Ion by minimizing symbol rate

Conclusion: Multilevel or M-ary signalling should be employed in the physical layer of sensor networks• i.e need to send more than 1-bit per symbol

Radio Energy Model: the Deeper Story….

Wireless communication subsystem consists of three components with substantially different characteristics

Their relative importance depends on the transmission range of the radio

Tx: Sender Rx: Receiver

ChannelIncominginformation

Outgoinginformation

TxelecE Rx

elecERFETransmit

electronicsReceive

electronicsPower

amplifier

Radio Energy Cost for Transmitting 1-bit of Information in a Packet

The choice of modulation scheme is important for energy vs. fidelity and energy tradeoff

level Modulation

scheme modulationary -Man for rate Symbol

synthesisfrequency for

circuitry electronic ofn consumptiopower

lengthheader packet

length payloadpacket

startup radio with dasssociateenergy

1*log*

)(

2

M

R

P

H

L

E

L

H

MR

MPP

L

EE

s

elec

start

S

RFelecstartbit

Examples

0

2000

4000

6000

8000

The RF energy increases with transmission range The electronics energy for transmit and receive are typically

comparable

0

100

200

300

0

200

400

600

TxelecE Rx

elecERFE TxelecE Rx

elecERFE TxelecE Rx

elecERFE

nJ/bit nJ/bit nJ/bit

GSM Nokia C021 Wireless LAN

Medusa Sensor Node (UCLA)

~ 1 km ~ 50 m ~ 10 m

Power Breakdowns and Trends

Analog electronics240 mW

Digital electronics170 mW

Power amplifier 600 mW

(~11% efficiency)

Intersil PRISM II (Nokia C021 wireless LAN)

Radiated power63 mW (18 dBm)

Trends: Move functionality from the analog to the digital electronics Digital electronics benefit most from technology improvements

Borderline between ‘long’ and ‘short’-range moves towards shorter transmit distances

What is wrong with this model?

Does not include many parameters• DC-DC converter inefficiencies• Overhead for transitioning from on to standby

modes• Different power consumptions for receiver and

transmitter• Battery discharge properties• Does not include the processor power and any

additional peripherals

Power Supply

Where does the Power Go?

Bat

tery

DC-DCConverter

Communication

RadioModem

RFTransceiver

Processing

ProgrammablePs & DSPs

(apps, protocols etc.) Memory

ASICs

Peripherals

Disk Display

DC-DC Converter Inefficiency

Current drawn from the battery

Current delivered to the node

Battery Capacity

Current in “C” rating: load current normalized to battery’s capacity

o e.g. a discharge current of 1C for a capacity of 500 mA-hrs is 500 mA

from [Powers95]

Microprocessor Power Consumption

CMOS Circuits(Used in most microprocessors)

Dynamic ComponentDigital circuit switching inside

the processor

Static ComponentBias and leakage currents

O(1mW)

clk2

ddlddscddleakageddstandby fVCVIVIVIP

Static Dynamic

Power Consumption in Digital CMOS Circuits

clk2

ddlddscddleakageddstandby fVCVIVIVIPower

standbyI

leakageI

scI

- current constantly drawn from the power supply

- determined by fabrication technology

- short circuit current due to the DC path between the supply rails during output transitions

lC - load capacitance at the output node

clkf - clock frequencyddV - power supply voltage

DVS on Low Power Processor

Maximum gain when voltage is lowered BUT lower voltage increases circuit delay

M

1k

2ddk VfCP

2TDD

DD

)V(VV

τ

CMOS transistor threshold voltageTransistor gain factor

Dynamic Power Component

Number of gates

Load capacitance of gate k

Propagation delay

Now Back to IEEE 802.15.4 MAC

MAC supports 2 topology setups: star and peer-to-peer Star topology supports beacon and no-beacon structure

• All communication done through PAN coordinator

Star: Optional Beacon Structure

Beacon packet transmitted by PAN Coordinator to help Synchronization of network devices. It includes:Network identifier, beacon periodicity and superframe structure

Generic Superframe Structure

GTS: Guaranteed timeSlots assigned by PANcoordinator

Star Network: Communicating with a Coordinator

Star Network: Communicating from a Coordinator

Beacon packet indicates that thereis data pending for a network device

Device sends request on a data slot

Network device has to ask coordinator if there is data pending.If there is no data pending the Coordinator will respond with a zeroLength data packet

Peer-to-Peer Data Transfer

Peer-to-peer data transfer governed by the network layer – not specified by the standard

Four types of frames the standard can use• Beacon frame – only needed by a coordinator

• Data frame – used for all data transfers

• ACK frame – confirm successful frame reception

• A MAC Command Frame – MAC peer entity controltransfers

Beacon Frame

ACK & Data Frames

ACK Frame

Data Frame

MAC Command Frame

Wrap-up Low Power MAC

You now have enough information to do a more detailed power consumption analysis for IEEE 802.15.4

Need to factor in different packet structures header and MAC overheads

What are the issues related with low power MAC protocols?

Design of low power schemes for peer-to-peer networking…

Concept of Primitives

Request: To initiate a service

Indication: Indicate an N-layer event that is significant to the used

Response: to complete a procedure previously invoked by an indication primitive

Confirm: conveys the results of one or more associated previous service requests

Next Lecture

Time Synchronization Read the paper

[Elson02] Fine-Grained Network Time Synchronization using Reference Broadcasts, Jeremy Elson, Lewis Girod and Deborah Estrin, Proceedings of the Fifth Symposium on Operating Systems Design and Implementation (OSDI 2002), Boston, MA. December 2002. UCLA Technical Report 020008.