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Stuff

New HW on the web later today No lab today Tests graded by Thurs

Last Time

CAN Bus

Intro Low-level stuff Frame types Arbitration Filtering Higher-level protocols

Today

Embedded wireless networking – 802.15.4

Characteristics Layers Spread spectrum intro PHY layer MAC layer Superframes Network layer

802.15.4

Design choices relative to 802.11 (WiFi) and 802.15.1

(Bluetooth)

Simpler Lower cost Shorter range Lower bandwidth Lower power

Intended applications

Home networking

• Motion detectors
• Thermostats
• Smoke detectors

Automotive networking Industrial networking

802.15.4

Defines only the PHY and MAC layers

Zigbee Alliance is an association of companies involved

with building higher-layer standards based on IEEE 802.15.4

Two device profiles

Full function – capable of being a network coordinator Reduced function – end-node

Characteristics

Single-Chip 802.15.4 + MCU: ChipCon 2430

8051 processor

Old-style 8-bit, originated at Intel 32 MHz, 8 KB RAM, 128 KB Flash

802.15.4 radio on-chip

Short-range, low-power wireless Zigbee is based on this

Low power: 25 mA for CPU + Zigbee receive Low cost: <$4 in large quantities This has potential to be big

Cheaply add wireless connectivity to any device

• Examples?

Big Picture

LLC == Logic Link Control

IEEE 802.15.4 MAC Upper Layers IEEE 802.2 LLC Other LLC IEEE 802.15.4 2400 MHz PHY IEEE 802.15.4 868/915 MHz PHY

Two PHY Options

2.4 GHz band has worldwide availability and

provides a transmission rate of 250 kb/s

The 868/915 MHz PHY specifies operation in the 868

MHz band in Europe and 915 MHz ISM band in the United States

Offer data rates 20 kb/s and 40 kb/s respectively

Channel Structure

Brief Spread Spectrum Intro

Idea:

Sender spreads the signal over a larger bandwidth than it

would ordinarily cover

• Do this by adding high-frequency pseudo-random noise

to the signal

Receiver “despreads” to recover original signal

Main goal: Increase resistance to interference This is direct sequence spread spectrum

Other SS techniques exist E.g. frequency hopping

Spread Spectrum

Spread Spectrum Example

Period of

• ne chip

Period of

• ne data bit

SS Keying

BPSK – Binary Phase-Shift Keying

0 leaves chips unmodified 1 flips sense of all chips Encodes 1 bit per symbol

QPSK – Quadrature Phase-shift Keying

Beyond the scope of today’s lecture Encodes 4 bits per symbol

Bandwidth Effects

Spreading: Despreading:

Resistance to Interference

More Good Things

Resistance to interception: This one doesn’t apply to 802.15.4! Resistance to multipath effects: Delayed paths are rejected (and can even be exploited)

802.15. 4 Modulation / Spreading

868 MHz PHY

BPSK chip modulation at 300 kchips/s Data rate 20 kbps Spread factor of 15

915 MHz PHY

BPSK chip modulation at 600 kchips/s Data rate 40 kbps Spread factor of 15

2.4 GHz PHY

O-QPSK chip modulation at 2.0 Mchips/s 62.5 k symbols/s, 4 bits/symbol Data rate 250 kbps Spread factor of 32

Physical Layer Packet Structure

Preamble Start of Packet Delimiter PHY Header PHY Service Data Unit (PSDU)

PHY Packet Fields

• Preamble (32 bits) – synchronization
• Start of Packet Delimiter (8 bits)
• PHY Header (8 bits) – PSDU length
• PSDU (0 to 1016 bits) – Data field

6 bytes 0-127 bytes

MAC Frame Structure

MAC Frame

Frame control field indicates the type of MAC frame

being transmitted, specifies the format of the address field, and controls the acknowledgment

Multiple address types

64 bit physical address Short 16 bit network assigned addresses

Address field size may vary from 0 to 20 bytes Payload field is variable size, at most 127 bytes FCS is used for integrity check using 16 bit CRC

MAC Protocol

CSMA/CA – Carrier sense with multiple access and

collision avoidance

Transmit when the medium is free

• In not free, back off

Exceptions:

• Beacons go out on a fixed time schedule
• Acknowledgments are synchronous
• Optional contention-free access

Superframes

Certain applications require dedicated bandwidth to

achieve low latency for this

Optional superframe mode

PAN coordinator transmits superframe beacons in

predetermined intervals

This interval divided into 16 time slots

PAN coordinator may assign time slots to devices

requiring dedicated bandwidth or low-latency transmissions

These assigned time slots are called guaranteed

time slots (GTS) and together form a contention-free period

Superframe Structure

15ms * 2n where 0 ≥ n ≥ 14 Network beacon Contention period Beacon extension period

Transmitted by network coordinator. Contains network information, frame structure and notification of pending node messages. Space reserved for beacon growth due to pending node messages Access by any node using CSMA-CA

GTS 2 GTS 1

Guaranteed Time Slot

Reserved for nodes requiring guaranteed bandwidth

Contention Access Period Contention Free Period

Network Layer

Two or more 802.15.4 nodes form a PAN

”Personal area network”

Two different types of devices

Full functional Device (FFD)

• Coordinator and simple node
• Any topology
• Talks to any device

Reduced Functional Device (RFD)

• Simple node only, either source or desination
• Star topology only
• Talks to network coordinator only

Star Topology

Mesh Topology

Combined Topology

Summary

Bluetooth – dying slowly? 802.15.4 / Zigbee – cool stuff

Simple protocols Very low power Cheap

801.15.4 might be big

Too early to tell Probably depends on whether incremental cost / power

requirements can be made very low