Transcript Document

ZigBee and 802.15.4
Copyright 2002 The ZigBee Alliance, Inc.
IEEE 802.15.4 Standard
Copyright 2002 The ZigBee Alliance, Inc.
IEEE 802.15.4 Basics
• 802.15.4 is a simple packet data protocol for lightweight
wireless networks
– Channel Access is via Carrier Sense Multiple Access with collision
avoidance and optional time slotting
– Message acknowledgement and an optional beacon structure
– Multi-level security
– Three bands, 27 channels specified
• 2.4 GHz: 16 channels, 250 kbps
• 868.3 MHz : 1 channel, 20 kbps
• 902-928 MHz: 10 channels, 40 kbps
– Works well for
• Long battery life, selectable latency for controllers, sensors, remote
monitoring and portable electronics
– Configured for maximum battery life, has the potential to last as
long as the shelf life of most batteries
Copyright 2002 The ZigBee Alliance, Inc.
Introduction to the IEEE
802.15.4 Standard
• IEEE 802.15.4 standard released May
2003
– Semiconductor manufacturers
• Sampling Transceiver ICs and platform
hardware/software to Alpha Customers now
– Users of the technology
• Defining application profiles for the first products,
an effort organized by the ZigBee Alliance
Copyright 2002 The ZigBee Alliance, Inc.
IEEE 802.15.4 standard
• Includes layers up to and including Link Layer Control
– LLC is standardized in 802.1
• Supports multiple network topologies including Star,
Cluster Tree and Mesh
• Features of the MAC:
Association/dissociation, ACK,
frame delivery, channel access
mechanism, frame validation,
guaranteed time slot management,
beacon management, channel scan
• Low complexity: 26 primitives
versus 131 primitives for
802.15.1 (Bluetooth)
Copyright 2002 The ZigBee Alliance, Inc.
ZigBee Application Framework
Networking App Layer (NWK)
Data Link Controller (DLC)
IEEE 802.15.4 LLC
IEEE 802.2
LLC, Type I
IEEE 802.15.4 MAC
IEEE 802.15.4
868/915 MHz PHY
IEEE 802.15.4
2400 MHz PHY
IEEE 802.15.4 MAC Overview
•
Employs 64-bit IEEE & 16-bit short addresses
– Ultimate network size can reach 264 nodes (more than we’ll probably
need…)
– Using local addressing, simple networks of more than 65,000 (2^16) nodes
can be configured, with reduced address overhead
•
Three devices specified
– Network Coordinator
– Full Function Device (FFD)
– Reduced Function Device (RFD)
•
•
•
•
•
•
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Simple frame structure
Reliable delivery of data
Association/disassociation
AES-128 security
CSMA-CA channel access
Optional superframe structure with beacons
GTS mechanism
Copyright 2002 The ZigBee Alliance, Inc.
IEEE 802.15.4 Device Types
• Three device types
– Network Coordinator
• Maintains overall network knowledge; most sophisticated of the three
types; most memory and computing power
– Full Function Device
• Carries full 802.15.4 functionality and all features specified by the
standard
• Additional memory, computing power make it ideal for a network router
function
• Could also be used in network edge devices (where the network
touches the real world)
– Reduced Function Device
• Carriers limited (as specified by the standard) functionality to control
cost and complexity
• General usage will be in network edge devices
• All of these devices can be no more complicated than the
transceiver, a simple 8-bit MCU and a pair of AAA batteries!
Copyright 2002 The ZigBee Alliance, Inc.
Data Frame format
•
•
•
•
•
One of two most basic and important structures in 15.4
Provides up to 104 byte data payload capacity
Data sequence numbering to ensure that all packets are tracked
Robust frame structure improves reception in difficult conditions
Frame Check Sequence (FCS) ensures that packets received are
without error
Copyright 2002 The ZigBee Alliance, Inc.
Acknowledgement Frame
Format
• The other most important structure for 15.4
• Provides active feedback from receiver to sender that
packet was received without error
• Short packet that takes advantage of standardsspecified “quiet time” immediately after data packet
transmission
Copyright 2002 The ZigBee Alliance, Inc.
MAC Command Frame format
• Mechanism for remote control/configuration of
client nodes
• Allows a centralized network manager to
configure individual clients no matter how
large the network
Copyright 2002 The ZigBee Alliance, Inc.
Beacon Frame format
•
•
•
Beacons add a new level of functionality to a network
Client devices can wake up only when a beacon is to be broadcast,
listen for their address, and if not heard, return to sleep
Beacons are important for mesh and cluster tree networks to keep all
of the nodes synchronized without requiring nodes to consume
precious battery energy listening for long periods of time
Copyright 2002 The ZigBee Alliance, Inc.
Copyright 2002 The ZigBee Alliance, Inc.
MAC Options
• Two channel access mechanisms
– Non-beacon network
• Standard ALOHA CSMA-CA communications
• Positive acknowledgement for successfully received packets
– Beacon-enabled network
• Superframe structure
– For dedicated bandwidth and low latency
– Set up by network coordinator to transmit beacons at
predetermined intervals
» 15ms to 252sec (15.38ms*2n where 0  n  14)
» 16 equal-width time slots between beacons
» Channel access in each time slot is contention free
– Three security levels specified
• None
• Access control lists
• Symmetric key employing AES-128
Copyright 2002 The ZigBee Alliance, Inc.
Non-Beacon vs Beacon
Modes
• Non-Beacon Mode
– A simple, traditional multiple access system used in simple peer
and near-peer networks
– Think of it like a two-way radio network, where each client is
autonomous and can initiate a conversation at will, but could
interfere with others unintentionally
– However, the recipient may not hear the call or the channel might
already be in use
• Beacon Mode
– A very powerful mechanism for controlling power consumption in
extended networks like cluster tree or mesh
– Allows all clients in a local piece of the network the ability to know
when to communicate with each other
– Here, the two-way radio network has a central dispatcher who
manages the channel and arranges the calls
• As you’ll see, the primary value will be in system power
consumption
Copyright 2002 The ZigBee Alliance, Inc.
Copyright 2002 The ZigBee Alliance, Inc.
Example of Non-Beacon
Network
• Commercial or home security
– Client units (intrusion sensors, motion detectors, glass break
detectors, standing water sensors, loud sound detectors, etc)
• Sleep 99.999% of the time
• Wake up on a regular yet random basis to announce their continued
presence in the network (“12 o’clock and all’s well”)
• When an event occurs, the sensor wakes up instantly and transmits the
alert (“Somebody’s on the front porch”)
– The ZigBee Coordinator, mains powered, has its receiver on all the
time and so can wait to hear from each of these stations
• Since ZigBee Coordinator has “infinite” source of power it can allow
clients to sleep for unlimited periods of time to allow them to save
power
Copyright 2002 The ZigBee Alliance, Inc.
Example of Beacon Network
• Now make the ZigBee Coordinator battery-operated also
– All units in system are now battery-operated
– Client registration to the network
• Client unit when first powered up listens for the ZigBee Coordinator’s
network beacon (interval between 0.015 and 252 seconds)
• Register with the coordinator and look for any messages directed to it
• Return to sleep, awaking on a schedule specified by the ZigBee
Coordinator
• Once client communications are completed, ZigBee coordinator also
returns to sleep
– This timing requirement potentially impacts the cost of the timing
circuit in each end device
– Longer intervals of sleep mean that the timer must be more accurate or
– Turn on earlier to make sure that the beacon is heard, increasing receiver
power consumption, or
– Improve the quality of the timing oscillator circuit (increase cost) or
– Control the maximum period of time between beacons to not exceed 252
seconds, keeping oscillator circuit costs low
– Application examples: environmental sensors in the forest
Copyright 2002 The ZigBee Alliance, Inc.
Growing the Network
•
•
•
In a beacon-environment, growing the network means keeping the
overall network synchronized
According to pre-existing network rules, the joining network’s PAN
Coordinator is probably demoted to Router, and passes along
information about its network (as required) to the PAN coordinator
Beacon information passed from ZigBee Coordinator to now-Router,
router knows now when to awake to hear network beacon
Joining Network
Existing
network’s
Coordinator
Demoted to
router
New link established
Copyright 2002 The ZigBee Alliance, Inc.
Frequencies and Data Rates
• The two PHY bands (UHF/Microwave) have
different physical, protocol-based and
geopolitical characteristics
– Worldwide coverage available at 2.4GHz at 250kbps
– 900MHz for Americas and some of the Pacific
– 868MHz for European-specific markets
Copyright 2002 The ZigBee Alliance, Inc.
ISM Band Interference and
Coexistence
• Potential for interference exists in every ISM band, not
just 2.4GHz
• IEEE 802.11 and 802.15.2 committees are addressing
coexistence issues
• ZigBee/802.15.4 Protocol is very robust
– Clear channel checking before transmission
– Backoff and retry if no acknowledgement received
– Duty cycle of a ZigBee-compliant device is usually
extremely low
– It’s the “cockroach that survives the nuclear war”
• Waits for an opening in otherwise busy RF spectrum
• Waits for acknowledgements to verify packet reception at
other end
Copyright 2002 The ZigBee Alliance, Inc.
PHY Performance
802.15.4 has excellent
performance in low
SNR environments
Copyright 2002 The ZigBee Alliance, Inc.
IEEE1451.5 Sensor Group
Wireless Criteria
• A survey was conducted mid-2002 on the characteristics
of a wireless sensor network most important to its users
• In order of importance, these characteristics are
1.
2.
3.
4.
5.
6.
7.
8.
Data Reliability
Battery Life
Cost
Transmission Range
Data Rate
Data Latency
Physical Size
Data Security
• How would you modify these requirements, if at all?
Copyright 2002 The ZigBee Alliance, Inc.
802.15.4 and the
• IEEE 802.15.4
– Composed of many of the individuals and companies that
make up the ZigBee Alliance
– Developed the basic PHY and MAC standard with the
requirement that 15.4 be simple and manageable and that
high-level functionality (networking, security key
management, applications) be considered
• The ZigBee Alliance is
– A consortium of end users and solution providers, primarily
responsible for the development of the 802.15.4 standard
– Developing applications and network capability utilizing the
802.15.4 packet delivery mechanism
– Addresses application and interoperability needs of a
substantial part of the market
Copyright 2002 The ZigBee Alliance, Inc.
Mission Statement
ZigBee Alliance members are defining
global standards for reliable, costeffective, low power wireless
applications. The ZigBee Alliance is a
rapidly growing, non-profit industry
consortium of leading semiconductor
manufacturers, technology providers,
OEMs and end users worldwide.
Copyright 2002 The ZigBee Alliance, Inc.
What is the ZigBee Alliance?
• Organization defining global standards for
reliable, cost-effective, low power wireless
applications
• A rapidly growing, worldwide, non-profit
industry consortium of
–
–
–
–
Leading semiconductor manufacturers
Technology providers
OEMs
End-users
• Sensors are one of the reasons for ZigBee!
Copyright 2002 The ZigBee Alliance, Inc.
What is ZigBee technology?
• Cost-effective, standards-based wireless
networking solution
• Developed for and targets applications that
need
–
–
–
–
Low to moderate data rates and low duty cycles
Low average power consumption / long battery life
Security and reliability
Flexible and dynamic network topologies
• Star, cluster tree and mesh networks
– Interoperable application frameworks controlled by
an industry alliance to ensure
interoperability/compatibility
Copyright 2002 The ZigBee Alliance, Inc.
The ZigBee Alliance Solution
• Targeted at
–
–
–
–
–
–
Industrial and Commercial control/monitoring systems
Wireless sensor systems
Home and Building automation and controls
Medical monitoring
Consumer electronics
PC peripherals
• Industry standard through application profiles
• Primary drivers
–
–
–
–
–
Simplicity
Long battery life
Networking capabilities
Reliability
Low cost
• Alliance member companies provide interoperability and
certification testing
Copyright 2002 The ZigBee Alliance, Inc.
Why do we need ZigBee
technology?
• ONLY standards-based technology that
– Addresses the unique needs of most remote
monitoring and control and sensory network
applications
– Enables the broad-based deployment of
wireless networks with low cost, low power
solutions
– Provides the ability to run for years on
inexpensive primary batteries for a typical
monitoring application
Copyright 2002 The ZigBee Alliance, Inc.
What kind of battery life can
a user expect?
• ZigBee protocol was designed from the ground
up to support
– very long life battery applications
• Users can expect
– Near-shelf life in a typical monitoring application
• Battery life is ultimately a function of
– battery capacity and application usage
• Many industrial applications are in harsh
thermal environments
– Batteries may include alkalines or Li-primaries
– Other forms of power generation might include solar,
mechanical, piezoelectric
Copyright 2002 The ZigBee Alliance, Inc.
The ZigBee Alliance Solution
• Targeted at home and building automation and
controls, consumer electronics, toys etc.
• Industry standard (IEEE 802.15.4 radios)
• Primary drivers are simplicity, long battery life,
networking capabilities, reliability, and cost
Copyright 2002 The ZigBee Alliance, Inc.
The Wireless Market
GRAPHICS INTERNET
HI-FI
AUDIO
STREAMING
VIDEO
>
LONG
TEXT
MULTI-CHANNEL
VIDEO
802.11b
< RANGE
SHORT
DIGITAL
VIDEO
LAN
802.11a/HL2 & 802.11g
Bluetooth 2
ZigBee
PAN
Bluetooth1
LOW
< DATA RATE
>
HIGH
Copyright 2002 The ZigBee Alliance, Inc.
Applications
security
HVAC
AMR
lighting control
access control
BUILDING
AUTOMATION
patient
monitoring
fitness
monitoring
CONSUMER
ELECTRONICS
TV
VCR
DVD/CD
remote
ZigBee
PERSONAL
HEALTH CARE
asset mgt
process control
environmental
energy mgt
Wireless Control that
Simply Works
INDUSTRIAL
CONTROL
RESIDENTIAL/
LIGHT
COMMERCIAL
CONTROL
Copyright 2002 The ZigBee Alliance, Inc.
PC &
PERIPHERALS
mouse
keyboard
joystick
security
HVAC
lighting control
access control
lawn & garden irrigation
Promoter Companies
Copyright 2002 The ZigBee Alliance, Inc.
ZigBee Alliance
Promoters
Participants
And more each month…
Copyright 2002 The ZigBee Alliance, Inc.
Development of the Standard
APPLICATION
ZIGBEE STACK
SILICON
• ZigBee Alliance
– 50+ companies
– Defining upper layers of
Customer
protocol stack: from network to
application, including
application profiles
ZigBee
Alliance
• IEEE 802.15.4 Working Group
IEEE
802.15.4
– Defining lower layers : MAC
and PHY
Copyright 2002 The ZigBee Alliance, Inc.
ZigBee Topology Models
Mesh
Star
ZigBee coordinator
ZigBee Routers
ZigBee End Devices
Cluster Tree
Copyright 2002 The ZigBee Alliance, Inc.
ZigBee and Bluetooth
Competitive or
Complementary?
Copyright 2002 The ZigBee Alliance, Inc.
ZigBee and Bluetooth
Optimized for different applications
• Bluetooth
• ZigBee
– Larger packets over small
– Smaller packets over
network
large network
– Ad-hoc networks
– Mostly Static
networks with many,
– File transfer; streaming
infrequently used
– Screen graphics, pictures,
devices
hands-free audio, Mobile
– Home automation,
phones, headsets, PDAs,
toys remote controls
etc.
– Energy saver!!!
Copyright 2002 The ZigBee Alliance, Inc.
ZigBee and Bluetooth
Address Different Needs
• Bluetooth is a
cable replacement
for items like
Phones, Laptop
Computers,
Headsets
• Bluetooth expects
regular charging
– Target is to use
<10% of host
power
Copyright 2002 The ZigBee Alliance, Inc.
ZigBee and Bluetooth
Address Different Needs
• ZigBee is better for
devices where the
battery is ‘rarely’
replaced
– Targets are :
• Tiny fraction of host power
• New opportunities where
wireless not yet used
Copyright 2002 The ZigBee Alliance, Inc.
ZigBee and Bluetooth
Air interface
ZigBee
Bluetooth
•
•
•
•
• FHSS
• 1 M Symbol / second
• Peak Information Rate
~720 Kbit / second
DSSS- 11 chips/ symbol
62.5 K symbols/s
4 Bits/ symbol
Peak Information Rate
~128 Kbit/second
Copyright 2002 The ZigBee Alliance, Inc.
ZigBee and Bluetooth
Voice
Application Interface
Network Layer
Service
Discovery
Protocol
(Serial Port)
L2CAP
Host Control Interface
Link Manager
MAC Layer
MAC Layer
Link Controller
Baseband
RF
PHY Layer
ZigBee
Stack
Fax
Telephony OBEX
Control
RFCOMM
Protocol
Data Link Layer
Silicon
Dial-up
Networking
Application
vMessage
Intercom
Headset
Cordless
Group Call
vCard
vCal
vNote
User Interface
Application
Silicon
Zigbee
Bluetooth
Stack
Applications
Bluetooth
Protocol Stack Comparison
Copyright 2002 The ZigBee Alliance, Inc.
ZigBee and Bluetooth
Timing Considerations
ZigBee:
• Network join time = 30ms typically
• Sleeping slave changing to active = 15ms typically
• Active slave channel access time = 15ms typically
Bluetooth:
• Network join time = >3s
• Sleeping slave changing to active = 3s typically
• Active slave channel access time = 2ms typically
ZigBee protocol is optimized for
timing critical applications
Copyright 2002 The ZigBee Alliance, Inc.
ZigBee and Bluetooth
AIR INTERFACE
PROTOCOL STACK
BATTERY
DEVICES/NETWORK
LINK RATE
RANGE
Bluetooth
ZigBee
FHSS
DSSS
250 kb
28 kb
rechargeable non-rechargeable
8
255
1 Mbps
250 kbps
~10 meters (w/o pa) ~30 meters
Comparison Overview
Copyright 2002 The ZigBee Alliance, Inc.
An Application Example
Battery Life & Latency in a Light Switch
• Wireless Light switch –
– Easy for Builders to Install
• A Bluetooth Implementation
would either :
– keep a counter running so that it
could predict which hop frequency
the light would have reached or
– use the inquiry procedure to find
the light each time the switch was
operated.
Copyright 2002 The ZigBee Alliance, Inc.
Light switch using Bluetooth
• Option 1: use counter to predict hop frequency
reached by light
– The two devices must stay within 60 us (~1/10 of a
hop)
– With 30ppm crystals, devices need to communicate
once a second to track each other's clocks.
– Assume this could be improved by a factor of 100 then
devices would need to communicate once every 100
seconds to maintain synchronization.
– => 900 communications / day with no information
transfer + perhaps 4 communications on demand
– 99.5% Battery Power wasted
Copyright 2002 The ZigBee Alliance, Inc.
Light switch using Bluetooth
• Option 2: Inquiry procedure to locate light each
time switch is operated
– Bluetooth 1.1 = up to 10 seconds typical
– Bluetooth 1.2 = several seconds even if optimized
– Unacceptable latency
Copyright 2002 The ZigBee Alliance, Inc.
Light switch using ZigBee
• With DSSS interface, only need to
perform CSMA before transmitting
– Only 200 µs of latency
– Highly efficient use of battery power
ZigBee offers longer battery
life and lower latency than a
Bluetooth equivalent.
Copyright 2002 The ZigBee Alliance, Inc.
Conclusion
• Bluetooth and 802.15.4 transceiver physical
characteristics are very similar
• Protocols are substantially different and designed for
different purposes
• 802.15.4 designed for low to very low duty cycle static
and dynamic environments with many active nodes
• Bluetooth designed for high QoS, variety of duty cycles,
moderate data rates in fairly static simple networks with
limited active nodes
Copyright 2002 The ZigBee Alliance, Inc.
ZigBee and Bluetooth
Conclusion
• ZigBee targets applications not addressable by
Bluetooth or any other wireless standard
• ZigBee and Bluetooth complement for a
broader solution
Copyright 2002 The ZigBee Alliance, Inc.
Reliability and Robustness
throughout the stacks of IEEE
802.15.4 and ZigBee
Copyright 2002 The ZigBee Alliance, Inc.
Reliability
• Consistently perform a given task
to the desired result despite all
changes of environmental behavior
• Without fail
• A necessary ingredient of trust
• “When the sensor measures its
environment; the controller always
knows that same value”
Copyright 2002 The ZigBee Alliance, Inc.
Reliability
• The wireless medium is not a
protected environment like the
wired medium, but rather, it is
fraught with degradations,
disruptions, and pitfalls such as
dispersion, multipath, interference,
frequency dependent fading,
sleeping nodes, hidden nodes, and
security issues.
Copyright 2002 The ZigBee Alliance, Inc.
Reliability
• Each of these degradations and
disruptions can be mitigated by
various mechanisms within the ISO
layers; but not all mechanisms are
compatible with all other mechanisms
or may negatively impact critical
performance attributes
• The system must be optimized for
the best performance in a realistic
environment
Copyright 2002 The ZigBee Alliance, Inc.
Reliability
• In addition to the previous
disruptions there is the case of
sending messages to devices that
are not receiving, e.g. they’re in
the “sleep” mode. When this
happens the message needs to be
buffered by another device that is
able to send the message when
the sleeping device wakes up.
Copyright 2002 The ZigBee Alliance, Inc.
Reliability
Interferer
Router
Multipath
XX
Sleeping Node
Network
Coordinator
Hidden Node
Copyright 2002 The ZigBee Alliance, Inc.
Reliability
• IEEE 802.15.4 has built upon the
successes of previous IEEE 802
standards by selecting those
mechanisms proven to ensure
good reliability without seriously
degrading system and device
performance.
Copyright 2002 The ZigBee Alliance, Inc.
Reliability
ISO Layers:
• PHY: Direct Sequence with
Frequency Agility (DS/FA)
• MAC: ARQ, Coordinator buffering
• Network: Mesh Network (redundant
routing)
• Application Support Layer: Security
Copyright 2002 The ZigBee Alliance, Inc.
Reliability
PHY Layers:
• Direct sequence: allows the radio to
reject multipath and interference by use
of a special “chip” sequence. The more
chips per symbol, the higher its ability
to reject multipath and interference.
• Frequency Agility: ability to change
frequencies to avoid interference from a
known interferer or other signal source.
Copyright 2002 The ZigBee Alliance, Inc.
IEEE 802 Direct Sequence
IEEE
802.
11
Chips/ 11
Symbol
11b
15.4
(900)
15.4
(2.4)
11
15
32
• As can be seen from above,
IEEE802.15.4/ZigBee has more
processing gain (chips/symbol)
than its predecessors
Copyright 2002 The ZigBee Alliance, Inc.
Direct Sequence and
Frequency Agility
Interferer
Over the Air
2.4 GHz
PHY
Desired Signal
After DS correlation
Channels 11-26
2.4 GHz
5 MHz
2.4835 GHz
Copyright 2002 The ZigBee Alliance, Inc.
Reliability
MAC:
• ARQ (acknowledgement request) is
where a successful transmission is
verified by replying with an
acknowledge (ACK). If the ACK is not
received the transmission is sent again
• Coordinator buffering is where the
network coordinator buffers messages
for sleeping nodes until they wake again
Copyright 2002 The ZigBee Alliance, Inc.
Reliability
Network:
• Mesh Networking: allows various
paths of routing data to the
destination device. In this way if a
device in the primary route is not
able to pass the data, a different
valid route is formed, transparent
to the user.
Copyright 2002 The ZigBee Alliance, Inc.
Reliability: Mesh Networking
ZigBee Coordinator (FFD)
ZigBee Router (FFD)
ZigBee End Device (RFD or FFD)
Mesh Link
Star Link
Copyright 2002 The ZigBee Alliance, Inc.
Reliability
Application Support Sub-layer(APS):
• Security: supports reliability by
keeping other devices from
corrupting communications.
• The APS configures the security
emplaced in the MAC layer and
also adds some of its own.
Copyright 2002 The ZigBee Alliance, Inc.
Robustness
• Let’s define robustness as the
ability to tolerate significant
degrading phenomena in the
physical medium
• Multipath and interference are
probably the most significant
degradations to the channel model.
Copyright 2002 The ZigBee Alliance, Inc.
Robustness
• Frequency hopping is a method that
allows the radio to periodically
change channels to over time
minimize the effect of a “bad”
channel. While this technique is
very effective in some circumstances
it creates other problems such as
latency, network uncertainty for
sleeping nodes, loss of the product
bandwidth x time, etc.
Copyright 2002 The ZigBee Alliance, Inc.
Robustness
• Direct Sequence with Frequency
Agility (DS/FA) combines the best
features of DS and FH without
most of the problems caused by
frequency hopping because
frequency changes aren’t
necessary most of the time, rather
they’re appropriate only on an
exception basis.
Copyright 2002 The ZigBee Alliance, Inc.
Robustness
The 802.11 Working Group couldn’t
agree upon which of the following
PHYs was the best: FH, IR, or DS.
So all three were standardized and
left to the market to decide.
Of the three PHYs; DS was the clear
market winner. DS provided
sufficient robustness with higher
overall performance.
Copyright 2002 The ZigBee Alliance, Inc.
Robustness
• Excess robustness does not
achieve higher performance,
rather it typically costs
performance
Copyright 2002 The ZigBee Alliance, Inc.
Conclusion
• IEEE 802.15.4/ZigBee have
addressed reliability throughout
the ISO stack with proven
mechanisms to minimize the
uncertainty of the wireless medium
Copyright 2002 The ZigBee Alliance, Inc.