ZigBee 802.15.4 and the ZigBee Alliance Motorola 802.15.4/ZigBee™ Platform
ZigBee and 802.15.4. Copyright 2002 The ZigBee Alliance, Inc.
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Transcript of ZigBee and 802.15.4. Copyright 2002 The ZigBee Alliance, Inc.
ZigBee and 802.15.4
Copyright 2002 The ZigBee Alliance, Inc.
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
IEEE 802.15.4 MAC
IEEE 802.15.4 LLC IEEE 802.2LLC, Type I
IEEE 802.15.42400 MHz PHY
IEEE 802.15.4868/915 MHz PHY
Data Link Controller (DLC)
Networking App Layer (NWK)
ZigBee Application Framework
• 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.
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)
• 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 standards-
specified “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
Demoted to router
New link established
Existing network’s
Coordinator
Joining Network
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 are1. Data Reliability2. Battery Life3. Cost4. Transmission Range5. Data Rate6. Data Latency7. Physical Size8. 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, cost-
effective, 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 MarketS
HO
RT
<
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AN
GE
>
L
ON
G
LOW < DATA RATE > HIGH
PAN
LAN
TEXT GRAPHICS INTERNET HI-FI AUDIO
STREAMINGVIDEO
DIGITALVIDEO
MULTI-CHANNELVIDEO
Bluetooth1
Bluetooth 2
ZigBee
802.11b
802.11a/HL2 & 802.11g
Copyright 2002 The ZigBee Alliance, Inc.
Applications
ZigBeeWireless Control that
Simply Works
RESIDENTIAL/LIGHT
COMMERCIAL CONTROL
CONSUMER ELECTRONICS
TVVCRDVD/CDremote
securityHVAClighting controlaccess controllawn & garden irrigation
PC & PERIPHERALS
INDUSTRIALCONTROL
asset mgtprocess controlenvironmental
energy mgt
PERSONAL HEALTH CARE
BUILDING AUTOMATION
securityHVAC
AMRlighting control
access control
mousekeyboardjoystick
patient monitoring
fitness monitoring
Copyright 2002 The ZigBee Alliance, Inc.
Promoter Companies
Copyright 2002 The ZigBee Alliance, Inc.
ZigBee Alliance
Participants
Promoters
And more each month…
Copyright 2002 The ZigBee Alliance, Inc.
Development of the Standard
• ZigBee Alliance
– 50+ companies
– Defining upper layers of protocol stack: from network to application, including application profiles
• IEEE 802.15.4 Working Group
– Defining lower layers : MAC and PHY
SILICON
ZIGBEE STACK
APPLICATION Customer
IEEE802.15.4
ZigBee Alliance
Copyright 2002 The ZigBee Alliance, Inc.
ZigBee Topology Models
ZigBee coordinator
ZigBee Routers
ZigBee End Devices
Star
Mesh
Cluster Tree
Copyright 2002 The ZigBee Alliance, Inc.
ZigBee and Bluetooth
Competitive or Complementary?
Copyright 2002 The ZigBee Alliance, Inc.
ZigBee and Bluetooth
• ZigBee– Smaller packets over
large network– Mostly Static
networks with many, infrequently used devices
– Home automation, toys remote controls
– Energy saver!!!
• Bluetooth– Larger packets over small
network– Ad-hoc networks– File transfer; streaming – Screen graphics, pictures,
hands-free audio, Mobile phones, headsets, PDAs, etc.
Optimized for different applications
Copyright 2002 The ZigBee Alliance, Inc.
• Bluetooth is a cable replacement for items like Phones, Laptop Computers, Headsets
• Bluetooth expects regular charging– Target is to use
<10% of host power
ZigBee and Bluetooth
Address Different Needs
Copyright 2002 The ZigBee Alliance, Inc.
• 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
ZigBee and Bluetooth
Address Different Needs
Copyright 2002 The ZigBee Alliance, Inc.
Air interfaceZigBee• DSSS- 11 chips/ symbol• 62.5 K symbols/s • 4 Bits/ symbol• Peak Information Rate
~128 Kbit/second
Bluetooth• FHSS• 1 M Symbol / second• Peak Information Rate
~720 Kbit / second
ZigBee and Bluetooth
Copyright 2002 The ZigBee Alliance, Inc.
Silicon
PHY Layer
MAC LayerMAC Layer
Data Link Layer
Network Layer
ZigBeeStack
Application
Application Interface
Application
Protocol Stack Comparison
Silicon
RFBaseband
Link Controller
Vo
ice
Link Manager
Host Control Interface
L2CAP
TelephonyControlProtocol
Inte
rco
m
Hea
dse
t
Co
rdle
ss
Gro
up
Cal
l
RFCOMM(Serial Port)
OBEX
BluetoothStack
Applications
vCar
d
vCal
vNo
te
vMes
sag
e
Dia
l-u
pN
etw
ork
ing
Fax ServiceDiscoveryProtocol
User Interface
Zigbee Bluetooth
ZigBee and Bluetooth
Copyright 2002 The ZigBee Alliance, Inc.
Bluetooth:• Network join time = >3s• Sleeping slave changing to active = 3s typically• Active slave channel access time = 2ms typically
ZigBee:• Network join time = 30ms typically • Sleeping slave changing to active = 15ms typically• Active slave channel access time = 15ms typically
Timing Considerations
ZigBee protocol is optimized for timing critical applications
ZigBee and Bluetooth
Copyright 2002 The ZigBee Alliance, Inc.
ZigBee and BluetoothBluetooth ZigBee
AIR INTERFACE FHSS DSSS
PROTOCOL STACK 250 kb 28 kb
BATTERY rechargeable non-rechargeable
DEVICES/NETWORK 8 255
LINK RATE 1 Mbps 250 kbps
RANGE ~10 meters (w/o pa) ~30 meters
Comparison Overview
Copyright 2002 The ZigBee Alliance, Inc.
An Application Example
• 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.
Battery Life & Latency in a Light Switch
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.
Conclusion• ZigBee targets applications not addressable by Bluetooth or any other
wireless standard
• ZigBee and Bluetooth complement for a broader solution
ZigBee and Bluetooth
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
Router
X X
Hidden Node
Interferer
Multipath
Sleeping Node NetworkCoordinator
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
• As can be seen from above, IEEE802.15.4/ZigBee has more processing gain (chips/symbol) than its predecessors
IEEE802.
11 11b 15.4(900)
15.4(2.4)
Chips/Symbol
11 11 15 32
Copyright 2002 The ZigBee Alliance, Inc.
Direct Sequence and Frequency Agility
2.4 GHz
Channels 11-26
2.4835 GHz
5 MHz
2.4 GHz PHY
Over the Air After DS correlation
Interferer Desired Signal
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 End Device (RFD or FFD)
ZigBee Router (FFD)
ZigBee Coordinator (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