The MAC Layer ZigBee and 802.15.4

2014

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

• Short range and low data rate

The Wireless MarketS

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PAN

LAN

TEXT GRAPHICS INTERNET HI-FI AUDIO

STREAMINGVIDEO

DIGITALVIDEO

MULTI-CHANNELVIDEO

Bluetooth1

Bluetooth 2

ZigBee

802.11b

802.11a/HL2 & 802.11g

Applications

ZigBeeWireless Control that

Simply Works

RESIDENTIAL/LIGHT

COMMERCIAL CONTROL

CONSUMER ELECTRONICS

TVVCRDVD/CDremote

securityHVAClighting controlaccess controllawn & garden irrigation

PC & PERIPHERALS

INDUSTRIALCONTROL

asset mgtprocess control

environmentalenergy mgt

PERSONAL HEALTH CARE

BUILDING AUTOMATION

securityHVAC

AMRlighting control

access control

mousekeyboardjoystick

patient monitoring

fitness monitoring

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

IEEE 802.15.4 Basics• 802.15.4 is a simple packet data protocol:

– CSMA/CA - Carrier Sense Multiple Access with collision avoidance

– Optional time slotting and beacon structure– 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

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• Channel scan for beacon is included, but it is left to the

network layer to implement dynamic channel selection

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

• Low complexity: 26 service primitives

versus 131 service primitives for 802.15.1 (Bluetooth)

IEEE 802.15.4 Device Types• Three device types

– Network Coordinator• Maintains overall network knowledge; most

memory and computing power– Full Function Device

• Carries full 802.15.4 functionality and all features specified by the standard; ideal for a network router function

– Reduced Function Device• Carriers limited functionality; used for 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!

ZigBee Topology Models

ZigBee coordinatorZigBee RoutersZigBee End Devices

Star

Star topology The communication is established between devices and a single central controller, called the PAN coordinator.

ZigBee Topology Models

ZigBee coordinator

ZigBee Routers

ZigBee End Devices

Mesh

Cluster Tree

Mesh topology There is also one PAN coordinator. In contrast to star topology, any device can communicate with any other device as long as they are in range of one another.

Cluster-tree network is a special case of a Mesh network

IEEE 802.15.4 PHY

• Features– Activation/Deactivation of radio Transceiver– Energy Detection (ED)– Link Quality Indication (LQI)– Channel Selection– Clear Channel Assessment (CCA)– Transmission/Reception of packets over physical

medium

Channels in Zigbee and WiFi

Network Formation:To form a new network, the first node -- ZigBee Coordinator, scan through the list of available channels so that the network will operate on the channel with least interference.

WiFi Co-Channel Interference is a major threat to ZigBee

Measured Results

Measured RSSI on the primary outdoors 802.15.4 network as a function of transmissions on the competing WiFi network

Signal Strength Distribution

• Measurement at ZigBee nodes when the WiFi data rate varies

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RSSI (dBm)

clear1Mb5Mb

10Mb15Mb22Mb

Bit-error distribution for ZigBee

• Bit-error distribution for 15.4 packets that failed the CRC check when the interfering 802.11 transmitter is in the symmetric region.

• Bit-error distribution for 15.4 packets that failed the CRC check when the interfering 802.11g transmitter is in the asymmetric region

Symmetric Region

• Packet corrupted at front

• Three techniques examined– Decrease correlation threshold

• Reduce the constrain

– Increase preamble length• Higher change to have valid preamble

– Multi-header

Asymmetric Region

• Forward error correction (FEC)– Apply error-correction code (ECC)

• Two ECCs– Hamming code

• Adding extra parity bits

• Can detect up to two bit errors and correct one bit error

– Reed-Solomon Code• Block-based error-correction code

• Divided message into x blocks of data and y blocks of parity

Frame cluster per 5s

Frame cluster per 1s

Frame cluster per 0.2s

Self-similarity of 802.11 frame cluster arrival process

21

White Space in Real-life WiFi Traffic

• Large amount of channel idle time

• WiFi frames are clusteredwhite space: cluster gaps that can be utilized by ZigBee

Basic Idea of WISE

• Sender splits ZigBee frame into sub-frames• Fill the white space with sub-frames• Receiver assembles sub-frames into frame

ZigBee

Time

WiFi frame cluster ZigBee sub-frames

ZigBee frame pending

sampling window

Channel Diversity

PRR over a duration of one hour for different channels on an IQ link.

at any given point of time, it is highly likely that at least one channel is operating in the good phase

Engineer” temporal white-spaces between WiFi transmissions

Sending WiFi compliant signals to refrain WiFi stations from transmitting

MAC Options• Two channel access mechanisms

– Non-beacon network• Standard CSMA-CA communications + ACK• Non-beacon mode is useful in situations where only light t

raffic is expected– Beacon-enabled network

• Superframe structure– Set up by network coordinator to transmit beacons at

predetermined intervals– 15ms to 252sec, slotted CSMA-CA

– In general, the ZigBee protocols minimize the time the radio is on, so as to reduce power use.

• In beaconing networks, nodes only need to be active while a beacon is being transmitted.

• In non-beacon-enabled networks, power consumption is decidedly asymmetrical: some devices are always active, while others spend most of their time sleeping.

Example of Non-Beacon Net• 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 co

ntinued presence in the network (“12 o’clock and all’s well”)• When an event occurs, the sensor wakes up instantly and tra

nsmits 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

Example of Beacon Network• Now make the ZigBee Coordinator battery-operated al

so– 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

• Application examples: environmental sensors in the forest

数据到主协调器的通信顺序

• 从设备监听网络的信标• 从设备与超帧结构进行同步• 从设备使用有时隙的 CSMA-CA 向主协调器发送

数据帧• 当主协调器接收到该数据帧后，将返回确认帧

Beacon-enabled network

数据到主协调器的通信顺序

Non-Beacon-enabled network

主协调器发送数据

• 主协调器发送网络信标中表明存在有要传输的数据信息。

• 从设备从网络信标中发现存在有主协调器要发送给它的数据信息时，采用有时隙的 CSMA-CA 机制，发送一个数据请求命令。

• 主协调器收到数据请求命令后，返回一个确认帧，并采用有时隙的 CSMA-CA 机制，发送要传输的数据信息帧

• 从设备收到该数据帧后，将返回一个确认帧，表示该数据传输事务已处理完成

• 主协调器收到确认帧后，将数据信息从主协调器的信标未处理信息列表中删除

Beacon-enabled network

• 非信标网络中传输数据给从设备时，主协调器存储着要传输的数据，由从设备先发送请求数据传输命令后，才能进行数据传输

主协调器发送数据 Non-Beacon-enabled network

Beacon-enabled network

• 时间划分成等间隔的周期，该周期由协调点发送的信标帧 (Beacon) 界定，通常由激活期和非激活期两部分构成。

• 所有节点在激活期进行业务的交互，在非激活期则转为低功耗模式。

• 如果节点有对实时业务的需求，可向网络协调点申请预留时隙 GTS( 保护时隙 ) ，形成免竞争期

激活期

第一个时隙传输信标

竞争期 免竞争期

Beacon-enabled network

• 当普通节点有数据发给协调点时，首先侦听网络信标，接收到信标帧后完成与协调点的同步

• 而后在 CAP 中采用时隙 CSMA/CA 发送数据帧• 协调点成功接收到该帧后回复相应的应答。

激活期 (或竞争期 )

第一个时隙传输信标

Beacon-enabled network

• 如果协调点有数据要发给普通节点，在信标帧中指出有将要发给某节点的的地址（ pending address field ）中。

• 普通节点周期性侦听网络信标，收到信标帧后，若发现信标帧的地址列表中有自身地址，则将采用时隙 CSMA/CA 机制向协调点发送数据请求命令帧，协调点给予应答。

激活期

Beacon Frame format

• Client devices wake up only when the beacon is broadcast, listen for their address, and if not heard, return to sleep

• Beacons keep all of the nodes synchronized • nodes need not listen all the time; save battery energy

ZigBee and Bluetooth

• ZigBee– Smaller packets over la

rge network– Mostly Static networks

with many, infrequently used devices

– Home automation, toys remote controls

– Energy saver!!!

• Bluetooth– Larger packets over small n

etwork– Ad-hoc networks– File transfer; streaming – Cable replacement for items

like screen graphics, pictures, hands-free audio, Mobile phones, headsets, PDAs, etc.

Optimized for different applications

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 / s• Peak Information Rate ~720 Kbit / second

ZigBee and Bluetooth

Silicon

PHY Layer

MAC LayerMAC Layer

Data Link Layer

Network Layer

ZigBeeStack

Application

Application Interface

Application

Protocol Stack Comparison

Silicon

RFBaseband

Link Controller

Vo

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Link Manager

Host Control Interface

L2CAP

TelephonyControlProtocol

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RFCOMM(Serial Port)

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BluetoothStack

Applications

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Fax ServiceDiscoveryProtocol

User Interface

Zigbee Bluetooth

ZigBee and Bluetooth

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

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

Conclusion• ZigBee targets applications not addressable by Blu

etooth or any other wireless standard

• ZigBee and Bluetooth complement for a broader solution

ZigBee and Bluetooth