Wireless Communication - Zigbee, Bluetooth

Amarjeet Singh

February 19, 2012

Logistics

2

Sample exam paper on the course website Derived from mid term exam of last year – will give you an idea

of what can be asked in the exam Will create projects for those who did not get it since all topics were

over Will create the presentation schedule for students by this weekend

Revision from last class - I

3

UART What are specific example interfaces? RS-232:

How do you make a minimum interface (with/without handshaking)?

How will you support RS-232 with UART in your microcontroller?

RS-422: How is it different from RS-232? How is it extended to support multi-device bus?

Infrared: How is it different from RS-232? What bit-encodings are used in infrared?

Revision from last class - II

4

USB: What does a USB packet contain? What are different types of token packets? What are different types of handshake packets? What are different types of data transfers supported? How is bandwidth distribution done in USB across different

types of transfers?

IEEE 802 Wireless Space

Data Rate (Mbps)

Ra

nge

ZigBee

802.15.4

15.4c 802.15.3

802.15.3c WPAN

WLAN

WMAN

WWAN

WiFi

802.11

0.01 0.1 1 10 100 1000

Bluetooth

802.15.1

IEEE 802.22

WiMax

IEEE 802.16

IEEE 802.20

IEEE 802.15.4

Approved by IEEE in 2003 with revision in 2006 for Wireless Sensor Networks

Specifies only 2 OSI layers - Physical (PHY) and Medium Access Control (MAC)

Only direct, single hop communication possible

Two types of devices are defined: RFD - Reduced Functionality Device

Contains limited features Can only communicate to FFD Requires little power, memory and processing

resources FFD - Full Functionality Device

Full set of features - capable to act as network coordinator

Communicate to both FFD and RFD Higher power, memory and processing resources

IEEE 802.15.4 Data Transfer Models

Star: Network Nodes (FFDs, RFDs) are connected to a

coordinator node (FFD)

Peer-to-peer: FFDs can communicate to all devices in

transmissions range RFDs can talk to FFDs they are associated with

IEEE 802.15.4 Physical Layer

Available in 3 frequency bands

2.4 GHz is most commonly used Available worldwide without license

Frequency (MHz)

Number of channels

Data rates (kbps)

2450 16 250

915 10 40, 250

868 1 20,100

Unlicensed Availability

Worldwide

America, Australia

Europe

Typical device (0 dB power) can transmit up to 200 meters outside and 30 meters inside

IEEE 802.15.4 MAC Layer

Key MAC layer responsibilities are: Data framing: Data to be sent is encapsulated in MAC frame

that is passed to RF transceiver Device addressing: Each device identified by 64-bit long MAC

address Channel Sense Management (CSMA-CA): Device scans

preconfigured channels and chooses one with least activity “listen before send” principle for managing access to single

physical channel among multiple devices

Device Association/Disassociation: Upon higher layer requests, MAC layer enters/leaves network

Zigbee Specification

Two lowest layers (PHY and MAC) are equal to those in IEEE 802.15.4

Higher layers in Zigbee stack are specified to allow efficient communication within entire network and on the application level

Routing mechanisms on network layer allow multi-hop data transmission, selection of best suitable path and rerouting

Application framework provides interface to enable simultaneous execution of multiple applications

Zigbee Node Types

Coordinator: Only mandatory node type in the network; Acts as root node and performs multiple network management activities

Only FFD in 802.15.4 terminology can act as network coordinator

Router: A 802.15.4 FFD node not acting as network coordinator;

Used to extend network coverage area beyond transmission range of single device; Increase network reliability by creating additional data routing paths

End device: Correspond to RFD in 802.15.4; Can directly

communicate with a single coordinator/router Node addressing: Each node that joins the zigbee network gets a 16

bit network address (How many bits was the MAC address?) Communication at network level is performed based on this

address; Direct communication between two neighboring devices is based on MAC address

Zigbee Network Topologies

Extends IEEE 802.15.4 transfer models by specifying tree and mesh topologies

Star Topology: Corresponds to 802.15.4 star topology Do you need a network layer in this case?

Tree Topology: Based on 802.15.4 peer-to-peer model Routers/Coordinators can have child nodes Direct communication possible in terms of parent-child Hierarchical routing without alternate paths

Star Topology Tree Topology

What will happen if a link fails in tree network?

Zigbee Network Topologies

Mesh Topology: Based on 802.15.4 peer-to-peer model Routers/Coordinators can have child nodes Direct communication possible between any FFD (coordinator/router)

in transmission range End device can only communicate with its parent Optimum and dynamic routing with alternate paths

Mesh Topology

ZigBee Mesh Networking

ZigBee Mesh Networking

ZigBee Mesh Networking

ZigBee Mesh Networking

ZigBee Mesh Networking

Zigbee Application Operation - I

Each application instance running on a node is a unique network entity where messages can originate/terminate

Termed as endpoint and has a unique address Each node can have up to 240 endpoints - A

endpoint is identified by network address of the node and its endpoint address on the node

Endpoint 0 reserved for Zigbee Device Object (ZDO)

This application has multiple roles - defines type of node (coordinator, router, end-point), initializes the node and participates in network creation

Zigbee Application Operation - II

Binding: Process of establishing a relationship between nodes that can communicate e.g. in Home Automation, which switches control which lights

Binding between two applications is specified by: Network address and endpoint of application where data is generated Network address and endpoint of receiving application Stored in a binding table (can be stored locally or on the coordinator

node) Binding types:

One-to-one One-to-many Many-to-one

Can you think of application scenarios in light control for different binding types?

Zigbee Operation - I

Network is initialized by the coordinator who pre-configures a number of network properties :

Network Depth - Maximum number of hops from coordinator to the farthest end-device

What is the network depth of star topology? Maximum number of child devices allowed per router Maximum number of child routers

Zigbee Operation - II

Forming a zigbee network by the coordinator Search for a suitable radio channel Start the network assigning a PAN ID to the network - can be pre-

determined or obtained dynamically Assigns itself the network id of 0x0000 Ready to respond to queries from other devices

Join process by other nodes: Search for network: Scan the available channels and find operating

networks (separated by their PAN IDs) Select parent: from (possibly) multiple routers and coordinator in

range Send join request Accept or reject join request (by the coordinator/routers)

Zigbee Operation - III

Message propagation: Message contains two destination addresses - Address of final

destination, Address of next hop node Are any of these obvious for any network topology?

Route discovery mechanism

Route discovery broadcast is sent by parent router of source end device - Contains the network address of destination end device

All routers receive the broadcast Parent router of destination end device replies back to parent router of

source end device As the reply traverses back, the hop count and signal quality measure

for each hop are recorded - Each router in the path can build a routing table containing best path to destination end device

Eventually each router in the path will have entry in the routing table

Zigbee Network Reliability

Zigbee employs a range of techniques for reliable communication Channel Selection: On initialization, channels with least activity are

selected CSMA-CA (Listen Before Sending): Before transmitting, node listens to

the channel to check if it is clear Data coding: Applies a coding mechanism ensuring higher probability

of successful transmission even in case of simultaneous transfer Acknowledgements: Receiving device acknowledges the successful

receipt of a message; retransmission if ack is not received in time Route discovery: In mesh network, possibility of finding an alternate

route if the default route is down Several mechanisms to ensure security:

Access control lists: Only pre-defined nodes can join the network 128-bit AES based Encryption Message freshness timers: Timed-out messages are rejected,

preventing message replay attacks

Can you give an example of replay attack?

Radio Characteristics Comparison

ZigBee technology relies

upon IEEE 802.15.4, which

has excellent performance

in low SNR environments

Advanced Metering Application

Building/Home Automation

Bluetooth Characteristics (I)

Short range radio links intended to replace cables No line of sight required unlike IrDA Short range: 0-30 feet (10 meters) with power consumption of 4

dBm (2.5 mW) Distance can be increased by amplifying the power

Operates in the unlicensed band at 2.4 GHz also used by other devices such as 802.11, garage door openers, microwave etc.

Higher probability of interference Bluetooth channel is divided into time slots each 625 uS in length

Devices hop through these timeslots making 1600 hops per second

Bluetooth Characteristics (II)

Uses 79 channels in the frequency range 2.402 - 2.480 GHz Uses Frequency Hop Spread Spectrum (FHSS) to avoid

interference Transmitter hops between available frequencies according to specified

algorithm Transmitter operates in sync with receiver A short burst of data is transmitted on a narrowband Transmitter then tunes to another frequency and transmits again -

capable of hopping its frequency over a given bandwidth several times a second

Requires much wider bandwidth than required to transmit the same information using one carrier frequency

Bluetooth Characteristics (III)

Supports two kinds of links Asynchronous Connectionless (ACL) for data transmission Synchronous Connection Oriented (SCO) for audio/video

Maximum effective rate around 700 kbps in asymmetric ACL link Symmetric ACL allows data rates of around 400 kbps

Piconets

Nodes can assume the role of master or slave One or more slaves can connect to a master, forming a piconet The master sets the hopping pattern for the piconet, and all

slaves must synchronize to that pattern: All units share the same channel

Maximum of 7 slaves controlled by a master (How many address bits are required?)

Bluetooth radios are symmetric - same device can act as both master and slave

Slaves are not allowed to talk to each other directly Other operational states (low power)

Parked: Device does not participate in the piconet, synchronized to the master and can be quickly reactivated

Standby: Device does not participate in the piconet, occasionally monitoring, not synchronized

Master

P

SB

SB

SB

Operational States

Slave

Parked*

Standby*

* Low power states

A piconet

M

S

S

S

S

Operational States

Initially, devices know only about themselves No synchronization Everyone monitors in standby mode (performs inquiry or page

scan for 10 ms every 1.28 seconds Power consumption in standby mode is reduced by over 98% All devices have the capability of serving as master or slave

Devices in this illustration are in the same mode but are not synchronized or coordinated - listening at different times and on different frequencies

D

A E

B C

F H

G

Forming a Piconet (I)

Unit establishing the piconet automatically becomes the master It sends an inquiry to discover other devices out there

Inquiry procedure: Enables a device to discover which devices are in range and determine the addresses and clocks for devices

Paging procedure: Establishes an actual connection; only bluetooth device address is required to setup the connection

Master and slave exchange packet using channel access code and master clock

Addressing Active devices are assigned a 3-bit active member address

(AMA) Parked devices are assigned an 8-bit parked member address

(PMA) Standby devices do not need an address

Forming a Piconet (II)

D

A

10 meters

H M

N

L

P

O

Q

B

C

F

K J

G

I

E

H

Note that a device can be “Undiscoverable”

Inquiry

disconnected

connecting

active

detach

standby

inquiry page

Transmit

AMA

Connected

AMA

Park

PMA

Hold

AMA

Sniff

AMA low power

States

Device in standby listens periodically

If a device wants to establish a piconet, it sends an inquiry, broadcast over all wake-up carriers

It will become the master of the piconet

If inquiry was successful, device enters page mode

Devices in standby may respond to the inquiry with its device address

It will become a slave to that master

standby

inquiry page

Transmit

AMA

Connected

AMA

Park

PMA

Hold

AMA

Sniff

AMA

Connecting to a Piconet

After receiving a response from devices, the master can connect to each device individually

An AMA is assigned

Slaves synchronize to the hopping sequence established by the master

In active state, master and slaves listen, transmit and receive

A disconnect procedure allows devices to return to standby mode

standby

inquiry page

Transmit

AMA

Connected

AMA

Park

PMA

Hold

AMA

Sniff

AMA

Page and Connect States

Sniff state

Slaves listen to the piconet at a reduced rate

Master designates certain slots to transmit to slaves in sniff state

Hold state

Only internal timer is running

Slave stops ACL transmission, but can exchange SCO packets

Park state

Slave releases its AMA

Still FH synchronized and wakes up periodically to listen to beacon

standby

inquiry page

Transmit

AMA

Connected

AMA

Park

PMA

Hold

AMA

Sniff

AMA

Low Power States

Piconets with overlapping coverage use different hopping sequences

Collisions may occur when multiple piconets use the same carrier frequency at the same time

Higher probability of collision with more piconets

Devices can participate in multiple piconets simultaneously, creating a scatternet

A device can only be the master of one piconet at a time

A device may serve as master in one piconet and slave in another

A device may serve as slave in multiple piconets

Scatternets (I)

J

F

I

E

A

G

D

M

O B

L

H

K

C

N

P

Q

Scatternets (II)