wireless# guide to wireless communications

Wireless# Guide to Wireless Communications

Chapter 5Low Rate Wireless Personal Area

Networks

Wireless# Guide to Wireless Communications 2

Objectives

• Describe a wireless personal area network (WPAN)

• List the different WPAN standards and their applications

• Explain how IrDA, Bluetooth, and ZigBee work

• Describe the security features of low-rate WPAN technology

What is a WPAN?

• Wireless personal area network (WPAN)– Group of technologies that are designed for short-

range communications– Eliminates the need for wires or cables to

interconnect multiple devices

• Applications for WPAN technology include:– Synchronizing PDAs, cellular, and Smartphones– Home control systems (smarthome)– Cordless telephones– Portable device data exchange

What is a WPAN? (continued)

• Applications for WPAN technology include (continued):– Industrial control systems– Location — smart tags used to locate people at home

or at the office– Security systems– Interactive toys– Inventory tracking

• Advantages– WPAN devices use very little power– Short range helps maintain security and privacy

Existing and Future Standards

• Institute of Electrical and Electronics Engineers (IEEE)– Currently developing various standards for WPANs

• Interoperability– Interoperability is of utmost importance– Following the OSI protocol model

• Allows manufacturers to ensure interoperability between their devices

Existing and Future Standards (continued)

• Interoperability (continued)– Open Systems Interconnect (OSI) model

• Developed by the International Organization for Standardization (ISO)

– Project 802• Started by the IEEE at about the same time ISO was

creating OSI

• Ensures interoperability among data networking products

• Widely used standards: 802.3 (Ethernet) and 802.5 (Token Ring)

Existing and Future Standards (continued)

Infrared WPANs (IrDA)

• Most common infrared connection today – Based on the IrDA specifications

• IrDA specifications– Define both physical devices and network protocols

• IrDA devices’ characteristics– Provide walk-up connectivity– Provide a point-to-point method of data transfer

• Between only two devices at a time

Infrared WPANs (IrDA) (continued)

• IrDA devices’ characteristics (continued)– Cover a broad range of computing and communicating

devices– Inexpensively implemented

• There are currently three published versions of the IrDA specifications– Plus a fourth currently under development

• IrDA PHY layer– Light emitting diodes (LEDs) send signals– Photodiodes receive signals

• Serial Infrared (Version 1.0)– Designed to work like the standard serial port on a PC– Uses a UART (Universal Asynchronous

Receiver/Transmitter)• Microchip that also controls a computer’s serial interface

– Uses a clock that is 16 times faster than the data rate– Transmitting a 0 using 7-3-6

• UART clock waits for seven clock cycles during the bit time (16 clock pulses)

• Send an infrared pulse for three clock cycles

• Then send nothing for six clock cycles

• Fast Infrared (FIR) (Version 1.1)– Extends the data rate to 4 Mbps– When two IrDA devices first communicate

• They both transmit using SIR

• Then they shift to FIR speed

– Uses 4-pulse position modulation (4-PPM)• Information is conveyed by the position of a pulse within

a time slot

• Two bits (or dibits) are transmitted for each pulse

Protocols Supported

• IrDA Physical Layer Protocol (IrPHY)– Controls hardware that sends and receives IR pulses

• IrDA Link Access Protocol (IrLAP)– Responsible for encapsulating the frames– Describes how the devices establish and close a

connection

• IrDA Link Management Protocol (IrLMP)– Detects the presence of devices offering a service– Checks the data flow

Protocols Supported (continued)

• IrDA Transport Protocols (TinyTP)– Manages channels between devices– Performs error corrections– Divides data into packets

• Optional extensions– IrWWW– IrTran-P (Infrared Transfer Picture)– Infrared printing (IrLPT)– Other extensions: IrFM, IrSimple, and IrOBEX

Other IrDA Considerations

• Factors to consider– Half-duplex transmission– Deflection angle– Ambient light– Ease of use– Security– Distance limitation

RF WPANs

• Offer functionality beyond that of IR devices

• Include– Bluetooth– 802.15.4 (ZigBee)

IEEE 802.15.1 and Bluetooth

• Industry specification– Bluetooth Special Interest Group (SIG)

• Defines small-form-factor, low-cost wireless radio communications

• IEEE licensed this wireless technology– To adapt and copy a portion of the specification as the

base material for 802.15.1

• 802.15.1 standard– Approved in March 2, 2002– Fully compatible with Bluetooth version 1.1

Bluetooth Protocol Stack

• Bluetooth RF layer– Defines how the basic hardware that controls the radio

transmissions functions– Data bits (0 and 1) are converted into radio signals

and transmitted

• Radio module– A single radio transmitter/receiver (transceiver)– Only hardware required for Bluetooth to function– Bluetooth can transmit at a speed of up to 1 Mbps– Bluetooth version 2.0 transmits at 2 or 3 Mbps

Bluetooth Protocol Stack (continued)

• Modulation technique– Bluetooth uses a variation of frequency shift keying

(FSK)– Two-level Gaussian frequency shift keying (2-GFSK)

• Uses two different frequencies

– To indicate whether a 1 or a 0 is being transmitted

– Modulation index• Amount that the frequency varies

• Between 280 KHz and 350 KHz

• Bluetooth Baseband layer– Lies on top of the RF layer– Manages physical channels and links– Handles packets, and does paging and inquiry

• To locate other Bluetooth devices in the area

• Radio frequency– 2.4 GHz Industrial, Scientific, and Medical (ISM) band– Bluetooth divides frequency into 79 different channels

• Spaced 1 MHz apart

– Bluetooth uses FHSS

• Radio frequency (continued)– Bluetooth uses the same frequency as IEEE 802.11b

WLANs– Devices that use Bluetooth can interfere with 802.11b

WLANs– Bluetooth version 1.2 adds a feature called adaptive

frequency hopping (AFH)• Further improves compatibility with 802.11b

• Network topologies– Piconet and scatternet

• Network topologies (continued)– Bluetooth devices

• Master, controls all of the wireless traffic• Slave, takes commands from the master

– Piconet• Bluetooth network that contains one master and at least

one slave and that uses the same channel– Each Bluetooth device is preconfigured with an

address• Needed when participating or not participating in the

piconet

• Network topologies (continued)– All devices in a piconet must change frequencies both

at the same time • And in the same sequence

– Bluetooth connection steps• Inquiry procedure• Paging procedure

– Multiple piconets can cover the same area• Each can contain up to seven slaves

– Bluetooth device can be a member of two or more overlapping piconets

• Network topologies (continued)– Scatternet

• Group of piconets in which connections exist between different piconets

– To communicate in each different piconet• Device must use the master device address and clock

of that specific piconet

– Bluetooth device can be a slave in several piconets • But can be a master in only one piconet

– A master and slave can switch roles in a piconet

• Bluetooth frames– Parts

• Access code (72 bits) — Contains data used for timing synchronization, paging, and inquiry

• Header (54 bits) — Contains information for packet acknowledgment, packet numbering, the slave address, the type of payload, and error checking

• Payload (0-2745 bits) — Can contain data, voice, or both

Bluetooth Link Manager Layer

• Link between Bluetooth devices– Types of physical links

• Synchronous connection-oriented (SCO) link

• Asynchronous connectionless (ACL) link

Bluetooth Link Manager Layer (continued)

• Link between Bluetooth devices (continued)– Error correction schemes

• 1/3 rate Forward Error Correction (FEC)

• 2/3 rate FEC

• Automatic retransmission request (ARQ)

– Bluetooth power usage• Bluetooth devices are designed to be mobile

• Conserving power is essential

• Power consumption varies depending on connection mode

• Link between Bluetooth devices (continued)– Bluetooth power usage (continued)

• Power-saving modes

– Active

– Sniff

– Hold

– Park

Other Layers and Functions

• Logical Link Control Adaptation Protocol (L2CAP)– Logical Link Control layer– Responsible for segmenting and reassembling data

packets

• Radio Frequency Virtual Communications Port Emulation (RFCOMM) data protocol– Provides serial port emulation for Bluetooth data

• LMP layer– Transmits control information

IEEE 802.15.4-Low Rate WPANs (ZigBee)

• ZigBee standard– Provides for the connectivity of simple fixed and

mobile devices• That require only low data rates between 20 and 250

• Consume a minimum amount of power

• Typically connect at distances of 33 feet (10 meters) to 150 feet (50 meters)

• ZigBee Alliance– Formed in 2002

ZigBee Overview

• ZigBee specification– Based on the relatively low-level performance

requirements of sensors and control systems

• ZigBee devices are designed to remain quiescent for long periods of time

• ZigBee transmissions are designed to be short in range

• Some ZigBee devices have the ability to route packets to other devices

ZigBee Overview (continued)

• Basic classes of devices in a ZigBee network– Full-function device– PAN coordinator– Reduced-function device

• ZigBee protocol stack– Based on the OSI seven-layer model– Defines only those layers that are relevant to

achieving specific functionality– Total of 27 channels across different frequency bands

• ZigBee protocol stack (continued)– Since DSSS transmission is used

• Carrier is modulated with a sequence of 15 chips

• In both the 868 and 915 MHz bands

– In the 2,450 MHz band, the technique employs 16 different 32-bit chip sequences

• Modulated using offset quadrature phase shift keying (O-QPSK)

– Uses two carrier waves that are exactly 90 degrees out of phase

• IEEE 802.15.4 PHY frame format

• 802.15.4 MAC layer– Handles all access from the upper layers to the physical

radio channel– Access to the medium is contention based

• Uses CSMA/CA

• 802.15.4 MAC layer (continued)– Superframe

• Mechanism for managing transmission time in a piconet

– Guaranteed time slots (GTS)• Reserved periods for critical devices to transmit priority

data between two beacons

– Beacons• Signal the beginning of a superframe

• Contain info about the type and number of time slots

– And the time synchronization frame for the network

• 802.15.4 MAC layer (continued)– Beacon frames are not required for device-to-device

communications– Procedures for associating with and joining a network,

routing, and so on, are embedded in the hardware– ZigBee devices are engineered to automatically

associate with and join the network– Device discovery

• Devices query other devices to identify them– Service discovery

• Identifies the capabilities of specific devices

• Coexistence with other standards– Relatively wideband interference, such as that

generated by IEEE 802.11b networks• Appears like white noise to an IEEE 802.15.4 receiver

– Impact of interference from Bluetooth (802.15.1) devices should be minimal

• Network addressing– The ZigBee specification defines several different

levels of addresses• For identifying devices within a PAN

• Network addressing (continued)– Levels of addresses

• IEEE address

• Network (PAN) address

• Node address

• Endpoint address

• ZigBee network topologies– Basic topologies

• Star, tree, and mesh

• ZigBee network topologies (continued)– In both tree and mesh topologies

• Alternate paths may be available for packets– In cluster tree networks

• Alternate paths are available– If another full-function device is within its radio range

– In a star topology• Network is controlled by the PAN coordinator

– Cluster tree topologies• Two or more tree topology networks that are

interconnected by full-function devices

• Power management in ZigBee networks– Packet routing requires a lot of processing overhead– ZigBee devices are designed to be very small

• Equipped with low-speed, power-efficient CPUs

– In a cluster tree network, only the devices that interconnect each different cluster tree incur overhead

– 802.15.4 standard favors battery-powered devices• Does not prevent devices from being connected to another

power source

– ZigBee must maintain certain parameter values• In case of a power failure

Low Rate WPAN Security• Security should be of little concern with WPANs• One of the most serious concerns is social

engineering• Designing security in WPANs

– Much more difficult task than in other networking technologies

• Public key infrastructure (PKI)– Unique security code, or key, provided by a certificate

authority

• Certificate authority– Private company that verifies the authenticity of users

Security in Infrared WPANs

• Limited to the line-of-sight characteristic of this type of connection

• IrDA specification makes no provision for encrypting data or protecting the connection– Users may encrypt a file before exchanging it

• IrDA devices support open access– So that anyone can transmit files without first notifying

the device’s owner

• Open access can be a major security concern

Security in Bluetooth WPANs

• Bluetooth provides security at the LMP layer– Using authentication

• Authentication is based on identifying the device itself

• Authentication scheme is a challenge-response strategy

• Encryption is the process of encoding communications– And ensures that the transmissions cannot be easily

intercepted and decoded

Security in Bluetooth WPANs (continued)

• Encryption modes– Encryption Mode 1 — Nothing is encrypted– Encryption Mode 2 — Traffic from the master to one

slave is encrypted• But traffic from the master to multiple slaves is not

encrypted– Encryption Mode 3 — All traffic is encrypted

• Levels of Bluetooth security– Level 1 — No security– Level 2 — Service-level security– Level 3 — Link-level security

Security in ZigBee and IEEE 802.15.4 WPANs

• ZigBee WPANs use symmetric keys for authentication and encryption

• IEEE 802.15.4 standard provides in addition:– Frame integrity, access control, and sequential

freshness security services• Frame integrity uses a message integrity code (MIC)• Access control is based on access control list (ACL)• Sequential freshness

– Security service used by the receiving device– Ensures that the same frames will not be transmitted

more than once

Security in ZigBee and IEEE 802.15.4 WPANs (continued)

• Security modes in the 802.15.4 standard– Unsecured mode– ACL mode (which uses access control)– Secured mode (which uses full authentication and

encryption)

Summary

• Network protocol is the set of rules for messages exchanged between communication devices

• The most common infrared connection today is based on the IrDA standard– IrDA devices cannot send and receive at the same

time– IrDA is easy to use

• Bluetooth is a wireless technology that uses short-range radio frequency (RF) transmissions– Supported by over 2,500 hardware and software

vendors

Summary (continued)

• Bluetooth uses two-level Gaussian frequency shift keying (2-GFSK) modulation

• Error correction schemes used in Bluetooth– 1/3 rate Forward Error Correction (FEC), 2/3 rate FEC,

and the automatic retransmission request (ARQ)

• ZigBee is a specification for low rate WPANs created by the ZigBee alliance– Includes full mesh networking capability

• ZigBee network topologies: star, tree, and mesh

Summary (continued)

• IEEE 802.15.4 standard frequency bands: 868 MHz,915 MHz, and the 2.4 GHz-ISM band

• 802.15.4 is designed to coexist easily with other WPAN and WLAN technologies

• Security in Bluetooth supports only device authentication and limited encryption

• ZigBee supports message integrity at the MAC layer– Can also check for the freshness of the message

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