Bluetooth Basics. Bluetooth Overview

Transcription

1 Bluetooth Basics Bluetooth Overview Wireless techology for short-rage voice ad data commuicatio Low-cost ad low-power Provides a commuicatio platform betwee a wide rage of smart devices Not limited to lie of sight commuicatio 1

2 Motivatio Digital Camera Computer Scaer Ikjet Priter Home Audio System PDA Cell Phoe Cordless Phoe Base Statio Bluetooth Applicatios Automatic sychroizatio betwee mobile ad statioary devices Coectig mobile users to the iteret usig bluetooth-eabled wire-boud coectio ports Dyamic creatio of private etworks 2

3 Sychroizatio Keep data o differet devices sychroized without usig a cable Example: Walk ito office ad have your PDA sych with your laptop o your desk without eve takig your PDA out of your briefcase Coectig to Iteret Beig able to gai access to the Iteret by usig Bluetooth access poits Access poit is used as a gateway to the iteret Both the access poit ad the device are Bluetooth-eabled A example of Service Discovery Protocol Access poit provides a service to the device 3

4 Ad Hoc Networks Up to 8 devices ca be actively coected i master/slave cofiguratio Picoets ca be combied to form scatterets providig ulimited device coectivity Protocol Stack 4

5 Bluetooth Radio Uses 2.4 GHz ISM bad spread spectrum radio ( MHz) Advatages Free Ope to everyoe worldwide Disadvatages Ca be oisy (microwaves, cordless phoes, garage door opeers) Frequecy Hoppig Spread Spectrum Iveted by Hedy Lamarr ad George Atheil durig 1941 Hedy kew that "guided" torpedos were much more effective hittig a target. The problem was that radio-cotrolled torpedos could easily be jammed by the eemy. Oe afteroo she realized "we're talkig ad chagig frequecies" all the time. At that momet, the cocept of frequecyhoppig was bor. Atheil gave Lamarr most of the credit, but he supplied the player piao techique. Usig a modified piao roll i both the torpedo ad the trasmitter, the chagig frequecies would always be i sych. A costatly chagig frequecy caot be jammed. 5

6 Frequecy Hoppig (cot.) Hops every packet Packets ca be 1, 3, or 5 slots log (a slot is 625µs) Packets are pretty short Basebad Layer Provides i-order delivery of byte streams Hadles Frequecy Hop Sequeces for Sychroizatio ad Trasmissio Establishes Liks Sychroous Coectio Orieted (SCO) Asychroous Coectio-Less (ACL) Provides fuctioality to determie earby Bluetooth devices 6

7 Coectio (Iquiry ad Pagig) Lik cotroller states durig coectio process Bluetooth: Hello, Ayoe Aroud? Iquiry Procedure Seds out a iquire, which is a request for earby devices (withi 10 meters) Devices that allow themselves to be discoverable issue a iquiry respose Ca take up to secods, after which the iquirig device should kow everyoe withi 10 meters of itself 7

8 Device Discovery Illustrated D F H G Note that a device ca be Udiscoverable M N A B P O E K J L I Q C 10 meters After iquiry procedure, A kows about others withi rage Issues with Iquire Messages Are the iquirer trasmittig ad the receiver listeig o the same frequecy? Sice they are ot yet coected, they are o totally differet hop sequeces, ad most likely o differet chaels If they are o the same frequecy, what if they are o a oisy chael? Bluetooth provides the capability for receivers to issue multiple iquiry resposes 8

9 Mai Idea Behid Iquire Iquirig device seds out a iquire o 16 differet frequecies (16 chael trai) Receiver (device i stadby mode), performs a iquire sca log eough for a iquirig device to sed the iquire o 16 frequecies Receiver does a iquire sca frequet eough so that it is guarateed to wake up durig a 16 chael trai Iquiry Hop Trai 16 Chael Trai Iquiry Chael Slot Series1 9

10 The Numbers Behid Iquire Each full sca of a 16 chael trai takes about 1.28 secods 16 chaels * 625us * 128 trais = 1.28 secods Oe full 16 chael trai takes 10ms. Receiver eters iquiry sca state at least oce every 1.28 secods, ad stays i that state for 10ms. What about oise? Devices always reply to received iquiry messages with a iquiry respose A iquirer is allowed to received multiple resposes from oe device I order to accout for the fact that chaels ca be oisy ad trasmissios ca get lost, the 128 trai sca is repeated up to 4 times for each trai (10.24 secods) Desiged to successfully commuicate at least oce with all devices withi rage 10

11 Iquiry Uses 32 iquire chaels to sed out iquiry messages Sed out iquiry o 32 chaels, broke up ito 2 iquiry hop trais (16 differet chaels to trasmit packets) Iteded to catch a device i iquiry sca mode o oe of the 32 iquire chaels Iquiry Sca A device periodically listes for iquiry packets at a sigle frequecy chose out of 16 frequecies Iquiry hop sequece depeds o device address Stays i the state log eough for a iquirig device to cover 16 frequecies Will re-eter iquiry sca state eve after respodig to a iquire 11

12 Iquiry Respose Whe radio receives iquire, it will wait betwee 0 ad.32 secods before sedig a FHS packet as a respose This is doe to avoid collisio with aother radio that also wats to sed a FHS packet FHS Packet cotais: Device ID Clock After iquirig radio is doe with iquirig procedure, it kows all of the radios (that are discoverable) withi rage Pagig: Will you coect to me? Very similar to iquire Still have ot sychroized clocks or frequecies Establishes actual Picoet coectio with a device that it kows about Coectio process ivolves a 6 steps of commuicatio betwee the the master ad the slave Step Message Directio Hoppig Patter Patter Source ad Clock 1 Slave ID Master to Slave Page Slave 2 Slave ID Slave to Master Page Respose Slave 3 FHS Master to Slave Page Slave 4 Slave ID Slave to Master Page Respose Slave 5 1st Master Packet Master to Slave Chael Master 6 1st Slave Packet Slave to Master Chael Master 12

13 Pagig Illustrated D F H G M N O J 10 meters E I A K C B L P Q Step 1: The Page Commad Device broadcasts a page message out to the device that it wats to set up a coectio with Does this i a similar maer as iquire messages (o 2 frequecy trais of 16 frequecies each) Oce the device receives a page respose, it will stop pagig ad move o to step 2 13

14 Pagig: Steps 2 & 3 Step 2: I the page respose, a ackowledgemet is set back to the master cotaiig the slave ID Step 3: I the master respose, the frequecy hoppig geerator is stopped ad the master issues a FHS packet to the slave Pagig: Step 4 The slave issues a fial slave respose trasmissio that is aliged to the slave s ative clock Usig the data from the FHS packet, the slave calculates adopts the master s frequecy hoppig patter ad sychroizes to its clock 14

15 Pagig: Step 5 Whe the master receives the packet, it jumps back to its frequecy hoppig patter ad assigs the slave a Active Member Address (AMA) for the picoet Master seds out a poll packet to esure that the slave is o its frequecy hoppig patter Pagig: Step 6 Oce the slave receives the poll packet, the slave replies with ay kid of packet to esure that it is o the right chael The ackowledgemet must be received by the Master withi the timeout period At the coclusio of step 6, a ew sychroized coectio is established betwee the master ad the slave 15

16 Lik Maager Performs all lik creatio, maagemet, ad termiatio operatios Resposible for all the physical lik resources i the system Hadles the cotrol ad egotiatio of packet sizes used whe trasmittig data Cotrols Operatio Modes for devices i a picoet Sets up, termiates, ad maages basebad coectios betwee devices Establishes differet types of liks depedet o requests from the L2CAP layer Sychroous Coectio-Orieted (SCO) Asychroous Coectio-Less (ACL) Asychroous Coectio- Less (ACL) Desiged for data traffic Packet switched coectio where data is exchaged sporadically as ad whe data is available from higher up the stack Data itegrity is checked through error checkig ad retrasmissio Oe ACL lik betwee a master ad a slave 16

17 Sychroous Coectio Orieted (SCO) Iteded for use with time-bouded iformatio such as audio or video Provides a circuit-switched coectio where data is regularly exchaged Retrasmissio is ot ecessary, sice data is real-time Up to 3 SCO liks per picoet ACL Liks vs. SCO Liks Iteded Traffic Type Retrasmissio Max # liks betwee master ad slave Supported durig hold mode Switched coectio type ACL Data Yes 1 No Packet SCO Time bouded ifo (Audio or Video) No 3 Yes Circuit 17

18 ACL Setup Uder LMP Establishig Picoets Wheever there is a coectio betwee two Bluetooth devices, a picoet is formed Always 1 master ad up to 7 active slaves Ay Bluetooth device ca be either a master or a slave Ca be a master of oe picoet ad a slave of aother picoet at the same time (scatteret) All devices have the same timig ad frequecy hoppig sequece 18

19 Scatterets Formed by two or more Picoets Master of oe picoet ca participate as a slave i aother coected picoet No time or frequecy sychroizatio betwee picoets Lik Maager Operatio Devices operate i stadby mode by default util they become coected to a picoet 4 Coectio Modes Active Hold Park Siff Modes allow devices to adjust power cosumptio, performace, ad the umber/ role of participats i a picoet 19

20 Active Mode Limited to 7 Active slaves for each master Three bit address (AM_ADDR) give to each active slave Uit actively participates o chael Ca receive commuicatios i ay give frame Active slaves are polled by master for trasmissios Uit operates o high-power Hold Mode Frees slave to Atted aother Picoet Perform scaig, pagig, or iquiry operatios Move ito low-power sleep Uit keeps active member address Uit does ot support ACL packets o the chael but may support SCO packets Master ad slave agree o a oe time hold duratio after which the slave revives ad sychroizes with chael traffic Uit operates o low-power 20

21 Siff Mode Very similar to hold mode Slave is freed for reoccurrig fixed time itervals Master ca oly commuicate durig arraged siff time slots Park Mode Parked uit gives up active member address ad is assiged 8 bit Parked member address (PM_ADDR) allows master to upark slave 8 bit Access request address (AR_ADDR) allows slave to ask master to upark it Uit stays sychroized to chael Operates i very low-power sleep 21

22 Park Mode (cot.) Provides the ability to coect more tha 7 devices to a master (8 bit PM_ADDR allows 255 parked devices) Active ad Parked slaves ca be switched i ad out to allow may coectios to a sigle picoet Park Mode (cot.) Master establishes a beaco chael ad beaco iterval whe a slave is parked Parked slave wakes up at regular beaco iterval to Maitai sychroizatio Liste for broadcast messages (packets with all zero AM_ADDR) Potetially make access request to master through (AR_ADDR) 22

23 Park Mode (cot.) Beaco slots must have at least ull master-to-slave traffic Master-to-slave trasmissios may exted over multiple beaco slots Security Lik maager provides mechaism used by devices at either ed of a lik for Negotiatig ecryptio mode Coordiatig ecryptio keys Basebad hadles ecryptio ad key geeratio 23

24 Host Cotroller Iterface (HCI) Most Bluetooth systems cosist of two processors: The higher layers of the protocol stack (L2CAP, SDP, RFCOMM) are ru o the host device s processor The lower layers of the protocol stack (Basebad ad radio) are ru o specific Bluetooth hardware HCI provides a iterface betwee the higher ad the lower layers of the protocol stack HCI Flow Cotrol Mai fuctio of the Host Cotroller Iterface May times higher layer protocols have data rates much larger tha data rate across Bluetooth radio ad air iterfaces Also eed to hadle the reverse situatio whe the host caot accept data as fast as the Bluetooth module ca sed it 24

25 Two Pieces of HCI Host cotroller resides o Bluetooth hardware acceptig commuicatios over the physical bus (radio ad air) HCI Driver resides o the host acceptig commuicatios from higher layer protocols The basic structure showig how the host cotroller layers are fitted ito the protocol stack 25

26 RFCOMM Cable replacemet protocol allowig applicatios built to iterface with serial port to fuctio seamlessly with bluetooth Emulates serial port over the L2CAP protocol by specifyig how a data stream ca be emulated RFCOMM actually hadles parallel data Emulatig the Serial Port Typically, the receive ad trasmit lies are coected to a UART (Uiversal Asychroous Receiver Trasmitter) Job of the UART is to covert betwee serial data set dow cables ad the parallel data processig which devices use Sice software that deals with serial ports view the data after it has bee through UART, it oly sees the parallel data RFCOMM protocol oly works with parallel data by coectig to the lower layers via L2CAP 26

27 Service Discovery Protocol (SDP) Idea: Traditioal LANs: Fid a coectio to a priter (or other resource) ad keep that coectio for a log time Bluetooth: Walk ito a area, fid a priter (or other resource), use it, the walk away forgettig ay details of the coectio SDP Cliet/Server Model SDP Server is ay Bluetooth device that offers services to other Bluetooth device (ex. Bluetooth-eabled priter, etc.) Each SDP Server maitais its ow database that cotais iformatio about the services that it offers SDP Cliet is ay Bluetooth device that uses the services offered by a SDP Server 27

28 SDP i the Bluetooth Protocol Stack SDP Query The SDP cliet queries a SDP server to fid out what services are available Uses the L2CAP lik that is set up betwee the cliet ad the server L2CAP lik provides iformatio o services but does t hadle ay coectio to services Need to specify a class of services that the cliet wats to use (e.g. pritig services) 28

29 SDP Database SDP Database is a set of records that describes the differet services that the server ca provide to aother Bluetooth device Whe the SDP server gets a query, it looks up the service that the cliet is requestig ad returs iformatio to the cliet o how to coect to the service Usig the Services The SDP cliet establishes a separate (o-sdp) coectio to use the service SDP coectio is oly used to determie service availability The L2CAP coectio uses to get iformatio for the service ca be dropped (if o more services are eeded) or retaied (if the cliet still eeds more services from the server) 29

30 Logical Lik Cotrol ad Applicatio Protocol (L2CAP) Performs 4 major fuctios Maagig the creatio ad termiatio of logical liks for each coectio through chael structures Eforcig ad defiig QoS requiremets Adaptig Data, for each coectio, betwee applicatio (APIs) ad Bluetooth Basebad formats through Segmetatio ad Reassembly (SAR) Performig Multiplexig to support multiple cocurret coectios over a sigle commo radio iterface (multiple apps. usig lik betwee two devices simultaeously) Segmetatio/Reassembly Basebad packet size is limited Ca hadle payload of 2745 bits L2CAP accepts packet size up to 64kb L2CAP segmets large packets ito smaller basebad maageable packets Smaller received basebad packets are reassembled comig back up the protocol stack 30

31 Quality of Service Applicatios may demad QoS o specific parameters Peak badwidth Latecy Delay variatio Toke rate Toke bucket size L2CAP provides requested QoS if possible ad otifies applicatio if lik ca ot support demads Protocol Multiplexig Applicatios may access L2CAP through differet support protocols Service Discovery Protocol (SDP) RFCOMM Telephoy Cotrol Protocol Specificatio (TCS) Basebad is ot cocered with operatio protocols meaig L2CAP must distiguish betwee them 31

32 Protocol Multiplexig Illustrated TCS RFCOMM SDP Audio LMP L2CAP Voice ACL SCO Basebad Summary Advatages of Bluetooth Low power cosumptio Low price o Bluetooth compoets No lie-of-sight Disadvatages of Bluetooth Wireless LANs offer faster data rates ad larger commuicatio rages Possibility of iterferece o 2.4GHz frequecy bad 32

33 Sources Bluetooth: Coect without Cables by Jeifer Gray Discoverig Bluetooth by Bret A. Miller 33