corso di reti mobili - cnrfly.isti.cnr.it/didattica/reti-mobili-wifi-bt.pdf · 2007-03-05 ·...
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Corso di Reti mobili
Wi-Fi and Bluetooth
Francesco Potortìhttp://fly.isti.cnr.it/didattica
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OverviewWi-Fi (802.11)
Accessing the networkAd hoc mode and infrastructure mode The MAC levelWMM: Wi-Fi Multimedia
Bluetooth (802.15.1)Accessing the networkConnection-oriented and connectionless linksThe MAC levelQuality of service
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Wi-FiWi-Fi Alliance for interoperabilityThe “wireless Ethernet”MAC address as in 802.x802.11 and b, a, g amendments802.11n: MIMO (2008?)
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Entering a BSS
Scan and selectAd hoc mode
optional authentication
Infrastructure modeauthenticationassociationWPA authentication
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MAC addressing
MAC address for the nodeMAC address for the APThe ESS802.11D, r, s: roaming
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DCF
Distributed coordination functionCSMA/CA methodoptional RTS/CTS
IFS: the interframe spacingif no one transmits, go onif the channel is busy, back off randomlyalso back off if want to transmit again after a transmission
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ARQ
Automatic Repeat RequestAfter transmitting, wait for ACKIf no ACK received, retransmitGive up after a maximum number of retransmissions
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WMM: QoS
Priority with guarantees8 priorities in four classes
voicevideobest-effortbackground
priority is obtained bychanging the IFS for each classchanging the backoff for each class
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Dynamic multi-rate switching
802.11: 1, 2 Mb/s802.11b: add 5.5, 11 Mb/s802.11g: add 6, 9, 12, 18, 24, 36, 48, 54 Mb/sswitching based on
RSSISNRframe loss ratio
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Bluetooth
Small devicesShort rangeWPAN: wireless personal area networkSeveral power ranges
Class 1 1 mW – 100 mWClass 2 0.25 – 2.5 mWClass 3 - 1 mW
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Accessing the network
A Piconet is made of a Master and up to seven slavesA Scatternet is made of several piconetsMaster and slave can exchange rolesAny node can belong to more than one piconet
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Start as master
Periodically send an Inquiry to discover new slaves and get their addressSend a Page request to bring the slave into the piconetAfter paging, Authentication optionally takes place
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Start as slave
Wait for an inquiry from the masterget the master's addresssyncronise the clock
Wait for a page request from the master
synchronise the hopping sequence
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Channel access
MAC address is 48 bitBand is 2.4 GHz, like Wi-FiFHSS (frequency hopping spread spectrum), 1 Mb/s79 channels in most of the world1600 hops/s: slot is 625μs long
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Hopping sequence
Unique for each piconetDepends on Master's address
Hopping sequence virtually infiniteMaster uses even slots, slave uses odd slotsMessage lasts for 1, 3 or 5 consecutive slots
Sequence stops during multislot
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Synchronous links
SCO (synchronous connection-oriented) link
guaranteed delay and bandwidthno retransmissionno CRCoptional 1/3 or 2/3 FEC protectionfixed 64 kb/s bandwidth, symmetric or unidirectional
eSCO up to 288 kb/s with ARQ
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Asynchronous links
ACL (asynchronous connectionless) linkSymmetric and asymmetric with different preset bandwidth up to 0.7 Mb/sBluetooth 2 + EDR (extended data rate) goes up to 3 Mb/s16-bit CRC, optional FEC and ARQ
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Quality of Service
A single ACL per slaveParameters for request
type of QoS (none, best effort, guaranteed best effort), token rate, token bucket size, peak bandwidth, latency, delay variation
Parameters on acceptancepoll interval, N
BC