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Chapter 15Chapter 15Wireless LANsWireless LANs
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IEEE 802 Standards Working GroupsIEEE 802 Standards Working Groups
WiMAX, an acronym that stands for Worldwide Interoperability for Microwave Access(802.16)
WiFi (Wireless Fidelity) : 802.11
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IntroductionIntroduction
Architecture
Physical Layer
MAC Layer
Addressing Mechanism
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15.1 IEEE 802.11 - Architecture15.1 IEEE 802.11 - Architecture
Basic Service Set (BSS)
Extended Service Set (ESS)
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Basic Service SetBasic Service Set
• made up of stationary or mobile wireless stations, and possibly a central base station (access point)
• BSS without an AP is stand-alone, cannot send data to other BSSs
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ESS – Extended Service Set
Made up of two or more BSSs with APs – connected through a distr
ibution system (usually a wired LAN)
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Station Types
No-transition mobility – either stationary or moving
only inside a BSS
BSS-transition mobility – can move from one BSS to
another, confined to one ESS
ESS-transition mobility – can move from one ESS to
another
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Physical layer specifications
802.11 defines the specs for the conversion of bits to
a signal
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FHSS – Frequency hopping spread spectrum
sender sends on one carrier frequency for a short amount of time, hops to another frequency for the same time, then another, and so on, eventually the cycle is repeated
makes it difficult for unauthorized person to make sense of data
2.4 GHz industrial, scientific, and medical (ISM) band – 79 subbands of 1MHz
modulation is FSK (two-level or four-level)
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Frequency-Hopping exampleFrequency-Hopping example
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DSSS – direct sequence spread spectrum
Each bit sent is replaced by a chip code (similar to CDMA, but this is at the physical layer, not a multiple-access data link layer method)
2.4 GHz ISM band
Modulation is PSK (BPSK or QPSK)
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802.11802.11
802.11 Infrared
Two capacities 1 Mbps or 2 Mbps.
Range is 10 to 20 meters and cannot penetrate walls.
Does not work outdoors.
802.11 FHSS (Frequence Hopping Spread Spectrum)
The main issue is multipath fading.
79 non-overlapping channels, each 1 Mhz wide at low end of 2.4 GHz ISM band.
Same pseudo-random number generator used by all stations.
Dwell time: min. time on channel before hopping (400msec).
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801.11a,b,g 802.11a OFDM – orthogonal frequency division multiplexing
5-GHz ISM band
48 subbands for sending 48 groups of bits at a time, 4 subbands for control information
PSK for 18Mbps and QAM for 54 Mbps
802.11b HR-DSSS – high-rate direct sequence spread spectrum
2.4 GHz band
similar to DSSS except for encoding, complementary code keying (CCK), encodes 4 or 8 bits to one CCK symbol
modulation BPSK and QPSK
4 data rates (1, 2, 5.5, and 11 Mbps) : DSSS for 1, 2 Mbps, BPSKllfor 5.5, QPSK for 11 Mbps
Range is 7 times greater than 11a.
802.11b and 11a are incompatible!!
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801.11a,b,g (cont’d)
802.11g OFDM
2.4 GHz band
54-Mbps data rate
Is backward compatible with 802.11b.
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MAC layers in IEEE 802.11 standard
PCF : Point Coordination Function DCF : Distributed Coordination Function
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CSMA/CA
Cannot implement CSMA/CD:
collision detection implies the ability to send and receive at the same time, which implies costly stations and increased bandwidth requirements
collision may not be detected because of a hidden terminal
distance could cause signal fading which could prevent a station from hearing a collision
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Hidden and Exposed TerminalsHidden and Exposed Terminals
(a)The hidden station problem (b) The exposed station problem
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CSMA/CA flowchart
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CSMA/CA and NAV
DIFS – distributed interframe spread NAV – network allocation vector
SIFS – short interframe spread collision only occurs during
RTS – request to send handshake period (RTS, CTS)
CTS – clear to send
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Frame format
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Subfields in FC(frame control) fieldSubfields in FC(frame control) field
Field Explanation
Version The current version is 0.
Type Type of information: management (00), control (01), or data (10).
Subtype Defines the subtype of each type (see ).
To DS Defined later.
From DS Defined later.
More flag When set to 1, means more fragments.
Retry When set to 1, means retransmitted frame.
Pwr mgt When set to 1, means station is in power management mode.
More data When set to 1, means station has more data to send.
WEP Wired equivalent privacy. When set to 1, means encryption implemented.
Rsvd Reserved.
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Control frames
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Values of subfields in control frames
type field : 01
Subtype Meaning
1011 Request to send (RTS)Request to send (RTS)
1100 Clear to send (CTS)Clear to send (CTS)
1101 Acknowledgment (ACK)Acknowledgment (ACK)
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Subfields in FC field
ToTo
DSDSFromFrom
DSDSAddressAddress
11AddressAddress
22AddressAddress
33AddressAddress
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00 00DestinationDestination
stationstationSourceSource
stationstationBSS IDBSS ID N/AN/A
00 11DestinationDestination
stationstationSendingSending
APAPSourceSource
stationstationN/AN/A
11 00ReceivingReceiving
APAPSourceSource
stationstationDestinationDestination
stationstationN/AN/A
11 11ReceivingReceiving
APAPSendingSending
APAPDestinationDestination
stationstationSourceSource
stationstation
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Addressing mechanism: case 1
•To DS = 0 and From DS = 0• Frame is not going to a distribution system• Frame is going from one station in a BSS to another• ACK sent to original sender
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Addressing mechanism: case 2
To DS = 0 and From DS = 1
Frame is coming from a distribution system
Frame is coming from an AP and going to a station
ACK sent to AP
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Addressing mechanism: case 3
To DS = 1 and From DS = 0
Frame is going to a distribution system
Frame is coming from a station to an AP
ACK sent to original station
address 3 contains final destination of frame
•
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Addressing mechanism: case 4
To DS = 1 and From DS = 1
Distribution system is wireless
Frame is coming from an AP going to another AP
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15.2 Bluetooth15.2 Bluetooth
• Architecture
• Radio Layer
• Baseband Layer
• Other Upper Layers
• L2CAP Layer
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Architecture - - Two Types of Networks
Piconets
Scatternet
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Piconet
up to eight stations, one is the master, rest are slaves
slaves synchronize their clocks and hopping sequence with master
only one master
communication between master and slave can be 1-1 or 1-many
can have up to eight additional slaves in parked state
slave in parked state is synchronized, but cannot communicate
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Scatternet
scatternet – combined piconets
slave in one piconet can be master in another
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Bluetooth layers
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Radio LayersRadio Layers
roughly equivalent to the physical layer of the Internet
model
2.4-GHz ISM band
FHSS – frequency hopping spread spectrum
modulation – sophisticated version of FSK, called
GFSK (FSK w/ Gaussian bandwidth filtering)
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Baseband LayerBaseband Layer
roughly equivalent to MAC sublayer in LANs
access method is TDMA
TDD-TDMA (time division duplexing TDMA) – a kind of half duplex communication
single slave communication – master uses even number slots, slave uses odd numbered
multiple slave communication – master uses even number slots, slave sends in the next odd-numbered slot if the packet in the previous slot was addressed to it
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Single-slave CommunicationSingle-slave Communication
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Multiple-slave communication
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Physical Links
SCO – synchronous connection oriented link –
avoiding latency is more important than integrity –
damaged packet is never retransmitted
ACL – asynchronous connectionless link – data
integrity is more important than avoiding latency – if
packet is damaged, it is retransmitted
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Frame Format TypesFrame Format Types
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L2CAP data packet format
Logical link control and adaptation protocol – roughly equivalent to the LL
C sublayer in LANs
used for data exchange on an ACL link
specific duties: multiplexing, segmentation and reassembly, quality of servi
ce, and group management
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Questions !Questions !