Chapter 15
Wireless LANs
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IEEE 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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Introduction
 Architecture
 Physical Layer
 MAC Layer
 Addressing Mechanism
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15.1 IEEE 802.11 - Architecture
 Basic Service Set (BSS)
 Extended Service Set (ESS)
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Basic 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
distribution 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 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 multipleaccess data link layer method)
 2.4 GHz ISM band
 Modulation is PSK (BPSK or QPSK)
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802.11
 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 Terminals

(a)The hidden station problem
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(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) 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)
1100
Clear to send (CTS)
1101
Acknowledgment (ACK)
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Subfields in FC field

To
DS
From
DS
0
0
Destination
station
Source
station
BSS ID
N/A
0
1
Destination
station
Sending
AP
Source
station
N/A
1
0
Receiving
AP
Source
station
Destination
station
N/A
1
1
Receiving
AP
Sending
AP
Destination
station
Source
station
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Address
1
Address
2
Address
3
Address
4
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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 Bluetooth
• Architecture
• Radio Layer
• Baseband Layer
• L2CAP Layer
• Other Upper Layers
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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 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 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 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 Types
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L2CAP data packet format
 Logical link control and adaptation protocol – roughly equivalent to the LLC
sublayer in LANs
 used for data exchange on an ACL link
 specific duties: multiplexing, segmentation and reassembly, quality of
service, and group management
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Questions !
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