15-441 Computer Networking
Lecture 4 – 802.11 and more
1
Readings
• Performance analysis of the IEEE 802.11
distributed coordination function, JSAC, Mar
2000
• Ultra-Wideband Technology for Short- and
Medium-range Wireless Communication,
Foerster, Green, Somayazulu, Leeper, Intel
Technology Journal, Q2 2001.
• "The Bluetooth radio system," J. C.
Haartsen, IEEE Pers. Commun. Mag., pp.
28--36, Feb. 2000.
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Lecture 4: 802.11 and more
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Outline
•
•
•
•
•
•
RF introduction
Modulation
Antennas and signal propagation
Equalization, diversity, channel coding
Multiple access techniques
Wireless systems and standards
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Outline
• Wireless systems and standards
• 802.11 Architecture
• 802.11 MAC
• Bluetooth
• WiMax
• UWB
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IEEE 802.11 Overview
• Adopted in 1997
Defines:
• MAC sublayer
• MAC management protocols and services
• Physical (PHY) layers
• IR
• FHSS
• DSSS
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802.11 particulars
• 802.11b (WiFi)
• Frequency: 2.4 - 2.4835 Ghz DSSS
• Modulation: DBPSK (1Mbps) / DQPSK (faster)
• Orthogonal channels: 3
• There are others, but they interfere. (!)
• Rates: 1, 2, 5.5, 11 Mbps
• 802.11a: Faster, 5Ghz OFDM. Up to
54Mbps
• 802.11g: Faster, 2.4Ghz, up to 54Mbps
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802.11 details
• Fragmentation
• 802.11 can fragment large packets (this is separate
from IP fragmentation).
• Preamble
• 72 bits @ 1Mbps, 48 bits @ 2Mbps
• Note the relatively high per-packet overhead.
• Control frames
• RTS/CTS/ACK/etc.
• Management frames
• Association request, beacons, authentication, etc.
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Overview, 802.11 Architecture
ESS
Existing
Wired LAN
AP
STA
BSS
AP
STA
STA
BSS
STA
Infrastructure
Network
STA
Ad Hoc
Network
STA
BSS
BSS
STA
Ad Hoc
Network
STA
BSS: Basic Service Set
ESS: Extended Service Set
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802.11 modes
• Infrastructure mode
• All packets go through a base station
• Cards associate with a BSS (basic service set)
• Multiple BSSs can be linked into an Extended
Service Set (ESS)
• Handoff to new BSS in ESS is pretty quick
• Wandering around CMU
• Moving to new ESS is slower, may require readdressing
• Wandering from CMU to Pitt
• Ad Hoc mode
• Cards communicate directly.
• Perform some, but not all, of the AP functions
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802.11 Management Operations
•
•
•
•
Scanning
Association/Reassociation
Time synchronization
Power management
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Scanning & Joining
• Goal: find networks in the area
• Passive scanning
• No require transmission  saves power
• Move to each channel, and listen for Beacon frames
• Active scanning
• Requires transmission  saves time
• Move to each channel, and send Probe Request frames to solicit
Probe Responses from a network
• Joining a BSS
• Synchronization in TSF and frequency : Adopt PHY parameters :
The BSSID : WEP : Beacon Period : DTIM
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Association in 802.11
1: Association request
2: Association response
3: Data traffic
AP
Client
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Reassociation in 802.11
1: Reassociation request
3: Reassociation response
5: Send buffered frames
Client
6: Data traffic
New AP
2: verify
previous
association
Old AP
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4: send
buffered
frames
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Time Synchronization in 802.11
• Timing synchronization function (TSF)
• AP controls timing in infrastructure networks
• All stations maintain a local timer
• TSF keeps timer from all stations in sync
• Periodic Beacons convey timing
• Beacons are sent at well known intervals
• Timestamp from Beacons used to calibrate
local clocks
• Local TSF timer mitigates loss of Beacons
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Power Management in 802.11
• A station is in one of the three states
• Transmitter on
• Receiver on
• Both transmitter and receiver off (dozing)
• AP buffers packets for dozing stations
• AP announces which stations have frames
buffered in its Beacon frames
• Dozing stations wake up to listen to the beacons
• If there is data buffered for it, it sends a poll frame
to get the buffered data
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Outline
• Wireless systems and standards
• 802.11 Architecture
• 802.11 MAC
• Bluetooth
• WiMax
• UWB
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IEEE 802.11 Wireless MAC
• Support broadcast, multicast, and unicast
• Uses ACK and retransmission to achieve
reliability for unicast frames
• No ACK/retransmission for broadcast or
multicast frames
• Distributed and centralized MAC access
• Distributed Coordination Function (DCF)
• Point Coordination Function (PCF)
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IEEE 802.11 DCF
• DCF uses RTS-CTS exchange to avoid
hidden terminal problem
• Any node overhearing a CTS cannot transmit
for the duration of the transfer
• Uses ACK to provide reliability
• Collision avoidance
• Backoff intervals used to reduce collision
probability
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Backoff Interval
• When transmitting a packet, choose a
backoff interval in the range [0, CW]
• CW is contention window
• Count down the backoff interval when
medium is idle
• Count-down is suspended if medium becomes
busy
• Transmit when backoff interval reaches 0
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DCF Example
B1 = 25
B1 = 5
wait
data
data
B2 = 20
cw = 31
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wait
B2 = 15
B2 = 10
B1 and B2 are backoff intervals
at nodes 1 and 2
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Backoff Interval
• The time spent counting down backoff
intervals is a part of MAC overhead
• Important to choose CW appropriately
• large CW  large overhead
• small CW  may lead to many collisions (when
two nodes count down to 0 simultaneously)
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Backoff Interval (Cont.)
• Since the number of nodes attempting to
transmit simultaneously may change with
time, some mechanism to manage
contention is needed
• IEEE 802.11 DCF: contention window CW
is chosen dynamically depending on
collision occurrence
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Binary Exponential Backoff
in DCF
• When a node fails to receive CTS in
response to its RTS, it increases the
contention window
• CW is doubled (up to an upper bound)
• More collisions  longer waiting time to reduce
collision
• When a node successfully completes a data
transfer, it restores CW to CWmin
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802.11 Overhead
Random
backoff
RTS/CTS
Data Transmission/ACK
• Channel contention resolved using backoff
• Nodes choose random backoff interval from [0,
CW]
• Count down for this interval before transmission
• Backoff and (optional) RTS/CTS handshake
before transmission of data packet
• 802.11 has large room for improvement
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DCF Operation
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Discussion
• RTS/CTS/Data/ACK vs. Data/ACK
• Why/when is it useful?
• What is the right choice
• Simulation vs. reality?
• Understand Fig 4.
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Outline
• Wireless systems and standards
• 802.11 Architecture
• 802.11 MAC
• Bluetooth
• UWB
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Bluetooth basics
• Short-range, high-data-rate wireless link for
personal devices
• Originally intended to replace cables in a range
of applications
• e.g., Phone headsets, PC/PDA synchronization,
remote controls
• Operates in 2.4 GHz ISM band
• Same as 802.11
• Frequency Hopping Spread Spectrum across ~
80 channels
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Bluetooth Basics cont.
• Maximum data rate of up to 720 Kbps
• But, requires large packets (> 300 bytes)
• Class 1: Up to 100mW (20 dBm) transmit power,
~100m range
• Class 1 requires that devices adjust transmit power
dynamically to avoid interference with other devices
• Class 2: Up to 2.4 mW (4 dBm) transmit power
• Class 3: Up to 1 mW (0 dBm) transmit power
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Usage Models
• Wireless audio
• e.g., Wireless headset associated with a cell phone
• Requires guaranteed bandwidth between headset and base
• No need for packet retransmission in case of loss
• Cable replacement
• Replace physical serial cables with Bluetooth links
• Requires mapping of RS232 control signals to Bluetooth messages
• LAN access
• Allow wireless device to access a LAN through a Bluetooth connection
• Requires use of higher-level protocols on top of serial port (e.g., PPP)
• File transfer
• Transfer calendar information to/from PDA or cell phone
• Requires understanding of object format, naming scheme, etc.
Lots of competing demands for one radio spec!
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Protocol Architecture
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Piconet Architecture
• One master and up to 7 slave devices in each Piconet:
• Master controls transmission schedule of all devices in the Piconet
• Time Division Multiple Access (TDMA): Only one device transmits at a
time
• Frequency hopping used to avoid collisions with other Piconets
• 79 physical channels of 1 MHz each, hop between channels 1600 times a
sec
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Scatternets
• Combine multiple Piconets into a larger Scatternet
• Device may act as master in one Piconet and slave in another
• Each Piconet using different FH schedule to avoid interference
• Can extend the range of Bluetooth, can route across
Piconets
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Baseband Specification
• 79 1-MHz channels defined in the 2.4 GHz ISM band
• Gaussian FSK used as modulation, 115 kHz frequency deviation
• Frequency Hopping Spread Spectrum
• Each Piconet has its own FH schedule, defined by the master
• 1600 hops/sec, slot time 0.625 ms
• Time Division Duplexing
• Master transmits to slave in one time slot, slave to master in the
next
• TDMA used to share channel across multiple slave
devices
• Master determines which time slots each slave can occupy
• Allows slave devices to sleep during inactive slots
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Time slots
• Each time slot on a different frequency
• According to FH schedule
• Packets may contain ACK bit to indicate successful
reception in the previous time slot
• Depending on type of connection...
• e.g., Voice connections do not use ACK and retransmit
• Packets may span multiple slots – stay on same frequency
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Physical and Logical Links
• Bluetooth supports two types of physical links.
• Synchronous Connection Oriented (SCO):
• Slave assigned to two consecutive slots at regular intervals
• Just like TDMA...
• No use of retransmission ... why??
• Asynchronous Connectionless (ACL)
• Allows non-SCO slots to be used for “on demand” transmissions
• Slave can only reply if it was addressed in previous slot by master
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Discussion
• Nice points
• A number of interesting low power modes
• Device discovery
• Must synchronize FH schemes
• Burden on the searcher
• Some odd decisions
• Addressing
• Somewhat bulky application interfaces
• Not just simple byte-stream data transmission
• Rather, complete protocol stack to support voice, data, video,
file transfer, etc.
• Bluetooth operates at a higher level than 802.11 and 802.15.4
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Outline
• Wireless systems and standards
• 802.11 Architecture
• 802.11 MAC
• Bluetooth
• UWB
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What is Ultra-Wideband?
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FCC
• UWB == bandwidth > 25% (center
frequency)
• UWB radiator must not exceed 500uV/m @
3m for every 1MHz band (-41.3 dBm/Mhz)
• Normal part 15 does not specify a per MHz
• Limits UWB to very short range
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What is Ultra-Wideband?
• Directly modulate impulses
• Occupies several GHz of bandwidth
• Much simpler radio design
• Great spatial efficiency
• 100’s Mbps
• Short-range (10m)
• Current application  cable replacement
(e.g. wireless USB)
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Pulse Waveforms
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•
Sinusoidal Gaussian pulse
that conforms to more
limited spectrum range
Commonly used Gaussian pulse shapes
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UWB Modulation
• Pulse amplitude modulation
• Pulse position modulation
• Data Sequence {1 -1 1 1 -1}
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Discussion
• Pros
• Speed
• Less affected by multipath distortion than conventional
RF
• Can measure precise timing for signal arrival
• Time synchronized senders – using time of arrival
• Cons
• Range
• Antenna design
• Other
• Multi-carrier (OFDMA) vs. pulse modulation
• FCC
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