1. Computer Networking
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2. Readings
Performance analysis of the IEEE distributed coordination function, JSAC, Mar 2000
Ultra-Wideband Technology for Short- and Medium-range Wireless Communication, Foerster, Green, Somayazulu, Leeper, Intel Technology Journal, Q
"The Bluetooth radio system," J. C. Haartsen, IEEE Pers. Commun. Mag., pp , Feb
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3. Outline RF introduction Modulation Antennas and signal propagation
Equalization, diversity, channel coding
Multiple access techniques
Wireless systems and standards
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4. Outline Wireless systems and standards Bluetooth WiMax UWB
Architecture
MAC
Bluetooth
WiMax
UWB
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5. 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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6. particulars
802.11b (WiFi)
Frequency: 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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7. 802.11 details Fragmentation Preamble Control frames Management frames
can fragment large packets (this is separate from IP fragmentation).
Preamble
72 1Mbps, 48 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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8. Overview, 802.11 Architecture
ESS
Existing Wired LAN AP AP
STA
STA
STA
STA
BSS
BSS
Infrastructure Network
STA
STA
Ad Hoc Network
Ad Hoc Network
BSS
BSS
STA
STA
BSS: Basic Service Set
ESS: Extended Service Set
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9. 802.11 modes Infrastructure mode Ad Hoc 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 re-addressing
Wandering from CMU to Pitt
Ad Hoc mode
Cards communicate directly.
Perform some, but not all, of the AP functions
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10. Components Station BSS - Basic Service Set ESS - Extended Service Set
IBSS : Infrastructure BSS
ESS - Extended Service Set
A set of infrastructure BSSs.
Connection of APs
Tracking of mobility
DS – Distribution System
AP communicates with another
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11. 802.11 Management Operations
Scanning
Association/Reassociation
Time synchronization
Power management
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12. 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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13. Association in 802.11 1: Association request 2: Association responseAP
3: Data traffic
Client
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14. Reassociation in 802.11 1: Reassociation request
New AP
3: Reassociation response
5: Send buffered frames
Client
2: verify previous association
6: Data traffic
Old AP
4: send
buffered
frames
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15. 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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16. 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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17. Outline Wireless systems and standards Bluetooth WiMax UWB
Architecture
MAC
Bluetooth
WiMax
UWB
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18. 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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19. IEEE 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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20. 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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21. DCF Example B1 = 25 B2 = 20 B1 = 5 B2 = 15 data wait data wait B2 = 10
B1 and B2 are backoff intervals
at nodes 1 and 2
cw = 31
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22. 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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23. Backoff Interval (Cont.)
Since the number of nodes attempting to transmit simultaneously may change with time, some mechanism to manage contention is needed
IEEE DCF: contention window CW is chosen dynamically depending on collision occurrence
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24. 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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25. Data Transmission/ACK
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
has large room for improvement
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26. DCF Operation
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27. Discussion RTS/CTS/Data/ACK vs. Data/ACK Simulation vs. reality?
Why/when is it useful?
What is the right choice
Simulation vs. reality?
Understand Fig 4.
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28. Outline Wireless systems and standards Bluetooth UWB
Architecture
MAC
Bluetooth
UWB
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29. 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
Frequency Hopping Spread Spectrum across ~ 80 channels
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30. 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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31. Usage Models Wireless audio Cable replacement LAN access File transfer
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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32. Protocol Architecture
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33. 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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34. 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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35. 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 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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36. 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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37. 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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38. Packet Formats Bluetooth supports 14 different payload formats!
Different formats for control, voice, and data packets
Frames can span 1, 3, or 5 slots
Different levels of error coding: No coding, 1/3, or 2/3 FEC
What is the maximum bandwidth that Bluetooth can achieve?
Counting only application payload bytes, no CRC or FEC
5-slot packet, no protection: 341 payload bytes
Total time = 5 * (0.625) ms = ms
But ... need to count an extra slot from the master for ACK!
Total bandwidth is therefore 341 bytes / (6 * ms) = 721 kbps
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39. Discussion Nice points Some odd decisions
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 and
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40. Outline Wireless systems and standards Bluetooth UWB
Architecture
MAC
Bluetooth
UWB
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41. What is Ultra-Wideband?
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42. FCC UWB == bandwidth > 25% (center frequency)
UWB radiator must not exceed 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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43. 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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44. Pulse Waveforms
Sinusoidal Gaussian pulse that conforms to more limited spectrum range
Commonly used Gaussian pulse shapes
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45. UWB Modulation Pulse amplitude modulation Pulse position modulation
Data Sequence { }
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46. Discussion Pros Cons Other 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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