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15-441 Computer Networking Lecture 4 – 802.11 and more.

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Presentation on theme: "15-441 Computer Networking Lecture 4 – 802.11 and more."— Presentation transcript:

1 Computer Networking Lecture 4 – and more

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 Lecture 4: and more

3 Outline RF introduction Modulation Antennas and signal propagation
Equalization, diversity, channel coding Multiple access techniques Wireless systems and standards Lecture 4: and more

4 Outline Wireless systems and standards Bluetooth WiMax UWB
Architecture MAC Bluetooth WiMax UWB Lecture 4: and more

5 IEEE 802.11 Overview Adopted in 1997 Defines: MAC sublayer
MAC management protocols and services Physical (PHY) layers IR FHSS DSSS Lecture 4: and more

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 Lecture 4: and more

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. Lecture 4: and more

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 Lecture 4: and more

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 Lecture 4: and more

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 Lecture 4: and more

11 802.11 Management Operations
Scanning Association/Reassociation Time synchronization Power management Lecture 4: and more

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 Lecture 4: and more

13 Association in 802.11 1: Association request 2: Association response
AP 3: Data traffic Client Lecture 4: and more

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 Lecture 4: and more

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 Lecture 4: and more

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 Lecture 4: and more

17 Outline Wireless systems and standards Bluetooth WiMax UWB
Architecture MAC Bluetooth WiMax UWB Lecture 4: and more

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) Lecture 4: and more

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 Lecture 4: and more

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 Lecture 4: and more

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 Lecture 4: and more

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) Lecture 4: and more

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 Lecture 4: and more

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 Lecture 4: and more

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 Lecture 4: and more

26 DCF Operation Lecture 4: and more

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. Lecture 4: and more

28 Outline Wireless systems and standards Bluetooth UWB
Architecture MAC Bluetooth UWB Lecture 4: and more

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 Lecture 4: and more

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 Lecture 4: and more

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! Lecture 4: and more

32 Protocol Architecture
Lecture 4: and more

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 Lecture 4: and more

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 Lecture 4: and more

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 Lecture 4: and more

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 Lecture 4: and more

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 Lecture 4: and more

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 Lecture 4: and more

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 Lecture 4: and more

40 Outline Wireless systems and standards Bluetooth UWB
Architecture MAC Bluetooth UWB Lecture 4: and more

41 What is Ultra-Wideband?
Lecture 4: and more

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 Lecture 4: and more

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) Lecture 4: and more

44 Pulse Waveforms Sinusoidal Gaussian pulse that conforms to more limited spectrum range Commonly used Gaussian pulse shapes Lecture 4: and more

45 UWB Modulation Pulse amplitude modulation Pulse position modulation
Data Sequence { } Lecture 4: and more

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 Lecture 4: and more


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