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802.11 Wireless Networks (MAC)
Kate Ching-Ju Lin (林靖茹) Academia Sinica
2016.03.18 CSIE, NTU
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Reference
1. A Technical Tutorial on the IEEE 802.11 Protocol By Pablo Brenner online: http://www.sss-mag.com/pdf/802_11tut.pdf
2. IEEE 802.11 Tutorial By Mustafa Ergen online: http://wow.eecs.berkeley.edu/ergen/docs/ieee.pdf
3. 802.11 Wireless Networks: The Definitive Guide By Matthew Gast
4. 802.11ac: A Survival Guide By Matthew Gast online: http://chimera.labs.oreilly.com/books/1234000001739
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We will cover … § Medium Access Control
– Infrastructure mode vs. Ad Hoc mode – DCF vs. PCF – CSMA/CA with exponential backoff – Hidden terminal
§ Physical Layer Basics – Packet Detection – OFDM – Synchronization
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We will cover … § Medium Access Control
– Infrastructure mode vs. Ad Hoc mode – DCF vs. PCF – CSMA/CA with exponential backoff – Hidden terminal
§ Physical Layer Basics – Packet Detection – OFDM – Synchronization – Modulation and rate adaptation
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Infrastructure Mode
§ Access point (AP) announces beacons periodically § Each station (STA) connects to an AP § An AP and its stations form a basic service set
(BSS)
AP
STA
BSS
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Infrastructure Mode
§ Several APs (BSSs) could form an extended service set (ESS)
§ A roaming user can move from one BSS to another within the ESS
AP
STA
APESS
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Infrastructure Mode
§ Issues – Inter-BSS interference: channel assignment – Load balancing: user association
AP
STA
APESS
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Ad Hoc Mode
§ Clients form a peer-to-peer network without a centralized coordinator
§ Clients communicate with each other via multi-hop routing
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We will cover … § Medium Access Control
– Infrastructure mode vs. Ad Hoc mode – DCF vs. PCF – CSMA/CA with exponential backoff – Hidden terminal
§ Physical Layer Basics – Packet Detection – OFDM – Synchronization
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Two Operational Modes
§ Distributed coordination function (DCF) – Stations contend for transmission opportunities in a
distributed way
§ Point coordination function (PCF) – AP sends poll frames to trigger transmissions
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DCF
§ Start contention after the channel keeps idle for DIFS § AP responds ACK if the frame passes the CRC check § Retransmit the frame until the retry limit is reached
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Prioritized Interframe Spacing
• SIFS > PIFS > DIFS • SIFS (Short interframe space): control frames,
e.g., ACK and CTS • PIFS (PCF interframe space): CF-Poll • DIFS (DCF interframe space): data frame
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Frame Format
§ Overhead of a 1500 byte packet (ignore contention, assume all bits sent at 1Mbps) = 1 – TData / (TDIFS + TPLCP + TMAC + TData + TSIFS + TACK) = 1 – (1500*8)/(50[DIFS] + 34*8 +1500*8 + 10[SIFS] + 14*8)
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Wireless Communications Breeze Wireless Communications Ltd. Atidim Technological Park, Bldg. 1, P.O.Box 13139, Tel Aviv 61131, ISRAEL Tel: 972-3-6456262http://www.breezecom.com Fax: 972-3-6456290
All 802.11 frames are composed of the following components:
Preamble PLCP Header MAC Data CRC
Preamble
This is PHY dependent, and includes:
n Synch: An 80-bit sequence of alternating zeros and ones, which is used by the PHY circuitry toselect the appropriate antenna (if diversity is used), and to reach steady-state frequency offsetcorrection and synchronization with the received packet timing.
n SFD: A Start Frame delimiter which consists of the 16-bit binary pattern 0000 1100 1011 1101,which is used to define frame timing.
PLCP Header
The PLCP Header is always transmitted at 1 Mbit/s and contains Logical information used by thePHY Layer to decode the frame. It consists of:
n PLCP_PDU Length Word: which represents the number of bytes contained in the packet. This isuseful for the PHY to correctly detect the end of packet.
n PLCP Signaling Field: which currently contains only the rate information, encoded in 0.5 MBpsincrements from 1 Mbit/s to 4.5 Mbit/s.
n Header Error Check Field: Which is a 16 Bit CRC error detection field.
MAC Data
The following figure shows the general MAC Frame Format. Part of the fields are only present inpart of the frames as described later.
Figure 5: MAC Frame Format
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Wireless Communications Breeze Wireless Communications Ltd. Atidim Technological Park, Bldg. 1, P.O.Box 13139, Tel Aviv 61131, ISRAEL Tel: 972-3-6456262http://www.breezecom.com Fax: 972-3-6456290
All 802.11 frames are composed of the following components:
Preamble PLCP Header MAC Data CRC
Preamble
This is PHY dependent, and includes:
n Synch: An 80-bit sequence of alternating zeros and ones, which is used by the PHY circuitry toselect the appropriate antenna (if diversity is used), and to reach steady-state frequency offsetcorrection and synchronization with the received packet timing.
n SFD: A Start Frame delimiter which consists of the 16-bit binary pattern 0000 1100 1011 1101,which is used to define frame timing.
PLCP Header
The PLCP Header is always transmitted at 1 Mbit/s and contains Logical information used by thePHY Layer to decode the frame. It consists of:
n PLCP_PDU Length Word: which represents the number of bytes contained in the packet. This isuseful for the PHY to correctly detect the end of packet.
n PLCP Signaling Field: which currently contains only the rate information, encoded in 0.5 MBpsincrements from 1 Mbit/s to 4.5 Mbit/s.
n Header Error Check Field: Which is a 16 Bit CRC error detection field.
MAC Data
The following figure shows the general MAC Frame Format. Part of the fields are only present inpart of the frames as described later.
Figure 5: MAC Frame Format
Data
ACK
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Fragmentation and Aggregation
§ Large frame – Reduced overhead, but less reliable – Packet delivery ratio of an N-bit packet = (1-BER)N
§ Fragmentation – Break a frame into into small pieces so that
interference only affects small fragments
§ Aggregation – Aggregate multiple small frames in order to reduce
the overhead
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We will cover … § Medium Access Control
– Infrastructure mode vs. Ad Hoc mode – DCF vs. PCF – CSMA/CA with exponential backoff – Hidden terminal
§ Physical Layer Basics – Packet Detection – OFDM – Synchronization – Modulation and rate adaptation (week 5: 03/17)
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ALOHA
§ First distributed access control (about 1970) § Transmit immediately whenever a node has
data to send § Do not sense the medium before transmission
Slotted ALOHA Original ALOHA
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CSMA/CA
§ Carrier sense multiple access with collision avoidance
§ STAs listen to the channel before transmission
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Exponential Backoff 1. Each STA maintains a contention window
– Initialized to CWmin = 32
2. Randomly pick a number, say k, between [0,CW-1]
3. Count down from k when the channel becomes idle
4. Start transmission when k = 0 if the channel is still idle
5. Double CW for every unsuccessful transmission, up to CWmax (1024)
Q: When will collisions occur?
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Theoretical Performance of DCF
Markov Chain model for the backoff window size
G.Bianchi,"PerformanceanalysisoftheIEEE802.11distributedcoordinaBonfuncBon,"SelectedAreasinCommunicaBons,IEEEJournalon18,no.3(2000):535-547
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We will cover … § Medium Access Control
– Infrastructure mode vs. Ad Hoc mode – DCF vs. PCF – CSMA/CA with exponential backoff – Hidden terminal
§ Physical Layer Basics – Packet Detection – OFDM – Synchronization – Modulation and rate adaptation (week 5: 03/17)
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Hidden Terminal
§ Two nodes hidden to each other transmit at the same time, leading to collision
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802.11’s Solution: RTS/CTS
§ Tx1 sends RTS whenever it wins contention § Rx broadcasts CTS § Nodes that receive CTS defer their transmissions
Rx
Tx1
Tx2
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802.11’s Solution: RTS/CTS
§ Usually disabled in practice due to its expensive overhead
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Recent Solutions to Hidden Terminals § Embrace collisions and try to decode collisions
– ZigZag decoding • S. Gollakota and D. Katabi
ZigZag decoding: combating hidden terminals in wireless networks ACM SIGCOMM, 2008
– Rateless code • A. Gudipati and S. Katti
Strider: automatic rate adaptation and collision handling ACM SIGCOMM, 2011
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Other Issues
§ Performance anomaly – M. Heusse, et al., "Performance anomaly of 802.11b," IEEE
INFOCOM, 2003
§ Expensive overhead due to increasing data rates – K. Tan, et al., "Fine-grained channel access in wireless
LAN," ACM SIGCOMM, 2011
– S. Sen, et al., “No time to countdown: migrating backoff to the frequency domain,” ACM MobiCom, 2011
§ Flexible channelization – S. Rayanchu, et al., ”FLUID: improving throughputs in
enterprise wireless LANs through flexible channelization,“ ACM MOBICOM, 2012
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rij=54 Mb/s ruv=6 Mb/s
t
p/b54 p/b6
b54=36.2 Mb/s when l54 sends alone c54=4.14 Mb/s as contending with l6
b6=5.4 Mb/s when l6 sends alone c6=4.37 Mb/s as contending with l54
Performance Anomaly
Channel is almost occupied by low-rate links
è Everyone gets a similar throughput, regardless of its bit-rate
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Summary § Nice properties of WiFi
– Distributed random access – No coordination – Ensuring fairness
§ Common issues – Expensive overhead – Collisions – No QoS guarantee
Every protocol balances the trade-off between performance and overhead
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Quiz § Compare the overhead of 1) enabling RTS/CTS and
2) disabling RTS/CTS for – A 1500-byte packet sent at 6mbps – A 1500-byte packet sent at 54mbps
§ Packet format in 802.11a (base rate = 6mbps) – Assume that the average backoff value is 16 slots – Time slot = 9 us – DIFS = 34us, SIFS = 16us – PLCP header: 13 symbols = 52us – RTS: 20 bytes – CTS: 20 bytes – ACK: 14 bytes – MAC header: 34 bytes