802.11 Wireless Networks (MAC)

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802.11 Wireless Networks (MAC)

Kate Ching-Ju Lin (林靖茹) Academia Sinica

2015.04.10 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 …

–  Modulation and rate adaptation (week 5: 03/17)

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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

AP ESS

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Infrastructure Mode

§  Issues

–  Inter-BSS interference: channel assignment –  Load balancing: user association

AP

STA

AP ESS

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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 …

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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 – T_Data / (T_DIFS + T_PLCP + T_MAC + T_Data + T_SIFS+ T_ACK)

= 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: 972-3-6456262

http://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 to select the appropriate antenna (if diversity is used), and to reach steady-state frequency offset correction 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 the PHY Layer to decode the frame. It consists of:

n PLCP_PDU Length Word: which represents the number of bytes contained in the packet. This is useful 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 MBps increments 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 in part of the frames as described later.

Figure 5: MAC Frame Format

Page 11

Wireless Communications Breeze Wireless Communications Ltd.

Atidim Technological Park, Bldg. 1, P.O.Box 13139, Tel Aviv 61131, ISRAEL Tel: 972-3-6456262

http://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 to select the appropriate antenna (if diversity is used), and to reach steady-state frequency offset correction 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 the PHY Layer to decode the frame. It consists of:

n PLCP_PDU Length Word: which represents the number of bytes contained in the packet. This is useful 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 MBps increments 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 in part 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 …

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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 CW_min = 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 CW_max (1024)

Q: When will collisions occur?

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Theoretical Performance of DCF

Markov Chain model for the backoff window size

G. Bianchi, "Performance analysis of the IEEE 802.11 distributed coordinaBon funcBon,"

Selected Areas in CommunicaBons, IEEE Journal on 18, no. 3 (2000): 535-‐547

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We will cover …

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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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r_ij=54 Mb/s

r_uv=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