Link Layer: Wireless Media Access Control

Link Layer:Wireless Media Access

Control

Y. Richard Yang

11/08/2012

2

Outline

Admin. and recap Wireless Media access control (MAC)

Slotted Aloha Hidden terminal Example: 802.11 Hidden terminal with 802.11 revisited

Admin.

Project check point I: next Tuesday

Feedback on topics to cover

3

4

Recap: Wireless Access Problem

Problem: single shared medium, hence if two transmissions overlap on all dimensions [time, space, frequency, and code], then it is a collision

Solution covered: Aloha (random access with collision recovery)

A

B

5

Recap: Slotted Aloha [Norm Abramson]

Time is divided into equal size slots (= pkt trans. time)

Node with new arriving pkt: transmit at beginning of next slot with probability p

If collision: retransmit pkt in future slots with probability p, until successful.

Success (S), Collision (C), Empty (E) slots

A

B

6

Recap: Slotted Aloha: Goodput vs. Offered Load Curve

S =

thro

ughput

= “

goodput

” (

succ

ess

rate

)

Define G = offered load = np0.5 1.0 1.5 2.0

Slotted Aloha

Goodput is optimal when offered load G (=p*n) = 1

Goodput is less than 1/e (~37%) at optimal

7

Recap: Slotted Aloha Dynamics

n backloggedeach transmits with prob. p

m-n: unbacklogged

each transmits with prob. pa

8

Recap: Slotted Aloha Dynamics

n: number of backlogged stations

0 m

successful transmission rate at

offered load np + (m-n)pa

new arrival rate:(m-n) pa

desirable stable point

undesirable stable point

Slotted Aloha can have an undesirable stable point

offered load = 1

- assume a total ofm stations- pa << p- success rate is thedeparture rate, the rate the backlog is reducing

dep.andarrivalrateofbackloggedstations

9

Summary of Problems of Slotted Aloha

Advantageso Simple, decentralized random access protocol

Issueso Low efficiency

o Only ~37% at optimal transmission rateo Even lower efficiency at non-optimal (fixed

p)o No rate allocation/fairness

10

Ethernet Fix for Efficiency

Introduce collision detection: instead of wasting the whole frame transmission time (a slot), we waste only the time needed to detect collision.

Introduce adaptive probability: reduce probability of trans. as # of collisions increases If more collisions => p is high => should reduce p

P

P: packet size, C: contention window

C C C

11

Ethernet Fix: Carrier-Sense Multiple Access /Collision Detection/Exponential Backoff

get a frame from upper layer;K := 0; n := 0; // K: control wait time; n: no. of collisionsrepeat: wait for K * 512 bit-time; while (network busy) wait; wait for 96 bit-time after detecting no signal; transmit and detect collision; if detect collision stop and transmit a 48-bit jam signal; n ++; m:= min(n, 10), where n is the number of collisions choose K randomly from {0, 1, 2, …, 2m-1}. if n < 16 goto repeat else give up else declare success

The Ethernet algorithm

Q: Does Ethernet alg work well in wireless?

Carrier sense

DetectCollision

AdaptProbability

12

Outline

Admin. and recap Media access control


13

The Hidden Terminal Problem

BA C

A is sending to B, but C cannot detect the transmission

Therefore C sends to B

In summary, A is “hidden” from C

DE

CSMA/CD + Hidden Terminals

14

get a frame from upper layer;K := 0; n := 0; // K: control wait time; n: no. of collisionsrepeat: wait for K * 512 bit-time; while (network busy) wait; wait for 96 bit-time after detecting no signal; transmit and detect collision; if detect collision stop and transmit a 48-bit jam signal; n ++; m:= min(n, 10), where n is the number of collisions choose K randomly from {0, 1, 2, …, 2m-1}. if n < 16 goto repeat else give up else declare success

Q: what is the outcome of CSMA/CD + hidden terminals,assume two senders with infinite backlog?

Hidden terminals => 0 goodput!

Hidden Terminals

Why cannot senders C and A detect collisions or potential collisions? Collision is spatially dependent

• C/A is at a different location than B Only receiver can detect a collision

happened or potential collisions

15A B C

Solution I: Receiver Notifies Collision Happened

Solution: receiver sends ACK to sender to indicate a collision happened or not If no ACK from receiver, sender assumes a

collision

16

17

Solution II: Receiver Signals Potential Collision

Receiver sends busy-tone

Used in CDPD (cellular digital packet data)

The base station sends a busy tone on the down link when receiving data

18

Solution III: Receiver Signals Potential Collision Using Virtual Carrier Sense/ACK

Short signaling packets (virtual carrier sense) Sender: RTS (request to send) Receiver: CTS (clear to send)

contain sender address, receiver address, transmission duration, called network allocation vector (NAV)

A node keeps quiet for NAV in CTS

BA CRTS

CTSCTS D

DATA

19

Comparisons: Media Access Techniques Handling Hidden Terminals

Slotted Alohavery simple to implement but low

efficiency

CSMA/CD (Ethernet alg.) hidden terminals can cause 0

goodput

CSMA/CD + ACK simple to implement low efficiency

20

Comparisons: Media Access Techniques Handling Hidden Terminals Busy tone

simple to implement but need a channel for busy signal

Virtual carrier sensing (RTS/CTS) higher efficiency when a collision occurs

(not waste the whole duration) But energy consumption can be high

because a node needs to monitor the environment all the time

• Idle:receive:send: 1:1.05:1.4 [Stemm and Katz]; Digitan 2 Mbps WLAN 1:2:2.5

many measurements show that overhead hurts performance

21

Outline



22

802.11: Infrastructure Mode

Architecture similar to cellular networks station (STA)

• terminal with access mechanisms to the wireless medium and radio contact to the access point

access point (AP)• station integrated into the

wireless LAN and the distribution system

basic service set (BSS)• group of stations using the same

AP portal

• bridge to other (wired) networks distribution system

• interconnection network to form one logical network (EES: Extended Service Set) based on several BSS

Distribution System

Portal

802.x LAN

Access Point

802.11 LAN

BSS2

802.11 LAN

BSS1

Access Point

STA1

STA2 STA3

ESS

23

The IEEE 802.11 Family

Protocol

Release Data

Freq. Rate (max)

Modulation Range (indoor)

Legacy 1997 2.4 GHz 2Mbps DSSS/FHSS ~20 m

802.11a

1999 5 GHz 54 Mbps

OFDM ~35 m

802.11b

1999 2.4 GHz 11 Mbps

DSSS ~38 m

802.11g

2003 2.4 GHz 54 Mbps

OFDM/DSSS ~38 m

802.11n

2009 2.4/5 GHz

540 Mbps

OFDM ~70 m

24

802.11 – MAC Traffic Services Asynchronous Data Service (ADS)

o Objective: exchange data packets based on distributed random access

o Approach: Implement ADS using distributed coordinate function (DCF):o DCF CSMA/CA (mandatory)

- collision avoidance via randomized “back-off“ -ACK packet for acknowledgements/detection

- DCF w/ RTS/CTS (optional)-additional virtual “carrier sensing

Time-Bounded Service (TBS)o Objective: Exchange of bounded delay serviceo Approach: implemented TBS using point (access point)

coordinated function (PCF)

25

802.11 CSMA/CA

CSMA: Listen before transmit Collision avoidance

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

26

802.11 Backoff

IEEE 802.11 contention window CW is adapted dynamically depending on collision occurrence after each collision, CW is doubled thus CW varies from CWmin to CWmax

802.11b 802.11a 802.11g

aSlotTime 20 usec 9 usec 20 usec (mixed);

9 usec(g-only)

aCWmin 31 slots 15 slots 15 slots

27

Congestion Avoidance: Example

data

waitB1 = 5

B2 = 15

B1 = 25

B2 = 20

data

wait

B1 and B2 are backoff intervalsat nodes 1 and 2cw = 31

B2 = 10busy

busy

28

802.11 – RTS/CTS + ACK

Sender sends RTS with NAV (Network allocation Vector, i.e. reservation parameter that determines amount of time the data packet needs the medium)

Receiver acknowledges via CTS (if ready to receive) CTS reserves channel for sender, notifying possibly hidden stations

Sender can now send data at once, acknowledgement via ACK Other stations store NAV distributed via RTS and CTS

t

SIFS

DIFS

data

ACK

defer access

otherstations

receiver

senderdata

DIFS

new contention

RTS

CTSSIFS SIFS

NAV (RTS)NAV (CTS)

29

Integrating PCF and DCF

Using different inter frame spacing values to implement priority SIFS (Short Inter Frame Spacing); 10 us in 802.11b

highest priority, for ACK, CTS, polling response PIFS (PCF IFS); 30 us in 802.11b

medium priority, for time-bounded service using PCF DIFS (DCF, Distributed Coordination Function IFS); 50 us in

802.11b lowest priority, for asynchronous data service

direct access if medium is free DIFS

t

medium busy SIFSPIFS

DIFSDIFS

next framecontention

30

802.11b 802.11a 802.11g

aSIFSTime 10 usec 16 usec 10 usec

aSlotTime 20 usec 9 usec 20 usec (mixed);9 usec (g

only)

aDIFTime (2xSlot+SIFS)

50 usec 34 usec 50 usec; 28 usec

802.11 – Inter Frame Spacing

31

802.11: PCF for Polling (Infrastructure Mode)

tNAV

polledwirelessstations

point coordinator

NAV

PIFSD

USIFS

SIFSD

contentionperiod

contention free periodmediumbusy

D: downstream poll, or data from point coordinatorU: data from polled wireless station

802.11b Frame Format

32

Sync SFD PLCP header CRC

2

Preamble (192 usec; or optional 96 short version) - Sync: alternating 0s and 1s (DSSS 128 bits) - SFD: Start Frame delimiter: 0000 1100 1011 1101

PLCH (Phsical Layer Convergence Procedure) Header - payload length - signaling field: the rate info. - CRC: 16 bit protection of header

MAC Data

preamble

33

802.11 – MAC Format

Types control frames, management frames, data frames

Sequence numbers important against duplicated frames due to lost ACKs

Addresses receiver, transmitter (physical), BSS identifier, sender (logical)

Miscellaneous sending time, checksum, frame control, data

FrameControl

Duration/ID

Address1

Address2

Address3

Sequencenumber

Address4

Data CRC

2 2 6 6 6 62 40-2312bytes

Protocolversion

Type SubtypeToDS

MoreFrag

RetryPowerMgmt

MoreData

WEP

2 2 4 1

FromDS

1

Order

bits 1 1 1 1 1 1

34

Example: 802.11b/ACK Timing Suppose TCP with 1460 bytes data payload

TCP data frame size (not including preamble)• 1536 bytes (1460 + 40 TCP header + 36 802.11 header)

TCP ACK data frame size (not including preamble)• 76 bytes

802.11b ACK frame size 14 bytes Suppose 802.11b at the highest rate

8 bits per symbol 1.375 Msps

35http://www.andrews.edu/~swensen/Wifi%20Throughput.pdf

802.11b/ACK Timing (1460B data)

TCP Data TCP Ack

36


TCP Data TCP Ack

DIFS (us) 50 50

37


TCP Data TCP Ack

DIFS (us) 50 50

802.11 Data (us) 192 + 1536 / 1.375= 1,310

192 + 76 / 1.375= 248

38


TCP Data TCP Ack

DIFS (us) 50 50

802.11 Data (us) 192 + 1536 / 1.375= 1,310

192 + 76 / 1.375= 248

SIFS (us) 10 10

39


TCP Data TCP Ack

DIFS (us) 50 50

802.11 Data (us) 192 + 1536 / 1.375= 1,310

192 + 76 / 1.375= 248

SIFS (us) 10 10

802.11 ACK (us) 192 + 14 / 1.375=203 203

40


TCP Data TCP Ack

DIFS (us) 50 50

802.11 Data (us) 192 + 1536 / 1.375= 1,310

192 + 76 / 1.375= 248

SIFS (us) 10 10

802.11 ACK (us) 192 + 14 / 1.375=203 203

Frame total (us) 1,573 511

Transactions total (us)

2,084

41

Example: 802.11g/ACK Timing Suppose 802.11g at the highest rate

(54Mbps) symbol duration: 4 usec; 216 bits/symbol 20 usec preamble; 6 usec “signal extension

time” at the end of each frame Suppose TCP with 1460 bytes data

payload data: 57 (=1536*8/216) symbols; ACK: 3

(=76*8/216) symbols 802.11b ACK frame size 14 bytes

1 symbol


802.11g Basic Timing (1460B data)

TCP Data TCP Ack

DIFS (us) 28 28

802.11 Data (us) 20 + 57 *4 + 6= 254

20 + 3 * 4 + 6= 38

SIFS (us) 10 10

802.11 ACK (us) 20 + 1 * 4 + 6=30 30

Frame total (us) 322 106


428

43

Example: TCP/802.11g + CTS

RTS/CTS uses 802.11b DIFS (50 usec) and long preamble (192 usec)

RTS/CTS uses 802.11b frame coding 20 bytes RTS 14 bytes CTS


802.11g + CTS Timing (1460B data)

TCP Data TCP Ack

DIFS (us) 28 -> 50 28 -> 50

CTS 192 + 14/1.375= 203 = 203

SIFS 10 10

802.11 Data (us) 20 + 57 *4 + 6= 254

20 + 3 * 4 + 6= 38

SIFS (us) 10 10

802.11 ACK (us) 20 + 1 * 4 + 6=30 30

Frame total (us) 322 106


428 -> 898

45

Summary

46

TechnologyTransactions

per secMbps of

TCPRelative to

802.11b

11b, 11Mbps 479 5.6 1

11a, 54 Mbps 2,336 27.3 4.9

11g, no CTS/RTS 2,336 27.3 4.9

11g, CTS 1,113 13.0 2.3

11g, RTS/CTS 750 8.8 1.6

47

Outline



Hidden Terminal with 802.11

Collision!

Alice Bob

No ACK

Hidden Terminal with 802.11

Alice Bob

One more CollisionRetransmission

802.11 Setting: Two Collisions of Two Packets

Pa

Pb

Pa

Pb

ZigZag Decoding

Exploits 802.11’s behavior Retransmissions

Same packets collide again Senders use random jitters

Collisions start with interference-free bits

∆1 ∆2Pa

Pb

Pa

Pb

Interference-free Bits

How Does ZigZag Work?

∆1 ∆2

Find a chunk that is interference-free in one collisions and has interference in the other

11

∆1 ≠∆2

Decode and subtract from the other collision

11

∆2

11

22

11

∆1



∆1 ≠∆2


∆211

2222∆1


33


∆1 ≠∆2


∆211

22 44∆1


33 33


∆1 ≠∆2


∆211

22 4444∆1


33 55


∆1 ≠∆2


∆211

66∆1


33 55 55

22 44


∆1 ≠∆2


∆211

6666∆1


22 44

33 55 77


∆1 ≠∆2


∆2

11

66 88∆1


22 44

33 55 7777


∆1 ≠∆2


Delivered 2 packets in 2 timeslotsAs efficient as if the packets did not collide

ZigZag Technical Issues

Collision detection Chunk subtraction Deployment

60

∆2

11

22

11

∆1

61

Outline



• Overall idea• Technical issues

– Collision detection

62

Collision Detection: How does the AP know it is a Collision and Where the Second Packet Starts?

Time

∆

Detecting Collisions and the Value of ∆

Time

AP received signal

Packets start with known preamble

AP correlates known preamble

with signalCorrelati

onTime

Correlate

∆

Matching Collision

Given (P1 + P2()) and (P1’, P2’(’)), how to determine that P1 = P’ and P2 = P2’ Determine offset first Correlation of P2() and P2’(’)

Pa

Pb

P’aP’b

65

Outline




– Collision detection– Subtracting chunks

How Does the AP Subtract the Signal?

• Channel’s attenuation or phase may change between collisions

• Can’t simply subtract a chunk across collisions

Alice’s signal in first collision

Alice’s signal in second collision

1122

1122

Subtracting a Chunk11

2211

22

What if AP Makes a Mistake?

∆1 ∆211 11

2222

Bad News: Errors can propagate

33

Can we deal with these errors?


∆1 ∆2


Good News: Temporal DiversityA bit is unlikely to be affected by noise in both collisions

Get two independent decodings

Errors propagate differently in the two decodings

For each bit, AP picks the decoding that has a higher PHY confidence

Which decoded value should the AP pick?

∆1 ∆2

11 11

2222

33

AP Decodes Backwards as well as Forwards

72

Outline




– Collision detection– Subtracting chunks– Deployment

Acknowledgement

Use as much synchronous acknowledgement as possible for backward compatibility

Implementation

• USRP Hardware• GNURadio software• Carrier Freq: 2.4-2.48GHz• BPSK modulation

USRPsTestbed

• 10% HT,

• 10% partial HT,

• 80% perfectly sense each other

802.11a

Throughput Comparison

802.11

Throughput

CD

F o

f co

ncur

rent

flo

w p

airs

Hidden Terminals

Partial Hidden Terminals

Perfectly Sense

Throughput Comparison

ZigZag

Throughput

CD

F o

f co

ncur

rent

flo

w p

airs

802.11

Hidden Terminals get high throughput

ZigZag Exploits Capture Effect

Capture EffectSubtract Alice and combine Bob’s packet

across collisions to correct errors

∆1 ∆2Pa1

Pb

Pa2

Pb

3 packets in 2 time slots better than no collisions

Extension of Zigzag Decoding

Focuses on single channel collisions, but many real time hidden terminals are due to overlapping channel collisions

Need same packet pair

Need timeshift to bootstrap decoding

79

∆1 ∆2Pa

Pb

Pa

Pb

8080

802.11 Background

Using 802.11g as an example Each channel has 4 groups of

subcarriers: C1 consists of G1, G2, G3, G4; C2 consists of G2, G3, G4, G5

C1 and C2 are overlapping adjacent channels;

C1 and C3 are overlapping non-adjacent channels

Bits are assigned to subcarriers

• E.g. bit sequences Ai is assigned to subcarrier Gi, i=1,2,3,4

Subcarrier GroupG1 G2 G3 G4

A1 A2 A3 A4

81

802.11g Overlapping-Channel Collision

Bob

APa on channel Ca

Collision!

Alice

APb on channel Cb

Collision!

Chuck

82

802.11g Overlapping-Channel Collision

Bob

APa on channel Ca

More Collision!

Alice

APb on channel Cb

More Collision!

Chuck

Retransmission

8383

Remap Basic Idea: Structured Permutation

Subcarrier Group

G1 G2 G3 G4

A1 A2 A3 A4Mapping π1




84

How Permutation Helps Non-matching collisions on adjacent

channels C1 and C2Subcarrier Group

G1 G2 G3 G4

A1 A2 A3 A41st transmission

2nd transmission A4 A3 A2 A1

A2 A1 A4 A33rd transmission

A3 A4 A1 A24th transmission

8585

How Permutation Helps (cont’d) Non-matching collisions on non-adjacent

channels C1 and C3

Subcarrier Group

G1 G2 G3 G4

A1 A2 A3 A41st transmission

2nd transmission A4 A3 A2 A1

86

Remap Basic Idea: Matching-collision setting

Collision!

Alice Bob

Collision!

APa on channel Ca

APb on channel Cb

Matching collisions on adjacent channels

87

Remap for Matching Collisions collisions at adjacent channels C1 and C2 : a time

and frequency view

Pb

∆1∆2

A1 A2

A3

A4

S1 S2Sn

Time

Freq

Pa

B5

B2

B3

B4

A4 A3

A2

A1

S1 S2Sn

B2

B5

B4

B3

P′b

P′a

G1

G3

G2

G5

G4

G2

1

59

13

410

14

3

711

2

68

12

Backup Slides

88

89

IEEE 802.11 Requirements

Design for small coverage (e.g. office, home)

Low/no mobility High data-rate applications Ability to integrate real time applications

and non-real-time applications (implications?)

Use un-licensed spectrum

90

802.11a Modulation

Use OFDM to divide each physical channel (20 MHz) into 52 subcarriers (20M/64=312.5 KHz each) 48 data, 4 pilot

Adaptive modulation BPSK: 6, 9 Mbps QPSK: 12, 18 Mbps 16-QAM: 24, 36 Mbps 64-QAM: 48, 54 Mbps

Link Layer: Wireless Media Access Control

Documents

time n

collision c

jam signal n

collision recoveryab

probability pif collision

frame transmission time

dimensions time

probability of