link layer wireless media access control (mac)...

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

ughp

ut =

“go

odpu

t”

(

succ

ess

rate

)

Define G = offered load = np 0.5 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 backlogged each 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 of m stations -  pa << p -  success rate is the departure rate, the rate the backlog is reducing

dep. and arrival rate of backlogged stations

9

Summary of Problems of Slotted Aloha ❒  Advantages

o  Simple, decentralized random access protocol

q  Issues o  Low efficiency

o  Only ~37% at optimal transmission rate o  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 collisions repeat: 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

Detect Collision

Adapt Probability

12

Outline

❒ Admin. and recap ❒ Media access control


13

The Hidden Terminal Problem

B A C

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

❒  Therefore C sends to B

❒  In summary, A is “hidden” from C

D E

CSMA/CD + Hidden Terminals

14

get a frame from upper layer; K := 0; n := 0; // K: control wait time; n: no. of collisions repeat: 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

15 A 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

B A C RTS

CTS CTS D

DATA

19

Comparisons: Media Access Techniques Handling Hidden Terminals

❒ Slotted Aloha ❍ very 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 q  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

q  Time-Bounded Service (TBS) o  Objective: Exchange of bounded delay service o  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

wait B1 = 5

B2 = 15

B1 = 25

B2 = 20

data

wait

B1 and B2 are backoff intervals at nodes 1 and 2 cw = 31

B2 = 10 busy

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

other stations

receiver

sender data

DIFS

new contention

RTS

CTS SIFS 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 SIFS PIFS DIFS DIFS

next frame contention

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)

t NAV

polled wireless stations

point coordinator

NAV

PIFS D

U SIFS

SIFS D

contention period

contention free period medium busy

D: downstream poll, or data from point coordinator U: 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

Frame Control

Duration/ ID

Address 1

Address 2

Address 3

Sequence number

Address 4 Data CRC

2 2 6 6 6 6 2 4 0-2312 bytes

Protocol version Type Subtype To

DS More Frag Retry Power

Mgmt More Data WEP

2 2 4 1 From DS

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

35 http://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



= 1,310 192 + 76 / 1.375 = 248

SIFS (us) 10 10

39



= 1,310 192 + 76 / 1.375 = 248

SIFS (us) 10 10 802.11 ACK (us) 192 + 14 / 1.375

=203 203

40



= 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 Transactions total (us)

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 Transactions total (us)

428 -> 898

45

Summary

46

Technology Transac.ons

per sec Mbps of TCP

Rela.ve 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 Collision Retransmission

802.11 Setting: Two Collisions of Two Packets

Pa Pb

Pa Pb

ZigZag Decoding Exploits 802.11’s behavior ❒  Retransmissions

àSame packets collide again ❒ Senders use random jitters

à Collisions start with interference-free bits

∆1 ∆2 Pa 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

1

∆1 ≠∆2

Decode and subtract from the other collision

1

∆2 1

2

1 ∆1



∆1 ≠∆2


∆2 1

2 2 ∆1


3


∆1 ≠∆2


∆2 1

2 4 ∆1


3 3


∆1 ≠∆2


∆2 1

2 4 4 ∆1


3 5


∆1 ≠∆2


∆2 1

6 ∆1


3 5 5

2 4


∆1 ≠∆2


∆2 1

6 6 ∆1


2 4

3 5 7


∆1 ≠∆2


∆2 1

6 8 ∆1


2 4

3 5 7 7


∆1 ≠∆2

Decode and subtract from the other collision Delivered 2 packets in 2 timeslots

As efficient as if the packets did not collide

ZigZag Technical Issues

❒  Collision detection ❒  Chunk subtraction ❒ Deployment

60

∆2 1

2

1 ∆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 signal

Correlation

Time

Correlate

∆

Matching Collision

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

Pa Pb

P’a P’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

1 2

1 2

Subtracting a Chunk

❒ Decode chunk into bits ❍ Removes effects of channel during first collision

❒ Re-modulate bits to get channel-free signal

❒ Apply effect of channel during second collision ❍ Use correlation to estimate channel despite

interference

1 2

1 2 What if AP Makes a Mistake?

∆1 ∆2 1 1

22

Bad News: Errors can propagate

3

Can we deal with these errors?

What if AP Makes a Mistake?

∆1 ∆2

What if AP Makes a Mistake?

Good News: Temporal Diversity A 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

1 1

22

3

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

USRPs Testbed

•  10% HT,

•  10% partial HT,

•  80% perfectly sense each other

802.11a Throughput Comparison

802.11

Throughput

CD

F of

con

curr

ent f

low

pai

rs

Hidden Terminals

Partial Hidden Terminals

Perfectly Sense

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9

1

0 0.5 1 1.5 2

Throughput Comparison

ZigZag

Throughput

CD

F of

con

curr

ent f

low

pai

rs

802.11

Hidden Terminals get high throughput

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9

1

0 0.5 1 1.5 2

ZigZag Exploits Capture Effect

Capture Effect ❒ Subtract Alice and combine Bob’s packet

across collisions to correct errors

∆1 ∆2 Pa1

Pb

Pa2 Pb

3 packets in 2 time slots à 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 ∆2 Pa

Pb

Pa Pb

80 80

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 Group G1 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

l Retransmission

83 83

Remap Basic Idea: Structured Permutation


A1

A2

A3

A4 Mapping π1

A4

A3

A2

A1 Mapping π2

A2

A1

A4

A3 Mapping π3

A3

A4

A1

A2 Mapping π4

84

How Permutation Helps ❒ Non-matching collisions on adjacent

channels C1 and C2 Subcarrier Group

G1 G2 G3 G4

A1

A2

A3

A4 1st transmission

2nd transmission

A4

A3

A2

A1

A2

A1

A4

A3 3rd transmission

A3

A4

A1

A2 4th transmission

85 85

How Permutation Helps (cont’d)

❒ Non-matching collisions on non-adjacent channels C1 and C3


A1

A2

A3

A4 1st 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

l 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 S2 Sn

Time

Freq

Pa

B5

B2

B3 B4

A4 A3

A2 A1

S1 S2 Sn

B2

B5

B4 B3

P′b

P′a

G1

G3

G2

G5

G4 G2

1

5 9

13

4 10

14

3

7 11

2

6 8

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 (mac)...

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