Design of a Multi-Channel MAC Protocol for Ad Hoc Wireless Networks H. Wilson So UC Berkeley /...

Design of a Multi-Channel MAC Protocol for Ad Hoc Wireless Networks

H. Wilson SoUC Berkeley / Siemens TTB

EE 228A Lecture2/21/2006

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Traditional Ad Hoc Network: Single Channel

Each device has 1 radio. All radios are tuned to the same channel.

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Typical Wireless Networks

t=0frequencySender 1

t=1frequencySender 2

t=2frequencySender 3

Each network uses 1 channel only.

Channel 1 Channel 2Channel 3

: :

Can we do better?

PowerDensity

4

Can we do better?

t=0frequency

PowerDensity

Sender 1

Simultaneous sending on different channels?

Channel 1 Channel 2Channel 3

Sender 3

t=1frequencySender 2 Sender 1 Sender 4

Sender 4

t=2frequencySender 3

: :

Sender 2Sender 4

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Goal Given a wireless network where:

M (>1) channels are available each node has 1 tunable radio each node has many neighbors

Design a Multi-Channel MAC protocol: increases total network throughput achieves low average delay robust, practical

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Scope of Project Single hop network (but as a

potentinal building block for multi-hop networks)

Ignore multi-hop forwarding / routing

No QoS guarantee

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Outline Indroduction

Why Multi-Channel MAC? Choose best approach

Classify / Compare (analysis, sim) Protocol design Implementation

Platform / Challenge / Results Conclusion

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Why Multi-Channel MAC? Why do we:

create multiple channels and then

design a MAC that uses multiple channels?

Two points of view: Theoretical Engineering

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Why Multi-Channel MAC?

t=0

t=1

frequency

frequency

Sender 1

Sender 2 Sender 1

Sender 3

Sender 4

Sender 4

t=0

t=1

frequency

frequency

Sender 1

Sender 2

Single “Super” Channel

Multi-Channel MAC

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Optimal M-Channel Schedule

Given some traffic demand. Schedule S gives optimal throughput.

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Optimal M-Channel Schedule

Given some traffic demand. Schedule S gives optimal throughput.

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Convert S (M-channel) into

S’(1 wide channel)

t=1

t=2

S Ch 1 Ch 2 ...

...

...

Ch M

1->3, 4->7,16->8, ...

5->2, 13->9,...

10->12,14->15, ...

S’

t=1t=2t=3t=4t=5

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Summary: Why Multi-Channel?

Theory: no gain to use multiple channels. Engineering: frequency management

and hardware limits give rise to multiple channels.

Conclusion: Channel should be as wide as practical. Multi-channel MAC can always take advantage

of additional spectrum to increase throughput.

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Outline Indroduction

Why Multi-Channel MAC? Choose best approach

Joint work with Prof. Jeonghoon Mo, ICU, Korea

Classify / Compare (analysis, sim) Protocol design Implementation

Platform / Challenge / Results Conclusion

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Core Design Issues

Q1: Which channel is receiver Y listening on?

Q2: Is channel i free?

time=t

time=t

frequency

frequencyFree ?

receiver Y

? ? ?

Chan i

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Multi-Channel MAC Protocols

(1) Dedicated Control Channel (2 radios) Dedicated control radio & channel for all control

messages DCA [Wu2000], DCA-PC [Tseng2001], DPC [Hung2002].

(2) Split Phase Time divided into alternate (i) channel negotiation phase

on default channel & (ii) data transfer phase on all channels

MMAC [J.So2003], MAP [Chen et al.]

(3) Common Hopping Sequence All idle nodes follow the same channel hopping sequence HRMA [Tang98], CHMA, CHAT [Tzamaloukas2000]

(4) Parallel Rendezvous Each node follows its own channel hopping sequence SSCH [Bahl04], McMAC (our own proposal)

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Protocol (1): Dedicated Control Channel

Ch3(data)

Ch2(data)

Ch1(Ctrl)

Time

Channel

RTS(2,3)

CTS(2)

RTS(3)

CTS(3)

Data Ack

Keys: 2 Radios/Node; Rendezvous on 1 channel; No time sync

Legend: Node 1 Node 2 Node 3 Node 4

Data

AckData

Ack ...

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Protocol (2): Split-Phase

Ch3

Ch2

Ch1

Time

Channel

Hello(1,2,3)

Ack (1)

Keys: 1 Radio; Rendezvous on a common channel; Coarse time sync

Control Phase Data TransferPhase

...

...

...Data AckRts Cts

DataRts Cts Ack ...

Hello(2,3)

Ack (2)

Unused

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Protocol (3): Common Hopping

Ch3

Ch2

Ch1

Time

ChannelKey: 1 radio; Non-busy nodes hop together; Tight time sync

Ch4

1 2 3 4 5 6 7 8 9 10 11

Data/Ack ...

Enough for RTS/CTS

RTS+CTS

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Potential Limitation of Approaches (1), (2), (3)

All nodes must contend on one channel at a time. Problem: contention channel saturates when:

(i) many short data packets and (ii) channels are numerous.

t=1 2 3 4 5 6 ...

ContentionChannel

Ch 1

Ch 2

: :

Slow contention => Many wasted channels !!!

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Protocol (4): Parallel Rendezvous e.g. McMAC (simplified)

t=1 2 3 4 5 6 ...

Ch 1

Ch 2

Ch 3

Ch 4

Sender needs to know the home channel of the receiver.

? ?

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McMAC (with hopping)

t=1 2 3 4 5 6 7 8 9

Ch1

Ch2

Original schedule

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McMAC (with hopping)

t=1 2 3 4 5 6 7 8 9

Ch1

Ch2

1. Data arrives 4. Hopping

resumes3. Hopping stopped during data transfer

2. RTS/ CTS/ Data

Original schedule

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Analytical Model Assumptions Discrete Time All pairs backlogged After rendezvous, each data

transfer lasts a geometric number of slots

Medium contention algorithm is slotted aloha w/ tx prob. p

Single collision domain Packet loss due to collision only

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Models: (1) Dedicated Channel & (3) Common Hopping Markov Model: # of ongoing

transfers as a function of time. Up: successful rendezvous (limited

by collisions and # channels) Down: existing transfer finishes

(affected by average transfer length)

Limiting distribution: yields the average throughput

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Model: (2) Split Phase Control Phase Model: How many

channel agreements can be made? Affected by: duration of control

phase

Data Phase Model: How much data is transferred on each channel?

Affected by: # of agreements in control phase, avg. transfer length.

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Model: (4) Parallel Rendezvous McMAC Markov Model: # of current

transfers as a function of time. Similar to (1) Dedicated Channel &

(3) Common Hopping ... but channel agreements can

happen simultaneously on many channels.

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Example Scenarios Scenario (B)

802.11b 2 Mbps x 3

channels 20 devices Avg transfer size:

1KB or 10KB Channel

switching: 100us

Scenario (A) 802.11a

6 Mbps x 12 channels

40 devices Avg transfer size:

1KB or 10KB Channel switching:

100us

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Analytical Results: 802.11b

Sim

Short Transfers (avg. 1KB) Long Transfers (avg. 10KB)

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Analytical Results: 802.11a

Sim

Short Transfers (avg. 1KB) Long Transfers (avg. 10KB)

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Throughput vs. Num of Channels

Short Packets Long Packets

Control Channel Congestion

Control Channel Congestion Delayed

Short Transfers (avg. 1KB) Long Transfers (avg. 10KB)

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Simulation Model

More realistic than analytical models

Can check packet delaysAnalytical Model

■ Symmetric Traffic

■ No Queueing

TEXT

Simulation Model

■ Variable degree of communication■ Queueing of Packets

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Simulation Results Highlight

Effects of Back-to-back Packets: Mixed real-time traffic + bulk transfer Delay of all traffic decreases with

increasing medium occupancy limits

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Medium Occupancy Limit: Delay vs. Throughput (802.11a)

3.3 ms max. 10.5 ms max.

32Mbps 40 Mbps

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Summary of Comparison Single control channel eventually

saturates.

Parallel rendezvous protocols (using 1 radio) perform surprisingly well compared to dedicated control channel

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Outline Indroduction

Why Multi-Channel MAC? Choose best approach

Classify / Compare (analysis, sim) Protocol design Implementation

Platform / Challenge / Results Conclusion

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Protocol Components Random Channel Hopping

Discovery

Synchronization

Rendezvous and Scheduling

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Challenge 1: Synchronization Challenge 1: Cannot assume

perfectly synchronized time slots

Option 1: Synchronize globally

Option 2: Synchronize pair-wise

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Challenge 2: Discovery (i) Discover existing nodes (ii) Discover a newcomer

Primary: beacon on channel 1 once every T1 seconds

Backup: nodes beacon every T2 seconds while hopping randomly. Takes: M * ln(N) * T2 (seconds)

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Outline Indroduction

Why Multi-Channel MAC? Choose best approach

Classify / Compare (analysis, sim) Protocol design Implementation

Joint work with Giang Nguyen Platform / Challenge / Results

Conclusion

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Implementation Why implement?

To prove that McMAC is pratical To uncover any hidden difficulty /

complexity Plan:

Choose a platform Evaluate the platform Implement subset of McMAC

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Platform: Mote (Telos) MCU: Ti MSP430

4MHz, 16-bit, 10K RAM

Radio: CC2420, 802.15.4 DSSS, 250Kbps

Peripherals: USB, 32KHz Crystal, sensors, LEDs

SW: TinyOS Cost: ~$80

Picture of TelosPhoto Credit: www.Moteiv.com

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4-Node Long Term Clock Drift

+9 ppm

-8 ppm

-0.3 ppm

Node 3’sClock

Good News! Almost a straight line!!

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Short Term Clock Instability

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SynchronizationSender’s Time Stamps

Receiver’s Time Stamps

Beacons received

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Regression: k Most Recent Time Stamps

Max. “Std Erro

r of E

stimatio

n”

Among 16x15 pairs

Using 5 Minutes of History

Median “Std Erro

r of E

stimatio

n”

Among 16x15 pairs

Std

. E

rr.

of

Est

imati

on

(3

0.5

us

tick

s)

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Practical Issues Keeping 30 points requires

240Bytes/neighbor. Can we use less memory?

Yes, by recursive estimation. Idea: give geometrically decreasing

weights to earlier time stamps. 8 numbers are enough to summarize history.

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Sync. Algorithm : receiver’s time stamp : sender’s time stamp : estimated sender’s time stamp

Minimize Error:

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Sync. Algorithm States

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Using Recursive EstimationS

td.

Err

. of

Est

imati

on

(30

.5u

s ti

cks)

Max. “Std Error of Estimation”

Among 16x15 pairsMedian “Std Error of Estimation”

Among 16x15 pairs

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Sync. Algorithm Details Problem: Overflow of sums Solution: Change of variable

Let y’ = y - k x’ = x - c Pick k, c carefully to avoid overflow. Repeat this for every sample.

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Implementation Techniques

Nbr 1

Nbr 3

Nbr 2

Recvr’ sLocal Clock

(i) Nbr 1’s Clock –Local Clock

(ii) Use approx. Receiver’s Clock

X X

X

XX

X

(iii) Outlier detection

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Experimental Setup No. of Nodes: 4, 8, or 16 No. of channels: 8 Beacon: every 1s or 5s Automatic discovery & sync.

Synchronization Metric: % beacons within 3 ticks (90us) % beacons within 6 ticks (180us)

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Time Slot Structure Mote Clock: 32,678Hz Tick = 30.5 us (i.e., 1/32768Hz) Big Slot (3.9 ms) = 128 ticks Small Slot (30.5us) = 1 tick

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Demo 8 nodes 4 channels ~1 minute timeout 3 sec between beacons

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Time Sync. Results

1s 1s 1s5s 5s 5s[ 4 Nodes] [ 8 Nodes][ 16 Nodes]

Beacon Interval:

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Outline Indroduction

Why Multi-Channel MAC? Choose best approach

Classify / Compare (analysis, sim) Protocol design Implementation

Platform / Challenge / Results Conclusion

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Conclusion Multi-channel MAC complements wide-

band radios. McMAC is practical!

~95% pkts delivered, >92% acked (light load)

Implementation is non-trivial!! Good throughput requires:

Good time stamps Fast controller Fine tuning

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Looking Forward McMAC as a building block for a multi-

hop ad hoc network! Challenges:

Hidden-nodes QoS guarantees Broadcast

Next 2 years: turning McMAC into a real product at Siemens, Berkeley.

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Thank You!! Wilson So so@eecs.berkeley.edu