Mitigating the Reader Collision Problem in RFID Networks with Mobile Readers

Presented ByShailesh M. Birari

Guided ByProf. Sridhar Iyer

2

Basic Working of RFID system

Uses radio frequency to identify & track items in supply chain and manufacturing

RFID readers and tags Active and Passive tags

Motivation for Mobile Readers

Cost : “Always on” Fixed reader may be an overkill

Convenience : Easy, faster deployment No wiring installation hassles

Example applications : Searching a particular book in library Counting the items on the shelves in a super market Showing the list of items in the vicinity of the

customer in a super market

Scenario under consideration

Super market, library Each customer has a RFID reader Readers form an ad hoc network All readers have unrestricted mobility Readers often join and leave the network All tags are passive

Reader Collision Problem (RCP)

Multiple Reader to tag Interference:

RCP (contd..)

Reader to Reader Interference:

RCP (contd..)

Hidden Terminal

Why a new protocol ?

TDMA : Interfering readers transmit in different timeslot Time synchronisation required Timeslot distribution is inefficient in a mobile

network CSMA : Sense channel before transmitting

RFID suffer from hidden terminal Collision happen at the tags and hence

collision detection is not possible by carrier sensing at the readers alone

Why a new protocol ?

FDMA : Interfering readers transmit at different frequency Tags do not have tuning circuitry Adding tuning circuitry to the tags will increase

the cost CDMA :

Requires complex circuitry at tags which will increase the cost of passive tags

CTSCTS

Why a new protocol ? (contd..)

RTS-CTS : Additional collision avoidance for CTS from

tags

A CTS from all the tags is required to ensure collision avoidance

RTS RTS T2T1 CTS CTSR1

T3RTST1

RTSR1RTSRTS R2T2

PULSE Protocol

Assumptions Dual channel : data and control channel Data channel : reader-tag communication Control channel : reader-reader communication A reader can receive simultaneously on both

channels but transmit on only one channel at a time

No inter-channel interference

PULSE Protocol Example

T1

Query

Beacon

Query QueryQuery QueryQuery QueryQuery

Beacon

R2T2 T3R1

QueryQuery QueryQuery

R1’s Read Range R2’s Read Range

PULSE Protocol Overview

Before communicating, a reader listens on the control channel for any beacon for Tmin time

If no beacon on the control channel for Tmin , start communication on the data channel

Reader periodically transmits a beacon on the control channel while communicating with the tags

Tmin

Contend_backoff

R1 chooses 2 BI, R2 chooses 5BI, R3 chooses 3BI

Tread2 1

5 4

3 2

TreadR1

R2

R3

2

5

3

Tmin

Tmin

Tmin

Tmin

Tmin3

2

5

1Tread

R1 chooses 3BI

Delay before beaconing

R2

R1

R3

R1‘s control channel Sensing range

R1‘s beacon range

Wait for control channel to get idle and then send beaconR1, R2, R3 are not in each others beacon rangeBoth R1 and R3 are communicating with tagsTransmit beacon immediatelyR1, R2 & R3 are communicating with the tagsChoose a small delay and then transmit

PULSE Protocol Flowchart

Simulation in QualNet

Simulation Setup

Simulation Setup (contd..)

Performance Metrics:

timeTotal

readers) all(by ly successfulsent queries Total Throughput System

readers allby collided) l(successfusent queries Total

100 readers allby ly successfulsent queries Total Efficiency System

Beacon Range Factor (BRF):

Poweron Transmissi Channel Data

Poweron Transmissi Channel Control BRF

Beacon Interval (BI) : interval after which beacon is sent Compared Protocols : CSMA, Colorwave, Aloha

System Throughput

25 Reader Topology :System Throughput with 25 Readers

0

1000

2000

3000

4000

5000

6000

7000

Aloha CSMA Colorw ave Pulse (BRF =28)

Mac protocols

Syste

m T

hro

ug

hp

ut

(Qu

eri

es/s

eco

nd

)

Static Readers

Mobile Readers

Pulse shows throughput improvement in both static and mobile networks

System Throughput (contd..)

Varying the number of readersSystem Throughput with Varying Number of Readers

0

1000

2000

3000

4000

5000

6000

7000

4 9 16 25 36 49 64

Number of Readers

Sys

tem

Th

rou

gh

pu

t (Q

uer

ies/

seco

nd

)

Aloha(Static)

CSMA(Static)

PULSE(Static)(BRF = 28)

Colorwave(Static)

Aloha(Mobile)

CSMA(Mobile)

PULSE(Mobile)(BRF = 28)

Colorwave(Mobile)

Pulse shows throughput improvement even at dense network of 64 readers

System Efficiency

25 Reader TopologySystem Efficiency with 25 Readers

0

10

20

30

40

50

60

70

80

90

100

Aloha CSMA Colorw ave Pulse (BRF =28)

Mac protocols

Sy

ste

m E

ffic

ien

cy

(Pe

rce

nta

ge

)

Static Readers

Mobile Readers

Pulse has system efficiency of above 95% which means Pulse is able to detect and avoid most of the collisions successfully

Optimal Beacon Interval (BI)

Effect of Beacon Interval on 25 reader topology

System Throughput with 25 Readers topology

0

1000

2000

3000

4000

5000

6000

7000

1 5 10 15

Beaconing Interval (msec)

Sys

tem

Th

rou

gh

pu

t (

Qu

erie

s/se

con

d)

Static Readers

Mobile Readers

System Efficiency with 25 Readers topology

98.4

98.6

98.8

99

99.2

99.4

99.6

99.8

100

1 5 10 15

Beaconing Interval (msec)

Syste

m E

ffic

ien

cy

(Perc

en

tag

e)

Static Readers

Mobile Readers

Variation in Beacon Interval does not show too much of difference in both system throughput and efficiency.

Optimal BRF

Throughput Vs BRF (Static Readers)System Throughput Vs BRF with Static Readers

0

1000

2000

3000

4000

5000

6000

7000

20 24 28 32

BRF for Pulse

Sys

tem

Th

rou

gh

pu

t (Q

uer

ies/

seco

nd

)

4 Readers

9 Readers

16 Readers

25 Redaers

36 Readers

49 Readers

64 Readers

BRF of 28 shows highest system throughput in almost all the networks

Optimal BRF (contd..)

Throughput Vs BRF (Mobile Readers)System Throughput Vs BRF with Mobile Readers

0

1000

2000

3000

4000

5000

6000

20 24 28 32

BRF for Pulse

Syste

m T

hro

ug

hp

ut

(Qu

eri

es/s

eco

nd

) 4 Readers

9 Readers

16 Readers

25 Redaers

36 Readers

49 Readers

64 Readers

BRF of 28 shows highest system throughput in almost all the networks

Optimal BRF (contd..)

Effect of Density of readers on networks with different BRFs

System Efficiency with Varying Readers

0

20

40

60

80

100

120

Number of Readers

Syste

m E

ffic

ien

cy

(Perc

en

tag

e)

Static BRF = 20

Static BRF = 24

Static BRF = 28

Static BRF = 32

Mobile BRF = 20

Mobile BRF = 24

Mobile BRF = 28

Mobile BRF = 32

Networks with BRF=28 maintain its efficiency above 95% even when the number of readers is increased to 64

Performance Modeling

Assume a beacon transmission is heard by all the readers

Backoff Decrement Interval: Interval after which backoff value is decremented

May contain a successful transmission by other reader

May contain a collision May be empty

Performance Modeling (contd..)

Cycle : Duration between two successful Tread transmission

by a reader Consists of BDIs

Calculate the average duration of a BDI Calculate the average number of BDIs in a

cycle Calculate the average duration of a cycle

Backoff Decrement Interval (BDI)

System Throughput

]E[

]E[ Throughput System

cycle

sT

T

BDIPQread

Comparison

Comparison results

Comparison of Analysis and Simulation Results

Simulation

Analysis

0

500

1000

1500

2000

2500

3000

3500

4000

4500

5000

4 9 16 25 36 49 64

Number of Readers

Sys

tem

Th

rou

gh

pu

t (

Qu

erie

s/se

con

d)

Analysis

Simulation

Conclusion

Mobile Readers reduce cost and improve convenience

Pulse shows an improvement in both the dimensions, system throughput and system efficiency

Pulse is effective even in dense mobile networks

References

[1] Daniel W. Engels. The Reader Collision Problem. Technical Report, epcglobal.org, 2002.

[2] J. Waldrop, D. W. Engels, and S. E. Sarma. Colowave: An anticollision algorithm for the reader collision problem. In IEEE Wireless Communications and Networking Conference (WCNC), 2003.

[3] QualNet Simulator 3.6. http://www.qualnet.com

[4] O. Tickoo and B. Sikdar. Queuing Analysis and Delay Mitigation in IEEE 802.11 Random Access MAC based Wireless Networks. In IEEE INFOCOM, 2004.

Thank you

Existing Work ETSI EN 302 208 (CSMA):

Sense the data channel for 100msec before communicating the with tags

Colorwave (TDMA) : Readers randomly select a timeslot to transmit Chooses a new timeslot if collision and announce it to

neighbors UHF Gen 2 Standard (FDMA):

Separate reader transmissions and tag transmissions spectrally

Readers collide with readers and tags collide with tags

Initial Results

Approaches Considered

Registration at the access point (query response) Transmit Neighbour information to AP along

with request to transmit AP scans the status of the neighbours and

responds accordingly

Centralised graph coloring at Access Point All nodes transmit neighbour information to the

AP AP applies a graph coloring to allocate time-

slots

Approaches Considered (contd.)

Interesting Features of RCP Readers may not be in each others sensing range; Tag cannot select a particular reader to

respond(unlike cellular systems) None of the readers can read the tag The passive tags, where the collision may take

place, are not able to take part in the collision resolution as in hidden terminal problem

Reduces the read rate of the RFID system