Mitigating the Reader
Collision Problem in RFID
Networks with Mobile
Readers
Presented By
Shailesh M. Birari
Guided By
Prof. Sridhar Iyer
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
2
Motivation for Mobile Readers

Cost :


Convenience :



“Always on” Fixed reader may be an overkill
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
Why a new protocol ? (contd..)

RTS-CTS :

Additional collision avoidance for CTS from
tags
T1 CTS

TCTS
1
RTS
RTS
R1
CTS
T2
A CTS from all the tags is required to ensure
collision avoidance
RTS R RTS
1
T2
RTS RRTS
2
CTST3
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
Beacon
T1
R1
T2
R2
Query
R1’s Read Range
T3
Query
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
Contend_backoff
Tmin
R1
2
2 1
Tread
Tmin
5
5 4
Tmin
Tread
Tmin
3
2
5
R2
Tmin
3
Tmin
3 2
R3
1
Tread
R1 chooses 2 BI, R2
5BI, R3 chooses 3BI
R1 chooses
chooses 3BI
Delay before beaconing
Wait
for
control
channel
tothen
get idle
and
then
send beacon
Transmit
R1,
Choose
BothR2
R1&
aR3
and
beacon
small
R3
R3
are
delay
are
immediately
communicating
and
transmit
with
with
the
tags
tags
R2,
are
not
incommunicating
each
others
beacon
range
R2
R3
R1
R1‘s beacon range
R1‘s control channel
Sensing range
PULSE Protocol Flowchart
Simulation in QualNet
Simulation Setup
Simulation Setup (contd..)

Performance Metrics:
Total queries sent successful ly (by all readers)
System Throughput 
Total time
Total queries sent successful ly by all readers  100
System Efficiency 
Total queries sent (successfu l  collided) by all readers

Beacon Range Factor (BRF):
BRF 


Control Channel Transmissi on Power
Data Channel Transmissi on Power
Beacon Interval (BI) : interval after which beacon is
sent
Compared Protocols : CSMA, Colorwave, Aloha
System Throughput

25 Reader Topology :
System Throughput with 25 Readers
7000
System Throughput
(Queries/second)
6000
5000
4000
Static Readers
Mobile Readers
3000
2000
1000
0
Aloha
CSMA
Colorw ave
Pulse (BRF =
28)
Mac protocols

Pulse shows throughput improvement in both static and mobile
networks
System Throughput (contd..)

Varying the number of readers
System Throughput with Varying Number of Readers
7000
System Throughput
(Queries/second)
6000
Aloha(Static)
5000
CSMA(Static)
PULSE(Static)(BRF = 28)
4000
Colorwave(Static)
Aloha(Mobile)
3000
CSMA(Mobile)
PULSE(Mobile)(BRF = 28)
2000
Colorwave(Mobile)
1000
0
4
9
16
25
36
49
64
Number of Readers

Pulse shows throughput improvement even at dense network
of 64 readers
System Efficiency

25 Reader Topology
System Efficiency with 25 Readers
100
90
System Efficiency
(Percentage)
80
70
60
Static Readers
50
Mobile Readers
40
30
20
10
0
Aloha
CSMA
Colorw ave Pulse (BRF =
28)
Mac protocols

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 Efficiency with 25 Readers topology
System Throughput with 25 Readers topology
100
7000
99.8
99.6
5000
4000
Static Readers
Mobile Readers
3000
2000
1000
99.4
Static Readers
99.2
Mobile Readers
99
98.8
98.6
0
1
5
10
Beaconing Interval (msec)

System Efficiency
(Percentage)
System Throughput
(Queries/second)
6000
15
98.4
1
5
10
15
Beaconing Interval (m sec)
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
7000
System Throughput
(Queries/second)
6000
4 Readers
5000
9 Readers
4000
16 Readers
25 Redaers
3000
36 Readers
2000
49 Readers
64 Readers
1000
0
20
24
28
32
BRF for Pulse

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
6000
System Throughput
(Queries/second)
5000
4 Readers
4000
9 Readers
16 Readers
25 Redaers
3000
36 Readers
49 Readers
2000
64 Readers
1000
0
20
28
24
32
BRF for Pulse

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 w ith Varying Readers
120
Static BRF = 20
System Efficiency
(Percentage)
100
Static BRF = 24
Static BRF = 28
80
Static BRF = 32
60
Mobile BRF = 20
40
Mobile BRF = 24
Mobile BRF = 28
20
Mobile BRF = 32
4
R
ea
de
rs
9
R
ea
de
rs
16
R
ea
de
rs
25
R
ed
ae
rs
36
R
ea
de
rs
49
R
ea
de
rs
64
R
ea
de
rs
0
Num ber of Readers

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
System Throughput 
QTrea d  Ps  E[ BDI ]
E[Tcycle ]
Comparison
Comparison results
Comparison of Analysis and Simulation Results
5000
4500
Simulation
4000
System Throughput
(Queries/second)

3500
3000
Analysis
2500
Analysis
Simulation
2000
1500
1000
500
0
4
9
16
25
36
Number of Readers
49
64
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):


Colorwave (TDMA) :



Sense the data channel for 100msec before
communicating the with tags
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
Approaches Considered (contd.)

Centralised graph coloring at Access Point


All nodes transmit neighbour information to the
AP
AP applies a graph coloring to allocate timeslots
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