Anti-Collision Algorithm for RFID Tags

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Anti
-
Collision Algorithm for RFID Tags


1

Anti
-
Collision Algorithm for RFID Tags

Selwyn Piramuthu

Information Systems and Operations Management, University of Florida,

Gainesville, FL32611, USA

E
-
mail: selwyn@ufl.edu

ABSTRACT
:

One of the advantages of RFID tags vis
-
à
-
vis bar codes is the ease wi
th which a batch of RFID tagged
objects can be simultaneously read. Although this leads to faster read rates, the accuracy of these reads may suffer
due to collisions, false reads, among others. We discuss collisions in general and present a modified algor
ithm.

Keywords

RFID, Collisions, Tree
-
Based Algorithm
.

INTRODUCTION

FID tags and bar codes have several common
characteristics from an automatic identification
perspective. While both bar codes and RFID tags have their
pros and cons, one of the advantag
es of using RFID tags is
that they can be simultaneously read as a batch. Clearly,
this is beneficial in applications such as supply chains
where the rate at which objects are read and identified is of
paramount importance. Although RFID tags can be read
m
uch faster than bar codes in principle, the reality is not
completely clear. For example, since RFID tags operate in
RF frequency, their read rate accuracy can be affected by
metallic shields or any other hostile media that may be
present between the tag a
nd reader (e.g., Tu and Piramuthu,
2008). The read rate accuracy may also be reduced due to
tag and/or reader collisions.

Collision, in the context of RFID tag reads, occurs when
either several tags or several readers are simultaneously
present in the same

field. Tag collision occurs when a
reader attempts to simultaneously read several tags.
Similarly, reader collision occurs when two or more readers
are simultaneously present in the same field (Piramuthu,
2008).

Several means to approach and alleviate som
e of the
problems associated with RFID tag/reader collision have
been proposed in the literature. A majority of them are
either based on the ALOHA protocol or some sort of tree
-
based algorithm. We provide a brief discussion on
ALOHA
-
based methods and then
consider a tree
-
based
approach to reduce RFID tag/reader collisions.

PROTOCOLS FOR REDUCI
NG COLLISIONS

Several researchers have examined the issue of collision in
RFID
-
tagged systems. The approaches taken in existing
literature in this area fall into two b
road categories: tree
-
based algorithms (e.g., Huang and Tran, 2007; Myung and
Lee, 2006) and variants of the ALOHA protocol (Chen and
Lin, 2006; Khandelwal
et al.
, 2007). There are also hybrid
algorithms that attempt to distill the beneficial aspects of
th
e two (e.g., Shin,
et al.
, 2007).

The tree
-
based algorithms operate by sequentially
traversing the tree from top to bottom, where each node of
the tree has an additional component of a tag’s information.
The root of the tree is common to all the tags, and

each leaf
node of the tree represents a specific tag. These algorithms
narrow down the tag of interest by successively muting tags
that are involved in collisions. Drawbacks of these
algorithms include the number of iterations involved before
the reader c
an identify any given tag, the complexities of
circuits required in the tag itself to be able to respond to
such readers, among others. The ALOHA
-
type of
protocols, on the other hand, let the tags choose when


(i.e., time slot) they respond. When col
lision occurs, the
tags involved in the collision participate in further iterations
until resolution occurs. The ALOHA
-
type of protocols
clearly take less time to complete, although there is a
positive probability for a tag to not be read within a given
nu
mber of iterations.

ALOHA
was developed in the 1970s for a packet radio
network
at

the University

of Hawaii. Whenever a station
has data, it transmits the data. The sender station finds out
whether the transmission was successful or experienced a
collision

by listening to the broadcast. If there is a collision,
the sender retransmits after a random wait period. This
protocol involves the simplest of all reader design where
the reader just listens.
The tags periodically send data
packets with random quiet pe
riods.
This protocol quickly
adapts to varying number of tags. The theoretically proven
maximum utilization with this pro
tocol (assuming Poisson
arrival
) is 18.4%.

Figure 1 provides an illustration of
ALOHA, with four tags and a reader. Collision occurs, a
s
illustrated by the dark rectangles at the reader side, when
two or more signals are received by the reader during the
same (overlapping) time period.

The
s
lotted
ALOHA

protocol was an improvement
where time is divided into discrete time intervals (slots)

and
a packet can only be transmitted at the beginning of a slot.
It reduced the collision duration. Here, packets either
R


Anti
-
Collision Algorithm for RFID Tags


117

collide completely or do not collide at all
,

i
.e., no partial
-
collisions like in the pure
ALOHA

case. This protocol
doubles the channe
l utilization of pure
ALOHA

to 36.8%.

Figure 1 provides an illustration of the Slotted ALOHA
protocol.


Fig
.

1
:

Collision and Multiple Access Protocol

The framed slotted
ALOHA

protocol incorporated
further discretization of time b
y grouping
medium
accesses
into frames, with N slots per frame. Within a frame, tags
transmit at most once in a randomly selected slot.
Thus
signal transmitted per tag can have at most one collision
within a given frame. Initially, frames were of uniform s
ize
with the same number of slots.
This was later extended to
incorporate
flexibility by letting the reader adaptively
expand and contract

the number of slots per frame in the
next upcoming round. The number of slots per frame is
selected as per the number

of tags in the field. There is no
deterministic way to handle this problem of adaptively
varying the number of slots per frame at any given point in
time. We address this issue in the next section.

The framed slotted
ALOHA

has been proposed as a
means to
alleviate problems associated with

collisions
when multiple RFID tags are simultaneously present in a
reader’s field. Almost all of

these approaches assume that
the reader first transmits its frame size to the tags. The tags
then

randomly pick a slot and r
eply in that slot. From the
reader’s side, in any given slot, it receives

(a) no signal,


(b) signal from one tag, or (c) signal from more than one
tag. The slot under case

(a)

is considered a loss since no
useful information was transmitted

during that slot. The
slot under

case

(b) is a successful case since the signal from
this tag is received by the reader. Collision occurs

in case

(c), and the number of tags that replied in this slot is
generally unknown.


It

i
s hard to estimate the initi
al number of tags present in
the field of the reader. The frame

length depends on the
number of tags.

Existing papers on RFID collision (as well
as
ALOHA
, in general) assume that the tags choose

a
uniform distribution to determine the slots in which to
rep
ly.

PROPOSED ALGORITHM

A majority of tree
-
based algorithms are based on assigning
an n
-
bit identifier to each tag, and exhaustively considering
every bit to distinguish and identify any given tag.
However, this can soon become combinatorially explosive
dep
ending on the depth of the binary tree. As seen in Figure
2, if


Fig.

2
:

An example binary tree

we

start

consider
ing from the root of the tree, the earlier
attempts are invariably going to be replete with collisions
since about half the tags (assuming th
at the tree is complete
and that there is a tag identifier at every leaf node) would
respond for the first bit from the top (of either 0 or 1).
Similarly, given the symmetry of this representation, the
bottom
-
most level of the tree would have the same
cons
equence since the number of 0s as 1s is the same. This
situation clearly necessitates an “intelligent” means to
address the issue.

The proposed method utilizes the density of entries at
any given level, and utilizes this information to reduce the
number o
f iterations that are required to completely
determine the identity of every tag in the field of the reader.

118


Mobile and Pervasive Computing (CoMPC

2008)

CONCLUSIONS

Due to lack of time, we were unable to completely describe
the proposed method. We expect to
be able to complete this
paper within the

next week or so, and we intend to re
-
send
the revised document when it’s completed.

REFERENCES

[1]

Wen
-
Tzu Chen and Guan
-
Hung Lin. An Efficient Anti
-
Collision Method for Tag Identification in a RFID System.
IEICE Transactions on Communications, Vol. E89
-
B, No
.
12, pp. 3386

3392, December 2006.

[2]

Christian Floerkemeier. Bayesian Transmission Strategy for
Framed ALOHA Based RFID Protocols. In Proceedings of
the IEEE International Conference on RFID, pp. 228

235,
2007.

[3]

Christian Floerkemeier and Matthias Wille. Com
parison of
Transmission Schemes for Framed ALOHA based RFID
Protocols. In Proceedings of the International Symposium on
Applications and the Internet Workshops (SAINT), pp.92

97, 2006.

[4]

Girish Khandel
wal, Kyonghwan Lee, Aylin Yener and
Semih Serbetli. ASAP:

A MAC Protocol for Dense and
Time
-
Constrained RFID Systems. In EURASIP Journal on
Wireless Communications and Networking, 2007.

[5]

Jihoon Myung and Wonjun Lee.
Adaptiv
e Binary Splitting:
A RFID Tag
Collision Arbitration Protocol for Tag
I
dentification.

Mob
il
e Networks and
Applications, Vol. 11,
pp. 711

722
, 2006
.

[6]

Selwyn Piramuthu. Adaptive Framework for Collisions in
RFID Tag Identification. Journal of Information &
Knowledge Management, 7(1),

pp. 9

14, May
,

2008.

[7]

Jae
-
Dong

Shin
, Sang
-
Soo Yeo, Tai
-
Hoon Kim an
d Su
ng
Kwon Kim. 2007. “Hybrid Tag
Anti
-
Collision Algorithms in
RFID Systems.” Proceedings of ICCS, Part

IV, LNCS 4490,
Springer
-
Verlag, pp. 693

700.

[8]

Yu
-
Ju Tu and Selwyn Piramuthu. Reducing False reads in
RFID
-
Embedded Supply Chains.

Journal of Theoretical

and
Applied Electronic Commerce Research, 3(2) August 2008.

[9]

Huang, Xu, a
nd Dat Tran.
Adaptive Binary Spli
tting for a
RFID Tag Collision
Arbitration via Multi
-
agent Systems
.
Proceedings of KES2007/WIRN2007, part III, pringer
-
Verlag, LNAI 4694, pp. 926

933
,

2007
.