rfid tag anti-collision protocol query tree with reversed ids

8/3/2019 RFID Tag Anti-Collision Protocol Query Tree With Reversed IDs

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RFID Tag Anti-Collision Protocol: Query Tree with Reversed IDs

Jung-Sik ChoSchool of Computer Science & Engineering,

Chung-Ang University,

Seoul, Korea

[email protected]

Jea-Dong ShinSchool of Computer Science & Engineering,


Seoul, Korea

[email protected]

Sung Kwon Kim

School of Computer Science & Engineering,


Seoul, Korea

[email protected]

Abstract

RFID system is a contactless automatic identification

system using small and low-cost RFID tag. It allows to rec-

ognize the information of tag via radio frequency attaching

RFID tag to an object such as a material object, human

or animal. Since RFID system has an advantage to rec-

ognize massive information simultaneously, it will be able

to replace the bar-code system. For this RFID system to

be widely spread, the problem of multiple tag identifica-

tion, which a reader identifies a multiple number of tagsin a very short time, has to be solved. So far, several anti-

collision algorithms are developed. We present an RFID

tag anti-collision protocol, called the query tree protocol

with reversed IDs (QTR protocol). This protocol works by

reversing the IDs of the tags and then applying the query

tree (QT) protocol. And we present the performances of the

QTR protocol by simulation. QTR protocol outperforms QT

protocol if the tag IDs are consecutive integers.

1. Introduction

RFID system is a contactless automatic identification

system attracting many attentions recently. The RFID sys-

tem consists of small low-cost RFID tag containing mi-

crochip and antenna, RFID reader and back-end server.

RFID tag has unique identification information, and it can

be attached to an object such as a material object, human

or animal. RFID reader can acquire the identification in-

formation from RFID tag via radio frequency in short dis-

tance. The RFID reader sends the identification information

acquired from RFID tag to back-end server, and can recog-

nize the information of an object attached the RFID tag.

The back-end server manages the identification information

that the RFID tag holds, and provides the information to the

RFID reader[3].

The advantages of RFID system are, RFID tag is small

and low-cost, massive RFID tags are recognizable simul-

taneously with radio frequency. Therefore, The RFID sys-

tem is expected to replace the current bar-code system used

in supply chain management[2][3]. A problem with this

single-reader-multiple-tag RFID system is that a tag withinthe readers range might not be identified if two or more tags

simultaneously answer (if a collision occurs).

To tackle the problem a RFID system employs a tag anti-

collision protocol which identifies the tags in a one-by-one

manner. Tag anti-collision protocols are partitioned into

two categories: ALOHA-based and tree-based protocol. In

ALOHA-based protocols [6], the reader broadcasts a mes-

sage with a frame size, then every tag receiving the message

selects a random slot number. A tag returns its ID when the

current counter becomes its slot number. The reader repeats

its broadcast until no collision is detected. Tree-based pro-

tocols are deterministic in the sense that no randomness is

involved. A tree based protocol works by implicitly travers-

ing a binary tree. An interesting combination of two proto-

cols is a hybrid protocol, which was shown to work faster

than existing protocols [7].

1.1. Motivation

A tag ID usually has several fields. For example, EPC

code assigned by EPCglobal [1] has four fields, header,

company ID, product ID, and serial number. The

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header defines the length and structure of EPC encoding,

which is usually fixed for a certain application. If a com-

pany uses tags with EPC codes for inventory and supply

chain management, the company will be assigned a unique

company ID by EPCglobal. Every product type produced

by the company will be assigned a unique product ID by

the company, and the products with the same product IDwill be given different serial numbers. Eventually, every

item will have a unique ID so that the ID uniquely repre-

sents the item. The ID is stored into an RFID tag and the

RFID tag is attached to the item at a factory where the item

is produced.

· Scenario I (Random IDs): At a retailer with RFID

check-out system, when a customer checks out many

items through a counter, the RFID reader at the counter

will automatically collect the tag IDs. These IDs are

random in the sense that they are from many different

product types from different companies.

· Scenario II (Consecutive I Ds): At a publisher, af-

ter printing, tens or hundreds of books with the same

title are usually packaged in a box for delivery and dis-

tribution. The books in a box will have a common pre-

fix in their IDs as they have the same company ID and

product ID. Furthermore, their serial numbers are con-

secutive as the books in the box are usually printed

almost at the same time and they are assigned serial

numbers in order. These IDs in the box will form con-

secutive IDs.

We shall present a tag anti-collision protocol which

works well in both scenarios. The protocol works much

faster in the second scenario than existing protocols. We

will demonstrate this by simulation.

Notations: Let b and c be two strings of bits. |b| denotes

the length of b. bR denotes the reversed string of b, which

reads b backwards. bc denotes the concatenation of b and c,

which is the string obtained by appending c at the end of b.

Obviously, |bc| = |b| + |c|.

2. Query tree protocol

A tag anti-collision system consists of a reader and a set

of tags. Each tag has a unique ID, which will be represented

as a bit string or as an integer representation of the bit

string. The query tree tag anti-collision protocol(QT

protocol) can be described by specifying what the reader

and the tags perform during an execution of the protocol[5].

READER: The reader maintains a queue.

• Initially the queue has two strings 0 and 1.

• While the queue is not empty do the following:

·· Delete the first string s from the queue.

·· Broadcast query string s to the tags and wait for

their responses.

·· If only one tag responds with its ID string a, then

output a.

·· If a collision is detected (i.e., if multiple tags re-

spond), then insert two strings s0 and s1 to the

queue.

·· If no tag responds, then do nothing

TAG: Every tag receiving the query string s from the

reader compares s with its own ID string a. If s matches the

prefix of length |s| of a, then the tag sends a to the reader.

After broadcasting a query, the reader waits for the re-

sponses from the tags for a certain amount of time. If itdoes not receive any response within the time limit, it con-

cludes that no tag responds. If only one tag sends its ID

within the time limit, the reader will correctly receive the

ID and identify the tag. If two or more tags send their IDs

simultaneously, the IDs from these tags will mix up and the

reader cannot recognize them. In this case there are a multi-

ple number of tags whose IDs start with s. To discriminate

these tags further, the query string is extended one bit by

appending a 0 and a 1 to s.

Figure 1 shows a table of the queries and responses com-

municated between the reader and a set of five tags with IDs

in {01001, 01010, 01011, 01100, 01101}.

ID = {01001, 01010, 01011, 01100, 01101}

Step Query Response

1 0 collision

2 1 no response

3 00 no response

4 01 collision

5 010 collision

6 011 collision

7 0100 01001

8 0101 collision

9 0110 collision10 0111 no response

11 01010 01010

12 01011 01011

13 01100 01100

14 01101 01101

Figure 1. Queries and responses between thereader and the tags.

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Figure 2 depicts a binary tree, called a query tree,

showing the complete communication of the table.

1

1

1

1

11

1

0

0

0

0

0 0

0

01001

01010 01011 01100 01101

Figure 2. Query tree of Figure 1.

Every “left” edge is labeled with a 0 and every “right”

edge is labeled with a 1. There is a one-to-one correspon-

dence between the query strings sent by the reader and the

nodes (except the root). Thus, the number of query strings

sent by the reader is equal to the number of nodes minus

one or the number of edges of the query tree. A gray node

means a collision was detected, a white node means no tagresponded, and a black node means one tag was identified.

The internal nodes are all gray and the leaves are either

white or black. The number of black leaves is equal to the

number of IDs. A query tree is a full binary tree, i.e., each

internal node has either two or no children.

3. Query tree protocol with reversed IDs

The query tree protocol with reversed IDs (QTR pro-

tocol) reverses the ID strings of the tags and applies QT

protocol to these reversed strings.

READER: The reader does the same as in the QT

protocol.

TAG: Every tag receiving the query string s from the

reader compares s with its own ID string a. If s matches

the prefix of length |s| of aR, then the tag sends a to the

reader.

In the example of Figure 1, the set of the reversed IDs

is {10010, 01010, 11010, 00110, 10110}. Figure 3 shows

the queries and responses communicated between the reader

and the set of reversed IDs. Figure 4 depicts the query tree

of Figure 3.

ID = {01001, 01010, 01011, 01100, 01101}Reversed ID = {10010, 01010, 11010, 00110, 10110}

Step Query Response

1 0 collision

2 1 collision

3 00 01100

4 01 01010

5 10 collision

6 11 01011

7 100 01001

8 101 01101

Figure 3. Queries and responses between thereader and the tags with IDs reversed.

Figure 4. Query tree of Figure 3.

4. Analysis of complexity

In this section we analyze the complexity of the com-

munication between the reader and the tags in terms of the

number of queries, assuming the scenarios where tag IDs

are consecutive integers and thus have a common prefix.

Let A = {b0, b1, . . . , bn−1} be a set of bit strings of

equal length. Define Q(A) to be the tree obtained after QT

protocol is applied to A. Note that Q(A) is uniquely de-

termined by A. Let e(A) be the number of edges of Q(A).

Then, e(A) is the number of queries sent by the reader.

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4.1. IDs having common prefix or suffix

Let c = α1 · · · αk be a string of length k, where

αi ∈ 0, 1. Prefix c to each string of A. Let A ={cb0, cb1, . . . , c bn−1}. Apply the query tree protocol to A

to obtain Q(A).

Lemma 1 e(A) = 2k + e(A).

Proof:If the reader broadcasts query string α1 · · · αi for

any 1 ≤ i ≤ k, a collision will be detected. So, Q(A) will

have a path with k edges. Let π denote this path. The nodes

of π are all gray. The edges of π are labeled with α1, · · · , αk

from the root.

If the reader broadcasts α1 · · · αi−1αi for any 1 ≤ i ≤ k,

then no tag will respond. αi is the complement of αi, i.e.,

αi = 1 − αi. So, each node of π except the last one has

another edge to a child, which is labeled with αi.

If the reader broadcasts query string cb where b is a bit

string and |b| ≤ |b0|, then it will receive the same response

as when it broadcasts b to the tags whose IDs are in A. So,

a copy of Q(A) will be hanged to the last node of π.

Hence, Q(A) has 2k + e(A) edges. 2

Now, append c at the end of each string of A and let

A = {b0c, b1c , . . . , bn−1c}. Apply the query tree protocol

to A.

Lemma 2 Q(A) and Q(A) are identical, and e(A) =e(A).

Proof: The communication between the reader and the

tags with IDs in A will be exactly the same as the one

between the reader and the tags with IDs in A. The reader

will not have a chance of broadcasting query strings of

length greater than |b0| as the tags will be identified before

that happens. So, Q(A) and Q(A) are identical.

Hence, e(A) = e(A). 2

4.2. IDs forming consecutive integers

Let H be the length of a tag ID. Then N = 2H is

the total number of possible tag IDs of length H . Con-

sider a set of n ≥ 2 consecutive nonnegative integers,

{a, a + 1, . . . , a + n − 1}. Let A = {b0, b1, . . . , bn−1},

where bi is the bit string of length H representing a +i for 0 ≤ i ≤ n − 1. Let c be the longest pre-

fix that is common to b0, b1, . . . , bn−1. In other words,

bi = cdi for 0 ≤ i ≤ n − 1 and d0, d1, . . . , dn−1 have

no common prefix. Let B = {d0, d1, . . . , dn−1}. Let

h = |d0|. d0, d1, . . . , dn−1 still form consecutive integers,

a mod 2h, (a + 1) mod 2h, . . . , (a + n − 1) mod 2h.

Define AR = {bR0 , bR1 , . . . , bRn−1}, and BR = {dR0 ,

dR1

, . . . , dRn−1}. In the example of Figure 1, B = {001,

010, 011, 100, 101} and BR = {100, 010, 110, 001, 101}Since A can be obtained by prefixing c to every string of

B and AR can be obtained by suffixing cR to every string

of BR, the following is from Lemma 1 and 2.

Lemma 3 (i) e(A) = 2(H − h) + e(B) , and (ii) e(AR) =e(BR).

Note that B has n strings that are of length h, have no

common prefix, and form consecutive integers.

Lemma 4 e(BR) = 2(n − 1).

Proof: If n = 2, then the first bits of dR0 and dR1 are

different as d0 and d1 are consecutive integers (one odd and

the other even). A query tree with two edges is sufficient to

discriminate dR0

and dR1

.

Let B0 (resp., B1) be the set of strings in B that end with

a 0 (resp., a 1). B0 contains the even integers from B and

B1 contains the odd integers from B. Let n0 = |B0| and

n1 = |B1|. Then, n0 ≥ 1, n1 ≥ 1 and n = n0 + n1. Let

C 0 (resp., C 1) be the set of strings obtained by deleting the

last bit of the strings in B0 (resp., B1). The strings in C 0(resp., C 1) have no common prefix. It is not difficult to see

that C 0 (resp., C 1) has n0 (resp., n1) strings and they form

consecutive integers. By induction hypothesis, e(C R0

) =2(n0 − 1) and e(C R1 ) = 2(n1 − 1).

Q(BR) consists of (i) the root, (ii) two edges from the

root, one of them is labeled with a 0 and the other with a

1, and (iii) two subtrees, Q(C R0 ) and Q(C

R1 ). The edge

labeled with a 0 connects the root and Q(C R0 ), and the edge

labeled with a 1 connects the root and Q(C R1

).

Hence, e(BR) = 2 + e(C R0

) + e(C R1

) =2 + 2(n0 − 1) + 2(n1 − 1) = 2(n − 1). 2

Lemma 4 implies that Q(BR) is mininal in the sense that

it has the least number of edges among the query trees for

a set of n IDs. A query tree is a full binary tree and has

n black leaves and some white leaves. A full binary tree

with 2(n − 1) edges has exactly n leaves. Thus, Q(BR) has

exactly n leaves and all of them are black.

Lemma 5 e(B) ≥ e(BR).

Proof: Since Q(B) is a query tree for a set of n IDs,

e(B) ≥ 2(n − 1). 2

We have proved the following theorem. Specifically,

e(A) ≥ 2(H − h) + 2(n − 1) and e(AR) = 2(n − 1).

Theorem 1 e(A) ≥ e(AR).

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5. Simulation

The performance of QT protocol and QTR protocol will

be examined by two simulations. The first simulation mea-

sures the number of queries used untill recognizing all

tags[7]. Figure 5 is the result of simulation in which QT

protocol and QTR protocol are applied to each two scenar-

ios above mentioned. In the diagram x axis is the number

of tags, and y axis is the number of queries to recognize all

tags.

Figure 5. Comparison of query number.

When tag IDs are random , QT protocol and QTR pro-

tocol have no difference in performance. In the case that,

however, tag IDs are consecutive IDs , the performance of

QTR protocol is better than QT protocol.The second simulation measures the transmitted bits

used untill recognizing all tags[7]. Figure 6 is the result

of simulation in which QT protocol and QTR protocol are

applied to scenario II . In the diagram x axis is the number

of tags, and y axis is the transmitted bits to recognize all

tags.

Figure 6. Comparison of transmitted bits.

6. Conclusions

This paper proposes a tag anti-collision protocol of RFID

system. This protocol works by reversing the IDs of the tags

and then applying the query tree protocol[5]. The motive of

this protocol has started from the idea that, when classifying

the several numbers of different bit strings, if the bit string

has consecutive or identical prefix, it is effective to classify

the suffix first. Actually, for an item generated from the

same company, the EPC code of EPCglobal has the same

header, company ID and product ID,and the serial numbers

are consecutive[1].

From this basis, the ID of most tags has identical and

consecutive prefix from manufacture to retail store on the

progress of supply chain management. Therefore,this pro-

tocol has effective performance from manufacture occupy-

ing the most on the progress of supply chain management to

retail store. Even though the ID of tag is random on the con-

trary to this, it brings almost same performance with the QTprotocol. The analysis and simulation in this paper proves

the fact in the above.

This protocol has improved its performance in certain

occasions by transforming a bit based on QT while having

the same performance in general. Therefore, it can deal with

QT protocol sufficiently.

7. Acknowledgements

This work was supported by the Korea Science and En-

gineering Foundation(KOSEF) grant funded by the Korea

government(MOST) (No. R01-2005-000-10568-0)

References

[1] EPCglobal Tag Data Standards Version 1.3,

http://www.epcglobalinc.org/standards/tds/TDS 1 3-

StandardRatified-20060308.pdf, 2006.

[2] EPC Radio-Frequency Identity Protocols Class-1

Generation-2 UHF RFID Conformance Requirements

Specification v.1.0.2, EPCglobal Inc, February 2005

[3] K. Finkenzeller, RFID Handbook Second Edition, Wi-

ley & Sons, 2002.

[4] R.L. Graham, D.E. Knuth, and O. Patashnik, Concrete

Mathematics, Second Edition, Addison-Wesley Pub-

lishing Company, 1994.

[5] C. Law, K. Lee, and K.Y. Siu, Efficient memoryless

protocol for tag identification, Proceedings of the 4th

International Workshop on Discrete Algorithms and

Methods for Mobile Computing and Communications,

pp.75-84, 2000.

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[6] F.C. Schoute, Control of ALOHA signaling in a mo-

bile radio trunking system, International Conference

on Radio Spectrum Conservation Techniques, IEEE,

pp. 38–42, 1980.

[7] J.D. Shin, S.S. Yeo, T.H. Kim, and S.K. Kim, Hy-

brid Tag Anti-Collision Algorithms in RFID Systems,The International Conference on Computational Sci-

ence 2007 (ICCS 2007), Lecture Notes in Computer

Science, vol.4490, pp.693-700, 2007.

ISBN 978-89-5519-136-3 -230- Feb. 17-20, 2008 ICACT 2008

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