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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
Jea-Dong ShinSchool of Computer Science & Engineering,
Chung-Ang University,
Seoul, Korea
Sung Kwon Kim
School of Computer Science & Engineering,
Chung-Ang University,
Seoul, Korea
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
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Generation-2 UHF RFID Conformance Requirements
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[6] F.C. Schoute, Control of ALOHA signaling in a mo-
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