Database Reference
In-Depth Information
5.3 Blocker Tags
An interesting solution for making it dicult to read tags in an unau-
thorized way is the use of blocker tags [41, 42]. Blocker tags exploit
the collision properties of RFID transmission, which are inherent in this
technology. The key idea is that when two RFID tags transmit dis-
tinct signals to a reader at the same time, a broadcast collision occurs,
which prevents the reader from deciphering either response. Such col-
lisions are in fact very likely to occur during the normal operation of
the RFID infrastructure. In order to handle this issue, RFID readers
typically use anti-collision protocols. The purpose of blocker tags is to
emit signals (or spam) which can defeat these anti-collision protocols,
thereby causing the reader to stall. The idea is that blocker tags should
be implemented in a way, that it will only spam unauthorized readers,
thereby allowing the authorized readers to behave normally.
Typically the anti-collision protocols which are used are also referred
to as singulation protocols, which allow the tag reader to systematically
explore all the tags in a certain order with the use of a tree-walking
protocol , which singles out all the tags for scanning in a specific order.
This is achieved by treating the binary code on each tag in the form of
a binary tree, where each node in the tree is considered a prefix of the
binary tree. The idea is that the reader has the capability to scan for
tags containing only a particular prefix, and ask all other tags to remain
“silent”. Tags which contain that particular prefix, transmit their bit
which comes just after that prefix. The algorithm starts at the root of
the tree, and scans the first bit of the tags. In the event that both 0 and 1
is transmitted, then a collision will occur, which is detected. This means
that both branches of the tree need to be explored, since there are tags
which contain both a 0 and a 1 in the first. Clearly, a collision is quite
likely to occur at the higher levels of the tree. On the other hand, if only
a 0 is transmitted, then the left branch of the tree needs to be explored.
Otherwise, the right branch of the tree is explored. This process is used
to recursively traverse the portion of the tree which is relevant to the
RFID tags being scanned. This recursive traversal finally reaches the
leaves of the tree, at which point, the tags are recorded uniquely by the
reader. It is clear that for a 96-bit Class 1 EPC tag, the portion of the
tree which is explored by the reader is an extremely tiny fraction of the
2 96 possible nodes in the tree, since the number of distinct tags being
present would be much smaller than 2 96 . In fact, the entire tree-size is
too large to be explored by the tree-walking algorithm.
The blocker tag takes advantage of this property and forces (mali-
cious) readers to explore the full tree of size 2 k , which would cause the
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