
TAG PROTOCOLS 79
person. “221b Baker Street London NW1 6XE, Great Britain” is an identity referring to
a particular place, just as “urn:epc:id:sgtin:00012345.054322.4208” is an identity refer-
ring to a particular widget.
How Tags Store Data
The high-level tag communications protocols know about the ID types that can be
stored on a tag and so know, at a general level, how data is stored on the tag. However,
since a reader only communicates with a tag via the air protocol, the actual physical lay-
out of memory on a tag is left to the tag manufacturer. A discussion of the physical lay-
out is beyond the scope of this book, but the logical or apparent layout of tag memory is
specified in the EPC Class 0 and Class I (Generation 1) standards. Figure 4-1 shows that
layout.
The CRC is a checksum (described in more detail in the sidebar “CCITT-CRC”), the EPC
is the ID on the tag, and the password is the “kill code” to disable the tag. Note that for a
tag to be EPC-compliant under Class 0 and Class I Generation 1 standards, it must never
transmit the password under any circumstances—not even in response to proprietary
diagnostic commands used only by the manufacturer.
This password is the same password described previously as a means of disabling (destroy-
ing) tags. (Under Gen2, this changed; we will describe Gen2 in more detail in the “EPC
UHF Class I Gen2” section of this chapter.)
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