
TAG PROTOCOLS 83
written into a tag’s memory, or physically realized through some other technology.
Figure 4-3 gives an example of encoding a pure identity as a bar code or an EPC tag.
Notice that the GID is used as the fundamental encoding for EPC tags. Other encodings
take an identity, which might be encoded in another form (such as a bar code), and con-
vert it to a form that fits into a GID with a syntax similar to “Header.GeneralManager-
Number.ObjectClass.SerialNumber.” This requires the EPC to take some liberties with the
GS1 SGTIN. For example, while the guidance from GS1 is that the GTIN should never be
parsed and treated as separate fields, an EPC must have a General Manager Number and
an Object Class for the Object Naming Service (ONS) to work, so the EPC-SGTIN divides
the GTIN. In this case, an SGTIN is an example of a pure identity. The EPC-SGTIN is the
encoding, and the bits on an EPC tag representing the EPC-SGTIN are the physical realiza-
tion of that encoding.
GS1 SGTIN Encoding
EPC readers and RFID middleware present tag data according to its EPC encoding. Inter-
facing with a reader or middleware requires at least a cursory understanding of tag encod-
ings for debugging code. An understanding of these encodings is also required to define
the events and filters introduced in Chapter 2, which we will describe in more detail in
Chapters 6 and 7. The SGTIN is a good example of an ...