
TAG PROTOCOLS 87
For each system identity, the specification describes an alternative encoding for 64-bit
tags. To help fit the identity into a 64-bit encoding, the Company Prefix is removed and a
Company Prefix Index takes its place in the encoding. This index is an offset into a Com-
pany Prefix table. A Company Prefix Index is supplied as needed by GS1 to those entities
that intend to use 64-bit tags. This table is limited to 16,384 entries, and this encoding
scheme is intended as a temporary solution until the industry universally adopts tags that
are 96 bits or larger. Note that the SGTIN-64 encoding is the only currently defined header
that contains a 1 in the MSB, allowing it to be only two binary digits long (11 is reserved
for other 64-bit encodings).
For headers, partition tables, and field types of the other EPC-defined 96-bit encodings,
see Appendix A.
Singulation and Anti-Collision Procedures
Our next topic concerns the way a reader and a tag use the air interface. There are many
different ways for readers and tags to communicate, but the different methods can all be
broadly categorized as Tag Talks First (TTF) or Reader Talks First (RTF). Logically, it would
seem simplest for a tag arriving on the scene to announce its presence to all concerned. In
practice, however, this is difficult unless tags are able to negotiate among themselves which
tag will speak first. Some high-end active ...