GS1 EPC Tag Data Standard 1.6 - Indicod-Ecr

489848994900490149024903490449054906490749084909491049114912491349144915491649174918491949204921492249234924492549264927492849294930493149324933493449354936493749384939494049414942I.9.2.3 Usage as a Presence/Absence DirectoryIn many applications, an Interrogator may choose to read the entire contents of any datacarrier containing one or more “target” data items of interest. In such applications, thepositional information of those data items within the memory is not needed during theinitial reading operations; only a presence/absence indication is needed at this processingstage. An ID Map can form a particularly efficient Presence/Absence directory fordenoting the contents of a data carrier in such applications. A full directory structureencodes the offset or address (memory location) of every data element within the datacarrier, which requires the writing of a large number of bits (typically 32 bits or more perdata item). Inevitably, such an approach also requires reading a large number of bits overthe air, just to determine whether an identifier of interest is present on a particular tag. Incontrast, when only presence/absence information is needed, using an ID Map conveysthe same information using only one bit per data item defined in the data system. Theentire ID Map can be typically represented in 128 bits or less, and stays the same size asmore data items are written to the tag.A “Presence/Absence Directory” Packed Object is defined as a Directory IDMPO thatdoes not contain a PO Index, and therefore provides no encoded information as to whereindividual data items reside within the data carrier. A Presence/Absence Directory can beconverted to an “Indexed Directory” Packed Object (see I.9.2.4) by adding a PO Index inan Addendum Packed Object, as a “child” of the Presence/Absence Packed Object.I.9.2.4 Usage as an Indexed DirectoryIn many applications involving large memories, an Interrogator may choose to read aDirectory section covering the entire memory’s contents, and then issue subsequentReads to fetch the “target” data items of interest. In such applications, the positionalinformation of those data items within the memory is important, but if many data itemsare added to a large memory over time, the directory itself can grow to an undesirablesize.An ID Map, used in conjunction with an AuxMap containing a PO Index, can form aparticularly-efficient “Indexed Directory” for denoting the contents of an RFID tag, andtheir approximate locations as well. Unlike a full tag directory structure, which encodesthe offset or address (memory location) of every data element within the data carrier, anIndexed Directory encodes a small relative position or index indicating which PackedObject contains each data element. An application designer may choose to also encodethe locations of each Packed Object in an optional ObjectOffsets subsection as describedabove, so that a decoding system, upon reading the Indexed Directory alone, cancalculate the start addresses of all Packed Objects in memory.The utility of an ID Map used in this way is enhanced by the rule of most data systemsthat a given identifier may only appear once within a single data carrier. This rule, whenan Indexed Directory is utilized with Packed Object encoding of the data in subsequentobjects, can provide nearly-complete random access to reading data using relatively fewdirectory bits. As an example, an ID Map directory (one bit per defined ID) can beassociated with an additional AuxMap “PO Index” array (using, for example, three bitsper defined ID). Using this arrangement, an interrogator would read the DirectoryPacked Object, and examine its ID Map to determine if the desired data item were presenton the tag. If so, it would examine the 3 “PO Index” bits corresponding to that data item,Copyright ©2005- 2011 GS1 AISBL, All Rights Reserved. Page 190 of 218