
94 CHAPTER FOUR
Singulated Command Mute
In this state, a tag waits quietly until it receives a data null, at which point it proceeds
to the Singulated Command Start state.
Slotted Terminal Adaptive Collection (STAC)
The STAC protocol is similar in many ways to Slotted Aloha, but some of the details of this
protocol make it more complex and otherwise different enough to warrant separate treat-
ment. STAC is defined as part of the EPC specification for HF tags. Because it defines up to
512 slots of varying lengths, it is especially well suited to singulation of large populations
of tags. This protocol also allows for the selection of groups of tags based on matching
lengths of EPC code beginning with the MSB. Because the EPC code is organized by
Header, Domain Manager Number, Object Class, and Serial Number (in that order) from
MSB to LSB, this mechanism can easily select only tags belonging to a particular Domain
Manager or Object Class. Since HF tags are often used for individual item identification,
this is especially useful if, for example, the application would like to know how many of
the items on a mixed pallet are boxes of A4 paper.
Like Slotted Aloha, STAC uses slots. Figure 4-11 illustrates how the slots are used.
The F or “fixed” slot is always present and is always the same length. Numbered, variable-
length slots follow. These slots must begin with a slot named “0,” and there must be ...