
TAG PROTOCOLS 89
Tag 5 puffs out his cheeks and sits in silence as Tags 1 and 2 roll their dice…. Rather than
using timed intervals, the reader may indicate the end of a slot using a signal, called a “slot
marker.” Anti-collision protocols that use these signals are called Slot Marker protocols. A
Slot Marker implementation of the Slotted Aloha protocol works much the same as that
described above, except that the end of a slot is indicated by a signal rather than by the
expiration of a set time. This allows some slots to be longer than others so that there is a
better chance of a read in each slot.
Adaptive Binary Tree
EPC Class 0 and Class I Version 1.0 (Generation 1) UHF tags use a slightly more compli-
cated approach to singulation and anti-collision known as an Adaptive Binary Tree proce-
dure. As the name implies, this procedure uses a binary search to find one tag among
many. Most of us are familiar with binary trees from other domains, but for the sake of
clarity, we will revisit the basics here. Then we’ll explain some of the nuances of doing a
binary tree search using a query/response approach similar to the previous description of
Slotted Aloha. Unlike with Slotted Aloha, with this protocol tags respond immediately.
The EPC specification for the air interface of UHF tags uses two separate subcarriers for 1s
and 0s in tag responses. Because the protocol doesn’t care how many tags responded ...