ThingMagic Mercury API v1.23.0 - Manual (Page 126)

How UHF RFID Works (Gen2)
126 Performance Tuning

Session “0”: Prepare to respond again as soon as RF power drops

Session “1”: Prepare to respond again between 0.5 and 5 seconds after first
response
 Session “2” or “3”: Do not respond again for at least 2 seconds
Selection of the session setting is nearly always based on the expected number of tags in
the field. You usually want all tags in the field to have had a chance to respond once
before the first tags start responding again. For ThingMagic’s default settings, the tag
read rate is around 200 tags per second. With these settings, you would want to use
session 0 up to around 100 tags in the field, and session 1 up to around 400 tags, and
session “2” (or “3”) for more than 400 tags.
The ThingMagic “target” setting is only important if you want to force tags to re-respond
more often than they otherwise would. There are two relevant choices, “A” and “A then B”.
“A” means that the reader always looks for tags that are in the “A” state. Tags held in their
“B” state by their session timer are ignored. “A then B” tells the reader to read all the tags
in the “A” state, then read all the tags in the “B” state, and keep repeating this process.
Tags in their “B” state will respond to a “B” query and return immediately to their “A” state,
regardless if there was time left on their session timer. This accelerated read rate is only
useful in two cases:
1. The application needs to read the same tag multiple times in order to determine more
than the tag’s identity, for example, if an attempt is being made to determine whether
the tag is moving toward or away from the antenna by monitoring the tag’s returning
signal level.
2. The user is attempting to estimate the reader’s performance in the presence of many
tags by reading fewer tags over and over.