ThingMagic Mercury API v1.23.0 - Manual (Page 122)

How UHF RFID Works (Gen2)
122 Performance Tuning
How UHF RFID Works (Gen2)
At its most basic level, the reader powers up the tag at the same time it is communicating
with it. A good analogy for this kind of system would be if two people are trying to send
messages from one mountaintop to another at night, where one has only a flashlight
(representing the reader) and the other has a mirror (representing the tag). The person
with the flashlight can send messages by turning the flashlight on and off, but when he
wants a response, he must keep the flashlight on so his partner can signal back with the
mirror. Layer on top of that the possibility that there may be many people on other
mountaintops, also with mirrors, ready to send messages back to the “reader”, and you
have an idea of the communication challenge the Gen2 protocol was designed to handle.
The settings which govern the tag’s behavior are all controlled by the reader via
messages it sends when it communicates with one or more tags. These settings do not
remain in effect long. Some last for a single exchange with the reader, some until all tags
present have responded, some until the tag powers down, and some for a fixed period of
time. After a few minutes, however, the Gen2 protocol expects all tags to fall back into
their default state so a second reader would automatically know what initial state the tags
are in when it encounters them.
There are quite a few Gen2 settings, but they can be grouped into 3 categories:
1. Settings that control how the reader communicates to the tags
2. Settings that control how the tag communicates back to the reader
3. Settings that control when tags respond relative to each other to avoid
communication collisions
Also, as you learn more about the Gen2 options, keep in mind which aspects of RFID
performance you are looking to optimize for your application. Typical choices are:
1. Maximum read distance
2. Minimum time to read every tag in the field at least once
3. Minimum time to read a single tag traveling through the field
4. Maximum number of responses from any tag in the shortest amount of time
The last case, 4, is rarely required in practical applications, but it is mentioned along with
the others because it is often the first thing users try to do when they test a reader in a lab
environment. Your goals may also include minimizing negative influences on
performance, such as reducing reader-to- reader interference when many readers are
located in close proximity. Gen2 settings can help there, too.