User's Manual

CAEN RFID API– User Manual
15
Sessions
When a tag is singulated by the “slotted ALOHA” algorithm described in the previous paragraph, it switches
automatically from an internal state A to another internal state B.
In the EPC C1G2 terminology these states are called “target A” and “target B” respectively; for our purposes we can
refer to target A as the non-inventoried state and to target B as the inventoried state: singulation makes therefore
switch a tag from the non-inventoried state to the inventoried state.
By default CAEN RFID readers looks for non-inventoried tags so once a tag has been inventoried it is no more
detectable by the reader until it returns back in the non-inventoried state.
Tags return in the non-inventoried state autonomously and the times needed to return detectable can be regulated
using the session parameter. This tuning can have a big impact on the inventory process performances especially in
case of large tags' population.
Gen2 protocol provides four different sessions: S0, S1, S2, S3. Each session has its own independent inventoried and
non-inventoried states with its specific persistence time as shown in the following table:
Flag
Required persistence
S0 inventoried flag
Tag energized: Indefinite
Tag not energized: None
S1 inventoried flag
Tag energized:
Nominal temperature range: 500ms < persistence < 5s
Extended temperature range: Not specified
Tag not energized:
Nominal temperature range: 500ms < persistence < 5s
Extended temperature range: Not specified
S2 inventoried flag
Tag energized: Indefinite
Tag not energized:
Nominal temperature range: 2s < persistence
Extended temperature range: Not specified
S3 inventoried flag
Tag energized: Indefinite
Tag not energized:
Nominal temperature range: 2s < persistence
Extended temperature range: Not specified
Tab. 3.2: Persistence Time
When session S0 is used, each time the reader switches off the radiofrequency field, the tags return back in the original
not-inventoried status so that they are again detectable with a successive inventory algorithm round. Since CAEN RFID
readers switch off the field at the end of each inventory round, all the tags are detected again each time the inventory
is repeated. This behavior can be desirable for testing purposes or when a certain level of redundancy is needed but it
is typically not efficient and generate a lot of useless information.
With session S1 the tags, once singulated, remain in the inventoried state for a time in the range between 500ms and
5s regardless if the tag is energized or not; during this period they are no more detectable by the subsequent inventory
iterations. On large tags' populations this behavior helps to reduce the number of tags simultaneously detectable by
the reader optimizing the speed of the inventory process and reducing the generated data.
Sessions S2 and S3 have a longer and not explicitly limited persistence time giving the opportunity to detect tags only
one time during the inventory process repetitions. These sessions are for advanced use only and are out of the scope of
this manual.
CAEN RFID API provides the methods to set and read back the session parameter: SetSession_EPC_C1G2 and
GetSession_EPC_C1G2.