Session

Session Flags in EPC Gen2

Session flags are a tag-side state mechanism defined by EPC Gen2 that controls how long a tag remembers it has been inventoried. They are critical for managing tag behaviour in multi-reader environments and for controlling re-read rates during singulation. Understanding sessions is essential for any deployment involving portal readers, conveyors, or overlapping read zones.

The Four Sessions

epc-gen2/" class="glossary-term-link" data-term="EPC Gen2" data-definition="UHF RFID air interface standard." data-category="Standards & Protocols">EPC Gen2 defines four independent session flags — S0, S1, S2, and S3. Each flag has two states, A and B. When a reader begins an inventory round, it targets tags in state A (or B). After a tag is successfully singulated, it flips its flag to the opposite state. The key difference between sessions is how long the tag retains its flipped state before reverting:

Practical Usage

Single reader, continuous scanning: Use S0 or S1. Tags revert quickly, so the reader can re-read the entire population every few seconds. This is ideal for cycle counting where an operator waves a handheld reader across a shelf.

Portal / conveyor: Use S2 or S3. When a tag passes through a portal reader and is read, it flips to state B and stays there. If the tag enters a second overlapping read zone, the second reader can be configured to target only A-state tags — effectively ignoring already-read items and reducing duplicate processing in middleware.

Multi-reader environment: Different readers can use different sessions simultaneously without interfering. Reader A inventories on S2, Reader B inventories on S3. Each reader's activity is invisible to the other's session flag, avoiding cross-reader contention.

Interaction with Anti-Collision

Session flags work hand-in-hand with the Q-algorithm and anti-collision mechanisms. By flipping tags out of the target state after successful reads, the effective tag population shrinks progressively during an inventory round, reducing collisions and accelerating throughput.

Common Pitfalls

Misconfigured sessions are one of the most frequent causes of poor RFID performance. Using S0 in a multi-reader environment causes duplicate reads and excessive middleware load. Using S2 or S3 in a single-reader handheld scenario causes tags to appear to "disappear" because their flags persist longer than the operator expects. Always match the session to the deployment topology and re-read requirements.