Inventory

Q Value

The Q setting in the UHF RFID system comes from the anti-collision procedure (Q-Algorithm) defined in the EPCglobal Gen2 / ISO 18000-Tuna3 standard.

Background

In UHF RFID, multiple transponders (tags) can be in the read range simultaneously. To ensure they don't all answer at the same time (which leads to collisions), a so-called Slotted ALOHA method is used. Here, time is divided into "slots." Each tag randomly selects a slot and transmits only during that slot.

Meaning of Q

Q is a parameter that determines the size of the "slot window." The number of slots = 2^Q. Example:

• Q = 0 → 1 Slot
• Q = 4 → 16 Slots
• Q = 8 → 256 Slots
• Q = 15 → 32,768 Slots

The higher the Q value, the more slots there are → fewer collisions, but longer reading time.

Fixed Q Values vs. Auto

Fixed Number (e.g., Q=4): The reader always uses the same number of slots, regardless of how many tags are present. This can be inefficient if the number of tags in the field varies significantly.

Q=Auto (dynamic): The reader adjusts the Q value itself, depending on how many tags it detects in the field.

• Few tags → small Q (faster detection)
• Many tags → larger Q (fewer collisions)

Practically, this means:

• Small Q = good for few transponders, fast identification.
• Large Q = necessary when many transponders need to be read simultaneously.
• Auto is the best choice in most cases because the device optimizes dynamically.

In short: "Q" stands for the size of the slot window in the anti-collision process. It regulates how tags "distribute" their responses so they don't talk over each other.

Target

The Target setting (A or B) in UHF RFID is directly related to the inventory process (Inventory Rounds) in the Gen2 protocol.

Background

In the EPCglobal Gen2 / ISO 18000-Tuna3 standard, there are so-called Inventory Flags (also called Inventoried Flags) for tags. Every tag in the field possesses two possible states, designated as A or B.

• These flags help the reader distinguish already inventoried tags from those not yet read.
• This prevents a tag from answering immediately again in every round (infinite loops/collisions).

How it works

1. Start of an Inventory Round
   • The reader specifies which Target it wants to work with (A or B).
   • Example: Target = A → only tags with Flag "A" are allowed to answer.
2. Tag Response Behavior
   • When a tag is successfully read, it switches its flag (e.g., from A to B).
   • As a result, the tag "withdraws" from the current round until the reader selects the other Target again.
3. Target Switch by the Reader
   • After a round, the reader can change the Target (e.g., from A to B) to query the other tags again.

Typical Settings

• Target = A (fixed) → The reader works only with tags in state A. This is often sufficient for simple readings.
• Target = B (fixed) → Corresponds to the other slot of the inventory flags.
• Auto / Toggle (depending on Reader) → The reader automatically switches between A and B so that all tags are reliably inventoried without manual intervention.

Practical Use

• A or B alone: Useful if you want to perform a quick reading just once.
• Switch / Toggle: Necessary if you want to ensure that truly all tags in the field are read at least once.

Summary: Target A/B controls which inventory flag on the tags the reader works with in the current round. Together with switching flags after a successful response, this prevents a tag from answering multiple times in the same round.

Session

The EPC Gen2 standard defines four sessions: S0, S1, S2, S3. Each tag maintains a separate A/B status for each session. When the reader selects a session, the Target setting (A/B) applies only to the selected session.

Difference between Sessions

The sessions differ mainly in how long the tags keep their A/B flag (persistence) after being read.

• Session S0: Flag switches back very quickly (within milliseconds). Tags become "immediately visible again." Good for continuous, fast reading in the same reader cycle.
• Session S1: Flags remain for a certain time (seconds range). Read tags remain in the other state and do not answer immediately again. Ideal for short inventories where you don't want to constantly read the same tag multiple times.
• Session S2 and S3: Flags remain for a very long time (seconds to minutes or longer, depending on the tag manufacturer). Often used for stock counts or multiple readers to prevent tags from being recorded multiple times.

Interaction with Target

• Target=A means: in the selected session, the reader queries only the tags that currently have "A".
• After a successful read, these tags switch their flag to "B" (in this session).
• In a second pass (same session, Target=A), these tags no longer answer because they are now B.
• If the reader now selects Target=B, it can specifically query the already read tags.

Practical Example

• S0 + Target=A: All tags answer, but if you read again immediately, they are there again (because the flag reverts quickly). → Good for live tracking with high update rates.
• S1 + Target=A: Tags that were read stay silent for a while. → Good for inventory, each tag is counted only once.
• S2/S3 + Target=A: Tags stay silent for a very long time. → Good when multiple readers monitor the same area (e.g., Logistics).

Summary: Target (A/B) = which flag state is queried in the selected session. Session (S0–S3) = determines how long the tags keep their flag after being read.

Overview RFID Session S0–S3

Combining Q, Target and Session

• Q Value (Anti-Collision): Regulates the slot size: the more tags, the higher the Q needed. Auto-Q: adjusts dynamically → almost always recommended. Important for resolving simultaneous answers. → Q ensures how many tags can answer simultaneously in an organized way.
• Target (A/B): Determines which tags are allowed to answer in the selected session. Target=A → Initial state: all tags answer. After a successful read, these tags switch to B → with Target=B you can query the already read tags again. → Target separates the "seen" from the "unseen" tags in the current session.
• Session (S0–S3): Determines how long the switch state A⇄B persists. S0: reverts immediately → good for real-time tracking. S1: stays silent briefly → one-time count per reading. S2/S3: stay silent very long → prevent multiple counts over time or across multiple readers. → Session controls the persistence of the flag and thus the "counting method."

Practical Combinations

Rules of Thumb

• Always use Auto-Q, unless you are optimizing very special scenarios.
• Target almost always = A → B only for special cases.
• Select Session according to the use case: S0 = Live data stream, S1 = Inventory / counting once, S2/S3 = Multiple readers / long-term counting

→ This allows you to set whether your reader should see all tags quickly and repeatedly, or if it should count each tag only once—and how long it remains silent.

Best Practices

Q Value (Anti-Collision)

• Activate Auto-Q → Reader adapts dynamically to the tag quantity.
• Fixed Q Value only for special cases: Few tags (<30) → small Q (2–4), Many tags (>500) → larger Q (8–12)

Target (A/B)

• Standard: Always Target=A → all tags start with Flag=A → readable immediately.
• Target=B: only useful if you specifically want to re-query already read tags.
• Toggle/Auto: if supported by the reader, ideal for complete inventory.

Session (S0–S3)

• S0: Flag reverts immediately → Tags reappear constantly. Ideal for real-time tracking / movement tracking.
• S1: Flag remains silent briefly (seconds). Ideal for inventory, each tag counted only once.
• S2 / S3: Flag remains silent for a long time (seconds–minutes, depending on manufacturer). Ideal for large stock counts or multi-reader scenarios to avoid multiple counts.

Combinations

• Real-time Tracking: Q=Auto, Target=A, Session=S0
• Inventory (fast and one-time count): Q=Auto, Target=A, Session=S1
• Large Warehouse Count with Multiple Readers: Q=Auto, Target=A, Session=S2 or S3
• Specifically addressing already read tags: Session as selected, but Target=B