A passive RFID reader energizes every tag in range. If several tags backscatter at the same reply time, the reader hears overlapping waveforms instead of one clean identifier.
RFID ALOHA Anti-Collision Workbench
Step through pure ALOHA, slotted ALOHA, and EPC Gen2-style frame slotted inventory rounds.
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rfid
aloha
networking
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Learner-ready RFID ALOHA anti-collision workbench showing reader queries, random tag replies, empty slots, collisions, successful reads, Q/frame-size tuning, and common RFID inventory misconceptions.
RFID
ALOHA anti-collision
step-through animation
EPC Gen2 context
RFID ALOHA Anti-Collision Workbench
Power several passive tags, let them choose reply times, and watch the reader separate empty slots, collisions, weak replies, and successful reads. The important learning point is not that collisions are bad; it is how frame size and random slot choice make dense inventory possible.
0/12tags identified
0collision slots
0empty slots
0%observed successes per slot
Q4 / 16current frame size
ALOHA-style protocols make tags choose random reply opportunities. Empty slots waste time, single-tag slots succeed, and multi-tag slots collide and must be retried later.
Use the Q/frame-size control. Too few slots creates collisions. Too many slots creates empty waiting time. A good inventory round balances both.
Reader inventory stage
1. QueryReader starts a round
2. ChooseTags pick reply times
3. ReplyCurrent slot opens
4. ResolveEmpty, success, or collision
5. AdjustQ and retry planning
Reader field and active replies
Current round diagnosis
Slot 0 waitingNo slot processed yet.
Load G = 0.75Approximate unread tags per slot.
Slotted max 36.8%Per-slot throughput limit for ideal slotted ALOHA.
Event log
Reply opportunities in this round
Tags reply whenever they are ready. A packet is vulnerable to overlapping starts before and after its own transmission, so the ideal maximum throughput is 1/(2e), about 18.4%.
Tags are aligned to slot boundaries. The vulnerable period is one packet time, giving ideal maximum throughput 1/e, about 36.8% per slot.
Tags choose one slot in a frame. EPC Gen2-style inventory uses a Q value where frame size is 2^Q. Successful tags are inventoried while colliding tags retry later.
Guided Practice
Select Frame too small, then step through the first frame. Which symptom appears first: empty slots or collision slots?
With 20 tags, increase Q until collisions fall without creating mostly empty slots. Explain why the best Q is not always the largest Q.
Select Weak edge tags. A missed reply is not a collision. What extra evidence would a real reader need before assuming a tag population estimate?
Technical Accuracy Notes
Q selects a frame size of 2^Q reply opportunities in this simplified inventory model.
Q is not the number of tags. It is a reader-side control for response probability.
A collision means more than one tag replied in the same opportunity, but the reader usually cannot infer the exact number of colliding tags from that slot alone.
Common misreadCounting one collision as exactly two tags gives false population estimates.
Pure and slotted ALOHA formulas describe successful packets per packet time or slot under ideal random-arrival assumptions.
Common misreadThey are not the same as the percentage of tags eventually inventoried after multiple rounds.
Adaptive Q uses empty, success, and collision observations to move the frame size toward the estimated unread population.
Common misreadIt does not guarantee no collisions, and RF loss can distort the estimate.
This page models the anti-collision idea. Full EPC Gen2 also includes sessions, inventoried flags, QueryRep, QueryAdjust, RN16, ACK, EPC transfer, timing, coding, and access commands.
Common misreadDo not treat this page as a full conformance simulator.
Formula And Source Links
The ideal ALOHA comparison in this workbench uses the standard offered-load notation G. Pure ALOHA throughput is modeled as S = G e^(-2G); slotted ALOHA is modeled as S = G e^(-G). The RFID frame model uses the same single-tag-per-slot success idea, then repeats rounds until tags are inventoried.