ALOHA
The first offered singulation protocol is the ALOHA protocol, originally used decades ago in ALOHAnet and very similar to CSMA/CD used by Ethernet. These protocols are mainly used in HF tags. In ALOHA, tags detect when a collision has occurred, and attempt to resend after waiting a random interval. The performance of such collide-and-resend protocols is approximately doubled if transmissions are synchronised to particular time-slots, and in this application time-slots for the tags are readily provided for by the reader. ALOHA does not leak information like the tree-walking protocol, and is much less vulnerable to blocker tags, which would need to be active devices with much higher power handling capabilities in order to work. However when the reader field is densely populated, ALOHA may make much less efficient use of available bandwidth than optimised versions of tree-walking. In the worst case, an ALOHA protocol network can reach a state of congestion collapse. The Auto-ID consortium is attempting to standardise a version of an ALOHA protocol which it calls Class 0 HF. This has a performance of up to 200 tags per second.
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