ISO IEC 29143:2011 pdf – lnformation technology —Automaticidentification and data capture techniques -Air interface specification for Mobile RFID interrogators.
7.3 Command Retransmission (Mandatory)
In case a communication collision is detected by the interrogator after a tag has already been separated to the current response slot, I.e. during the period of tag acknowledgement and tag access, the affected command shall be reissued in order to resolve possible Multiple Interrogators to Tag- or Interrogator to Interrogator Collisions.
During the phase of tag anti-collision, commands shall not be retransmitted unless a Multiple Interrogators to Tag- or Interrogator to Interrogator Collision can be determined by the affected interrogator through any adequate means, I.e. command retransmission is prohibited for all Interrogators not equipped with a collision diagnostics function capable of discriminating among the three different types of communication collisions described In Annex A. Instead, tag anti-collision shall be applied to address the common case of Tag on Tag Collisions caused by several tags responding in the same cornmun.cahon slot.
The number of possible retransmission attempts due to collisions is unlimited and may be adapted to fit to the needs of the application. Retransmission may last until a valid tag response Is received. The introduction of an upper limit for unsuccessful retransmission attempts in case of emerging collisions is encouraged in order to avoid perpetual communication traffic and resulting communication collisions.
If a receiver timeout is detected by the interrogator (no tag response is received within a predefined time) commands shall be retransmitted only once.
It Is important that with respect to emerging receiver timeouts, commands are not repeatedly retransmitted. Instead, anti-collision shall be applied if the receiver timeout persists, e.g. the number of slots may be decreased. if there are still undetected tags expected to be In the interrogation zone to ensure that there is at least one tag allowed to answer the interrogator in the current slot before a command is reissued again to recover from a possible Multiple Interrogator to Tag CollisIon.
Additionally, it has to be considered that command retransmission makes only sense if the Involved tag(s) remain susceptive to the command, i.e. do not change their internal state in the meanwhile due to a receiver timeout. In case of the UHF air interface specified by ISO1IEC 18000-6 Type C, tags are forced to switch back to state Arbitrate in case of a T2 timeout in the Reply- or Acknowledged states, which implies that reissuing a command after a wait time greater equal T2 would be ineffective as it would never result in a tag response. Therefore commands shall only be retransmitted if the expected internal state of the tags addressed by the command does not implement a timeout or the selected wait time (see 7.4) for the command retransmission is smaller than the minimum value of the timeout as specified by the air interface specification. Based on the additional constraint that interrogators are only suspended for a random wait period in the early phase of an inventory round, i.e. are not longer suspended if one or more tags have already been separated, this potential fatality can be automatically ruled out in advance, see 7.4.
Figure 12 shows the whole situational command retransmission scheme in overview, including the application of the random wait time specified In detail In 7.4.
7.4 Random Wait Time (Mandatory)
7.4.1 General
To mitigate the negative effects of concurrent command transmissions in Mobile RFID environments, a random wait time TB is introduced aimlig at breaking the regular transmit flows ruled by the underlying basic air interface specification and the resulting sequences of colliding interrogator commands.
To avoid a livelock (starvation) where two competing interrogators block each other with recurrent (re)transmission attempts, a random wait time siall be applied between an unsuccessful transmission of a message and Its retransmission if all further preconditions regarding the time of applying the random wait time specified in 7.4.2 are satisfied
Additionally, a random wait time shall be applied between two subsequent inventory commands dedicated to the process of tag anti-collision, e.g. between a Query and a QueryAdjust in case of ISOIIEC 18000-6 Type C. if the type of the current collision cannot be unambiguousty determined or it the interrogator is not equipped with a cotislon diagnostics function, and If all further preconditions regarding the time of applying the random wait time specified in 7,4.2 are satisfied.
7.4.2 When to Apply the Wait Time
Interrogators shall be suspended for a random wait time before retransmitting a command during the phase of tag selection or tag inventory if no tag has yet been acknowledged (tag singulated and ACK transmitted but no response received) nor inventoried in the course of the current inventory pass.
In contrast, interrogators shall never be suspended during tag access.