In order to communicate between many simple transponders and a reader, a set of instructions(rules) implemented in the transponders and readers is needed. This set of instructions is known as the RF protocol. RF protocols have been developed which allow up to 1000 transponders to communicate reliably with the reader on a single frequency, even if the all have the same identity, and yet preserve their properties of frequency agility and simple design.

There are basically two types of protocols for implementing multi-transponder situations, namely "Tag-talks-first(TTF)" and "Reader-talks-first(RTF)".

Tag talks first protocol
In a "tag-talks-first" situation, the reader puts out an energising field which is generally a carrier wave signal with no modulation at the operating frequency of the system. Tags entering the field, collect this energy in their transponder aperture, convert it to operating energy, and communicate their identity to the reader. In some cases the reader communicates the successful receipt of the message back to the transponder by a short burst of modulation on the energising field. These systems use minimal RF bandwidth for the energising field and hence cause minimum interference for other users of the RF spectrum. Many readers can operate at distances of a few meters from each other using this type of protocol with small frequency offsets between readers. The identitiy information is communicated by the tags which operate at power levels of up to 1 million times less than that of the energising field of the reader.

Reader talks first protocol

In a "reader-talks-first" situation the reader puts out an energising field which is modulated with call messages to the transponders. Tags entering the field, collect this energy in their transponder aperture, convert it to operating energy, and listen for messages from the reader which is modulated on the energising field. The reader polls for transponders in its reading field by asking "Transponder number 1, are you there?" If transponder number 1 is there, then it replies "Yes" and its identity is known to the reader. This is repeated for all the possible numbers of transponders present, or a more compact "treeing" method is used. The disadvantage of this system is that the reader has to be continually calling for transponders, modulating its high power signal and causing widespread interference around the reader, even if no transponders are in the field at the time. Also this interrogation has to be repeated faster than the time it would take for a transponder to pass through its field so as not to possibly miss a transponder in transit through the field. This requires high bandwidth and widespread interference to other radio users.

To explain the difference between reader-talks-first and tag-talks-first protocols - imagine you were at a picnic in a quiet park trying to whisper to your companion, but close by you have another group of picnickers with a radio playing rock music full blast - that is the situation with reader talks first. Now imagine that back in the radio station the music source is disconnected or comes to an end of the music track - the radio is still receiving a signal but is quiet as there is no modulation on the carrier signal - that is tag-talks-first. In both situations the radio is still broadcasting a signal, the difference being the amount of interference it causes.

The following table compares the two technologies for UHF transponder systems

	"Tag-talks-first"	"Reader-talks-first"
Energising signal	Carrier wave	Modulated energising signal
Modulation bandwidth	10kHz	Up to 1 Mhz
RF power of modulated signal	20uWatts	0.5 to 4 watts
Interference zone for other readers	4 meters	1 km
Speed of transit of zone to detect tag	300kph	3kph
Interference when no tags present	Carrier wave only	Modulated energising signal
Transponder receiver complexity	Very simple	Needs to have good dynamic range

The choice of protocol impacts many decisions regarding the use of multiple RFID reader systems in close proximity. TTF systems are able to operate two readers within only four metres of each other due to the very small bandwidth(10kHz) used and the stable transmitted signals.

Note The EPC Global Gen 2 protocol approved in December 2004 is a RTF type protocol with high interference levels. The EPC Global has approved three levels of designs regarding interference levels for readers - namely

Level 1 - no other reader operating within 1 kilometer

Level 2 - A few readers operating within 1 kilometer

Level 3 - More than 50 readers operating within 1 kilometer

In addition RTF designs are very susceptible to interference from cell phones - whereas TTF protocols generally work well with cell phones and allow readers to operate together with spacing as close as 4 meters. In the long term both the EPC Global and the ISO 18000-6 specifications will be forced to migrate to TTF type protocols due to the high levels of interference caused.