Data Integrity - RFID: Fundamentals and Applications in Contactless Smart Cards and Identification - page 201

The polling procedure requires a list of all the transponder serial numbers that
can possibly occur in an application. All the serial numbers are interrogated by the
reader one after the other, until a transponder with an identical serial number responds.
This procedure can, however, be very slow, depending upon the number of possible
transponders, and is therefore only suitable for applications with few known transpon-
ders in the field.
Binary search procedures are the most flexible, and therefore the most common,
procedures. In a binary search procedure, a transponder is selected from a group by
intentionally causing a data collision in the transponder serial numbers transmitted
to the reader following a request command from the reader. If this procedure is to
succeed it is crucial that the reader is capable of determining the precise bit position
of a collision using a suitable signal coding system. A comprehensive description of
the binary search procedure is given in Section 7.2.4.

7.2.4 Examples of anticollision procedures

In the following subsections some of the more frequently used examples of anticollision
algorithms are discussed. The algorithms in the examples are intentionally simplified
such that the functional principle of the algorithm can be understood without unnec-
essary complication.

7.2.4.1 ALOHAprocedure

The simplest of all the multi-access procedures is the ALOHA procedure, which got
its name from the fact that this multi-access procedure was developed in the 1970s
for ALOHANET — a radio network for data transmission on Hawaii. As soon as
a data packet is available it is sent from the transponder to the reader. This is a
transponder-driven stochastic TDMA procedure.
The procedure is used exclusively with read-only transponders, which generally
have to transfer only a small amount of data (serial numbers), this data being sent
to the reader in a cyclical sequence. The data transmission time represents only a
fraction of the repetition time, so there are relatively long pauses between transmissions.
Furthermore, the repetition times for the individual transponders differ slightly. There
is therefore a certain probability that two transponders can transmit their data packets
at different times and the data packets will not collide with one another.
The time sequence of a data transmission in an ALOHA system is shown in
Figure 7.14. The offered load G corresponds with the number of transponders trans-
mitting simultaneously at a certain point in time t 0 (i.e. 0, 1, 2, 3, ... ). The average
offered load G is the average over an observation period T and is extremely simple to
calculate from the transmission duration τ of a data packet:

n

τ n
T · r n

G = ( 7 . 1 )

1