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With Aloha protocols, messages are sent whenever needed without checking the communication
channel (Wieselthier et al. 1989). Collisions lead retransmission with a random delay. According to
the protocol, whenever a terminal has a radio packet to transmit, it transmits the packet without
checking the channel. Possible collisions lead to retransmission of packets with random delay. With
the use of slotted Aloha protocol, time is divided in to slots of one packet duration. Each tag may
reply at most once in a slot. Framed Aloha protocol uses time frames that are divided into a number of
slots. Now each tag may reply at most once in frame (Wieselthier et al. 1989). Figure 2 illustrates the
average duration of identification slots of tag populations varying from 10 to 100. 64 bit tag identifiers
were used in calculations. Framed Aloha shows slightly superior performance, being however clearly
less efficient that EPC tree (see Figure 2).

,
n 40 4
o
i
Aloha
t 30 ° Aloha
a
r
u Slotted Aloha
d - *

20
D s 20
Framed Aloha
I Framed Aloha

e - - • '
g 10
a « - =
r
e 0 - • • - - • — —
v
A
10 20 30 40 50 60 70 80 90 100
70
60
80
100
90
20
10
30
50
40
Number of tags
Number of tags
Figure 2 Average identification duration with Aloha, slotted Aloha and framed Aloha protocols
MEASUREMENTS
Measurements were taken in TUT RFID laboratory, Tampere, Finland. The arrangement is shown in
Figure 3. Tags were placed on a specific grid that has either 64 or 16 blocks (see Figure 3, right side).
The tag populations included 4, 9, 16, 25, 36, 49 and 64 tags. The reader and tags were commercially
available, operating under Class I specification (Auto-ID Labs 2001). The centre of the block of tags
and the centre of the reader antenna were placed at the same height. The tag grid and the reader
antenna face each other at a distance of either 1 or 2 m. Measurement data was collected during 1 and
5 minutes.
m E m
m
R m E " 0
READER 3 i 0
0
5
1 I
TAG
300 mm
l "* 150 mm
GRID 150 mm • m m300 n
Figure 3 Measurement setup (left) and tag grids (right)
The studied factors were the influence of the identification range, the tag population, tags' mutual
alignment, time used for identification, and identification reliability. First, reducing the range clearly
increases the number of successful identification cycles. However, the number of tags identified is not
increased. Second, decreasing the number of tags increases the identification certainty. Third, the
identification performance decreases when tags' are located closer to each other. Fourth, increasing
the time does not increase the number of tags identified, but the number of successful identification
cycles multiplies by the same factor as the time is multiplied. Finally, Figure 4 shows the percentage
of identified tags as a function of the size of tag population for each measurement cases, where lines
correspond squares and dashed lines correspond triangles, coloured correspondingly.