wilamowski-b-m-irwin-j-d-industrial-communication-systems-2011

12-8 Industrial Communication Systems
12.5.5 Coexistence in the 2.4 GHz Band
Friendly coexistence between WLANs and WSNs is an absolute requirement for the successful deployment
of both solutions in the same area of a plant or facility, as both the IEEE Std 802.11b/g/n and the
IEEE Std 802.15.4 define operation in the 2.4.GHz ISM-band.
The IEEE Std 802.11b/g/n divides the 2.4.GHz band into 14 overlapping channels with a bandwidth of
20.MHz [802.11], although to follow national regulations, only channels 1–11 are available in the United
States and Canada, and channels 1 through 13 in the rest of the world except Japan.
To avoid interference between neighboring channels in an IEEE Std 802.11b/g/n network and at the
same time enable friendly coexistence with IEEE Std 802.15.4 based networks, the nonoverlapping
channels 1, 6, and 11 should be used for WLAN installations. As illustrated in Figure 12.3, this allows
the IEEE Std 802.15.4 channels 15, 20, 25, and 26 (only channel 15, 20, and 25 in the United States and
Canada) to operate in-between the IEEE Std 802.11b/g/n channels without suffering major interference,
and vice versa. As the IEEE Std 802.11b/g/n has both higher maximum-allowed transmit power and a
wider channel bandwidth compared to the IEEE Std 802.15.4, the risk, and the consequences, of interference
between coexisting WLAN and WSN are higher for the WSN than for the WLAN. Experiments
performed in [ABFS08] and [PDA08] show that the packet loss for the IEEE 802.15.4 networks increase
significantly when coexisting with active IEEE 802.11b/g/n networks.
When it comes to the three international standards described in the previous section, they each
employ different mechanisms for coexistence with IEEE 802.11 networks. ISA-100.11a utilizes adaptive
frequency hopping and will blacklist channels which suffer from high packet loss due to noise/interference.
An ISA-100.11a network will therefore after a period of time stop using the channels which are
subject to interference from the WLAN networks, and might, in a worst-case scenario, end up using the
interference-free channels 15, 20, 25, and 26 (as illustrated in Figure 12.3).
dB
IEEE Std 802.11b/g
Channel 1 Channel 6 Channel 11
f
dB
2.412 GHz 2.437 GHz 2.462 GHz
IEEE 802.15.4
11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26
2.425 GHz 2.450 GHz
2.475 GHz
f
FIGURE 12.3
IEEE Std 802.11b/g and IEEE Std 802.15.4 coexistence.
© 2011 by Taylor and Francis Group, LLC