## 5 Mechanisms Used by Industrial Technologies to Improve Performance Metrics
### Channel Hopping Techniques  
Channel hopping is often used to mitigate _external interference_ and _multipath fading_. The proper reception of wireless signals may be prevented by other radio signals generated by the devices outside the network. This kind of interference is known as _external interference_. Signals in the same frequency range can be generated by Bluetooth devices, microwave ovens, other external networks (such as the IEEE 802.11 network) or many unintended sources of radio interference, such as other high-power interference sources. Channel hopping techniques are a way to mitigate _external interference_ and _multipath fading_. Figure 3 provides a classification of the different channel hopping techniques as well as the standards which use each of these techniques. There is a tradeoff between using _blind channel hopping_ and _adaptive channel hopping_ (ACH). In the former, if the node switches to another congested channel or switches from a good channel to a congested one, this hopping does not help to mitigate the interference and just wastes energy [30]. However, in spite of this disadvantage, blind channel hopping has less overhead as the hopping pattern is already known by the network devices. In addition, if the system manager decides to blacklist a particular channel, nodes in the network still hop to the channel, but simply remain idle in that time period. Thus the larger the number of blacklisted channels, the more time is lost by nodes idling on blacklisted channels. Blind channel hopping techniques ensure that while two communicating nodes hop in unison, neighboring node pairs never use the same frequency at the same time in order to prevent hidden terminal problems. This is shown in Figure 4.  
ACH differs from blind hopping in the sense that unlike in blind hopping, nodes do not keep on changing from one operating frequency to another at regular time intervals. In other words, nodes _only_ change their frequencies when interference is detected on the current operating channel. However, nodes need to collaborate to decide which channel to switch to and this can introduce a significant overhead since nodes need to continuously scan all channels for interference levels and also because nodes need to ensure that while communicating nodes choose the same frequency, neighboring node pairs use different channels [61]. In WIA-PA in each cluster/star network, the cluster head and each  
Figure 4: Two pairs of nodes using different communication channels. Figure 3: Channel hopping techniques. node irregularly change their channel on a link-by-link basis only when channel conditions require it to. However to the best of our knowledge, there are currently no algorithms indicating strategies to be followed for ACH approaches in the multi-hop network. Both WirelessHART and ISA100.11a networks use blacklisting techniques to mitigate external interference or multipath fading. WISA and ZigBee Pro do not have this capability. ZigBee Pro, however, uses \"frequency agility\". This mechanism is not as tolerant to fluctuating wireless conditions as WirelessHART, WIA-PA, and ISA100.11a. In this technique, the network channel manager collects interference reports from all the nodes. If external interference is detected, the network channel manager scans for a better channel and moves the _entire_ network to a new channel. This technique requires network formation to be carried out again and thus introduces inconvenient delays. Note that ACH is clearly a better technique as it only requires nodes facing interference to make changes to their operating frequencies--it does not affect the entire network. ISA100.11a tries to separate the successive channels in the hopping pattern by at least 20 MHz. That means that at least a three-channel separation exists in each consecutive hop and, in the case of retries in the next hop, that they will not encounter the same IEEE 802.11 channel. The way by which this standard can coexist with the IEEE 802.11 standard has been predefined.