To many of us, 2.4GHz sounds like the traffic of Interstate 10 – or whatever freeway is close to you. It is overcrowded. WiFi, cordless phones, microwave ovens, and many other wireless technologies are taking advantage of this license-free frequency band. How could WirelessHART possibly transmit messages reliably over this seemingly overcrowded band? The developers of WirelessHART found a way. First you simplify, and then you improve.
In this second article in our technical discussion of WirelessHART, we examine the use of frequency hopping spread spectrum (FHSS) to maximize the available bandwidth and increase the reliability of the network. Application of this technique goes a long way to improving not only the reliability, but also bandwidth utilization.
When specifying the physical layer for a wireless implementation of the HART network protocol, designers looked for a well-established standard to use as a foundation. They found it in IEEE’s 802.15.4-2006 wireless standard. The same standard used for WLANs.
But they didn’t require the entire standard, so they simplified it. First, they restricted it to just one band, the 2450MHz ISM band. Then they eliminated one channel from that band (channel 26) because the frequency was not allowed in all locations. They also reduced and simplified the control signals (service primitives) used in the physical layer.
Next, the designers of WirelessHART went to work improving on the IEEE802.15.4-2006 standard to make it more function in a field environment. This is where frequency hopping comes in. Unlike many other wireless technologies, however, every time a transmission occurs between links in WirelessHART, the channel is switched. This ensures channels with interference won’t be used to transmit all packets. In contrast, Zigbee doesn’t have frequency hopping, and all Zigbee Pro transmissions stay on the same channel unless the entire network decides to hop to another channel.
Transmission-level frequency hopping has an additional bonus. Transmissions are synchronized in 10ms slots. During each slot, all available channels can be utilized by the various nodes in the network. This allows 15 packets to be propagated through the network at a time while minimizing the risk of collisions.
Each network device keeps an active channel device. Because channels subject to interference may be eliminated from use due to blacklisting, the number of avaible channels may be less than 15. Each slot is assigned a vector which includes not only source and destination information, but also a channel offset. The actual channel to be used for transmission is determined from the formula:
ActualChannel = (ChannelOffset + ASN) % NumChannels
The actual channel number is used as an index into the active channel table to get the physical channel number. ASN the absolution slot number which starts at 0 when the network is created is constantly increasing. The “%” represents a modulo division, meaning the same channel offset may be mapped to different physical channels in different slots. By assigning a different channel offset for each transmission scheduled to occur in a given slot, the entire 2450MHz band can be utilized.
For example, assume all 15 channels are available and the channel offset is 5. For an early transmission when the ASN is 16, the actual channel will be 6. For a later slot when the ASN is 37, the actual channel will be 12.
Like changing lanes on the freeway, the use of frequency hopping spread spectrum techniques in the physical layer of the WirelessHART communication protocol not only increases the reliability of the network, it also increases the bandwidth utilization and traffic throughput. Next time, our technical discussion turns to the use of Time Dimensional Multiple Access by the data link layer.