Why are WirelessHART devices slow to join the network?

It takes years for devices to join the network. Well, I am exaggerating a bit. It takes minutes. But if you are a technician in the field and wait for the device to show up on the Gateway, it does feel like years. It is one of the frustrating experiences people have with the current WirelessHART devices.

So why is this process so slow and does it have to be this slow?

To answer these questions, we need to look first at how a device joins the network.

A WirelessHART device hops channels (15 physical channels in total) every 10ms in a random fashion. So overhearing the first transmission is a kind of cat-and-mouse game. The new device will continuously listen on a physical channel for a period of time and then continue to the next channel, so it takes some patience for the cat to catch the mouse. Statistically, there is a chance that the device will not hear an advertisement within a certain length of time, so to raise the chances of success, a large number is used for the Timeout value.

Furthermore, a device doesn’t immediately send out a join request even after it finds one neighbor. Instead, it will keep listening for an additional amount of time, trying to find more neighbors. Remember, the reliability of a WirelessHART network largely comes from the fact that every device has multiple neighbors. When one fails, the device can quickly switch to another.

In Network Management Specification HCF_SPEC-85, this amount of time is called TimeAdWaitTimeout and is defined as the amount of time waiting while attempting to receive additional advertisements. Version 1.2 03/09/2009 set the default value to 300 seconds, which is 5 minutes. This is the value most of the current WirelessHART devices use and do not vary from. So 5 minutes is by design in a way.

So what if we change the 300 seconds to say, 3 seconds? We can do it, but there are some catches. First, the device is likely not reliably connected to the network. You may not be able to get reliable data at the DCS. Second, you need to make sure the device has many discovery links so that it can keep finding more neighbors and inform the network manager.

The good news is that in the latest Network Management Spec, 2.0, published by the HCF on 06/18/2012, the value has been revised to 30 seconds. It may take a while for products on the market to adopt this new setting, but we will surely see devices that are able to join faster and faster.

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