A friend of mine says I need to say something about “fabrics,“ so here goes. (This is one reason I don’t like to get into super technical discussions outside of engineering forums: you’re jumping down the rabbit hole, for sure. )
I posted this on another site in response to a power user author, who had talked about joining another thread network, which is not actually what happens in matter.
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The following is supertechnical, but I do think the definition of “fabric” is a little different than what you have described. It’s a term that the Thread group has used for a long time to deal with the fact that the same security key is shared among devices on completely different networks, like a Wi-Fi device and a thread device. Or even just between two thread partitions that happen to be on different frequencies. They called it “fabric“ because devices on different networks could be woven together using the same security key.
(note that it’s not the networks themselves being woven together, it’s in the application layer above that. You could have two Wi-Fi smart plugs on the same Wi-Fi network, but only one of them is matter-compatible, and only that one would participate in the “fabric“ established by the Matter Controller acting as admin.)
Here’s the official thread border router white paper, which explains:
In matter, devices are paired to a controller known as an admin. Devices become interoperable when they are paired with the same admin. {this is when the security key is exchanged.} examples of Admins are popular smart home platforms and apps that are interested in configuring, controlling, and listening to smart home devices. Groups of such devices are known as a “fabric“ of devices, since the IPv6 messages can be woven across many different physical networks, like Wi-Fi, thread, and ethernet.
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This woven nature of matter fabrics is why thread border routers are so crucial to the success of matter with Thread devices, in particular. In principle, a thread mesh without a border router can use matter if configured properly; however, in practice, a border router is required to enable the majority of use cases. This is especially true today, since the vast majority of smart speakers, smartphones, and computers do not have direct Thread capabilities, so they must communicate with Matter over thread devices through a thread border router.”
So a Matter “fabric” is all the devices that are allowed to send messages to each other, regardless of their specific network, because they have all been enrolled to the same admin, and are using the same security key.
the “multi admin” feature in matter means that one device can be enrolled to multiple fabrics at the same time. Note that it might be on the same WiFi or Thread network the whole time, but it’s enrolling individually with different admins, like once with Apple home, and once with home assistant. Again, that’s because matter is in a layer above the transport network.
So it’s not that the thread with matter smart plug is joining a new network when you enroll it with a new controller. It’s joining a new fabric. It’s on the same physical network it was before, but now it has permission to exchange messages with all of the Matter devices that have been enrolled with that new admin, regardless of their individual networks. So that’s why we needed a new term, in this case “fabric,“ because we are talking about a Message exchange method which operates above the level of the physical Networks.
I hope that’s not too technical. Again, all of this will be invisible to most people once matter is fully deployed. But some people may find the engineering details Interesting.
@Automated_House