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Reading about Zigbee, I see it described as a technology for creating personal area networks. I've also been reading about 6lowPAN, which seems to crop up in mesh settings, cementing the idea, at least in my mind, that mesh is limited to PAN applications.

Is there something about mesh networking that makes it inherently limiting in terms of network size?

Since I already have some home automation gadgets using Zigbee, I already know that a Zigbee network is good at least for an apartment sized wireless network with ten to fifteen nodes.

If I extended my Zigbee network to, say, provide smart lighting for an apartment block of 100 apartments, would I start to feel some limitations?

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    The same "Mesh Networking" wikipedia page I linked to in the question mentions the Meraki meshed Wi-Fi solution which, being "optimized for long-distance communication" can provide coverage of over 250 meters. So perhaps distance is a limiting factor rather than number of nodes? – Chris Steinbach Sep 14 '17 at 5:31
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    Thinking about it again, maybe it would be better to split this into the two questions I answered, because they are quite different. Especially, if I misunderstood you and your question was not about industrial applications at all, you should clarify what you mean by PAN and we move the industry part to a new question. – koalo Sep 14 '17 at 7:54
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    @koalo PAN means 'in the vicinity of a person', or range at around maybe 10 metres - not limited to the purpose of the network. Industrial robots could use a PAN... See the size heirarchy, PAN < LAN < WAN. Next step up is inter-planetary. – Sean Houlihane Sep 14 '17 at 10:37
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    @SeanHoulihane You are right, that is a very common definition. However, for IEEE 802.15.4 a PAN just describes the network itself (regardless from range or use case). And there is the definition of "a network for interconnecting devices centered on an individual person's workspace" -> Wikipedia. So I would be interested in the definition of Chris :-) – koalo Sep 14 '17 at 10:44
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    @koalo My question assumes PAN relates to network size in terms of supported number of nodes, or perhaps coverage area. It might just as well have something to do with security but I didn't consider that. Maybe passing mixed user traffic over multiple untrusted nodes makes some mesh technologies unsuitable for anything other than personal use. I was able to relate to your discussion of industrial applications, so I believe you were able to grasp the gist of my question, nebulous as it is :) – Chris Steinbach Sep 14 '17 at 11:16
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+100

That is a very good question and actually my current research topic. I try to give an adequate but concise answer¹.

I will focus my answers to networks based on the IEEE 802.15.4 standard (Zigbee and 6LoWPAN both use it as physical and data link layer) as well as networks where most traffic goes through a common gateway. Most arguments also hold for peer-to-peer networks, but it is more difficult to define what a network is (e.g. if your neighbor and you both use Zigbee devices).

¹ I will hopefully be able to fill a whole PhD thesis by answering the question ;-)

Is there something about mesh networking that makes it inherently limiting in terms of network size?

There are several factors that limit scalability in wireless mesh networks:

  • There is only a given amount of traffic that can be handled by a single node and this especially holds for the gateway. Thus, in general, if you double the number of nodes, every device can at most transmit half of the number of packets per time.
  • With more nodes in a mesh network, there is more chance for complex arrangements that generate many problems in the routing layer and the link layer. One example is the hidden node problem that increases the probability of overlapping (and therefore failed) transmissions.
  • The more hops a packet has to take to its destination, the higher is the probability that it is lost. Much simplified: If the probability is 99% that a packet transmission fails, the probability is 0.99^h for h hops.
  • More nodes are more difficult to maintain. For example, updating the software manually is OK for 10 devices, but not for 1,000. So you need some kind of remote software management.

So there is no inherent limit to the network size itself (apart from maybe your address space...), but a reliable communication gets increasingly complex and error-prone. Of course, deploying a network with thousands of nodes should be possible if every node only sends one message per day. But a network of 1,000 nodes where every node sends many messages per second will overload the channel.

That said, this does not make wireless mesh networks per se worse than any other wireless technology. Cellular networks can only serve thousands of devices because the providers own a large amount of the wireless spectrum, they clutter the area with basestations and only few devices want to communicate at the same time². And the LoRa technologies certainly have their applications, but in the claimed highly scalable networks the throughput is far from what a IEEE 802.15.4 mesh network can deliver.

And to address your comment: No, overall distance is not the actual problem. In fact, the main advantage of mesh networks is that a larger distance can be bridged without increasing the power or reducing the data rate.

² I do not want to say that cellular networks are bad, but just that you can not compare the scalability of a city-scale cellular deployment with a wireless mesh network with a single gateway. (And multiple gateways are possible.)

Are wireless mesh networks limited to PAN applications?

The definition of the term PAN is quite fuzzy, depending on the context. If the only distinction is a low number of nodes, the answer is already given by the previous part. However, as the name indicates, it can also mean "a network for interconnecting devices centered on an individual person's workspace (Wikipedia)". So this question has another dimension, that is if wireless mesh networks (according to IEEE 802.15.4) are limited to personal applications.

Are industrial wireless mesh networks feasible?

I was contributing to a research project that assessed the feasibility of a wireless mesh network in the context of a solar tower power plant. And as far as I can say, using a wireless mesh network in this application is very promising. While we did not had the opportunity to deploy a very large network, there are other very promising industrial deployments, for example

T O’donovan, J Brown, F Büsching, A Cardoso, J Cecı́lio, J D Ó, P Furtado, P Gil, A Jugel, W-B Pöttner, et al., The GINSENG System for Wireless Monitoring and Control: Design and Deployment Experiences. ACM Trans. Sen. Netw. 10(1), 4:1–4:40 (2013).

There are many reasons for using wireless technology, but you only need wireless when movement is involved. This is especially the case for all kinds of vehicles. Furthermore, there is a current trend to replace cable carriers by using wireless technology, but that is only a point to point connection, no need for mesh.

A second argument can be a cost consideration: The deployment of cables can be quite cost intensive, especially outdoors, so for example in solar tower power plants wireless devices would actually reduce the investment costs (maybe a lot) even if it is not required because no mobility is involved. That might or might not be applicable to other industrial applications. Clearly, choosing the right number of basestations and thus the size and performance of a single mesh network is a trade-off that involves cost considerations, too.

So, why are there so few industrial wireless mesh deployments?

Short answer: Because cables are great and well-proven!

At first, finding applications where wireless technology really shines is not that obvious (see last section). Then there is the energy aspect: There are applications where energy harvesting work very well (including solar tower power plants), but otherwise you have to rely on batteries (that might induce high maintenance costs) or power cables. Cables only for power might actually also be beneficial, for example when retrofitting old factories that provide power everywhere, but there is no space for additional data cables.

Certainly, in the future there will be much more industrial wireless applications and also wireless mesh networks will find their niches, but it is obvious that new technologies find their application first in personal environments instead of industrial environments where problems will have much larger consequences.

EDIT: I just noticed that the IEEE changed the title of the IEEE 802.15.4 standard from IEEE Standard for Low-Rate Wireless Personal Area Networks (WPANs) to IEEE Standard for Low-Rate Wireless Networks according to this correction sheet published last year. The reason for this was probably that "personal" does not really fit to the standard anymore as I argued above.

  • I could argue with some of your "claims", they are lacking some dimensions. – MatsK Oct 12 '17 at 4:08
  • As I wrote, this answer can not cover all aspects of the question, but I would be more than happy to discuss them with you. I fear the comments section is not the right place for that. Any suggestions? Mail? Chat? – koalo Oct 12 '17 at 5:28
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If you consider that the internet is a mesh network of sorts, you should see your answer in the broadest terms.

Asking is a specific mesh network has any scalability issues is slightly different. There is plenty of scope to architect a modified network protocol to address a specific implementation, with the right sort of optimisations.

At some point, you might also want to consider if the mesh approach is best, or maybe a hybrid approach has some value.

Factors to consider are:

  • Latency requirements (related to number of hops)
  • Cross Network bandwidth (e.g. how much traffic crosses from one half to the other, as opposed to just talking to a neighbour). Saturation of key nodes might be a limiting factor.
  • Deployment structure. If you have regularly distributed nodes (e.g. on streetlamps), this is different to a distribution that has sparse areas.

It feels that the example you describe should scale to an apartment block. Any one second shouldn't see more than a handful of data transactions. It might scale to a few blocks, but not a street or city. Proving an architecture at the scale of millions of devices or transactions is hard.

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