Mesh and open standards
No wireless network can function without access to spectrum, and one of the most important factors promoting the growth of mesh networks... has been the growing availability of unlicensed spectrum.
So far, the commercially successful mesh products have tended to use proprietary mesh protocols. Meraki, for example, uses 802.11 for the lowest levels of its networking stack, but runs a homebrew mesh routing protocol on top of it. Biswas argued that this is a reflection of the primitive state of mesh networking protocols.
"We update our routing protocols almost on a quarterly basis to improve performance," he said. "It doesn't feel like it's come to the point where it's stabilized enough to have an interoperable standard."
One possible exception is the 802.11s standard, a WiFi variant which is now under consideration by an IEEE task force. Biswas predicted that 802.11s "will fit in really well to the lower end of the market where you're talking about a smaller number of devices and interoperability matters a lot more."
Biswas predicted that 802.11s-based devices were unlikely to work well for larger networks. "There's nothing that specifies 'here's how you deal with lossy links,'" he said. "It works well in a basic scenario, but when you start building networks with hundreds of devices, you start to run into a lot of scaling issues that 802.11s doesn't address."
Open mesh networks and the importance of incentives
All of the mesh networking applications we've discussed so far have been cases where a single organization owns and manages all the nodes in the mesh. A more ambitious goal is to build heterogenous mesh networks that are open to the general public.
To understand the challenge of building public mesh networks, Ars talked to Jinyang Li, a computer science professor at New York University who has been working on mesh networks for over a decade. Li and Biswas were colleagues when both were working on mesh networking technologies at MIT.
In a conversation with Ars, she pointed out that "the original vision of the mesh was that all of the nodes would cooperatively forward traffic for each other whenever there was a global benefit to doing so." The cell phone market seems like a place where this model could fit particularly well; short-range radio transmissions consume dramatically less power than long-range transmissions, so in principle cell phones could save energy by sending their packets to cell phone towers via multi-hop routes that include nearby cell phones.
In practice, this strategy has several major problems. Most obviously, achieving reliable routing with rapidly moving devices is difficult. A customer used to the reliability of single-hop wireless connections is unlikely to be interested in a network that could fail if the owner of a key node in the local mesh walks out of range. Moreover, cell phones that participate in mesh networks would have greater variability in battery life. The total battery life might improve on average, but this will be little consolation to a user who needs to make an urgent phone call and finds that his phone has run low on power because it forwarded too many packets earlier in the day.
Li pointed out that current mesh protocols don't give nodes incentives to forward one another's packets. This isn't a major problem in academic, corporate or government settings where all the nodes can be assumed to have the same owner. But in open, heterogenous networks, each user will be tempted to "cheat" by modifying his own device to take advantage of other nodes' forwarding services without reciprocating. "It's still an open question whether we'll be able to solve this incentive problem," Li told Ars.
A related concern is security. Users should worry about the security implications of entrusting their packets to random strangers. Things would get even more dicey if routing protocols were enhanced to add monetary incentive schemes such as micropayments; in that case, security flaws in mesh networks could allow unscrupulous mesh participants to generate bogus traffic in order to siphon money away from their neighbors.
Li also pointed to another niche application for mesh networking: sensor networks. In sensor networks, a large number of tiny computers form an ad hoc network in order to relay data back to a centralized base station. Mesh networking techniques work well for sensor networks because two key design goals of such networks is often to minimize cost and energy consumption.
A company called Silver Spring Networks uses mesh networking techniques to automatically report utility meter readings. In this application, power consumption isn't a major issue, but low cost of deployment and high reliability are crucial. And the relatively low throughput of mesh networks isn't a serious problem, because the amount of data being transmitted is tiny.
No wireless network can function without access to spectrum, and one of the most important factors promoting the growth of mesh networks—and WiFi-based networks more generally—has been the growing availability of unlicensed spectrum. Biswas says that the amount of unlicensed spectrum available for use by Meraki products has increased from 80MHz to 300MHz since he started working on mesh networks. The recent opening of the 5GHz band gives mesh network vendors a new, less crowded swath of spectrum to work with.
Biswas told Ars that continued expansion of available spectrum would further improve the performance of mesh networks. He strongly endorsed recent proposals to allow unlicensed use of white space spectrum in the television bands. This spectrum has much better propagation characteristics than the 2.4GHz and 5GHz bands currently available: transmissions in the television bands are better able to pass through walls and other obstructions, which would improve coverage.
Biswas was less enthusiastic about the prospect of auctioning off more licensed spectrum. "The customer needs to go get the licensed spectrum, not the device vendor," he said. "Site licenses are expensive. We have customers that are community colleges; they're not going to be able to spend money on something like that."
Jinyang Li was more bullish on licensed spectrum. "If you look at city-wide deployment of a mesh network, the unlicensed band is both a blessing and kind of a curse," she said. "It's a blessing because you don't have to license the frequency band. But it's a curse because it's hard to guarantee quality. There could be ham radio operators blasting at high power, and there's nothing you can do about it."
Li suggested that opening up both more licensed and unlicensed bands could promote the development of mesh networking technologies.
Mesh networking is still a young technology. Engineers are still discovering new applications for it and learning how to squeeze maximum performance out of existing ones.
Mesh networking is best understood as a way to supplement and enhance other networking technologies rather than a replacement for them. The inherent limits of wireless transmission make it unlikely mesh networks will replace fiber and coax networks any time soon. Because mesh networks tend to rely on incumbent networks for backhaul, there doesn't seem to be much prospect that the growth of mesh networks will change the terms of the broadband policy debate.
But mesh networks do underscore the importance of spectrum reform. Whether you prefer unlicensed spectrum or flexible licensed spectrum, the growth of mesh technologies underscores the need for additional bandwidth for digital networks. Such allocation is being bitterly opposed by incumbent broadcasters, but as mesh networks mature, the argument for making more spectrum available grows stronger.