Julien Herzen, a PhD student at Swiss École Polytechnique Fédérale de Lausanne’s (EPFL) Laboratory for Communications and Applications, has a bright future. Herzen, along with Ruben Merz, senior innovation engineer at Swisscom, and Patrick Thiran, full professor at EPFL’s School of Computer and Communication Sciences, has solved a major challenge now plaguing Wi-Fi networks.
People operating a Wi-Fi network (WLAN) are realizing the importance of carefully selecting the operating channel — in particular, when several WLANs are in close proximity, such as an apartment complex. Having to share some portion of the finite spectrum available on any given Wi-Fi band improves the odds of WLANs interfering with each other and reducing their individual operating capacity.
It turns out this is becoming more important as Wi-Fi technology advances. Recent Wi-Fi norms (802.11n and 802.11ac) promise improved throughput by increasing the amount of frequency spectrum available for each channel. Good news, except doing so reduces available space on already crowded Wi-Fi bands.
In the spirit of compromise
The team explains their solution in the paper Distributed Spectrum Assignment for Home WLANs. The researchers introduce Spectrum Assignment for WLANS (SAW), an algorithm that automatically and repeatedly determines what channel a connection should use and the amount of bandwidth that can be allocated to avoid interference with neighboring WLANs.
Like current Wi-Fi networks there is a controlling Access Point (AP) whether self-contained or part of a Wi-Fi router. “SAW runs on the AP (depicted to the right) of a WLAN and relies exclusively on inter-neighbor measurements, without generating additional traffic,” mentions the paper. “It is transparent and operates while the network is up and running.”
During an email conversation, Herzen wrote, “The goal is not only to minimize interference, but also maximize the spectrum usage (using as wide a channel as possible).”
Details of how SAW works
I asked Herzen what the difference was between SAW and existing AP technology, mentioning that typical APs already choose channels to avoid congestion. “Yes, most APs have the intelligence to determine a channel: except, in most cases, this channel uses a fixed bandwidth, and is only chosen once when the AP boots (even APs with auto-channel),” explains Herzen. “Whereas SAW continuously adapts the spectrum band, taking into account both this new bandwidth parameter, as well as the intensity with which neighboring WLANs interfere.”
I was curious what happens when a WLAN uses the same channel; does SAW automatically search around and decide what to do? “In order for communication to take place, both ends have to tune to the same channel,” writes Herzen. “Because deciding which spectrum chunk to use is made at the AP, the clients have to be notified when a change occurs.
“In our current prototype implementation, we use a custom-made signaling system, where the AP sends a series of packets to the clients before a switch occurs. These packets contain precise timing information that the clients can lock on, so that everybody switches to the new band at the same time (or within a reasonable delay).”
Herzen mentions that their channel-changing system is not perfect — that would require modifying existing 802.11 drivers. He adds, “It is our hope Wi-Fi chip manufacturers will take our algorithm and implement it using a cleaner and more efficient signaling scheme.”
The paper’s conclusion mentions, “We have conducted performance evaluation using simulations and test bed experiments. SAW provides drastic capacity and fairness improvements, even when a large proportion of the APs behave selfishly.”
I asked Herzen what “fairness improvement” and “behave selfishly” meant. “It is a kind of technicality,” explains Herzen. “It refers to APs that optimize their spectrum bands without taking into account the interference they could produce on their neighbors.”
Herzen adds, “It turns out that this aspect is needed to mathematically prove that SAW makes efficient decisions that lead to optimal spectrum assignments. However, large performance gains are also obtained even when the APs behave selfishly.”
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