I’m commonly asked, “How does the SkyPilot protocol compare with 802.11″? The most fundamental difference is that our protocol is synchronous, meaning that nodes share a common clock reference, while 802.11 is asynchronous. When nodes want to communicate in a synchronous protocol, they need to coordinate which node transmits at which time, while devices using an asynchronous protocol simply sense the channel and transmit when it’s idle (such as with 802.11’s CSMA/CA).
While there are many medium access schemes for synchronous protocols (even ones that combine a common clock reference with CSMA), the SkyPilot protocol uses a bandwidth allocation scheme where all data transmissions are coordinated, avoiding collisions. Coordinated transmissions are critical in an outdoor environment, since interference happens at the receiver, not at the transmitter. A carrier-sensing transmitter may sense an idle channel, while the channel at the receiver is not idle.
In addition to the coordination of transmissions to avoid interference, the SkyPilot protocol uses its synchronous nature to schedule bandwidth for QoS, to compensate for the speed of light (5 microseconds per mile adds up), and most importantly, to point directional antennas (but more on these items later).
Common disadvantages of synchronous protocols include cost, latency, and complexity. The SkyPilot system addresses cost by utilizing in-band over-the-air clock synchronization, which avoids the use of GPS in customer premise equipment and doesn’t require an expensive clock crystal. Latency is addressed by utilizing contention, normally associated with asynchronous protocols, but only using it within small, periodic, synchronous bandwidth request opportunities. And, once a node successfully contends for bandwidth, subsequent requests are piggy-backed on the regular scheduled data transmissions, avoiding further contention. Complexity has been dealt with during the 8 years of development.
While an asynchronous protocol such as 802.11 normally has low latency in a lightly loaded system with few nodes, the most common problem is inefficiency and variable performance due to collisions once the load and number of nodes increase. In an outdoor environment, and especially in a mesh network, this problem is greatly increased due to the presence of many “hidden nodes”, where two transmitters can’t sense each other so their transmissions end up colliding. After collision, CSMA devices perform a random exponential back-off, so performance can suffer quickly and drastically. Collision avoidance is sometimes used in 802.11 to deal with these hidden nodes, where a two-way handshake is performed prior to transmission, but not only does this consume bandwidth, but it causes an “exposed node” problem, where devices incorrectly avoid transmission, also decreasing efficiency.
This is an extremely simplified comparison of SkyPilot’s synchronous protocol with 802.11’s asynchronous protocol. While we believe there are enough fundamental advantages in a synchronous protocol to justify the effort involved in its development, the greatest advantages of the synchronous protocol come from the additional technologies that it enables. But that’s for a future blog…
In a previous blog post, I mentioned the relationship between low cost, long range, and high capacity. As always, it costs extra to get extended coverage and better capacity since higher-gain antennas and higher-power radios cost more.
