This diagram shows how a traditional single radio mesh system has its bandwidth reduced due to a large interference domain allowing only a single device to transmit at a time (note: the circles show the communication range, while the interference range will usually have a radius many times larger).

A multi-radio system could use several frequencies to allow multiple transmissions to take place at the same, reducing some of these interference conditions (however, not only does this require multiple clean channels, but there are some pitfalls that will be analyzed in a future post).

So an obvious question is, “How does SkyPilot’s dynamic antenna switching affect system capacity?” The answer is that even though the SkyPilot system uses a single backhaul radio, it can still provide 1/n the channel capacity simultaneously to each device due to the dynamic antenna switching.

In addition to all of the previously discussed benefits of dynamic antenna switching, such as higher link budget, interference avoidance and point-to-point power levels, the largest benefit is probably from something called “spectral re-use”. Basically, spectral re-use is a benefit of using dynamically switched high-gain antennas where multiple transmissions can take place simultaneously, on the same frequency, in very close proximity.

For example, the dynamic point-to-point link formed by the high-gain antennas allows a first-hop transmission to not interfere with a third-hop reception, even on the same channel. And while one first-hop device is relaying, spectral re-use allows many other devices to simultaneously communicate, such as allowing the gateway to transmit to another first-hop device. That is why we always recommend at least 2 first-hop devices. This allows the gateway, and most other devices within the mesh, to be continuously active, so the capacity of the overall system is equal to the capacity of the gateway radio.  This allows at least 1/n to be delivered to each device simultaneously, equivalent to the multi-radio mesh system and much higher than traditional single radio systems.

And by only consuming a single channel, additional channels can be employed in order to multiply overall system capacity (plus, it is often difficult to find the multiple clean channels that multi-radio architectures require). But, the use of multiple radios in context of traditional mesh networks and the SkyPilot system will be explored in a future post.

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• The radio’s transmit power
• The transmit antenna gain
• The receive antenna gain

For this discussion, we’ll ignore other common link budget parameters, such as cable and path losses, since we are looking at power and antenna gain on a particular link.

Most regions restrict a device’s EIRP (Effective Isotropic Radiated Power), which is essentially the radio’s transmit power plus the transmit antenna gain.  To comply with EIRP limits, each device must either reduce its radio’s output power, reduce its transmit antenna gain, or both.

Since SkyPilot’s products use high gain antennas, the radio’s output power must be reduced to comply with EIRP limitations.  Due to this, there is a common misconception that SkyPilot’s high-gain antennas are not beneficial in EIRP-restricted regions.

However, since the SkyPilot system uses high-gain directional antennas on both transmit and receive, the link budget is still increased due to the antenna gain.  Let’s look at an example by comparing two identical links, with different antennas, in a region restricted to 30 dBm EIRP:

• Link 1: radio output power of 24 dBm + transmit antenna gain of 6 dBi + receive antenna gain of 6 dBi = 30 dBm EIRP and comparable link budget of 36 dBm
• Link 2: radio output power of 12 dBm + transmit antenna gain of 18 dBi + receive antenna gain of 18 dBi = 30 dBm EIRP and comparable link budget of 48 dBm

So, even though the use of a high-gain antenna on link 2 resulted in the radio’s output power needing to be turned down by 12 dB in order to meet the 30 dBm EIRP limit, the actual link budget is 12 dB higher than link 1 due to the receive antenna gain.  In free-space, this would result in 4 times the range (or an increase in modulation, depending on how you want to spend the link budget).

To put it more simply, EIRP restrictions limit how big your mouth is, but not how big your ear is.  And of course there are still the other benefits of using directional antennas, such as causing less interference and being less susceptible to interference.

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• Clocks need to be synchronized: Devices participating in a synchronous protocol obviously needed synchronized clocks. This can be provided in several ways, including external clock sources such as GPS or over-the-air clock synchronization. SyncMesh uses a combination of the two, which leverages the accuracy of GPS clocks with the low cost of over-the-air synchronization.
• Clocks need to be very accurate: This usually requires expensive clock crystals that are accurate over a wide temperature range. SyncMesh provides an extremely accurate clock source by utilizing an over-the-air calibration protocol along with an internal calibration algorithm that maintains accuracy even with inexpensive crystals.
• Inefficiencies: Many synchronous, slotted protocols are inefficient due to their simple Time Division Multiple Access (TDMA) MAC layers, which assigns fixed slots to each user. To overcome this, SyncMesh uses a dynamic slot allocation scheme which assigns all slots in real time.
• Lack of interoperability with other systems: Since many outdoor wireless systems leverage unlicensed frequencies, multiple systems may need to share the spectrum. Carrier sensing systems may be able to (in theory) share the spectrum by avoiding simultaneous use, while more complex synchronous systems will probably not understand each other. However, we’ve already seen that carrier sensing has issues, and many systems ‘tweak’ their carrier sensing and back-off protocols to get an unfair advantage over other users of the spectrum. SyncMesh handles multiple users of the spectrum by pointing antennas – the high link budget point-to-point link can avoid interference from other systems, while its directional nature minimizes interfering with other systems. And with a dynamical directional system, if one path is not idle, others likely will be.
• Complexity: WiMAX-like synchronous systems are much more complex than asynchronous 802.11 systems. That is a large reason why WiMAX CPEs are more expensive than 802.11 clients, and why WiMAX base stations are significantly more expensive than 802.11 access points. SyncMesh has been developed over a period of 6 years and runs on top of off-the-shelf 802.11 silicon, which lowers cost.

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Some of the problems associated with carrier sensing (CSMA) and collision avoidance (CA) protocols include:

• Acknowledgment overhead: This is compounded over long distance links due to propagation time.
• Exponential back-off: This is compounded in outdoor networks, where re-transmissions are common due to interference, which causes latency to increase exponentially.
• “Hidden Nodes”: This is a classic problem with 802.11 CSMA, where carrier sensing at the transmitter does not sense interference at the receiver. This is greatly compounded in outdoor networks, where obstructions and long distances between the transmitters normally results in them not being able to hear each other.
• “Exposed Nodes”: This is a classic problem with 802.11 CA, where the RTS message between a transmitter and receiver causes other potential transmitters to become idle when they could have transmitted successfully to a different receiver. This is greatly compounded in a mesh network, where there are normally many active receivers.
• CA overhead: The collision avoidance overhead due to the RTS-CTS-Data-ACK exchange requires 4 propagation times, which results in large overhead on long-distance links.
• CSMA failures: In a small office or cafe, all stations can normally hear each other, which allows them to properly carrier sense and avoid collisions. In an outdoor wireless network, many stations can not normally hear each other, resulting in collisions which cause nodes to experience exponential back-off.
• Ad-hoc architecture: When connecting to an access point in a small office or cafe, all communications occur between the stations and the access point (which is called infrastructure mode) and not directly between stations. This means that most of the transmissions will never collide since all downlink transmissions are from a single device, the access point. In a mesh network using either ad-hoc mode or infrastructure mode there are many simultaneous transmitters and receivers, and all transmissions may collide.
• Unfairness: Another classic problem with 802.11 is MAC layer unfairness, and the problem greatly increases in outdoor networks. Due to the increasing back-off during retransmissions, nodes with fewer retransmissions are more likely to gain access to the channel than nodes that are retransmitting. Additionally, nodes that sense the channel becoming idle earlier are more likely to get access to the channel, and over long distances this results in unfairness to some nodes due to their location.

These problems are basic issues with asynchronous protocols such as 802.11, and all of these problems are drastically increased in outdoor wireless networks. Most people have experienced performance problems related to these issues in offices or cafes, but in outdoor mesh networks the impact of these problems is greatly increased, sometimes resulting in a complete collapse of the MAC layer.

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