Archive for the ‘Protocol’ Category

Point to MultiPoint Vs Dynamic Antenna Switching

Friday, February 20th, 2009

Point-to-MultiPoint (PtMP) systems typically require multiple frequencies in order to avoid self-interference (interference among base-stations within the same network, or among sectors of a single base-station). The degree that multiple frequencies are re-used within the network is called “frequency re-use”, and is quantified by a frequency re-use factor. The frequency re-use factor will vary based on the number of available frequencies, the deployed technology and the network architecture.  The network architecture generally falls into two categories: omni-directional systems and sectorized systems.

For omni-directional PtMP systems, the frequency re-use factor is basically how often a frequency gets reused within the overall network, and re-use factors of 3, 4, 7, 9 and 12 are common. A frequency re-use factor of 4 means that 4 different frequencies are used, with base-stations that have adjacent coverage each operating on a different single frequency, and each frequency is reused on each 4th base-station.

freq-reuse

A problem with this type of network is that obviously many frequencies are required, which may not be possible in many limited frequency bands such as 3.65 GHz or 4.9 GHz. And since only a fraction of the total available bandwidth is used at each base-station, the capacity of each base-station is reduced. Additionally, since each base-station is only providing a single frequency, there is no frequency or base-station redundancy at the subscriber level, so interference or a failure of a base-station will cause a complete outage for any affected subscribers.

For sectorized base-stations, a frequency re-use factor of 3 is commonly used, and adjacent sectors do not use the same frequency. For instance, if a base-station has 3 sectors, each sector would be 120 degrees wide for 360 degree coverage and each sector would use a different frequency from a total of 3 frequencies. Problems with this architecture include:

  • Lack of frequency diversity at the subscriber: a subscriber physically resides in one primary sector (and frequency), so if that frequency is being interfered with by a different base-station (in a licensed band) or a different network (in an unlicensed or “lightly” licensed band) then the subscriber could lose service. And if directional antennas are used at the subscriber, which is almost always the case in order to increase the link gain, then redundancy is not even availabe from other base-station locations.
  • Lower antenna gain: the frequency re-use factor dictates the sector beam-width (antenna gain is directly related to beam-width), and, in order to get 360 degree coverage, wide antenna beam-widths are needed. And even if a single frequency were used multiple times on a single base-station (which usually requires some sort of coordination), such as in a F1,F2,F3,F1,F2,F3 pattern, each sector would still only be at most 60 degrees. In a dynamically switched antenna system, like SkyPilot’s, this constraint does not exist, and the antenna beam-width can be much smaller which results in higher antenna gain.
  • Multiple frequencies are needed: just like in the omni-directional case, in some bands there are a limited number of available frequencies (or frequencies are expensive in licensed bands).  And in unlicensed bands there may not be multiple clean channels. And if a single channel is sub-divided, which many systems do not even support, each sector would only have a fraction of the total bandwidth.

With SkyPilot’s dynamic antenna switching, 8 high-gain 45 degree sectors are shared using a single radio, so a single frequency can be provided with 360 degree coverage while still providing the benefits of a high-gain antenna. Even though SkyPilot provides the resiliency of a mesh networking architecture, this spectral reuse flexibility has allowed many service providers to deploy large PtMP deployments in which each base-station provides synchronous connectivity to  low-cost subscriber equipment.

freq-reuse-sect

In situations where multiple channels are available, an omni-directional PtMP system loses any extra capacity that could have been gained, due to the required frequency re-use. By simply using multiple base-stations with the SkyPilot system, all of the additional channel capacity can be provided at each base-station location. And, each channel is provided over 360 degrees, compared to sectorized PtMP architectures which only provide each frequency on particular sectors, so with the SkyPilot equipment, there is frequency and base-station equipment redundancy to each subscriber (even if the subscriber uses a high-gain directional antenna).

And, of course, there is the additional benefit of meshing for additional range, routing around obstructions, and increasing system capacity by relaying through shorter high-modulation links (instead of wasting base-station bandwidth by communicating to a long range subscriber at low modulation, a high-modulation relay can be used).  But, these benefits are all extra, since even in a pure PtMP environment there is significant benefit from dynamic antenna switching.

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Mesh Capacity (Part 2): The Multi-Radio Myth

Wednesday, February 18th, 2009

When we were designing the SkyPilot multi-hop scheduling protocol, our task would have been much easier if we simply used one radio to talk to the parent node and another radio to talk to the child nodes. However, there are several reasons why we chose to tackle the much more difficult problem of single-radio multi-hop scheduling. Obvious reasons to use only a single radio include cost (radios might but cheap, but high power, industrial grade radios and the additional interconnect are not), power, size and the inability to find many clean channels…, but the main reason is that using multiple radios simply doesn’t work!

Focusing on that last claim, simultaneous transmissions and receptions over long-distance links simply do not work in the real world. This is based on physics. If a high-power radio is transmitting at +30dBm while another co-located radio is trying to receive a signal at -90dBm, then the +30dBm transmission will completely swamp the -90dBm reception. That’s a 120dB difference in signal levels, and to put that in perspective, the transmitted signal level is 1 trillion times stronger than the received signal level.

To combat this problem, multi-radio systems have traditionally tried using combinations of:

  • filtering (but filtering is expensive and can not provide anywhere near the needed 100dB+ of isolation)
  • physical separation of antennas (but using extremely long cables to externally mounted antennas is lossy, expensive, and doesn’t fit onto a pole – so it’s impractical to get the level of isolation required)
  • increasing received signal strength by only allowing very short links (but this is still insufficient, and would only allow for very short links)
  • lab demos (it’s common to see cabled multi-radio setups showing simultaneous active radios in the lab, but this is just smoke and mirrors where the cables are providing artificial path isolation and are allowing unrealistically high received signal levels)
  • lowering modulation (where the radios are allowed to interfere and simply drop modulation and rely on CSMA – but then there is no capacity benefit to using multiple radios)
  • requiring channel separation (but even skipping an entire channel is not enough, which you can see from radio vendors’ published “alternative channel rejection” values)
  • and (in theory), trying to schedule around tx/rx situations (but, this requires symmetric upstream and downstream traffic, and is impractical)

Even if different channels are used, a typical wireless specification like 802.11a only requires adjacent channel rejection of up to 16 dB and alternate channel rejection up to 32 dB. So even different channels don’t help, and different frequency bands are sometimes recommended (for instance, one channel at 5.2Ghz and the other at 5.8Ghz) which is impractical due to power restrictions and availability.

Even the combination of many of these techniques is nowhere near sufficient to allow for simultaneous transmissions and receptions in a single device over reasonable link distances. So, while a multi-radio story might sound extremely compelling, and has somehow even found its way into some RFP requirements, there are many hidden technical challenges that make it not feasible in the real world.

And there is another important factor in the single versus multiple radio debate – in an “access network”, where the majority of traffic is flowing to and from a gateway node, the bottleneck is almost always the gateway. So, in order to increase capacity of the overall mesh, multiple radios can simply be used at the gateway (which I’ll talk about how to most effectively do in regard to SkyPilot devices in another post). So, if you happen to have access to a live mesh “access network”, try monitoring the utilization of all of your non-gateway radios versus your gateway radios, and this will show you how much money you’ve stranded on your poles.

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Common Misconceptions #2: SkyPilot’s products are less applicable in EIRP-restricted regions like Europe

Tuesday, January 27th, 2009

Given a particular link and radio technology, the primary variable components of the link budget are:

  • 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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Common Misconceptions #1: The technology’s main benefit is long links

Monday, January 12th, 2009

A common misconception from the early days of SkyPilot is that our products’ high link budget is only applicable to long-range communications. At the time, we had customers spanning mountain tops across state boundaries, so it was understandable that users didn’t see how the equipment would apply to denser deployments like municipal Wi-Fi or smart metering networks. But the core technology of scheduling communications with high-gain directional antennas on both sides of the link has several benefits beyond long range, specifically in the areas of interference avoidance and capacity.

Regarding interference, transmitting with a directional antenna reduces the interference caused to other devices. Receiving with a directional antenna reduces the amount of interference received from other devices. Both of these compound and allow for many simultaneous devices in an extremely dense area such as municipal and utility smart grid networks. It is common to have two pairs of nodes communicating simultaneously just a few blocks apart.

Capacity is also increased since multiple communications can take place simultaneously within the same network. For instance, while a gateway is communicating to one first hop node, other first hop nodes in the same network can communicate to second hop nodes. In this way, the gateway is always active and overall system capacity is increased, which is important for very dense networks with many users in a small area.

And since both long range communications and dense node clustering are enabled by the same underlying technology, these characteristics can coexist in a single network. Many of our customers’ networks contain a wide range of node densities and it is common to see long range links connecting dense pockets of users in downtown areas or apartment complexes, along with individual paths to office buildings or power substations – all in a single network. Since our protocol dynamically schedules communications and controls the pointing of directional antennas, it applies equally well to many levels of network density.

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Why Not Synchronous?

Friday, December 5th, 2008

We’ve analyzed the benefits of synchronous protocols and the disadvantages of asynchronous protocols in outdoor wireless networks, but what are the disadvantages of using a synchronous protocol? Here are a few disadvantages, and potential solutions:

  • 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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Why Not Asynchronous?

Wednesday, December 3rd, 2008

To understand the benefits of a synchronous protocol, it helps to look at the disadvantages of an asynchronous protocol. When a node using an asynchronous protocol such as 802.11 wants to transmit a frame, it normally will simply transmit the frame after it senses the channel is idle for a period of time (which is called Carrier Sense Multiple Access, or CSMA). If a collision is determined, due to the lack of an acknowledgment frame, the frame is re-transmitted after waiting an amount of time that increases exponentially for each retransmission. In order to minimize the impact of a collision and to maximize the chance of a successful reception of the data frame, 802.11 includes an optional collision avoidance (CA) function where a short Request-To-Send/Clear-To-Send (RTS/CTS) exchange is first performed, which causes devices overhearing those frames to not access the channel for a period of time. This collision avoidance function may be beneficial in some situations, but it comes with a large overhead, and it introduces problems of its own, and the impact of these problems is greatly increased in a long-range outdoor system.

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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Why Synchronous? (Part 3)

Thursday, October 23rd, 2008

To summarize so far, there are two primary reasons to use a synchronous, scheduled protocol within a mesh network: MAC layer coordination and to point directional antennas.

Regarding the latter, to avoid the challenges of dynamically pointing antennas, some multi-antenna systems use a separate radio for each antenna (or subset of antennas). This has several problems, with the most obvious problem being cost. Even though there is now the availability of inexpensive 802.11 radios, these radios have many hidden costs due to:

  • amplifiers
  • increased processing power and processor interconnect
  • increased node size
  • increased power consumption

However, there is a bigger problem with using multiple radios – self-interference. Even if the radios each use separate frequencies and guard bands (which is impractical due to the limited number of channels in many frequency bands), all radios interfere on some level. This can be seen by looking at an 802.11 radio’s published adjacent channel rejection values, which is basically the amount of interference from communications on an adjacent non-overlapping channel. The problems due to this self-interference are magnified by the characteristics of outdoor wireless, such as high levels of external interference and weak signal reception due to long links and high amounts of obstruction.

To address the issues of cost and limited channel availability, a reduced number of radios is sometimes used. For instance, some systems use 2 or 3 radios per node. However, a reduced number of radios means a reduced number of antennas, which means either very low gain antennas are used, or 360 degree coverage is not provided. Both of these restrictions are a large problem for an outdoor mesh system.

To mitigate the interference issues, the most obvious solution is to provide high levels of isolation between the radios and between the antennas. Traditionally, this would mean expensive filters and large amounts of physical shielding which is expensive and increases node size. However, it is impractical to cost effectively provide a sufficient amount of isolation in a mesh node, given typical outdoor wireless scenarios where the received signal may be under -90dBm while the transmissions might be at +30dBm. Adjacent, or even alternate, channel rejection along with filters and physical isolation are not enough to provide anywhere near the level of isolation required. So, interference between the radios is not addressed, and results in decreased link modulation and reduction in link range, which are the two main reasons one would use a directional antenna in the first place.

Another general technological issue with using a radio per directional antenna is that such a system can’t take advantage of steerable (adaptive beam-forming) antennas. Steerable antenna technology allows an antenna’s pattern to be electronically adjusted, so a radio per beam can not be used since there are no fixed beams.

All of these issues can be addressed by using a synchronous protocol to coordinate all transmissions so that a single radio can be switched among many antennas (or between beam-steering weights). And even though a single radio architecture may not seem to have the capacity of a multiple radio architecture, a multiple radio system can not take advantage of additional radio capacity due to self-interference. And, the real bottle-neck of a mesh network is almost always at the bandwidth injection point (gateway), which means the use of multiple radios in the majority of nodes in a mesh network is wasted money.

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Why Synchronous? (Part 2)

Friday, September 26th, 2008

Beyond the reasons mentioned in Part 1, there is another equally important, if not more important, reason to use a synchronous protocol for broadband wireless mesh – to point antennas.

One of the most effective tools an RF engineer uses to improve a wireless link and to minimize a link’s impact on others is to use directional antennas. The benefits of directional antennas include:

  • increased link budget (both on transmit and receive), which allows higher modulation and longer range 
  • less susceptible to interference from others 
  • causes less interference to others 
  • increased power allowed in many regions

However, the challenge with using directional antennas is just that – they are directional, which requires manual pointing and alignment. In mesh networks, it’s advantageous to have 360 degree omni-directional coverage. 360 degree coverage from every node provides easy installation, maximizes redundancy, and avoids expensive and time-consuming system engineering of the mesh.

To provide a node with 360 degree coverage using directional antennas, multiple antennas are needed, and as the gain of the antennas increases the number of antennas needed to provide 360 degree coverage also increases. This basic relationship applies no matter what antenna technology is used, from fixed sectors to beam-forming arrays – each of these antenna designs focuses RF energy, and as the antenna gain increases, the RF energy is more focused, decreasing the coverage angle. And while some advanced beam-forming techniques do not use fixed antenna sectors, the RF energy is still focused in a particular direction, so the antenna angle needs to be varied in order to provide 360 degree coverage.

So, most 802.11 mesh networks with directional antennas use manual pointing, where 360 degree coverage is not provided, and the network must be engineered link-by-link. There has been some research around dynamically pointing antennas with 802.11, but its asynchronous nature makes this extremely difficult. One challenge with an asynchronous protocol is that some of the transmissions need to be made with omni-directional antennas (such as omni-directional Request-To-Send messages), since transmissions are not naturally pre-coordinated. While such a method may allow for higher modulation transmission of the actual data frames, it suffers from decreased range, increased interference and increased overhead due to the coordination (the latter can be very significant in an outdoor wireless system due to high modulations and the speed-of-light propagation). Alternatively, an asynchronous system could simply use a directional antenna only for transmissions, and use a separate omni-directional antenna for receptions. The challenge here is that interference is an issue with the receiver, and an omni-directional receive antenna neither increases the desired signal nor decreases the interference or noise. And, range is limited due to the lack of receive antenna gain. Additionally, when only a single side of a link uses a directional antenna, it is not normally classified as a point-to-point link, and many regions limit the effective output power of the link.

By using a fully synchronous protocol, such as SyncMesh, where every communication is coordinated (even bandwidth request opportunities and network entry points), antennas can be pointed on both transmit and receive. This provides all of the benefits of a system consisting entirely of point-to-point links, while still providing the redundancy and simple installation of an omni-directional system. While these benefits are significant, there are some challenges around creating a fully synchronous mesh protocol, but those will be discussed some other time.

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Why Synchronous? (Part 1)

Wednesday, September 10th, 2008

The most obvious reason that someone would choose a synchronous protocol for an outdoor wireless network is to have the ability to schedule transmissions. However, there are actually some crude ways to implement a scheduled system without being synchronous, such as by simple polling. In fact, 802.11 includes an optional Point Coordination Function (PCF) that uses polling (and 802.11e extends this functionality in its optional Hybrid Coordination Function). Additionally, 802.11 even includes some synchronous features in its base specification, specifically its Time Synchronization Function (TSF), which allows devices to periodically align their clocks, which can then be used by functions such as power-save where a sleeping device can periodically wake up at the right moment to see if there is data for it.

However, there are many reasons that 802.11 is not considered a synchronous protocol. Some features traditionally associated with synchronous protocols, such as WiMAX or SkyPilot’s SyncMesh protocol, include:

  • Contention-less data transmissions: 802.11’s base Distributed Coordination Function (DCF) normally puts data in contention, meaning that multiple nodes may transmit simultaneously. WiMAX and SyncMesh schedule data transmissions within time slots, avoiding the contention of data, allowing more bounded latency.
  • Ranging: DOCSIS (the cable modem standard), 802.16 and SyncMesh all include a time ranging function, which determines how far apart nodes are in order to compensate for RF propagation at the speed-of-light. This maximizes efficiency, since inter-frame spaces then do not have to allow for the time of the RF propagation. Synchronous protocols that do not support ranging suffer from this overhead, and polling protocols pay the propagation penalty twice. While the speed of light is normally considered fast, on long distances links the 10’s of microseconds start to add up, especially as the frame transmissions times decrease at higher bandwidths and modulations.
  • Periodic time slot grants: DOCSIS and SyncMesh include the ability to grant recurring time slots. This means that nodes can be granted extended rights to communicate on certain time slots, which increases efficiency. Asynchronous polling protocols do not provide this. Periodic time slot grants is probably the feature that most people think of when they think of a synchronous protocol, and it’s useful for providing higher classes of service for applications like voice.
  • Clock Precision: The features of a synchronous protocol benefit from very precise clocks, which means continually adjusting for phase between time sync messages (or signals from an external clock source), or using very frequent sync messages (SyncMesh performs the former since it is more efficient).

These advanced MAC features are just some of the benefits of using a synchronous protocol, but there are others…but more on that next time.

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How does the SkyPilot protocol compare with 802.11?

Monday, September 1st, 2008

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…

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