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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Just a few days ago, we launched the SkyConnector Mini and SkyConnector Pro and, due to the 50% price reduction we were able to achieve, we posted a side-by-side feature and pricing comparison with the Motorola Canopy CPE. The response that we have received has been exceptional and we have had many wireless ISPs who have been using competitive broadband wireless products request assistance with upgrading to SkyPilot.

As a result of these requests, we have announced a promotion where WISPs can trade-in a base station from another vendor, such as Alvarion, Motorla, Proxim, or Redline, and get a savings of USD$2,000 towards a new SkyGateway. The promotion is designed to help lower the costs to upgrade to SkyPilot and take advantage of the new SkyConnector Mini and SkyConnector Pro broadband wireless CPE.

The program is online at http://www.skypilot.com/promotion/basestationtradein.html. If you are interested and have questions, please contact us through e-mail at sales@skypilot.com.

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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.

With the SkyConnector Mini, we’ve been able to essentially replicate the SkyConnector Classic but at a lower cost. Today, SkyPilot is also announcing the SkyConnector Pro, which has the same the same price as the SkyConnector Classic but with substantially better wireless performance.

The Mini and Pro are designed to be complements. The Mini is focused principally on price-sensitive residential subscribers and the Pro is focused on the service-focused business subscribers. The Pro has a high-gain antenna (22 dBi) and a high-power radio (26 dBm) to provide the link budget required to ensure wireless connectivity and resiliency even in demanding conditions.

Both the Mini and Pro work with all other SkyPilot equipment, so there is no hardware changes required to current deployments.

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With our announcement today of the SkyConnector Mini, SkyPilot solves one of the biggest concerns for any wireless ISP - per-subscriber cost.

The most consistent feedback we get from our WISP customers relates to the cost of customer premise equipment (CPE). This is not too surprising, since every ISP - from DSL to cable to wireless - needs to keep per-subscriber costs low in order for their business model to work. Some simple math shows that decreasing per-subscriber costs reduces the time to profitability, which substantially improves the overall financial model.

Over the past few years, we’ve engineered the SkyConnector - our broadband wireless CPE - to keep costs down while still maintaining a focus on providing broadband speeds even over long-range wireless links. Today, we announced the SkyConnector Mini to substantially lower per-subscriber costs while still maintaining the commitment to long-range and high-capacity wireless.

The SkyConnector Mini essentially cuts the cost of our “classic” SkyConnector by 50%; however, it still provides the same fundamental features and capabilities. It has a 14 dBi antenna and still provide high-power transmissions at 24 dBm. We’ve provided a side-by-side comparison of the SkyConnector Mini with the SkyConnector Classic to show how similar the two products from a feature perspective, but the Mini will be able to get under US$200 in our volume packs. To our current WISP customers - and hopefully to a bunch of WISP who are not yet customers - this price point represents a milestone.

There is a lot of information relating to the Mini on our website. Please browse through this information and let us know if there is any additional information you’d like.

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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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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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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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