Posts Tagged ‘mesh’

Mesh Capacity (Part 1)

Thursday, February 5th, 2009

There has been an ongoing discussion in the mesh community about how much capacity is lost due to the relaying of data within a wireless mesh network. Proponents of multi-radio architectures have argued that they can deliver close to 1/n (where n is the number of hops) of the capacity of a radio simultaneously to each mesh device, while single radio architectures are closer to 1/2^n. For instance, a 4-hop path in a multi-radio system (assuming several clean channels are available) could deliver on the order of 1/4 the capacity of a radio simultaneously to all mesh devices, while a single-radio system may only be able to deliver 1/2^4, or 1/16, the capacity of a radio, due to multi-hop interference.

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).

Single Radio Mesh

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.

Dynamically Switched Directional Antennas

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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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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