Posts Tagged ‘asynchronous’

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 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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