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.
