Why Not Synchronous?

Randy Frei — Fri, 05 Dec 2008 23:24:50 +0000

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 Synchronous?