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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Tags: asynchronous, mesh, synchronous, wifi, wimax 802.11, wireless
Hi Sir ,
Your protocol ( SyncMesh ) is like hiperlan/2 . Am’I wrong ?
Thank’s
Marco
The MAC layers of SyncMesh and HiperLAN/2 both perform centralized TDMA scheduling, and they have similar capabilities. But the actual protocols are different since SyncMesh performs multi-hop scheduling in order to mesh/relay.
The capabilities of the SyncMesh MAC layer are also very similar to WiMAX (many members of the protocol team came from cable modem backgrounds), but the protocol was designed to run over 802.11 PHYs which are very different than 802.16 PHYs.
Thank’s for replay
Of course , also Hiperlan/2 use 802.11 PHYs with TDMA/TDD.
Regard’s
Marco
Hi ,
Why not implement SyncMesh firmware for centrino ? And , why not in 2.4Ghz ?
Thank’s’ in advance.
Marco
Good questions. We’ve considered running SyncMesh right on a user’s PC, but general purpose platforms like Centrino are missing the “hard real-time” performance that our protocol needs.
Regarding 2.4Ghz, we’ve found that even though propagation loss is lower at 2.4Ghz than 5Ghz, the decreased noise level at 5Ghz more than makes up for it. Plus, there are hundreds of Mhz available at 5Ghz in many regions, compared to 3 channels at 2.4Ghz. So, it comes down to decreased noise and more bandwidth.
Thank’s for answer.
So , about mobility ( at low speed , 10MPH ) , your system how works ?
Marco
Mobility support hasn’t been added to the synchronous protocol. We’ve currently decided to leverage the WiFi interfaces of the DualBand and TriBands (2.4, 4.9 Ghz) for mobility.