WAN: Why Synchronous? (part 1)

WAN: Why Synchronous? (part 1)

The most obvious reason that someone would choose a synchronous protocol for an outdoor wireless network is to have the ability to schedule transmissions. However, there are actually some crude ways to implement a scheduled system without being synchronous, such as by simple polling. In fact, 802.11 includes an optional Point Coordination Function (PCF) that uses polling (and 802.11e extends this functionality in its optional Hybrid Coordination Function). Additionally, 802.11 even includes some synchronous features in its base specification, specifically its Time Synchronization Function (TSF), which allows devices to periodically align their clocks, which can then be used by functions such as power-save where a sleeping device can periodically wake up at the right moment to see if there is data for it. However, there are many reasons that 802.11 is not considered a synchronous protocol. Some features traditionally associated with synchronous protocols, such as WiMAX or SecureMesh WAN, include:

• Contention-less data transmissions: 802.11’s base Distributed Coordination Function (DCF) normally puts data in contention, meaning that multiple nodes may transmit simultaneously. WiMAX and the SecureMesh WAN schedule data transmissions within time slots, avoiding the contention of data, allowing more bounded latency.
• Ranging: DOCSIS (the cable modem standard), 802.16 and the SecureMesh WAN all include a time ranging function, which determines how far apart nodes are in order to compensate for RF propagation at the speed-of-light. This maximizes efficiency, since inter-frame spaces then do not have to allow for the time of the RF propagation. Synchronous protocols that do not support ranging suffer from this overhead, and polling protocols pay the propagation penalty twice. While the speed of light is normally considered fast, on long distances links the 10’s of microseconds start to add up, especially as the frame transmissions times decrease at higher bandwidths and modulations.
• Periodic time slot grants: DOCSIS and the SecureMesh WAN include the ability to grant recurring time slots. This means that nodes can be granted extended rights to communicate on certain time slots, which increases efficiency. Asynchronous polling protocols do not provide this. Periodic time slot grants is probably the feature that most people think of when they think of a synchronous protocol, and it’s useful for providing higher classes of service for applications like voice.
• Clock Precision: The features of a synchronous protocol benefit from very precise clocks, which means continually adjusting for phase between time sync messages (or signals from an external clock source), or using very frequent sync messages (the SecureMesh WAN performs the former since it is more efficient).

These advanced MAC features are just some of the benefits of using a synchronous protocol, but there are others…but more on that next time.