Ping Latency Improvement#

Using the Wi-SUN Network Measurement application in its default configuration (Lowest speed Wi-SUN PHY, Network size set to small, and default broadcast dwell interval), the ping latency performance is not optimal. This chapter describes ways to optimize the solution ping latency and explains some of the trade-offs associated with the modifications.

AN1330 Application Note#

AN1330: Silicon Labs Wi-SUN Mesh Network Performance reports the ping latency internal tests performed by Silicon Labs in real-world deployments. The tests focus on characterizing the expected Wi-SUN stack latency in point-to-point communications but also in round trip messages traveling over several hops. The tests are performed with the Wi-SUN stack default (non-optimized) configuration using the 50 Kbps North America Wi-SUN PHY. The results provide a good reference point to compare your own test results. Keep in mind that using a different region PHY impacts the results in the following figure.

Neighbor Ping Latency HistogramNeighbor Ping Latency Histogram

Wi-SUN PHY Impact#

The first critical configuration impacting the ping latency is the Wi-SUN PHY which is used by every device in a Wi-SUN network. A point-to-point ping is composed of four messages over the air. A ping with a payload of 512 bytes triggers 1332 bytes exchanged on the air. It represents approximately 30 milliseconds spent sending messages on the radio medium (see the following figure).

512-byte Ping Diagram512-byte Ping Diagram

The Wi-SUN PHY also impacts the transmissions triggered to manage the Wi-SUN network. Both the speed and number of channels available in the PHY factor into the penalty that is applied to a ping latency sent close to a network management event. The two main time-consuming network management mechanisms are the PAN advertisement and PAN configuration events. For example, a PAN advertisement sent using the 50 kbps North America PHY using 128 channels holds the radio for approximately two seconds (about 15 milliseconds spent on each channel). The PAN advertisement and configuration events explain the periodical delays that can be noticed during a long ping test. Using a Wi-SUN PHY with a higher modulation speed and fewer channels highly reduces the impact of those network management events on the ping latency (for example, 300 kbps North America PHY with 41 channels). The drawback or trade-offs are the effective distance for a good communication between nodes in the network is reduced. Finally, you need to consider how changing the network size setting can affect the network management event intervals. See Network Configuration and Size Setting below.

In addition to the PHY speed itself, the frequency hopping mechanism and the associated channel switches can delay transmissions of packets to prevent sending a message on a channel change. The default unicast dwell interval is 255 milliseconds.

Network Configuration and Size Setting#

The network configuration plays an integral role in the way packets flow through the Wi-SUN network. These network configurations impact the ping latency:

  • The border router broadcast interval defines at which interval a broadcast dwell interval happens. By default, the border router broadcast interval is set to 1 seconds and 20 milliseconds. You can modify it on the border router side. The configuration of the border router broadcast interval propagates throughout the Wi-SUN network and every device part of the network uses the same interval. If an application focuses on unicast messages, Silicon Labs recommends that you increase the border route broadcast interval.

  • The broadcast dwell interval defines the time of a broadcast frequency hopping slot (by default, 255 milliseconds equal to the unicast default dwell interval). The longer the broadcast dwell interval, the less time remains for unicast communications. Silicon Labs recommends that you reduce the broadcast dwell interval for a unicast-focused application. You change this configuration on the border router side.

  • The network size setting dictates the intervals between network management events among other things. To optimize the ping latency, Silicon Labs recommends that you use the large or medium configuration depending on your network deployment size to increase the network management exchanges interval. Be aware of the potential trade-off with the network connection speed and network configuration update speed.

Considering all the options available for a Wi-SUN solution developer, you can greatly improve the ping latency from the initial results retrieved from the default solution configuration. Keep in mind that every setting change is a trade-off with another Wi-SUN network performance component.