Wi-SUN Metrics

To provide a better understanding on the Wi-SUN stack behavior the stack maintains internal statistics.

The Wi-SUN Network Measurement application monitors several metrics during the tests:

 

• Lifetime

• MAC transmission count

• MAC transmission failed

• RPL rank

• Expected Transmission Count (ETX)

• Received Signal Level OUT (RSL out)

• Received Signal Level IN (RSL in)

 

The metrics are maintained for each neighbor device in the routed network

(that is, the parent and all of the children in the RPL meshtree).

 

 

 

1. Lifetime

The lifetime is a counter maintained with each neighbor to evaluate the state of the connection with this device.

After each successful unicast communication with the neighbor, the counter is reset to 2200 seconds.

When no unicast communication is exchanged with the neighbor, the counter is decreased each second.

 

 

 

2. MAC Transmission Count

The MAC transmission count (or MAC TX count) represents the number of packets transmitted over the air.

A single ping attempt can result in several MAC-level transmission if the first attempts have not been successful.

The number also considers packets not related to the ping test itself. Packets routed by the device to its parent

and network maintenance related communications also increase this number.

 

 

 

3. MAC Transmission Failed

The MAC transmit failed (or MAC TX failed) represents the number of unsuccessful transmission attempts.

In the Wi-SUN FAN specification, the peer must acknowledge every unicast packet.

If a packet is not acknowledged within a given timeout, the transmission is considered failed.

The packet is retransmitted by the Wi-SUN stack MAC layer until it reaches the maximum number of transmission attempts.

 

 

 

4. RPL Rank

The RPL rank represents the rank or hop distance from the tree root (that is, the Wi-SUN border router).

For more information, see section 3.5 of RFC 6550.

 

 

 

5. Expected Transmission Count

The Expected Transmission Count (ETX) is an Exponentially Weighted Moving Average (EWMA) of the number of expected packet

transmissions required for error-free reception at destination.

The ETX is calculated as (frame transmission attempts)/(received frame acknowledgements) *128 with a maximum value of 1024,

where 0 received frame acknowledgments sets ETX to the maximum value.

This metric is maintained as part of the RPL protocol (RFC 6550) and participates in the best parent selection.

 

 

 

6. Received Signal Level OUT(RSL out)

The Received Signal Level out (RSL out) is an EWMA of the received signal level for the node-to-neighbor direction.

The RSL is calculated as the received signal level relative to standard thermal noise (290°K) at 1 Hz bandwidth or -174 dBm.

This provides a range of -174 (0) to +80 (254) dBm.

 

 

 

7. RSL: Received Signal Level IN(RSL in)

The Received Signal Level in (RSL_in) is an EWMA of the received signal level for the neighbor-to-node direction.

The RSL is calculated as the received signal level relative to standard thermal noise (290°K) at 1 Hz bandwidth (= -174 dBm).

his provides a range of -174 (0) to +80 (254) dBm in 1 dBm steps.

The RSL_in value is communicated through an information element in the Wi-SUN packet exchanges between the device and its neighbor.