Fixing some values in the ZEP frame sent to Wireshark

[mikomarrache]

When using Wireshark to display the packets, the raw IEEE 802.15.4 packet received by the Ember module is first encapsulated in a "TI CC24xx" frame format, then in a ZEPv2 (ZigBee Encapsulation Protocol version 2) frame format before being sent using UDP. Using the "TI CC24xx" frame format permit passing the RSSI value but has also limitations:

• The "RSSI" value is correctly sent using a signed integer value in dBm
• The "FCS Valid" field is always set to true as the Ember module discards invalid packets
• The "LQI Correlation Value" is limited to a range of 0 to 127 (whereas the Ember module and the norm are defining this value for the range 0 to 255), so the displayed value is divided by 2.

The real LQI value reported by the module in the range 0 to 255 should be displayed in the ZigBee Encapsulation Protocol section, but due to a bug, this isn't actually the case (see https://bugs.wireshark.org/bugzilla/show_bug.cgi?id=16369).

Due to a limitation in the new Ember modules based on the EFR32, the LQI value isn't based on the bit error rate of the demodulator but is related to the RSSI value (see https://www.silabs.com/community/wireless/zigbee-and-thread/knowledge-base.entry.html/2017/08/15/lqi_in_silicon_labs-vvSq)

Credits to @sraillard

[sraillard]

@cdjackson, if this PR is accepted, is-it possible to add the text description at the end of the README.md at the repository root? This will give some information about the information provided.

[cdjackson]

I will take a look at this - if you want to add to the README then please feel free to incorporate it in this PR. I'm sure I will accept this, but will of course review first and I may have comments (or not) ;)

[cdjackson]

Thanks @mikomarrache