Iot-Device Firmware - Espressif ESP8266 Technical Reference

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Two data packets can be separated by a specific packet. The specific packet works
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like separative sign.
Packet with redundant data so that packet can verify each other.
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Set flag-packet at the beginning of sequence. Then phone APP can be cyclic
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sending the whole sequence.
Only need to send the lowest 2 Bytes of AP's BSSID ( MAC address ), IOT-device can
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still get it. If AP will broadcast its SSID, then phone APP need not to send AP's SSID
either. So AP beacon need to be analyzed to check if the AP will broadcast its SSID.
• Length of UDP packet need to be multiply by 4. Because when phone APP sent a
AMPDU packet which only has one sub-frame, packet length will be filled to be a
multiple of 4.
For Scenario 1, phone APP can send packets as fast as possible.
Phone APP won't know it is Scenario 1 or Scenario 2 for IOT_device.

14.4. IOT-device Firmware

For Scenario 2, IOT-device should notice:
Search the channel which has strongest signal first, according to RSSI.
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Filter useless packets according to RSSI. Considering 10 ~ 15db fluctuations in the
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air, some packets may be decline 10db or more. We could search the strongest
signal at first, then extend the range since find the target sequence.
Check the Aggregation bit of HT-SIG to distinguish AMPDU packet.
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AMPDU packet can only be encrypt by CCMP(AES).
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To design the length of packet that works as separative sign, different QoS, different
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encryption algorithm and AMPDU packet will be a multiple of 4, all of these should be
taken into consideration.
Use relative value to transmit information, for example, the value that the length of
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data packet minus the length of packet that works as separative sign.
Espressif 85 86 ! /!
14. Sniffer Introduction
2016.05