Sorin Bora

Time slotted channel hopping MAC feature and applicability in the sub-GHz US 902-928 MHz band for FCC compliance

Discussion created by Sorin Bora Employee on Oct 3, 2014

FCC part 15.247 allows spread spectrum transmitters operating in the US frequency band 902-928 MHz to operate using as high as +30dBm output power under certain conditions:

  • If the 20dB bandwidth of the channel is less than 250 kHz, the channel hopping list shall contain at least 50 frequencies and in a 20 second period a frequency will not be occupied more than 0.4 seconds
  • If the 20dB bandwidth of the channel is between 250 and 500 kHz, the channel hopping list shall contain at least 25 frequencies and in a 10 second period a frequency will not be occupied more than 0.4 seconds

 

Frequency hopping mechanism is performed in the MAC layer as described by the 802.15.4e Time Slotted Channel Hopping feature in conjunction with the PHY modes listed in the 802.15.4g for the US 902-928 MHz Frequency Band.

 

The network layer of a PAN Coordinator configures the channel hopping sequence, communication links between nodes and timeslot parameters. Timeslots are grouped in slotframes and repeat periodically in a pattern described by the size of the slotframe.

Each link is a pairwise communication between two nodes and is assigned to a timeslot inside a slotframe and a channel offset in the hopping sequence. Thus, a slotframe can accommodate up to a number of timeslots multiplied by the number of channel offsets of point to point communications between nodes.

 

TSCH parameters are advertised periodically by the PAN Coordinator through the use of special MAC frames, the Enhanced Beacons, in order to allow new nodes to scan a range of channels and join the network timeslot and hopping structure. Once the network synchronization is performed, the device will communicate following the PAN Coordinator hopping, timeslot and link configuration and can advertise these parameters at the discretion of the network layer.

 

The network layer can set the size of a slotframe in timeslots. Multiple slotframes can be configured, so that multiple communication patterns can be followed inside a network. Also, the size of the channel hopping sequence list can be up to or even greater than the maximum supported channels on a specific PHY mode, as a channel can appear more than one time in the pattern.

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