Technical Specs

Frequency Hopping
RMEs implement frequency hopping between up to quantity 31 800 kHz channels, PHY data rates of 50 kbps, 200 kbps, 400kpbs,800
kbps and 1200kbps in the 902-to-928 MHz ISM band. The frequency hopping protocol maximizes the use of the available spectrum
by allowing multiple sender-receiver pairs to communicate simultaneously on different channels. The frequency hopping protocol
also mitigates the negative effects of narrowband interferers.
RMEs allow each communication module to follow its own channel-hopping schedule for unicast communication and synchronize
with neighboring nodes to periodically listen to the same channel for broadcast communication. This enables all nodes within a RME
PAN to use different parts of the spectrum simultaneously for unicast communication when nodes are not listening for a broadcast
message. Using this model, broadcast transmissions can experience higher latency than with unicast transmissions.
When a communication module has a message destined for multiple receivers, it waits until its neighbors are listening on the same
channel for a transmission. The size of a broadcast listening window and the period of such listening windows determine how often
nodes listen for broadcast messages together rather than listening on their own channels for unicast messages.
RMEs implement a leading-edge frequency hopping scheme developed by Cisco. Currently, neither IEEE 802.15.4 nor any
other industry standard defines a frequency hopping protocol.
Note
Unicast Listening Schedule
The unicast schedule supports unicast communication used for communicating MAC commands and IPv6 unicast datagrams.
Each node maintains its own channel-hopping schedule for receiving unicast messages. A unicast schedule is defined by the following
parameters:
• Channel Sequence—A list of channels indexed by a 16-bit integer that a mesh interface follows when listening for unicast
transmissions.
• Slot Duration—The equal-sized time slots of the unicast schedule. A node listens to a single channel for the entire duration of
a slot before switching to the next channel in the unicast schedule for listening.
Broadcast Listening Schedule
The Layer-2 broadcast schedule supports broadcast communication used for communicating Layer-3 IPv6 multicast datagrams. The
broadcast schedule is established on a CGR and disseminated to all nodes in the PAN using a Trickle-based dissemination protocol.
All nodes in the PAN synchronize to only one broadcast schedule. There is no coordination of broadcast schedules between PANs.
The following parameters define the broadcast schedule:
• Channel Sequence—Lists channels indexed by a 16-bit integer the mesh interface follows when listening for broadcast
transmissions.
• Slot Duration—Specifies equal-sized time-slots for the broadcast schedule.
• Broadcast Listen Window—Specifies how long a node listens for broadcast messages within a broadcast slot. Broadcast packets
must start their transmission within the Broadcast Listen Window to ensure that all neighboring nodes are listening for the
broadcast transmission. The Broadcast Listen Window must specify a time that is no greater than the Slot Duration. At the
beginning of each broadcast slot, the node switches to the next channel in the broadcast schedule to listen for broadcast
transmissions. At the end of the Broadcast Listen Window, the node returns to listening for unicast transmissions until the start
of the next broadcast slot. The unicast schedule is free running and the timing remains unaffected by the broadcast schedule. In
other words, the broadcast schedule is overlaid on a node unicast schedule.
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