A shared resource calls for a shared convention

Sub-band P

Mesh networks all end up on the same portion of the ISM band: the sub-band known as P, from 869.4 to 869.65 MHz. It is the only one that allows 500 mW of effective radiated power and a 10% duty cycle; elsewhere, non-specific short range devices are capped at 25 mW and at 0.1 to 1%.

It is 250 kHz wide. Divided into slices of 62.5 kHz, the width MeshCore uses, it holds four of them. This division is indicative: no allocation has been decided.

Two mesh protocols transmit there in France today. Meshtastic, with the setting recommended by Gaulix, occupies 125 kHz centred on 869.4625 MHz. MeshCore occupies 62.5 kHz centred on 869.618 MHz. Between the two, 61.75 kHz carry no mesh, with no margin on either side. 155 kHz separate the two centre frequencies.

Those 61.75 kHz are not available for all that. Sub-band P also carries the downlink of infrastructure networks: LoRaWAN places its RX2 window there, 125 kHz centred on 869.525 MHz, and Sigfox comes down on the same frequency. That downlink covers the whole of the interval without mesh and the upper half of the Meshtastic block. These networks came there for the same reason as the others: it is the only portion at 500 mW and 10%.

Each community has chosen its settings on its own side. A third network today has no reference point for settling in without disturbing the first two: that is what the convention has to establish.

The regulatory framework

The 863-870 MHz band falls under short range devices, governed at European level by CEPT ERC Recommendation 70-03 and by the ETSI EN 300 220 standard. In France, transposition falls to the ANFR and the ARCEP.

The sub-band most used by mesh networks is the one known as P, from 869.4 to 869.65 MHz. It allows 500 mW of effective radiated power and a 10% duty cycle. It is the most generous of the band: the others are capped at 25 mW and at 0.1 to 1%.

That 10% ceiling amounts to 360 seconds of transmission per hour, over a sliding window. It applies to each piece of equipment, but its effect is collective: several nearby devices share the same physical channel.

See the regulatory references, with the texts and their dates.

What this means in practice

With the parameters used by French mesh networks, that is 869.618 MHz over 62.5 kHz of bandwidth, spreading factor 8 and coding rate 4/8, the time a packet occupies can be calculated exactly.

A complete exchange, message and acknowledgement, therefore occupies about 1,444 milliseconds. Set against the 360 seconds available, that amounts to about 250 exchanges per hour and per repeater, all users combined.

The real cost of a message

In a mesh network using flooding, every repeater that hears a packet retransmits it once. The cost of a message therefore does not depend on the distance travelled, but on the number of repeaters that overlap along its path.

In a dense area, a short message occupies the channel eleven times. Equipment density therefore does not improve capacity beyond a certain threshold: it reduces it.

The absence of a convention

The protocols deployed have message scope mechanisms. A message can be marked to travel only within a department, a region, or a country. A repeater can limit the number of retransmissions and cap its transmission time.

These mechanisms are little used. Three reasons contribute to this.

• Default configurations are not suited to the territory. They were defined for deployments of a few dozen devices, and have not kept up with the growth of national networks.
• The consequences are not explained. A recommendation without justification is not applied. A quantified consequence is.
• No framework exists for building a common convention. Recommendations circulate through informal channels, without coordination between the technologies present on the sub-band.

The remit

The Centre deals with the sub-band, not with a technology. The convention concerns every protocol that shares these 250 kHz: today MeshCore and Meshtastic, alongside the downlink of infrastructure networks, tomorrow others.

See the technologies of the band, their sites and their repositories.