Shared 868 MHz Airtime: The Commons Is Real, but Measure It Properly
Shared 868 MHz Airtime: The Commons Is Real, but Measure It Properly
A crowded mesh can punish every user, yet neither a “sent” indicator nor a passive packet log proves who exceeded a legal limit. Good stewardship starts by measuring the right quantity.
A passive 868 MHz logger can reveal a busy channel, repeated frames and surprising reception paths. That is valuable evidence. It is not, by itself, a regulator-grade finding that named transmitters exceeded a 10% duty-cycle limit. The difference matters because shared-spectrum engineering has three separate ledgers: what each transmitter emits, what one receiver observes, and what the network successfully delivers.
Regulatory note: technically reviewed 29 August 2026. This is engineering guidance, not legal advice. Check the current national interface, the exact sub-band and the conformity evidence for the equipment and operating mode.
The Sunday-Logger Observation
Set a node to receive only—no beacons, telemetry or forwarding—and its log becomes a useful local spectrum diary. It may show frames attributed to distant nodes, bursts of repeated traffic and periods when the channel appears heavily occupied. Those observations can justify a better measurement campaign.
They do not yet prove the distance of a radio path, the identity of the physical transmitter or its legal duty cycle. A displayed node position can be stale or self-reported. One receiver misses packets below its sensitivity, packets hidden by collisions and transmissions blocked by local noise. Flooded or relayed frames can also make one application message appear many times.
Cumulative on-air time from one physical transmitter over the specified observation period and frequency range.
The fraction of time energy or decodable traffic is present at one measurement location, subject to receiver bandwidth and threshold.
Useful messages delivered with an acceptable delay after forwarding, duplicates, acknowledgements, collisions and retries.
These ledgers influence one another, but they are not interchangeable. A receiver can observe high occupancy while every visible node remains within its own duty-cycle ceiling. A hidden transmitter can breach a limit without appearing in that receiver's decoded log. And a network can deliver poorly long before the channel looks continuously busy.
The Commons Analogy—Useful, but Not a Law of RF
The “tragedy of the commons” describes an incentive problem: one participant receives the immediate benefit of consuming more of a shared resource, while much of the cost is distributed among everyone. On a shared radio channel, the private benefit can be another advertisement or retry; the distributed cost is more collision risk, longer queues and less airtime headroom for nearby users.
That is a good governance lens. It is not a deterministic claim that every open 868 MHz network must collapse. Elinor Ostrom's work on economic governance—recognised by the 2009 prize in economic sciences—showed that communities can govern shared resources through suitable rules, monitoring and participation. Radio networks likewise differ in traffic, topology, coordination and protocol design.
The engineering translation is simple: incentives affect offered traffic, but physics decides whether transmissions overlap at a receiver and protocol rules decide how many transmissions one delivered message costs.
“868 MHz” Does Not Mean “10% Everywhere”
The current harmonised EU framework contains several non-specific short-range-device entries between 868 and 870 MHz. Under Commission Implementing Decision (EU) 2025/105, the examples below are different regulatory options, not one interchangeable band:
| Frequency range | Maximum e.r.p. | Harmonised access condition |
|---|---|---|
| 868.0–868.6 MHz | 25 mW | Qualifying spectrum-access and interference-mitigation techniques, or duty cycle ≤1% |
| 868.7–869.2 MHz | 25 mW | Qualifying techniques, or duty cycle ≤0.1% |
| 869.4–869.65 MHz | 500 mW | Qualifying techniques, or duty cycle ≤10% |
| 869.7–870.0 MHz | 5 mW without an additional access requirement; 25 mW conditionally | At 25 mW: qualifying techniques, or duty cycle ≤1% |
The familiar 10% figure belongs to the 869.4–869.65 MHz entry often used by EU_868 mesh profiles. Belgium's BIPT publishes current short-range-equipment interfaces with national conditions that must be checked for the exact equipment and frequency.
Disabling a software limiter is not the legal test. Compliance depends on the emissions and the route relied upon: exact frequency, category, radiated power, occupied bandwidth, duty cycle or qualifying access technique, and the applicable equipment-conformity evidence. Changing a limiter can make operation non-compliant, but the button itself is not the regulation.
The current ETSI EN 300 220-2 V3.3.1 also makes clear that polite spectrum access is a defined method with assessment, deferral and transmission controls. Ordinary carrier sensing or a marketing label is not automatically an exemption from a duty-cycle route.
What Duty Cycle Actually Counts
Duty cycle is a property of one transmitter's cumulative on-air time over a defined observation period and frequency range. Unless another value is specified, the familiar calculation uses a continuous one-hour observation period.
d_i = Σ T_TX,i / T_obs
With T_obs = 3600 s, a 10% ceiling corresponds to at most 360 seconds of transmitter-on time per hour for that device under that compliance route.
Every emission by that transmitter consumes the account: user messages, advertisements, telemetry, route maintenance, forwarded packets, acknowledgements and retries. It is not a pool shared by the network, and one quiet node cannot donate unused seconds to a busy relay.
MeshCore's current official CLI documentation exposes get dutycycle and set dutycycle, lists a 50% default and states that the operator must select a value appropriate to the jurisdiction and channel plan. That documentation is a useful configuration check, not a substitute for verifying the radio's actual emissions and the national rules.
What a Passive Logger Can—and Cannot—Establish
| A defensible observation | What still needs proof |
|---|---|
| Frames were decoded at this receiver at recorded times. | Whether every frame came directly from the named physical node rather than a relay or duplicate path. |
| The receiver saw a certain decoded-frame rate. | Total channel occupancy, including undecodable, collided and below-threshold transmissions. |
| A node record advertised a distant location. | The transmitter's actual location, antenna, power and propagation path. |
| One identity appeared frequently in the log. | Its cumulative transmitter-on time over the legally defined observation interval and bandwidth. |
For a credible duty-cycle investigation, establish physical-device identity, synchronise time, know each packet's complete time on air, cover the whole observation period and applicable band, account for hidden nodes and missed packets, and distinguish original transmissions from forwarded copies. A calibrated spectrum recording or multiple coordinated receivers may be needed. Even then, legal conclusions belong to the competent authority.
A local channel-occupancy study is still worthwhile. State the receiver location, antenna, bandwidth, detection threshold, observation duration, firmware and decoding rules. Publish the uncertainty alongside the result. “This receiver observed 18% energy occupancy between these thresholds” is far more useful than “many nodes are illegal.”
Why Congestion Can Accelerate
Shared-channel load is better described in packet-seconds than in node count. For transmissions that contend at a receiver, a simple attempted-load quantity is:
G = Σ λ_i T_i
λ_i is attempted transmissions per second and T_i is their time on air. Include every relay copy, acknowledgement and retry.
As offered load rises, overlaps waste a growing fraction of packet time. Lost packets may cause retries, and retries add more offered load: a positive-feedback loop. Flood routing can multiply one message across several transmitters; a high relay can hear and forward traffic from a wider region; hidden nodes can both sense an apparently clear channel and still collide at a common receiver.
But there is no universal “50% and the mesh dies” threshold. Capture, spreading factor, bandwidth, coding rate, packet length, clear-channel assessment, duplicate suppression, topology and spatial reuse all change the result. The correct question is not how many nodes exist, but how many attempted packet-seconds arrive in each collision domain and how many useful messages are delivered at the required latency.
Govern the Network with Measurements
- Freeze the legal profile. Record country, exact frequency range, power, bandwidth and the compliance route the equipment relies upon.
- Measure time on air. Use the actual payload, preamble, bandwidth, spreading factor, coding rate and firmware.
- Separate origin from forwarding. Count advertisements, telemetry, floods, acknowledgements, retries and duplicate suppression explicitly.
- Map collision domains. A nationwide mesh is not one universal channel, but a high site may join several local traffic regions.
- Publish occupancy honestly. State receiver bandwidth, threshold, location, antenna, clock, duration and the traffic the decoder could not see.
- Set a budget below the legal ceiling. Reduce unnecessary telemetry, advertisements, flood scope, retries and hop depth using controls documented for the exact firmware.
- Coordinate infrastructure. Known maintainers, contact details, change records and a response process reduce both accidental abuse and recovery time.
- Test the busiest credible hour. Compare attempted packets, delivered packets, duplicates, retries, latency, queue depth and per-device transmit time.
Stewardship rule: a legal maximum is a ceiling, not a traffic target. Leave headroom for growth, fading, bursts and other lawful users, and reduce traffic where measurement shows it buys little delivered service.
Would Amateur Spectrum Solve the Commons Problem?
Amateur radio can provide identity, operator competence and established coordination practices. Those are useful governance tools. It is not automatically congestion-free, and an IARU band plan is not the same thing as national authorisation.
A mesh on an amateur allocation must still satisfy the country's allocation, operator and station authorisations, identification requirements, emission limits, content restrictions, automatic-station rules and interference obligations. Belgium's current BIPT guidance places a material restriction on unattended fixed amateur stations outside the APRS exception. Holding a callsign therefore does not by itself authorise an unattended mesh backbone.
The fair conclusion is narrower: known participants and agreed operating rules can make coordination easier, but the service change introduces different legal gates and does not remove airtime physics.
The Practical Verdict
The commons problem is real whenever a participant can gain locally by consuming more shared airtime while distributing the cost. It is a warning about incentives, not proof that a particular node broke the law or that every mesh must fail.
Use the exact SRD entry. Measure each transmitter's on-air time separately from local channel occupancy. Audit the protocol's forwarding and retry cost. Then govern the network with visible data, conservative budgets, known maintainers and changeable rules.
Configure your node as if the band belongs to everyone—because it does. Measure it as if someone else must reproduce the result—because that is how responsible engineering turns a good analogy into useful evidence.
Primary and project references checked
- Commission Implementing Decision (EU) 2025/105 — harmonised SRD conditions
- ETSI EN 300 220-2 V3.3.1 — non-specific SRD spectrum access
- BIPT — Belgian short-range-equipment interfaces
- BIPT — conditions for unmanned fixed amateur stations
- MeshCore official CLI documentation — duty-cycle controls
- Prize in Economic Sciences 2009 — economic governance of the commons
Mini-FAQ
- Is every EU 868 MHz device limited to 10% duty cycle? No. The conditions vary by frequency range, device category, power and access route. The familiar 10% alternative belongs to the 869.4–869.65 MHz non-specific SRD entry.
- Does disabling a software duty-cycle limiter automatically prove illegality? No. The legal test is the equipment's actual operation against the applicable frequency, power, bandwidth, access and conformity conditions. The change can cause non-compliance, but the setting alone is not the complete evidence.
- Can a passive packet logger prove a transmitter's duty cycle? Not by itself. It may miss hidden, collided or weak packets and may confuse origin traffic with forwarded copies. Device identity, complete on-air time, observation bandwidth and a full observation period must be established.
- Why can a mesh become less reliable as traffic grows? More offered packet-seconds create more overlap. Collisions can trigger retries, while flooding and relaying multiply transmissions. Near saturation that feedback can reduce delivered traffic even as attempted traffic rises.
- Do amateur bands automatically solve mesh congestion? No. Identification and coordination can improve governance, but national authorisation, automatic-station rules and shared-channel capacity still apply.