Published: June 2, 2026 by Roombanker Engineering Team
When a door sensor triggers, a signal travels through walls, floors, and interference to reach the hub. Most installers never think about what happens in those milliseconds — until something goes wrong.
A customer calls because the alarm did not sound. Or it sounded ten minutes late. The app shows “sensor offline” for no obvious reason. The sensor is there. The battery is full. The distance looks fine. But the signal did not arrive.
To diagnose problems like these — and to prevent them — you need to understand what happens between the moment a sensor triggers and the moment the hub acts. This guide walks through the entire communication chain, explains the key concepts, and shows how Mediterranean building materials affect every stage.
The Basic Flow: Five Stages
Every wireless alarm transmission follows the same five-stage path.
Stage 1 — Sensor trigger. The sensor’s microcontroller wakes from low-power sleep (3-8 milliseconds) and prepares a data packet containing the sensor ID, event type, battery status, and a checksum for error detection.
Stage 2 — RF transmission. The sensor activates its radio and sends the packet on a specific frequency. The transmission lasts 20-50 milliseconds.
Stage 3 — Hub receipt. The hub’s receiver picks up the packet, checks the checksum, confirms the sensor ID, and measures signal strength.
Stage 4 — Processing. The hub’s firmware evaluates the event in microseconds — is this an immediate alarm, an entry delay, or a notification-only event?
Stage 5 — Action. The hub sounds the siren, sends a push notification, reports to the monitoring station, logs the event, or some combination.
Each stage can fail independently. The most common failures happen at Stage 2 (the RF transmission never reaches the hub) or Stage 3 (the packet arrives too weak to decode). Understanding why requires a closer look at the radio communication itself.
Key Concepts Explained Simply
Frequency Bands and Modulation
Think of frequency bands as radio lanes. Different wireless technologies stay in different lanes. Most European alarm systems use the 868 MHz band (863-870 MHz), part of the Sub-GHz spectrum below 1 GHz. This band is reserved for short-range devices like alarms and smart meters. It is less crowded than 2.4 GHz (used by WiFi and Bluetooth), and its lower frequency penetrates walls better.
Some older or cheaper systems use 2.4 GHz, the same band as WiFi. The alarm signal competes directly with every WiFi router, Bluetooth device, and microwave oven in the building. A sensor at 868 MHz can penetrate two or three concrete walls reliably. A 2.4 GHz sensor may struggle with one.
Modulation is how the sensor encodes data onto the radio wave. Modern alarm systems use frequency-shift keying (FSK) or Gaussian FSK (GFSK). These methods are power-efficient and resistant to noise. Roombanker’s RBF protocol uses GFSK, which filters the signal to reduce sideband interference — important in crowded RF environments.
RSSI — Received Signal Strength Indicator
RSSI measures signal strength at the hub, in dBm. Numbers closer to zero are stronger:
• -40 dBm: Excellent (sensor next to hub)
• -75 dBm: Adequate (through one wall)
• -85 dBm: Marginal (through two walls or a concrete floor)
• -95 dBm: Weak (edge of reliable reception)
• Below -100 dBm: Likely communication failures
Every receiver has a sensitivity threshold. For Roombanker hubs it is approximately -110 dBm. The difference between the received signal and that threshold is called link budget — the buffer that keeps sensors connected when conditions worsen.
Packet Acknowledgment
When a sensor sends a packet, the hub replies with an acknowledgment — a short “message received” transmission. If the acknowledgment does not arrive within milliseconds, the sensor assumes the packet was lost and retransmits. This two-way handshake is the foundation of reliable alarm communication.
Supervision
Supervision is the system’s heartbeat. Every enrolled sensor checks in with the hub at regular intervals (15-30 minutes in modern systems) even when no alarm occurs. The check-in says “I am still here, my battery is fine, my tamper is intact.” If the hub misses two or three consecutive check-ins, it declares that sensor offline. This catches problems before a break-in, rather than discovering a dead sensor during an event.
One-Way vs Two-Way: Why Bidirectional Matters
One-way communication means the sensor transmits only. It sends its signal and hopes it arrives. There is no acknowledgment. These systems are cheaper but fundamentally insecure. A jammer can block the transmission, and neither the sensor nor the hub knows anything is wrong.
Two-way communication means the sensor and hub exchange messages in both directions. The sensor transmits, the hub acknowledges, and the sensor retries if no acknowledgment arrives. This is the minimum for Grade 2 certification under EN 50131.
All Roombanker systems are two-way at the sensor level. Beyond basic acknowledgment, bidirectional communication enables remote firmware updates, configuration changes, on-demand self-tests, and detailed diagnostics. One-way sensors are black boxes that either work or silently fail.
What Happens During Interference
Interference is normal in real-world RF environments. What matters is how the system responds.
When a sensor transmits and receives no acknowledgment, it follows a programmed escalation path:
First retry. The sensor waits a random 10-50 milliseconds and retransmits on the same frequency. The random delay prevents two sensors that transmitted simultaneously from colliding again.
Channel change. If the first retry also fails, the sensor switches to an alternate channel within the same band. Most 868 MHz systems have multiple channels. If interference is on channel 1, channel 4 may be clear.
Supervision escalation. If brief interference prevents a single alarm transmission but the sensor eventually succeeds, the event is logged. If interference is sustained enough to cause supervision failures, the hub reports a communication fault to the user and monitoring station.
The entire chain runs in seconds. The key difference between a robust system and a weak one is channel diversity. A system that only retries on the same frequency fails against interference on that channel. A system that changes channels dynamically has a much higher probability of delivery. For more on deliberate interference, see our article on RF Jamming Detection Explained. For guidance on maintaining signal quality through real-world building materials, see Signal Stability vs Signal Range.
What Protocols Actually Do at the Communication Layer
Most installers hear protocol names — Z-Wave, Zigbee, RBF — and assume they are interchangeable. They are not. Each makes different decisions about frequency, topology, and reliability.
Z-Wave operates at 868 MHz in Europe and uses a mesh topology — devices relay messages for each other. This extends range but introduces latency. Every mains-powered device may end up relaying neighbor traffic, draining batteries in portable sensors. Maximum 232 nodes.
Zigbee operates at 2.4 GHz and also uses mesh networking. Its frequency has much poorer building penetration than 868 MHz. In Mediterranean buildings with reinforced concrete walls, Zigbee sensors often need multiple intermediate devices to reach the hub. Zigbee was designed for home automation, not security — its priority is low-power connectivity, not guaranteed alarm delivery.
RBF (Roombanker RF Protocol) is purpose-built for security. It operates at 868 MHz in a star topology — every sensor talks directly to the hub. No device depends on another to relay its signal. RBF implements two-way acknowledgment at the hardware level, frequency agility (automatic channel switching during interference), and adaptive power control (sensors adjust transmit power based on link quality to save battery).
The practical difference: Z-Wave and Zigbee were designed for convenience. RBF was designed for reliability. For a deeper comparison of Sub-GHz versus 2.4 GHz and why frequency choice matters on site, see our Sub-GHz vs 2.4 GHz Wireless Alarm Frequency Guide. For the business side of wireless technology selection — what distributors need to know about stocking and selling different protocol devices — see our guide on How Security Distributors Make Money.
Mediterranean Building Context
The communication stages above work differently depending on construction. Mediterranean buildings have specific characteristics that affect RF at every stage.
Reinforced concrete. Buildings across Greece, Turkey, and Italy use reinforced concrete frames with steel rebar that acts as a reflector for 868 MHz signals. A signal passing through a reinforced concrete wall loses 15-25 dB — reducing a -65 dBm signal to -85 dBm or worse. This directly affects Stage 3: the packet arrives, but at marginal strength.
Stone walls. Older buildings in city centres from Athens to Izmir use solid stone 40-60 cm thick. Stone is dense and absorbs RF energy. A single stone wall can attenuate by 25-35 dB. A sensor one room away may have a clear path, while a sensor two rooms away cannot reach the hub at all.
Steel security shutters. Mediterranean homes and shops use metal roller shutters on windows and doors. When lowered, they create a reflective surface that blocks or redirects RF. A door contact may show RSSI of -70 dBm during installation (shutter raised) and -95 dBm in the evening (shutter lowered). This timing matches exactly when most false “sensor offline” reports occur — at night, when shutters are down.
How to compensate. Hub placement is the single most important decision. Mount it centrally, at waist height or higher, away from large metal surfaces and concrete columns. When a sensor struggles through multiple walls, use a wireless repeater. Run a full walk test with RSSI logging over 24 hours — not a quick pair-and-leave. Our guide on Signal Stability vs. Signal Range includes step-by-step methods for measuring link quality on site and catching sensors at risk of intermittent failure.
Close
An alarm system is only as reliable as its weakest communication link. A sensor that reads -75 dBm through air but -95 dBm through a steel shutter and concrete wall is not faulty — it is asking for more link budget than the installation provides.
When you understand the five-stage flow, RSSI and link margin, the difference between one-way and two-way protocols, and how Mediterranean materials affect RF transmission, you stop guessing and start diagnosing. A problematic placement becomes obvious before the customer calls. A marginal RSSI reading during walk test becomes a decision to move the hub or add a repeater, not a hope that it will hold.
Understanding how the signal travels helps you install systems that never drop.
For installers evaluating wireless systems in Mediterranean markets, our Exclusive Territory Distribution Guide covers market-specific building types and installer requirements across Turkey, Greece, and Romania that directly affect communication planning. For security businesses looking to partner with a manufacturer that provides full technical support, explore Roombanker’s engineering and manufacturing approach. To discuss technical compatibility or request a site survey demonstration, contact our engineering team.
For the full picture of how Roombanker’s technology platform supports reliable installations, explore our Technology Hub or browse the Wireless Alarm FAQ.
Roombanker wireless alarm systems use the RBF protocol at 868 MHz, with two-way acknowledgment, dynamic channel switching, and adaptive power control — designed for the real conditions Mediterranean installers face every day. For more technical resources, visit the Roombanker Installer Knowledge Base.