Understanding AIS Transponders: Class B vs. Class B+ (SOTDMA)
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Automatic Identification System (AIS) transponders are essential marine safety tools that let vessels broadcast their identity, position, speed and course over VHF to other craft and shore stations. While Class A AIS systems are mandated on larger commercial vessels, recreational and smaller craft typically use Class B systems.
Class B AIS – The Baseline Standard
A Class B AIS transponder is designed for leisure craft and non-SOLAS vessels:
- Lower transmit power: 2 W output, giving shorter range than commercial units.
- CS-TDMA technology: uses Carrier Sense TDMA, meaning it waits for a free slot before transmitting rather than reserving one in advance.
- Update rate: typically transmits position reports about every 30 seconds, regardless of speed.
- Basic data: transmits essential static and dynamic data (MMSI, position, COG/SOG, vessel dimensions).
Class B units offer a cost-effective way to be visible to other nearby vessels, but in busy waters or fast-moving boats they can miss transmission opportunities or appear less frequently on other vessels’ displays.
Class B+ (SOTDMA) – An Enhanced Standard
Class B+ (also called Class B SOTDMA or “5 W Class B”) upgrades traditional Class B by incorporating key features from Class A:
- Higher transmission power: 5 W instead of 2 W – improving range and reliability.
- SOTDMA slot reservation: uses Self Organising TDMA, guaranteeing a reserved time slot even in crowded traffic, reducing missed messages.
- Faster update rate: transmission frequency adapts to vessel speed, with reports as fast as every ~5 seconds at higher speeds.
- Better satellite reception: higher transmit power improves visibility to space-based tracking platforms where supported.
In practical terms, this means Class B+ devices provide a more consistent “safety picture” on other vessels and monitoring systems than legacy Class B units, especially for offshore use or in high-density waters.
| Feature | Class B | Class B+ (SOTDMA) |
|---|---|---|
| Transmit Power | 2 W | 5 W |
| Time Slot Method | CS-TDMA | SOTDMA |
| Update Rate | ~30 s | Adaptive (faster with speed) |
| Slot Priority | Lower | Higher (reserved) |
Class B+ is often seen as a “sweet spot” for serious sailors, commercial sport boats or anyone who wants strong performance without the cost and complexity of Class A hardware.
What is VDES? The Next-Generation of AIS
As AIS usage has grown dramatically, its original communication channels (two VHF maritime slots) are becoming congested. To handle more data, improve reliability, and support future marine communications, the industry is moving toward the VHF Data Exchange System (VDES) – widely considered AIS 2.0.
VDES Explained
VDES stands for VHF Data Exchange System, a new communication standard building on AIS, defined under international telecom and maritime bodies.
Here’s what makes VDES a big step forward:
- Greater bandwidth: VDES uses additional terrestrial and satellite VHF channels to offer many times the data capacity of current AIS systems – supporting richer information exchange and future services.
- Bidirectional data exchange: Unlike classic AIS, which is broadcast-only, VDES supports two-way communication between ship-to-shore and ship-to-ship, enabling remote updates, route exchange, and more advanced maritime data sharing.
- Satellite coverage: With dedicated satellite links, VDES extends global reach – including offshore and polar areas, overcoming AIS’s line-of-sight limitations.
- Supports e-Navigation and security: VDES enhances situational awareness with higher data throughput and improved cyber-security approaches, supporting modern navigation frameworks.
Instead of replacing AIS immediately, VDES includes AIS functionality and builds on it – so existing AIS infrastructure and deployment experience are preserved while transitioning to a richer system.
Quark-elec and the Future of Marine Communication
At Quark-elec, we’re excited about the future of maritime connectivity and how VDES will transform marine safety, tracking, and operational communication. That’s why we are actively developing our own next-generation VDES device – designed to bring the benefits of two-way global VHF IP data exchange to vessels of all sizes.
Stay tuned for updates on our VDES transponder program!
