The Advantages of Synchronous Protocols in Wireless Networks

The Pitfalls of Asynchronous Communication

Before appreciating the merits of synchronous protocols, it's crucial to understand the drawbacks of their asynchronous counterparts. Asynchronous protocols, such as those based on the 802.11 standard, rely on a method known as Carrier Sense Multiple Access (CSMA) for transmitting data. This approach involves a node transmitting a frame after sensing that the channel is idle. However, if a collision occurs—when two nodes transmit simultaneously—the frame must be retransmitted, leading to potential delays and inefficiencies.

Key Challenges with Asynchronous Protocols:

• Acknowledgment Overhead: Long-distance links exacerbate the time taken for acknowledgments, increasing overhead.
• Exponential Back-off: Outdoor networks often face retransmissions due to interference, causing latency to rise sharply.
• Hidden Nodes: Transmitters may not detect interference at the receiver's location, leading to failed transmissions.
• Exposed Nodes: The RTS/CTS mechanism can prevent nodes from transmitting when they could have done so successfully.
• Collision Avoidance Overhead: The RTS-CTS-Data-ACK sequence introduces significant overhead on long-distance links.
• CSMA Failures: In outdoor networks, nodes often cannot hear each other, leading to frequent collisions and back-offs.
• Ad-hoc Architecture: In mesh networks, the potential for collisions increases due to multiple simultaneous transmitters.
• Unfairness: Nodes with fewer retransmissions or those sensing an idle channel earlier can dominate channel access.

These issues are not just theoretical; they have practical implications. For instance, in outdoor mesh networks, the compounded problems can lead to a breakdown of the Media Access Control (MAC) layer, severely impacting network performance.

The Case for Synchronous Protocols

Synchronous protocols offer a structured approach to wireless communication, which is particularly beneficial for outdoor networks covering large areas. By scheduling transmissions, these protocols improve spectral efficiency and enhance Quality of Service (QoS) through better latency control, rate control, and traffic prioritization.

Advantages of Synchronous Protocols:

• Contention-less Data Transmissions: Protocols like WiMAX and SyncMesh™ avoid data contention by scheduling transmissions within designated time slots.
• Ranging: Time ranging functions in synchronous protocols account for the distance between nodes, optimizing efficiency by minimizing RF propagation delay.
• Periodic Time Slot Grants: Recurring time slots can be allocated, improving efficiency and supporting higher service classes for applications like VoIP.
• Clock Precision: Synchronous protocols benefit from precise clocks, ensuring phase alignment and reducing the need for frequent synchronization.

One of the most compelling reasons to use a synchronous protocol is the ability to dynamically point antennas. Directional antennas are crucial for improving wireless links and minimizing interference, but they require precise alignment. Synchronous protocols like SyncMesh coordinate every communication, allowing for the precise pointing of antennas on both transmit and receive, combining the benefits of point-to-point links with the redundancy of an omnidirectional system.

Challenges and Solutions in Synchronous Protocols:

• Clock Synchronization: Devices need synchronized clocks, which can be achieved through GPS or over-the-air synchronization.
• Clock Accuracy: High accuracy is required, often necessitating expensive crystals. SyncMesh addresses this with an over-the-air calibration protocol.
• Efficiency: Fixed slot assignments can lead to inefficiency, but SyncMesh uses a dynamic slot allocation scheme to optimize usage.
• Interoperability: Synchronous systems may struggle to coexist with other systems in shared spectra, but directional antennas can mitigate interference issues.
• Complexity: Synchronous systems are more complex and potentially more costly than asynchronous ones, but SyncMesh leverages off-the-shelf 802.11 hardware to reduce costs.

The Hidden Costs of Multiple Radios

Using multiple radios per node to accommodate directional antennas introduces several problems, including increased cost, self-interference, and higher power consumption. Moreover, the complexity of providing sufficient isolation between radios and antennas in outdoor scenarios can lead to reduced link modulation and range—precisely what directional antennas aim to improve.

Conclusion

Synchronous protocols offer a structured and efficient approach to wireless communication, particularly in outdoor and long-range networks. By addressing the limitations of asynchronous protocols and leveraging the benefits of directional antennas, synchronous protocols like SyncMesh can significantly enhance network performance. Despite their complexity, the advantages they bring to the table often outweigh the challenges, making them a compelling choice for robust and reliable wireless networking.

For further reading on the benefits of synchronous protocols and the challenges of asynchronous communication, consider exploring resources from the Institute of Electrical and Electronics Engineers (IEEE) and WiMAX Forum.