Why Ethernet Is Less Reliable Over Long Distances

Ethernet is undoubtedly one of the most widely used technologies for local area network (LAN) connections globally, appreciated for its simplicity, cost-effectiveness, and relative ease of installation. However over long distances, certain technical challenges arise that can impact its reliability significantly. The main reason relates to its flow-control mechanism and how it interacts with the inherent physical and protocol limitations of Ethernet cabling and signalling.

This article explores why Ethernet becomes less reliable as distance increases, with a particular focus on flow-control and related limitations that contribute to performance degradation in longer Ethernet runs.

Understanding Ethernet and Its Typical Distance Limits

First, it’s important to grasp the physical constraints of Ethernet over copper twisted-pair cables, which are the most common form of Ethernet wiring. The Ethernet standards, especially those covering typical cables such as Cat5e, Cat6, and Cat6a, specify a maximum cable length of approximately 100 meters (328 feet). This length is a well-established industry standard designed to ensure that the electrical signals used for data transmission remain strong and coherent enough for error-free communication.

Beyond this 100-meter limit, several problems arise due to signal attenuation—the gradual loss of signal strength as electrical pulses travel through the cable. 

Attenuation increases electrical resistance, causes signal distortion, and introduces latency. These physical signal degradations lay the foundation for the network performance issues Ethernet experiences at extended distances.

Flow-control in Ethernet: How It Works and Why It Matters

Flow-control in Ethernet is a protocol feature designed to prevent data loss during temporary congestion or when a receiving device is overwhelmed with packets. The IEEE 802.3x standard introduces "pause frames," which allow a receiver to temporarily stop the sender from transmitting data until it is ready to process more packets. This mechanism aims to avoid packet drops by regulating the data flow between devices.

While flow-control is crucial for managing congestion under ideal conditions, it has serious implications on the reliability of Ethernet under long-distance use:

1.    Propagation Delay and Feedback Loop Timing:
Over longer cable distances, electrical signals naturally experience propagation delay—the time it takes for a signal to travel from sender to receiver. This delay affects the responsiveness of flow-control, as pause frames sent from receiver to sender also take time to traverse the cable. If the cable length extends beyond the ideal limit, the feedback loop controlling the flow becomes less effective and can cause excessive and untimely pausing or, conversely, longer periods of unchecked transmission.

2. Increased Risk of Congestion and Buffer Overflows:

The inefficiency introduced by delayed flow-control feedback can lead to buffer overruns in network devices. If a device near the receiving end cannot signal the sender quickly enough to pause transmission, data packets may be lost, forcing retransmissions that degrade throughput and increase latency.

3. Amplified Effects of Signal Degradation:

At longer distances, signal attenuation can corrupt packets, causing greater retransmission rates. Even with flow-control in place to prevent congestion, the physical degradation means more errors occur, affecting the overall reliability of the connection.

Why flow-control Does Not Fully Solve Long-Distance Challenges

Flow-control was intended as a local traffic management tool—not a solution for physical layer limitations. Unfortunately, in long Ethernet cable runs, the electrical and timing impairments are outside the scope of what flow-control can efficiently manage. flow-control cannot correct or prevent:

• Physical Signal Loss: Attenuation and interference degrade the actual transmitted data regardless of pause frames.
• Electromagnetic Interference (EMI): Running Ethernet cables parallel to sources of strong electromagnetic fields (like power cables) can induce noise, leading to corrupted frames and transmission errors.
• Latency Induced by Long Cables: Longer cables increase end-to-end delay, which flow-control can only react to after the fact, never prevent.

The Standard Ethernet Distance Limit: A Practical Compromise

The 100-meter maximum cable length has been carefully standardized to balance the electrical properties of copper cables with the timing requirements of Ethernet protocols, including flow-control. Staying within that limit allows flow-control to function optimally, ensures low error rates, and maintains stable connections.

Stepping beyond this limit often leads to:

• Decreased Signal-to-Noise Ratio: More noise means higher error rates.
• Problems with Carrier Sense and Collision Detection: For half-duplex Ethernet, the extended round-trip time complicates collision detection.
• Reduced Effectiveness of flow-control: The latency introduced disrupts the synchronization of pause frames.

Alternatives and Solutions for Long-Distance Ethernet Use

To extend Ethernet beyond its native limits without losing reliability, various technologies and strategies are employed:

• Ethernet Extenders and Repeaters: These devices regenerate and amplify signals to overcome attenuation but introduce additional latency themselves.
• Fiber Optic Cabling: Using fiber optics can extend distances to kilometers with minimal signal loss and no electromagnetic interference, although at a higher cost.
• Media Converters: Convert Ethernet signals to fiber for long-distance transmission and back to Ethernet at endpoints.
• Network Switches: Deploying intermediate switches every 100 meters can segment the cable runs and refresh the signal.
• Lower Speeds Over Longer Distances: Reducing transmission speeds (e.g., from 1 Gbps to 10 Mbps) can allow longer cable runs with fewer errors but sacrifices bandwidth.

Ethernet’s flow-control mechanism is vital to preventing packet loss due to congestion but inherently depends on timely feedback between sender and receiver. Over long cable distances, signal attenuation and physical delays introduce latency in this feedback loop, reducing flow-control effectiveness. Combined with increased error rates from signal degradation and electromagnetic interference, these factors explain why Ethernet becomes less reliable the further it stretches beyond its specified cable limits.

While Ethernet standards and flow-control were never designed to handle very long physical transmissions, solutions such as fiber optics and Ethernet extenders address these shortcomings. Understanding the technical reasons behind these limitations helps network designers select appropriate technologies to maintain reliable connectivity over long distances without sacrificing performance.

Ethernet over very long distances faces natural physical and protocol challenges, with flow-control playing a limited role beyond a certain point—making Ethernet inherently less reliable for such scenarios. Following Ethernet specifications for cable lengths and using proper extension technologies is essential to preserving network reliability and performance.