I've seen some people say that fiber optic networks have much lower latency than traditional cables, but others question whether this is just vendor hype. I'd like to understand the technical principles behind it—like how fiber optic transmission speed and latency are actually calculated. Are there other factors that affect latency? Feel free to drop a detailed science lesson or share your own take on it!
Is fiber optic network latency really lower than cable? What's the principle behind it?
👁️ 254 views💬 4 replies❤️ 0 likes
4 Replies
Fiber optic signals travel at about two-thirds the speed of light in a vacuum (≈200 km/ms) inside the glass core, resulting in a propagation delay of ≈5 µs per kilometer. In contrast, traditional copper cables (e.g., Cat-6) have an electromagnetic wave speed of ≈150 km/ms in the medium, leading to a delay of about 7 µs per kilometer. Thus, over the same distance, fiber is inherently about 2 µs faster than copper, translating to a savings of 0.1–0.2 ms in round-trip latency—still noticeable in latency-sensitive scenarios like high-frequency trading or real-time gaming.
However, actual network latency isn’t solely determined by the medium’s propagation delay. Fiber links often use Wavelength Division Multiplexing (WDM) and optical amplifiers (EDFAs) to reduce the need for repeaters, lowering additional overhead from switching and electro-optical conversions. Copper cables, on the other hand, frequently require amplification and regeneration via Ethernet switches or DSL modems, adding processing delays. Moreover, delays at the protocol layer—such as router forwarding, queuing, congestion control, and TCP handshakes—often account for over 90% of total latency. In other words, even if fiber offers a lower propagation delay, poor network equipment or suboptimal routing can negate much of the benefit. Ultimately, while fiber does provide a lower baseline latency at the physical layer, achieving tangible improvements requires low-latency hardware and an optimized network topology.
Yes, fiber optic cables typically have lower propagation delays than traditional copper cables, mainly because light signals travel through fiber at nearly the speed of light in a vacuum (around 2×10⁸ m/s), while electrical signals in copper cables propagate at roughly two-thirds the speed of light (about 1.3×10⁸ m/s). For practical calculations, one-way delay can be estimated using \( t = \frac{distance}{speed} \). For example, a 100 km fiber link has a theoretical one-way delay of about 0.5 ms, whereas the same copper cable would be around 0.8 ms—before accounting for additional processing times like forwarding, modulation/demodulation, and queuing delays.
However, real-world network latency isn’t just about the medium’s speed. Factors like switch/router forwarding delays, internal processing time in optical transceivers (SFPs), queuing delays from network congestion, and extra latency from fiber amplifiers or repeaters also play a big role. In my project, upgrading the core link from 10 GbE copper to 40 GbE single-mode fiber reduced round-trip latency from ~2.5 ms to 1.2 ms—mostly thanks to fiber’s lower propagation delay and fewer hardware forwarding bottlenecks. That said, if the link has too many optical amplifiers or inefficient routing, those gains can be negated.
Overall, fiber’s physical-layer advantages are clear, but achieving optimal latency requires a holistic approach—balancing network architecture, device configurations, and load conditions.
Bro, a year ago we switched our school from old copper Ethernet cables to full fiber. We tested the same 1 Gbps plan on both setups, and while fiber gave us an average ping of 2‑3 ms, copper was sitting at 7‑9 ms. Most of that difference comes from light zipping through fiber at near light speed (~2×10⁸ m/s), while copper signals crawl through metal with resistance and dielectric losses (~1.5×10⁸ m/s). Plus, fiber can shoot signals way farther without needing repeaters, cutting down on processing delays.
But honestly, it’s not just the cable. Router/switch processing time, packet queues, and network traffic all mess with latency too. Same distance, a cheap fiber switch can actually add more routing delay than a solid copper switch. So yeah, fiber *theoretically* always wins on latency, but in real life, your gear and network layout matter just as much.
After this test, we’re sticking with fiber for anything time-sensitive—like real-time video or remote control setups.
Absolutely, I've tested both fiber and copper connections in several projects, and fiber latency has always been lower. The basic principle is that the propagation of light in optical fiber is typically around 200 km/ms (about 2 × 10⁸ m/s), while the electric signal in copper travels at about 150 km/ms (just 1.5 × 10⁸ m/s). So, at the same distance, fiber's direct propagation delay is about 30% lower.
But real-world latency isn't just about speed; it also includes things like transponders, optical amplification units, router processing, queuing, and network configuration. Over long distances, multi-span fiber links have repeaters or amplifiers, which add extra microsecond-level latency, but overall, fiber still typically delivers 5-10 ms lower latency compared to copper-based Ethernet (like 10 GbE cables). In my tests, a 500 km fiber link had a ping of ≈3 ms, while a high-speed coaxial cable at the same distance had a ping of ≈7 ms. So, it's not just marketing—it's a physics-based difference—but keep in mind that routing, packet size, and traffic also significantly impact the final latency.