Deep physics of twisted pair and several common classifications of network cables

The maximum transmission distance for twisted pair cables is commonly known as the "insurmountable" 100-meter limit. Whether it's Category 3 (10M), Category 5 (100M), or Category 6 (1Gbps), all of them have a maximum effective transmission range of 100 meters. According to integrated wiring standards, horizontal cabling should not exceed 90 meters, and the total link length must stay under 100 meters. This means that 100 meters is the physical limit for wired Ethernet connections, from the network card to the hub or switch. But where did this 100-meter limit come from? Why does twisted pair have such a restriction? The answer lies in the physics of signal transmission. When data travels over a twisted pair, it’s an electrical signal that faces resistance and capacitance, leading to signal attenuation and distortion. These factors can degrade the signal quality, making it unreliable beyond a certain distance. To understand how the 100-meter limit was calculated, we look at the Ethernet specifications. For example, in Fast Ethernet (100Base-TX), the bit time is 10 nanoseconds per bit. The minimum Ethernet frame size is 64 bytes (512 bits), which takes 5120 nanoseconds to transmit. During this time, the system must detect any collisions on the network. If a collision occurs after the signal has traveled too far, the devices won’t be able to detect it in time, leading to packet loss. Ethernet uses CSMA/CD (Carrier Sense Multiple Access with Collision Detection) to manage shared communication channels. In a collision domain, if two devices send data simultaneously, the collision must be detected before the entire frame is transmitted. This is why the maximum allowable delay is set to 5120 nanoseconds for 100 Mbps networks. Adding up delays from cables, repeaters, and other components, the 100-meter rule ensures proper collision detection. In practice, when using Power over Ethernet (PoE), the same 100-meter rule applies. While some installations may go slightly beyond due to cable quality or lower speeds, it’s not recommended. Even if the network works initially, long-term reliability can suffer. For instance, upgrading to higher bandwidths might cause issues if the cable exceeds 100 meters. Cable quality also plays a big role. Category 5 cables, while widely used, vary in performance depending on the manufacturer. Some cheaper cables use copper-clad iron or steel instead of pure copper, reducing their effectiveness and causing instability. Super Category 5 (Cat 5e) improves on Cat 5 by reducing crosstalk and improving signal integrity, making it ideal for Gigabit Ethernet. Category 6 (Cat 6) offers even better performance, supporting frequencies up to 250 MHz and providing more headroom for high-speed applications. Although the 100-meter limit is a technical standard, real-world installations often aim for 80-90 meters to ensure stability. At lower speeds, like 10 Mbps, the distance can be extended to 150-200 meters, but this isn't guaranteed to meet official standards. Ultimately, using high-quality cables is essential for reliable PoE and network performance, especially in demanding environments.