Question

Coaxial cable Ethernet was limited to a maximum of \(500 \mathrm{~m}\) between repeaters, which regenerate the signal to \(100 \%\) of its original amplitude. Along one \(500-\mathrm{m}\) segment, the signal could decay to no less than \(14 \%\) of its original value \((8.5 \mathrm{~dB})\). Along \(1500 \mathrm{~m}\), then, the decay might be \((0.14)^{3}=0.3 \%\). Such a signal, even along \(2500 \mathrm{~m}\), is still strong enough to be read; why then are repeaters required every \(500 \mathrm{~m}\) ? $$ \begin{array}{l|l} \hline \text { Item } & \text { Delay } \\ \hline \text { Coaxial cable } & \text { propagation speed } .77 c \\ \text { Link/drop cable } & \text { propagation speed .65c } \\ \text { Repeaters } & \text { approximately 0.6 } \mu \text { s each } \\ \text { Transceivers } & \text { approximately 0.2 } \mu \text { s each } \\ \hline \end{array} $$

Short answer

Repeaters are required every 500 meters to maintain signal strength and quality, minimizing delay and ensuring reliability.

Step-by-step solution

Understanding Signal Decay

First, note that a signal decays to 14% of its original value over 500 meters. This level of decay corresponds to approximately 8.5 dB.

Calculate Cumulative Decay

Over 1500 meters, the signal decays to \((0.14)^3 = 0.027216\), which is about 0.3% of its original value.

Understand Signal Readability

Despite significant decay, even a 0.3% signal over 1500 meters is still readable with the correct equipment.

Reason for Repeaters

Repeaters are required every 500 meters because they regenerate the signal to 100% of its original amplitude, ensuring proper signal strength and quality throughout the network, preventing excessive delay and ensuring reliability.

Consider the Propagation Delay

Coaxial cables have a propagation speed of 0.77c, while repeaters introduce an additional delay of approximately 0.6 microseconds each. This means that to ensure the timely and accurate transmission of data, repeaters are necessary every 500 meters.

Key concepts

Signal Decay

In network communications, *signal decay* refers to the reduction of signal strength over a distance. In a coaxial cable Ethernet network, the signal can drop to 14% of its original value over 500 meters. This happens due to the inherent properties of the cable material and the electromagnetic interference that occurs. The amount of reduction is measured in decibels (dB), and in this case, it corresponds to approximately 8.5 dB.
When signals travel over longer distances, the decay becomes more pronounced. For example, along 1500 meters, the cumulative decay would be \( (0.14)^3 = 0.0027 \,\equiv\,\text{or\,}.3\%!\.\) Despite significantly reduced strength, this signal can still be read with the right equipment.

Repeaters

In long-distance network communications, *repeaters* play an essential role. These devices are installed along the network to regenerate signals back to their original strength. For coaxial cable Ethernet specifically, repeaters are placed every 500 meters.
This frequent placement ensures the signal remains strong and clear, facilitating reliable data transfer. By boosting the signal to 100% of its original amplitude, repeaters prevent the issues that come with significant signal decay. They also help in maintaining network performance and preventing excessive attenuation.

Propagation Delay

Every network transmission experiences some delay as the signal travels along the cable. This delay is known as *propagation delay*. With coaxial cables, the propagation speed is 0.77c, where *c* is the speed of light. This implies that the data does not travel instantaneously but at a significant fraction of the speed of light.
Additional delays can occur due to network components like repeaters. Each repeater introduces an approximate delay of 0.6 microseconds. When designing a network, accounting for propagation delay is crucial to ensure timely and accurate data delivery from one point to another.

Network Reliability

Maintaining *network reliability* is a cornerstone of effective network design. Reliability refers to the network’s ability to consistently perform its required functions without failure. By using repeaters every 500 meters on a coaxial cable Ethernet network, the integrity of the data signal is maintained throughout its path. This helps prevent data loss and ensures that all network communications are accurate and timely.
Without repeaters, the decaying signal could lead to corrupted data, increased errors, and ultimately, network downtime. Therefore, installing repeaters is a crucial measure to uphold the reliability of the network.

Signal Regeneration

*Signal regeneration* is the process through which repeaters amplify weak signals back to their original strength. This is crucial for maintaining the quality and readability of the data being transmitted over long distances.
When a signal travels through a coaxial cable, it loses strength due to absorption and interference. By placing repeaters at intervals (like every 500 meters), the network ensures that the signal does not degrade beyond a readable level. Regenerated signals reduce the chances of data errors and help maintain a smooth flow of information across the network.
This continuous cycle of signal decay and regeneration enables the network to operate efficiently over extended distances.