Introduction

At the physical layer, all digital networking eventually becomes electrical, optical, or radio signals that travel over some medium. In this chapter you focus only on electrical signals on copper media and how they are used to represent the bits that higher layers work with.

You do not need to understand every detail of electronics to work with networks, but you should be comfortable with the basic ideas of voltage, current, and how simple electrical patterns become digital 0s and 1s.

Voltage, Current, and Circuits

An electrical signal needs a path to travel. In networking with copper cables this path is a circuit that connects two devices, for example a PC and a switch.

Two core quantities describe what happens in that circuit.

Voltage is an electrical potential difference between two points. You can think of it as electrical pressure that pushes charges along a conductor. It is measured in volts (V). In a twisted pair Ethernet cable the voltage between two wires changes over time to carry information.

Current is the flow of electric charge through a conductor. It is measured in amperes (A). In networking cables the current is small, but it still exists whenever there is a voltage difference and a closed path.

The relationship between voltage, current, and resistance in a simple circuit is often described by Ohm’s law.

You use this relationship mainly to reason about how much current will flow for a given voltage and cable resistance, and why long or poor quality cables can affect signal quality.

Analog vs Digital Signals

Electrical signals in the real world are naturally analog. This means their voltage can vary smoothly over a continuous range and change over time in a continuous way.

Digital signals use discrete levels that represent logical states. In networking the simplest digital system uses two states which represent bit 0 and bit 1.

In practice, a digital signal on a wire is still an analog electrical signal, but devices that send and receive the signal only care if it is within the range that corresponds to a 0 or within the range that corresponds to a 1. Small variations inside those ranges are ignored.

A simple conceptual mapping is:

The exact numbers depend on the electrical standard in use, but the idea is the same. There is a defined low range and a defined high range. Values between them are undefined and may indicate noise or errors.

Encoding Bits as Electrical Levels

To carry data over a copper cable, you must map each bit to some pattern of voltage over time. This mapping is called line coding or signaling.

A very simple idea is to use a constant voltage level for the duration of a bit period. Time is divided into equal slices, each slice is called a bit interval or bit time. For each bit interval, the transmitter sets the line to the correct level for that bit.

For example, in a simple non return to zero (NRZ) like concept:

The receiver samples the voltage at the center of each bit interval and decides if it is low or high, then reconstructs the bits.

Real Ethernet versions use more advanced encoding schemes to improve reliability, reduce the required frequency, and support higher speeds. However, the basic idea is always that bits are mapped to controlled changes of voltage over time.

Time, Frequency, and Bit Rate

To interpret a signal, the receiver must know how fast bits are sent. The number of bits transmitted per second is the bit rate. If the bit rate is $R_b$ bits per second, then the duration of one bit is

For example, at $1$ Mbps:

$t_b = \dfrac{1}{1{,}000{,}000} \approx 1$ microsecond per bit.

Faster bit rates require the electrical signal to change state more quickly. This is related to the frequency content of the signal. Higher frequencies are more easily absorbed or distorted by the cable, which is one reason that high speed standards have strict cabling requirements and distance limits.

Single Ended vs Differential Signaling

There are two main ways to use electrical signals in copper networks.

Single ended signaling uses one wire to carry a varying voltage relative to a common reference, usually ground. The receiver measures the voltage on that wire compared to the reference. This method is simple but more sensitive to noise, because any noise added to the single line changes the measured voltage directly.

Differential signaling uses a pair of wires. The transmitter sends equal and opposite signals on the two wires. If one wire is at +v and the other is at −v, the receiver subtracts one from the other and looks at the difference. External noise usually affects both wires in a similar way, so when they are subtracted, much of the noise cancels out.

Twisted pair Ethernet uses differential signaling on pairs of wires. This improves noise immunity and allows higher bit rates over longer distances than would be practical with a single ended design.

Noise and Signal Integrity

An electrical signal on a cable is never perfect. Various sources of interference add unwanted variation to the voltage. This unwanted variation is called noise.

Some common noise sources in copper networks are electromagnetic interference from power lines, motors, fluorescent lights, or radio transmitters, and crosstalk from neighboring pairs inside the same cable.

The goal is not to remove noise completely, but to keep it small enough that the receiver can still decide correctly whether each bit interval should be interpreted as a 0 or a 1.

One helpful concept is signal to noise ratio, often abbreviated SNR. It compares the strength of the desired signal to the strength of the background noise.

Higher SNR means the signal stands out more clearly from the noise, so the receiver can decide bit values more reliably. Good cabling practices, twisting pairs, shielding, and grounding are all used to improve SNR.

Attenuation and Signal Levels over Distance

As an electrical signal travels along a copper cable, it loses strength. This loss of power is called attenuation. It is caused by resistance in the conductors and imperfect insulation, and increases with distance and with frequency.

Attenuation is usually measured in decibels (dB). A negative value means loss.

If the transmitted power is $P_{\text{in}}$ and the received power is $P_{\text{out}}$, then

Because $P_{\text{out}}$ is smaller than $P_{\text{in}}$, the result is negative. For example, if power is reduced to one tenth, the attenuation is $10 \log_{10}(0.1) = -10$ dB.

More attenuation reduces the received signal level, which reduces SNR and makes bit errors more likely. This is one of the main reasons why copper Ethernet standards specify maximum cable lengths.

Timing and Clock Recovery

To decode bits correctly, the receiver must know when to sample the signal. If it samples too early or too late within the bit interval, it can misinterpret transitions and produce errors.

In some simple systems, devices share a separate clock line. In most modern copper network links, the timing information is embedded into the data signal itself. The receiver uses the patterns in the incoming signal to recover a clock. This process is called clock recovery.

If a line coding scheme sends long runs of identical bits without transitions, clock recovery becomes difficult. For this reason, practical encoding methods used by Ethernet and similar systems are designed to make regular transitions and to limit the number of consecutive identical bits. This improves the reliability of timing and therefore the correct interpretation of bits.

Thresholds and Decision Making

At the receiving end, hardware must decide for each bit interval whether the signal corresponds to a 0 or a 1. It does this by comparing the measured voltage (or, in differential systems, the measured difference) to one or more thresholds.

A simple conceptual scheme might be:

Noise and attenuation can push the measured voltage toward the middle area. Good physical layer design aims to keep the signal far from the threshold under normal conditions, which is often called having a good noise margin.

If the receiver detects too many invalid or inconsistent values, higher layers may have to request retransmission of data, or the link may be considered down.

Summary

Electrical signals are the physical representation of bits on copper cables. They use changing voltage and current in a circuit, and digital systems interpret those changes in discrete time intervals to reconstruct data. Concepts like analog vs digital levels, differential signaling, noise and SNR, attenuation, and timing are all key to understanding how a physical link can carry reliable digital information, even though the underlying signal is subject to imperfections.

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