Curso de Redes. 2.2.2. Cables metálicos. Trenzado

Coaxial and Twisted Pair Cables: An Overview

Coaxial Cable Characteristics

• Coaxial cable is less commonly used today but offers superior shielding against electromagnetic interference (EMI).

• It protects not only from external interference but also prevents the emission of EMI, which can lead to signal loss.

• Coaxial cables exhibit lower attenuation over long distances, making them ideal for applications like cable television networks.

• Despite its advantages, modern cable TV networks primarily use fiber optics for main runs and coaxial for shorter branches.

• The higher cost and complexity of handling coaxial cables compared to twisted pair cables have led to its decline in usage.

Twisted Pair Cable Insights

• Twisted pair cables (TP), often referred to as "twister," are more prevalent due to their cost-effectiveness and ease of manipulation.

• TP cables can be shielded or unshielded; unshielded twisted pairs (UTP) are most common in networking.

• They are limited to 100 meters in length due to excessive attenuation, adhering to structured cabling standards.

• The twisting of wires minimizes EMI, enhancing signal quality by reducing interference between pairs within the cable.

• The physical length of a twisted pair is shorter than its electrical length due to the nature of the twisting.

Historical Context and Development

• Different twisting densities among wire pairs help prevent synchronization issues that could increase interference.

• The concept of twisting wires originated in 1881 when Alexander Graham Bell noted improved telephone call quality with alternating wire positions on poles.

Understanding Electromagnetic Interference in Data Transmission

The Need for Full Duplex Communication

• To achieve full duplex communication, a pair of wires is required for each direction of data transmission. This discussion focuses on analyzing one direction at a time.

Impact of Electromagnetic Interference

• Data is transmitted through electrical signals resembling high-frequency alternating current, which can be disrupted by external electromagnetic interference (EMI), such as from fluorescent tubes.

• Traditional fluorescent tubes are particularly prone to generating EMI when they are turned on, causing fluctuations that can affect nearby cables.

Induction Effects on Cables

• When EMI occurs, it induces currents in both the red and green cables involved in the circuit. The proximity of the interference source affects the intensity of this induction.

• If one cable is closer to the interference source, it will experience more induced current than the other cable, leading to an imbalance.

Net Flow and Noise Generation

• The induced currents from both cables result in a net flow that can introduce noise into data communication; if one current is stronger than the other, it creates unwanted signal distortion.

• In cases where two unshielded cables are used, this noise becomes significant enough to disrupt effective communication between devices.

Advantages of Twisted Pair Cables

• Using twisted pair cables helps mitigate EMI effects because any induced currents tend to cancel out due to their alternating positions along the length of the cable.

• Ideally, if distances remain equal for both wires throughout their lengths, any induced noise would effectively neutralize itself.

Importance of Cable Geometry

• A well-made twisted pair with uniform geometry enhances cancellation effects against EMI. Deformations or physical pressure on cables can lead to increased susceptibility to interference.

• Small defects in cable structure may seem trivial but significantly impact data transmission quality at high speeds by increasing electromagnetic interference.

Real-world Implications and Security Concerns

• External sources of interference not only affect individual pairs within a cable but also pose risks for data security; sensitive information can be intercepted without direct contact with transmission lines.

• In espionage scenarios, it's possible to capture signals from adjacent wires using sensitive equipment without needing physical access or contact with those wires.

Enhanced Security Measures

• For maximum security during copper transmissions—especially in military contexts—copper cables are encased in steel tubes filled with pressurized gas. This setup prevents unauthorized access while triggering alarms if tampered with.