Two major classifications of communication cables

Communication cables are designed for the transmission of various electrical signals, including telephone calls, telegrams, fax documents, television and radio programs, and data. These cables are typically constructed by twisting two insulated wires together, forming a pair that ensures efficient signal transfer. Compared to overhead open lines, communication cables offer greater communication capacity, improved transmission stability, better confidentiality, and reduced susceptibility to natural conditions and external interference. **Symmetrical Cable** A symmetrical communication cable consists of two wires arranged symmetrically, forming a balanced circuit. There are two types: high-frequency and low-frequency. High-frequency cables can support frequencies up to 800 kHz, allowing for up to 180 simultaneous telephone connections in a loop. Low-frequency cables, on the other hand, operate at frequencies below 252 kHz, supporting around 60 telephone connections. However, the electromagnetic field of these cables remains open, leading to higher attenuation, greater mutual and external interference, and limitations in increasing transmission frequency or capacity. Long-distance symmetrical communication cables are made from four-wire groups with different numbers and insulation structures. Common configurations include star-shaped groups and double-twisted pairs. Insulation materials vary, such as paper tape, paper-rope, polyethylene rope-belt, polystyrene rope-belt, and foam polyethylene. High-frequency cables require more advanced structural performance, often using a rope-insulated star-twist design with materials like polystyrene or polyethylene. Low-frequency cables typically use tape insulation. **Telephone Cable** Telephone cables are used for local communication within cities, suburbs, and industrial areas. They are commonly referred to as local telephone cables and feature a large number of wire pairs—sometimes up to several thousand. Due to their shorter distances and lower frequency usage, they have thinner conductors, usually 0.5 mm in diameter. The structure includes twisted pairs, star-type twisted groups, and double-pair twisted groups. Telephone cables are available in different types based on insulation and sheathing materials, such as paper-insulated lead-sheathed cables, polyethylene-insulated cables, and waterproof grease-filled cables. In telephone cables, two insulated wires are twisted together at a specific pitch to form a pair. These pairs are often color-coded (e.g., red and white) for easy identification during installation. The core structure is typically either concentric or unit-based, with varying twist pitches between adjacent pairs to reduce crosstalk. Larger cables may include spare pairs for replacement if needed. Unit-based cables consist of multiple basic units, each containing a group of twisted pairs, which are then combined into the final cable structure. **Overhead Laying and Waterproofing** Telephone cables are often self-supporting, with steel cables bearing the weight. Since the 1960s, waterproof cables filled with grease have been developed to protect against moisture. In the 1970s, integrated cable sheaths became widely used in urban telephone systems. Three main types include Alpeth, Stalpeth, and Lepeth, each offering different levels of protection and durability through layers of polyethylene, aluminum, and lead. **Coaxial Cable** Coaxial cables are composed of an inner and outer conductor, both insulated and concentric, forming a coaxial pair. They are primarily used as trunk lines for long-distance communications, supporting multi-carrier services, TV programs, and efficient data transmission. Due to their concentric design, coaxial cables minimize signal loss and external interference, making them highly effective at frequencies up to 10–100 MHz or higher. Coaxial cables come in various sizes, such as micro, small, medium, and large, defined by the dimensions of the inner and outer conductors. The inner conductor is typically copper, sometimes reinforced with a steel core for added strength. The outer conductor is often made of copper tape, with corrugated, indented, or lock-type designs for flexibility and stability. The insulation must have low dielectric loss and sufficient mechanical strength to maintain concentricity. A shielding layer is also used to prevent interference, often made of double-wrapped steel strips.

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