Two major classifications of communication cables

Communication cables are used for transmitting a wide range of signals, including telephone calls, telegrams, fax documents, television and radio programs, data, and other electrical signals. These cables are typically constructed by twisting two insulated wires together. Compared to overhead open lines, communication cables offer greater transmission capacity, higher stability, better confidentiality, and less susceptibility to natural conditions and external interference. **Symmetrical Cable** A symmetrical communication cable consists of two wires arranged symmetrically. There are two types: high-frequency and low-frequency. The high-frequency type can transmit up to 800 kHz, supporting around 180 telephone lines, while the low-frequency type usually operates below 252 kHz, suitable for about 60 lines. Due to its open electromagnetic field, this type of cable experiences significant attenuation and interference at higher frequencies, making it difficult to increase transmission capacity or frequency. Long-distance symmetrical communication cables are made from four-wire groups with different insulation structures. Common configurations include star-shaped groups and double twisted pairs. Insulation materials vary, including paper tape, polyethylene, polystyrene, and foam polyethylene. High-frequency cables require more advanced structural designs, often using a rope-and-insulated star twist configuration. Low-frequency cables typically use tape insulation. **Telephone Cable** Telephone cables are designed for local communication in urban areas, suburbs, and industrial zones. They are also known as local telephone cables. These cables have a large number of wire pairs, sometimes reaching several thousand, though commonly hundreds or tens. Since they operate at lower frequencies and over shorter distances, their wire diameter is usually 0.5 mm. The structure includes twisted pairs, star-type groups, and double-pair groups. Materials used for insulation and sheathing vary, including paper, polyethylene, and waterproof compounds. The cable core is typically arranged in a concentric or unit-based structure. To reduce crosstalk, adjacent pairs in the same layer are twisted at different pitches. Each layer has color-coded flags for easy identification. In cables with 80 pairs or more, there are backup pairs to replace faulty ones. Unit cables consist of basic units of 50 or 100 pairs, which are then twisted into the final cable core. **Overhead Laying** Telephone cables laid overhead are self-supporting, with steel cables bearing the weight. Since the 1960s, waterproof cables filled with grease have been developed to prevent moisture ingress. **Integrated Sheaths** Since the 1970s, integrated sheaths have been widely used in city telephone cables to improve moisture resistance and stability. Three common types are Alpeth, Stalpeth, and Lepeth, each offering different layers of protection and insulation. **Coaxial Cable** Coaxial cables are formed by one or more coaxial pairs, consisting of an inner and outer conductor that are concentric and insulated. They are mainly used for long-distance communication, providing multi-carrier services and efficient data transmission. Their design minimizes signal loss and interference, with transmission frequencies reaching up to 100 MHz or higher. Coaxial cables come in various sizes, such as micro, small, medium, and large. The inner conductor is typically copper, sometimes reinforced with a steel core for added strength. The outer conductor is often made of copper tape, and the insulation must have low dielectric loss and sufficient mechanical strength. Shielding layers are used to maintain concentricity and reduce interference between coaxial pairs.

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