"Optical Cable Types and Model Numbering Explained"

1. Basic Structure of Optical Cables

An optical cable consists of bare fiber plus protective layers; the core goal is "to keep the fiber transmitting stably in all kinds of environments." It generally comprises three parts:

Cable core — made up of fiber units (single or multiple fibers); multi-fiber cables require color-coded fibers for identification. The interior is also filled with grease or water-blocking material to prevent moisture migration from degrading attenuation. Sheath — divided into an inner sheath (polyethylene or PVC) and an outer sheath (steel-tape armor etc. added depending on installation conditions), providing mechanical protection, pressure resistance, moisture resistance, and chemical resistance. Strength member (central strength element) — bears the tension during installation and operation, typically located at the center of the cable core (stranded and groove-type designs) or around the periphery (central tube design). Materials are metallic steel wire and FRP (fiber-reinforced plastic); FRP non-metallic strength members effectively prevent lightning strikes.

2. Classification of Optical Cables

Optical cables can be classified in many ways; the classifications that matter most in actual engineering are:

The most commonly used categories in current projects: outdoor (field) cables (direct-buried/duct/aerial/underwater), indoor (office) cables, flexible cables (excellent bend performance), in-equipment cables, and submarine cables.

3. Characteristics of the Four Mainstream Cable Structures

1. Stranded (Layered) Structure Cable

Multiple secondary-buffered fibers in loose or tight tubes are stranded around a central metallic strength member into a round, regular cable core, longitudinally wrapped with laminated aluminum tape and a polyethylene inner sheath, plus water-blocking tape and double-sided film-coated steel (aluminum) tape and a polyethylene outer sheath (buried cables add an armor layer).

Advantages: accommodates a large fiber count; fiber excess length is easy to control; good mechanical/environmental performance; suitable for direct burial, duct, and aerial installation. Disadvantages: structure and process equipment are relatively complex; production involves many steps; higher material consumption.

2. Central Loose Tube Structure Cable

The fibers sit in one large loose tube at the cable center; the strength element is not at the center but placed outside the loose tube (serving both as strength member and mechanical protection). The cable is longitudinally wrapped with water-blocking tape and double-sided plastic-coated steel (aluminum) tape, with two parallel phosphated carbon steel wires or FRP round rods embedded in the polyethylene sheath.

Advantages: simple structure and straightforward manufacturing process; good lateral crush resistance, improving network transmission stability; small cross-section and light weight, especially suitable for aerial installation; fiber count inside the bundle tube is flexible. Disadvantages: fiber count should not be too high (loose fiber ≤12 fibers, fiber bundle ≤36 fibers, fiber ribbon ≤216 fibers); fiber excess length is hard to control; the loose tube may shrink back in the finished cable.

3. Groove-Type (Slotted Core) Structure Cable

Single or multiple fibers are placed in the helical grooves of a slotted core, with the strength element at the center of the core; groove cross-sections are V-shaped, U-shaped, or rectangular. In China it is limited to dry ribbon cables — the fiber ribbon is laid in matrix form into U-shaped helical grooves of the slotted core, with water-blocking tape wrapped outside and a steel tape + polyethylene outer sheath.

Advantages: excellent fiber protection; good lateral crush strength; compact structure and small cable diameter, suitable for duct installation; high fiber density, up to a thousand to several thousand fibers; splicing requires no removal of water-blocking grease, so jointing is efficient. Disadvantages: complex manufacturing equipment, many process steps, and high production difficulty.

4. Butterfly Drop Cable (Flat Drop Cable)

A dedicated cable for the subscriber drop segment, generally with 1/2/4 fibers. Common fiber categories are B1.3 (extended-wavelength, non-dispersion-shifted single-mode fiber, i.e., G.652.D), B1.3a (low-loss, large effective area type), and B6 (bend-insensitive single-mode fiber, i.e., G.657). The sheath material is generally low-smoke zero-halogen olefin or PVC. Standard delivery lengths are 500 m/1000 m/2000 m; custom lengths are negotiable.

Mainly used for: fiber to the home (FTTH) and fiber to the building (FTTB). Single-fiber is recommended for residential subscribers; business subscribers may choose 2 or 4 fibers as needed.

4. How to Read Optical Cable Model Numbers (YD/T 908—2020)

An optical cable model consists of three parts — type code + specification code + special performance designation (optional). Understanding the model tells you the basic attributes of a cable.

Type Code (Five Parts)

Part 1: Classification code

Part 2: Strength member

Part 3: Structural features (fiber organization + secondary buffering + cable core structure + water-blocking structure)

Common combination examples: stranded (no symbol), groove-type (G), bundle-type (R), central tube (no symbol but different position), fiber ribbon (D). Water-blocking structure: (no symbol) = fully dry, H = semi-dry, T = gel-filled.

Part 4: Sheath (structure and material)

Part 5: Outer sheath (two digits: armor layer + outer sheath)

Common: 33 = double round steel wire armor, 4 = stainless steel tape armor, 5 = chrome-plated steel tape armor, 7 = non-metallic strand armor.

Specification Code

Consists of fiber count + fiber category code, e.g., "12B1.3" means 12 fibers of G.652.D single-mode fiber.

Model Examples

The source document is derived from communications and radio & TV engineering training material, organized per the YD/T 908—2020 standard. The specific numbers in cable models (e.g., GYTA 12B1.3+6B4) are standard examples, not actual product model numbers.