Multimode fiber is a type of optical fiber that is designed to carry multiple light signals at the same time. It is commonly used in short-distance data transmission applications such as Ethernet and fiber channel. The most common type of multimode fiber is 850nm, which is used for short-distance data transmission up to 550 meters. It is made of a glass core surrounded by a cladding layer, which helps to keep the light signals from escaping. The core is typically larger than single-mode fiber, allowing more light signals to be transmitted at once. 850nm multimode fiber is also more cost-effective than single-mode fiber, making it a popular choice for many applications.