The low transmission rate and short transmission distance can take advantage of the characteristics of bandwidth and transmission loss of multimode fiber which are inferior to that of single-mode fiber. But single-mode fiber is cheaper and better than multimode fiber, so why not use single-mode fiber in the network? This is because in the above network characteristics, the loss is large when there are many bends, and the optical power splitting is frequent when there are many nodes, which requires sufficient optical power transmission inside the optical fiber. The multimode fiber has larger core diameter and larger numerical aperture than the single-mode fiber, which can couple more optical power from the light source. In that network, the use of connector and coupler is large, the passive device of single-mode fib is more expensive than that of multimode fib, and it is relatively precise and has small tolerance, so the operation is not as convenient and reliable as that of multimode fib. Single-mode fibers can only use a laser (LD) as a light source, which is much more expensive than light-emitting diodes (LEDs) used in multimode fibers.