A network system includes P upper switches, Q lower switches, and a first mapping device. Each upper switch of the P upper switches includes a plurality of upper ports. A group of upper switches selected from the P upper switches includes P1 upper switches. Each lower switch of the Q lower switches includes a plurality of upper ports. The first mapping device includes P1 upper adapter terminals coupled to a part of upper ports of the P1 upper switches, and P1 lower adapter terminals coupled to lower ports of a part of Q lower switches. The first mapping device is used for allocating a plurality of transmitting channels and receiving channels received by each upper adapter terminal to the P1 lower adapter terminals.