Onboard communication system and communication method

A communication method in an in-vehicle communication system including,

• an optical coupler,
• a first in-vehicle device group composed of a plurality of in-vehicle devices connected to a first end of the optical coupler, and
• a second in-vehicle device group composed of a plurality of in-vehicle devices connected to a second end of the optical coupler,
• the method comprising:
• transmitting optical signals from the in-vehicle devices in the first in-vehicle device group to the in-vehicle devices in the second in-vehicle device group via a common transmission path in the optical coupler;
• and
• transmitting optical signals from the in-vehicle devices in the second in-vehicle device group to the in-vehicle devices in the first in-vehicle device group via a common transmission path in the optical coupler.