A simple rule is that each device needs two cores—one for sending and one for receiving data. Understanding Fiber Cores: Core: The central glass fiber that transmits light signals. Single-mode: A. The total number of cores for a 1pc fiber patch cable is calculated as the number of branches multiplied by the number of cores per branch (if there are no branches, the number of branches = 1). For example, an MTP®-8 trunk cable with four branches and eight. The number of optical cores in an optical fiber is the total number of equipment interfaces multiplied by 2, plus 10% to 20% of the spare quantity, and if the communication mode of the equipment has serial communication and equipment multiplexing, you can reduce the number of cores.