This IEEE Standard is part of the C37 Family on Switchgear, Substations & Protective Relays. Project scope: This standard describes the interconnection details for N times 64 kilobit per second connections of teleprotection equipment to digital multiplexers using optical fiber. Requirements for both physical connection and the communications timing are also included. Project purpose: The purpose of this standard is to define an interconnection of different vendors' relays with different vendors' multiplex equipment, without any restriction on the ocntent of the N times 64 kilobit per second data, using up to 2 kilometer of 50 micrometer or 62.5 micrometer multimode fiber.