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. 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. New IEEE Standard - Superseded. 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.