In a SONET or SDH frame, the overhead dedicated to control of data transport between network elements (NEs).TOH comprises Section Overhead (SOH) and Line Overhead (LOH). SOH of nine octets is dedicated to the transport of status, messages, and alarm indications for the maintenance of links between add/drop multiplexers (ADMs). LOH of 18 octets controls the reliable transport of payload data in the Synchronous Payload Envelope (SPE) between any two NEs. See also ADM, frame, link, NE, octet, overhead, payload, SDH, and SONET.