ISO 22671:2011 defines the Communications Link Transmission Unit (CLTU) service in conformance with the transfer services specified in CCSDS 910.4-B-2 (equivalent to ISO 15396). The Forward CLTU service is a Space Link Extension (SLE) transfer service that enables a mission to send Communications Link Transmission Units (CLTUs) to a spacecraft.

ISO 22671:2011 defines, in an abstract manner, the Forward CLTU service in terms of the operations necessary to provide the transfer service, the parameter data associated with each operation, the behaviors that result from the invocation of each operation, and the relationship between, and the valid sequence of, the operations and resulting behaviors.

ISO 22671:2011 does not specify individual implementations or products, the implementation of entities or interfaces within real systems, the methods or technologies required to radiate data to a spacecraft and to acquire telemetry frames from the signals received from that spacecraft for extraction of the Operational Control Field, the methods or technologies required for communications, or the management activities necessary to schedule, configure, and control the Forward CLTU service.