ISO 22671:2007 defines the communications link transmission unit (CLTU) service in conformance with the transfer services specified in ISO 15396:1998. 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 22671:2007 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.

It 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.