This document defines the Communications Link Transmission Unit (CLTU) service in conformance with the transfer services specified in reference [1], Cross Support Reference Model—Part 1: SLE Services.  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.

This document defines, in an abstract manner, the Forward CLTU service in terms of:

1. the operations necessary to provide the transfer service;
2. the parameter data associated with each operation;
3. the behaviors that result from the invocation of each operation; and
4. the relationship between, and the valid sequence of, the operations and resulting behaviors.

It does not specify:

1. individual implementations or products;
2. the implementation of entities or interfaces within real systems;
3. 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;
4. the methods or technologies required for communications; or
5. the management activities necessary to schedule, configure, and control the Forward CLTU service.