This thesis describes the process of implementing the software part of a communication sub-system for a student built satellite (CubeSat). The implementation consist of a communication protocol in the data link layer that utilizes AX.25 frames for communicating with a ground station based on amateur radio equipment. The development and usage of this communication protocol are based on data communication theory, link budget calculations, efficiency and availability analyzes together with practical tests. The steps in how this software has been developed and evaluated are described, and can be useful for other project that could benefit from this communication protocol. Source code listings are included. The reader should be familiar with communication theory, data communication, programming and electronics.