OTMA has similarities to network protocols. Several architectural models for networks exist. The following figure shows two. The simplified four-layer model shown on the right is often used in descriptions of UNIX networks. In the open systems interconnection (OSI) model, shown on the left, OTMA is the session layer. Both models have a Transport, Network, and Data Link layer. The OSI model also includes layers for Application, Presentation, and Session, and the simplified model includes a process layer.