a) enables system description, design, development, installation, operation, improvement, and maintenance to be performed at a given layer or layers in the hierarchical structure,

b) allows each layer to provide a set of accessible functions that can be controlled and used by the functions in the layer above it,

c) enables each layer to be implemented without affecting the implementation of other layers, and

d) allows the alteration of system performance by the modification of one or more layers without altering the existing equipment, procedures, and protocols at the remaining layers.

NOTE 1 Examples of independent alterations include

a) converting from wire to optical fibres at a physical layer without affecting the data-link layer or the network layer except to provide more traffic capacity, and

b) altering the operational protocols at the network level without altering the physical layer.

NOTE 2 Open systems architecture may be implemented using the Open Systems Interconnection-Reference Model (OSI-RM) as a guide while designing the system to meet performance requirements.

2. Non-proprietary systems architecture.