A programmable layered sub-system interface includes an extension sub-layer module, a physical coding sub-layer module, a physical media attachment module, an input module, an output module, a 1.sup.st switch module and a 2.sup.nd switch module. The 1.sup.st switch module is coupled between the physical media attachment module and the physical coding sub-layer module. The 2.sup.nd switch module is operably coupled between the physical coding sub-layer module and the extension sub-layer module. The input and output modules are operably coupled to the 1.sup.st and 2.sup.nd switch modules. The 1.sup.st switch module provides various combinations of coupling between the physical media attachment module, the physical coding sub-layer module, the input module and the output module. The 2.sup.nd switch module provides combinations of coupling between the extension sub-layer module, the physical coding sub-layer module, the input module and the output module.