Normally the application programmers are in charge of layer 5 protocols (the application layer), while the layer 3 and 4 protocols are services provided by the TCP/IP stack in the operating system. Microcontroller firmware in the network adapter typically handle layer 2 issues, supported by a driver software in the operational system. Non-programmable analog and digital electronics are normally in charge of the physical layer, typically using a application-specific integrated circuit (ASIC) chipset for each radio interface or other physical standard.
However, hardware or software implementation is not stated in the protocols or the layered reference model. High-performance routers are to a large extent based on fast non-programmable digital electronics, carrying out layer 3 switching. In modern modems and wireless equipment, the physical layer may partly be implemented using programmable DSP processors or software radio (soft radio) programmable chipsets, allowing the chip to be reused in several alternative standards and radio interfaces instead of separate circuits for each standard, and facilitating. The Apple Geoport concept was an example of CPU software implementation of the physical layer, making it possible to emulate some modem standards.
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