Q & A on Physical and Protocol Layers Q: 1) It is said that all the layer in TCP/IP or OSI are independent of each other, while,MAC & physical layer are not independent because each protocol in MAC layer (802.3-5) require different physical layer. Please clarify. A: TCP/IP and OSI were developed separately and were not created to work together. Thus, the layering models should be viewed as competing, not part of single system. The term ``MAC'' was coined to describe protocols for media access. Each MAC protocol is indeed designed to work with a certain medium (physical layer). For example, a MAC protocol that is designed to work over copper might use the CSMA/CD technique, but a MAC protocol designed to work over RF will need a modification known as CSMA/CA. Q: 2) Does the operating system has all the required protocol to run a LAN (802.3-5) or we need to install one. A: A network interface card (NIC) contains hardware that implements the MAC protocols. All an operating system needs is a device driver that provides access to the NIC. One has to install a device driver (most operaing systems detect hardware and install a driver automatically; older systems require a system administrator to configure the driver manually). Q: 3) Can we write our own protocol for the MAC layer to run a LAN. A: No, not with most hardware. The MAC protocols are located in ROM on the network interface card and cannot be changed.