The Network Access Layer and the OSI Model
As Hour 2, "How TCP/IP Works" mentioned, TCP/IP is officially independent of the seven-layer OSI networking model, but the OSI model is often used as a general framework for understanding protocol systems. OSI terminology and concepts are particularly common in discussions of the Network Access layer because the OSI model provides additional subdivisions to the broad category of network access. These subdivisions reveal a bit more about the inner workings of this layer. The OSI model has been influential with computer networking vendors, and the recent trend toward multiprotocol standards such as NDIS and ODI (discussed later in this section) has accentuated the need for a common terminology that the OSI model provides.
As Figure 3.1 shows, the TCP/IP Network Access layer roughly corresponds to the OSI Physical and Data Link layers. The OSI Physical layer is responsible for turning the data frame into a stream of bits suitable for the transmission medium. In other words, the OSI Physical layer manages and synchronizes the electrical or analog pulses that form the actual transmission. On the receiving end, the Physical layer reassembles these pulses into a data frame.
Figure 3.1 OSI and the Network Access layer.
The OSI Data Link layer performs two separate functions and is accordingly subdivided into the following two sublayers:
Media Access Control (MAC)This sublayer provides an interface with the network adapter. The network adapter driver, in fact, is often called the MAC driver, and the hardware address burned into the card at the factory is often referred to as the MAC address.
Logical Link Control (LLC) This sublayer performs error-checking functions for frames delivered over the subnet and manages links between devices communicating on the subnet.
In real network protocol implementations, the distinction between the layers of TCP/IP and OSI systems has become further complicated by the development of the Network Driver Interface Specification (NDIS) and Open Data-Link Interface (ODI) specification. NDIS (developed by Microsoft and 3Com Corp.) and ODI (developed by Apple and Novell) are designed to let a single protocol stack (such as TCP/IP) use multiple network adapters and to let a single network adapter use multiple upper-layer protocols. This effectively enables the upper-layer protocols to float independently of the network access system, which adds great functionality to the network but also adds complexity and makes it even more difficult to provide a systematic discussion of how the software components interrelate at the lower layers.