The frame header contains the physical addresses
required to complete the data link functions, and the frame trailer contains the FCS.
The physical layer provides a service to the data link layer, encoding the data link frame
into a pattern of 1s and 0s (bits) for transmission on the medium (usually a wire) at Layer 1.
Network devices such as hubs, switches, and routers work at the lower three layers. Hubs
are at Layer 1, switches are at Layer 2, and routers are at Layer 3.
The TCP/IP Protocol Stack
The TCP/IP suite is a layered model similar to the OSI reference model. Its name is actually
a combination of two individual protocols, Transmission Control Protocol (TCP) and
Internet Protocol (IP). It is divided into layers, each of which performs speci?¬?c functions
in the data communication process.
Both the OSI model and the TCP/IP stack were developed by different organizations at
approximately the same time as a means to organize and communicate the components that
guide the transmission of data.
Although the OSI reference model is universally recognized, the historical and technical
open standard of the Internet is the TCP/IP protocol stack. The TCP/IP protocol stack,
shown in Figure 1-24, varies slightly from the OSI reference model.
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