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Figure 1-12 shows what a typical frame looks like. Different network implementations define frames in very different, highly specific ways, but the following data items are common to all implementations:
These items are used to perform fundamental tasks that underlie every network transmission: to take the needed information, send it to the proper destination, and ensure that it is received successfully. Using Frames That Contain Other FramesThe layering of networking protocols within a single frame is a powerful concept that makes network communication possible. The lowest layer knows how to tell the network adapter to send a message, but that layer is ignorant of file servers and file redirection. The highest layer understands file servers and redirection but knows nothing about Ethernet or Token Ring. Together, though, the layers give you the full functionality of a local area network. Frames always are layered (see Figure 1-13).
When a higher level file redirection protocol gives a message to a midlevel protocol (such as the Network Basic Input Output System, or NetBIOS, for example), it asks that the message be sent to another PC on the network (probably a file server). The midlevel protocol then puts an envelope around the message packet and hands it to the lowest level protocol, implemented as the network support software and the network adapter card. This lowest layer in turns wraps the (NetBIOS) envelope in an envelope of its own and sends it out across the network. In Figure 1-14, you see each envelope labeled header and trailer. On receipt, the network support software on the receiving computer removes the outer envelope and hands the result upward to the next higher level protocol. The midlevel protocol running on the receivers computer removes its envelope and gives the messagenow an exact copy of the senders messageto the receiving computers highest-level protocol.
The primary reason for splitting the networking functionality into layers in this manner is that the different hardware and software components of the network are manufactured by different companies. If a single vendor produced every product used on your network, from applications to operating systems to network adapters to cabling, then they could arrange the communications however they wanted, and still be assured of the interoperability of the different parts. This is not the case, however. Different vendors may split the LAN communications functions in slightly different ways, but they all have to rely a common diagram of the overall process to ensure that their products will successfully interact with all of the others used on a typical LAN. One such diagram is called the OSI Reference Model. Using the OSI Reference ModelThe International Organization for Standardization (cryptically abbreviated as the ISO), has published a document called the Open System Interconnection (OSI) model. Most vendors of LAN products endorse the OSI standard but few or none implement it fully. The OSI model divides LAN communications into seven layers. Most NOS vendors use three or four layers of protocols, overlapping various OSI layers to span the same distance. The OSI model describes how communications between two computers should occur. It calls for seven layers and specifies that each layer be insulated from the others by a well-defined interface. Figure 1.17 shows the seven layers. Various development projects over the years have attempted to create a networking system that is fully compliant with the OSI architecture, but no practical product has emerged. The OSI model remains a popular reference tool, however, and is a ubiquitous part of the education of any networking professional.
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