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1.5 The OSI Network Management Architecture

The ISO/OSI model [1-1] has been a benchmark for computer networking since it was first published in 1978. Figure 1-6 shows the familiar seven-layer structure. Following is a summary of the seven layers:

Layer Description

Physical Provides the physical transmission medium for carrying the raw data, such as electrical or optical impulses, from one network node to the next.
Data Link Provides reliable communications on the link; that is, it creates the channel between adjacent nodes on a LAN, MAN, or WAN. Functions include addressing, framing, and error control on the link.
Network Provides communications functions for an internetwork. These include tasks such as the global addressing, routing, and switching that take data from its source to its destination via an internetwork of LANs, MANs, and WANs.
Transport Assures the reliable end-to-end delivery of data. Its functions include error control and sequence control.
Session Establishes the logical connection between end-user applications. These functions include mechanisms that synchronize the data transfer once a connection is established.
Presentation Represents the application data so that it can be properly interpreted at the distant location. Examples of these functions include data compression/decompression, encryption, or ASCII to EBCDIC code conversion.
Application Includes the functions responsible for end-user applications, such as file transfer, electronic mail, or remote terminal access. SNMP is an Application layer protocol.


Figure 1-6.  Network management within the OSI framework

While the entire OSI model has yet to be widely implemented, elements of its architecture are finding their way into multivendor systems such as electronic mail, directory services, and network management applications. History may prove, however, that the real value of the OSI model is in the organizational structure it provides for multivendor networks rather than in specific protocol implementations.

Reviewing Figure 1-6, note that the network management application manages internetwork and local network functions.

What do these seven layers have to do with managing a real network? Consider the network shown in Figure 1-7. Suppose that a network management console (called the manager) needs to retrieve entries from a routing table. To do so, the manager generates a request message and passes it down through the layers of the protocol stack. The Data Link layer inserts the message (or fragments of a long message) into a frame for transmission on the local network. (Figure 1-7 shows an Ethernet LAN, so an Ethernet frame would contain the request message.) The Data Link layer would then convert the frame into a bit stream and transmit it over the physical network to the intended receiver. At the distant end, the network management command would trigger another network management process (the agent) to perform the requested function, and build a response message. The response would follow a similar, but opposite, journey back to the console.


Figure 1-7.  Network Management protocol stack operation

The ISO/OSI model extends beyond the seven layers, however, for the management of open systems. OSI standards include a model of network management and a network management protocol.


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