STANDARD &PROTOCOL

LANs are complex systems that implement many different services in order to provide communication between all of the types of devices that can be connected to them. A communications model called the Open Systems Interconnect (OSI) reference model was developed by the International Standards Organization (ISO) to define all of the services a LAN should provide (see Figure 8). This model defines seven layers, each of which provides a subset of all of the LAN services. This layered approach allows small groups of related services to be implemented in a modular fashion that makes designing network software much more flexible. A network software module that implements services at the Network and Transport Layers of the model can be paired up with different Physical and Data Link Layer modules depending on the requirements of the user's application. But the OSI model doesn't say how these services should actually be implemented in LAN equipment. The "how to" part has been defined in a number of different protocols that have been developed by international standards bodies, individual LAN equipment vendors, and ad hoc groups of interested parties. These protocols typically define how to implement a group of services in one or two layers of the OSI model. For example, Ethernet and Token Ring are both protocols that define different ways to provide the services called for in the Physical and Data Link Layers of the OSI model. They have both been approved by the Institute of Electrical and Electronics Engineers (IEEE), an international communications standards body.
	The International Standards Organization (ISO), the primary standard-setting body in the data communications industry, developed a framework for LAN standards called the Open Systems Interconnect reference model. This reference model represents a standard approach to communicate information throughout a network so that a variety of independently developed computer and communications devices can interoperate.
Because they are approved and published by the IEEE, both the Ethernet and Token Ring protocols are said to be industry standards. Any company can acquire the specifications and design Ethernet or Token Ring NICs. Users can purchase an Ethernet NIC, for example, from any vendor and be assured that it will operate in a network with Ethernet NICs from other vendors. This degree of interoperability is highly desirable. However, there are many more protocols for providing services at the higher layers of the OSI model and very few of them have been approved by an international standards bodies. In fact, most upper layer LAN protocols are incorporated into proprietary network operating systems, such as Novell's NetWare, IBM's LAN Server, and Microsoft's LAN Manager. A user has to buy only that vendor's products in order to be assured that they will interoperate on a LAN. Network Operating Systems Ethernet and Token Ring technologies are just one part of a complete LAN. They provide the services specified in the Physical and Data Link Layers of the OSI model, but several other services must be added on top of the connectivity of Ethernet or Token Ring. Network operating systems (NOSs) are most often used to provide the additional communications services. A NOS defines client and server systems. Clients are individual user workstations attached to the network where application programs are run and data is generated. Servers are shared network resources that provide hard disk space for users to store files, printer services, and a number of other network services. The network operating system provides a set of protocols in software that run on both servers and client systems and allow them to communicate with each other, share files, printers, and other network resources.