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Inter-router protocols are router-to-router protocols that can operate over heterogeneous networks. The protocol passes routing information and keep alive packets during periods of idleness. Examples include RIP (Routing Information Protocol) from Novell and OSPF (Open Shortest Path First) from the Internet Engineering Task Force (IETF).

Serial line protocols provide for communications over serial or dialup links connecting unlike routers. Examples include HDLC, SLIP (Serial Line Interface Protocol), and PPP (Point-to-Point Protocol). More discussion of these specific protocols is provided in Chapter 13 which deals with the Internet.

Protocol Stack Routing and Bridging Protocols advise the router as to which packets should be routed and which should be bridged.

Gateways

A term originally applied to what we now call routers [9-20], gateways are at the top of the LAN food chain. Gateway routers perform all of the functions of bridges and routers, including protocol conversion at all seven layers of the OSI Reference Model. Generally consisting of software residing in a host computer, such a midrange or mainframe, gateway technology is expensive but highly functional.

Protocol conversion, rather than encapsulation, can serve to fully convert from Ethernet to Token Ring, to FDDI or any other standard or proprietary LAN protocol. Additionally, protocol conversion can address higher layers of the OSI model, perhaps through Layer 7, the Application Layer. IBM SNA, DECnet, Internet TCP/IP and other protocols can be converted from network-to-network. As the process of protocol conversion is complex, gateways tend to operate rather slowly, compared to bridges and routers. As a result, they may create bottlenecks of congestion during periods of peak usage.

LAN Segmentation

LAN segmentation is a concept that simply involves dividing a single LAN into two or more physical segments. A LAN segment typically involves a workgroup that communicates intensively on the basis of station-to-station, stations-to-specific servers, or stations-to-specific peripherals such as local printers. As the usage is intensive, as the traffic patterns are well understood, and as the traffic is highly localized, a LAN segment can be effected through the use of a filtering bridge (which function may be included in a router or intelligent hub). The physical LAN still exists in its physical and logical entirety—it is just segmented.

The advantage of segmentation is that the relatively intensive traffic within the workgroup is confined to a physical and logical segment (domain) of that LAN. Therefore, that traffic does not affect the users on the balance of the LAN. When a LAN is divided into many, relatively small segments, the concept is known as microsegmentation. While the cost of the bridges may be of concern, the performance advantages often outweigh cost considerations.

LAN Operating Systems

A LAN Operating System, or Network Operating System (NOS), is software that provides the network with multiuser, multitasking capabilities. The operating system facilitates communications and resource sharing, thereby providing the basic framework for the operation of the LAN. The operating system consists of modules which are distributed throughout the LAN environment. Some NOS modules reside in servers, while other modules reside in the clients. International Data Corporation estimated that the total sales of LAN operating systems increased 54% from 1989 to 1990 to $1.14 billion and was expected to reach $2.8 billion in 1995; Novell NetWare accounts for three million users and a 65 to 77% market share [9-14] and [9-2].

Digressing for just a moment, the client/server model originated with the development of the U.S. Department of Defense ARPANET in the 1960s. As the cost and size of computer systems decreased and as the capabilities of those systems and the networks increased, the embodiment of client/server changed. A contemporary client is an application that generally resides on a microcomputer. The application can be word processing, a spreadsheet, or a database. The client runs against a server, a multiport computer that contains large amounts of memory, allowing multiple clients to share the servers’ resources, while performing certain functions independently. Servers are database engines capable of processing client requests for information. Servers also manage the data. As an example, client/server continues to be used extensively in the Internet. When accessing America Online, Prodigy, CompuServe, or another service provider, one uses the Graphic User Interface (GUI) and browser software that resides on the client PC. When initiating an Internet session, that software runs with software installed in the service provider’s communications server. Through this approach, the two devices communicate effectively without requiring that the software to be downloaded from the server as part of every Internet session. Because the graphic files are huge, a great deal of time and bandwidth would be wasted. Once connected to the communications server, the user subsequently can accesses a large number of database servers on a point-and-click basis, courtesy of the GUI. Only the target data is transmitted across the Internet.

In addition to supporting multitasking and multiuser access, LAN operating systems provide for recognition of users based on passwords, user IDs, and terminal IDs. On the basis of such information, LAN OSs can manage security using access privileges. Additionally, a LAN operating system provides multiprotocol routing, as well as directory and message services. DOS-based LAN operating systems include Novell NetWare and Sun Microsystems’ TOPS/DOS. OS/2 and UNIX-based LAN operating systems include Banyan VINES, IBM LAN Server, Microsoft LAN Manager, and Novell SFT NetWare.


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