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The discussion of PC access to the CRDSS follows the issue list presented above. One section is <br />presented for each issue; however, there is a strong relation between different issues so references <br />between sections are common. <br />Communications Hardware and Software <br />In the ideal world, all computers are able to communicate with each other, allowing data to flow <br />easily from site to site. In the real world, computers are widely distributed and communication <br />avenues are fragmented. For example, sites may consist of a single PC or may consist of a Local <br />Area Network (LAN) of several PCs and/or workstations which communicate using a common <br />protocol such as IPX (Novell) or TCP/IP (UNIX). <br />Dedicated Wide Area Networks <br />A Wide Area Network (WAN) is a network that allows LANs to communicate and can be configured <br />in many different ways. For example, two PC-based networks can be linked by providing a <br />dedicated link between the two networks using a phone line. Dedicated WANs use connections that <br />are on-line 24 hours a day. Such a connection is usually transparent to users (although the <br />connection may be slow) and requires that hardware be purchased as outlined in Figure 1 (different <br />configurations are possible based on the telecommunications technology that is used, but the diagram <br />illustrates the basic configuration): <br />Telecommunications <br />RouterRouter CSU/DSUCSU/DSU <br />LAN LAN <br />Figure 1. Basic LAN to LAN connection <br />In this configuration, each LAN has a router (which understands the communications protocol for the <br />particular LAN) and a CSU/DSU (Channel Service Unit/Digital Service Unit). The CSU/DSU <br />converts signals to a form that is compatible with and robust enough to be transmitted across <br />telecommunications lines. Additional features in the router may be required depending on the type <br />of communications used (e.g., analog line, frame relay, ISDN). This configuration is reliable, can <br />provide fast communications, but can be expensive (on the order of thousands of dollars to start up <br />and hundreds of dollars per month to maintain). Connecting two LANs in the manner shown above <br />still results in a closed system in that the resulting WAN is isolated from the ?world.? <br />Another solution for connecting LANs is to rely on a WAN provider to do most of the configuration <br />and maintenance work. For example, an Internet provider such as Colorado SuperNet can be used to <br />provide the link between two LANs. The Internet is a very large WAN that relies on the TCP/IP <br />communication protocol. Two LANs are connected via the Internet using the same configuration as <br />shown in Figure 1. The difference is the ?world? is now also connected because some of the <br />telecommunications actually consists of the Internet (security measures can be implemented to <br />screen traffic entering each LAN). <br />2 <br />A275 06.29.94 1.05-29 Malers <br />