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Capacity planning also involves considering the costs for lines and equipment. This will vary depending upon your organizations buying power and current WAN infrastructure. Today's remote-access servers can accommodate a number of different media for both inbound and outbound lines. The modem racks themselves are capable of hosting a number of telecommunication equipment besides data modems-fax boards, voice boards, ISDN and remote-access servers, for instance.

Line input comes in three basic flavors:

• Analog is the lowest cost option, in general, for up to two dozen lines. Installation is generally a flat rate to the demarcation point, and from there you're responsible to get the phone lines to the modems. The down side with analog is the mass of cables tha t come with it. Even a well-implemented cable management system is hard to work with.

• T1 is a digital circuit that delivers 24 lines through a single cable. T1 circuits require special installation and extra equipment (such as a CSU/DSU or channel bank device) in addition to provisioning from the central office, but T1s provide a more robust and flexible line solution. Unlike analog lines, you don't have the same cable management issues because you have collapsed 24 lines into one. However, with the additional equipment comes more items to manage and there a greater loss of service if the T1 cable is severed, or the CSU/DSU is down. Often, remote-access devices such as modem racks and servers can be equipped with internal CSU/DSUs.

• ISDN PRI is a digital circuit that provides 23 64-Kbps channels and a 24th channel used for signaling. ISDN's fast call-setup time and reliable links offer practical connection on demand dial-up solutions for end users. Selecting a network application initiates the connection. Since most network activity is bursty with (relatively) long idle times, costs can be controlled by connecting only when necessary. ISDN users most likely won't even know they are reconnecting. ISDN PRI also has the advantage of routing analog and ISDN calls from the PSTN to the appropriate device automatically. Expect to pay higher line charges for ISDN over analog lines or T1 service, but the added flexibility may be advantageous for critical business applications.

Of course, depending on your current arrangements with your telecommunications provider, one, two or a mix of all three service types may be available and cost effective. Additionally, the costs of converting from one type of service to another may preclude going with the less expensive option now for a cost savings that will be gained in the future. Beyond 24 lines, you might be better off getting a T1 or ISDN PRI.

For example, from the graph above, at app roximately 20 analog lines, the cost per line exceeds the cost for the T1. As you continue to increase port density, you'll find the cost to add lines will slowly drop as you begin to gain volume. You'll likely have equipment that is capable of handling more than one T1 span, reducing investment costs, plus the learnig curve of managing digital service will be lower due to your prior experience. The cost analysis reveals that the move from analog to T1 can't be made on a purely monthly cost basis.

Looking at the graph above, the crossover point isn't reached at the 24-port density point. In fact, unless you can get discounted rates for T1, the savings from monthly costs may never be reached over a long period of time. The cost per port of analog lines will see-saw over the cost of a T1 because the analog line cost will increase steadily while the T1 line charges will leap at each density increase. You need to carefully examine how your lines are tarriffed and calculate when it is most advantageous to move to T1 lines.

The advantages of digital service provided by T1 trunks such as high reliability and flexibility may far outweigh the pure cost savings of analog. The costs of installing a T1 line and the extra equipment on your premises, may be high initially, but the costs of converting from analog to T1 may be more costly as your required port density increases because you'll have to abandon or reallocate analog equipment and train managers in telephony management. What you need depends entirely upon your specific situation. The output side of the remote-access service bears some consideration. You'll need to make sure that the segment(s) off of which your devices will hang will need to be able to handle the additional traffic without hurting overall performance.

Updated January 17, 1997

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