Analog modems are becoming faster--the top-end V.34-modem speed is 28.8 Kbps--and less expensive (street prices for V.34 modems are about $200, while basic ISDN hardware costs around $500). So, are analog modems a better choice than ISDN?
Your first consideration should be how you'll use your high-speed communications line. You may find V.34 modems the most cost-effective solution if you aren't doing a lot of interactive work over the line. For instance, V.34 should work fairly well for batch-oriented tasks, such as downloading E-mail or Usenet news articles. But it may not be the best solution for using a WWW (World Wide Web) browser.
If speed is your primary concern, consider that an ISDN connection can be up to six times
faster than a V.34-modem link. For example, look at the theoretical raw
transfer times for a 100-Kb file
in the figure. (Note that many other factors, including protocol overhead, compression techniques, and data type, also affect transfer times.) Thirty-four seconds may not sound like a long time to transfer 100 Kb of data, but if you're using a WWW client, the difference between that and 12 1/2 seconds seems significant.
Does fast connection time matter for your application? In addition to its fast data transfer rates, an ISDN line only takes a few milliseconds to bring up a connection. If you're running TCP/IP over your ISDN connection and your hardware supports on-demand connections, you can leave your connection down most of the time and bring it up only when you need it.
You might want to do this if you're paying for metered service for either your ISDN connection or your TCP/IP connection on the other end (or both). In any event, connection time for ISDN is negligi
ble, which is not the case for analog modems, which must dial and negotiate the correct speed before the communications links connect.
High-Bandwidth Circuits
If you're communicating with just one service provider, you might consider a permanent 56-Kbps leased circuit, which compares closely in performance with ISDN. Compared to ISDN, leased circuits usually have higher installation costs and recurring costs. Recurring costs include charges for the mileage from your POP (point of presence) to the provider of your services.
By contrast, many RBOCs (Regional Bell Operating Companies) don't charge for mileage for ISDN switched circuits. In some areas, ISDN usage is metered, but many RBOCs offer either a flat-rate monthly charge (all day) or less expensive off-peak rates. Leased lines generally carry a flat rate, which can be several hundred dollars per month, depending on the speed of the circuit and the distance of your calls. However, if ISDN isn't available, or if it's met
ered and you are a high-usage customer, the leased-line flat rate might cost less. Also, many companies prefer to budget for a fixed recurring charge rather than gamble on a variable cost.
Hardware costs are similar for both ISDN and leased lines. With leased lines, you'll need a CSU (channel service unit) and a DSU (data service unit), as well as the appropriate hardware (e.g., routers, bridges, and concentrators) to transport the signal to the data equipment. The CSU and DSU handle A/D and D/A conversions, offer limited loopback capabilities, and provide the leased-line interface to the other DCE (data communications equipment). You can think of them as the "modem" for leased lines.
There is a break-even point where using leased lines becomes more cost-effective than using several ISDN BRI (Basic Rate Interface) lines because of the more sophisticated hardware required to do the inverse multiplexing of all the lines to gain the necessary bandwidth. For instance, assume you need a 512-Kbps unco
mpressed bandwidth. For ISDN, you'd need four BRIs (i.e., eight B channels). Start-up charges, which include installation for each of the four BRIs and an eight-port router that can do inverse multiplexing, come to about $6000. For the full bandwidth to be used, there have to be eight physically separate calls to the remote end, which also requires four BRIs and an eight-port router. If your RBOC meters your service, you'd be paying eight times the charges of a single B channel, since all the lines would be used.
To use leased lines, you would need a fractional T1 circuit with a 512-Kbps bandwidth. The start-up charges would include a single installation charge, a CSU/DSU (which costs approximately $1500), and a small router (which starts at $5000). The ongoing cost is a flat monthly rate for the fractional T1 circuit.
As you can see from these examples, in general, the higher the bandwidth required, the more attractive leased lines become.
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High-Speed Alternatives
