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Until very recently, users and service providers have had to deal with two 56-Kbps standards, X2 and f56flex; however, a new standard, V.90 (originally called V.pcm), has been introduced and vendors have been quick to adopt it.
Hitting the Slopes For our quantitative analysis, we pitted a ZyXEL Prestige ISDN router against a 3Com Dual Analog X2 router for our home office use (a V.90 standard solution for the analog test was not available at the time of the tests). The routers were connected to a home office computer through a standard Ethernet interface.
Our setup consisted of a U.S. Robotics/3Com Corp. NetServer connected to our San Mateo lab's local area Ethernet network on one end and to our service provider's ISDN for the remote end. The NetServer u
ses BRI lines for both the 56-Kbps modem technology and ISDN technology connections, making it a flexible solution for your office's remote-access server.
Both solutions can use the same algorithms for compression, so to make it fair, we turned off compression for our tests. Both of our routers were set to dial on demand and bring up a second line once a given traffic threshold was reached.
 The Results
As expected, we found that ISDN outperformed our dual-analog solution. Call setup times were much faster, throughput was greater and latency was lower.
Most literature on the subject admits that, in practice, it's difficult to obtain optimal speeds with 56-Kbps modems. Most users will experience connection speeds up to 48 Kbps. With the X2 Dual Analog router, we could not obtain connection speeds of 56 Kbps. In our lab tests, 32 Kbps was the best per modem speed we could attain on the receiving side, and 28.8 Kbps was as high as we went on the se
nding side. However, this is a problem with our phone switch and not a problem with the modem itself.
On the other hand, ISDN is a digital technology and does not have to negotiate connection speed the way that modems do. Bearer channels always connect at 64 Kbps.
We measured call setup times for our first line by pinging a remote host in our lab until the remote host answered. ISDN calls were initiated in roughly three seconds; modem calls took approximately 30 seconds.
Next we tested FTP throughput. After ensuring that both Multilink PPP lines were up, by initiating traffic between the home office and our lab, we transferred a 1-MB file both to and from our home office. For FTP puts, we transferred the file at 4.45 KB per second with the analog router, and at 15.37 KB per second with the ISDN router.
For FTP gets, we transferred the file at 6.3 KB per second with our analog router. If we were to extrapolate our analog results linearly, estimating analog performance at top analog speeds (112 Kbps
when bonded), our 6.3 KB per second would scale to approximately 11 KB per second. This is compared to 15.21 KB per second with our ISDN router.
In addition, we found latency to be greater with our analog solution. If you're operating remote terminal connections or other highly interactive work, this is important to know.
Living With Your Decision
The performance of ISDN comes at a price. A survey of Network Computing editors found the average monthly bill for an ISDN connection is in the $100 per month range. Installation fees on the West Coast are about $125--not including the extra lines.
In contrast, a second phone line for an analog solution adds another $30 per month. For a dual-analog solution, you'll probably want two lines in addition to your regular phone line. However, most homes are equipped to handle a second phone line, but not a third. So, you'll need to install the third line, plus another $60 per month for the second and third lines.
Once installed, ISDN reliability is us
ually good, though problems do occur. For example, this past winter AT&T stopped long-distance service to ISDN customers in the Bay area because of billing problems. The customers were left on their own to figure out what happened. After verifying home addresses and faxing information back to AT&T, service was returned.
This sort of problem can be frustrating and the steps to determining where to lay the blame are not straightforward. And unfortunately, the telcos don't always have solutions right away. Service providers often make changes without notifying you or other service providers. For example, your local phone service provider may have changed your SPIDs without telling you, your long-distance provider, or the ISP.
Although you make performance and latency concessions with an analog solution, you don't have to worry about changes effecting your service.
Ski Under Control
It's easy to blame a provider for inadequate service. It's also easy to rack up huge bills if you're getting
charged by the minute for your connections. Network Computing's editors have had their own share of errors. For example, an IPX configuration problem went unnoticed for a week, nailing up an ISDN connection for the entire period. And it's easy to accidentally leave a telnet session to a home router open over a long weekend. If you're paying several cents per minute, you won't be very happy when your bill arrives.
ISDN has been around for at least 10 years. Will it withstand the push for DSL (Digital Subscriber Line) technologies by Compaq Computer Corp., Intel Corp. and Microsoft Corp., or some other younger technology? It's not guaranteed that any of the newer technologies will deal any better with the problems of poor phone support, installation delays or outages. These will also experience growing pains.
Meantime, ISDN is available in most of the country (at least most urban areas), and it's a worthy, reliable service, provided you've got the time and energy to invest, or money to have the installati
on done for you. We feel that in most cases, the performance is worth the cost. However, it is still not a simple solution for the masses.
Mike Lee can be reached at mlee@nwc.com.
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