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Communications of the ACM

Communications of the ACM

On Site: the Need For Speed

One of the limiting Internet growth factors in the consumer area is the lack of affordable high-speed access options for the home. The most widely used technique for Internet access is still an analog modem running at V.34 speeds of 28.8Kbps or 33.6Kbps. However, there are emerging technologies becoming widely available to consumers at relatively low cost. Some of the techniques leverage the existing telephone lines found in almost everyone's house; others require a significant investment in new infrastructure.

I've now had the opportunity to use three different types of Internet access technologies: 56Kbps modems, digital subscriber lines, and cable modems. 56Kbps modems provide a modest improvement over standard dialup modems. Digital subscriber lines and cable modems can provide connection speeds that match or better the traditional speeds of dedicated and expensive T1 lines. Despite these technological differences, all three methods share the potential for significantly affecting areas such as distance learning and Net-centric computing.

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56Kbps Modems

The widespread deployment of 56Kbps modems has given users Internet connection speeds comparable to the 64Kbps offered by single-band Integrated Services Digital Network (ISDN). The first 56Kbps modems appeared in the spring of 1997. I began using one a few months after their introduction.

These modems operate in an asymmetrical fashion; the download speeds can (theoretically) reach 56Kbps,1 but the upload speeds are limited to 33.6Kbps. Asymmetric bandwidth is common for consumer-oriented, high-speed Internet access technologies. This is due primarily to the limitations of the so-called "last mile" of the network—from the external infrastructure to inside the home. Given the nature of Internet use, asymmetric access is usually sufficient; most data travels downstream from the Internet to the client, while comparatively little is sent upstream. There are exceptions, such as bandwidth-intensive two-way video conferencing.

There are two proprietary protocols (X2, K56Flex) and one standard (V.90) currently in use for 56Kbps modems. Both proprietary protocols can deliver 56Kbps performance, but they are incompatible with one another. The recent V.90 standard supercedes both the X2 and K56Flex proprietary protocols. Most Internet service providers (ISPs) are in the process of upgrading their systems to support V.90 in addition to whichever proprietary protocol they have adopted. It's important to select your ISP to match your modem.

However, just checking the speed of your ISP is not enough. You also have to check the quality of your telephone line. Not all lines are capable of supporting 56Kbps. Too much noise on the line means the increased digital traffic becomes too lossy and the protocol shifts down to lower speeds as needed. To see if your telephone line is capable of 56Kbps speeds, you can call a toll-free number operated by one of the major modem suppliers. A program will perform some checks and inform you if your line is clean enough to support a 56Kbps link.

Luckily, my home telephone line passed the test. I routinely connect to my ISP at about 52Kbps using a PC card modem that supports both the X2 and V.90 protocols. My ISP has several hundred access points in North America and many more around the world. Most support 56Kbps access, which makes high-speed connections possible when I'm away from home.

In my opinion, upgrading to 56Kbps is something everyone should do, irrespective of any other available networking solutions. Although 56Kbps modems only offer connection speeds less than twice as fast as the V.34 standard, I have found that using a 56Kbps link seems nearly as fast as working on a LAN, especially for browsing the Web. With compression, downloads are up to four times faster than a 28.8Kbps connection.

The biggest advantage of 56Kbps modems over other high-speed access techniques is low cost. Unlike ISDN, no new telephone lines are needed. As long as your telephone line can pass the quality test, you're ready to connect at speeds approaching single-channel ISDN, but for the same monthly rate of about $20 per month.

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Digital Subscriber Lines

The second type of high-speed Internet access technology is DSL. In late spring of 1998, I had DSL service installed in my home office. DSL is offered by local telephone companies. The appeal of DSL is that it offers users high bandwidth (up to 7Mbps) without requiring any new wiring; it uses the existing copper-based telephone line infrastructure.

Like 56Kbps modems, DSL is also asymmetric. However, it is much faster, moving data upstream at 1.5Mbps and speeds approaching 7Mbps downstream. For this reason, DSL is sometimes called ADSL, the "A" standing for "asymmetric." In fact, there are several variants of DSL, so it is also known as XDSL.

DSL uses a different part of the telephone line's frequency for different types of data. This means the same connection can be used for simultaneous Internet access and voice or fax communication. DSL also has the attraction of constant availability; there is no need to dial your ISP every time you want to go online.

For me, installation of DSL was relatively simple. It involved a short visit by a local telephone service representative. A "modem" that connects to the regular telephone line was installed. I already had an Internet browser installed—all I needed was some standard ISP information, such as the name of the mail server. The entire installation took about 15 minutes.

The "modem" is about the size of a hardcover book, turned on its end. Calling it a modem is really a misnomer; it's actually an Ethernet-to-ADSL router. It uses the asynchronous transfer mode message-packaging format over an ADSL physical layer to support point-to-point protocol networking. I already had a second telephone line used for 56Kbps access, so I just switched that line into the back of the router, connecting the device to telephone company's wide-area network. The router also has a standard network interface jack, which I connected to my computer's Ethernet card. With two simple steps, I was online with DSL.

DSL is just the physical layer of the Internet connection. You still need an ISP for the usual services, such as domain name resolution, email delivery, and so on. Moreover, with DSL you need an ISP that supports DSL. This is similar to finding an ISP that supports your type of 56Kbps modem. In my case, I chose the same telephone company that provided the DSL service, since it was the easiest combination, and they provided service in my area.

The cost for all the new technology is relatively small. Installation was $110. The cost of the DSL line is $40 per month. This includes the concurrent use of the line as a regular telephone or fax line. The ISP cost is extra, currently $19.95 a month for unlimited access. Thus, for about $60 a month, I have high-speed Internet access and simultaneous use of a second telephone line. So far, I've been very pleased with the performance. I have the lowest DSL speed offered, which is nominally 256Kbps both upstream and downstream. However, the modem has a diagnostic mode that lets you measure actual throughput, and I am usually connected at about 640Kbps downstream and 256Kbps upstream.

From the end-user perspective, I can say that using this DSL connection seems almost the same as a corporate intranet. Web pages load almost instantaneously, files download very rapidly, and large email attachments are no longer a problem. A side benefit of the modem acting as a router is that it can be plugged into a hub, so I can share the Internet connection with other computers on my office network.

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Cable Modems

More recently, I've been using a third technology—a cable modem. In the fall of 1998, I had cable modem service installed in my home office. So far, it's been an acceptable Internet access solution. It's reasonably inexpensive, relatively simple to install, and offers a satisfactory connection speed.

Installation of the cable modem involved a visit from two representatives of my local cable company. The first representative was a traditional cable installer; he ran new wiring around my office from an existing outlet. He also split the line at its main entry, providing a new live feed to my home office in addition to the existing feed to the television in the family room.

This type of line splitting is one example of how cable modems differ from DSL. The DSL connection can coexist with a regular telephone line, allowing both lines to be used simultaneously without any noticeable signal degradation. The cable connection suffers from a signal loss every time it is split. For example, splitting one feed means each of the new feeds can have a -4db drop in signal strength. This loss is not very important for television viewing, since the picture quality does not noticeably change. However, for a cable modem, the loss of signal strength means a lower signal-to-noise ratio, which can directly affect connection throughput. If you are in a home that already has the main feed split more than once, say to two televisions, then the signal loss is even greater, usually -7db per connection. To counter signal loss, I installed a powered +10db signal amplifier between my office outlet and the cable modem.

The second cable representative helped with the initial setup of the "modem" itself.2 The modem looks like a small pizza box turned on its end. It has four connections—power, cable input, Ethernet, and analog modem. The cable signal connects directly to the modem; it provides the downstream connection. For my particular cable modem service, the digital data flow is not just asymmetric, it's one-way; all upstream communication takes place over a regular 33.6Kbps analog modem housed within the cable modem. This means a telephone line is needed to use the cable modem, a factor I view as a significant detriment to its widespread adoption. The computer connects to the cable modem using a standard Ethernet card.

As with DSL, the cable modem provides just the physical layer of the Internet connection. You still need an ISP for the usual services. My cable company has an arrangement with a large national ISP, which means I have a regular ISP email account, space for a home page on the Web, and so on. It also means I can use any of the ISP's access points through a regular dial-up connection.

The cost for all the new technology is slightly lower than the cost of DSL (although actual costs will vary from location to location). For me, the cost of installation was $25. The cost of the cable modem, which includes the ISP service, is currently $44.95 per month. However, I still need a second telephone line for the analog portion of the setup, which in my area costs about $20 a month for unlimited use. Setup charges for the second telephone line were about $85. The total cost for cable modem service is nearly identical to DSL service.

So far, I've been reasonably pleased with the performance of the cable modem. I'm told the actual maximum downstream speed for my service is about 400Kbps, far below what cable modems are capable of handling. The upstream speed is limited to 33.6Kbps because of the analog connection. My experience has been that downloading large files is quite fast.

Choosing between the different high-speed digital Internet access technologies is not always simple. It usually involves a compromise involving cost, availability, and speed. The limited deployment of DSL, for example, means this option might not be a viable alternative, even if you were ready to pay for the service.

56Kbps modems have the advantage of being inexpensive and widespread. DSL has the advantage of offering relatively high-speed connections using existing telephone wiring and allowing the simultaneous use of the same line for either voice or fax traffic and Internet connection. Cable modems have the advantage of offering potentially high-speed connections using existing television cabling.

For now, I'd have to say I prefer DSL to cable modems when connection rates are low and the cable setup is so complicated. Until then, I'll continue experimenting with new technologies as they become available.

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Scott Tilley ( is an assistant professor in the Department of Computer Science at the University of California, Riverside.

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1FCC regulations currently limit connection speeds to 53 Kbps due to a restriction on line voltage.

2As with the DSL "modem," the cable "modem" is really more than a simple analog modem, so calling it a modem is again a bit of a misnomer. However, the name has stuck.

©1999 ACM  0002-0782/99/0700  $5.00

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