Life in the Digital Subscriber Lane



Scott R. Tilley (Florida Institute of Technoloy)

This article was originally published in News at SEI on: September 1, 1998

In the first Net Effects column, I talked about the era of net-centric computing (NCC) and its profound and far-reaching effects in the office and elsewhere. Recall that the underlying principle behind NCC is a distributed environment where applications and data are downloaded from network servers on an as-needed basis. NCC relies on portable applications than run on multiple architectures (“write once, run anywhere”), high bandwidth (for downloading applications on demand), and low-cost thin clients such as the network computer (NC), the NetPC, and Windows-based terminals (WBT).

However, there is much more to NCC than just thin clients. NCC is not a technology per se. Rather, it is a collection of technologies that, taken together, characterize this new paradigm. Table 1 illustrates some of the separate technologies (and specific instances) that together constitute net-centric computing. As with most things in computing, terminology related to NCC is full of acronyms. From Table 1, Orb refers to a CORBA (Common Object Request Broker Architecture) server, DOT stands for Distributed Object Technology, CBS means Component-Based System, XML is the eXtensible Markup Language, and TPS is a Transaction-Processing System. There are of course other technologies involved in net-centric computing; this is just a representative sample.


















fat, thin,  lean


Web,  Orb
















Table 1: NCC Technologies

The first column focused on the client aspects of NCC, in particular on thin clients. This column focuses on some of the infrastructure aspects of NCC, in particular on my experiences with one type of high-speed Net access technology that is becoming widely available at relatively low cost: digital subscriber lines (DSL). In early June, I had US West’s “Megaline” DSL service installed in my home office. To paraphrase Austin Powers, “It’s fast baby, yeah!

High-speed Net access

There are now several high-speed Net access options. Table 2 summarizes some of the more common methods available to consumers. The most widely used technique for Net access is still analog modems running at either 28.8K or 33.6K. (Note: Throughout this article, when used in the context of Net speeds, the suffix ‘K’ refers to thousands of bits per second and the suffix ‘M’ refers to millions of bits per second.) The more recent deployment of 56K modems has given users the capability to connect at speeds comparable to single-band ISDN (Integrated Services Digital Network). Note that 56K modems operate in an asymmetrical fashion: the download speeds can (theoretically) reach 56K, but the upload speeds are limited to 33.6K. Asymmetric bandwidth is common for consumer-oriented high-speed Net 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. Asymmetric access is usually sufficient, given the nature of Net use. Most information is coming downstream from the Net (or the Web) to the client; comparatively little is sent back upstream, except with bandwidth-intensive applications such as video conferencing.


Net Connection


Upload Speed 


Download Speed


Dial-up modem






Bonded modems












Satellite dish






Power lines


















Cable modem






Table 2: High-Speed Net Access Options

Modem bonding is an esoteric development that allows two modems to be coupled to operate as one virtual modem. This setup requires two telephone lines and an Internet service provider (ISP) that supports this type of multi-link connection. When it works, it offers speeds comparable to dual-band ISDN (128K). ISDN itself has a checkered history of high cost and notoriously difficult setup. Although it has been available for some time, it is being supplanted by newer technologies.

Some of the newer technologies are satellites, digital subscriber lines, and cable modems. These technologies provide connection speeds that are equal to or better than the traditional speeds of dedicated and expensive T-1 lines. Satellite hookups operate in an asymmetric fashion, using the satellite dish to download data at speeds ranging from 400K to 10M. Data uploads are still done using a regular analog modem. The wide discrepancy in speed for satellites reflects the different types of technology used. Some satellite systems receive information from space-based satellites in stationary orbit. Others receive signals from ground-based transmitters. Although the ground-based setups can offer much higher connection speeds, the receiver needs to be in a line-of-sight with the transmitter. For flat areas such as Phoenix, this works very well. For hilly areas such as Pittsburgh, the use is more limited. For most people, affordable and high-speed two-way satellite communication is still a few years away. Satellite constellations like Iridium or Teledisic may change that.

Recently, Nortel and Norweb collaborated to offer 1M Net connections over standard power lines. This means one could literally plug the computer into the wall socket and voila!—high-speed network access without any extra wiring. For the moment, this technology is limited to trials in the UK, where transformer setups are different from those in North America. However, it is expected that the technology will become available here as it matures. The allure of powerline-based networking to homes and small offices is obvious: no changes in the infrastructure are required to go online.  Even now, you can buy smaller versions of powerline-based networks for the SOHO (single office, home office) use, allowing computers and printers to be networked using small adapters plugged into wall sockets.

For higher speed access, cable modems are becoming more widespread. They currently offer up to 10M connection speeds downstream and up to 2M upstream. They have the advantage of using the existing wiring from television cable companies, thereby reducing the infrastructure requirements. This is one of the main reasons that AT&T recently purchased the cable giant TCI. The disadvantage of cable modems is that the connection is shared, or pooled, among users in the neighborhood. Although there may be 10M of bandwidth available from the cable company, if 10 people are all using the service at the same time, each will receive about 10% of that bandwidth, reducing throughput to 1M—still quite fast, but not as fast as one might like.

Digital subscriber lines

The main competition to the cable modems offered by the cable companies is digital subscriber line (DSL) technology, offered by the local telephone companies. The appeal of DSL is that it offers users high bandwidth (up to 7M) without requiring any new wiring; it uses the existing copper-based infrastructure, also known as POTS (Plain Old Telephone Service). In fact, the same line can be used for simultaneous Net access and voice or fax communication. This is because the DSL system uses a different part of the telephone line’s frequency for different types of data. Unlike cable modems, the DSL line is not shared with other users; you have all the bandwidth to yourself.

DSL is also asymmetric, moving data at up to 1.5M upstream and up to 7M downstream. For this reason, DSL is sometimes called ADSL. In fact, there are several variants of DSL, so it is also known as xDSL. The biggest stumbling block to widespread adoption of DSL is (again) the existing infrastructure. Because DSL uses the existing copper telephone lines for the last mile, special equipment must be installed between the phone company’s central office and local neighborhoods. This equipment creates “access loops” for customers. Unfortunately, the loop’s length is limited to a couple of miles. This means a significant portion of U.S. phone customers live too far away from their local phone offices to get DSL.

My experience with DSL

Unlike ISDN, which requires extensive knowledge on the part of the consumer and the installer, installation of DSL is relatively simple. For me, it involved a short visit by a representative from US West’s Megaline service. He installed a “modem” by NetSpeed (a subsidiary of Cisco Systems) that connects to the regular telephone line. I already had the other software they offer, such as an Internet browser, installed. So all I needed was some standard ISP information, such as the name of the mail server, and that was it. The entire installation took about 15 minutes.

The NetSpeed device is a SpeedRunner 202. It’s 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 ATM (asynchronous transfer mode) message-packaging format over an ADSL physical layer to support PPP (point-to-point protocol) networking. I already had a second telephone line that I was using for 56K access, so I just switched that modem line into the back of the SpeedRunner, connecting the device to US West’s WAN. The SpeedRunner also has a 10/100Base-T interface, into which I plugged my computer’s Ethernet card. With two simple connections, I was online with DSL.

I should point out that DSL is just the physical layer of the Net 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 56K modem access, x2 or K56Flex. In my case, I chose, 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 Net access and simultaneous use of a second phone line. So far, I’ve been very pleased with the performance. I have the lowest DSL speed offered, which is nominally 256K both upstream and downstream. However, the SpeedRunner has a diagnostic mode that lets you measure actual throughput, and I seem to be connected at about 640K downstream and 256K upstream.

From the end-user perspective, I can say that using this DSL connection feels almost the same as sitting on a 10M corporate local area network (LAN). Web pages load almost instantaneously, files download very rapidly using ftp, and large email attachments are no longer a problem. A side benefit of the SpeedRunner router is that it can be plugged into a hub, so I could share the Net connection with other computers on my office LAN if I so chose.

Implications of wide-spread deployment of DSL

If the use of DSL for SOHO access to the Net becomes more common, the implications may be significant. (The same could be said for other high-speed Net access options, such as cable modems.) DSL can support a variety of high-bandwidth applications, such as Web access, telecommuting, virtual private networking, and streaming multimedia content for on-demand video or teleconferencing. These services were either not possible to support or were ineffectively supported by conventional dial-up data delivery technologies such as 33.6K analog modems. Since DSL is based on existing standards (PPP, ATM, and so on), a competitive marketplace is being created to offer a variety of networking solutions. Besides offering high-speed Net access, the “always on” feature is very attractive: there is no need to dial to connect to your ISP; your Net connection is always ready.

For office use, DSL lends itself to branch office connectivity, effectively replacing expensive leased lines. As mentioned before, most business PC applications (e.g., file access, email, terminal emulation, Web browsing) perform asymmetric communication, making DSL a suitable technology to connect a SOHO to the enterprise. With more organizations exploring the benefits of telecommuting, the importance of high-speed Net access technologies like DSL will increase.

Although there are significant security issues that need to be addressed, technologies such as DSL can provide the infrastructure for virtual private networks (VPN), also known as “Extranets.” A VPN is a secure extension of an Intranet to existing global network backbones, such as the Internet. Nodes on the Extranet can use DSL to connect at high speeds through corporate firewalls using secured tunneling protocols such as PPTP (point-to-point tunneling protocol). Eliminating geographical distances between points on the network, and hence fostering collaboration between people, is just one way DSL can be used to improve the net-centric computing experience.

About this column

The focus of the Net Effects column is the impact of net-centric computing on a wide variety of issues, including computer science, information technology (IT), and software engineering.

About the author

Scott Tilley is a visiting scientist with the Software Engineering Institute at Carnegie Mellon University, an assistant professor in the Department of Computer Science at the University of California, Riverside, and principal with S.R. Tilley & Associates, a strategic and tactical information technology consulting boutique.

The views expressed in this article are the author's only and do not represent directly or imply any official position or view of the Software Engineering Institute or Carnegie Mellon University. This article is intended to stimulate further discussion about this topic.

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