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WE'VE ALL HEARD the come-ons: "Blazing speed!" "Connect 100 times faster than over the phone!" Unfortunately, two very important phrases are left out: "sometimes" and "up-to." Most of the hype that we hear about high-speed, broadband access is driven by the marketers rather than by the techies. But for the reasons mentioned below, that hype is rarely delivered. Lest this sound too negative, I'm pleased to note that the cable modem provider in the Newtown, Conn, area has taken a much more realistic approach to its marketing and is promising what it can deliver. [delivering what it promises?]

The Reality

After switching to cable modem access, many people discover that they rarely achieve the kinds of speeds that they are promised in the marketing hype. Typically their access speed is significantly improved over their old POTS connection (Plain Old Telephone Service), although in some cases when things get very bad this may not be true. The basic problem with achieving high speeds is that the cable modem connection is shared, and the speed that is achieved is determined by how many other users are competing for the pipe. In addition, one important consideration that is often overlooked is the end-end view of a connection on the Internet. The cable modem represents only the "last mile" between the home and the cable head-end (essentially, the ISP). From that point to the server providing the content, the packets flow over the ISP connection to the Internet, the Internet backbone, and whatever connection the server is using. In many cases, one of these is the rate-limiting link: No matter how fast the last mile is, data will not flow any quicker. In addition, users quickly discover that bandwidth is only a part of the equation. As outlined below, other factors (especially the "always-on" characteristic of cable modem connections) become more important.

Feature attractions

Besides bandwidth, cable modems do offer many attractive features. One of the first things that new subscribers notice is that they regain a telephone connection. When using POTS modems for the Internet, the phone line is tied up.

Probably of more importance is the "always-on" nature of the Internet connection. Whenever the computer is on, it is connected to the Internet. Initially for those coming from a connection-oriented service, this may not seem like much of an advantage. But once habits are unlearned, this characteristic becomes quite important, as illustrated by the following two examples:

• E-mail: With an always-on connection, e-mail becomes less of a batch process and more like checking a telephone answering machine. I've begun leaving my mail client running whenever my PC is, automatically checking the server for new e-mail every 10 minutes. Whenever I walk by the computer, I check for a new-mail indication (similar to looking for the blinking light on an answering machine) - or listen for the "mails-in" sound (similar to the phone ringing).
• CD player: A feature that many people aren't aware of for many CD-audio players under Windows is the capability of looking up a CD in a database located somewhere on the Internet. I was quite amazed the first time I played a CD after installing my cable modem: shortly after closing the CD drive tray, I was presented with not only the name of the CD I'd inserted, but also a list of all the songs on the disk.

Even when their service is oversubscribed, resulting in slow speeds, numerous studies have shown that few people are willing to give up their cable modem and go back to their old access.

The Alternatives

Today, there are essentially two alternatives to cable modem technology for high-speed Internet access -- T1 and ADSL access. Both of these are phone technologies using twisted-pair copper connections to the telco's central office. Some may remember ISDN, which although quite successful in Europe has come and gone in the U.S. without much impact. T1 technology has been in existence for a long time and provides approximately 1.5 mbps dedicated bi-directional connections. Unfortunately, it is also extremely expensive and requires a high-quality line between the subscriber and the central office. The combination of these two factors takes T1 connections out of the realm of possibility for most home users.

ADSL (Asymmetric Digital Subscriber Line) is a lower-bandwidth technology that is considerably cheaper and more forgiving of line conditions. ADSL connections coexist with your telephone connection without tying up the line (you can talk and compute at the same time). Typically, ADSL subscriptions are sold as multiples of 256 Kbps, with prices comparable to cable modem access. Unlike cable modems though, ADSL connections are not shared, so network loading will not affect your speed. Outside of the high-tech areas of the country, it is rare to have a choice between ADSL and cable modem connections. This makes the decision quite easy: You subscribe to what is available. In this area of Connecticut, cable modem seems to be the choice.

The Technology

Cable Modem signals travel on the same coax that delivers cable television service. In order to understand some of the limitations of cable modem service, it is necessary to understand something about the topology of the cable network. Today's systems use an infrastructure called hybrid fiber-coax, which essentially means that all signals from the cable head-end travel along fiber optic connections to a network node. At the node, the signal is converted to RF, which travels through coaxial cable running along the telephone poles to the home. While each node may be fed different information, all of the signals from the node to the homes are the same (two different houses on the same coax line will see exactly the same information).

There are typically two different paths with quite different characteristics for the data delivered to you (the "downlink") and the data you send (the "uplink"). The downlink channel is provided by modulating digital information on one of the 6 Mhz television channels within the spectrum provided by the cable system. As mentioned above, this channel is shared amongst all of the users. Depending on the technology used, a single 6Mhz analog channel can carry up to about 27 mbps of digital data (typically much less - often 10 mbps). Only the information for a given user is presented at the interface from the cable modem to the ethernet card in the computer. As you can readily see, the bandwidth available to an individual user is a function of how many others are trying to get information at the same time. If there are 100 others vying for the same pipe, then each user will get 100 Kbps, far from the total capacity of the pipe. Matters aren't quite as grim as they seem though, since most Internet connections are connection oriented: A brief flurry of data is sucked down, followed by a quiescent period while the information is digested (for example, reading a Web page). In addition, some of the providers are being careful about how they provision their networks, essentially guaranteeing a high level of service.

The uplink channel is a bit more problematic, since it is typically carried in the "low-split" region (the frequency band below the television signals). The main reason for using this band is historical: This was the only way to sneak a return path by the amplifiers on what was originally a one-way distribution system.

The low-split region has much less data-carrying capability and is much more susceptible to noise (before today's technology matured, there were numerous stories about how turning on a vacuum cleaner knocked out cable modem service for an entire neighborhood). The lack of bandwidth for the return path is usually not a problem, since the most common use of Internet connections is for Web browsing, which has very asymmetric characteristics--a short request brings a flood of data.

Running a web server behind a cable modem connection is a different story--very little data on the downlink and a large amount of data on the uplink, which often swamps the return connection and destroys performance for other users. This is the reason most cable modem contracts prohibit running servers.

Security

As usual, there's a dark side to the always-on characteristic of cable modems -- the issue of security. Unfortunately, there are people lurking on the Internet who are looking for openings they can exploit. A common practice among hackers is to run port probes on machines in a network, looking for openings. A cable-modem connection has no additional security exposures over any other form of Internet connection, except that the connection lasts longer. The pervasive connection makes it more likely that a poorly protected machine can be found by someone running port probes, and once a hole is found, it is more likely that the machine will be available for mischief later.

How to deal with the security issue is the subject of another article. But for now, here are the major points to remember:

• Don't run any unnecessary daemons (for example telnet or ftp).
• Don't enable any unnecessary protocols over the cable modem connection (TCP/IP should be all that is necessary).
• If you are using file/printer sharing, change the workgroup name from the default and don't enable the protocol for the cable modem.
• Consider running a firewall to protect the connection.