Cooking with Cable

Volume 2 Number 7 October 31, 1998

RealAudio

by Scott Tilley

Are you looking for high-speed connections to the Net? I’ve used 56K modems, Digital Subscriber Lines (DSL), and now cable modems. Based on my experience with the cable modem, I wouldn’t say it cooks—or is fully cooked—just yet.

I’ve now had the opportunity to use three different types of digital Net access technologies. These consumer-oriented high-speed connections are becoming widely available at relatively low cost. The first technology was a 56K modem. My experiences of just over a year ago with the Megahertz x2-based modem are documented in SIGPC Volume 1, Number 20:  "56K Modems."

The second technology was Digital Subscriber Lines (DSL). In early June, I had US West’s "Megaline" DSL service installed in my home office. My experiences with DSL are documented in  my Net Effects column in SEI Interactive Volume 1, Issue 2: "Life in the Digital Subscriber Lane."

More recently, I’ve been using a third technology: a cable modem. In September, I had Charter Communications’ "Pipeline" cable modem service installed in my new home office. So far, it’s been an acceptable Net access solution. It’s reasonably inexpensive, relatively simple to install, and offers a satisfactory connection speed. However, I wouldn’t say it cooks—or is fully cooked—just yet.

Digital Telephone Technologies

The most widely used technique for Net access is still analog modems running at either 28.8K or 33.6K. However, several technologies available offer high-speed digital connections. A few leverage the existing telephone lines found in almost everyone’s house. The widespread deployment of 56K modems has given users the capability to connect at speeds comparable to the 64K offered by single-band ISDN (Integrated Services Digital Network). 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.

For most consumers, the most advanced offering from the local telephone companies is digital subscriber line (DSL) technology. 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. A DSL line is not shared with other users; you have all the bandwidth to yourself.

DSL is 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 obstacle to widespread adoption of DSL is the present telecommunications infrastructure. Because DSL uses the existing copper telephone lines for the last mile, special equipment must be installed between the telephone 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 many customers live too far away from their local telephone offices to get DSL.

Cable Modems

The main competition to digital subscriber line (DSL) technology offered by the local telephone companies are the cable modems offered by the cable companies. Cable modems 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 deployment costs and some of the adoption barriers.

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. For most locations, that is not yet a problem. However, if you happen to live in a neighborhood with many cable modem users, the bandwidth can drop noticeably.

Think of the cable company’s main feed as a fire hydrant. In this analogy, the line going to your home is like a fire hose. When more and more fire hoses are connected to the same hydrant, the flow drops from a rush to a trickle. Eventually, the connection into your house is more like a garden hose than a fire hose. With too many cable modems connected to the same main feed, your 10M fire hose can eventually become a 100K garden hose—still faster than a 33.6K straw, but not exactly the kind of bandwidth you are paying for.

Like the DSL variant ADSL, most cable modem connections are asymmetric. The current infrastructure of mixed copper/fiber makes it difficult to reengineer the traditionally uni-directional flow of information, from the cable company to your home, to handle bi-directional data. This means the downstream data is flowing through the digital fire hose, but the upstream data is limited to the analog straw. In some cases (as with my cable modem service), the digital data flow is not just asymmetric—it’s completely one-way: all upstream communication takes place over a regular 33.6K analog modem.

As with the telephone companies and DSL, the cable companies provide the physical network link; they don’t necessarily provide ISP service. It may be that many cable companies will begin to offer such service in the future, but for now a separate service is often required. In my case, that meant setting up the cable modem and creating an account with Earthlink.

My Experience with Cable Modems

Installation of my cable modem involved a visit from two representatives of Charter Communications. The first representative was a traditional cable installer: he ran new wiring from an existing outlet, around my office, to my desk and computer. He also had to split the line at the main entry outside my house, 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 another way cable modems differ from DSL. The DSL connection can co-exist 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 into two means each of the new feeds can have a -4db drop in signal strength. This loss is not very important for television viewing; the picture quality does not noticeably change. However, for a cable modem, the loss of signal strength means a lower signal to noise ratio and this directly affects the 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 the signal loss, I installed a powered +10db signal amplifier between my office outlet and the cable modem.

The second representative was there to help with the initial setup of the modem itself, a "SB1200 Cable Modem" from General Instrument. It 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. The upstream connection is handled through a regular analog modem that is 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.

The cable modem’s settings are accessed through a Web interface. To go online, you bring up the proper page and select "Connect to the SURFboard Network", which is the Internet. The modem dials up the ISP using the analog modem, and establishes a PPP connection. From that point on, your computer acts as if it’s on a LAN, communicating to the Net using the Ethernet card. The analog modem in the cable modem is not visible to your computer; you never see any type of "Dial-Up Networking" applet.

The cable modem provides 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. Charter Communications has an arrangement with Earthlink to act as your ISP. This means you have a regular earthlink.net email account, space for a home page on the Web, and so on. It also means you can use any of the Earthlink POPs through a regular dial-up connection, for example if you are traveling and don’t have access to your cable modem. Unlike many ISPs now, the Earthlink connection is truly unlimited; no maximum number of usage hours per month.

There is one hitch using the cable modem under Windows 95 when you also use your computer on another LAN: the network settings for the cable modem need to be fixed. The cable modem is actually operating as a router for your computer; all TCP/IP packets are sent and received from the 192.168.100.1 gateway, which is the default address of the modem. Your computer needs to specify its own fixed IP address, which by default is 192.168.100.2, and a gateway mask of 255.255.255.0.

The settings for domain services, such as host name and DNS servers, must also be hard coded. It would make things much easier if these values were all dynamically set upon connection, for example by using DHCP, because then you can have multiple TCP/IP to Ethernet bindings that don’t conflict with one another. As it is, I use a separate Ethernet card to use my computer outside of my home office. I’ve managed to get the settings working well enough such that I don’t have to fiddle with them each time I move from one location to another. This was no mean feat in Windows 95, which requires a system reboot when the DNS settings change.

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 (the first three months are half price). However, I still need a second phone 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 phone line was about $85. This means a total cost of about $65 a month for cable modem service, which is nearly identical to what I was paying for DSL service.

So far, I’ve been reasonably pleased with the performance. I’m told the actual maximum downstream speed is about 400K, far below what cable modems are capable of handling. The upstream speed is limited to 33.6K because of the analog connection. My experience has been that downloading large files, say via FTP, is quite fast. The download speed is usually a sustained 30K bytes per second. Accessing the Web does not always seem noticeably faster than a regular 56K link.

If the cable modem was bi-directional, negating the need for a second phone line, and if the connection rate was more like 7-10M, I’d be very happy. For now, I’d have to say that I prefer DSL to cable modems when the connection rates are this low and the setup is this involved. Now if only my local telephone company would make DSL available…