Cable Modems and CATV Networks / Upgrading and Repairing PCs

For many users, cable modem service—which piggybacks on the same cable TV (CATV) service lines that bring your TV many channels—represents both a big boost in speed from that available with ISDN and a major savings in initial costs and monthly charges. Unlike ISDN, cable modem service normally is sold as an "all you can eat" unlimited-access plan with a modest installation charge (often waived) and a small monthly fee for the rental of the cable modem. Because more and more cable networks support a single standard, you can also choose to buy your cable modem from any of several vendors in some cases, saving even more money over the long term.

Connecting to the Internet with a "Cable Modem"

As with ISDN, the device used to connect a PC to a CATV network is somewhat inaccurately called a modem. In fact, the so-called "cable modem" (a name I will continue to use, for the sake of convenience) is actually a great deal more. The device does indeed modulate and demodulate, but it also functions as a tuner, a network bridge, an encryptor, and an SNMP agent. To connect your PC to a CATV network, you do not use a serial port as with analog modem technologies or ISDN terminal adapters. Instead, the most typical connection today uses 10/100 Ethernet or USB (10/100 Ethernet is faster). If your computer doesn't include an Ethernet adapter or 10/100 Ethernet card, you can install a 10/100 Ethernet adapter into a PCI slot or connect most recent cable modem models to a USB port on your computer. Some older cable modem services utilize an internal adapter for one-way service and use the conventional modem for uploads; this type of service is called telco return. Even though you get fast downloading, telco return systems tie up your phone line and are not recommended.

Tip

For maximum speed and ease of sharing, I recommend that you connect your cable modem to a 10/100 Ethernet card or port or use a USB 2.0 port (built into many new machines or available as an add-on card for older systems). USB 1.1 ports limit the throughput of your cable modem.

The Cable Modem and the CATV Network

The cable modem connects to the CATV network using the same coaxial cable connection as your cable TV service (see Figure 19.1). Thus, the cable modem functions as a bridge between the tiny twisted-pair network in your home and the hybrid fiber/coax (HFC) network that connects all the cable customers in your neighborhood.

A few cable modem CATV systems have been built using the older one-way (download-only) coax cable, but this type of cable is much slower for cable modem use and is obsolete for both CATV and data communications. The industry has largely replaced coax with HFC. Before you sign up for CATV Internet service, find out which type of service is being offered. Only the two-way, HFC-based systems allow you to use the Internet independently of the telephone system; one-way cable modem service requires an analog modem for uploading page requests, files, and email. The modem can be built into your one-way cable modem (these are called bundled cable modems) or be a separate external analog modem. In either case, going online with a one-way cable modem ties up your phone line.

Digital CATV service, which brings your TV many more channels and a clearer picture, requires the cable TV provider to upgrade to an HFC physical plant. Thus, digital CATV service is a precursor to two-way cable modem service; pure-coax CATV systems can't be used to transmit digital service or handle two-way cable modem traffic. CATV systems that have been upgraded to digital service are capable of providing two-way cable modem service, after suitable head-end equipment is installed at the CATV central office. A good rule of thumb, therefore, is that CATV systems that don't offer digital cable TV might offer only one-way cable modem service or no cable modem service at all. A typical two-way cable modem connection is shown in Figure 19.1.

Originally, cable modems were not sold to users of CATV Internet access but were leased by the CATV companies offering Internet access to their cable modem customers. This is because each cable modem on a particular CATV network had to match the proprietary technology used by the network. In late 1998, DOCSIS-compliant cable modems began to be used by some CATV companies. DOCSIS refers to devices that meet the Data Over Cable Service Interface Specification standards established by Cable Television Laboratories, Inc. (CableLabs). Modems that meet DOCSIS standards are now referred to as CableLabs Certified cable modems. Visit the CableLabs Web site at www.cablelabs.org for a complete list of cable modems that are CableLabs Certified.

Many vendors of traditional modems and other types of communications products, such as Zoom Telephonics, 3Com, GVC, General Instruments, Philips, Motorola, Cabletron, Toshiba, Cisco, D-Link, and many others, now make CableLabs Certified Cable Modem–compliant hardware. The models supported by your CATV Internet provider might vary according to the DOCSIS standard it supports. Table 19.1 provides a brief overview of the differences in these standards.

Table 19.1. DOCSIS Standards Overview
DOCSIS Standard	Benefits	Notes
1.0	Basic broadband CATV (cable modem) service	Original DOCSIS version
1.1	Supports tiered service (different speeds at different costs), faster uploading, home networking, and packet telephony while reducing costs	Backward-compatible with DOCSIS 1.0
2.0	Faster performance for downloading and uploading compared to DOCSIS 1.0 and 1.1; supports high-speed two-way business services	Backward-compatible with DOCSIS 1.1, 1.0

Although most cable modems are now available for about $100, compared to $30–$70 for typical analog modems, you should check with your CATV Internet provider before purchasing one to determine which models are supported by your provider and whether your CATV Internet provider still requires you to lease the cable modem.

If you plan to keep cable modem service for more than a year, I recommend purchasing a CableLabs Certified cable modem, but if you're unsure of your long-term plans, leasing isn't all that expensive. Typical lease costs for the device add only about $10/month to the monthly rate of $30–$40 for cable modem service.

Types of Cable Modems

Cable modem models that must be used with (or include) an analog modem are designed for older "one-way" CATV Internet connections; these will tie up your phone line when transmitting page requests as well as when sending uploads.

Note

For more comprehensive coverage of cable modem service types, installation, sharing, security, and use, see the Absolute Beginner's Guide to Cable Internet Connections by Mark Edward Soper (published by Que, 2002).

CATV Bandwidth

Cable TV uses what is known as a broadband network, meaning the bandwidth of the connection is split to simultaneously carry many signals at different frequencies. These signals correspond to the channels you see on your TV. A typical HFC network provides approximately 750MHz of bandwidth, and each channel requires 6MHz. Therefore, because the television channels start at about 50MHz, you would find channel 2 in the 50MHz–56MHz range, channel 3 at 57MHz–63MHz, and so on up the frequency spectrum. At this rate, an HFC network can support about 110 channels.

For data networking purposes, cable systems typically allocate one channel's worth of bandwidth in the 50MHz–750MHz range for downstream traffic—that is, traffic coming into the cable modem from the CATV network. In this way, the cable modem functions as a tuner, just like your cable TV box, ensuring that your PC receives signals from the correct frequency.

Upstream traffic (data sent from your PC to the network) uses a different channel. Cable TV systems commonly reserve the bandwidth from 5MHz to 42MHz for upstream signals of various types (such as those generated by cable TV boxes that enable you to order pay-per-view programming). Depending on the bandwidth available, you might find that your CATV provider does not furnish the same high speed upstream as it does downstream. This is called an asymmetrical network.

Note

Because the upstream speed often does not match the downstream speed (and to minimize noise, which tends to accumulate because of the tree-and-branch nature of the network), cable TV connections usually are not practical for hosting Web servers and other Internet services. This is largely deliberate because most CATV providers are currently targeting their traditional home user market. As the technology matures, however, this type of Internet connection is likely to spread to the business world as well. There are now specialized domain name services that can be used to "point" Web surfers to your cable modem or DSL connection.

Some cable ISPs require you to switch to a more expensive business plan if you want to host a server (including a P2P server such as Gnutella) on your cable modem connection. In fact, hosting a server on a residential cable modem service could be a violation of your contract and could lead to cancellation of your service.

The amount of data throughput that the single 6MHz downstream channel can support depends on the type of modulation used at the head end (that is, the system to which your PC connects over the network). Using a technology called 64 QAM (quadrature amplitude modulation), the channel might be capable of carrying up to 27Mbps of downstream data. A variant called 256 QAM can boost this to 36Mbps.

You must realize, however, that you will not achieve anything even approaching this throughput on your PC. First of all, if you are using a 10BASE-T Ethernet adapter to connect to the cable modem, you are limited to 10Mbps. A USB 1.1 port is limited to 12Mbps, but even this is well beyond the real-life results you will achieve. As with any LAN, you are sharing the available bandwidth with other users in your neighborhood. All your neighbors who also subscribe to the service use the same 6MHz channel. As more users are added, more systems are contending for the same bandwidth, and throughput goes down.

In November 1999, ZDTV (now TechTV) tested five brands of cable modems in typical operations and found that the overhead of CATV proved to be a major slowdown factor. The cable modems were first connected directly to the server to provide a baseline for comparisons. Some of these tests showed speeds as high as 4Mbps. However, when the CATV cable was connected and the same tests were run, the best performer dropped to just 1.1Mbps, with others running even slower.

Tip

You can improve the performance of your cable modem or other broadband Internet access device by making changes to your system Registry that affect the size of the TCP receive window and other settings. I recommend the system tweaks available at the SpeedGuide.net Web site (www.speedguide.net). You can find tweaks for all Windows versions from Windows 95 through Windows XP, along with instructions on how to make the changes manually and easy methods for setting your system's Registry back to its default values.

Widespread reports from cable modem users across the country indicate that rush hour–type conditions occur at certain times of the day on some systems, with big slowdowns. This rush hour is due to increasing use of cable modem systems in the late afternoon and early evening, as daytime workers get home and pull up the day's news, weather, stocks, and sports on their Internet connections. Because cable modems are shared access, this type of slowdown is inevitable and becomes exceptionally severe if the CATV Internet provider doesn't use a fast enough connection to the rest of the Internet. To minimize this problem, many CATV Internet providers use caching servers at their point of presence connections to the Internet. These servers store frequently accessed Web pages to enable users to view pages without the delays in retrieving them from the original Web sites. By adding multiple T1 or T3 connections to the Internet backbones and using caching servers, ISPs can minimize delays during peak usage hours.

CATV Performance

The fact that you are sharing the CATV network with other users doesn't mean the performance of a cable modem isn't usually spectacular. Although the CATV network takes a big cut out of the maximum speeds, you'll still realize a throughput that hovers around 512Kbps, almost 10 times that of the fastest modem connection and four times that of ISDN. You will find the Web to be an entirely new experience at this speed. Those huge audio and video clips you avoided in the past now download in seconds, and you will soon fill your hard drives with all the free software available.

Add to this the fact that the service is typically quite reasonably priced. Remember that the CATV provider is replacing both the telephone company (if you have two-way service) and your ISP in your Internet access solution. The price can be about $40–$50 per month (including cable modem rental), which is twice that of a normal dial-up ISP account, but it is far less than ISDN, does not require a telephone line (if your provider has two-way service), and provides 24-hour access to the Internet. The only drawback is that the service might not be available yet in your area. In my opinion, this technology exceeds all the other Internet access solutions available today in speed, economy, convenience, and widespread availability. Its nearest rival is DSL, which is still not as widely available geographically and is plagued with poor coordination between ISPs and telephone companies. Because cable modem Internet service providers provide the physical plant, provide ISP services, and can provide equipment, you can get service installed in just days and avoid the finger-pointing common with other types of broadband Internet service.

CATV Internet Connection Security

Because your PC is sharing a network with other users in your neighborhood and because the traffic is bidirectional on systems using two-way cable modems, the security of your PC and the network becomes an issue. In most cases, some form of encryption is involved to prevent unauthorized access to the network. CableLabs Certified (DOCSIS) cable modems have built-in encryption, but older one-way modems might not have this feature.

If you use an operating system such as 32-bit Windows that has built-in peer networking capabilities and your provider doesn't use CableLabs Certified cable modems or some other form of encryption, you might be able to see your neighbors' computers on the network. The operating system has settings that enable you to specify whether other network users can access your drives. If these settings are configured improperly, your neighbors might be able to view, access, and even delete the files on your hard drives. Be sure the technician from the cable company installing the service addresses this problem if your cable modem hardware doesn't provide encryption. If you want to use a cable modem along with sharing access on your computer (for printing, file storage, and so on), I'd recommend that you use passwords for any shared drives, but you're even safer if you disable file and printer sharing on the system you connect to the cable modem.

For more information on securing any type of Internet access, see "Securing Your Internet Connection," p. 1046.

Upgrading and Repairing PCs