DSL and Cable Modems

The next chapter is devoted to using cable modems for Internet access. Both DSL and cable modems offer a larger bandwidth connection than you will ever get with a dial-up modem. However, it is important to understand the main difference between cable and DSL technology.

As mentioned earlier, most home users can't afford to spend the several hundreds or thousands of dollars a month it would cost to put in a T-class of service. For many years , ISDN (discussed in the preceding chapter, "Dedicated Connections") was the primary digital method for connecting home offices or those who have large bandwidth requirements to the Internet or to a company's remote access servers. ISDN, however, has become almost outdated for most of today's remote users for several reasons. One reason is that if you can get DSL or cable access in your area, you will probably pay less for those than for ISDN service.

However, sometimes ISDN is a good choice for a connection, but it does have some drawbacks:

• ISDN is a dial-up technology. To connect to the Internet or a remote site, you must place a call, just like you do with a telephone.

• ISDN BRI (Basic Rate Interface service), with two 64Kbps channels you can use for data transmission, although cheaper than a T-class of service, is still priced beyond the reach of most typical home users or small businesses. This pricing discrepancy will drive away most customers who can get DSL or cable modem access. And although DSL/cable providers have pretty much stabilized to offer services in a small range of prices, the same cannot be said for ISDN ”or for the T-carrier lines either. You will find that the cost of ISDN service varies widely from one location to another.

• Newer technologies have been developed over the past 10 years that are not only faster than ISDN, but also much cheaper to provide. These digital services are provided by either your local cable television company through a cable modem, or by one or more providers in your area using the same twisted-pair telephone wires that connect your telephone to the local central office. This is, of course, the topic of this chapter: DSL technology.

With the advent of faster modems, the price that telephone companies charge for ISDN's basic maximum of 128Kbps (two 64Kbps lines) service isn't really justifiable for most users when you consider that current modem technology (with multilink capabilities, as you'll find in Windows operating systems) can easily come close to this speed. However, in such a situation the cost of multiple phone lines coupled with the difficulty of finding a service provider that supports multilink would probably be prohibitive factors when compared to DSL.

Topological Differences Between Cable and DSL

Both cable modems and DSL are digital broadband technologies, but that's about all they have in common. The first major difference is the physical connection. DSL provides a direct point-to-point connection to a termination point in the telephone company's central office using the same twisted-pair wiring used for telephone services. Because DSL uses copper wires and the signal attenuates (weakens and degrades) the farther you get from the central office, DSL has a limited distance, usually a maximum of 18,000 feet of cable. Another thing to keep in mind is that the farther you get from the central office, the slower the speed you will be able to get from a DSL line.

Note

The term broadband has many definitions. The most basic definition is that broadband connections (DSL, cable, and ISDN, for example) use more than one frequency for data transmissions, and can thus multiplex data streams across multiple frequencies at the same time. Compared to a "nonbroadband" or " narrowband " connection (such as your 4MHz telephone channel), the multiple frequencies allow a larger data pipe. Some definitions rely on the actual bandwidth achieved, no matter what the method.

Because most people don't live next door to a central office, and because most telephone companies have been trying to extend profitable service offerings to more and more of their customer bases, a solution that creates a "mini" central office in the field is being rapidly deployed in many areas. This involves using high-capacity lines (usually fiber optic) from the central office to connect a digital concentrator out in the field. The fiber- optic cable enables digital communications back to the central office from your neighborhood. Using this method, it's easy to deliver digital services in the "last mile" by moving the digital equipment out of the central office and closer to subscribers. Building out this fiber network is to the benefit of telephone companies, because the fiber can be used for many services, such as telephone service, cable TV, and DSL.

Back at the central office, the data portion of the signal is split off and sent directly to a high-speed digital connection to the Internet. The voice portion of the signal (the first 4Mhz) is directed to the public switched telephone network (PSTN). See Figure 17.1 for an example. This means that your connection is shared by no one and, after the service has been provisioned and installed, you can expect to get the same speed no matter what time of day from your connection to the telephone company's central office. Cable systems work in a similar manner, separating Internet traffic from television data at the cable company's front end.

Figure 17.1. DSL uses frequencies that are left unused on a typical voice-grade copper cable.

The communication path for cable modems is a shared access medium, similar to an Ethernet network. The coaxial cable that snakes through your neighborhood delivering cable television services is used for both television and cable-modem communications. When you get DSL advocates and cable-modem aficionados in the same room arguing, the DSL guys will tell you that the shared access coaxial cable is a choke point that can limit the speeds you can obtain. This argument is based on the notion that as more homes in your neighborhood begin to use the cable-modem service, the shared coaxial cable can become saturated . Although most cable modems, when working at top speed, can easily surpass a DSL connection, your actual speed using a cable modem will vary from the maximum capacity depending on how many other users are using the same cable at the same time. In practice, however, cable systems offer a higher bandwidth than many DSL systems, especially if you don't live close to a central office or there isn't a digital connection sitting in a box somewhere in your neighborhood. And, as cable segments become heavily populated , cable companies can split the cable into different segments and run a line back to a router that connects to the Internet. Because DSL tends to be the most flexible service, in that you can choose from various speeds for upload and download capabilities, it is probably the choice for a business. Cable modems typically offer only one or two types of service ”which are probably due to their physical capabilities ”and cable service providers won't generally allow you to do such things as maintain a Web site and keep a static IP address. DSL business class, however, can give you these features.

If you have only a small office that uses another provider to offer your Web page, and you just need a fast connection to the Internet to check email and so on, then a cable modem connection can serve you very well, and usually at a much less expensive price than a DSL connection. The next chapter covers cable modem technology in more detail. However, after you read that chapter, another question you must ask yourself when making a decision about using a cable modem or DSL is which company provides you better service now? When you call your local telephone company or cable company, which is the quickest to respond? Which company provides better service? How important is that Internet connection to your business or for recreational use? Today, access to the Internet is becoming a business necessity. Before choosing an access method (or a backup method), be sure to evaluate how important this access is to your business.

DSL Services

Just as you can obtain phone service from more than one company, using the same telephone lines, the same goes for DSL services. Although your local telco might not offer the service, other providers in the same area might. Many telephone companies invested heavily in the equipment needed to provide ISDN services several years ago, charged a lot for the service, and found few customers. Some are hesitant to invest in the DSL Access Multiplexer (DSLAM) and other equipment needed to provide DSL services. However, that doesn't stop third parties from co-locating their own DSL equipment at the local telco office and offering you the service. Today DSL service is booming, due in part to the demand from both home users and businesses that cannot afford T-carrier and other networking services.

Keep in mind that for both cable modems and DSL connections the high-speed capacity they both provide is only on the link from your computer to the ISP Internet connection. This is very important to consider. Both cable and DSL modems will do nothing to make the Internet itself or Web servers work faster . So if your favorite Web site is being swamped with a large number of hits, you may not notice any difference between a dial-up connection and a digital connection. And if a large number of your customers are trying to connect to your Web site, it is not just your bandwidth between your network and the local ISP that matters. When it comes down to it, the lowest common denominator is the speed of the Internet itself. If everyone is downloading the newest update to a popular software product, you can expect to experience slower transfer rates and lower bandwidth availability due to circumstances beyond your local connection to the telco or cable modem provider.

For these reasons, don't expect DSL to be the absolute solution to bandwidth on the Internet. The Internet is far too complex for such a simple solution.

A Quick Primer on the PSTN

Before you can understand why DSL technology and cable modems are revolutionizing high-speed access to the Internet, it is important that you understand how your ordinary telephone line works. Given that knowledge, you can understand how DSL takes advantage of the remaining capacity inherent in the telephone voice cabling, and why cable modems can offer a service using another type of cabling.

Ordinary voice-grade service, called plain old telephone service (POTS), has its roots back in the 1930s when it was discovered that human beings can hear frequencies of up to about 20,000KHz but normally talk in frequencies that range up to only about 3,500KHz. Because of this, the original telephone network was designed to transmit in channels of only 4,000KHz. However, those ordinary copper wires used in the last mile to the home can transmit signals using much higher frequencies. The problem with using the PSTN to provide high-speed data communications comes from the fact that although the copper wiring can handle transmission at greater frequencies, the techniques used at the central office to digitize voice-grade traffic cannot. The incoming analog voice signal is sampled 8,000 times per second and coded into 8 bits for transmission on the digital portions of the PSTN.

This means that the effective data throughput is limited to 64Kbps. Because most telephone circuits use one of the bits for network-management purposes, the actual data throughput shrinks to only 56Kbps.

Communications between central offices and long-distance lines make use of more modern techniques, stacking multiple voice circuits on top of each other in 4KHz segments and sending them across high-capacity fiber-optic lines. The Internet backbone is composed of similar high-capacity lines capable of extremely fast transmission of large amounts of data. The limiting factor for dial-up modems is the old, switched telephone network that sets the ceiling at 8,000 samples per second.

The maximum speed of today's dial-up modems is achieved by using sophisticated coding techniques on the wire on which data is transmitted as symbols, coupled with other compression algorithms. Yet analog modems are limited to using only that first 4KHz of bandwidth on the copper wire (the voice channel ”which is why you can hear the squealing and clicking of modem communications if you pick up a voice line while a data connection is active). Because the twisted-pair telephone wire can support higher frequencies, it's only natural to assume that there would be a digital solution to the speed roadblock that the voice-grade telephone circuit forces you to use.

And ordinary analog modems on a voice circuit may not achieve the highest rate for that modem. Because of noisy lines, and other interference, it is often the case that a modem connects to an ISP or another modem at a rate lower than the highest rated speed.

xDSL

The term xDSL is usually mentioned when discussing DSL because there isn't just one type of DSL; instead, you can choose from an assortment of similar technologies, depending on your needs. For example, perhaps the most popular xDSL technology is ADSL (Asymmetric Digital Subscriber Line). This service provides a fast download speed to your computer or network, but you'll have a much smaller data path back to the provider of this service.

One thing to keep in mind about xDSL technologies is that their usability is limited by the distance from the telephone company's central office. This is due to several factors, including attenuation of the signal as it travels down the copper wire and interference caused by signals from wires in a cable bundle interfering with other wires in the same bundle. Because telephone cables were initially laid down with no thought for offering digital services, it's possible in many locations that you won't be able to get any kind of xDSL connection until local lines are upgraded.

Tip

One factor that may help you make a decision on which vendor to select is the availability of email accounts and USENET access. When you obtain a DSL connection, there is an associated ISP that connects your digital line to the Internet. Newsgroup access (USENET) may not be important to many companies (but it can be a very useful research tool), and if it isn't available, there are various Web-based solutions (such as http://groups.google.com) providing access to most of the nonbinary newsgroup postings. However, email is usually a very important factor, and one that may be a deciding factor when it comes to making a decision about connecting to the Internet. Check the number of email accounts your DSL ISP provides. If it is not sufficient to support your business users, don't panic. You can always use another service on the Internet (such as www.activewebhosting.com) that will provide you a Web page, FTP storage, and a number of email mailboxes. If your ISP limits you on email, consider using a third-party provider instead, and your own domain name. The last feature, your own domain name , is an attractive feature if you really want to project a presence on the Internet.

These are some of the more common offerings you'll find in the xDSL world:

• ADSL ” Suited best for a home user or a small office where data requires a large download speed (1,500Kbps to 8,000Kbps) and a slower upload speed (32Kbps to 1,088Kbps). ADSL can potentially reach up to 18,000 feet from the central office.

• RADSL ” Rate-Adaptive Digital Subscriber Line is a variation of ADSL. The modems on this type of connection can test the line to determine what speed it will support. RADSL usually provides for a longer distance from the central office (about 21,000 feet) but provides for slower speeds. Expect download speeds from 600Kbps to 7,000Kbps. Typical upload speeds are from 128Kbps to 1,000Kbps.

• SDSL ” This Single-line Digital Subscriber Line allows for symmetric bidirectional communications. Unlike ADSL, SDSL gives you the same bandwidth in either direction. Generally, you must be within 10,000 feet of the central office. Transmission rates (depending, of course, on distance) range from 160Kbps to 2,084Kbps.

• HDSL ” High bit-rate DSL is an early entry into the xDSL market. HDSL also gives you the same bandwidth for both directions.

• G.Lite (also called Universal ADSL) ” This form of ADSL is easier to install than the traditional ADSL. Traditional ADSL requires installing a splitter at the consumer premises, which effectively splits the lower voice-frequency channel from the higher frequencies used for digital transmission. G.Lite operates at a lower speed than ADSL (1,544Kbps downstream, 384Kbps upstream) but does so without using a splitter.

There are still other xDSL technologies, but some are proprietary and the solution you end up choosing will depend on what your local provider offers. There's even a version called IDSL, which offers an ISDN service using xDSL technology. Of course, the higher-rate services are going to cost more. For most small offices or home users, the advent of cheap ADSL and G.Lite services can be an ideal solution for remote connections.

DSLAMs, CAP, and DMT

Implementing ADSL services in a central office is not nearly as costly as implementing ISDN. Using ADSL, the network traffic never actually enters the PSTN like dial-up modem analog connections do. At the central office, the incoming signal is split, sending the voice-grade frequencies (the first 4KHz channel) to the normal telephone switch, and the higher frequencies used by DSL to a device called a DSL Access Multiplexer (DSLAM). The DSLAM is responsible for concentrating traffic it receives and passing it to the Internet by using higher-capacity technologies, such as T1 “T3 lines, or to an ATM switched network, for example. Lucent and Nortel make telephone line cards that can be used in the central office which combine digital and voice functions. If these cards are used, it isn't even necessary to rewire your phone line at the central office when DSL is provisioned, making it even easier for telephone companies to offer the service. One of the first drawbacks of getting DSL service is how it will be supported by the local telephone company. No matter who the "provider" of this service might be, much depends on how the telephone company handles the digital signal after it's split off from the voice signal.

And it does get just a little more complicated. For ADSL, two basic methods use those higher frequencies on copper wires: Carrierless Amplitude Phase (CAP) and Discrete MultiTone (DMT). Both of these methods use frequencies that are well past the 4KHz voice channel, and because of this are called passband technologies. Both voice and data can be carried on the same twisted-pair wires at the same time, operating in full-duplex mode. The voice-grade frequency range is not used for data (it is "passed by"), and there is a small amount of separation of frequencies before the ADSL upstream channel starts and another separation before the downstream channel starts, depending on whether your DSL is implemented using CAP or DMT. This separation helps to reduce the probability of one channel interfering with another.

The main difference between CAP and DMT is the way in which they use the higher frequencies on the ordinary copper wire pair. CAP uses two different carriers , a smaller frequency range for the upstream data and a larger frequency range for the downstream data path, as you can see in Figure 17.2. CAP uses a small part of the frequencies above the voice channel for uploading data to the Internet, and a much larger range of frequencies for the downstream data path.

Figure 17.2. CAP uses two ranges of frequencies, with the larger range used for the downstream path.

However, a drawback of CAP is that any noise on the line can easily cause the signal to become corrupted. This can be caused by wiring that has degraded, interference from other wires in a cable bundle, or bridge taps that haven't been removed. To overcome this, CAP-based modems usually try to measure the line when they start up to determine the defects inherent in it. The modems then try to use equalizers to create mirror images of the line distortion that can be used to clean up the signal.

DMT uses channels above the voice channel just like CAP does. However, as you can see in Figure 17.3, DMT divides the upper frequencies into many discrete, separate 4KHz channels, and assigns some for upstream communications and others for downstream data flows. Each channel is called a bin .

Figure 17.3. DMT creates multiple channels in the upper-frequency range and assigns them for upstream or downstream communication.

DMT can monitor each channel to determine which ones are having problems with interference, and not use those small segments of the available frequency range, or use the troublesome bins for a lower rate of data transmission. Bins in the lower frequency range are used for the upstream data path, and those in the higher frequency ranges are used for the downstream data path. DMT constantly monitors each bin and adjusts their use depending on performance. Bins in the lower frequency range also can be used as bidirectional communication channels. If this is done, echo-canceling circuitry is necessary, making DMT a more expensive technology.

CAP was originally deployed by ADSL vendors early in the game, and much of the equipment is proprietary. That is, a modem you get from one provider might not work with another provider's system. This really shouldn't matter, however, if you intend to keep the same provider for a while.

The ITU selected DMT as the standard method to be employed for ADSL. However, a lot of equipment using CAP has been installed in the past and can continue to have a useful life for some years to come. When it comes to choosing a provider, you might not have many choices in your area. You might have to take what you can get ”if you can get it. You might be located too far from the central office to make DSL a solution for your home or small-business connection. Or the building in which you are located might have been wired years ago and contain a lot of bridge taps and deteriorating cables.

However, it is important to know the difference between CAP and DMT. Because DMT is a standard now, most new equipment that will be produced is likely to use DMT. Of course, your best protection against changing standards is to use service providers that include the DSL equipment with the DSL service.

ADSL

ADSL is ideal for home users for Internet surfing because it's usually priced reasonably, and the bandwidth use for home users is generally asymmetrical . That is, you click the mouse button on a link and then wait for a large download of data to your computer. Most Internet Web page communication for the home user involves a minor amount of data being sent upstream to the Internet, with a large amount of data being sent downstream to the computer in response to the request. With multimedia applications, such as streaming audio and video, the large data pipe that ADSL provides will make the Internet appear to be lightning fast to a home user, or a business user performing research.

However, the opposite is not true. If you operate a Web server from your house or business, keep in mind that ADSL provides a much smaller data pipe back to your ISP. And many DSL providers will try to prevent you from using your connection for a Web server by using DHCP to assign your address. If you are a business customer, you may be able to get a business class of service that includes a faster upstream link and a static IP address. Otherwise , expect to be treated as a "home" customer. If you want to place a server on the Web, and you expect to send out large volumes of data to the Internet (or receive a very large number of hits), you'll probably have to settle for another xDSL technology and pay the telco a little more for the dedicated address.

Note

The Dynamic Host Configuration Protocol ”see Chapter 29, "BOOTP and Dynamic Host Configuration Protocol (DHCP)" ”is used to dynamically assign network information, such as an address, to clients of the network. Because of its dynamic nature, a client may acquire a different address if it disconnects and then reconnects later. This is because the DHCP server may have reused the older address and it has been assigned to another client. This is why you shouldn't consider using a DSL or cable modem service that uses DHCP if you want to create a Web presence from a computer on your network. Instead, use a service provider to host your Web page, and use your DSL link to manage the Web site.

If a business has users who work from home, ADSL might be a good way to establish connectivity back to the main office through the Internet. For example, a help-desk technician can use the voice channel on the telephone line to take customer help calls. To assist the worker in his job, the ADSL connection can be used to allow the technician to search Web site pages or a database residing on a server back at the business's main office. As long as it isn't necessary for the worker to send large amounts of information back to the office, ADSL can be a good fit here.

Another aspect to examine for this type of service is a work-from-home user who needs to download larger amounts of data to work with, and then upload the changes. Using the fast download provided by ADSL, graphic files, programming files, and so on can be obtained quickly. Then, using a slower upload path, the home worker can send the files back to the main network.

A practical example of the use of ADSL is the food service industry. When branch office managers need to consult the headquarter's database to obtain data about price quotes or discounts , or to simply download a listing of company products, the capacity provided by ADSL can prove very beneficial. This assumes, of course, that the headquarter's site also uses the bandwidth necessary to communicate with a large number of branch offices. Yet because each branch office will probably upload much less data to produce the reports necessary for headquarters to make delivery computations , this can be an expensive service.

Think here about which direction your data flows. Do you have more incoming or outgoing traffic when connecting to the Internet (or another intranet)?

G.Lite DSL

Concerns over putting too many wires in a cable bundle to use for high-frequency DSL services spurred the development of a version of DSL called G.Lite. This version of DSL doesn't require a splitter (to separate the voice channel from the ADSL channels) and operates at lower frequencies. Lower frequencies allow for downstream data flows of only about 1Mbps to 1.5Mbps. Upstream capacity usually runs at 100Kbps to 300Kbps. However, download capacity can range up to 6Mbps and upload speeds to 384Kbps. By using a lower range of frequencies, it's possible for telephone companies to offer a more reliable service, over a longer distance, because attenuation of the higher frequencies (and the cross-talk they can cause) is less of a factor. The speeds provided by G.Lite recognize that the Internet itself, like the PSTN, can be a limiting factor when it comes to speed. It doesn't matter how fast your particular connection to the Internet is if the Web site you're trying to access is on a slow server, or connected by a slow link. Most users won't notice the difference between 1.5Mbps and 8Mbps that can be attained with more expensive DSL technologies.

G.Lite was developed by the Universal ADSL Working Group (UAWG). Its mission was to develop a lower-cost version of ADSL, based on standards, that could be easily installed. The ITU approved G.Lite as a standard in 1999 (no. G-992.2). G.Lite is a good solution to a technological problem. G.Lite doesn't require a "truck roll," which means that the phone company can mail you the package and you can install it without the company's help. Each time a telco truck has to visit a customer, the expense is a serious cost to the telephone company.

Note

Note, however, that for those versions of DSL that don't require a splitter on the home end of the connection, it might be necessary to plug in inexpensive filters for every telephone in the house. This helps reduce the noise that telephones can introduce into the wire, when you take them off the hook, for example. In addition, these filters remove the higher-frequency signals used by the DSL connection before they reach the telephone, because some users are otherwise able to hear a very high-pitched whine resulting from the high-frequency data communication. Without filters, you might find that a previous modem dial-up line no longer works after you've installed a DSL modem. In this case, installing a filter between the modem and the telephone faceplate should also solve this problem.