Using the Phone Line for Datathe DSL Way

Modems have a great purpose in the world of networking. Almost everywhere that you might put a PC, there's probably already a phone line nearby. Using a PC with a modem allows almost every computer the capability to connect to the Internet.

Modems do have some drawbacks, with one of the biggest having to do with talkative teenagers, roommates, or other family members. Modems use the phone line to make what the telco thinks is a voice call, so you can't make another call while you're surfing the Internetor, in other words, you can't use the Internet with your modem while your teenager is using the phone! Another big negative is the speed. Although 56 Kbps for a single user might seem fairly fast, it's not fast enough for some people. Today, e-mails often have attached files with them; those e-mails might take a fairly long time to pull down onto a computer. Also, websites often have tons of graphics and animation today, all of which require a lot of bandwidth.

What the world needs now is a faster way to send and receive data using that same old ubiquitous phone linewhile being able to make a simultaneous phone call. What the world got was exactly that, and it's called digital subscriber line (DSL). DSL is an alternative technology for sending and receiving data to and from an ISP, using the same old phone line, but running at much faster speeds. Also, you can make phones calls and surf (send IP packets) at the same time. But, of course, it's a little more expensive.

Dr. Analog Voice and Mr. Hiding Digital

Dr. Jekyll and Mr. Hyde had a serious split personality issue in the classic book by Robert Louis Stevenson. Dr. Jekyll used advanced chemistry to create the split. DSL creates a split personality of sorts for the local phone line (local loop) using some advanced technology. DSL allows the same old analog voice signal to be sent over the line by a phone; at the same time, DSL allows a separate digital signal to go over the same phone line.

The telephone generates analog signals at frequency ranges between 0 and 4000 Hz. Most speech ranges from 300 Hz to 3300 Hz in frequency. The folks who created DSL defined the standards so that the digital DSL signal uses frequencies higher than 4000 Hz, which means they do not interfere with human speech or hearing. To appreciate what happens, take a look at Figure 16-6.

Figure 16-6. DSL Connection from the Home

The figure shows an analog phone and a DSL modem connected to a single wall plate with two receptacles. (Most people's home phones plug into a wall plate with just one receptacle; to use DSL, you can buy a little plastic device that will create two receptacles for about $5 retail at most office supply stores.) Physically, you connect your DSL modem to a wall socket just like any of the phones in your house. You can pick up your same old phone and make a call, just like always. Simultaneously, the DSL modem can send data over the same phone line to the router in the ISP network, as shown in the figure.

DSL uses the same local loop wiring that's already run between the CO and your house, but now the CO connects the local loop wiring to a device called a DSL access multiplexer (DSLAM). The DSLAM splits out the digital signal and analog signal from the local loop. The DSLAM gives the analog voice signalthe frequency range between 04000 Hzto a telephone switch. The voice switch treats that signal just like any other analog voice line.

Conversely, the DSLAM does not pass the digital electrical signal to a telephone switch. The DSLAM forwards the data traffic to a router owned by the ISP that provides the service in this figure. Notice that the ISP's router is actually depicted as being resident in the local telco's COthat is actually true in many cases. To support DSL, the ISP works with the local telco to install some of the ISP's gear in the COa process called co-location, or co-lo for short. The local telco DSLAM forwards and receives the IP packets to and from the ISP router, while the telco maintains control over the local voice traffic.

If you can get some binary digits from a PC, through a DSL modem, to the DSLAM, and finally to a router at an ISP, you can get IP packets to and from that router. Incidentally, DSL uses PPP as the Layer 2 protocol. Likewise, because DSL can get an analog electrical signal from the phone, over the local loop, to the DSLAM, and then to the telco switch, you can make voice calls.

Interestingly, with DSL, the ISP is often not the same company that provides local telephone service. Typically, the consumer requests DSL high-speed Internet access from an ISP, the ISP charges the customer for the service, and then the ISP pays the local telco some cut of the fee. You still need to pay for the local phone services as well. You end up with two services, basicallyphone service and Internetand you pay two bills, assuming that you get the Internet service from someone besides the local telco.

Faster Is Better

With all the types of physical networking standards covered in this book, there have been options for different speeds. Even Ethernet has several options. With WAN options, there are many more variations for speed, and DSL is no exception.

DSL standards come in many flavors to meet different needs. (Most of the standards come from the ITU, which also owns most of the modem standards.) For instance, DSL has limits on how long the local loop can be. (The length of the local loop is simply the length of the combined cables that stretch from a house to the CO.) Some DSL variants allow the local loop to be much longer than some other DSL variants, whereas others only allow for a shorter local loop. The distances tend to range from less than a mile to 45 miles. For the standards with a shorter local loop, the transmission rates tend to be much higher, which is a simple design tradeoff.

The speeds for DSL can go as high as 50 Mbps, but you must have a short local loop to the CO. The slower DSL speeds are approximately 384 Kbps, but that allows you to have a much longer local loop.

Interestingly, the speed at which data goes from the Internet to the home is faster than the other directiona feature called asymmetric transmission rates. As it turns out, a lot more data goes toward a PC at home than in the reverse direction for most TCP/IP applications. For instance, when you ask for a web page, the HTTP request might be a few hundred bytes, but the web page might be millions of bytes of data. So, rather than send at the same speed in both directions, it's better to have more bandwidth toward the home, where most of the bandwidth is needed.