Internet Access: DSL

DSL is most commonly known as a digital subscriber line, or digital subscriber loop. DSL is a way to connect to the Internet using the regular telephone line without interrupting the telephone dial tone signal, unlike dial-up connections, which utilize the same part of the telephone line as telephones do and require the dial tone to ‘dial’ up to an Internet phone number.

DSL ranges in download speeds from 256 kilobits per second to 24,000 kilobits per second. The download speed varies depending on the technology you are using, the conditions of the line and the service level implemented on your phone line. ADSL, or asymmetric digital subscriber line has a slower upload speed than download speed and symmetric digital subscriber lines have equal upload and download speeds.

DSL Internet carries different frequencies than are required for voice transmissions over your telephone line. Telephone lines have been using the same basic set up for over a century and the lines that are running, a pair of copper wires to each house, can carry much more bandwidth than is used for a telephone voice conversation. DSL Internet takes advantage of the unused bandwidth in the regular telephone wires to transmit the digital signal, without interfering with the other frequencies and transmissions being carried out on the same telephone line.

As with all technology, DSL has both advantages and disadvantages.

Some of the advantages of DSL include:

The ability to leave your Internet connection open while using the phone line for regular voice telephone calls.

DSL provides a higher speed than a regular dial-up modem.

You can use the phone line that already exists in your home and you don’t necessarily need new wiring.

You usually get the equipment you need, such as the DSL modem from the company that offers the DSL Internet access.

Some disadvantages of DSL include:

The closer you are to the provider’s central office, the better your DSL connection works.

It’s faster to receive data on DSL than it is to send it over the same line.

DSL service isn’t available everywhere.

Perhaps the most important factor to consider when deciding on DSL Internet access is to find out if DSL is available in your area. In more rural and remote areas, DSL is not available, and you may be stuck using a dial-up modem or satellite Internet. If DSL is available in your area, then it’s safe to say that the choice between DSL and dial-up is an easy one to make. DSL has come down a lot in price with competition rearing its head all over the country and world. No longer is DSL too much more expensive than dial-up and the DSL modem that is required is more readily available to consumers.

The biggest limitation to DSL Internet access and why it’s not available everywhere is because of the distance limitations. As stated in the ‘disadvantages’, the closer you are to the central office of the company that provides your DSL service, the better your signal quality will be. The overall limit for ADSL service is approximately 18,000 feet. The further away you are, the lower your connection speed will be. You may be wondering why the distance limitation exists for DSL Internet access but not for voice calls if DSL and telephone voice calls use the same lines. The reason for this lies in the small amplifiers called loading coils that are installed in the telephone lines to boost the voice signals.

These loading coils are incompatible with DSL signals, so if there is a loading coil in the line between your location and the central office of the telephone company, you can’t receive DSL.

Other limitations for DSL are:
Bridge taps—extensions between you and the telephone company central office that extend service to other customers.
Fiber-optic cables—DSL can’t pass through the conversion of analog to digital and then back to analog if a portion of your telephone circuit is on a fiber optic cable.

You won’t know if distance is a limitation for you until you contact your local phone company and find out. Phone companies don’t advertise their central office locations, so until you call to find out if you qualify for DSL from the phone company, there’s really no other way to tell.

There are two basic standards for DSL Internet access—discrete multi-tone (DMT) and carrier-less amplitude/phase (CAP). Most DSL equipment installed uses DMT while the earlier DSL systems use the CAP system.

The CAP system divides your telephone line into three bands—voice conversations, upstream and downstream. Voice conversations are carried in the band that is for zero to four KHz (kilohertz), upstream channels (from you to the server) is carried in the band for 25 to 160 KHz and downstream (from the server to you) begins at 240 KHz upward to a maximum of approximately 1.5 MHz, depending on various conditions. The three channels in the CAP system are spread apart widely to minimize the interference with other channels on the line or between signals on different lines.

The DMT system divides the signals into separate channels as well, but doesn’t use only two broad channels for upstream and downstream data. DMT divides data into 247 channels which are each 4 KHz wide. Each of these 247 channels is monitored and the signal shifts to other channels if the quality on one channel is impaired. The DMT system shifts signals around to search for the best channels to use for transmission and reception.

When you install your DSL in your home, you are given small filters to attach to the phone outlets that go to your telephones and to lines that don’t give a signal to your DSL modem. These filters are ‘low pass filters’ that block out all signals above a certain frequency. These filters make it so that the DSL signals don’t interfere with your voice frequency on your phone lines—the filters block out everything above the standard 4 KHz voice conversation signals.

DSL Internet access requires equipment on two ends—the user end and the service provider’s end. At the user end there is a DSL transceiver and on the service provider’s end there is the DSL Access Multiplexer, or DSLAM.

The transceiver on your end is also known as a DSL modem. This is where your computer is connected to the DSL phone line. Most DSL modems connect to your computer through a USB port or 10 base-T Ethernet connections. Of course, other than the DSL modem, you can also have a network router, network switch or other networking equipment on the same circuit as your DSL modem.

The DSLAM is the equipment on the ISP’s end that allows DSL to take place. The DSLAM takes connections from many DSL customers and compiles them into a single connection to the Internet with a high-capacity connection.

Other than this basic equipment, you need a regular phone line running to your home. This phone line will be the basis for your DSL connection to the Internet. In most cases, your regular phone line will do to connect you to the Internet using DSL connection. You will also need the small low pass filters for the outlets that you connect your telephones to so that the DSL frequencies don’t interfere with your voice conversations on your telephone line.

ISDN stands for Integrated Services Digital Network. An ISDN is a circuit-switch telephone network system, such as the one that you have running to your home, that allows for digital transmissions of voice calls and data over the telephone’s copper wires. ISDN technology allows for better quality signals and higher speeds.

The ‘integrated services’ is ISDN’s capabilities of allowing the delivery of at least two simultaneous connections in a combination of voice, video, fax and data over a single line—such as the ability to transmit a voice phone call and your DSL Internet connection at the same time.

‘Digital’ simply means that ISDN is a digital transmission, instead of analog transmissions as was used in POTS (plain old telephone service). When you connect to the Internet with an ISDN there’s no conversion of analog to digital signals, which allows for clear transmission quality.

‘Network’ means that the ISDN works from your local telephone exchange to each remote user and includes all of the switching and telecommunication equipment in between the two points.

As you know by now, DSL technology uses your existing telephone lines. DSL speeds can range from 256 Kbps to 1.5 Mbps. Cable Internet connections transmit your Internet signal over coaxial cable that also sends your television signals. Cable modems are a separate interface that connects to the coaxial cable where it comes into your home and then connects to a network interface card your computer.

Cable modems can offer a higher data transfer rate that can go up to 10 Mbps. The biggest problem with cable modem technology is that it is ‘shared technology’ which means that you share the service connection with all the other users in the neighborhood who have a connection and use it at the same time. This shared connection can result in slow connection service, especially when lots of people are using the service and are downloading large files.

Another method of Internet connection is satellite connections. This is a good option for people who can’t access DSL or Cable Internet connections and still want something faster than a standard dial-up connection. The satellite Internet connection will allow for a similar download link that is equivalent to that of a cable modem, the downside is that satellite dishes can’t send uplinks so you must use your telephone to make uplink commands. There is also a lag of about 2 seconds for everything you want the Internet to do which can make Web browsing slow and irritating.

If you are using a DSL connection and modem, it’s important to take some extra steps to protect your computer. The type of connection that you use to connect to the Internet, a standard phone line, is easy for hackers to find and access your computer through the connection. Therefore, if you are using a DSL connection with your computer it is excessively important to install and use a firewall at all times.

A firewall is a system installed on your computer that prevents unauthorized access to your private network connection. A firewall can be implemented in software or hardware, or a combination of both. The firewall on your computer examines the messages sent to and from your computer and examines them—the firewall then blocks the messages that don’t meet specific security criteria. There are different techniques for firewalls, which are:

Packet filters: Each packet entering or leaving your network is examined and either accepted or rejected, based on rules defined by the user.

Application gateway: Security mechanisms are applied to specific applications such as FTP servers.

Circuit-level gateway: Security mechanisms are applied to TCP or UDP connections that are established. When these connections are made, packets can flow through between the hosts without any further checking by the firewall.

Proxy server: A proxy server intercepts all of the messages that enter or leave your network and hides the true network addresses.

Most firewalls use two or more of these techniques together to establish further safety nets for your computer.

It is relatively easy to choose when you are faced with the variety of choices for Internet connections. DSL is better than a dial-up connection, however in some applications and places, cable connections are better than DSL, however the shared type of technology employed by cable connections make DSL still a better bet to ensure your transmission quality to maintain an adequate speed at all times. Satellite technology is great if you don’t have access to cable or DSL connections, but if you have the choice, DSL is still the way to go, because you will be required to use your regular phone line to uplink to the Internet with a satellite connection.