Home Technology Networking Dial-up, DSL and Cable Modems

Dial-up, DSL and Cable Modems Print E-mail
Article Index
Dial-up, DSL and Cable Modems
Page 2
All Pages


As the internet becomes an everyday necessity for communicating with others and providing entertainment, the speed or the total bandwidth available to a user becomes a greater concern. Currently the last mile loop or the connection between user and Internet Service Provider (ISP) is the slowest part of the internet connection-connecting user with the content they are trying to access. Several last mile carriers can provide internet connection to users. The slowest option for connecting to the internet is the use of an analog dial-up connection using a modem. Two faster options for connecting the last mile to the internet is to use digital subscriber lines (DSL) and cable internet. These three options vary in the maximum theoretical speed possible, distance a user must be from the internet providers’ central office, and the cost of each option.


Analog dial-up connections through a modem provide the best option for people and businesses that live or work in rural areas and are far from a phone companies central office or do not have access to cable internet access. The central office of telephone exchange is where telephone connections occur and where DSL connections terminate into a digital subscriber line access multiplexer (DSLAM). Dial-up connections allow for a maximum possible connection speed of 56 Kbps. The actual speed of the connection is limited by the distance between two modems and the quality of the phone lines. Dial-up modems work by using the 3000 hertz of bandwidth available through a phone line to send an analog single which is converted back to digital by a modem on the other end of the line. While modern modems can receive at speeds up to 56 Kbps, they can only reach this speed in perfect conditions. Real-world environments rarely enable a modem to connect at speeds greater than 48 Kbps.

An advantage of modems is their incorporation of gradual degradation. Gradual degradation is used between two modems as they send data to each other. While sending data the modems test the phone line to determine if the current speed will allow the modems to remain the current speed. If the error rate is high, the modems will slow the connection down to compensate for line noise or other problems with sending and receiving the signals. Under most conditions slowing the connection speed will sufficiently compensate for line noise, however, if line noise is too high modems will disconnect and reconnect with the hope of using another circuit that is better quality and has less line noise. If the connection appears to be good quality, modems will increase the connection speed.

Current modems used in desktop computers today and first introduced by USRobotics are Winmodems. What differentiates a Winmodem from a traditional modem is how much work required for modulating and demodulating the signals is done with hardware, and how much is done by software. In the last few years as computers have become faster they are now able to do tasks in real-time that once required specialized hardware to be done in real-time. Winmodems work with the Microsoft Windows operating system and perform many of the tasks traditionally done with hardware modems in software. The result of using Winmodems is fewer computer chips are required to build a modem and are cheaper to produce than traditional modems.

Early Winmodems were plagued with problems ranging from buggy drivers and slowing down computers. Since their main steam adoption, computers have increased in speeds to compensate for extra CPU cycles required to operate the newer modems and modem drivers have matured eliminated most problems. People today notice little difference between using a Winmodem and a traditional hardware modem. The difference in price between the two types of modems is noticeable. Winmodems cost less than $15 while it is difficult to find a traditional hardware modem.

Asymmetric Digital Subscriber Line (ADSL) is an alternative to using a dial-up modem. The ‘D’ in ADSL is actually a misnomer because ADSL still uses an analogue connection to the central office just as traditional modems do except ADSL uses higher symbol rates and faster handshaking between modems. There are other variations of DSL such as SDSL (symmetric DSL), however; ADSL is currently the most popular variation in the US. Unlike ADSL, SDSL does use a digital signal and a router for its connection but requires its own copper telephone line while ADSL is able to share the same telephone lines used for voice telephone service. With a standard modem, a user must connect before they can use the internet however with DSL the user is always connected and does not have to dial a phone number first.

DSL uses copper twisted pair wires just as regular telephone lines and ADSL typically shares the same wires voice communication uses. Normal telephone service often referred to as POTS (Plain old telephone service) uses the frequency range between 0 and 3,400 Hertz. Copper lines used for POSTS can handle a significantly higher range of frequencies, which transfers to how much data is possible over the wires. DSL uses some of the unused frequencies not used by POTS to transfer data. Because DSL has a larger amount of frequencies available to it, DSL is able to send and receive more data than a standard modem can that is limited to the 0 to 3,400 Hz frequency range.

There are two different standards for ADSL, Discrete multitone (DMT) and carrierless amplitude/phase (CAP). The most commonly used standard today is DMT. DMT works by splitting signals into 247 separate channels, each 4 KHz wide. POTS uses a single 4KHz channel to send voice signals so ADSL has the same number of channels available to it as 247 modems, this is the reason why a regular modem using POTS is significantly slower than ADSL.

While SDSL typically has the same upload and download bandwidth, ADSL has a relatively slow upload bandwidth and a faster download bandwidth. Downstream rates begin at 256 kbps and usually go up to 1.5Mbps however speeds can go as high as 6.0 Mbps in the US and other parts of the world have variations of ADSL that can reach 52Mbps. Upload rates begin at 64 kbps and typically go as high as 256 kbps however it is possible for ADSL to go up to 768 kbps.

The biggest limiting factor as to how fast an ADSL line can go is the distance between the central office and other end of the line. The closer a user of ADSL is to the central office the faster the speed is possible due to attenuation as the distance increases. Downstream bandwidth is only possible if a user is 100 meters from the central office, which means they must be virtually next to the central office to see speeds close to 52Mb/s. For nine Mbps speeds, a user must be 1000 feet from the central office. For ADSL to be a possible option at any speed a user must be 5,460 meters from the central office in ideal conditions, however due to quality of most telephone lines most DSL providers will only provide service to users well within 5,460 meters. It is worth noting the distance between the central office and the user is not the distance as the crow flies but rather how the telephone lines are setup. Based on a map a user can be 500 feet from a central office however depending on how telephone lines run in the neighborhood the total length of telephone line between central office and user could significantly more. While a dial-up connection using a standard modem is available virtually anywhere in the word with a standard telephone line, DSL is not available everywhere, noticeably in rural areas where there is significant distance between user and the closest central office. DSL is primarily only available in large cities and well-populated suburbs.

Amplifiers called loading coils are the reason why voice signals can travel over far distances in rural areas to a central office, however because loading coils are not compatible with ADSL, signals lines with loading coils cannot be used to transmit signals for DSL. Loading coils work by filtering out the same high frequency ranges ADSL uses to transmit data.

The use of fiber-optic cables also prevents users from using ADSL to access the internet. Telephone companies use fiber-optic cable to carry several voice signals at the same time. Signals sent through fiber-optic cables are digital signals sent with pulses of light while ADSL signal is analog. Since ADSL signals cannot convert to digital signals back to analog, users living in areas where telephone companies use fiber-optic cables to carry voice signals to the central office cannot use ADSL and must use another option for high-speed internet access.

Cable internet access through a cable modem is another option for high-speed internet access. Cable modems uses the same coaxial cable used for cable TV (CATV) to provide high-speed internet access. Coaxial cable used to provide television signals to a TV is able to carry hundreds of megahertz of signals. Each TV station sent through a coaxial cable only uses a 6 MHz channel so even with 100 stations fed through the same cable there is still a large range of frequencies left for other uses such as internet access. Cable internet access uses spare 6 MHz channels to download data and uses 2 MHz to upload data using an analog signal.

Internet data sent through the cable line looks the same as TV signals. When the internet signal reaches the CATV tuner, the modulated internet signal goes to a demodulator that can be either an external cable modem or a modem inside the computer.

Cable internet access works in either of two ways depending on the company providing the internet access. The first way it to use one set of frequencies generally between 42 and 850 MHz for downloading and another set of frequencies in the 5 to 42 MHz range for uploading. A different method some companies use is to only use the cable line for download traffic and use a dial-up modem using POTS for upload traffic.>

One of the advantages of cable internet access is while DSL requires a straight connection of copper twister pair wires; cable can use any combination of mediums to transmit the signal from user to the central office. Many cable companies use fiber optics to connect neighborhoods together and only use copper coaxial cable to run from users’ homes to the street where the fiber-optic cable runs. Unlike DSL, distance is not a major factor determining if a user can get cable internet access. In most places if a user is able to receive cable TV they are also able to receive internet access through the cable connection.

DSL connections are dedicated connection while cable connections are shared connections. A single user is connected to each line running from their ADSL modem to the central office while a cable internet user shares the coaxial line with up to 1000 other users. As a result, a DSL user will always experience the same connection speed from their ADSL modem to central office while a user with a cable modem will experience different speeds depending on how many people are using their cable modem on the same local loop. While cable internet access can operate at up to 40 Mbps, it is possible to a user to experience internet speeds no faster than a dial-up modem using POTS due to multiple users being required to share the total bandwidth available. Users who are one of the first to connect to a cable internet local loop will experience significantly faster internet speeds than a DSL user however as more people start using the cable internet local loop each users will experience a decrease in internet bandwidth available to them.