What is ADSL

 ADSL is an abbreviation for Asymmetric Digital Subscriber Line, and is a technology for high-speed data communication using general telephone lines (metallic cables).

 The frequency band used to carry phone voice is low frequency, below 4 kHz. Telephone lines have the ability to transmit higher frequencies, and ADSL communicates using a high frequency band that is not used for voice.

 ADSL is a type of xDSL technology. xDSL is a technology that transfers data at high speed by using a frequency band higher than voice, and there are types such as ADS, SDSL, HDSL, and VDSL depending on the data transfer method.

 ADSL, as the name suggests, is an asymmetric digital subscriber line, with different upload and download speeds. Downlink has higher transmission capacity than uplink.

 This is because when using the Internet in general, downloads are used much more often than uploads.

 In addition to ADSL, there are other broadband connections such as FTTH and cable TV, but FTTH and cable TV require indoor wiring work and installation of a dedicated modem, making it a little more difficult to introduce than ADSL. or costly.

 ADSL has the advantage of being easy to introduce, as telephone lines can be used as is, wiring work is simple, and service charges are low.

 Therefore, ADSL is widely used as a broadband connection that can be easily used. Also, compared to FTTH and cable TV, the speed was slow, but high-speed ADSL has appeared, and it can achieve a download speed of 50 Mbps and an upload speed that is much faster than conventional ADSL. Some can go fast.

The biggest advantage of ADSL is that you can get a high-speed broadband environment at a reasonable price.

How ADSL works

An ADSL connection configuration example is shown below.


 ADSL uses telephone lines. , there are both low-frequency signals for phone calls and high-frequency signals for Internet communications on the phone lines. Therefore, it is necessary to separate the signal used for telephone and the signal used for ADSL. A splitter is a device that performs this separation.

*Some cases do not require a splitter. A splitter may be installed in the equipment that collects telephone lines in apartments and condominiums.

- ADSL modem

 ADSL modems modulate computer digital signals to analog signals, and demodulate telephone office analog signals to computer digital signals. A phone line is used to connect the ADSL modem to the splitter. Longer telephone lines can cause noise and slow down communication speeds.

DSLAM (DSL Access Multiplexer) and BAS (Broadband Access Server)

A DSLAM that bundles xDSL is installed in the telephone office. Connect to the Internet via BAS.

 ADSL modems and BAS communicate using PPPoE and PPPoA, which encapsulate PPP. By doing so, it is possible to identify each user individually, assign an IP address, or assign them to the ISP they have a contract with.

 FLET'S ADSL uses notation similar to e-mail addresses for user authentication, and collectively processes the connection destination provider and the provider's user name.


 ADSL uses telephone lines. , there are both low-frequency signals for phone calls and high-frequency signals for Internet communications on the phone lines.

 Telephone voice transmission uses the low frequency band below 4 kHz, and data communication uses the high frequency band that is not used for voice above 4 kHz. It is necessary to separate the signal used for telephone and the signal used for ADSL. A splitter is a device that performs this separation.

 The connectors on the splitter are all RJ11 and connect the phone line. Connect the ADSL modem side of the splitter and the ADSL modem. Also connect the telephone side of the splitter and the telephone. Longer telephone lines can cause noise and slower communication speeds, so care must be taken when installing.

 In conventional data communications, a modem was directly connected to the phone line, and the line could not be shared between calls and data communications, but with ADSL, by separating the frequency bands used, data communications can be carried out even during calls. is possible.

ADSL (for DMT)

 In Japan, 1.5 Mbps services based on the G.992.2 (G.lite) standard were the mainstream for ADSL at first, but Yahoo! BB started 8 Mbps services based on the G.992.1 (G.dmt) standard in August 2001. As a result, other companies have started 8 Mbps services one after another, and ADSL speed competition has begun, and now even 50 Mbps services have appeared.

 Here, we will focus on G.992.1 (G.dmt). G.dmt divides the band used every 4MHz into units called bins, and transmits data in units of bins.

Even if one bin is affected by noise, the remaining bins can be used for transmission, so it has a noise-resistant feature.

 Uplink uses a frequency band of 26 kHz to 138 kHz, with a maximum transmission rate of 1.1 Mbps.

 With ADSL, a dedicated ADSL modem is required because the transmission standards and incidental standards differ depending on the service. Be careful when using an ADSL modem.

Drawbacks of ADSL

●Best-effort service

 With ADSL, you can choose a maximum communication speed such as 1.5 Mbps, 8 Mbps, 12 Mbps, 24 Mbps, 40 Mbps, 47 Mbps, 50 Mbps, etc. and make a contract for the service, but the maximum communication speed guarantees that you can always achieve the contracted maximum communication speed. to say that it is nothing.

 The maximum communication speed is only a theoretical value. ADSL may not be able to provide satisfactory communication speeds due to factors such as noise effects and signal attenuation.

●Vulnerable to noise

 ADSL is relatively susceptible to noise, and the signal may be attenuated by crosstalk from ISDN or noise from AM radio. In particular, ADSL with Annex A specifications is extremely affected by ISDN. Indoor telephone lines can also be affected by noise.

●The speed decreases in proportion to the distance from the host station.

 As the distance from the telephone station line accommodating station increases, the communication speed decreases proportionally. This is because the higher the frequency band, the greater the attenuation of the signal.

 Even if you have a contract for a high-speed ADSL service such as 50 Mbps, you will only be able to connect at a low speed if you are far from the receiving station. Before signing up for ADSL, it is necessary to check the distance from the telephone office.

- Slow upload speed

 With ADSL, the uplink and downlink speeds are different. In order to prioritize the downlink communication speed, the downlink communication speed is much faster than the uplink communication speed.

 In other words, the uplink communication speed is slow, so if you frequently upload websites or upload large files such as videos, you should use FTTH (optical fiber), which has the same communication speed for uplink and downlink. It might be better.

●Separate work may be required

 ADSL cannot be used if there is an optical fiber in the middle of the route. Separate work is required. If you are using a digital line (ISDN), you will need to switch back to an analog line, so additional construction costs will be required. At that time, the phone number may change.