ADSL

Digital service from your telephone company is commonly called ADSL, for asymmetric digital subscriber line. It is a broadband Internet interface using copper wire, typically sending to the customer at 1.5 to 9 Mbps, and taking from the customer at a slower rate of 16 to 800 Kbps. The lite version is slower.

Asymmetric refers to the common practice of having two transfer rates: a fast one (typically 1.5 Mbps) for information flow into your network (the downlink) and a slower uplink speed (360 Kbps) for data flow out to the Internet. Most users don’t notice the difference, except when they forward a long mail message. (Sending it takes longer than receiving it.) Asymmetric service is cheaper. A variety of speeds are available up to 6 Mbps, and you can order a symmetrical DSL line if you think you need one.

Developed in 1994, ADSL technical capabilities are standardized by organizations that include the American National Standards Institute (ANSI), the European Telecommunications Standards Institute (ETSI), and the International Telecommunications Union-Telecommunications Standards Sector (ITU-T). ADSL technology supports the consolidation of data, video, and voice traffic for transmission over the local loop and provisions QoS (Quality of Service) assurances. ADSL leverages the in-place infrastructure to enable applications such as video security monitoring, interactive television programs, Web exploration, and videoconferencing, and concurrently supports the continuation of telephone conversations or fax transmissions. Officially known as full-rate ADSL, this technology is standardized in ITU-T Recommendation G.992.1. Endorsed by the ITU-T in 1998, this specification also supports utilization of a single terminal interface at the subscriber location.

ADSL is the most widely available option today, because not all homes have cable TV systems, but almost all have telephones. Limits exist here also, however. You must be within three miles of a switching office, and the office must be equipped to handle digital service. If you call your telephone provider, they can tell you if you are a candidate and also send a test signal down your line to see if it is ready to carry the digital signal. Not all of them are ready. Some need to be rebuilt, with voice devices such as bridge taps and load coils removed. Some phone companies are unwilling to spend the labor and will simply turn you down as a DSL customer. It may take a month or more for the DSL modem to arrive . When the DSL modem arrives, you are expected to connect it to your phone system yourself, but that is a simple matter not much more complicated than plugging in another telephone. Then you must add microfilters between your existing phones and the wall outlets to isolate the phones from the digital signal. That is also a simple process, which does not require tools. As with cable modems, you must have an NIC installed in your PC, and then connect that card to the modem. Software that will introduce your computer to the network must then be loaded onto the PC also, just as in any industrial LAN.

ADSL employs a pair of modems or transceivers that are located on either end of the local loop, specifically at the local telephone exchange and at the subscriber site. ATU-R (ADSL Terminal Unit-Remote) refers to ADSL transceivers at subscriber venues. ATU-C (ADSL Terminal Unit-Central Office) refers to ADSL transmission equipment at the local telephone exchange. The ITU-T G.994 Recommendation establishes handshaking procedures for enabling dependable data exchange between ATU-R (ADSL Terminal Unit-Remote) and ATU-C (ADSL Terminal Unit-Central Office) devices.

ADSL implementation requires the installation of equipment that includes a pair of transceivers (or transmitters and receivers) at the subscriber site and the local telephone exchange. This installation serves as the foundation for virtual point-to-point DSL dedicated network connections that provision fast access to high-performance applications via the local loop. At the local telephone exchange, DSL transmissions are redirected to high-speed backbone networking configurations or the public Internet by DSLAMs (DSL Access Multiplexers). DSLAMs also redirect voice calls to and from the PSTN, thereby eliminating the need for time-consuming dial-up operations required by conventional analog telephone modems. In addition to traffic routing and distribution services, DSLAMs support transmission of streaming media from the Internet or other high-speed backbone network to designated DSL subscriber venues.

ADSL downstream speeds depend on the distance of the subscriber site from the local telephone exchange, wire gauge or thickness, and the condition of the inplace wireline plant. ADSL supports downstream rates reaching 8 Mbps (Megabits per second) at 9,000 feet, 6.312 Mbps at 12,000 feet, 2.048 (E-1) at 16,000 feet, and 1.544 Mbps (T-1) at 18,000 feet or 5.5 kilometers. In parallel with other DSL technologies, ADSL is a high-speed always-on digital switching, routing, and signal processing technology that enables voice calls and fax transmission in RF bands between the 300 Hz and 3.4 kHz frequencies and multimedia transmission in the upper frequency range. Conventional voiceband modems compress voice, video, and data into a narrow range of frequencies for supporting information transmission via PSTN service. By contrast, ADSL employs Digital Signal Processing (DSP) for creating high-speed digital channels to optimize bandwidth capacity of copper telephone lines.

ADSL service to a subscriber premise is provisioned via Unshielded Twisted Pair (UTP). Depending on the ADSL solutions available from the local telephone exchange, a POTS or ISDN channel can be used instead of an ADSL channel for information transport on the return path in the upstream direction.

ADSL lines are typically much faster downstream than upstream and can appear to choke when simultaneous up- and downloads are required, as they are in a two-way videoconference.