DSL. Digital Subscriber Line. A general term for any local network loop that is digital in nature; technically, DSL equates to ISDN BRI, but this is decreasingly enforced terminology. DSL technology is available in several varieties. See also ADSL, HDSL, IDSL, RADSL, SDSL, VDSL, xDSL.

• ADSL. Asymmetric Digital Subscriber Line. A term for one-way T1 transmission of signals to the home over the plain old, single twisted-pair wiring already going to homes. ADSL modems attach to twisted pair copper wiring. ADSL is often provisioned with greater downstream than upstream rates (hence "asymmetric"). These rates are dependent on the distance a user is from the central office and may vary from as high as 9 Mbps to as low as 384 Kbps.
• HDSL. High bit-rate Digital Subscriber Line. The oldest of the DSL technologies, HDSL continues to be used by telephone companies deploying T1 lines at 1.5 Mbps and requires two twisted pairs.
• IDSL. ISDN Digital Subscriber Line. IDSL provides up to 144-Kbps transfer rates in each direction and can be provisioned on any ISDN capable phone line. Unlike ADSL and other DSL technologies, IDSL can be deployed regardless of the distance the user is from the central office.
• RADSL. Rate Adaptive Digital Subscriber Line. Using modified ADSL software, RADSL makes it possible for modems automatically and dynamically to adjust their transmission speeds. This often allows for good data rates for customers residing greater distances from the CO.
• SDSL. Single-line Digital Subscriber Line or Symmetric Digital Subscriber Line. A modified HDSL software technology, SDSL is intended to provide 1.5 Mbps in both directions over a single twisted pair. However, the distance over which this can be achieved is less than 8,000 feet.
• VDSL. Very high-rate Digital Subscriber Line. The newest of the DSL technologies, VDSL can offer speeds up to 25 Mbps downstream and 3 Mbps upstream. Similar to SDSL, the gain in speed can be achieved only at short distances. These maximum speeds can be achieved only up to 1,000 feet. Sometimes also called broadband digital subscriber line (BDSL).
• xDSL. A generic term for the suite of digital subscriber line (DSL) services, where the "x" can be replaced with any of a number of letters. See also DSL, ADSL, HDSL, IDSL, MDSL, RADSL, SDSL, VDSL.

DSU/CSU. Data Service Unit/Channel Service Unit. The devices used to access digital data channels are called DSU/CSUs (Data Service Unit/Channel Service Units). At the customer's end of the telephone connection, these devices perform much the same function for digital circuits that modems provide for analog connections. For example, DSU/CSUs take data from terminals and computers, encode it, and transmit it down the link. At the receive end, another DSU/CSU equalizes the received signal, filters it, and decodes it for interpretation by the end-user.

DS1/DS3. Like T1 or T3 connections.

• T1. A digital transmission link with a capacity of 1.544 Mbps. T1 uses two pairs of normal twisted wires, the same as found in most residences. T1 normally handles 24 voice conversations, each one digitized at 64 Kbps. But, with more advanced digital voice encoding techniques, it can handle more voice channels. T1 is a standard for digital transmission in the United States. T1 lines are used to connect networks across remote distances. Bridges and routers are used to connect LANs over T1 networks. There are faster services available. T1 links can often be connected directly to new PBXs and many new forms of short haul transmission, such as short haul microwave systems.
• T3. A T3 line consists of 28 T1 lines or 44.736 million bits per second (commonly referred to as 45 Mbps). A T3 line can handle 672 voice conversations. T3 runs on fiber optic and is typically called FT3. See also T1.