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Table Of Contents  CertiGuide to A+ (A+ 4 Real)
 9  Chapter 13: Basic Networking Terminology
      9  Modem, IDSN, DSL and More

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DSL (Digital Subscriber Line) or xDSL is a general name of the group of related technologies, which today are actively advanced and continuously developed in many countries of the world. These technologies designed for high-speed transfer on existing telephone wires allow by the cheapest way to solve a problem of last mile separating the consumer from the supplier of services.

DSL promotes distribution of high-speed Internet, and creation VPN (Virtual Private Network), that is corporate network, which basis is Internet or other public network. The data in such network pass through the special tunnel inside the usual channel and are accessible only to participants of the virtual private network.

The speeds of transfer in different technologies DSL vary and also some technologies function asymmetrical, they have different speeds for sending and receiving, but as DSL advances as service for access in Internet, asymmetric property of the traffic is not large problem, as in the basic users of Internet receive from a network much more data, than transfer.

The DSL connection is constantly active, that is similar to the dedicated line. Besides, the low prices and the high speeds of transfer have resulted that these services become more and more popular. Some technologies DSL use frequencies lying outside of a range of voice communication that enables to use the voice traffic with the data traffic through the one pair of telephone wires. For the purpose splitters are used – devices of division frequencies of a signal on a vote and data.

However and this perfect technology has lack for multi-service users - absence of an opportunity of Quality of Service, that complicates creation of voice, video and data networks on the basis of the given technology.

Table 34: Common DSL Technologies and Attributes




Simultaneous Voice and Data

Where to use

(High-rate DSL)

1,544Mbps (2pairs) 2,048Mbps (3 pairs)

100 - 3000 m


Instead of T1/E1 for Internet; LAN & PBX;

(Symmetrical DSL)

1,544Mbps or 2,048Mbps (1pair)

100 - 7000 m


Instead of T1/E1 for Internet; LAN & PBX; VoIP

(Asymmetrical DSL)

1,544 – 8,448 Mbps for downstream and 640 - 1,544 Kbps for upstream

100 - 5000 m


Internet, Intranet, VPN, video on demand, VoIP

(Very-high-rate DSL)

12,96-51,84Mbps for downstream and 1,6 – 2,3 Mbps for upstream

100 - 1500 m


Access to multimedia resources; HDTV

From the equipment for creation of DSL networks, it needs a usual telephone line POTS and DSL modems on both ends of a line. Sometimes, when the point to multipoint connection is required DSLAMs are used this device combining many DSL modems in one case. Such devices usually used by Internet providers. They place them directly on PBX for communication with the clients. All DSLAMs connect among themselves with the help of backbone and routers. A network of modern DSL ISP looks thus.

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