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ADSL Modems

In the implementation and deployment of a high-speed wide area public digital network, the most challenging part is the link between subscriber and network: the digital subscriber line. With billions of potential endpoints worldwide, the prospect of installing new cable for each new customer is daunting. Instead, network designers have sought ways of exploiting the installed base of twisted-pair wire that links virtually all residential and business customers to telephone networks. These links were installed to carry voice-grade signals in a bandwidth from 0 to 4 kHz. However, the wires are capable of transmitting signals over a far broader spectrum—1 MHz or more. The asymmetric digital subscriber line (ADSL) is the most widely publicized of a family of modem technologies designed to provide high-speed digital data transmission over ordinary telephone wire. ADSL is offered by a number of carriers.

Figure 2 depicts the ADSL configuration for Internet access. The telephone central office can provide support for a number of ISPs, each of which must support the ADSL modem technology. At the central office, the ISP data signal is combined with a voice signal from the ordinary telephone voice switch. The combined signal can then be transmitted to/from a local subscriber over the subscriber line. At the subscriber's site, the twisted pair is split and routed to both a PC and a telephone. At the PC, an ADSL modem demodulates the data signal for the PC. At the telephone, a microfilter passes the 4 kHz voice signal.

Figure 2 ASDL modem application.

The data and voice signals for ADSL are combined on the twisted-pair line using frequency-division multiplexing (FDM) techniques. There are two elements of the ADSL strategy:

  • Reserve the lowest 25 kHz for voice, known as POTS (plain old telephone service). The voice is carried only in the 0 to 4 kHz band; the additional bandwidth is to prevent crosstalk between the voice and data channels.

  • Allocate two bands—a smaller upstream band and a larger downstream band. Use FDM within the upstream and downstream bands. In this case, a single bit stream is split into multiple parallel bit streams and each portion is carried in a separate frequency band.

Table 2 compares the performance of the various modems discussed above with each other and with ISDN access, which uses a digital signaling technique.

Table 2 Speeds for Internet Access Methods

Access Method

Upload Speed

Download Speed

Download Time (10 megabit file)

Dial-up modem

33.6 Kbps

56 Kbps

3 minutes

ISDN basic rate (two channels)

128 Kbps

128 Kbps

1.3 minutes


16 to 640 Kbps

1.5 to 9 Mbps

1.1 to 6.7 seconds

Cable modem

400 Kbps

10 to 30 Mbps

0.3 to 1 second

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