16.1 The History of DSL and Telephony

DSL technology and telephony have been intimately associated from the invention of the very first digital line protocols. The ISDN basic rate interface (ISDN BRI) was developed to carry both digital voice and data (at rates between 64 Kbps and 128 Kbps) over the same phone line to environments such as residences, small businesses, and larger business sites using Centrex-type services. T1 carrier systems were originally created to provide digital trunk lines between telephone company facilities. Their use in what may have been the first "digital subscriber line" service to business customers for providing both private voice and data services to business customer premises was originally seen as providing telephony services. The ISDN primary rate interface (ISDN PRI), which provides (in North America) twenty-three 64 Kbps channels for voice or data, is an extension of the T1, explicitly to provide combined voice and data to larger businesses. HDSL (high-speed digital subscriber line), the first of the technologies to be given the title DSL, was developed to provide improved and lower cost support for T1.

As illustrated in Figure 16.1, ADSL supports both traditional analog telephony and a DSL connection over the same loop. ADSL was specifically developed to support broadband access to a personal residence. By working on the same loop as a traditional analog POTS line, ADSL allows the conservation of loops in a telephone company's outside plant. This is often a scarce resource, especially in residential areas. Additionally use of the same loop for broadband access and traditional voice services means that in many cases the same wiring at the customer's premises can be used for both services. Current practice in North America is to use "in-line" filters (sometimes known as microfilters) to protect the voice and data services supported on the same inside wire pair from interfering with each other. Because these filters, which attach to the user 's telephone equipment, can be installed by most residential users, operational costs for the telephone company are considerably reduced.

Figure 16.1. ADSL multiplexed with POTS.

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In Europe, especially Germany, ADSL service is provided on the same loop as basic rate ISDN. Using the functionality provided in ITU-T G.992.1 Annex B [3], the ADSL physical layer (PHY) is transported on the same loop as the ISDN BRI. As with "ADSL over POTS," the voice service and the ADSL service are separate; they share the same physical media in the outside plant but are provided by separate networks that are operated independently. In the case of both ADSL over POTS and ADSL over ISDN, the two services are frequency multiplexed on the same facilities. The POTS or ISDN operates at the same low frequencies (up to 4 kHz for POTS and up to about 100 kHz for ISDN) that they use when the loop does not also support ADSL. The ADSL operates at frequencies above 26 kHz in the case of POTS and 138 kHz in the case of ISDN. The carrier's networks supporting the broadband and voice services are entirely separate once the two frequency bands are separated in the central office (CO). Except for the addition of splitters or microfilters, the user continues to receive their phone service identically to how they received it before they had the ADSL service installed.

In the United States, the FCC has mandated line sharing since mid-2000. When line sharing is implemented, the incumbent telephone company provides the loop for both the telephone service and DSL and also provides POTS service to the customer. The DSL service on the same loop can be provided by a competitive provider, who leases only the ability to carry DSL above the voice band . Thus, two separate services provided by separate companies can be provided over the same loop.

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