The local access network today comprises transmission facilities that link customer premises equipment and networks to local-exchange switches. Traditionally, the local access network has included a combination of analog local loops and circuit-based digital transmission facilities.

Incumbent Local Access

Customers with fewer than 12 to 16 lines are served generally by means of analog local loops. Where the customer is located within about 20,000 feet from a central office, the local loops run directly from the customer premises to the central office (CO). Customers located beyond the reach of direct analog connections are served by outside plant cabinets that contain digital loop carrier (DLC) systems. A DLC connects analog loops from customer premises to digital transmission facilities, such as repeated T-1 or E-1 lines that link the DLCs back to the CO.

Customers with more than 16 to 20 lines may be served more economically by means of direct digital transmission facilities into the customer premises, most often with T-1 or E-1 lines. These facilities may be connected directly to digital private branch exchanges (PBXs) or key systems, or they may be dropped off as analog lines by means of a conversion device known as a channel bank.

Competitive Local Access

Access to unbundled network elements, including copper loops, and the ability to co-locate transmission equipment in the COs of incumbents have enabled competitive service providers to offer local phone services using a variety of traditional, circuit-based, access techniques.

For business subscribers requiring more than 20 lines, competitive service providers can lease a T-1 or E-1 access line from the incumbent. These lines may run directly from the customer premises to the competitive service provider’s local switching end office, or they may be groomed into higher-speed facilities such as T-3 or E-3 lines using collocated multiplexing equipment.

T-1 and E-1 access lines generally do not provide a cost-effective solution for subscribers requiring 16 lines or less. To date, competitive service providers have had to serve these subscribers using an individual unbundled loop for each line, with co-located DLC systems to concentrate the traffic onto digital facilities for transport to their local switching end offices.

Emerging VoDSL solutions provide a far more economical solution for subscribers requiring 16 lines or less. VoDSL leverages the high-speed transmission and packet transport of the DSL access network, delivering multiple lines of telephony plus high-speed data over a single unbundled loop. Another advantage of VoDSL is that voice and data traffic can share common asynchronous transfer mode (ATM)–based local network facilities for traffic concentration as well as backhaul to the competitive service provider’s local switching end office. VoDSL can provide a cost-effective solution for competitive local access for as few as four lines of telephony.

Access Network Evolution

The emergence of the packet-based broadband transmission technologies, collectively known as DSL, is having a profound impact on the evolution of the access network. From a pure transmission standpoint, DSL has been around for a number of years: integrated services digital network (ISDN) is, technically speaking, a variety of DSL. But ISDN and other related variants of DSL, such as high–bit rate DSL (HDSL), made use of traditional time division multiplexing techniques, which limited their appeal.

DSL technologies owe their rapid success rate, then, to their exclusive use of packet-based transport, which provides a cost-effective solution for data applications, such as Internet access.

VoDSL leverages the economies of broadband packet transmission in the access network and enables service providers to deliver integrated voice and data services over a common infrastructure. Many industry observers are now coming to the conclusion that the future of the telephony access network lies with packet VoDSL.

Figure 1. VoDSL Architecture for Competitive Service Providers