"Rethinking the Access Network"
To deliver high-speed data services, service providers need a new approach to the access network. Broadband serving areas may be the next step.
RANDY SHARPE
Residential access networks, designed primarily for voice services, experienced remarkably little change during much of the last century. Today, however, new factors are having the most profound effect on the access network since the introduction of loop electronics in the 1970s. The Internet, the accompanying demand for high-speed access and the technologies developed to meet that demand, compel network planners to rethink conventional access network design rules.
Limitations of the existing network--particularly the data performance constraints with twisted pair--for addressing the needs of the future are becoming increasingly evident to those involved in the public network. To address these limitations, network planners must reconsider the access network, shifting emphasis away from traditional definitions of carrier serving areas (CSAs) and into the emerging concept of broadband serving areas (BSAs).
Evolution of the access network
Loop plant design has varied over the years. Today's network is the result of a myriad of engineering practices that directly affect the ability to deliver high-speed data services such as asymmetrical DSL (ADSL). Approximately 15% of access lines have load coils preventing their use for ADSL services. Revised resistance design rules have been popular with network planners for central office (CO)-served access lines since ISDN was introduced in the mid-1980s. Approximately 90% of unloaded access lines comply with these rules. However, not all revised resistance design access lines adequately support ADSL services.
The layout of loop plant also has changed over the years. In the 1960s, network planners followed a design plan known as the serving area concept. Under this concept, access lines served from a CO were divided into several serving areas, with each area containing a single interface point back to the CO. After the introduction of carrier systems in the 1970s, network planners developed a CSA design plan for carrier system-fed access lines.
As network planners began moving to the CSA approach, digital loop carriers (DLC) became the equipment of choice for service providers.
A 1991 Bellcore survey found that more than 60% of DLC loops met CSA guidelines. However, CSA compliance does not guarantee a satisfactory level of ADSL performance. Table 1 summarizes the revised resistance design and CSA loop plant design rules.
CSAs often are divided into one to six distribution areas (Figure 1). A DLC system's remote digital terminal provides telephony services to subscribers within the CSA. Cable pair groups feed a single feeder distribution interface in each distribution area where cross-connections are made.
Historically, deployment of DLC systems was predicated primarily on an economic analysis of the relative costs to reinforce or replace feeder cables vs. the cost to install a DLC system. Several factors influenced the decision to deploy a DLC system, including the distance from the switch, the nature of the switch interface (universal or integrated, concentrated or non-concentrated) and the cost, size, reliability and power consumption of the terminal equipment.
These factors have led many service providers to deploy DLC systems, causing the economic prove-in distance from the CO to decline. Consequently, the percentage of access lines served by DLC systems is increasing. DLC systems now represent approximately 40% of all access lines, and the percentage is increasing.
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