Digital Loop Carrier Overview
As urban sprawl continues with new housing developments in suburban and rural areas, distances from homeowners to their COs can exceed RRD and CSA ranges. Building COs to accommodate these distant users is not a good economic option. Therefore, a new class of lower-cost facilities called Digital Loop Carrier (DLC) systems are implemented to multiplex traffic back to the central office.
The motivations for DLC systems are to reduce pressures on existing voice switches, reduce the number of expensive COs, and deploy switching equipment closed to the user. The following section overviews these motivators and the architecture of DLC.
Benefits and Business Rationale of DLC
A more distributed architecture arises from the development of DLC technology. As the number of telephone wires grows, the binder groups become unwieldy. DLC was devised in the 1980s by Bellcore and AT&T to multiplex traffic on the wires in the binder group onto four wires, thereby saving lots of copper; in turn, this means savings in weight and costs. DLC also provides improved operational characteristics due to reduced inventory of discrete lines.
Consider a neighborhood of 24 homes that are served by a common CO. Normally, 24 pairs of wires are connected from the homes to the CO. An alternative approach is to use DLC technology.
Instead of 24 pairs of wires backhauled from the homes to the CO, the 24 pairs are terminated in the neighborhood in a DLC. From the DLC, four wire pairs carry traffic back to the CO using T1 or HDSL, which saves 22 wire pairs. AT&T Bell Labs (now Lucent) improved on this with an SLC-96 product. SLC-96 served 96 homes over five T1s (four active T1s and one spare T1). SLC-96 reduced the need from 96 pairs to 10 pairs. The DLC can be located at pedestals, vaults, telephone poles, or office buildings. Figure 4-2 shows the major components of a DLC connection between a CO and homes.
Figure 4-2. DLC Schematic
In Figure 4-2, Jimmy lives close to the CO, so his local loop directly connects to the CO. Rosie lives far from the CO, so she cannot be directly connected. Instead, her local loop connects to a DLC that multiplexes her loop back to the CO. Mom lives farther out still. She is connected to the DLC as well, but because she lives outside the range of the CSA, she cannot receive advanced services, such as ISDN, without special equipment such as repeaters.
The connection between the CO and the remote terminal can be a T1 circuit but is more likely to be fiber in modern DLC installations. Examples of such systems are the Lucent SLC-2000 and DSC Litespan 2010. Roughly 20 percent of lines in the United States use T1 or fiber DLC systems. The remote terminals are installed in controlled environmental vaults (CEV), which are underground sites, or street-side pedestals. The CEVs are more compact, less costly facilities than COs. In this architecture, a remote switching terminal can switch calls from Rosie to Mom.
Telcos' investment in existing plants is in the hundreds of billions of dollars, with more than $20 billion spent annually on upgrades. Because they are enhancing their phone wires with new DLC technologies, which introduce fiber into the neighborhood, it makes business sense to investigate using fiber-based transmission coupled with phone wire to offer high-speed services.