Carrier serving area plan
The carrier serving area (CSA) plan calls for each feeder route to be marked so all loops less than 12 kft use twisted pair and can carry analog or wideband (ISDN or xDSL) signals with no special conditioning. Beyond 12 kft from the wire center, the loop plant is divided into CSAs. Each CSA is structured so that all customer loops in it are less than 9 kft from a location where a remote terminal (RT) can be housed. Service from the RT to the wire center is provided over a digital carrier. A digital loop carrier (DLC) system is deployed at the RT site. The DLC is essentially a channel bank that also performs other functions, such as providing a source of ringing and test access to the copper loops. From the remote site to the subscriber, the loop is twisted pair. Because the length is less than 9 kft, there is no need for load coils.
This plan was developed to support 56 kbps and T-1 service, but also supports ISDN BRI transmission and more recently, DSL transmission. The design works well for telephony, but is less suitable for ISDN and DSL services. In particular, DSL is a high cost proposition for the telephone company to deploy. To be successful, there must be a reasonable number of customers with high probability of signing up for the service to justify the deployment cost.