The number of additional wire pairs that may be served by applying some technology to existing telephone lines (wire pairs). For example, with the installation of digital loop carrier equipment at the telco office and at the customer site, one cable pair can be turned into two phone lines. Two cable pairs can be turned into many more lines using T1 and other technologies. Every channel bank, digital loop carrier and DSLAM is a pair gain system. See channel bank, digital loop carrier and DSLAM.