<P> In the 1970s, Bell Labs developed higher rate systems . T1C with a more sophisticated modulation scheme carried 3 Mbit / s, on those balanced pair cables that could support it . T - 2 carried 6.312 Mbit / s, requiring a special low - capacitance cable with foam insulation . This was standard for Picturephone . T - 4 and T - 5 used coaxial cables, similar to the old L - carriers used by AT&T Long Lines . TD microwave radio relay systems were also fitted with high rate modems to allow them to carry a DS1 signal in a portion of their FM spectrum that had too poor quality for voice service . Later they carried DS3 and DS4 signals . During the 1980s companies such as RLH Industries, Inc. developed T1 over optical fiber . The industry soon developed and evolved with multiplexed T1 transmission schemes . </P> <P> DS1 signals are interconnected typically at Central Office locations at a common metallic cross-connect point known as a DSX - 1 . When a DS1 is transported over metallic outside plant cable, the signal travels over conditioned cable pairs known as a T1 span . A T1 span can have up to + - 130 Volts of DC power superimposed on the associated four wire cable pairs to line or "Span" power line repeaters, and T1 NIU's (T1 Smartjacks). T1 span repeaters are typically engineered up to 6,000 feet (1,800 m) apart, depending on cable gauge, and at no more than 36 dB of loss before requiring a repeated span . There can be no cable bridge taps or Load Coils across any pairs . </P> <P> T1 copper spans are being replaced by optical transport systems, but if a copper (Metallic) span is used, the T1 is typically carried over an HDSL encoded copper line . Four wire HDSL does not require as many repeaters as conventional T1 spans . Newer two wire HDSL (HDSL - 2) equipment transports a full 1.544 Mbit / s T1 over a single copper wire pair up to approximately twelve thousand (12,000) feet (3.5 km), if all 24 gauge cable is used . HDSL - 2 does not employ multiple repeaters as does conventional four wire HDSL, or newer HDSL - 4 systems . </P> <P> One advantage of HDSL is its ability to operate with a limited number of bridge taps, with no tap being closer than 500 feet (150 m) from any HDSL transceiver . Both two or four wire HDSL equipment transmits and receives over the same cable wire pair, as compared to conventional T1 service that utilizes individual cable pairs for transmit or receive . </P>

When did t1s and pris stop being used for data transmission ​