<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> <P> DS3 signals are rare except within buildings, where they are used for interconnections and as an intermediate step before being muxed onto a SONET circuit . This is because a T3 circuit can only go about 600 feet (180 m) between repeaters . A customer who orders a DS3 usually receives a SONET circuit run into the building and a multiplexer mounted in a utility box . The DS3 is delivered in its familiar form, two coax cables (1 for send and 1 for receive) with BNC connectors on the ends . </P> <P> Twelve DS1 frames make up a single T1 Superframe (T1 SF). Each T1 Superframe is composed of two signaling frames . All T1 DS0 channels that employ in - band signaling will have its eighth bit over written, or "robbed" from the full 64 kbit / s DS0 payload, by either a logical ZERO or ONE bit to signify a circuit signaling state or condition . Hence robbed bit signaling will restrict a DS0 channel to a rate of only 56 kbit / s during two of the twelve DS1 frames that make up a T1 SF framed circuit . T1 SF framed circuits yield two independent signaling channels (A&B) T1 ESF framed circuits four signaling frames in a twenty four frame extended frame format that yield four independent signaling channels (A, B, C, and D). </P>

When did t1 and pri stop being used for data transmission