Editing T-carrier (section)

==Digital signal cross-connect==
[[Digital Signal 1|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 supply power to line or "Span" signal repeaters, and T1 NIU's (T1 Smartjacks). T1 span repeaters are typically engineered up to {{convert|6000|ft|m}} apart, depending on cable gauge, and at no more than 36&nbsp;dB of loss before requiring a repeated span. There can be no cable [[bridge tap]]s or Load Coils across any pairs.

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&nbsp;Mbit/s T1 over a single copper wire pair up to approximately twelve thousand (12,000) feet (3.5&nbsp;km), if all 24 [[American wire gauge|gauge]] cable is used. HDSL-2 does not employ multiple repeaters as does conventional four wire HDSL, or newer HDSL-4 systems.

One advantage of HDSL is its ability to operate with a limited number of bridge taps, with no tap being closer than {{convert|500|ft|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.

DS3 signals are rare except within buildings, where they are used for interconnections and as an intermediate step before being multiplexed onto a [[Synchronous optical networking|SONET]] circuit. This is because a T3 circuit can only go about {{convert|600|ft|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 [[coaxial cable|coax]] cables (1 for send and 1 for receive) with [[BNC connector]]s on the ends.<ref name="ANSI T1.403">ANSI T1.403</ref><ref>ANSI T1.231</ref><ref>ANSI T1.404</ref><ref>ANSI T1.510</ref>