The United States Bell System activated the first commercial digital carrier system in 1962 in Chicago, Illinois, where electrical noise from high-tension lines and automotive ignitions interfered with analog systems. The system was designated T1, with the T standing for Terrestrial to distinguish the land transmission from satellite transmission. (Bell Laboratories also launched Telstar I, the first communications satellite, in 1962.) T-carrier was designed for a four-wire twisted-pair circuit, although the DSX-1 interface is medium-independent, i.e., can be provisioned over any of the transmission media, at least at the T1 rate of 1.544 Mbps. At the T3 rate of 44.736 Mbps, twisted pair is unsuitable over distances greater than 50 feet due to issues of signal attenuation. As the first digital carrier system,T-carrier set the standards for digital transmission and switching, including the use of pulse code modulation (PCM) for digitizing analog voice signals. (Note: T-carrier uses the Âµ-law (mu-law) companding technique for PCM.) T-carrier not only set the basis for the North American digital hierarchy, but also led to the development of E-carrier in Europe and J-carrier in Japan.The fundamental building block of T-carrier is a 64-kbps channel, referred to as DS-0 (Digital Signal level Zero). Through time-division multiplexing (TDM), T-carrier interleaves DS-0 channels at various signaling rates to create the services that comprise the North American digital hierarchy, as detailed in Table T-1.
Table T-1: North American Digital Hierarchy: T-carrier
|Digital Signal (DS) Level||Data Rate||64-Kbps Channels (DS-0s)||Equivalent T1s|
|DS-1 (T1)||1.544 Mbps||24||1|
|DS-1C (T1C)||3.152 Mbps||48||2|
|DS-2 (T2)||6.312 Mbps||96||4|
|DS-3 (T3)||44.736 Mbps||672||28|
|DS-4 (T4)||274.176 Mbps||4,032||168|
|DS-5 (T5)||400.352 Mbps||5,760||250|
See also analog
, fractional T1
, Âµ-law, signaling rate
, and transmission rate
. See also digital signal hierarchy
for a side-by-side comparison of the North American, European, and Japanese digital hierarchies.