ABSTRACT
T-l (also spelled Tl), means ‘Tier 1.” It represents the original transmission rate (1.544 Mbps) of a high speed, copper-wire based, Pulse Code Modulation (PCM) and Time Division Multiplexing (TDM) digital network for voice transmission called the T-Carrier system (T stands for trunk). It was developed by AT&T starting in 1955, was demonstrated (somewhat disastrously) in 1958 and suc cessful trials finally took place in 1961. Commercial T-l service debuted on July 25, 1961. The Bell System then began the arduous task of converting their Frequency Division Multiplexing (FDM) trunks to TDM trunks. The digital T-l and the T-carrier system improved the signal/noise ratio on multi-line phone trunks and eventually completely replaced the backbone of what for nearly 100 years had been a fully ana log telephone system. T-l was used internally by the phone companies to interconnect Central Offices (COs) until it was made available to public individual subscribers as a tariffed point-to-point dedicated (“on all the time”) service in 1983, thanks in part to the break up of AT&T also occur ring at that time. AT&T first offered T-l service under the brand called the High-Capacity Terrestrial Service, then renamed it ACCUNET around 1985 and offered additional functions such as network cross-connect reconfiguration capability. Today you can order T-l service under the AT&T’s trademark of “ACCUNET” Southern Bell’s “MEGALINK,” and other brand name telco services.
