T1 - T3 Service

T1 technology is proving to be a cost-effective means of linking voice and data, both inter-office and intra-office, and serves as an alternative to high speed modems for data transport. There is significant discussion these days about "T1 Gateways" and "T1 trunks" as the cost from the various phone companies of these services goes down. Users are discovering that it costs less to have a T1 trunk than a series of leased telephone lines in a point-to-point topology. This increase in the use of T1 requires a fundamental understanding of the technology.

T1 is a high speed digital network (1.544 Mbps) developed by AT&T in 1957 and implemented in the early 1960's to support long haul pulse code modulation (PCM) voice transmission. The primary innovation of T1 was to introduce "digitized" voice and to create a network fully capable of digitally representing what was up until then, a fully analog telephone system.

T3 (which is a combination of 28 T1 lines) operates at 44.736 Mbps and T4, operating at 274.176.  These are known as "supergroups" and their operating speeds are generally referred to as 45 Mbps and 274 Mbps respectively.

It is described as a "two-point, dedicated, high capacity, digital service provided on terrestrial digital facilities capable of transmitting 1.544 Mb/s. The interface to the customer can be either a T1 carrier or a higher order multiplexed facility such as those used to provide access from (fiber optic) and radio systems."

So in the basic definition there is the discussion that there is a "higher order" or hierarchy of T1.  There is T1 which is, as we have discussed, a network that has a speed of 1.544 Mbps and was designed for voice circuits or "channels" (24 per each T1 line or "trunk"). In addition, there is T1-C which operates at 3.152 Mbps. There is also T-2, operating at 6.312 Mbps, which was implemented in the early 1970's to carry one Picture phone channel or 96 voice channels.