It seems like technological innovation is moving at a breakneck speed in these modern times. For example, the growth in the interconnection of personal computers, terminals and telephones in the business environment has been increasing exponentially. 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.

T1 is a high speed digital network developed by AT&T in 1957. T1 was put into use in the early 1960's to support long-haul pulse-code modulation voice transmission. The primary innovation of T1 was to introduce "digitized" voice and to create a network fully capable of digitally representing what was once an entirely analog system.

In the basic definition of T1there is a hierarchy of services. Each tier of the hierarchy is designated by a speed in Mbps. There is T1 which is simply a network with a speed of 1.544 Mbps and was designed for voice circuits or "channels. 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 Picturephone channel or 96 voice channels. There is T-3, operating at 44.736 Mbps and T-4, operating at 274.176 Mbps. These are known as "supergroups" and their operating speeds are generally referred to as 45 Mbps and 274 Mbps respectively.

For mathematical reasons, a voice channel was selected to be at 64 Kbps. Twenty four of these channels is a composite of 1.536 Mbps, not 1.544 Mbps. Why is there a difference? The reason is that after a byte of data is sent from each channel there is an extra bit used for synchronizing called a Frame bit - hence 193 bits are sent and this increase of 1 bit per 192 causes the speed to increase to 1.544 Mbps.

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.

Perhaps the way to really begin this discussion is to discuss the AT&T Digital Carrier System referred to as "ACCUNET T1.5". 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."

A convenient way to think of T1 is from the International Standards Organization and Open System Interconnect model: the Physical and Logical layers. The Physical layer focuses on the electrical characteristics such as signal shape, voltage levels, etc. The logical layer deals primarily with the format issue.

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