T1 and the T Carrier

In the 1960s, the Bell System introduced digital technology into its network. By then the network’s connections had outstripped the abilities of analog technology. The amassed noise of millions of connections, thousands of miles of trunk and hundreds of thousands of switches had created an endless and expensive burden of maintenance and upgrading.

Analog Transmission Basics

Because voice communications was the first service carried, analog modulation of information onto a simple twisted wire pair was enough to do the job.  But as electrical energy travels down the wire pair, the degrading effects of simple physics pile up. In telecommunications, these effects show themselves as noise.

Every component of any network contributes its own noise into its environment.  The wire itself, and every connection and switch, contribute a minuscule amount of thermal noise through the mechanics of their own resistance. Wires act as an antenna and attract added noise (static or cross talk).  Because electromagnetic signals dwindle (attenuate) over distance, amplifiers installed at regular intervals upheld signal strength. Not only do amplifiers produce their own noise, they also amplify everything that has come to them from upstream, including the amassed noise that is undistinguishable from any other signal the amplifiers receives.  After nearly 100 years of growth, the original Bell network was literally defeating its own purpose.
   

The Evolution to Digital

T-Carrier technology enabled the first successful digital voice transmission.   Originally developed to address the problems of noise accumulation, T-carriers allowed engineers to increase call capacity and improve call quality.  T-carrier was important for many reasons:

• By being the first digital transmission medium, it was able to fix standards for digital switching, transmission, and modulation techniques.
• The transmission rate was 1,544,000 bps.
• The T-carrier technology defined rules of design and for subsequent telecommunications products, including those of competitive equipment manufacturers.

The quality of transmission improved dramatically, and efficient use of plant increased dramatically.  A four-wire twisted strand could now carry 24 simultaneous conversations.

The Movement to End Users

Over time, digital technology has migrated further downstream, so digital voice and accompanying digital features are available to all but the most remote end users.  To promote this migration, so-called Individual Case Basis (ICB) tariff arrangements were introduced to deliver digital capacities further and further down the line from the Central Offices.  Technical upgrades to outside plant traveled this same path to the end user. Moving from retrofitting digital components into all Central Exchanges, to replacing amplifiers with digital repeaters, the Bell network has evolved into a fully digital telecommunications facility.

         Analog transmissions use amplifier spacing of 18,000 feet.  The higher frequencies used in digital transmission need amplification every 2,000 to 6,000 feet (depending on frequency) to ensure good call quality. Since digitized information is not vulnerable to noise accumulation or interference, the increased number of amplifiers this calls for does not degrade signal quality.  The first use of T-Carrier service was for digitized voice, but data has become the dominant application for this technology.