T3 and E3 Technology
As the technology in telecommunications advanced, so did the requirements for customer circuits. As more and more businesses and users were connected to the infrastructure, it became apparent that even DS1 and E1 circuits would no longer be sufficient. Higher bandwidth circuits were required to allow for growth and to provide more services to subscribers. DS3 carrier systems, just as DS1 carrier systems, allow for a level of pair gain.
NOTE Remember, pair gain is the function of gaining pairs by deploying multiple circuits over a time-division multiplexing (TDM)-based circuit such as DS1 or E1. Because two pairs can serve 24 or 30 circuits, you are said to be gaining pairs when you are actually just avoiding having to deploy them.
DS2 is an intermediary step that is covered in the "M12 Multiplexing" and "E12 Multiplexing" sections later in this chapter. T2 and E2 were developed as the second digital hierarchy level but were hardly ever used. Particularly in the U.S., T2 saw only developmental and internal service provider deployments. This step is important because although not used, it is included within the T3/E3 framework. Just as you would multiplex DS0s into a T1 or E1, you multiplex a number of T1/E1s and T2/E2s into a T3 or E3, respectively.
This chapter discusses the framing, line coding, and operation of the T3 and E3 carrier systems. Within each section, the deployment of these circuits is analyzed and accompanied by configuration examples. Furthermore, there is a brief discussion of the higher levels within each digital hierarchy.
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