T1: A Survival Guide

Chapter 3. Basic Digital Transmission on Telephone Networks

The dead govern the living.

—Auguste Comte

As mentioned in Chapter 1, T1 is a time-division multiplexed stream of 24 telephone calls. Each call is carried by a 64-kbps digital stream called a DS0. Several meanings are ascribed to the acronym DS; you may hear any combination of the words data, digital, service, stream, speed, and signal. DS0 is the bottom rung of the T-carrier hierarchy. Higher levels of the hierarchy are built by multiplexing lower levels together. Understanding the T-carrier hierarchy starts with understanding DS0 transmissions.

AT&T’s initial digital leased-line offering was called the Digital Dataphone Service (DDS). DDS was offered at several different speeds, ranging from 2,400 bps to 56 kbps. Service initially topped out at 56 kbps because a portion of the signal is required for timing overhead. DDS circuits formed the Internet backbone in December 1969. Traffic growth eventually overwhelmed the limited-circuit capacity, and the Internet backbone was upgraded to T1 circuits in the late 1980s. Mushrooming traffic led to a further network upgrade to T3 in the early 1990s. Eventually, economics conspired to kill DDS as a standalone service. T1 is not much more expensive, so companies that required more throughput, higher reliability, and guaranteed service levels shifted to T1, while budget-conscious users migrated to cheaper technologies such as DSL.^[7]

Introduction to DS0

The telephone ...