Book excerpt: Legacy Technology: All About T1 This module provides detailed information on the carrier system technology called T1. T1 used to be a principal subject of telecommunications courses, but is now relegated to the back of the book, as T1 is a technology running at 1.5 Mb/s on copper wires, replaced with Gigabit/s Optical Ethernet on fiber. That said, there are thousands of T1 circuits installed and in use, and some readers of this book have picked it precisely to learn about T1 because they have been tasked with supporting it, or auditing an existing installation. Power companies, the military and government still have T1s in place, along with T1s at big organizations that are no longer being used, but have been forgotten and are still being paid for each month... a small but appreciated part of any phone company’s revenue. This chapter on T1 also provides a very detailed explanation of synchronous Time-Division Multiplexing, framing and channels. Since the principles of operation of legacy SONET fiber-optic transmission systems are the same as T1, learning about T1 is also learning about SONET. Telecom Module 18 Detailed Outline 18 Legacy Technology: All About T1 18.1 T1 History and Applications 18.2 T1 Circuit Components 18.3 Operation 18.4 T1 Framing ..... 18.4.1 Superframe Format ..... 18.4.2 ESF 18.5 Pulses and Line Code: AMI ..... 18.5.1 Repeaters 18.6 Synchronization: Bit-Robbing 18.7 56 kb/s for Data 18.8 B8ZS and 64 kb/s Clear Channels 18.9 How T1 Is Provided ..... 18.9.1 HDSL 18.10 Fractional T1, DACS and Cross-Connects 18.11 Subrate Data Circuits 1.2 kb/s to 56 kb/s ..... 18.11.1 CSUs, DSUs and CSU/DSUs