T1: A Survival Guide

Chapter 2. T1 Architectural Overview

The world hates change, yet it is the only thing that has brought progress.

—C. F. Kettering

One of the complaints that many data-networking veterans have when venturing into the telecommunications world is the bewildering number of acronyms and strange terminology that await them. Before diving into small details about different components of a T1, some background with the technology is essential. This chapter introduces the terms and basic structure of a T1 circuit so that successive chapters can delve into detail on the most important components.

Telecommunications Puzzle Pieces

Figure 2-1 shows a high-level diagram of the link between an Internet service provider (ISP) and a customer, delivered over an archetypal T1 circuit. Due to the number of components required to build a T1, Figure 2-1 is divided into four parts. In the classic case, two wire pairs, one each for reception and transmission, combine to form a T1. Devices called repeaters are used to allow for transmission of the signal over long distances. Signals degrade as they travel along the path, but repeaters recover the digital input data and retransmit the digital data at full strength. In theory, this allows for perfect transmission of data because a digital signal can be perfectly recovered and an exact bit-for-bit copy sent on to the destination. Repeaters divide the circuit into a series of spans, or paths between repeaters.

As technology marched on, the technology ...