T1: A Survival Guide

Preface

In the past, T1 links formed the backbone of the telephone network in the U.S.; they were later used as the data circuits for the Internet backbone. Now, T1 is the standard for medium-speed access to the Internet. Widespread adoption is not to be confused with technical excellence, though. T1 is not elegant, efficient, easy to use, or even all that well suited to the needs of data transmission, largely because its development is intimately tied to the U.S. telephone system architecture of the late 1960s.

In spite of T1’s limitations, no competing technology has presented a significant challenge. T1 is proven, reliable technology that currently fills the need for medium-speed Internet access for institutions of many sizes, especially those not in metropolitan areas. Newer technologies require widespread availability of fiber and extensive regional networks, and the population density in rural areas is not well suited to the expense of building large fiber networks. For Internet access off the beaten path, traditional leased lines are the only solution.

In spite of its age, T1 technology is often learned by apprenticeship, filing away little troubleshooting tidbits until they may be needed again. For technology older than many of its users, this is an inexcusable state of affairs. As more networks use T1s, more engineers will require practical information on how to set up, test, and troubleshoot them. This book is intended to be a practical, applied reference for networkers interested in using T1 as a data transport. It covers the following broad topics:

• How the history of the telephone network led to the development of T1 and how T1 was adapted to the needs of data transport

• What components are needed to build a T1 line, and how those components interact to transmit data effectively

• How to adapt the T1 physical layer to work with data networks by the use of standardized link layer protocols

• How to troubleshoot problems that arise, including how to work with all the vendors: the telephone company, equipment manufacturers, and Internet service providers

Audience

This book maintains a blatant, and sometimes overwhelming, focus on the U.S. In many ways, this concentration is a result of the fact that the T-carrier system was developed at Bell Labs and deployed by AT&T. European digital carrier systems took a different direction (and came out with a better result, in some respects). Nevertheless, this book is not likely to be useful for engineers who do not deal with American digital carrier systems.

This book approaches T1 from the fairly narrow perspective of data networking. Although T1 can be used for voice or video transport, or as a flexible network that can respond to changing demands, this book treats it as simply another type of network link. Many of the examples deal with Internet access explicitly, although several of these could easily be applied to private leased-line networks. Network engineers must frequently deal with T1 for a variety of reasons: it can be used to provide a single pipe for Internet access or 24 voice circuits for modem aggregation, or even as the distribution layer or core of a far-flung WAN.

Coordinating T1 networks involves a large cast. Telco employees are responsible for providing the T1s. ISPs may order T1s on behalf of their customers and support them directly with backup by the telco. Small businesses with a need for medium-speed Internet access now frequently turn to T1 technology to fill that need. In recognition of the popularity of T1 as a data transport, most network equipment vendors now offer interface cards with internal T1 CSU/DSUs. Engineers supporting T1 interfaces must be familiar with details as well as the “big picture” so that they can guide new T1 users through the steps of troubleshooting and contacting carriers and service providers to bring up T1s.

T1 may not be glamorous, but it fills a need for medium-speed, high-reliability Internet connectivity, and nothing is likely to displace it from that role in the near future.