Chapter 6. Mysteries of theCSU/DSU

A man is as old as his arteries.

Pierre J. G. Cabanis,Epigrams

When bringing up a T1, a CSU/DSU is required by FCC part 68 to protect the telco network from your equipment. If the CSU/DSU does not function correctly, whether due to component failure or misconfiguration, the line will not come up. Unfortunately, producing clear, understandable printed documentation has never been a goal of data communications equipment vendors. CSU/DSU manuals have improved to the point of basic intelligibility, but even the best manuals require a solid understanding of telco networking.

Line Build Out: Moving BetweenTheory and Practice

In a perfect theoretical world, wires have no resistance and voltage pulses can travel forever. The real world, however, is rarely anything like the world of theory. (There is a reason that physicists joke about spherical frictionless cows, after all.) Repeaters and signal regenerators along T1 spans are the first admission of a practical world. Line build out (LBO) is the second concession to practicality.

Two different types of connections are used on T1 lines. A long-haul connection is made to the telco network. The first repeater may be up to 3,000 feet away from the end-user location. CSUs must be able to drive the pulse up to 3,000 feet, so the arriving pulse at the first repeater is still strong enough to be regenerated. Long-haul line build out is used to avoid problems that arise from having multiple T1 customers in an ...

Get T1: A Survival Guide now with O’Reilly online learning.

O’Reilly members experience live online training, plus books, videos, and digital content from 200+ publishers.