The synchronous optical network/synchronous digital hierarchy (SONET/SDH) standards for optical telecommunication networks are the most comprehensive means of implementing transport infrastructures. The increased configuration flexibility and bandwidth availability of SONET/SDH provides significant advantages over older telecommunication systems. However, as the data capacity of SONET/SDH fibre optic telecommunication networks increases, equipment manufacturers, network operators and end consumers would experience significant losses if a system were to fail.

Testing the signals that go into the network and analysing the signals that come out becomes ultimately important. The so-called “tributary rate” signals (e.g., optical OC-12/STM-4 and electrical DS-1 and E-1) must be tested to ensure that the signals conform to standards before they are distributed on the broadband network.

A key element of tributary signal testing is testing the equipment’s physical layer. The physical layer is the lowest layer in the open systems interconnection (OSI) network model.

In this article, we’ll discuss how to use a digital phosphor oscilloscope (DPO) to perform physical layer testing of tributary signals. The measurements discussed highlight some of the oscilloscope’s capabilities for design debugging, verification of compliance with industry standards, and signal characterization.