Electrical Sonet testing

As fiber optic transmission systems began to proliferate in the mid- to late-1980s, forward-thinking standards organizations joined forces to ensure that the technology would be implemented in an orderly and standardized fashion. The result of their efforts-the synchronous optical network standard known in the U.S. as Sonet and internationally as synchronous digital hierarchy-defines optical carrier levels and their electrically equivalent synchronous transport signals, providing a straightforward, common infrastructure for both current and future implementation of fiber optic telecommunications.

The logical structure of the Sonet hierarchy is a boon for maintenance and installation personnel: Simply access the information embedded in the signal and you have the data you need to identify payload types and accurately pinpoint errors. Access, however, has been one of the main challenges.

Many organizations cannot afford to invest in equipment designed to access optical signals, particularly in the early stages of Sonet deployment. And although Sonet rings invariably coexist with traditional point-to-point infrastructure, using test equipment designed only for existing networks also would lose many of Sonet's advantages, such as the capability to extract a low-speed signal without demultiplexing the entire signal.

Fortunately, almost all the most critical overhead information needed to effectively troubleshoot a Sonet circuit can be found and readily accessed at an electrical rather than optical level, enabling some economical and practical approaches to Sonet testing.

Sonet network devices The basic building block of a Sonet network is the synchronous transport signal, Level 1 (STS-1). This electrical signal is equivalent to the basic OC-1 optical signal.

Higher-order signals are formed by byte-interleaved multiplexing of these lower-order signals-and because the interleaving is the only signal processing required apart from the necessary optical-to-electrical conversion, the low-speed signals remain visible even at higher-order transmission rates.

Thus, any STS-1 signal can be extracted directly from the STS-3, OC-12, OC-36 or OC-48 level.

Similarly, the payload of each STS-1 signal is composed of multiplexed virtual tributary signals containing lower-speed asynchronous signals. Differing VT sizes are defined to accommodate different types of signals within the STS-1 format.

The 27-byte VT1.5 frame, for instance, accepts a DS-1 signal; a 1028-byte VT-6 frame contains mapping of a DS-2 signal; and DS-3 signals are mapped directly into the payload envelope.

This architecture makes switching in a Sonet environment much less complicated than in earlier, asynchronous DS-1/DS-3 based networks, in which all tributary channels had to be demultiplexed before any switching operation. In a Sonet network, only the channels requiring switching need to be extracted (Figure 1). This is done using an add/drop multiplexer.

The ADM can add, drop or cross-connect tributary channels at any Sonet rate, dropping and inserting only the desired channel and passing remaining traffic through the network element undisturbed.

A Sonet digital cross-connect system performs a similar function, typically at a Sonet hub, and it may be used to interconnect a much greater number of STS-1s than an ADM.

Terminal multiplexers for combining DS-1/DS-3 and Sonet signals onto a single Sonet bearer, and repeaters for long-distance transmissions, also may be part of a typical Sonet network.

Within the Sonet frame The frame structure of an STS-1 signal is divided into two distinct, readily accessible areas: the synchronous payload envelope and the transport overhead.

The synchronous payload envelope is the area of the signal that carries the data for customer services. It and the payload it contains can be transported and switched without having to be examined or demultiplexed at intermediate nodes. In an STS-1 signal, the payload typically transports 28 DS-1s or one DS-3.

Of particular importance for testing purposes are transport overhead bytes, since they contain critical information regarding the performance of the Sonet circuit on several levels, including the section, line and path.

Section overhead provides the diagnostic information needed to support and maintain the transportation of the synchronous payload envelope between regeneration points such as repeaters. Functions in this layer include framing, error monitoring, data communications and a local orderwire for point-to-point voice communication within the section.

Line overhead addresses the synchronous payload envelope's movement between adjacent network nodes and includes information for monitoring individual STS-1s, line alarm indications and far-end receive failure indications.

Path overhead addresses the end-to-end Sonet circuit between termination points such as ADMs and channel service units, and it includes remote alarm indicators as well as information on framing.

The appearance of an error condition in one or more of these overhead bytes helps the technician determine not only the severity of a problem, but also its probable location. Specific functions contained in these bytes enable effective in-service testing of a Sonet network by passing on failure indicators between sections, lines and paths.

For example, a loss of signal, frame or pointer would cause alarm indication signals to be transmitted downstream (Figure 2). Maintenance signals would then be sent upstream in response to the alarm indication signals.

After a line alarm indication signal, loss of signal or loss of frame is detected, a far-end receive failure message would be sent upstream in the line overhead. Remote alarm indicators would be sent upstream in the path overhead in response to path alarm indication signals. And finally, in the VT path, a remote alarm indicator would be sent upstream in the VT path overhead.

The Sonet overhead bytes provide a wealth of information that allows the technician to reconstruct the sequence of alarms leading to the failure and thus to pinpoint its source. This information is useful for the maintenance technician troubleshooting in-service lines or for the installation technician performing routine adds, drops and changes.

Real-world testing All that remains is to readily access the Sonet overhead bytes. This can be a difficult proposition in the optical environment, where individual elements of the signal cannot be easily extracted without disturbing the rest of the circuit. Introducing splitters, which allow access, involves expensive components and requires great care in installation. It also introduces the potential for signal degradation.

When the Sonet signal is converted from the optical to the electrical environment, however, testing becomes much simpler. None of the signal information is lost because there has been no intermediate signal processing.

Testing at this level can be accomplished using familiar methods and commonly encountered in-service monitoring points. And because it does not need to be designed to cope with the delicacy of optics, the test equipment itself is an economical investment, easy-to-maintain and built for rugged, frequent use.

Sonet rings typically meet the existing infrastructure through a DCS and ADM at central office sites (Figure 3). Most of these DCSs include access to the STS-1 signal through an asynchronous transport signal cross-connect 1 (STX-1) point, which can be inexpensively added to existing DS-3 bays using cabling, connectors and hardware.

The STX-1 point allows simple access to overhead byte information for precise fault location. Technicians can get as much or as little information about those overhead bytes as they need.

Many overhead byte functions translate directly into messages or LED indicators that are both common and familiar-loss of signal or loss of pointer LEDs, for instance-and should be available on most transmission test sets. Where greater detail is needed, full displays of overhead bytes at all levels can permit in-depth analysis.

A number of traditional tests can be conducted from the STX-1 access port to broadly verify circuit performance. For example, a full loopback can be performed to the DS-1 or DS-3 network interface unit to verify performance not just of the Sonet circuit, but of its demultiplexed elements on the DS-1 or DS-3 service. This is useful when an STS-1 circuit is being delivered to the customer and is broken into multiple DS-1s or a DS-3 before it is handed off.

Selecting different channels for transmitting and receiving STS-1 enables verification of mapping from one STS-1 port to another on a DCS. When transmitting and receiving are set at different rates, it is also possible to test across both the high- and low-speed ends of a multiplex (Figure 4), and to verify correct mapping of a tributary signal.

Most Sonet terminals include built-in optical diagnostics that continually monitor the health of the optical circuit. But connection to the customer premises occurs at the electrical level.

A technician equipped with both STS-1 and DS-3/DS-1 capabilities is better prepared to pinpoint the location of a fault within any of the common digital circuits delivered to the customer.

Because accessing optical circuits is so complex, most optical test sets may have little application on a day-to-day basis. Routine add/drop activity does not require a testing approach built around optical access.

The technology for electrical testing, however, is affordable. It is similar to traditional transmission testing, making the learning curve shorter so that technical staff can be more effective more quickly.

Electrical test sets often are designed with hand-held packages and battery operation to simplify usage for field technicians. The meshing of STS-1, DS-3 and DS-1 electrical interfaces in a single set allows the field technician to continue testing traditional asynchronous digital circuits in addition to the growing number of Sonet circuits.

By making electrically-based testers available to every technician, and by providing a shared optical tester for installation purposes, an operation may achieve the optimum mix of test capabilities. Relying extensively on electrically-based testers also provides a significant cost savings compared with investing strictly in optical test technology.

Ray Chong is Director of Marketing-Asia for Sunrise Telecom Inc., San Jose.