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Optical/IP

Testing Time For Digital Wrappers

A key enabler of G.709, formerly known as "digital wrappers", emerged yesterday when Agilent Technologies Inc. (NYSE: A) announced the first test gear capable of checking compliance with the new standard (see Agilent Unveils G.709 Test Set).

ITU-T G.709 OTN, to give it its full name, is part of a wider group of standards that will enable what's called the Optical Transport Network (OTN). It's billed as "the next major evolution in the transport network," according to Agilent.

In a nutshell, the OTN is a new multiplexing scheme designed for optical crossconnects and optical add-drop multiplexers. It allows low-speed channels to be extracted from Sonet payloads without having to convert the signal back into the electronic domain, explains Ronnie Neil, product marketing manager for Agilent's telecom networks test division.

This is achieved by having a very rigidly-defined frame structure, making it possible to know exactly which time slots in the frame contain a particular tributary data stream. Those time slots can then be switched onto a different pathway by an optical switch fabric, such as a MEMS mirror or inkjet bubble.

"Once you can identify the start of a frame, then you can identify every lower rate channel within that frame, says Neil.

The bottom line is that G.709 eliminates the need for a lot of multiplexing equipment and thus is likely to win widespread acceptance by carriers.

The G.907 portion of the standard was ratified by the International Telecommunication Union (ITU) in February 2001 (see ITU Approves Optical Standards), and the first chips that support it have already appeared on the market (see, for example AMCC Intros 10-Gig 'Digital Wrapper').

Initially G.709 was called "digital wrapper", a term reportedly coined by Lucent Technologies Inc. (NYSE: LU). But that has now been dropped in favor of the moniker OTN, in keeping with the industry's penchant for giving everything a meaningless and totally impervious acronym.

G.709 also contains bells and whistles for network monitoring. In addition, Forward Error Correction is an integral part of the standard. This adds an encoding overhead, which can be used to correct errors at the receiving end of the link. The end result is to make it possible to send signals further between regeneration points (see the new beginner's guide on Digital Wrappers and Forward Error Correction (FEC) for more details).

Agilent's tester can generate a G.709 compliant signal, with or without errors for use in bit error rate testing, or it can receive a signal and then check it for compliance. The tester, called OmniBER OTN, is available to order now, will start shipping in February 2002, the company says.

Given the importance of G.709, all major network test vendors will be developing gear that checks compliance with the standard, including Acterna Corp. (Nasdaq: ACTR), Anritsu, and Tektronix Inc. (NYSE: TEK). Agilent is not likely to not be sole occupier of this particular test equipment spot for long.

— Pauline Rigby, Senior Editor, Light Reading
http://www.lightreading.com
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