OTN (G.709) Pocket Guide
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System Activation and Commissioning

WDM System

The last but critical step before turning up the network is to simulate true DWDM traffic and analyze its behavior as signals travel through the fiber, but also through multiplexers, filters and amplifiers. It is important to see what impact transmitting several wavelengths simultaneously will have on the network. Careful testing and monitoring of each wavelength/channel, optical parameters and bit error rate is a must.

A typical WDM point-to-point link

Figure 1: A typical WDM point-to-point link

An optical spectrum analyzer is required to measure a WDM system's total performance, as well as analyze each communication channel separately. The OSA is usually combined with a tunable laser source and a variable optical attenuator.

Test Method


Recommended Equipment

Optical spectrum analysis
The optical spectrum analyzer allows users to measure the power of each individual wavelength in a CWDM or DWDM system. An OSA is also used to measure the individual optical signal-to-noise ratio (OSNR) of every channel or to compare the spectrum of a signal at different stages of a network such as at an EDFA or ROADM.
Tunable laser
A tunable laser source will give the best accuracy for filter characterization when combined with a power meter by emitting only the selected wavelength. It can also be used to emulate a system transmitter when characterizing a ROADM system, for example.
Variable attenuation
During system turn-up or network commissioning, variable attenuation will reduce the power level of the signal. The most common application is to combine a variable attenuator with a bit-error-rate tester (BERT) to verify the power headroom of a transmission system.
Synchronization analysis
A comprehensive wander and network synchronization analyzer supports all standard telecom rates and wander parameters (TIE, MTIE, TDEV); it is also remotely accessible for monitoring applications.

Test Parameters

Before testing can begin, it is important to check that each WDM transmitter and receiver is functioning properly. Once that has been ensured, test parameters must be determined. On the WDM system, specific test parameters will depend on the testing point, which can typically be at the multiplexer/demultiplexer, at the EDFA or at OADM.

Some of the main test parameters that are measured in the WDM system include:

  • Central wavelength: DFB laser must remain within the system channel bandwidth.
  • Channel spacing: Channels must keep proper distance between each other or signal degradation will occur.
  • Power level, flatness and gain: Channels must keep similar power levels, which can be affected by repeated amplification; proper functioning of EDFAs must be assessed to validate each channel’s gain. Total optical power level of the system, if too high, may generate non-linear effects.
  • Signal-to-noise ratio (SNR): EDFA amplification is the major noise contributor (ASE); a low signal-to-noise ratio will generate impairments at the signal reception.
  • Crosstalk (unwanted interactions between optical channels)
  • Wavelength drift over time


Testing the Protocol Layers

As the legacy world of circuit-switched transport integrates increasing volumes of frame-based data traffic, protocol-layer testing conducted during provisioning focuses on validating the interoperability of next-generation SONET/SDH network elements and multiservice provisioning platforms (MSPPs).

Today, ITU-T G.709, commonly known as optical transport network (OTN) or digital wrapper, is providing an efficient way to multiplex different services, including Ethernet, storage, digital video, as well as SONET/SDH over a single optical transport unit (OTU) frame, either OTU-1 at 2.7 Gbit/s, OTU-2 at 10.7 Gbit/s or OTU-3 at 43 Gbit/s.

In the metro network, high-speed Ethernet backhaul networks converge with legacy SONET/SDH networks, so a wide variety of test capabilities are required to adequately turn up and service these networks.

FTB-200 Compact Platform

Test Type


Recommended Equipment

Next-generation SONET/SDH
Next-generation SONET/SDH testers validate the functionality and performance of next-generation SONET/SDH networks through GFP performance characterization, VCAT conformance testing, differential delay measurement and dynamic bandwidth adjustment (LCAS) verification.
SONET/SDH testers verify basic SONET/SDH functionality including power-level verification, frequency-offset testing, automatic protection switching (APS), stress testing and long-term bit-error-rate testing (BERT).
Ethernet testers perform end-to-end Ethernet performance validation (RFC 2544), reliability (BERT) and TCP throughput testing.


EXFO test modules are hosted in the following portable platform:

FTB-500 Platform
FTB-500 Platform
The ultimate multimodule platform for network experts involved in optical and protocol testing

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