Craig A. Armiento and Yuri A. Yudin
Wavelength managers enable distributed monitoring and centralized reporting of all DWDM channels at multiple network locations. These subsystems can manage bandwidth in metro networks because they embed intelligence in the optical layer and automate control of network devices.
The economic imperatives of metro networks will drive two trends: dynamic bandwidth allocation and optical protocol transparency. Services will be transported on individual wavelengths along paths (lightpaths) to the end user that will be reconfigured dynamically to make efficient use of the network's wavelength capacity. The time for reprovisioning wavelengths will be on a scale of hours or minutes, versus the months it can take to provision services on current networks. Flexibility in the transport of multiple protocols requires the use of optically transparent network elements, whereby channels are not constrained to specific protocols and/or bit rates by optical-electrical-optical (OEO) conversion electronics.
The trends toward dynamic lightpath reconfiguration and optical transparency create new operational challenges. Unlike SONET rings, which perform OEO conversion at every node, transparent network elements will not convert signals to the electrical domain at every switching or multiplexing point. Traditional techniques such as bit-error rate (BER) testing will not be available to monitor signal integrity at strategic points in the network. Although end-to-end BER testing can be performed at the client layer, different approaches are required to monitor and manage the integrity of the optical transport layer.
Distributed optical monitoring will thus become essential for ensuring that lightpaths are properly configured throughout the network. Characterization of all optical channels at multiple points will be required, as well as the ability to report these data to a network operations center. Rapid changes to the configuration of optical network elements may also require localized intelligence to control or manage these network elements. Components such as variable optical attenuators or tunable lasers may require local control to ensure that power and/or wavelength are maintained within acceptable levels.
Several devices have been developed to monitor optical signals.1 Although optical channel analyzers can provide optical metrology in the form of power, wavelength, and optical signal-to-noise ratio (OSNR), these devices do not typically provide the means for communication to a network operations center. The need for distributed optical monitoring, combined with network-wide communication of distributed optical metrology, has prompted development of wavelength manager subsystems.
Wavelength managers will enable critical operational tasks for wavelength-based networks, including system commissioning, bandwidth expansion, and preemptive network maintenance. Maintenance will be greatly enhanced by the ability to communicate monitoring data to the network operations center.
Wavelength manager subsystems comprise several optical and electronic functions that expand the basic capabilities of optical channel monitors and optical channel analyzers (see Fig. 1). The optical input, typically a 0.5% to 1% tap of the fiber power, is launched into the spectrometer module. An optical switch can be incorporated into the wavelength manager to monitor multiple fibers. The spectrometer function is typically implemented by using a scanning Fabry-Perot (FP) filter or a planar diffraction grating.
In the first scenario, a FP filter is mechanically scanned to serially measure the entire spectrum using a single photodetector. The grating-based approach, on the other hand, measures all optical channels in parallel by dispersing the spectrum across a multielement indium gallium arsenide photodetector array. Unlike its scanning FP counterpart, a grating-based spectrometer has no moving parts that might degrade reliability or robustness. The parallel approach is faster than the serial technique, requiring between 10 and 100 ms (versus seconds for scanning FPs) to characterize the entire spectrum with regard to wavelength, power, and/or OSNR. Spectrum acquisition time would be important for monitoring or managing network elements in transient conditions or if monitoring many fibers with a single wavelength manager.
The spectrometer must have sufficient dynamic range and wavelength resolution to distinguish between closely spaced channels with wide power differences. This is important because optical channels will have different power levels based on their path differences and their specific service. A dynamic range of 50 to 60 dB and a wavelength accuracy of 20 pm are sufficient for protocol-diverse DWDM networks with 100-GHz channel spacing.
In addition to the optical elements of the wavelength manager, the subsystem must also provide functional control of data acquisition and analysis as well as network-wide communication. The network operator must be able to define network channel plans, create alarms for each channel, and log events. Communication with the unit must be implemented both locally and over the entire network. Network management protocols such as SNMP and TL1 should be available for integrating wavelength managers with the network operating system.
These embedded communication capabilities allow the system operator to quickly integrate measurement and reporting capabilities into the network. Alarm capabilities for individual channels are also important to identify channels that are drifting beyond an acceptable power, wavelength, or OSNR range. The wavelength manager should also have an intuitive GUI that allows straightforward interpretation of network data at the network operations center.
Critical monitoring points in DWDM networks include nodes where wavelengths are multiplexed, switched, or amplified to influence the path and/or optical properties of channels. We created a three-node network consisting of a multiplexer, optical add/drop module (OADM), and demultiplexer to demonstrate monitoring of transparent network elements in a multiprotocol environment using OC-48, OC-192, and QAM signals (see Fig. 2).
The 40-channel multiplexers/demultiplexers were based on passive diffraction-grating technology with a channel spacing of 100 GHz.2 These devices had an insertion loss of 3.5 dB, a passband of 0.20 nm, and a Gaussian filter profile. The distance between network nodes was 40 km and both links used erbium-doped fiber amplifiers to compensate for fiber and multiplexer losses. The wavelength manager at node 2 monitored both the input and output channels of the OADM. Communication with each wavelength manager was implemented over a network with a computer acting as a network operations center via Ethernet local network or SNMP.
The spectrum view for the OADM input demonstrates that optical power levels of the channels can vary significantly due to different path lengths and protocol requirements (see Fig. 3, top). The wavelength manager spectrometer and embedded software must be able to distinguish between individual channels in the spectrum for monitoring and/or management purposes. The channel at 1538.19 nm, which has a power level 22 dB below its adjacent channel, is resolved by the spectrometer.
The channel view of the OADM input enables easy visualization of the weaker channel, which is shaded in red to indicate that a power and wavelength alarm has been triggered. The ability to monitor both ports of the OADM allows the operator to determine whether channels are correctly dropped, added, or expressed through the OADM. Comparison of the OADM input and output channels shows that three 2.5 Gbit/s (OC-48) channels were dropped, three OC-48 channels were added, and five QAM channels were added (on 200-GHz spacing; see Fig. 3, center and bottom). One of the QAM channels is shaded in yellow to indicate it has triggered a wavelength alarm. These GUI screens demonstrate how individual channels can be easily tracked from a remote network operations center.
The wavelength manager also allows the network operator to set alarms that trigger corrective measures if channels deviate from acceptable wavelength and/or optical power conditions. A simple example of element management using a wavelength manager was performed using the network previously depicted in Fig. 2. A transmitter with a nominal wavelength (1560.60 nm) was launched at node 1, expressed through the OADM at node 2, and monitored by the wavelength manager at node 3. An alarm GUI for this channel displays the measured laser wavelength, along with minor, major, and critical alarms set by the operator in Fig. 4. Alarm levels can be set during the commissioning process based on the acceptable wavelength and power limits for the service carried on that channel.
Control of a drifting transmitter was demonstrated by writing a simple program to communicate with the wavelength manager using the RS232 interface (SNMP or TL1 could also be used). Automatic control of the laser wavelength was initiated when triggered by a major alarm from the wavelength manager (see Fig. 4, top). When the program is triggered, it automatically tunes the laser back to its nominal wavelength value by querying the wavelength manager for continuous wavelength measurements.
The experiment was initiated by manually increasing the laser wavelength until a major alarm (1560.65 nm) was triggered. The program then took control, returning the laser back to its nominal wavelength. A second manual adjustment was then made to drive the wavelength in the opposite direction until the lower major alarm (1560.55 nm) was reached and the program returned the laser to its nominal value (see Fig. 4, bottom).
This experiment is a simple demonstration of localized control of a network element using wavelength managers. Other devices such as variable optical attenuators can be controlled in a similar manner to keep optical power within acceptable limits. Lightpath reconfiguration will invoke changes in the characteristics of optical channels due to changes in the devices along the optical path. Dynamic use of wavelengths in the network will require source tuning (power and/or wavelength) to ensure the acceptable BER and OSNR over the new path. Wavelength managers can provide the distributed monitoring capability that will be a key element of managing bandwidth in metro networks.
- J. Sirkis and A. Kersey, WDM Solutions, 75 (August 2001).
- W. Emkey, Deploying a transparent DWDM network, CED (November 2000).
Craig A. Armiento is director of optical network engineering and Yuri A. Yudin is a senior optical systems engineer at Lightchip Optical Networking, 27 Northwestern Drive, Salem, NH 03079. Craig Armiento can be reached at firstname.lastname@example.org.