Monitoring optical networks using SLM systems

Sept. 1, 2001

Recent advances in physical-layer monitoring in conjunction with newly developed standards-based network-management software are making service-level management systems a reality.


The communications market is undergoing a metamorphosis. Increased competition among the surviving carriers and service providers is mandating lean and effective operations as well as the delivery of services at highly competitive rates. In this environment, service providers must implement management systems that minimize the consumption of critical resources without compromising the quality of service.

One emerging technology that enables cost-effective operations and service reliability is the optical service-level management (OSLM) system. Such a system combines hardware and software functions and operates at the physical layer, independent of the service protocol. A highly developed standards-based OSLM system can be integrated with other operational support systems (OSSs) using OSS-specific adapters.
Figure 1. A typical optical transmission link starts with the transmitted data, followed by electrical-to-optical conversion, fiber-optic transmission, optical-to-electrical conversion, quantization, and finally the recovered data. Analog symbol monitoring is performed prior to signal quantization.

An OSLM system enables service providers to create and proactively maintain service-level agreements (SLAs), manage remote network elements (NEs) without an extensive IP infrastructure, and troubleshoot and isolate problems quickly.

Analog symbol monitoring (ASM) is the underlying technology that provides an optical-networking system with unique physical layer SLA capabilities, enabling service providers to create and monitor SLAs based on the bit-error rate (BER), regardless of what digital protocol is transported on a wavelength. ASM integrated into a software platform allows service providers to accomplish the following tasks:

  • Create SLAs per customer, network, service, link, and wavelength.
  • Monitor and report SLA performance and threshold violations
  • Graphically show real-time and historical BER metrics.
  • Compute available time against set SLA parameters.
  • Generate periodic reports for billing or trouble-ticket usage.

Service providers can use this monitoring tool to generate advance warnings of potential SLA violations, allowing them sufficient time to prevent further degradation of service.

ASM is the measuring of the signal integrity of the received optical signal before its quantization-the process of determining "1" or "0." Most fiber-optic networks use a transmission standard called on/off keying, which simply means that a digital 1 is represented by the presence of light and a 0 by the absence of light (see Figure 1). When this modulation technique is used for fiber-optic transmission, the optical signal loses its digital characteristics because of basic properties of physics-finite frequency response of the transmitter, attenuation, and the dispersive characteristics of optical fiber. By the time the signal reaches the optical receiver, it has altered into an analog signal, losing its digital discreetness. It's the job of the receiver circuitry to convert this analog signal back to a digital signal by measuring the relative presence or absence of light power, which it translates into 1s and 0s; these 1s and 0s are then interpreted by processors as the information originating from the transmitter. During the quantization conversion, a significant amount of information about what the optical signal experienced between transmission and reception is lost, however. The lost information would have provided a wealth of information such as indications of when bit errors may occur.
Figure 2. An example of how an optical service-level management system provides inform ation on multiple customers' link availability based on bit-error rate.

Just as doctors' focus on their patients' well-being, service providers worry about the health of their networks. ASM is essentially a technique to monitor the pattern of an optical signal. To a doctor's trained eye, heart irregularities can be observed by viewing the electrocardiograph chart or display. It is impractical, however, for a dedicated expert to read multiple patients' charts continuously, for example, in an intensive care facility. Instead, health-care staff at the nurses' station responds to monitoring sensors attached to every patient. The same principle is true when monitoring multiple network nodes and links from a network operations center.

Robust software can simultaneously analyze the network's nodes and links, detect any subtle irregularities, and trigger alarms when problems are detected. Early detection of subtle problems or degradation trends calculated based on past data can lead to corrective action by the NEs or the service provider. Figure 2 illustrates how an OSLM system can interpret the data from ASM and provide information on the integrity of the optical signal to network operations staff.

Management information is communicated between nodes via a telemetry signal, preferably out-of-band but in-wavelength. Thus, remote nodes can be fully managed from a central site without the need for an external management infrastructure. Using in-wavelength-management (IWM) techniques, service pro viders can manipulate, upgrade, and manage remote NEs using the same fiber that carries the customers' data, without introducing any overhead into the data stream. Additionally, service providers can perform the same functions over IWM as they would if the remote NEs were connected to an IP infrastructure. This technology can save service providers the cost of potentially large initial capital expenditures.

According to industry estimates, network management consumes 9% of a service provider's budget. While calculating the cost of operations, service providers include the resources and time required to perform several management functions:

  • Provisioning services.
  • Identifying and isolating problems.
  • Upgrading and maintaining NE software.
  • Retrieving data from vendors' element-management systems (EMSs) and feeding it into other OSSs.
  • Sustaining a standalone EMS that does not integrate with other OSSs.

Service providers are accustomed to thinking in terms of truck rolls or the number of mouse clicks needed to accomplish a task. Generally, if the provider has to roll a truck once, it breaks even; if two truck rolls are required, the provider loses money. An OSLM system enables providers to remotely provision, manage alarms, troubleshoot problems, and maintain equipment.

To remotely provision a service, service providers can change the line-rate corresponding to standard protocols of the optical links with a click of the mouse or "on the fly." That's required only on the local side, reducing provisioning time by 50%.
Figure 3. An optical service-level management (OSLM) system combines hardware and software functions and operates at the physical layer, independent of the service protocol. A standards-based OSLM system can be integrated with other operational support systems (OSSs) using OSS-specific adapters.

Each NE maintains a sophisticated current-alarm table, which is accessible through all the interfaces. The alarm table gives the technician information on the health of the element. At the network level, a network-management system (NMS) aggregates alarms from all the managed NEs (via IWM) and presents this information for viewing via a graphical user interface (GUI). The affected nodes appear in a "Failed Object" category, providing single-click accessibility. Additionally, the NMS allows alarms to be escalated per business rules. Once the affected node is identified, the user can remotely run diagnostics and further isolate problem areas. The entire scenario is managed with only a few mouse clicks.

A necessary feature of an OSLM system is the ability to maintain and upgrade remote hardware via NMS and IWM technology. The technician can select the NEs, configure the site by retrieving the software, and set the time of download and activation. An OSLM using a client-server architecture-based NMS allows for easy access from any location at any time. A GUI or command-line interface prompt allows technicians in the field to use their laptops to connect to the NMS server through a dial-up connection. This flexibility assists the technician with isolating and resolving problems and can reduce problem resolution time.

Many service providers develop OSSs in-house. An OSLM with advanced Common Object Request Broker Architecture (CORBA) and Extensible Mark up Language (XML) interfaces allow integration with existing OSSs (see Figure 3). Any functionality available through the system's GUI can also be accessed through the CORBA and XML interfaces. Information can be retrieved automatically from the OSLM and imported into the OSS, such as statistics and SLA data generated by ASM (BER, errored seconds, severely errored seconds, unavailable intervals), accounting, asset reports, alarms, security data, and configuration reports. The OSLM allows periodic report generation for billing usage as well as automatic trouble-ticket report generation. These systems are designed to work with multiple OSSs simultaneously at a service-provider site.

In short, the OSLM system enables service providers to provide the highest level of service while reducing the associated costs.

Eugene Park is marketing director at LuxN (Sunnyvale, CA).