Transport/core networks require new look at test

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by Juan Masmela

The move to next-generation networks (NGN) is anything but straightforward. Evolving technology, unprecedented customer expectations, and the challenges of a legacy infrastructure combine to present some daunting tasks. Yet with advanced, highly integrated test equipment, telecom operators can overcome these challenges while effectively managing capital and operational expenditures.

Transport networks are evolving from traditional SONET/SDH/DWDM to OTN/DWDM, and transmission bit rates in core networks are increasing to 40 and 43 Gbits/sec. At the same time, telecom operators face tougher competition and are expected to deliver the highest quality of experience – essentially, error-free transmission across long- and ultralong-haul networks. Concurrently, to support the higher bandwidths required to carry NGN services like voice, data, and video for long distances, service providers must upgrade existing infrastructures and extend the length of their networks' optical links.

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Figure 1. Forward error correction increases the signal-to-noise ratio by approximately 6.2 dB.

Traditionally, carriers have preferred SONET/SDH "transport containers" that move both voice and fixed-rate data in long-haul networks. This is because the basic format of a SONET/SDH signal enables many different services to be carried in its Synchronous Payload Envelope (SPE) or Virtual Container (VC). However, the dramatic global growth of Ethernet and IP-based services such as voice over IP (VoIP) and IPTV, as well as deployment of NGNs, has compelled service providers to refurbish long-haul transport networks to improve network performance, reduce expenses, and support bandwidth-intensive, delay-sensitive applications. In addition to the multimedia services that compose NGN offerings, service providers must deliver packetized voice and data from wireless connections. Thus, carriers require seamless transmission at higher bandwidths on both the access and transport sides of the network.

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Figure 2. New test systems enable verification of end-to-end connectivity and the FEC capabilities of elements in an OTN network.

The global standard commonly called Optical Transport Network (OTN) or "digital wrapper" — ITU-T recommendation G.709 defines the framing structure — addresses these challenges. As a result, OTN and DWDM are becoming the dominant technologies in long- and ultralong-haul networks interconnecting distant regions and countries.

OTN technology enables multiple networks and services, such as legacy SONET/SDH, to be combined seamlessly into a common infrastructure for data, voice, video, and storage applications. OTN "wraps" the client data (SONET OC-768/OC-192/OC-48, SDH STM-256/STM-64/STM-16 and 10-Gigabit Ethernet LAN and WAN) into a new payload that completely preserves each service and transports the signal across the long-haul network. This safeguards the client's original signature and enables thorough performance monitoring across the entire network.

In applying the digital wrapper, OTN adds forward error correction (FEC) using a Reed Solomon 239/255 algorithm defined in ITU-T G.709/Annex A. This technique increases the signal-to-noise ratio by approximately 6.2 dB and enables the receiver to correct detected errors. FEC can extend the length of the link, reduce the number of amplifiers in the OTN, and ultimately lower operational expenses.

An OTN's bandwidth management capabilities extend the performance of legacy SONET/SDH networks to the DWDM side, where users can track the performance of every channel inside the DWDM network. FEC allows service providers to expand the length of the optical link without having to regenerate the signal, which reduces equipment requirements and the potential for errors.

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Figure 3. For out-of-service testing of network elements, error correction capabilities and real-time network performance can be measured.

An OTN signal comprises a frame and client data. To calculate FEC bytes, the OTN frame and data are divided into 16 sub-multiframes, each with 255 bytes (239 for the frame and payload and 16 for FEC). Using the Reed Solomon algorithm, FEC bytes are calculated for each sub-multiframe that corresponds to the payload (Fig. 1).

After data is transmitted, the receiving network element examines the frames and identifies errors. In most cases, errors are fixed at the receiving end and the signal continues to be transmitted along the network. For a significant burst of errors, the receiver will seek another transmission or generate an alarm.

Like SONET/SDH, OTN offers a mechanism for alarming, signaling, protection, switching, and remote management through overhead bytes. As networks evolve, OTN provides a significant advantage by extending the same management capabilities to new technologies like Gigabit Ethernet (GbE), 10GbE, Fibre Channel, Enterprise Systems Connection, and Fibre Connection. By leveraging OTN technology, new services can be transported over longer distances at higher levels of performance with less equipment. The net for telecom operators is lower capital expenditures, fewer network issues, and an overall better customer experience.

In addition, OTN combines the administrative, maintenance, and protection capabilities of traditional SONET/SDH with the bandwidth management capabilities of a DWDM network. OTN enables service providers to monitor the performance of each channel traversing different wavelengths and explore the details of each wavelength throughout the entire network. The ability to monitor and test both the client side and the physical-layer DWDM allows technicians to examine performance down to the data and payload levels. Without this capability, troubleshooting is extremely difficult and all the monitoring has to be performed at the client SONET/SDH side.

Changes in transport network technologies require test equipment manufacturers to design and deliver feature-rich NGN test equipment that integrates comprehensive test suites for multiple technologies into a single, compact unit. Service providers can leverage these systems to test emerging technologies such as OTN, Ethernet over SONET/SDH, and next-generation SONET/SDH (VCAT, GFP, and LCAS), while reducing capital expenditures by eliminating the need for multiple instruments.

OTNs require both out-of-service and in-service testing. Out-of-service testing is completed during network or network element commissioning and is designed to ensure that a network is fully functional before transmitting live traffic. In-service testing includes continuous network monitoring for alarms and errors, and troubleshooting to remedy problems without affecting service delivery. Both are critical to optimizing the customers' quality of service (QoS).

The OTN characteristics of advanced test instruments are optimized for installation and maintenance and enable technicians to perform both out-of-service and in-service testing on transport and access networks, both legacy and next-generation, with a single product. Using these systems, technicians can verify end-to-end connectivity (Fig. 2) and the FEC capabilities of network elements, as well as their asynchronous/synchronous mapping of SONET/SDH client signals to determine conformance to ITU-T G.709 recommendations.

Out-of-service tests qualify the performance of network elements in the field by stressing the network element's error-correction capabilities and testing real-time network performance in increasingly longer intervals (Fig. 3). After setting up the network equipment, technicians send errors and alarms from the client and verify they are received using remote indicators (Fig. 4). With OTN networks, technicians can test send and receive performance in both directions. If the test system identifies problems in either direction, technicians can ensure that network management software re-routes the data or seeks retransmission.

Advanced test equipment ensures that errors are identified and that network management directions are recognized at both ends of the network. If testing indicates clear transmission, the technician can insert errors to determine how the network element and network management software respond. Before commissioning and acceptance tests are performed, all the network elements must be verified to ensure proper performance. This task is executed by the network equipment manufacturer's field installation team and verified by the service provider's engineer.

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Figure 4. Network element testing also can be performed remotely.

Another common out-of-service test performed during the installation and integration of OTN terminal equipment is the digital wrapper or OTN mux test. This test ensures that a SONET/SDH client signal is properly wrapped into an OTN signal and that the client payload from the OTN can be extracted without errors or alarms (Fig. 5).

Once every element in the network is functioning properly, technicians are ready to perform bit-error rate tests on the system using pseudo-random binary data or sequence, also known as test pattern. Initially, technicians will connect to a payload for approximately 15 minutes to identify any immediate and obvious errors. The connection time increases to several hours and then to a recommended maximum of seven days. Depending on the length of the test, the performance thresholds change. ITU M-2401 and G-8201 provide recommended thresholds that validate network performance based on the length of the tests.

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Figure 5. Digital wrapper (or OTN Mux) testing ensures that a SONET/SDH client signal is properly wrapped and can be successfully extracted.

In-service testing is used to maintain the network by quickly identifying defects or anomalies that can degrade the performance and responding with immediate corrective actions. Once the network is brought into service, any number of factors (noise, fiber problems, timing, and/or clock recovery failures) can affect the performance of a link. Identifying whether errors or alarms are from the optical transport unit (OTU), optical data unit (ODU), or optical payload unit (OPU) and comparing the BIB-8 parity bytes enables technicians to evaluate network performance against defined thresholds.

OTN overhead includes additional information, such as full time forward location (FTFL) and automatic protection switching/protection communication channel (APS/PCC) bytes. FTFL helps identify faults and their locations. Non-intrusive maintenance tests enable the network engineer to evaluate the protection switching protocol by monitoring and decoding APS/PCC bytes.

In-service testing or non-intrusive monitoring can be completed in the following two ways: by connecting the test set receiver to the output of an optical splitter or by inserting the test set in transparent mode between two network elements so the live signal is not modified. Connecting the test set between the network elements will temporarily interrupt the circuit under test.

Telecommunications service providers in most developed countries have been using 10-Gbit/sec transport technology for some time. As bandwidth demand increases and enterprise campuses expand, 10 Gbits/sec is becoming a bottleneck for some applications. Typically, more 10-Gbit/sec connections that require additional fiber and equipment are added to alleviate the bottleneck.

From the telecom operators' perspective, increasing the speed to 40 Gbits/sec (SONET OC-768/SDH STM-256) is a better option. Moving to 40 Gbits/sec enables operators to transport a higher number of tributaries, using less space and at lower costs. Transmission at 43-Gbit/sec OTN (OTU3), especially when combined with DWDM, can provide sufficient bandwidth to carry all services through the network, error free.

The eventual migration to 40 and 43 Gbits/sec underscores the need for accurate testing during initial commissioning. Properly testing networks enables service providers to understand the strengths and weaknesses of a network – where alarms occur and their causes – and to create plans for expansion and upgrades to higher speeds. Testing with the latest full-featured equipment and the ability to plan accurately enables service providers to deliver the highest QoS today while anticipating and investing in the networks they need for the future.

Juan Masmela is senior product marketing manager, Telecom Products Group, at Sunrise Telecom (

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