OTN Multi-Channel Testing with the Viavi Solutions™ … OTN Multi-Channel Testing with the Viavi...

OTN Multi-Channel Testing with the Viavi Solutions™ ONT Solution

Application Note

Major network operators have adopted optical transport network (OTN) technology as the preferred technology for the construction of new optical networks for its error correction capability and new types of mapping better suited for today’s data-heavy traffic. In addition to synchronous optical network/synchronous digital hierarchy (SONET/SDH), client traffic such as Ethernet and Fibre Channel (FC) can now be efficiently mapped into the OTN payload via ODU0 and ODUflex, creating high-speed OTN signals that contain many smaller bandwidth signal channels, as shown in Figure 1. Muxponders will soon map client signals of all kinds into bigger OTN pipes, as Figure 2 illustrates, and OTN cross-connects will route traffic in various directions, enabling the construction of mesh topology networks based on OTN technology. Each low-speed channel can carry a variety of client signals and, therefore, network equipment manufacturers and their suppliers must demonstrate that they can perform all mapping/de-mapping, multiplexing and de-multiplexing according to international standards. Likewise, the bit-error ratio (BER) and service disruption time (SDT) for each of their product channels must fall within required specifications to ensure quality of services (QoS).

2 OTN Multi-Channel Testing with the Viavi Solutions™ ONT Solution

Service provider laboratories tasked with evaluating a candidate’s network solutions must accurately know the equipment’s actual performance. Higher numbers of channels within the high-speed signal pipe poses challenges to measuring BER and SDT with conventional test tools.

Figure 1. Example of an OTN signal

Figure 2. Example of an OTN muxponder

4GFC

GE

STM-16

GESONET/SDH

10GEODUk

FC

N xOTU2/2e/1eGFEC/EFEC

N x 10G

Challenges of Testing Multiplexed OTN Signals

BER Measurement

Simulating actual equipment or network performance requires a test configuration that reflects actual network scenarios where all channels are properly loaded. Evaluating BER performance for all channels using conventional tools requires measuring the BER of each channel individually, which does not reflect the actual application scenario and is extremely time-consuming. The total test time equals the sum of the total test time for each channel, so testing time increases proportionally to the number of channels tested.

One way to save time testing is to daisy-chain all the channels, which requires tediously configuring the equipment or the network. Also, because the test pattern comes from the same source, it lacks the necessary independent pattern necessary to rigorously measure error performance.

Loading each channel with one BER tester is too expensive for many companies. Some devices under test (DUT), such as backplane switches, require loading all channels and testing BER simultaneously. In the absence of special tools, this may be the only alternative.

Errors and Alarms

Network equipment must be robust and capable of withstanding stressed working conditions. When unexpected events occur, the equipment to which it is connected both directly and indirectly may generate concurrent alarm signals. In these conditions, the equipment must respond appropriately without amplifying the damage by becoming nonfunctional or by sending out inappropriate alarms. Thorough stress tests can give service providers confidence in equipment and network performance. Conventional testers cannot typically generate errors and alarms on multiple channels as required and, thus, are unsuitable for these applications.


SDT

To minimize damage or lost revenue resulting from service interruptions, many customers will pay a premium for protected connections, making SDT an important parameter in service-level agreements (SLAs). Past experience proves that the ability of network elements and networks to deliver protected, error-free services can be compromised when all or part of the network experiences excessive errors and alarms. Network equipment manufacturers and operators must perform exhaustive tests to ensure the QoS meets specified requirements. Challenges associated with the total test time and cost of test equipment that equipment manufacturers and service providers face in measuring BER also apply here.

The Viavi Solution—ONT Multi-Channel Test SolutionsThe OTN multi-channel test features within the Viavi ONT platform address these issues. The OTN multi-channel test can generate and receive high-speed OTN signals that contain multiple channels, as described in Figure 3, and lets users measure BER in parallel, SDT in parallel, and stress-test DUTs to verify their performance under severe conditions. This capability greatly reduces the total test time and minimizes test equipment expense. Involving more channels saves on the overall test time with the OTN multi-channel test solution supported on the Viavi ONT platform. It also can be used to test application-specific integrated circuits (ICs), network equipment, and an entire network.

Figure 3. ONT in multi-channel testing

Highlights of the ONT Multi-Channel Test Functionalities

y Detects the status of all containers in a single overview page; fully simulates the composition of the signal to be tested with pseudorandom binary sequence (PRBS) clients, with mixed mapping and multi-stage multiplexing testing support

y Provides parallel testing of BER, alarms, and SDT for each channel, significantly reducing the total test time for required resources

y Independently adjusts channel frequency offset to test the ability of the DUT to accommodate signals derived from various clock sources

y Independently generates channel errors and alarms to test the performance of the DUT under stressed conditions

y Measures best-in-class SDT

– Parallel measurement triggers ensures catching every event, saving valuable test time

– User-configurable separation time provides control over SDT measurements

y Ability to automate tests accelerates regression tests with easy-to-use scripting support

ONT-603

GE in ODU0

GE in ODU0

4G FC inODUflex


Simulation of Test Signal Structure

Test results are most relevant when the test signal simulates the actual working condition of the DUT. The OTN multi-channel test features provided with the Viavi ONT can generate test signals as needed.

An OTU2 signal contains a total of 8 x 1.25 G time slots, each of which can be used to carry one ODU0 which is the smallest unit of OTN optimized to carry Gigabit Ethernet. Several 1.25 G time slots can be grouped together to form an ODUflex to carry traffic that requires more than 1.25 G, such as 4G FC signal. An OTU2 signal running within a network may carry just one 10 G client or, as Figure 4 shows, 4 x ODU1, 8 x ODU0, or a mix of any number of ODU1 and ODU0 as long as the total bandwidth does not exceed 8 x 1.25 G. The Viavi OTN multi-channel feature lets users create test signals that are composed for use by a DUT, fill the payload areas of each subchannel with PRBS, and perform BER testing on all channels simultaneously.

Client areaFAS

FECOTU-OH

ODU- OH

OPU-OH

ODU0

ODU0

ODU0

ODU0

ODU0

ODU0

ODU0

ODU0

8x 1.25 G timeslots = 10 GTS 1 TS 8TS 7TS 6TS 5TS 4TS 3TS 2

ODU1


ODU1ODU1ODU1

ODUflex with 3 timeslots


ODU1ODU0

ODU0

ODU0 5 existing sub-channels


ODUflexwith 3 timeslots ODU1

ODU0

ODU0

ODU0 4 existing sub-channels level 1ODU1

ODU0

ODU0 4 additional sub- channels level 2

Figure 4. Examples of OTU2 signal compositions

Client signals contained in a high-speed signal can come from various sources and, therefore, must be allowed different frequency offsets than their nominal line rates. To simulate these conditions, the ONT lets users independently adjust bit rates for all client signals within their industry-specified limits. Figure 5 illustrates how to achieve this on the Viavi ONT graphical user interface (GUI). Without this capability, the tester is greatly limited in its ability to evaluate the performance of a DUT.

Figure 5. Users can independently control stuffing for each channel


Concurrent Multi-Channel BER Testing

Unlike conventional testers that requires users to flip channels one after another to examine each channel’s status, the Viavi OTN Multi-Channel Test Solution lets users test the errors and alarms for all channels simultaneously and report bit-error counts and ratios for each channel. It reduces the test time for an OTU2 signal with 8 ODU0s to one-eighth of that required without this multi-channel capability. Figures 6 and 7 provide overviews of the signal without and with errors, respectively.

Figure 6. System performance overview without errors

Figure 7. System performance detecting errors and alarms

Stress-Testing Network Performance with Alarm and Error “Flooding”

Unexpected network events can cause the DUT to receive alarms from other directly connected equipment. Client channels contained in the high-speed signal may also contain alarms generated by their terminating equipment. Each alarm event may vary making it critical to have a test tool that can independently generate error conditions and alarm signals for each channel.

The Viavi ONT Multi-Channel Test Solution can independently generate errors and alarms on each channel. The thoroughness of a test depends upon the design and work users put into each test case, letting them examine the responses of the DUT and determine its performance. Figure 8 shows that alarms have been generated on four different channels. Users can then check to see how the DUT responds. Figure 9 reports alarms in the received signal, and based on the alarms reported for channel 5, users can check its operation at the client input. Users may also need to check the path of channel 1 to see which segment caused the bit interleaved parity (BIP) errors.

Figure 8. Selective channel alarm generation and detection


Figure 9. Multi-channel errors and alarms report

Best-in-Class SDT Measurement

For services, a disruption in any segment of a multi-hop connection is merely that, a disruption. Lab qualification requires advanced testers that can quickly and effectively detect disruptions on the link to which it is directly connected as well as those occurring on intermediate links. Defects upstream from standards-compliant equipment in the intermediate link send alarm signals downstream, such as ODU-OCI (ODU open connection indication), ODU-AIS (ODU alarm indication signal), or other alarms based on the actual disruption and the standards response requirement for such circumstances.

The Viavi OTN Multi-Channel Test Solution allows for a wide selection of errors and alarms, as shown in Figure 10, to trigger the SDT measurement. More importantly, those triggers can be configured in parallel so that the occurrence of any one of the selected events will trigger an SDT measurement. This capability eliminates needing to know the type of event prior to test as well as repeating test to discover what triggered the measurement, greatly reducing the test time, especially for multi-channel applications. Furthermore, it sees the termination of the previous error or alarm that persists at the end of the disruption, regardless of what initiated the measurement, ensuring SDT measurement accuracy.

Figure 10. Flexible SDT measurement triggers

SDT measurements can quickly detect a number of events caused by the bouncing effect from a restoration effort or resulting from switching the test signal to multiple paths before finally landing on a good one. Users can choose to see these as one single event or as multiple events, because the ONT lets users determine the separation time. Time intervals of two events separated by time intervals less than the separation time are considered one disruption.


The following example shows the same service disruption scenario measured with different separation time settings to highlight the effects that various settings have on the test result:

a. Separation time set at 300 ms (default): the total service disruption time is 40 ms

b. Separation time set at 10 ms: reports two events one with an SDT=8 ms, and one with an SDT=20 ms.

Figure 12 shows a service disruption on channel 1 and channel 5, where the SDT for channel 1 is within the specification, but exceeds the limit on channel 5.

SummaryThe latest development in the OTN technology introduces challenges for evaluating performance using conventional test equipment. The Viavi OTN Multi-Channel Test Solution solves this problem by measuring BER and SDT in parallel for each subchannel showing the full performance of the DUT in the actual working environment under stressed conditions. The intuitive information the GUI provides lets users immediately know the error and alarm status for all channels and pinpoints problems to a particular channel(s). Its robust automation makes the ONT multi-channel solution extremely applicable for research laboratories and network equipment manufacturers where both performance and automation are vital.

Figure 11. Effects of separation time on measurement results Figure 12. Measuring SDT

© 2015 Viavi Solutions Inc. Product specifications and descriptions in this document are subject to change without notice. otntesting-an-opt-tm-ae 30173179 900 0312

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