Don’t Let OTN Testing Test Your Patience


Achieving 100 Gbps Optical Network Transmissions and Ensuring QoS

The rapid emergence and deployment of new broadband services has  significantly impacted the traditional fiber optic network infrastructure. Bandwidth-dependent client services such as cloud data, streaming video, social media, and real-time applications supported by wireless devices, have created new revenue opportunities for operators, but current network infrastructure is unable to maintain the increased data demands. To achieve optimal throughput and latency requirements necessary for new services, operators and network providers are evolving existing optical networks from 10 Gbps to 100 Gbps.

Migration to 100 Gbps presents challenges in optical network hardware deployment, data management, and network reliability. T1/E1 lines are being replaced and the Core network is advancing with reconfigurable optical add-drop multiplexer (ROADM), Optical Transport Network (OTN), and Multiprotocol Label Switching-Transport Profile (MPLS-TP) technologies. Over time, these more robust alternatives will make SDH/SONET obsolete. This creates a challenge for operators. While Core networks are moving to OTN, services such as voice over SDH/SONET and Voice-over-IP (VoIP) are continuing to be sent on Ethernet.

This melding of technologies is providing a glimpse into the future of the optical network. Operator or Metro networks that operate at lower rates below 1 Gbps will be managed by Ethernet, while Core, Metro, and Access that operate above 1 Gbps will be managed by OTN.

Operators are seeking cost-effective methods to achieve this next-generation optical network that will operate at 100 Gbps. Recent designs are focusing on increasing port density, as it can reduce network cost by making rack space more efficient. To accomplish this, optical transceivers are being downsized using advanced technology. Century Form Pluggable (CFP)/CFP2 transceivers are predominantly being used but the next version — CFP4 — is much more compact, and is currently under development and being evaluated.

OTN Versatility
One reason OTN is being selected is the flexibility of its mappings, as shown in Figure 1. It can use Asynchronous Mapping Procedure (AMP), Bit-synchronous Mapping Procedure (BMP), and Generic Mapping Procedure (GMP). OTN also accepts clients with different bit-rate tolerances, such as Ethernet (+/-100 ppm) and SDH/SONET (+20 ppm).

Figure 1. ITU-T G.709 OTN Hierarchy

As OTN moves into Metro networks, it will provide operators with better visibility and control closer to the network edge. Network management will be simplified, as OTN will place user traffic on the Core-managed infrastructure earlier. The network itself will be simplified by encapsulating legacy STM/OC-type interfaces, as well as newer Ethernet technologies. Another benefit is that OTN allows operators to better monitor and service large customers.

Testing OTN
Achieving all these advantages, of course, requires OTN to be properly deployed and network operation to be optimized. Testing OTN, therefore, becomes imperative. Forward Error Correction (FEC) performance tests using Poisson distribution random errors was adopted by ITU-T O.182. These tests are important because the FEC section is one of the most vital areas of the network frame. It allows for greater dB range between equipment by correcting errors within the frame at the receiver end.

Reproducible, accurate FEC error correction tests are performed by generating truly random signal errors that can stress OTN FEC. This allows for a much lower BERT measurement to ensure testing to the limit or beyond the switching equipment’s ability. This is required to test the true performance and threshold of an OTN.

The Section Monitoring (SM) and Path Monitoring (PM) of an OTN have different alarms and error detections, which are called maintenance signals. They send feedback on issues that occur at the network far end and offer an indication of the layer in which it occurred.

Among the indications are:
Backward Defect Indication (BDI) — indicates signal fail in the upstream.
Backward Error Indication (BEI) — indicates the number of errors detected in the upstream.
Incoming Alignment Error (IAE) — detects error by BIP-8 code in the OTU layer.
Backward Incoming Alignment Error (BIAE) — counts the IAE errors in the upstream in the OUT.

Using these OTN maintenance signals allows engineers and technicians to quickly and correctly locate an issue, so they know the position in the network where testing should begin. It also allows issues to be prioritized; Layer SM is a higher priority as it is likely a core issue, compared to a PM problem that is possibly a customer situation.

Taking this Tandem Connection Monitoring (TCM) approach makes it easy to identify which customer or segment level is affected. With this information, operators can ensure that issues affecting high-priority customers are corrected first. Additionally, understanding the different TCM levels that are being analyzed or injecting errors identifies the actual section where there is a problem.

Dividing the Network
When embarking on testing, it is important for an engineer to divide the network into logical sections supporting both testing from end-to-end and section-to-section, as well as between sections or end points. Segmenting the network in such a manner allows issues to be quickly isolated and identified. They can be divided into core-to-core, metro-to-metro, customer-to-customer, or any combination of the three. (See Figure 2.)

Figure 2. OTN Digital / Optical Network Diagram

End-to-end circuit testing is the most common test conducted by an operator, and is often required to commission a customer circuit. Usually this test is run by generating the same traffic as a customer would produce, and is completed customer edge to customer edge. The most common tests are RFC 2544 or Y.1564.

If a customer site has a larger contracted data rate than other offices, it can be difficult to run a commission test from the head office connection. However, it is quite simple to test from each regional office to the head office by emulating multiple traffic streams for each regional office via Y.1564. This allows the operator to confirm it can supply the contracted throughput to the regional offices.

Confirming whether the contracted throughput to the head office has been difficult for an operator to document, mainly because the customer side of testing must be connected to an Ethernet interface while the operator side is done on an Ethernet over OTN interface. It is important to be able to establish an Y.1564 or RFC 2544 test on one section of the network and receive it on another. Consequently, testing from the customer side is completed using an Y.1564 test over Ethernet while testing on the operator side is completed using an Y.1564 test over Ethernet encapsulated over OTN. In this application, a solution that tests over a multistage OTN as shown in Figure 3 should be used.

Figure 3. Anritsu MT1000A Network Master OTN field test platform measurement

Broadband services provide operators with new revenue opportunities but to reap the benefits they must invest in network upgrades. An OTN network that can accommodate 100 Gbps transmissions is a solution but requires certain tests to be performed to ensure QoS. Conducting the appropriate tests with the proper instruments will help 100 Gbps networks operate according to specification, and provide operators with the infrastructure necessary to deliver high-bandwidth client services.



About Author

Daniel Gonzalez is the Senior Business Development Project Manager for Anritsu’s Optical Testing solutions portfolio. He has more than 19 years of experience in Telecommunications and Fiber Optical Transport. For more information, please email or visit: Follow Anritsu on Twitter @anritsu.

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