400G ZRx Part 3: The Which, When and Why of Coherent Pluggables

In the Part 1 of our blog series on 400G ZRx transceivers, we covered different types of 400G coherent pluggable transceivers, industry standards for these pluggable form factors and how they came into being.  Part 2 focused on 400G ZRx applications including grey links, amplified vs unamplified, and how to perform basic power link budget calculations.  Now in Part 3 comes the fun part:  Which type of 400G ZRx transceiver do we use, when, and why for a particular application?  As most everyone involved with optical networks is likely aware, coherent technology brings many benefits in terms of capability and performance.  Coherent is also more complicated than direct detect and although there are standards, certain intricacies and details around the implementation of 400G ZRx transceivers into routers and switches require special knowledge and expertise.

To aid customers in the process of determining which 400G ZRx transceiver is right for a particular application, Precision OT has developed some simplified decision maps;  one for grey link applications and one for DWDM/Amplified Links.  While these decision maps are by no means a substitute for the robust network design process needed to develop end-to-end solutions, they can assist network designers and teams by providing an introduction to the use of coherent pluggables. 

400G ZRx Decisions: Which Transceiver is Best for My Grey Link Application? 

Determining which transceiver to use for any application starts with first understanding a customer’s needs.  The first question is “Are your needs unique to 400G or are there other data rates needed for your application?”   The decision tree shown in Figure 1 was created to help navigate through this type of discussion.   Behind this simplified tree diagram is a significant amount of technical knowledge and experience vital to helping make an educated decision about which transceiver to use, when, and why. 

Figure 1  400G ZRx Transceiver Decision Map – Grey Link 

For example, if you are interested in grey link lengths greater than 40 km, we know that by performing some simple power budget calculations and taking into account key factors such as fiber loss, we can estimate a range of theoretical max link distances particular transceivers can achieve.   Transceiver product datasheets typically provide the TX power and RX sensitivity we use in our power link budget calculation:  

Power Link Budget =  (TX Power) – (RX Sensitivity) 

Usually, the minimum TX power is used for direct detect transceivers, although in the case of variable TX power transceivers, we can use the highest value that can provide the greatest power link budget.  The PRE-QSFP56DD-ZRPHT,  for example, can have a max TX power of +1 dBm.  Or the 400G ZR can be varied up to -8 dBm.  These are the kinds of intricacies one needs to know (or should get help to determine) when deciding which transceiver to use for a particular network application.  New host platforms with new SW will allow the user to configure and set the TX power via CLI (command line interface) or their Network Management System (NMS). Some older legacy platforms and SW versions, however, won’t allow the user to do that. In such cases, Precision OT can customize the transceiver to initialize at a certain TX power that fits the need of the customer’s application. 

DWDM/Amplified Links:  Which 400G ZRx is Best & Why?

Similarly, with DWDM/Amplified links, we start by determining whether it’s a single application (400G) or multi-rate application (100G – 400G).  Then we consider the TX power.  In Optical Line Systems (OLS), companies have designed their TX optical input power to be a certain value. Depending on their optical network design, it may require Low TX power input (typically between –10dBm and –8 dBm) or High TX power input (typically between 0dBm and +2 dBm).  Especially in brownfield applications, there’s a need to get all the optical signals going through the OLS at the same level so that the OLS can perform amplification and equalization with all signals coming in at the same signal strength.

In DWDM Amplified networks the max reach will also depend on the transceiver chosen and optical network design. The PRE-QSFP56DD-ZR and  PRE-QSFP56DD-ZRHT follow the OIF 400ZR which uses C-FEC and will only be able to reach max 120km regardless of your optical network design. The PRE-QSFP56DD-ZRP and   PRE-QSFP56DD-ZRPHT support all the OpenZR+ applications, using O-FEC, and can reach up to 100s of km depending on the optical network design: number of link spans being amplified, link span distances, type of amplifiers (EDFA vs Raman), fiber quality, etc. 

Figure 2  400G ZRx Transceiver Decision Map – DWDM/Amplified Link

The Transceiver Decision Maps above give you an idea of how the discussion will go when working with an experienced optical network solutions partner.   We’ll walk you through the data rates, FEC, Max reach and other application questions above and also discuss Network design (topology? Line system? Minimum input power requirement? Pluggable optics interoperability? Type of deployment?), host platform, testing and timeline.  Precision OT knows the importance of delivering reliable, interoperable high-quality products designed to help you bring together all the systems within your network.   Contact us about 400G ZRx applications and all your optical network needs!