40/100G BiDi Transceiver – Some Sequels Are Better Than The Original

In earlier blog posts (Looks Like We’re Upgrading Again! Dual-Rate 40G/100G BiDi Transceiver and 40/100G QSFP BiDi Transceiver’s Backward Compatibility With 40G BiDi), we introduced the dual-rate 40/100G QSFP BiDi transceiver and described how Cisco uniquely offers 40G capability and backward compatibility. Let’s review why the QSFP+ 40G BiDi was such a big hit in the first place when it was released back in 2013, and how the BiDi value proposition still makes plenty of sense.

It’s all about fiber cable infrastructure. To appreciate this, we need to understand a brief history of short reach transceivers. For “short reach” links (on the order of a couple hundred meters), multi-mode fiber cable is used. It’s preferred not because it’s cheaper than single-mode fiber – it’s actually more expensive – but because it supports the use of VCSEL (Vertical Cavity Surface Emitting Laser) based transceivers, which are much cheaper than transceivers with lasers for single-mode fiber, like Fabry-Perot lasers and DFB (Distributed Feedback) lasers.

When the IEEE defined the 40GBASE-SR4 optical interface standard for short reach, it required a transceiver design that essentially consists of four 10Gb SR transceivers packed into a single pluggable form factor like QSFP+ (Quad Small Form Factor Pluggable). This means that each of the miniature 10G transceivers within the QSFP+ housing requires its own fiber pair. And this in turn requires the QSFP+ SR4 to run over parallel ribbon fiber to support the four channels, instead of a single fiber pair. Hence, the need for an MPO connector on the QSFP+.

What this means to a data center operator is that when they upgrade from 10G SR to 40G SR4, they need to add optical fiber to their cable infrastructure. This can cause all sorts of disruptions and add complication to the upgrade process, not to mention add cost. The QSFP+ 40G BiDi solved this problem by supporting the same 40G reach as SR4, but over a single fiber pair. No new fiber required!

Now that we have a 100G BiDi (which can also operate as a 40G BiDi) available, we can enjoy the same advantages when upgrading to 100G. Instead of 100G SR4, which also runs over parallel ribbon fiber, the 40/100G BiDi needs only a single fiber pair. So you can upgrade easily from 40G BiDi to 100G BiDi, or you can go straight from 10G SR to 100G BiDi.

And still, no new fiber… as we are reminded in the video below: