40GBASE-PSM4: A Simple Route to Higher Density

PSM4 | FluxLight Optical Transceivers

PSM, or Parallel Single Mode, is a new type of interface with primary applicability today on 40G and 100G optical transceivers. These transceivers include a female MPO optical interface. The most popular PSM, the PSM4, utilizes an MPO-12 single mode fiber cable in which 8 of the 12 fibers are used. So, in the case of the 40GBASE-PSM4, a single QSFP+ 40Gbps module delivers four full-duplex 10Gbps links over 8 single mode fibers (SMF) in a single cable. When I first came across these interfaces I thought, “is anyone really going to install multi-fiber ribbons over SMF link distances…up to 10km (over 6 miles)?” Well, it took one of our customers to show me the ‘light’.

A large Independent School District (ISD) in Texas was doing a major infrastructure upgrade this year and needed the higher density of 40G QSFP+ modules at its central datacenter with connectivity at 10Gbps to a number of remote campuses. They had no intention of running a multi-fiber single mode cable between these endpoints. Rather, they wanted to use the existing 2-fiber SMF connections they already had deployed (previously running at 1Gbps). The QSFP+ 40GBASE-PSM4 deployed with an MPO-12 to 4X dual-LC breakout cable was the perfect solution.

Network Upgrade Story

The challenges involved in achieving a major network speed upgrade economically is not unique to this Texas ISD. The most expensive and time-consuming aspect of upgrading any distributed campus network is the deployment of the fiber plant connecting it all together. If the existing fiber plant can be utilized, then that is a huge savings in time and money.

Before PSM4 | FluxLight Optical Transceivers
Figure 1

Our ISD needed to upgrade campus connectivity from the 1Gbps links they had been running on for many years, to 10Gbps to each campus. In their existing central datacenter they had an aging switch with multiple 1Gbps SFP slots that was really of no use in the upgraded network. The cleanest solution was to pull that switch out (it was 1RU high) and drop in the newer, faster switch in its spot. Ideally the same fiber pairs that were running 1Gbps would now be providing 10Gbps connectivity.

PSM4 Implementation | FluxLight Optical Transceivers
Figure 2

For the central District datacenter location, a switch with enough individual 10Gbps SFP+ slots to dedicate a port to each campus link was considered. However, the final decision was the Extreme Networks® X770 Series, a higher density switch equipped with multiple 40Gbps QSFP+ slots. While there are 10Gbps SFP+ switches with enough ports to handle the needs for this year, with the ISDs considerable projected growth over the next few years, the Extreme QSFP+ 40G solution was the most cost-effective.

The only wrinkle was how to utilize the high-density 40Gbps QSFP+ slots to get individual 10Gbps links to each campus. As mentioned earlier, that is where the 40GBASE-PSM4 comes in. This QSFP+ is equipped with an MPO-12 interface which actually terminates four independent 10Gbps SMF links in a single connector. The solution was a simple SMF breakout jumper cable. One equipped with an MPO-12 on one end and 4 dual-LC connectors on the other. The MPO end is inserted into the 40GBASE-PSM4 QSFP+ module and the dual-LCs, each carrying one 10Gbps full-duplex link, connect at the patch panel to the existing fiber pairs to each campus.

PSM4 Breakout Cable & Optical Module | FluxLight
Figure 3, Figure 4

Figure 3 shows this MPO to 4x Dual-LC breakout cable, and Figure 4 is an example of the 40GBASE-PSM4 optical transceiver module. 

This elegant solution allowed the School District to achieve a 10x link speed upgrade with no modifications to their existing fiber plant. Also, the new, higher-speed networking equipment fit into the same rack space previously occupied by the older 1Gbps gear. Using FluxLight’s incredibly affordable 40G and 10G transceivers allowed another customer to build it bigger, faster and sooner than they ever dreamed possible!


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