Friday, December 10, 2010

Is SFP+ the Optical RJ-45?

By Lisa Huff

For those of you that have been in the networking industry for what seems to be 100 years, but is really about 25 years, you know that the one “connector” that hasn’t changed much is the RJ45. While there have been improvements by adding compensation for the error that was made way back when AT&T developed the wiring pattern (splitting the pair causing major crosstalk issues), the connector itself has remained intact. Contrastingly, optical connectors for datacom applications have changed several times – ST to SC to MT-RJ to LC. They have finally seemed to settle on the LC and perhaps on a transceiver form factor – the SFP+. The SFP was originally introduced at 1G, was used for 2G and 4G and with slight improvements has become the SFP+ and the dominant form factor now used for 10G. Well, it is in the process of getting some slight improvements again and promises to make it all the way to 32G. That’s six generations of data rates – pretty impressive. But how?

The INCITS T11.2 Committee's Fibre Channel Physical Layer – 5 (FC-PI-5) standard was ratified in September. It specifies 16G Fibre Channel. Meanwhile, the top transceiver manufacturers have been demonstrating pre-standard 16G SFP+ SW devices. But, wait a minute – short-wavelength VCSELs were supposed to be very unstable when trying to modulate them at data rates above 10G right? Well, it seems that at least Avago and Finisar have figured this out. New microcontrollers and adding at least one clock and data recovery (CDR) device in the module to help clean up the signals have proven to be keys. Both vendors believe it is possible to do this and not add too much cost to the modules. In fact, both also think that possibly by adding electronic dispersion compensation (EDC) they can push the SFP+ to 32G as well - which is the next step for Fibre Channel - hoping to stop at 20G and 25G to cover developments in Ethernet and InfiniBand.

And what about long wavelength devices? It has always been a challenge fitting the components needed to drive long distances into such a small package mainly because the lasers need to be cooled. But not anymore – Opnext has figured it out. In fact, it was showing its 10km 16G FC SFP+ devices long before any of the SW ones were out (March 2010). Of course, this isn't surprising considering Opnext has already figured out 100G long haul as well.

These developments are important to datacom optical networking for a few of reasons:  

  1. They show that Fibre Channel is not dead.
  2. The optical connector and form factor "wars" have seemed to subsided so transceiver manufacturers and optical components vendors can focus on cooperation instead of positioning.
  3. They will impact the path other networking technologies are taking – Ethernet and InfiniBand are using parallel optics for speeds above 10G – will they switch back to serial?
Stay tuned for more on these points later.

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