For those of you that have been in the 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:
- They show that Fibre Channel is not dead.
- 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.
- 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.
Some of you may have noticed that in my last post I neglected to talk about three other 10-Gigabit Ethernet variants – LX4, LRM and ER. That’s because the content was already long enough and I wanted to focus on the volume data center applications. Now, I’ll discuss the others.
10GBASE-ER runs over SMF at 1550nm for up to 40km. While there are a few service providers that might choose to do this, the vast majority of them choose WDM through the OTN instead. There may be some private networks that have need for this variant as well which is what prompted the IEEE include it in the standard. These transceivers are priced out of typical budgets for the average enterprise at over $4000.
10GBASE-LX4 was originally introduced to address installed base of FDDI-grade (not laser-optimized) MMF, but can also be used with higher-grades of LOMF as well as SMF. It uses CWDM to send four wavelengths (thus the X4) between 1269.0 to 1355.9nm running at data rates of 2.5 Gbps each. LX4 modules are available in both X2 and XENPAK form factors and from at least a couple of sources such as Emcore and Hitachi Cable. As you can imagine, because these devices have four lasers and four detectors with their associated electronics, they cost appreciably more than either the SR or LR transceivers. The module alone retails for about $2000, which means that the per port cost would probably around $2500. But, you most likely wouldn’t fill your switch with these modules of course; you would only use them as needed where you wanted to re-use existing installed fiber.
In response to the high priced LX4, 10GBASE-LRM variant was developed. It was enabled by chip companies such as Clariphy and Scintera with some new technology, electronic dispersion compensation (EDC), that could push 10G serial to longer distances on MMF. It took a while to develop the standard and the consequence was that LX4 really took much of the market it was intended for. However, once products were released and supported by the top-tier transceiver manufacturers (Avago, Finisar, Opnext), it has really taken much of the business away from LX4. LRM modules are now available in SFP+ packages as well, which clearly indicates the vendors think there will be an ongoing market for them.
One note of caution – if you intend to use either the LX4 or the LRM modules, you need to make sure that both ends of your channel have them, otherwise it won’t work.
