Monday, April 2, 2012

Next Generation 100G Ethernet (with corrections)

Ever since the 40/100G Ethernet standard was completed in 2010, the IEEE standards group has been working on ways to improve it. In my opinion, there were two very serious holes in the original standard. The 40G long-reach variant did not match the existing telecom standard for 40G so some type of conversion equipment would be needed. This was fixed when the IEEE 802.3bg 40GBASE-FR single mode fiber standard was released in 2010. The second concern, which still exists, is the 100GBASE variants. Four standardized and one MSA currently exist and are shown in the following table.

Data rate
Min. Reach (meters)
Form Factors
IEEE 802.3
CXP Direct Attach Copper
Twinax Copper

LOMF 850nm
SMF 1310nm

SMF 1310nm
SMF 1550nm
Not supported

40/100G, the IEEE did not want to make the mistake of too many variants and form factors again (like they did for 10G) so consciously limited them. But, in our opinion, may have restricted them too much. By reducing the laser-optimized multi-mode fiber (LOMF) optical reach to 100m over OM3 and 150m for OM4, the IEEE left a huge gap in distance covered for data center applications – in fact, a two orders of magnitude gap – from 100m to 10km. This results in an enormous difference in cost as well. For example, a 100GBASE-SR10 CXP module average selling price is about $200, while the 100GBASE-LR4 average price is more than $20,000. So it is currently cost-prohibitive to design a data center with connections longer than 100m. This is not realistic. In order to address this shortcoming, the top transceiver manufacturers are working on SR4 products that have the potential to reach to 300m. Recently, the IEEE has recognized this issue and is looking to address it in its next generation study group. It is called the Next Generation 100Gb/s Optical Ethernet Study Group and its charter is to investigate 25G-per-lane standards and to explore lower-cost solutions to cover reaches perhaps up to a kilometer.

Any 100G variant using 25G signaling is still under development. While the optical devices are almost ready to go, there are long-term projects to ascertain how 25G is going to run on a printed-circuit board (PCBs) or on twinax cable. The group that was studying this has just officially been named a task force in the IEEE – the P802.3bj 100 Gb/s Backplane and Copper Cable Task Force. There are chip sets available to run 25G signals over PCBs that will be available in the coming months. Texas Instruments was demonstrating this at SC11 and Altera, Amphenol, Semtech/Gennum, IBM, Inphi, TE Connectivity and Xilinx showed 25G products in the OIF booth at OFC/NFOEC 2012.

Notice in the table above that there are different signaling schemes and form factors between 100GBASE-CR10, SR10 and 100GBASE-LR4. The CXP that was chosen for short-reach copper and LOMF is not suitable for longer-reach SMF operation. Even though most of them were involved in the IEEE process, equipment manufacturers are not happy about this because that means their products must support two different form factors at the same time. It may also doom CXP to only the initial products until another, better form factor can be developed that will cover both cost effectively – maybe a CFP2 or CFP4? Or the 25G signaling matures and the SR4 and CR4 variants are created in the QSFP28 (now being worked on in the SFF committee) is used.

The LR10 variant is not standardized, but is backed by a consortium of vendors and end users – including Google and Facebook. Whether this will take hold in the industry at large remains to be seen, but some of the industry leaders are boasting that it is actually selling very well currently at more than 2,500 units already.
So, while we talk about Terabit Ethernet being on the horizon and there have been multi-vendor demonstrations of 25G signaling for 100G operation, plenty of work remains to bring 100G to fruition prior to the next speed bump.