Global Sources
EE Times-Asia
Stay in touch with EE Times Asia
EE Times-Asia > Interface

Optical forum completes electrical interface work

Posted: 16 Jun 2005 ?? ?Print Version ?Bookmark and Share

Keywords:optical forum? electrical interface? backplane? physical layer? pci express?

Electrical interfaces for 6Gbps and 11Gbps backplanes have been firmed up by the optical internetworking forum, completing work started three years ago by the High-Speed Backplane Initiative.

The forum completed two implementation agreements for its common electrical I/O (CEI): 11G-LR, for 10Gbps to 11Gbps interfaces in backplanes and long-reach fiber links, and CEI-P, for protocols like FEC over physical links.

Now, four physical-layer CEI standards cover 5-6Gb interfaces for both short- and long-reach fiber and the equivalent fiber standards for the 10Gb to 11Gb realm.

Mike Lerer, chairman of the forum's physical and link layer working group, said that while attention has centered on 10Gb interfaces because of their commonality with Ethernet, the initial focus of the High-Speed Backplane Initiative was on lower speeds. "We see continued interest in the 5Gb to 6Gb range for those upgrading from legacy systems, since it represents an easier step up from Xaui interfaces than going straight to 10Gb," he said.

The common electrical I/O (CEI) specs provide a wide range in physical-layer speedsfrom 4.976Gb to beyond 6Gb at the lower rate and from 9.95Gb to beyond 11Gb at the higher rateto allow for the system overhead necessary to implement features like FEC. The short-reach version of the standard is intended for serial links up to 200mm, or up to 8 inches of PCB trace; the long reach takes links to 1m.

The internetworking forum is working with the PCI SIG on PCI Express and Express Advanced Switching, with the rapid io Trade Association and with the IEEE 802.3ap working group on backplane Ethernet. The groups' activities have accelerated with the adoption of the XFP optical module form factor in carrier and data center applications.

"There is continued pressure from telecom equipment manufacturers to add more ports in less space," said Lerer. Forum standards governing the Serdes framer interface, the system packet interface and the time-division multiplexing fabric-to-framer interface will all be tweaked to meet the new CEI standards, he said.

After that, all of the forum's working groups will have accomplished most open standards for interfaces up to 10Gbps and will suspend work until new requirements emerge at higher speeds.

The CEI buffers will use high-speed, low-voltage logic with nominal differential impedance of 100. The connections specify point-to-point balanced differential pairs, using unidirectional signaling. But Lerer said the working group kept process technology, device pin-out, management interfaces and similar characteristics deliberately undefined, so that IC makers have many degrees of freedom in differentiating their products.

Simulated receive eye

The CEI group developed a special simulator at the receiver end for dealing with high speeds. Traditionally, the "eye diagram" is open at the receiver end, and standards bodies specify an eye mask. At multigigabit rates, however, the eye is closed at the receiver, making it difficult to do conformance testing.

The CEI group specified transmitter compliance and channel compliance through S-parameters, then created a simulated receive eye based on a minimally compliant transmit and receive. The forum has published mathematical algorithms for the simulator.

The CEI-P for protocols specifies the use of FEC. Lerer said FEC designers should pay attention to the need to handle more than single-bit errors, since advanced equalization may change several design characteristics.

"Because higher-speed receivers will be based on fairly exotic equalization methods, the characteristics of link errors will change, and FECs must be able to handle burst errors," Lerer said. "The clock-recovery and error-detection protocols used in the industry may have to be reconsidered."

- Loring Wirbel

EE Times

Article Comments - Optical forum completes electrical i...
*? You can enter [0] more charecters.
*Verify code:


Visit Asia Webinars to learn about the latest in technology and get practical design tips.

Back to Top