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Optical communications migrate to SFP+

Posted: 01 Apr 2008 ?? ?Print Version ?Bookmark and Share

Keywords:interface chip vendors? SFP+? optical module? optical transceiver? single-mode fiber?

Interface chip vendors in growing numbers are tailoring chips to support the migration of datacom and telecom routers and switches to SFP+, the latest small form-factor pluggable (SFP) optical module for use in 10Gbit/s Ethernet and 8.5Gbit/s Fibre Channel systems.

SFP+ offers a more compact form factor than X2 and XFP packages and runs on less than 1W. It also offers higher mounting densities than current 10Gbit/s devices. A more streamlined design has made SFP+ the same size as the SFP industry standard used for data rates up to 4Gbit/s.

The newer standard reduces cost by removing everything except the optics from the modules. Signal-conditioning functions are moved from the module to the host card, as are the Serdes, media access controller, clock and data recovery (CDR) and electronic dispersion compensation (EDC) functions. The result is a simplified transceiver module with electrical-to-optical and optical-to-electrical polarization-mode dispersion (PMD) functions.

Inside Avago's 10Gbit/s SFP+ transceiver are its own 850nm VCSELs.

Three SFP+ ranges have been standardized for 10Gbit/s nets: 10GBase-SR (30-300m with OM3-grade multimode fiber), 10GBase-LR (10km across single-mode fiber) and 10GBase-LRM (220m with FDDI-grade multimode fiber).

New solutions
In January, Avago Technologies Ltd began offering production quantities of a 10Gbit/s Ethernet SFP+ short-reach optical transceiver. At the core of the AFBR-700SDZ are Avago's own 850nm vertical-cavity surface-emitting lasers (VCSELs), which leverage technology from its 4G VCSEL design. According to the company, that earlier design has undergone more than 2 billion device-hours of testing without failure. The AFBR-700SDZ uses multimode fiber and extends coverage to 300m.

The transceiver supports the SFF-8431 specification for enhanced 10Gbit/s SFP+, and SFF-8472 for the digital monitoring interface in optical transceivers. The RoHS 6-compliant, Pb-free device delivers real-time temperature, supply voltage, laser bias current and laser average output power, and it receives input power information through a two-wire interface. The digital diagnostic interface allows disabling of the transmitter to check for faults, and for the monitoring of potential loss of signal in the receiver.

Last year, Applied Micro Circuits Corp. claimed to be the first vendor to ship a fully integrated dual-port SFP+ transceiver. The QT1215 is a 9.95-10.6Gbit/s CDR circuit with fully adaptive EDC capability. It not only provides an interface between a 10Gbit/s Serdes and an SFP+ optical module but also converts a single differential input XFI data stream into a serial 9.95-10.5Gbit/s output data stream, in compliance with the SFP+ standard.

In the SFP+ side receive direction, the QT1215 contains a sophisticated EDC engine that complies with both the SFP+ and LRM standards. The EDC permits the recovery of signals degraded by differential mode delay, chromatic dispersion and PMD. Using internal algorithms, the chip continuously adapts to channel characteristics. The EDC engine in the QT1215 uses a low-power mixed-signal architecture that instantly adapts to fiber impulse response changes. The transceiver was designed to exceed the performance specified in 10GBase-LRM, thereby providing a robust product for multimode fiber applications, said the company. The QT1215 includes a standard two-wire interface for communicating with external EEPROM for the embedded microprocessor. A separate two-wire interface provides control and status capability for the IC. The integrated embedded controller provides EDC adaptation control and can be programmed for various value-added capabilities.

A typical network application for AMCC's QT1215 is shown: a fully integrated, two-port, 9.95-10.6Gbit/s CDR circuit with fully adaptive EDC capability.

Other innovations
Sumitomo Electric Industries Ltd has launched a line of SFP+ transceivers for 10Gbit/s applications that is available in production quantities. The SPP series is designed to meet IEEE 802.3ae and 802.3aq specifications for 10G Ethernet. Sumitomo released three transceivers in the SPP5000 series. SPP5101SR, for 10GBase-SR, uses an 850nm VCSEL for operation over multimode fiber at distances up to 300m. SPP5101LM is for 10GBase-LRM, using a 1,310nm Fabry-Perot laser for operation over multimode fiber at up to 220m. SPP5101LR is for 10GBase-LR and uses a 1,310nm DFB laser for operation over single-mode fiber (SMF) at up to 10km.

In December, Vitesse Semiconductor Corp. released the VSC8242, a CDR chip with EDC. Although the device was initially aimed at X2, the dominant optical module for 10Gbit Ethernet shipments today, its lead application is enabling the migration of datacom and telecom routers and switches to the SFP+ optical module form factor. The VSC8242 supports all three SFP+ range applications and complies with 802.3aq, 802.3ae and SFF-8431 electrical specifications. That functionality allows existing module designs to move quickly and cost-effectively to the SFP+ form factor.

The VSC8242 incorporates two bidirectional 10Gbit/s paths to support the increased port densities. Additional features include various lookbacks, a complete set of programmable Rx and Tx path functionality (such as output preemphasis) and full GbE data rate support.

Finisar Corp. has introduced 8Gbit/s long-wavelength SFP+ transceivers for advanced storage networking applications. The 1,310nm transceivers support 1.4km and 10km transmission over SMF and are well-suited for fiber connectivity and disaster-recovery storage applications.

- Ismini Scouras

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