QSFP specification expands to support 40 Gb/s applications [QSFP

Abstract

The Quad Small Form Factor Pluggable (QSFP) specification is complete, and vendors have released modules, connectors, and cages that support up to 40 Gb/s/port. Each QSFP port currently support four independent channels with speeds up to 10 Gb/s/channel, and optical modules are expected to support these speeds in the near future. Passive cables combined with QSFP connectors and cages are already running at 10 Gb/s in backplane and shortreach applications. QSFP optical modules running to 5 Gb/s are currently in production, and 8 and 10 Gb/s modules are expected in late 2007. With the QSFP packing four independent optical channels via an MPO 12fiber ribbon cable for 40 Gb/s applications, the QSFP module offers the highest pluggable bandwidth density in the world. The QSFP is designed for many applications including Ethernet, Fibre Channel, InfiniBand, and synchronous digital hierarchy/synchronous optical network (SDH/SONET). QSFP spans 1‐10 Gb Ethernet applications as well as 1, 2, 4, 8, and 10GFC applications for Fibre Channel. The QSFP is also being proposed for the emerging 20 and 40GFC applications that are being defined for Fibre Channel ganged links. With InfiniBand primarily using quad links, QSFP is suited for single data rate (SDR at 2.5 Gb/s/channel), double data rate (DDR at 5 Gb/s/channel), and quad data rate (QDR at 10 Gb/s/channel). In the telecom space, the QSFP is applicable to OC-3, OC12, OC-48, and OC-192 speeds. Each channel of the QSFP module can run independently or in parallel, depending on the application, to support bandwidth to 40 Gb/s. The QSFP module combines the best features of the SFP and XFP modules. The QSFP was originally designed to replace four SFP modules running up to 5 Gb/s by using an MPO 12-fiber ribbon connector. Although the electrical connector for the module supported 10 Gb/s from the start, the module vendors doubted that 10 Gb/s speeds could be supported in a module that dissipated less than 3.5 W of power in the small package. With recent reductions in integrated circuit (IC) power consumption and the desire to support higher speeds, the module vendors agreed to open up the throttle on the specification so that 10 Gb/s modules can be supported in short-reach applications. The QSFP module borrows heavily from the XFP specification and uses a similar latching mechanism, cage, and memory map, but does not include a retimer of the XFP to keep the modules low cost. Supporting speeds up to 10 Gb/s/channel, the QSFP can also replace four SFP+ modules, which replace the SFP at speeds above 4 Gb/s. Several modifications were needed for the QSFP to support the four channels through its pluggable 38-pin electrical connector. While the SFP has separate connections for LOS, Rate Select, TxFault and Tx Disable, the QSFP could not support 16 pins for these functions when 16 pins are required for the high-speed differential signals and 12 more for grounding. Individual channel controls were incorporated into the processor on the module for control and digital diagnostics. The sophistication of the two-wire serial interface to the transceiver is significantly more complex than previous single-channel transceivers since multiple channels need to be controlled through the single interface. Through the combined efforts of the consumers and supplier of the QSFP, the QSFP specification has yielded a valuable solution that meets the highspeed and high-density needs of the networking community. Some users are finding QSFP to be very effective in dense backplane applications where passive copper cables can be used to interconnect the QSFP cages. When longer distances are required in the application, optical QSFP modules can replace these passive copper cables to increase scalability of the implementation. With density and speed being pushed to the limits, the QSFP solution relieves the pressures that have been building up in networking applications. For more information on QSFP, please visit http://www.qsfpmsa.org.