Small Form Factor Defines Many Products
By Bob Hult, Bishop & Associates Inc.

Small Form Factor is a catchy name that sounds good and conveys the intended attribute, which is why it’s often applied to a variety of products. But this can cause some confusion in the industry, especially when users expect I/O plug compatibility between equipment.

As a case in point, mention small form factor (SFF) connectors to a group of engineers and they may each visualize a different image. Yet each image will technically meet the SFF description.

	A copper interface commonly found in networks, servers, workstations, and remote storage equipment is referred to as a SFF 8088 or mini-SAS connector.
	At the same time, a SFF 8470 interface is an entirely different connector, typically used in Infiniband 4X applications.
	Just to confuse things a little more, a connector physically identical to the SFF 8470 is used in 10 Gbit Ethernet applications and is referred to as the CX4 connector.
	Mention small form factor to a designer of embedded computers, and he or she will describe a variety of standardized printed circuit boards of different shapes and sizes. These boards may be single board computers, stackable computer modules, or computer-on-modules, each of which define specific connectors that provide stacking, power, or I/O interconnect.

Small form factor optical connectors are another animal. These multi-fiber interfaces were introduced to address market demand for higher speed and port density I/O in data center applications.

	They include MT-RJ connectors from TE Connectivity
	FJ connectors from Panduit
	VF-45 connectors from 3M
	Lucent duplex LC connectors

These SFF fiber optic connectors have become industry standards, and are widely utilized in networking applications today.

Small form factor optical connectors, such as LC duplex, may be mated to Small Form Factor Pluggable (SFP) transceivers, also known as mini-Gigabit Interface Converters (GBICs), another entirely different class of interfaces that enable both copper and fiber I/O.

The transceiver modules and I/O cables that mate with them are referred to as Small Form Factor Pluggable interfaces.

SFF pluggable connectors are one of the fastest-evolving interfaces available for high-speed/high-density interfaces.

Demand for greater I/O bandwidth has been an ongoing quest since the introduction of networked systems. Terabytes of data are constantly flowing through the Internet and to attached storage systems, requiring ever-larger pipes to handle the traffic. The performance of specific interface protocols, such as Ethernet and InfiniBand, are being constantly upgraded to keep pace with demand, while faceplate real estate is shrinking. The near logarithmic increase in Internet traffic, driven by a host of new mobile access devices and HD video, shows no signs of slowing.

The I/O panel of network components, including switches and routers, can be a very congested space. Large network farms and data centers have become a wiring nightmare.

Thanks to greater chip integration, the envelope of electronic devices continues to shrink, allowing manufacturers to pack more ports in the same enclosure. The problem becomes how to bring an increasing number of high-speed I/O cables to a smaller I/O panel.

Small form factor pluggable I/O interfaces address these needs.

Small form factor pluggable interfaces offer greater I/O customization and flexibility to optimize performance and extend the useful file of data center equipment by providing a simple migration path to increased capacity and bandwidth. Field installable modules are hot pluggable and adapt the system to a variety of signaling protocols using either copper or fiber I/O cables.

Development of standardized connectors, cage assemblies, latching mechanisms, and pluggable modules with defined mechanical form factors has set the stage for a continuing series of performance upgrades.

A carefully designed high-speed edge connector is located at the rear of a stamped metal cage assembly that provides module alignment, as well as protection from electromagnetic interference.

Modules plug into the cage assembly that converts I/O signals to a variety of standard protocols in both copper and fiber optic media. The SFP interface has proven to be a cost effective, multi-sourced high-bandwidth solution to support Fibre Channel, Gbit Ethernet, InfiniBand, and Sonet data transfer protocols.








Starting in 1999, multi-source agreement (MSA) groups have developed a progression of new small form factor modules and cage assemblies, each offering improvements in performance packaging density and cable reach.

This evolution continues today with the two current leading contenders being SFP+ and QSFP+.

The high-speed SFP format has been upgraded over the past few years through a series of connector and PCB launch refinements to provide 10 Gb/s data transfer rates in the same high-density SFP envelope.




SFP+ connectors and modules meet the performance requirements of the SFF-8431 specification, as well as supporting 10 Gbit Fibre Channel and 10 Gbit Ethernet standards.  The separable host connector is a 20-position surface-mount receptacle.









The SFP specification also defines copper cable assembles that mate directly to the PCB cage connector assembly. These direct-attach copper (DAC) cables can be either passive or incorporate active signal conditioning devices that improve signal integrity at longer distances. The shells of these cable connectors are typically die cast zinc for shielding effectiveness, as well as mechanical durability.

SFP+ optical modules utilize 850nm VCSEL laser drivers that couple to duplex LC optical connectors. Optic link distances can extend up to 300 meters.






PCB cage assemblies are available in stacked and ganged configurations for maximum packaging density. Light pipes for status indication, EMI gaskets, and heat sink accessories are also available.







Continued fine tuning of the SFP+ host connector and recommended PCB launch have pushed the bandwidth to support 16X fiber channel at 14 Gb/s. SFP+ connectors, including the Amphenol SFP+ ExpressPort™ and zSFP+ from Molex and TE Connectivity, are examples of these advances.
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Quad Small Form Factor Pluggable (QSFP+) was the next progression in the line of SFP profiles. QSFP+ connectors are approximately 30% larger than SFP+, but offer a major improvement in faceplate I/O density. While the SFP+ interface provides a single 10-14 Gb/s channel, QSFP+ provides four 10 Gb/s transmit and receive channels in the same connector.





Copper cables and optic transceiver modules plug into a 38-position host connector and cage assembly. Passive direct-attach copper cables are offered, as well as those featuring active compensation and equalization features.





 

Twelve fiber MTP optic cables plug into the optic converter modules for long-reach applications. QSFP+ modules are hot-pluggable, simplifying the configuration process.

Cable assemblies consisting of a QSFP+ connector at one end, breaking out to four SFP+ terminated cables have become a popular item.

A key characteristic of all SFP connectors has been backward compatibility. Suppliers have upped the bandwidth of these interfaces without changing the mechanical specification, allowing older modules and cables to successfully mate with the most current iterations of the PCB host connector. As speeds increase, this constraint has required advanced signal integrity analysis and innovative manufacturing techniques to achieve these new bandwidth targets.

The latest version of the QSFP+ host connector from TE Connectivity and Molex, trade named zQSFP+, supports next generation 100 Gb/s InfiniBand and Ethernet data transfer requirements. Four 28 Gb/s channels can transmit up to five meters via a passive copper cable for top of rack applications, or up to five kilometers on optical fiber. This new host connector maintains the same mating face dimension, but adds 2.2mm to the depth of the connector. The E Series QSFP+ connector from Amphenol High Speed Interconnects features 28 Gb/s with no connector dimensional changes.

Issues of power consumption and the resulting thermal challenges, as well as signal integrity and EMI protection, are pushing the limits of standard SFP+ and QSFP+ interfaces.

A new contender in the market is the CXP pluggable interface. CXP was created to satisfy requirements for high-density connections in the data center, and offers 12 transmit and 12 receive channels operating at 10 Gb/s each, for a total of 120 Gb/s total bandwidth. The profile of this connector is slightly larger than QSFP+. The Molex iPass+ HSC CXP system is one example of this new interface.

Rather than a single row of contacts, the mating face is split into two rows, allowing a narrower profile. The one-piece, 84-circuit host connector/cage assembly provides robust shielding. Active and passive copper cables can be plugged directly into the header.

Standard MTP/MPO optical cables with 24 fibers can be attached via pluggable optical modules. Fiber cables up to 300 meters in length satisfy InfiniBand performance requirements.

Active optical cable assemblies that can transmit 10 Gbit signals up to one kilometer are available.

CXP connectors are expected to find use in high-performance computing, data centers, and network installations where massive amounts of data must be quickly routed among servers and attached storage. CXP connectors have been specified as the 12X Quad Data Rate (QDR) InfiniBand Interface. CXP has also been defined in the IEEE 802.3ba 100 Gbit Ethernet standard.

The CFP is a larger pluggable module designed specifically for long haul optical transmission. The CFP MSA has defined the original CFP module to support emerging 40 and 100 Gb/s applications. The 148-position, two-piece CFP host connector assembly from TE Connectivity offers a system solution, including a receptacle, cover, guide rails, external bracket assembly, backer plate, and riding heat sink.

Development work continues to push the bandwidth higher and packaging envelope smaller, with a MSA roadmap that includes CFP2 and CFP4 form factors. The QUADRA host connector will offer eight CFP2 ports per line card, while CFP4 will provide up to 16 ports per line card.

The race to ensure I/O is capable of supporting increasing processing speeds continues to spur innovation in both copper and fiber interconnects.

Bishop & Associates Comments:

• The term Small Form Factor applies to a wide range of products, which can cause some confusion.

• System designers want to ensure performance headroom capable of supporting anticipated network growth. Small Form Factor Pluggable interconnects offer a cost-effective method of adapting data center network equipment with increasing demand for higher bandwidth and I/O port density.

• Standard Small Form Factor Pluggable interfaces are continuing to evolve to higher-speed performance and port density while maintaining backward compatibility.

• The continuing evolution to 10 Gbit Ethernet and beyond is driving the transition to pluggable module I/O interfaces such as QSFP+, CXP, and CFP. The new target appears to be 100 Gbit.

• Pluggable modules offer media flexibility and provide a clear migration path to support higher data throughput at both the initial installation and field updates.

• SFP interfaces are finding applications in switches, routers, high-performance computing, and attached storage equipment.

• Standard one-piece edge connectors used as host interconnects have reached their high-speed performance limits, forcing connector suppliers to consider a transition to two-piece connectors. Two-piece interconnects are also perceived as being more reliable.

• Although not currently defined as a SFP interface, CFP connectors fit the description of pluggable interfaces and offer additional long haul, high-speed optical options.

• The availability of signal integrity equipment and expertise required to develop and refine these high-density/high-speed connectors may become a limiting factor to smaller manufacturers without these internal capabilities.

Bishop & Associates, Inc. © 2011