Optical Interconnects: The Long and Short of it.

VPX at Light Speed—Optical Brings 100 Gigabits to Backplane Architectures

A new generation of connectors and interfaces is taking its place in the world of VPX to handle the growing demands of data transfer between cards and systems.


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With the emergence of backplane data communication at PCI Express (PCIe) Gen3 and 10/40 Gigabit Ethernet (GbE) speeds, it’s becoming more and more likely that backplane I/O will require support at similar bandwidths. Certainly chassis-to-chassis connections will need to be accomplished at the same bandwidth, and because copper cables of useful lengths are not practical above 3 Gbit/s, applications will be turning increasingly to optical cables for high-speed external data connections.

The need for high-speed I/O is not directly related to the communication from slot to slot within a backplane. Also, it is reasonable to point out that individual sensors do not yet have bandwidth requirements in the 8 to 10 Gbit/s range. What will drive the need for optical I/O will be arrays of sensors with local analog to digital conversion. In addition, this need will also be driven by the bandwidth requirements for attached storage and chassis-to-chassis communication.

VPX is the first embedded backplane architecture specifically designed to allow optical I/O through the backplane. The optical backplane connector is defined within ANSI-VITA 66.0 66.1 and 66.3. The VITA 66 base standard defines a suitable family of optical interconnects for use on VITA 46.0 plug-in modules and backplanes, with VITA 66.1 identifying the mechanical transfer (MT) style contact variant and VITA 66.3, the Mini Expanded Beam contact variant.

The VPX backplane’s optical I/O is capable of serving several different purposes such as card-to-card data connections, chassis-to-chassis data connections and I/O connectivity to sensors and sensor arrays. Until now, most signaling within and between backplanes was at speeds that could always be supported by copper cabling.  

Cables from the Front Panel

One of the most popular optical interfaces today is the QSFP+, originally developed by the SFF committee that addresses storage industry needs. This interface supports a pluggable module inserted in a standard socket or cage that can be either an active optical cable or a copper cable.

QSFP+ active optical cables are in accordance with SFF-8436, which defines the QSFP+’s electrical (copper), optical and mechanical characteristics and are readily available from such suppliers as 3M, Amphenol, Finisar, Mellanox, Molex, Samtec and TE. The cables support 10 GbE as well as InfiniBand FDR and QDR, which are 14 Gbit/s and 28 Gbit/s, respectively. Each cable consists of two electrical-to-optical transponders that plug into a QSFP cage connected by optical fibers.

Similar active optical cables are available to support proprietary PCIe Gen3 solutions. The interfaces to these cables are almost exclusively on the front panel or the edge of integrated motherboards. The locations of these interfaces were initially chosen to best serve the connection requirements common in the networking, telecom and storage industries.

Because cable PCIe is already an important feature of VPX embedded systems, the industry will be watching to see how PCIe cable I/O is accomplished for Gen3 speeds. At the present time, the PCI-SIG technical committee is making a decision regarding the format of a standard Gen3 optical connector.

This copper or optical interconnect will be called OCuLink, supporting 8 Giga transfers/second (GT/s) and faster connection speeds. Although the mechanical implementation is still being decided, the electrical requirements have been established. There will be a strong bias toward an affordable interconnect, such as QSFP+, to ensure a successful PCI Express Gen3 optical interface.

No Place for Front Panel I/O

For most 19” rackmount installations, the front rack panel has long been an acceptable surface for optical interfaces. However, this is not the case for typical industrial or military enclosures, such as Air Transport Rack (ATR) boxes per ARINC 404. These units always have provisions for front covers and any I/O is usually accommodated by a combination of MIL-C-38999 or other similar sealed cable end connectors arranged as bulkhead mounted I/O panels.

Backplane I/O can reach the bulkhead I/O panel in a number of ways, however. It can be cabled from separable cable connectors on the backplane. It can be routed through a riser board installed like a plug-in card, but at the very edge of the backplane. Or a rigid PCB supporting the bulkhead connectors can be connected with an integral, flexible PCB section that is part of the backplane, as well.

This rigid-flex-rigid method is favored because it is reliable, takes less labor to install and can handle a wide variety of signals. The optical fibers need to exit behind the backplane and terminate at the same bulkhead location as the circular electrical connectors (Figure 1).

Figure 1
A typical rigid-flex-rigid ATR I/O Assembly used for I/O in an ATR enclosure.

Although rear panel optical I/O connectors are almost a necessity for ATR-style enclosures with front covers, this same arrangement could also simplify cabling for conventional installations. Backplane optical I/O could help control the tangle of front panel cables that make replacing front cards more time-consuming. Backplane-mounted connectors are also likely to be more reliable because there are no cables to flex when installing a card and no cable connectors to be handled.

MT Optical Ribbons to the Rescue

In contrast to the QSFP+ active optical cables, the VITA 66.1 solution moves the optical transducer onto the plug-in card or onto an XMC or FMC mezzanine module carried by a VPX plug-in card. VITA 66.1 defines the MT variant for backplane optical connections on 6U VPX cards. VITA 66.4 will define the same thing for 3U VPX cards when it is completed.

A fiber ribbon attaches directly to the optical transducer, but terminates in an MT ferrule positioned on the rear card edge, typically in the P6 position. This arrangement moves the optical interface to the backplane and allows for inexpensive passive fiber ribbons rather than active optical cable assemblies. The VITA 66.1 module is the same size as the MultiGig module that populates the standard 6U VPX card edge positions P1 thru P5. This P6 VITA 66.1 module holds two MT ferrule assemblies that can contain between 8 and 24 optical fibers each (Figure 2).

Figure 2
Left to Right - MPO cable end (a), VITA 66.1 BP module (b) and proposed 3U VITA 66.4 module (c).

With the optical transducer located on the card or a mezzanine, all that is needed to connect to the outside world is a passive optical ribbon terminated with an MT (MPO or MTO) ferrule on each end. One end of the passive optical cable plugs into the VITA 66.1 backplane module and the other end typically plugs into a MIL-C-38999 MT housing on the bulkhead of an ATR enclosure.

A similar passive cable would then connect to the same MIL-C-38999 from the outside and extend from one or two meters up to 1,000 meters to another device equipped with an MT interface. Although VITA 66 defines MT ferrules with up to 24 fibers, MT ferrules are available today that support 72 fibers arranged in six VITA 66-compatible ribbons.

VITA 66 Everywhere

There is one little noted aspect of the VITA 66 family of backplane mounted optical connectors.  Because the VPX form factor conforms to an Enhanced Eurocard Standard (IEEE 1101.10) backplane and daughter card design and an IEEE 1101.11 rear transition module (RTM) design, any backplane connector developed for VPX can potentially be utilized in other Eurocard-based designs.

With PCIe Gen3 being targeted by both CompactPCI Serial and CompactPCI Express R2, the VITA 66.1 optical backplane connector is a potentially viable optical I/O in those architectures, as well. The VITA 66.1 module could also be used within the AdvancedTCA architecture. Although ATCA utilizes a unique mechanical form factor, the engagement and seating dimensions of the backplane connector are the same as the IEC 61076-4-101 and IEC 61076-4-113 connector families, which are used with IEEE 1101.10 Enhanced Eurocard architectures.

VITA 66.1 and 66.3 solutions could potentially find themselves in Zone 3 ATCA-based applications as well, since this user-defined area can hold a special backplane to interconnect boards with signals not defined in ATCA. This could be particularly inviting as PICMG 3.1 R2 brings ATCA applications firmly into the 40 GBase-KR4 world (Figure 3).

Figure 3
Edge of daughter card to backplane: The similarity between the mounting space for VPX, CompactPCI Serial and AdvancedTCA enables possible use of VITA 66 connectors across different platforms.

100 GbE Demands Optical

At the time of the writing of this article, IEEE 802.3bj is in negative ballot resolution; the first working group ballot having been successful. This Ethernet standard, the latest IEEE 802.3 standard, will define 100 GbE comprised of four 25 Gbit/s channels. It is well on its way to a November 2013 sponsor ballot and a second quarter 2014 release.

It is a sobering thought to consider how much work will be required to implement 25 Gbit/s channels on a backplane. With silicon available, however, there is no doubt that our industry will rise to the challenge. One thing is quite certain: I/O at this data rate will not be accomplished in copper cable.

For those who will try to predict when trans-backplane optical I/O will first appear in deployed systems, you might want to watch the defense industry. The requirement for enclosed systems with front covers will drive optical I/O to the backplane. VITA 66.1 is likely to be the vehicle.

When ANSI-VITA 66.1 becomes a familiar solution within VPX, we will probably see it next in AdvancedTCA applications, followed by ATCA extensions. CompactPCI Express and PXI Express may then follow with CompactPCI Serial as well. After that, we may begin to see commercial 19” rack-based systems implement rear optical. VITA 66.1 may eventually be used to implement slot-to-slot optical backplane connections.

The data processing demands made on backplanes will continue to increase, making it impossible for standard copper I/O interfaces to keep pace with these evolving requirements. Fortunately, VPX has made provisions for a backplane interface that supports backplane optical I/O, and can potentially be used in other technology platforms as well.  This will serve as a great asset in system design requiring high-speed data communications. Table 1 presents links to details on the various standards and specifications mentioned in this article and beyond. 

Table 1
Untangling the Standards.

Elma Bustronic
Fremont, CA.
(510) 490-7388