VXS gigabit serial technology delivers extreme system performance

As embedded system components steadily advance in clock rates and I/O speeds, designers struggle to keep ahead of increasing interboard data transfer requirements. New standards using VXS gigabit serial links beat conventional backplane performance by several orders of magnitude.

Through evolutionary enhancements and new technology insertion, VMEbus still serves as the dominant bus structure for high-performance, real-time embedded systems. When VMEbus was first introduced, interboard transfer rates of 30 or 40 MBps of its shared bus backplane were more than adequate for most applications.

As requirements grew, VME acquired new interfaces including VSB, RACEway, RACE++, VME64, VME320, and 2eSST, thus ensuring a healthy community of suppliers and a new stream of backward-compatible products. This continues to be especially important to risk-averse government customers selecting products for long-term programs requiring support and maintenance for a dozen years or more.

Now after more than a quarter century of widespread deployment of VMEbus, the recent adoption of the VXS gigabit serial interface clearly represents the most significant leap of performance throughout its entire history. Following a review of the VXS standard and a look inside a representative product, the VXS system example presented offers ample evidence of this superior system architecture.

VXS: Switched fabric for VME

Motorola's VME Renaissance announcement in 2003 unveiled a new standard called VXS, officially designated VITA 41 by the VITA Standards Organization. It defines the implementation of gigabit serial technology for VME in a logically layered specification, while carefully preserving the legacy VMEbus form factors and bus operations. At the top layer, VITA 41.0 defines the connectors, pin designations, dimensions, and mechanical structures for cards and backplanes – completely free of any mandates for specific protocols or fabrics.

One of the biggest challenges for developers of the VXS standard was choosing a suitable connector that could handle serial signals at bit rates up to 10 GHz, while not interfering with existing mechanical structures such as legacy VME P1 and P2 connectors and modules. It also had to be readily available from one or more vendors at a reasonable price. The solution was Tyco's MultiGig RT2 connector supporting 16 differential gigabit signal pairs and 4 single-ended signals.

The 16 pairs are usually ganged into two 4X links, each with four transmit signals and four receive signals. Each link forms a full duplex channel capable of delivering data at 1.25 GBps in both directions simultaneously when operating at bit rates of 3.125 GHz on each serial lane.

VXS defines two types of cards: the payload card and the switch card. Both utilize the same mechanical outline as a standard 6U VMEbus card, thus preserving the extensive investment in VME card cage tooling and system packaging. The payload card fits the MultiGig RT2 connector in between the existing P1 and P2 connectors in a position designated P0. Most legacy VME backplanes provide clearance for the new P0 connector, thus allowing insertion of VXS cards for backwards compatibility even though the VXS interface is not engaged.

VXS backplanes have one mating MultiGig RT2 connector for each payload card slot but the VITA 41 specification does not dictate any specific backplane topology, leaving plenty of flexibility for various architectures. The simplest configuration is the VXS switchless mesh backplane. For example, three VXS cards can be joined in a ring configuration, with each card connected to the other two through two 4X links hard wired in copper as part of the backplane design. At a serial bit rate of 3.125 GHz, this system supports simultaneous data transfers between the cards in both directions at an aggregate rate of 7.5 GBps.

As a comparison, a traditional three-card VMEbus system shares a common backplane bus that allows one data transfer at a time in one direction at a time, and at rates up to only about 70 MBps – more than two orders of magnitude slower than its VXS counterpart.

However, instead of dedicated direct mesh wiring of the 4X serial links between cards, VXS also defines an alternative backplane topology that uses a switch card to route the 4X serial links. Although the switch card still retains the VME 6U form factor, it differs from the payload card in that it does not have P1 and P2 connectors. In their place are additional MultiGig RT2 connectors offering as many as 22 serial 4X links to the backplane for connection to multiple payload cards and other switch cards.

Figure 1 shows a typical "star" topology backplane with two 4X links from each payload card wired in copper to one switch card. Now each payload card can connect to any two other payload cards according to the routing paths provided by the switch card. The switch card can be dedicated to a particular serial protocol, which determines the switch path routing through inspection of packet header routing information. Alternatively, the switch card can be "transparent" to the serial traffic protocol, where the switch ports are assigned by some other method such as an onboard controller receiving commands over an Ethernet link.

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Figure 1
(Click graphic to zoom by 1.7x)

In either case, unlike the dedicated paths of the switchless backplane, the switch card topology offers excellent flexibility in assigning a data flow within the system. This can be a static data flow pattern established during initialization or dynamic routing during runtime. For high-reliability applications, two switch cards provide redundancy because payload cards can be joined through either switch.

Maximum data transfer rates in this switched star VXS system are 2.5 GBps in each direction for each payload slot, for a full duplex aggregate rate of 100 GBps for a 21-slot system. Compared to a legacy 21-slot VMEbus with its shared backplane operating at 70 MBps, this represents a thousand-fold data rate improvement. Unlike VME with its fixed maximum backplane speed regardless of the number of cards, VXS systems gain extra backplane bandwidth with each additional card.

Figure 1 also shows another key extension to VXS: GbE control plane links. Apart from the dual 4X data links, each payload MultiGig connector also supports two 1X GbE-X ports, each using two differential pairs, one for each direction. Multiple GbE-X ports from the payload cards are sent to one of the MultiGig connectors on the switch card, where they are connected with an Ethernet switch or hub. By allowing Ethernet commands to be passed between boards, these GbE-X ports are intended to replace the control plane function of the VMEbus.

VXS products exploit serial connectivity

Now only five years after its introduction, dozens of embedded computer hardware vendors have developed hundreds of VXS products. They include switchless mesh backplanes, switched star backplanes, complete card cages and chassis, switch cards, and converters, software radio cards, PMC and XMC carriers, single board computers, and RISC array processors, cards, and memory cards, as well as copper and optical LAN, WAN, and disk storage adapters.

VXS fully supports PMC mezzanine modules, allowing systems integrators to take advantage of the enormous base of existing I/O products available prior to VXS. Concurrent with the introduction of VXS, the gigabit serial extension to PMC known as XMC was introduced and later defined under VITA 42. XMC modules connect to the VXS carrier card with two or four 4X links, supporting transfer rates up to 5 GBps and eliminating the legacy PCI bus bottleneck.

As an example, Figure 2 illustrates the Pentek 4207 VXS PowerPC I/O processor with dual XMC sites and a wealth of critical interfaces for high-performance embedded systems. At its heart is an onboard fabric transparent gigabit serial crossbar switch, highlighting the vital role of serial technology for interconnecting resources within the board and to other boards across the backplane.

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Figure 2
(Click graphic to zoom by 1.3x)

VXS system example

Taking advantage of the tremendous bandwidth afforded by VXS, the 16-channel beamforming signal processor and recording system shown in Figure 3 digitizes 16 antenna IF inputs using four 4-channel A/D converter modules. Installed FPGA IP cores on the XMC modules perform digital down conversion from IF to baseband, producing four 15 MHz baseband signals, each requiring a data transfer rate of 75 MBps, or 300 MBps for each XMC.

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Figure 3
(Click graphic to zoom by 1.3x)

Model 4207, serving as a dual XMC carrier, delivers data from the two XMC modules across one 4X VXS link across the backplane to the VXS switch card at 600 MBps. A second identical 8-channel subsystem completes the front-end functions of digitizing and delivering data for all 16 channels.

Data streams from each I/O processor subsystem are alternately delivered through the switch card into one 4X port of a DSP VXS card where the beamforming algorithms are performed. After processing, the 1,200 MBps input is reduced to 600 MBps and delivered out across the second 4X port, through the switch card, into a third I/O processor on the right. Here, the data is buffered in memory, formatted into a Fibre Channel stream, and then delivered to a Fibre Channel disk array for real-time storage.

Aggregate bandwidth within this five-slot VXS system is 3.6 GBps, far exceeding traditional VMEbus capability, but yet comfortably below the 10 GBps limit for this VXS configuration.

VXS: Here to stay

With this elegant, high-performance technology for both data and control planes, the VXS standard opens up a whole new range of applications previously unattainable with the legacy VMEbus because of backplane bottlenecks. These applications include synthetic aperture array radar, military countermeasures systems, wideband communications, signals surveillance and intelligence, and high-resolution imaging and target identification systems. Supported by an active community of dozens of COTS VXS card vendors offering the market hundreds of products, VXS is assured a secure role in future embedded system solutions. CS

Paul Mesibov, cofounder of Pentek, serves as director of engineering. He has more than 26 years of experience in the electronics industry and is responsible for the design and implementation of Pentek's DSP, software radio, and products. He is currently an active member of , VITA, PCI-SIG, the SDR Forum, and IEEE. Prior to his current position, he was a design engineer at AP Circuit designing active signal processing filters and at Wavetek designing benchtop signal processing filter products. He holds Bachelors and Masters degrees in Electrical Engineering from the City University of New York. He can be contacted at paul@pentek.com.

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