The backplane and hardware module standards help to increase part commonality and the reuse of components in different system architectures and applications, but this is only one part of the system design challenge. While the specified footprint, backplane format, and electrical signal characteristics help the design of modular hardware and open architectures, they still tell very little about how modular (and unambiguous) the interfacing among functions and their interactions are. This aspect is covered at the system integration (network) layer.

Figure 1. Logical View of VPX Backplane with external Ethernet backbone, computing modules, and remote IO.

VPX, as a switched fabric, supports the design of advanced integrated systems using technologies such as deterministic Ethernet, which can be used in backplane and backbone applications. In cases where functional interrelationships and Ethernet network bandwidth sharing is deterministic and all logical links among critical function have configurable quality of service with guaranteed timing, the complexity challenges in design of advanced integrated architectures can be much simpler to handle and mitigate. This enables design of truly open and flexible modular embedded systems, which can host hard real-time, real-time, and soft functions at lower system lifecycle costs. Incremental modernization is fully supported, and new functions can be added without influencing already integrated ones.

VME and the VPX Market

Table 1. Real-Time and Bandwidth Performance of VPX Backplane.
VME has been used for over 30 years in different industries, and its latest VME64 64-bit backplane can move 40MByte of data per second between plugged cards. Over the years, many extensions have been added to the VME interface (VME64x), providing “sideband” channels of communication in parallel to VME itself, to enhance bandwidth capabilities.

VPX (ANSI/VITA 46.0-2007) is positioned as a VME successor and backplane standard for design of rugged modular systems, designed by the VITA and its 100+ members. The VME embedded market today in aerospace and defense applications is estimated to be $600M (~20% is VPX). The difference in market numbers comes from the fact that VME boards are used for upgrades to existing systems, while VPX is used in new designs.

The VPX standard enables integration of high-speed serial switched fabric interconnects such as PCI Express, RapidIO, Infiniband, and 10 Gigabit Ethernet to satisfy high bandwidth requirements in a minimized footprint. Around this VITA (www.vita.com) standard, an ecosystem of COTS products has developed since 2008, with over 400 board products provided by major COTS board suppliers, such as Curtiss-Wright, GE IP, Kontron, Mercury, and over 30 other suppliers.

VITA 68 Bandwidth Extensions for VPX Backplane

Figure 2. Gigabit-Ethernet switches with different Ethernet traffic classes (QoS Layer 2 services).
As VPX was designed, the network bandwidth was significantly lower for many of the networking technologies used only 7-8 years ago. VITA 68 was introduced to support the integration of extended bandwidth and integrity guarantees for high-bandwidth networks into the VPX backplane format.

VITA 68 defines a VPX compliance channel, including common backplane performance criteria, required to support multiple fabric types across a range of defined baud rates and bit error rates (BER) for different fabric types. With this standard, the latest Ethernet physical layers (1000T-KX, 10GT-KX4), sRIO (6.25Gbps), Infiniband DDR (5Gbit/s) and QDR (10Gbit/s), and future technologies (100Gbps Ethernet 802.3bj) are and can be supported in VPX. The excess bandwidth does not assure deterministic operation, but new high-bandwidth Ethernet variants with 10+ Gbps and Layer 2 QoS extensions can enable the design of deterministic distributed functions and well-defined use of shared networking resources.

In the VPX standard, there is the difference between extension plane (PCIx, S-ATA, ...), control plane (Gigabit Ethernet), data plane (Inifiniband, serial RapidIO( sRIO), 10G Ethernet), and utility plane (clock sync, power …). In Figure 1, a logical view of inter-module connectivity and integration with Ethernet VPX backplane and backbone is presented.

While InifiniBand and sRIO offer high bandwidth for distributed processing of large data volumes, they do not provide temporal guarantees for design of real-time systems. With Gigabit-Ethernet at a control plane, it is also not trivial to design hard RT systems and ad vanced integrated architectures. However, with Gigabit-Ethernet switches, which support deterministic QoS Layer 2 services, VPX plays the role of key platform technology for the design of advanced integrated systems with time-critical (hard RT), mission-critical, and safety-critical systems, which are simpler to design, integrate, maintain, verify, certify, and reuse. Essentially, the deterministic Gigabit-Ethernet switching devices for VPX backplane are not different from any standard Ethernet switch. Critical functions can take advantage of QoS services, while for all other less critical functions the network operates as any other switched Ethernet network.