The main advantage of switched fabric interconnects is speed, followed by redundancy, high availability and scalability. To increase total throughput of a parallel bus, it must either be sped up or widened, both of which tend to put more limits on the number of devices that can be effectively connected to the bus. With a switched fabric, each device is connected to every other device in the system through a network of connections. Therefore, several devices can communicate simultaneously. The point-to-point connections provide built-in redundancy. Multiple links to other nodes ensure that if one point goes down, there are other links throughout the system backing them up. Point-to-point connections are also inherently friendly to device insertion and removal. When the user wishes to scale up the system, additional points are easily added. Replacing the serial bus architecture with a switched fabric interconnect requires throwing out the VME bus as we know it. The evolution of integrated circuitry, mainly the high power requirements of processors operating at gigahertz speeds, requires several changes: from the mechanical to the cooling to the power supplies.

Starting with the subrack, VITA 34's initial developments look to have a module pitch on the backplane of 1.2". The modules are 4U or 8U high and 220mm deep. They are enclosed with sheet metal for several reasons. The enclosure will keep EMI from the electronics inside the module to a minimum and will keep external EMI from disrupting the circuitry within the module. An enclosed module allows a spray cooling application if desired. The mechanical task group is discussing how to assign the connector areas on the module. The current feeling in the task group is to have as many I/O pins as possible, with areas set aside for power, fiber optic connectors, keying and guide/ESD pins.

Due to the higher speeds of the signals on the backplane and modules, a new connector must be specified. The switched fabrics use differential pairs for signaling. Several connector manufacturers have stepped forward with their high-speed connectors designed specifically for differential pair signaling. Tyco and ERNI have presented the ZD connector, FCI and ITT Cannon the Metral line and 3M recently presented the HSHM connector. The task group will settle on one of these connectors in the next 3 months.

A power connector will also be chosen by the task group. Due to the lowering of supply voltages for integrated circuits and the corresponding rise in current requirements, it is no longer feasible to supply large amounts of 5V and 3.3V power directly to the modules. The current thinking is that the power connector will supply 48V to the module. AMP and FCI have bladed connectors the task group could choose, while Positronic Industries has developed a PCI series style connector for consideration.

Several other design considerations have not been discussed in detail by the VITA 34 task group. The group has seen presentations of spray cooling applications. While it is a viable solution to dissipating large amounts of heat, we'll have to see if the group tries to implement it in initial developments. Fan cooling will most likely remain the predominant cooling technique. A software task group has been formed, but action in that group will remain limited until the mechanical framework has been decided. It is unclear at this point whether a particular switched fabric will be chosen or if the framework will strive for a fabric agnostic approach.

This is an exciting time in the evolution of the next generation of communication methodologies. While standard VME won't be going away any time soon, VITA is looking toward the future of switched fabric technologies and preparing a framework.