Two-stage switching, used for cards in opposite planes (Fig. 4b), would progress as follows:
- The source function card in the first plane communicates with the switch card in the second plane.
- The switch card in the second plane communicates with the switch card in the first plane.
- The switch card in the first plane communicates with the destination function card in the second plane.
While the dual switch cards in each plane are necessary for redundancy in the switch-oriented system, these cards could also be used in tandem for load sharing.
The two system-area networks of Figures 5 and 6 indicate another possible advantage. Figure 5 depicts a general description of a system-area network, or data center. Figure 6 shows how this could work in a single system using the virtual midplane approach.
All functionality of a system-area network, at least at a first level of detail, can be built into one system via the virtual midplane structure. Redundant paths, and one- and two-stage switching, are utilized throughout the example. These are all available with the interconnect structure being described and by using the switch-fabric approach of Figure 5.
Some system architects may choose to add direct connections between function cards in opposite planes to realize the shortest path between critical functions. Redundancy would still be available if the switch path re-mained. For a system with a fixed definition of function cards for each card slot, the direct function-card-to-function-card connections would be a straightforward exercise.
But for a system that needs flexible scalability (like the ability to add routers, servers, controllers, and memory, as a given application requires), it can become a complex issue that necessitates methodical planning. That's because the location of the connection between two cards varies slot by slot. Unless a connection is made at every intersection of cards, the connection location on a given card might not correspond to the correct card in the opposite plane. Or even more disconcerting, the connection on the opposing card at the expected intersection point may not exist.
An alternative for the flexible structure described here is to use only direct connections between cards, possibly at every card intersection, rather than switch cards. This solution presents its own complexities, which must be carefully studied and evaluated during system architecture design. In an extremely complex or extremely high-performance system, the solution could ultimately prove very powerful.
An example application may involve a high-end communications system. The interconnect at each card would, effectively, be a semi-mesh network, where each card would be connected to all cards in the opposite plane. This might work especially well if all traffic is always sent from one plane to the other. A switch on each card would, of course, allow for connections of all cards.
Issues: Because the virtual midplane structure is new and radically different from the norm, there are still a number of critical issues to be resolved:
- What applications are most suited for this structure?
- How can this configuration be optimized for a particular application?
- How can connectors and cages be implemented that don't require the traditional midplane or backplane printed-circuit card?
- What is the best configuration for connectors?
- How will cards be inserted?
- What is the best way to provide power distribution?
- How can adequate airflow be achieved?
Consider the airflow issue. If the card cage is arranged so one plane of cards is vertical while the other is horizontal, one would have easy access to the cards from two sides of a cabinet. But the horizontal cards would have little or no natural airflow. Horizontal fans would be necessary for one side of the card cage.
Another option would be to place the card cage so that all cards are oriented vertically, similar to the orientation in Figure 1a. Airflow may not be an issue in this case, but access to the cards from the top and the bottom would be required. Also, the size of the connectors may be too big, thus blocking the airflow. While this vertical option may be possible for a single-cage system, it would require some innovation in a rack-mount version. A card cage that has a sliding access, for instance, could be used for this configuration.
Another suggestion orients the card cage so that every card is at a 45° angle. Though somewhat restricted, airflow would be possible in this configuration.
Also, some wasted space would exist within the rack. Testing would be required to ensure adequate airflow. In the final analysis, horizontal fans for the horizontal cards and vertical fans combined with natural airflow for vertical cards may be the most straightforward solution for card access and viable airflow.
Power-distribution concerns point, once again, to the switched solution. It may be possible to distribute power from the switch cards, which would always be present in a working system. This approach also provides redundancy for the power, as well as the switching. Of course, placing a midplane card for interconnect would also allow for power distribution.
Without a doubt, the virtual midplane is a most intriguing and promising configuration for the ultimate card-to-card interconnect solution in system networking. Whether it can deliver all that it promises remains to be seen.