PCI Express CONTENDS for communications role: scalable performance and low cost are making PCI Express attractive for communications-device designs, challenging proprietary-bus structures for next-generation design wins.(Cover story)

Replacing PCI as a peripheral bus in general-purpose computers, PCIe (PCI Express) is now seeking a role in communications. It offers the raw performance for communications and has significant cost advantages over today's popular proprietary buses. Its legacy link to the PC may limit its communications success, however, unless proponents can solve critical shortfalls in its architecture.

Decades after it surfaced, the IBM PC is still having a ripple effect throughout the electronics industry. The PC's immense popularity and resulting production volumes have made PC-centric technology both inexpensive and widespread. These advantages, in turn, have made the technology appealing to a variety of other applications. The PC's bus structures have spun off a number of derivatives, including PC/104, PXI, and CompactPCI, which make the PC's processor, peripheral devices, and software elements available to non-PC applications.

PCIe is the most recent version of the PC's peripheral-bus structure to begin finding its way into other applications. As with the earlier PCI and AT buses, PCIe is generating interest because it allows embedded-computing developers to use the proven, powerful, low-cost, and widely available technology that arises from desktop computing. Unlike the previous buses, however, PCIe offers performance levels that match the processor's--with room to grow. At the same time, PCIe retains software compatibility with the PCI bus, preserving the cost advantages that previous generations of PC technology have enjoyed (see box "PCI Express basics").

This combination of high performance and low cost has caught the attention of the communications industry. Traditionally, communication developers have used proprietary-bus structures to handle their highest performance needs. Cost and time-to-market pressures, however, have made proprietary approaches increasingly unattractive.

Still, the home that PCIe will find in the communications market is not yet certain. Communications devices have a broad range of needs, varying with their position in the network hierarchy. This hierarchy spans multiple levels with differing mixes of control and data-handling requirements (Figure 1).

VARYING NEEDS

The transport tier lies at the high-data-rate end. This tier provides long-distance data transport over high-capacity channels. Statically configured transport-tier devices do not interact significantly with the data they are transporting. As a result, these devices are not strong candidates for PCIe. Devices start to become aware of the data they handle at the core tier, although their interaction with the data is still limited. Core devices prioritize their functions based on the tags and labels added to the data in the lower tiers. …

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