Communications - June 2009 - Research Highlights (Page 50)

The capacities and economics of computer systems do not advance smoothly, nor are the rates of improvement of various components synchronized. The resulting interactions produce dramatically different trade-offs in system partitioning that evolve over time. What is correct today may not be right after the next technology improvement. An example will illustrate the point.

Once upon a time, disk storage was expensive—really expensive—but it also exhibited significant economy of scale. At that time, LAN connectivity and processor performance were sufficient to make it desirable to share large disks among multiple workstations, giving rise to the diskless workstation. This lasted for a number of years, but as disks slid down the learning curve, the decreasing cost per megabyte of disk space overwhelmed the operational complexity of diskless workstations so they became diskfull, and they have been ever since—until relatively recently. Today the typical large organization averages the better part of one PC per employee, so the operational grief of administering all those desktop PCs is substantial. This cost is now high enough that the diskless workstation has been rediscovered, this time named thin clients. All the storage is elsewhere; nothing permanent exists on the desktop unit. History is busily repeating itself. Why? Because the various cost curves have moved enough, relative to each other, to the point where centralization makes sense.

The same thing happens with NFEs. At a point in time, systems don’t have enough network “go-fast” to deliver the performance required, so just add a dedicated processor to the network interface to make up for it. The economics of that are fleeting at best, however. Between chip design and system-integration complexity, an NFE will need to be an economically attractive solution for quite some time to recoup the development costs. Unfortunately, the relentless improvements in processor, memory system, and system interconnect in the base PC platform make that window of advantage a shrinking, fast-moving target. Does anyone else remember the HiFN file compression processor chip? It was built into PC systems for a very short time. Proces-

sors quickly improved enough to do compression/decompression on the fly, however, and that was the end of HiFN’s dream—well before the dropping cost of disk storage would have killed it.

Any effort to question the efficacy of NFEs should include a caveat for one particular case that merits a special mention because it indeed makes a compelling case for a particular style of NFE.

The proliferation of microcontrollers in devices such as thermostats, light switches, toasters, and almost everything else with more than a simple on/off switch has created a real opportunity for NFEs. Almost all of these microcontroller applications are typified by intense cost pressure, which usually translates into extreme limitations on available computing resources. It is simply out of the question to put a network stack in the vast majority of these systems, but the desirability of remote management of these devices increases daily.

This has created a new breed of NFE: the network communications adapter (NCA) that specializes in the simplicity of the protocol between the microcontroller host and the NFE— serial ASCII. Most microcontrollers have some serial port ability, so by looking like a terminal, the NCA can play the role of translator, speaking serial out one side and TCP/IP out the other. The NCA appears as a host on the TCP network, often containing a simple Web server that vends state information and may provide certain other management functions that get translated into simple ASCII exchanges with the microcontroller system.

An NCA is usually implemented in one of the more powerful microcontrollers that have been designed to provide an Ethernet interface and support enough RAM and ROM to contain a simplified network stack. The NCA is now available as an off-the-shelf module designed for easy integration no more difficult than a modem on a serial port.

The question of which is the tail and which is the dog comes to mind in many of these applications. From the TCP network’s point of view, the NCA is the host and the microcontroller is being managed. From the point of view of

the lighting controller, the NCA looks like just one more switch, albeit a chatty one. This distinction is usually irrelevant—it just makes hash of pedantic layering diagrams. There’s something quite satisfying about that.

Conclusion

Rather than debate the religious propriety of NFEs, particularly the TOE variety, I have examined the architectural issues that have produced their recurring rise and fall. The TOE-style NFE is best viewed as a tactical tool with a limited expected lifetime of economic viability, not an enduring architectural approach. This is just another example of the recurring ebb and flow of functions between specialized peripherals and the system CPU(s), as the economics slosh back and forth interacting with system requirements. The limited lifetime of the NFE’s advantages makes it difficult to justify the significant development costs for any but the highest-value applications.

That said, the inexpensive NCA is likely to be an approach that does endure. It literally transforms network communication into an inexpensive, pluggable physical component. By doing so, it provides an avenue for dealing with the extreme cost pressure inherent in microcontroller applications while providing an incremental option of genuine network citizenship when the customer will pay for it.