This standard will encompass two levels of interface, defining operation over distances less than 10 m. The physical layer will specify electrical, mechanical, and thermal characteristics of connectors and cards. The logical level will describe the address space, data transfer protocols, cache coherence mechanisms, synchronization primitives, control and status registers, and initialization and error recovery facilities. The preceding statements were those submitted to and approved by the IEEE Standards Board as the definition of the SCI project. These goals have been met and exceeded: support for message-passing was added, and the operating distance is not limited to 10 m. (The intent of that limitation was to make clear that this is not yet-another Local Area Network.) The real distinction between SCI and a network has more to do with the memory-access-based model SCI uses and the distributed cache-coherence model. The practical operating distance depends more on the throughput and performance needed than on any absolute limit built into the specification. Very long links would yield unacceptable performance for many users (but perhaps not all). In particular, the fibre-optic physical layer can extend the SCI paradigm over distances long enough to link a computer to its I/O devices, or to link several nearby processors. No arbitrary length limit would be appropriate, but practical considerations including the throughput requirements and the cost of transmitters and receivers will set the lengths that people consider useful. A very-high-priority goal was that SCI be cost-effective for small systems as well as for the massively parallel ones mentioned in the purpose statement above. SCI's low pin count and simple ring implementation make medium-performance, few-processor systems easier to build with SCI than with bused backplane systems; a two-layer backplane should be sufficient, and three layers should be enough to support the optional geographical addressing mechanism. The SCI interface, complete with transceivers, fits into a single IC package that includes much of the logic needed to support the cache-coherence protocols. This economy for small systems leads to the expectation that SCI processor boards will be built in high volume, making them inexpensive enough to be assembled in large numbers for building supercomputers at low cost. SCI also simplifies the construction of reliable systems. SCI Type 1 modules are well protected against electrostatic discharge and electromagnetic interference, and can be safely inserted while the remainder of the system remains powered. SCI supports live insertion and withdrawal by using a single supply voltage (with on-board conversion as needed) and staggered pin lengths in the connector to guarantee safe sequencing. Note, however, that system software plays an important role in live insertion or removal of a module because the resources provided by that module have to be allocated and deallocated appropriately. In systems where several modules share a ringlet, the removal of one module interrupts all communication via that ringlet, so the resources on those modules also have to be deallocated. A similar situation arises in any system that may have multiple processors resident on one field-replaceable board: all have to be deallocated when any one is replaced. The system software for handling the deallocation and reallocation of these resources is outside SCI's scope. Although SCI does not provide fault tolerance directly in its low-level protocols, it does provide the support needed for implementing fault-tolerant operation in software. With this recovery software, the SCI coherence protocols are robust and can recover from an arbitrary number of detected transmission failures (packets that are lost or corrupted). The SCI paradigm removes the limits that bus structures place on throughput, but its latency is of course limited by the speed of signal propagation (less than the speed of light). Ever-increasing throughput can be expected as technology improves, but the organization of hardware and software will have to take into account the relatively constant latency (delay between request and response), which is proportional to the physical size of the system. The last generation of buses approached the ultimate limits of performance, leading to the concept of an ultimate standard. However, the initially defined SCI physical layers are likely just the first of a series of implementations having higher or lower performance levels. The 1 Gbyte/s link speed specified for the initial ECL/copper-backplane implementation was chosen based on a combination of marketing and engineering considerations. From a marketing point of view, it was necessary to define a territory that did not disturb the markets for present 32-bit standards or present networks, and from an engineering point of view this link speed was near the edge of what available signalling technology and integrated circuit technology could support. New technologies, such as better cables, connectors, transceivers; IC packages with more pins or higher power-dissipation capabilities; or faster ICs, could make it practical or desirable to implement SCI on new physical-layer standards. Such standards, with different link widths or bit rates, will be developed from time to time. However, packet formats and higher level coherence protocols will be the same across all these physical implementations. That should make the problem of interfacing one SCI system to another relatively simple – SCI already includes the necessary mechanisms to cope easily with speed differences.