Page 4 of 18 Fibre Channel: Why another interface? March 2001 Unfortunately, electrical and mechanical constraints limit the physical width and length of the cable and the number of devices that can be connected on a single SCSI bus. Increasing the number of parallel data lines to increase the data transfer rate requires wider cables and larger connectors. Increasing the bus clock rate requires that the data pulses be spaced ever closer together, making it more difficult to detect the end of one pulse and the beginning of the next. As the transfer rates increase, the effect of these constraints becomes more severe. Today’s manufacturers are striving to make devices smaller and less expensive. These efforts often conflict with the design changes needed to achieve the higher transfer rates. The larger, more complex cables and connectors are expensive, bulky, and prone to failure. The additional signal-processing needed to increase the clock rates also adds cost. When multiple devices are attached to the bus, complex arbitration and addressing protocols are required to control access to the bus and manage data flow. Furthermore, the available bandwidth must be shared among all devices. The increased processor overhead required to manage the bus and the need to share bandwidth degrade the performance of all the devices on the bus. The following table summarizes the advantages and disadvantages of a SCSI bus. In many cases, the disadvantages of parallel interfaces like SCSI outweigh the advantages. SCSI interfaces that function well in direct, one-to-one connections between servers and storage cannot support the levels of performance, manageability, and scalability required by today’s storage environments. SCSI bus advantages and disadvantages Advantages Disadvantages •    High-speed data transfer (up to 160 MB per second) •    Support for large data transfers •    Simple data transfer protocol •    In-sequence data delivery possible •    Low error rates •    Hardware-based signal processing and error correction •    Restricted to short distances (1.5 to 25 meters) •    Limited number of devices per bus (2 to 16) •    Single protocol •    Bulky, expensive cables and connectors •    Complex arbitration and addressing •    Shared bandwidth •    Not scalable •    Difficult to manage Electrical and mechanical constraints and complex data management issues make increasing the performance of a SCSI bus technically difficult and expensive to implement. p1   p2   p3   p4   p5   p6   p7   p8   p9   p10   p11   p12   p13   p14   p15   p16   p17   p18