Page 3 of 18 March 2001 Fibre Channel: Why another interface? The bus approach A bus usually operates in a closed, structured, and predictable environment where the host “knows” all of the devices that can communicate with it in advance. A bus is designed to transfer large amounts of data between the host and an individual peripheral device. These data transfers can be many thousands of bytes in size. Because the transfer occurs over a direct one-to-one connection, it is fast and has inherently low error rates. Any needed error detection and correction is handled in hardware; very little intervention by the system CPU is required. The familiar SCSI bus, often used to connect tape drives and libraries to servers, is a parallel data bus that uses multiple lines to transmit blocks of data in multi-bit pulses. In addition to multiple lines for transferring data, a parallel data bus includes lines carrying signals that define the type of information on the data bus and signals that take care of flow control messaging between the system and the device. The SCSI standard defines both the characteristics of the physical interface used to transfer data and the signaling protocol used for device control. During a parallel bus data transfer, the bits making up each data pulse, along with error detection information, are transferred simultaneously from the source to the target. Because the connection between the sending device and the recipient is direct, the transmitted data bits arrive at their destination in the same order as they were sent. This eliminates the need for expensive processor overhead to reassemble the data stream and makes data transmission extremely fast and reliable. Over the years, the SCSI interface has evolved to provide ever-increasing data transfer rates. The increased transfer rates are achieved either by using a wider data bus (more parallel lines) to increase the number of data bits that can be transferred at one time or by decreasing the amount of time between successive data pulses (faster bus clock rate). A SCSI bus transmits multiple bits of data over multiple parallel lines SCSI host bus adapter Control Signals Data Bus Server Buffer CDROM drive Tape drive Disk array A SCSI bus provides fast, relatively error-free transfer of large amounts of data over a dedicated connection between a single server or workstation and one or more peripheral devices, such as tape and disk drives. p1   p2   p3   p4   p5   p6   p7   p8   p9   p10   p11   p12   p13   p14   p15   p16   p17   p18