Storage Networking Building Blocks

Mar 8, 2002

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IP SANS: A Guide to iSCSI, iFCP, and FCIP Protocols for Storage Area Networks: A Guide to iSCSI, iFCP, and FCIP Protocols for Storage Area Networks

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3.2 Legacy SCSI Cabling

SCSI cabling is a parallel wiring plant analogous to common personal computer parallel printer cabling. In the SCSI cable, each bit of a data byte is given its own wire. A SCSI cable may have 8 (narrow SCSI) or 16 (wide SCSI) data lines, plus control lines to manage device selection and data transmission. The effective transmission rate is governed by the number of data lines provided and the clock rate at which bytes of data are sent concurrently. Although new parallel SCSI designs such as high-voltage differential and low-voltage differential (LVD) have enabled higher throughput, the overall distance limitation of SCSI cabling is 25 m, and device support is limited to 15 devices on a SCSI chain.

In a parallel SCSI configuration, the sending party places the 8 or 16 bits of data on the cable plant and toggles a control line to indicate transmission. Small differences in propagation delay may occur as the data bits are sent, so it is essential that the receiving device be able to capture all bits accurately within a certain window of time. This window is known as skew. The greater the differences in propagation delay along the length of the cable, the wider the window must be for all data bits to be captured. Parallel transmission thus favors shorter cable lengths to minimize skew and to avoid data corruption. To achieve an effective bandwidth greater than serial gigabit transports, the SCSI cable must be less than 12 m long.

Distance limitations and limited device population have relegated SCSI technology to the middle and low end of the storage market, where resource sharing and high availability are not required. Additionally, adds, moves, or changes in a SCSI configuration may necessitate downtime and disruption to users. This is less likely to be the case in gigabit storage networking.

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