SCSI is a layer 2 deterministic channel protocol. It’s initials stand for “Small Computer Systems Interface.” It is a standard protocol for computer systems to connect to storage systems. SCSI protocol runs on Fibre Channel (called fibre channel protocol or FCP), Infiniband, and layer 2 Ethernet.
The cost of layer 2 networks was substantially higher that standard TCP/IP Ethernet. TCP/IP Ethernet has become the ubiquitous networking standard for computer to computer interconnect. This created a ground swell of demand to run block storage (SCSI storage) on TCP/IP Ethernet. Running SCSI protocol on the non-deterministic layer 3 TCP/IP Ethernet required a different standard. SCSI is a deterministic protocol. TCP/IP is a non-deterministic protocol which means variable latency. ANSI, SNIA, and the IEEE working groups mapped the SCSI protocol to TCP/IP to enable the SCSI protocol to work on layer 3 TCP/IP Ethernet. This standard is called iSCSI. The result is a protocol that is not as low latency as SCSI running on Fibre Channel, Infiniband, or Converged Enhanced Ethernet (data center bridging protocol or DCBx), but it has become the most popular shared storage SAN or block storage interconnect in the market.