The SCSI protocol regulates the reservation procedure of the peripheral devices and determines the data format on the SCSI bus. The connection of the peripheral devices to the SCSI bus takes place in daisy chaining. Depending on the SCSI version, up to 16 peripheral devices can be managed via this daisy chain, whereby each unit can have up to eight Logical Unit Numbers( LUN) with its own ID number. The ID numbers are arranged hierarchically according to priority, with the SCSI host adapter having the highest priority.
Behind the SCSI protocol is a three- layer set of rules with command, protocol and interface levels. The core of SCSI is formed by the SCSI Primary Commands( SPC). All devices must be able to handle these commands. Based on this, there are special commands for individual device groups, the Controller Commands (SCC) for host adapters, the Block Commands( SBC) for hard disks and the Stream Commands( SSC) for tape drives, which make up the command level.
Below the command level, at the protocol level, are the protocols for the various interface types. These logical protocols control how a SCSI command is mapped out of the respective interface and how the individual devices communicate with each other. These protocols include the Serial Storage Protocol( SSP) for Serial SCSI( SSA) and the Generic Packetized Protocol( GPP), which help vendors of other interfaces gain access to SCSI. The lowest level is the interface level, where the various physical transmission media are defined.
The SCSI protocol has been modified in conjunction with the various SCSI versions, primarily to achieve higher performance. Thus, with newer protocol versions, SCSI commands can be issued without having to wait for the previous command to be acknowledged. In addition, several write and read commands can be issued simultaneously.