SAN: FC vs SAS vs iSCSI

Question

I am currently preparing for an exam.
I thought FC (Fibre Channel) was the transport protocol for SCSI and SAN.
But now I read, that you have to decide if you want to use FC (SCSI), SAS or iSCSI in an SAN-Network.
Why can't you use SAS with Fibre Channel?

And that leads me to another question:
FC is designed for serial transport while SCSI is a parallel protocol. If the transport is serial and you have a point to point connection via FC-Switches, isn't that SAS?
How do the two protocols (FC+SCSI) fit together?

Can someone help me with this topic, since I'm really confused right now...

Many thanks in advance! :)

Kind regards,
Nic

Solution

"SAN" is a term for a network used to provide access to (block) storage. It can be anything.

SCSI is a family of storage protocols that all share a common core but have more or less subtle differences (apart from the obvious, physical ones). Examples include Parallel SCSI (obsolete), SSA (obsolete), Fibre Channel, SAS, iSCSI.

SAS and Fibre Channel are two different SCSI variants that each define their own physical protocols and thus, infrastructure.

iSCSI is another SCSI variant but instead of defining a physical implementation, it sits on top of TCP/IP which most often runs over Ethernet. iSCSI is hardware-agnostic in general.

Since all three are SCSI implementations, there are ways to make them compatible which each other (by multi-protocol bridging) or at least use them on the same physical infrastructure (e.g. FCoE). Bridging is costly however and most often you use just a single flavor or sometimes multiple in parallel.

PS: Parallel SCSI (SPI) is ancient and long obsolete. It was the foundation for the modern variants though and logically, all things SCSI use the same command protocol.