Networks use a protocol as the language that each device on the network understands so the network doesn’t become a Tower of Babble. Each device using the network needs to speak a common language. Think of it this way: Ifeveryone in your neighborhood spoke a different language, communicating with each other would be virtually impossible. If you pick up the phone and call someone in France, you’d better be able to speak French or your conversation won’t get very far.
And just as different dialects of the same language can be spoken in different parts of the same country, so can different protocols be used by similar devices to describe the same thing. Take United States English vernacular,for instance. If you were in a grocery store in New York and wanted something to put your food in, you’d ask for a bag; if you were in Georgia, you’d ask for a sack. Both mean the same thing but are called something different depending on where you come from. Computer networks and storage networks have a similar need for all the components to speak the same language so they can communicate effectively.The process of using a protocol is similar to how the telephone network operates.Wires coming into your house connect your phone to the telephone network. The only way you know you can use that network to make a call is when you hear a dial tone. No dial tone, no call. Therefore the dial tone in a phone network acts as the basic protocol (between you and your phone) for understanding whether a connection can be established. When you use a cellular phone, you still hear a dial tone, but cellular phones use a completely different type of network. So the dial tone acts as the signal that lets you know that the basic network is functional and a call can be placed. A dial tone on a phone network is a basic signal, no matter whether it’s a copper wire network or wireless network.When you dial a number and someone picks up the phone on the other side, a higher-level language comes into play; which one it is depends on the country you’re in, and on the country you’re calling. In France, the language would be French; in the United States, the language may be English; and so on. The spoken language over a phone network works at a higher level than a basic dial tone. The dial tone serves as a low-level protocol that lets you know the network is working. The spoken language serves as a higher-level protocol that you can use to get things done. Many higher-level languages can be spoken over a phone network; the network doesn’t need to understand the higher-level language for the network to work.The phone network itself just provides the transport mechanism for moving the spoken words from one place to the next. All you need for a conversation to be successful is that the people on each end of the network understand
the same higher-level language being used. The same holds true for computer networks. Each device on the network needs to understand the same higherlevel language for a conversation to take place between devices.
Continuing with this analogy, in a storage network, the underlying “dial tone” is the Fibre Channel protocol. The “spoken language” that your applications use to talk to the hard drives is the SCSI (Small Computer System Interface) protocol. The SCSI protocol, as a “higher-level language,” is layered on top of the Fibre Channel protocol “dial tone.”
Two types of “dial tones” (that is, protocols) are used in Fibre Channel
storage networks:
✓ Fibre Channel-Arbitrated Loop (FC-AL) protocol helps establish data communication in network loops, which are created by connecting the
devices with hubs.
✓ Fibre Channel-Switched (FC-SW) protocol helps establish data communication in the SAN fabrics created by connecting devices with switches.
Source: Storage Area Networks For Dummies
