Storage Area Networks For Dummies®

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Par t I: SAN 101
When you were a kid, did you ever try to create your own little phone network
by using a string to connect two paper cups? Although it was probably
a bit hard to hear your friend on the other end, the “phone” actually
worked. The vibrations from your voice were transmitted over the string (the
medium) to the paper cups (the devices), which allowed your friend to hear
your voice, and you took turns talking and listening (the protocol); taken
together, the cups and string were really a simple network.
Okay, that was fine for only two people, but what if little Johnny across the
street also wanted to play? You could always tie on another string and paper
cup — but if all three of you spoke at the same time, distinguishing one voice
from another would be really hard. Not only that, but string probably didn’t
transmit vibrations very well. If more kids were going to play, you needed a
better way.
Electrical engineers figured out that using copper wires to carry electrical
signals through transmitters and receivers worked a lot better than string
and paper cups. In a common telephone handset, the transmitter is the little
device you speak into that converts your voice into the electrical signals that
get sent over the wire to someone at the other end. The receiver converts the
electrical signals representing your voice back into the vibrations that the
person on the other end hears as your voice through the handset speaker.
Today’s phone networks tie together billions of people. With all that talking
going on, every conversation has to be intelligently routed to the person
you are trying to call. The intelligence is implemented as a protocol — a set
of rules that all the devices follow. It’s a common language for the devices,
if you will; it lets all those conversations take place without getting jumbled.
The phone network is nothing more than a well-organized bunch of wires and
switches connecting all the phones together, but with intelligence built in to
control how everything works.
Telephone networks for voice communication operate very similarly to
the Internet for computer communication (in fact, part of the Internet still
uses phone lines). In storage communication, the network is made of servers
and storage arrays (the devices) connected together via optical cables
and switches (the medium) — and the rules for communicating between the
devices (the protocol) is Fibre Channel. Everything together makes a storage
area network, or SAN.
Moving Data at the Speed of Light
Using copper as a network connection medium has its limitations. First,
copper is heavy; the poor guy who runs all the cables gets a real workout.
Second, electrical signals traveling over copper wires lose strength when