Storage Area Networks For Dummies®

Chapter 2: SAN Building Blocks
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knows it’s connected to that port. The WWN is also sometimes called a
WWPN, or World Wide Port Name.
The WWN and a WWPN are the exact same thing, the actual address
for a Fibre Channel port. In some cases, large storage arrays can also
have what is known as a WWNN, or World Wide Node Name. Some Fibre
Channel storage manufactures use the WWNN for the entire array, and
then use an offset of the WWN for each port in the array for the WWPN.
I guess this is a Fibre Channel storage manufactures way of making the
World Wide Names they were given by the standards bodies last longer.
You can think of the WWNN as the device itself, and the WWPN as the
actual port within the device, but in the end, it’s all just a WWN.
The name server is like a telephone directory. When one device wants
to talk to another in the fabric, it uses the other device’s phone number
to call it up. The switch protocol acts like the telephone operator. The
first device asks the operator what the other device’s phone number is.
The operator locates the number in the directory (the name server) in
the switch, and then routes the call to the port where the other device is
located.
There is a trick you can use to determine whether the WWN refers to a
server on the fabric or a storage port on the fabric. Most storage ports’
WWN always start with the number 5, and most host bus adapters’ start
with either a 10 or a 21 as the first hexadecimal digits in the WWN. Think
of it like the area code for the phone number. If you see a number like
50:06:03:81:D6:F3:10:32, its probably a port on a storage array. A
number like 10:00:00:01:a9:42:fc:06 will be a servers’ HBA WWN.
✓ SCSI: The SCSI protocol is used by a computer application to talk to its
disk-storage devices. In a SAN, the SCSI protocol is layered on top of
either the FC-AL or FC-SW protocol to enable the application to get to
the disk drives within the storage arrays in a Fibre Channel SAN. This
makes Fibre Channel backward-compatible with all the existing applications
that still use the SCSI protocol to talk to disks inside servers. If the
SCSI protocol was not used, all existing applications would have needed
to be recompiled to use a different method of talking to disk drives.
SCSI works a bit differently in a SAN from the way it does when it talks to
a single disk drive inside a server. SCSI inside a server runs over copper
wires, and data is transmitted in parallel across the wires. In a SAN, the
SCSI protocol is serialized, so each bit of data can be transmitted as a
pulse of light over a fiber-optic cable. If you want to connect older parallel
SCSI-based devices in a SAN, you have to use a data router, which acts as a
bridge between the serial SCSI used in a SAN and the parallel SCSI used in
the device. (See “Data routers,” earlier in this chapter, for the gory details.)
Although iSCSI and Infiniband protocols can also be used in storage networks,
the iSCSI protocol is used over an IP network and then usually bridged into