Specifications
Escala Tower PL & S, E, T System Hardware
Chapter 3: External Storage and SAN Subsystems 83/116
9es3s1c3.doc
Rev 5.9
02/12/2003
6.3.4.3. Switched Loops
Switched loops are the synthesis of switching technologies and fibre channel private loop protocol.
They enable private loop devices, which do not support fabric connection, to benefit of improved
performance compared to "simple" loops.
With this technology, several small private loops, called
looplets
are interconnected by a switching
device called
switched hub
or
storage switch
. This device creates a single logical private loop
compliant with the FC-AL standard (with up to 126 devices), but enables several simultaneous
communications between couple of ports, like a fabric switch. The device intelligently controls loop
protocols, like the initialization and the arbitration protocols and enables several devices to acquire the
loop simultaneously, as long as they are not in the same looplet.
looplet
looplet
looplet
looplet
looplet
Single logical loop
switching
hardware
storage switch enabling
several independent
communications
This kind of features can be available in managed hubs relying on an internal switching architecture, or
can be emulated on fabric switches. Depending on the implementations, the logical loop can be limited
to a single hub/switch, or can spawn across multiple hubs/switches.
A storage switch increases the performance compared to hubs, without using fabric protocols. But it is
more limited in term of addressing, interconnection and routing.
6.3.4.4. Mix of Private Loops and Fabrics
The connection to a fabric, either in point-to-point or through hubs requires specific protocols that are
not yet available on all the devices. Thus, fabric switch vendors have implemented various proprietary
solutions to support on the same hardware, full fabric operations, private loop emulation, and bridging
capabilities between fabric capable devices and "private loop" devices.
The characteristic of each piece of equipment must be carefully checked before using these features in
a SAN implementation.