Arbitrated loop(AL) vs Switched fabric (SW)

Rambo

Arbitrated loop, also known as FC-AL, is a
Fibre Channel

topology
in which devices are connected in a one-way loop fashion in a
ring topology
. Historically it was a lower-cost alternative to a
fabric topology
. It allowed connection of many
servers
and
computer storage
devices without using then very costly
Fibre Channel switches
.
As of 2007
the cost of the switches dropped considerably, so FC-AL is rarely used
for a server-to-storage communication. It is however still commonly
utilized on backend of some
disk array controllers
.

• It is a
  serial
  architecture that is compatible with
  SCSI
  , handling up to 127 ports (devices). One port may optionally connect a loop to fabric switch port.
• The bandwidth on the loop is shared among all ports.
• More than 2 ports on the loop can communicate at the same time (there is no concept of a "token").
• Arbitrated loop with only 2 ports is a valid one, and while having the same physical topology as
  point-to-point
  still acts as a loop protocol-wise.
• Arbitrated loop can be physically cabled in a ring fashion or using
  a hub. The physical ring ceases to work if one of the devices in the
  chain fails. The hub on the other hand, while maintaining a logical
  ring, allows a star topology on the cable level. Each receive port on the hub is simply passed to next active transmit port, bypassing any inactive or failed ports.
• Fibre Channel ports capable of arbitrated loop communication are NL_port (node loop port) and FL_port (fabric loop port), collecively referred to as the L_ports. Physical connectors on the hub are not considered ports in terms of the protocol.
• An arbitrated loop with no fabric port (with only NL_ports) is a private loop.
• An arbitrated loop connected to a fabric through FL_port, is a public loop.
• NL_Port must provide fabric logon (FLOGI) and name registration
  facilities to initiate communication with other node through the fabric
  (to be an initiator).

Switched fabric in Fibre Channel
In the Fibre Channel switched fabric topology (called FC-SW), devices are connected to each other through one or more
Fibre Channel switches
.
This topology allows the connection of up to the theoretical maximum of
16 million devices, limited only by the available address space (224). Multiple switches in a fabric usually form a
mesh network
, with devices being on the "edges" ("leaves") of the mesh. While this topology has the best
scalability
properties of the three FC topologies, it is also the most expensive, the only one requiring a costly
fibre channel switch
.
Visibility among nodes in a fabric is typically controlled with
zoning
.
Most Fibre Channel network designs employ two separate fabrics for
redundancy
.
The two fabrics share the edge nodes (devices), but are otherwise
unconnected. One of the advantages of this topology is capability of
failover
, meaning that in case one link breaks or a switch is out of order,
datagrams
can use the second fabric.


Example topology of a
Fibre Channel
switched fabric network
I
               
               
               

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