Technical data

Figure 6–3: Arbitrated Loop Topology
NL_Port
NL_Port
Node 1
Node 2
NL_Port
NL_Port
Transmit
Receive
Transmit
Receive
Transmit
Receive
Transmit
Receive
Node 3
Node 4
Hub
ZK-1535U-AI
6.2 Fibre Channel Topology Comparison
This section compares and contrasts the fabric and arbitrated loop topologies
and describes why you might choose to use them.
When compared with the fabric (switched) topology, arbitrated loop is a
lower cost, and lower performance, alternative. Arbitrated loop reduces
Fibre Channel cost by substituting a lower-cost, often nonintelligent and
unmanaged hub, for a more expensive switch. The hub operates by collapsing
the physical loop into a logical star. The cables, associated connectors, and
allowable cable lengths are similar to those of a fabric. Arbitrated loop
supports a theoretical limit of 127 nodes in a loop. Arbitrated loop nodes are
self-configuring and do not require Fibre Channel address switches.
Arbitrated loop provides reduced cost at the expense of bandwidth; all
nodes in a loop share the bandwidth (100 MB/sec per loop), and bandwidth
degrades slightly as nodes and cables are added. Nodes on the loop see
all traffic on the loop, including traffic between other nodes. The hub can
include port-bypass functions that manage movement of nodes on and off the
loop. For example, if the port bypass logic detects a problem, the hub can
remove that node from the loop without intervention. Data availability is
then preserved by preventing the down time associated with node failures,
cable disconnections, and network reconfigurations. However, traffic caused
by node insertion and removal, errors, and so forth, can cause temporary
disruption on the loop.
Using Fibre Channel Storage 6–7