Specifications

Escala Tower PL & S, E, T System Hardware
Chapter 3: External Storage and SAN Subsystems 79/116
9es3s1c3.doc
Rev 5.9
02/12/2003
The bandwidth and connectivity limitations are inherent to the protocol used. The risk of loop
disruption is addressed by the use of Fibre Channel hubs, which implement a loop within the device, in
conjunction with bypass circuits for each hub's port. The bypass circuits maintain the loop integrity in
case of unused port or faulty external devices. They also simplify the cabling, converting the loop
cabling to a star cabling.
External star cablin
g
Rx
Tx
Rx
Tx
Rx Tx
bypass circuits
inactive port
bypassed
inactive port
bypassed
Hub’s internal
Internal loop
Interconnected devices
In practice, a hub may contain one or more logical loops. The repartition of ports in several loops is
performed either with hardware switches, that control loop continuity or loopback, or through software
configuration.
6.3.3.3. Fabrics and Switches
A fabric is a network where, contrarily to loops, several couples of devices can communicate
simultaneously, and can benefit of the whole bandwidth. The fibre channel fabric can be compared to
other switched networks, like Ethernet, ATM or frame relay networks. Each switch's port analyses the
frame headers, selects the destination port, and puts the frame in the appropriate transmission queue.
Non blocking designs and advanced forwarding algorithms allow a very low latency (few micro
second), and a full bandwidth per port, regardless of effective traffic conditions.
The advantages of switches versus hubs are:
•
they enable the deployment of large networks
•
they offer higher performance and scalability (no bandwidth sharing, selection of the best past, …)
•
they provide better fault isolation
•
switches and fabrics are more stable that loops (less reconfigurations and traffic interruptions)
•
they have built-in management functions (directory, …)
The drawback is usually a higher price per port.