HP Mainframe Connectivity Design Guide

send more data than the ISL or path bandwidth can accommodate. ISL trunking can help
balance the load between physical ISL ports and prevent oversubscription.
• Fabric interconnect speeds
FICON supports 16 Gb/s, 8 Gb/s, 4 Gb/s, 2 Gb/s, and 1 Gb/s speeds. The highest
performance is attained by configuring a fabric with all components at the same,
highest-available speed. Additional factors such as distance, number of buffers, and device
response times can also affect performance.
Currently, no CHPIDs or CUs support 16 Gb/s. However, 16 Gb/s speeds can be supported
as ISLs.
• Mixed FICON speeds
For fabrics consisting of 16 Gb/s, 8 Gb/s, 4 Gb/s, 2 Gb/s, and 1 Gb/s directors and
devices, the fabric segment connections negotiate the speed at which devices communicate.
The presence of lower-speed devices in a fabric does not force other independent higher-speed
devices or paths to a lower-speed. FICON requires that:
◦ All 16 Gb/s ports be able to negotiate to 8 Gb/s and 4 Gb/s speeds
◦ All 8 Gb/s ports be able to negotiate to 4 Gb/s and 2 Gb/s speeds
◦ All 4 Gb/s ports be able to negotiate to 2 Gb/s and 1 Gb/s speeds
Director ports or device ports in a fabric communicate at the highest mutually-supported speed.
Performance guidelines
By following the rules and recommendations in this guide, you can maximize performance.
Recommendations on the number of ISLs ensure that adequate bandwidth is available across the
fabric, minimizing oversubscription.
To maximize FICON SAN performance:
• Implement dual-fabric FICON SANs.
• Use the highest speed available for all infrastructure components and devices.
• Match director port speeds to CHPID and CU port speeds.
• Ensure that the ISL port speed is the same (or higher) as that used for the CHPID or CU port.
• Distribute CHPIDs from a Sysplex across multiple boards in the director.
• Distribute CU ports from a storage system across multiple boards in the director.
• Distribute ISLs between a pair of directors across multiple boards in each director.
• Connect devices that communicate frequently to the same FICON director.
• In a cascaded fabric, connect the directors with the highest port speeds to the channels.
84 FICON SAN fabric connectivity and director interoperability rules