HP Surestore E Tape Library Model 6/60 HP SureStore E Tape Library Models 2/20 - Page 209

NOTE Introduction to Fibre Channel Fibre Channel Technology the combined hubs still form one loop. This is called cascading hubs. (See "Zoning" on page C-12 for dividing Fibre Channel topologies within the same physical configuration.) Some hubs do not work with more than a certain number of hubs cascaded in a series. Typically, the maximum number of hubs between any two devices is three, due to a lack of good signal regeneration. If too many hubs are cascaded, the Fibre Channel signal degrades to the point where data is not being transferred reliably. In addition to physically creating a loop with its ports, hubs also provide isolation for failed or disconnected ports. Hubs allow loops to be created without requiring that all ports are connected and all devices are turned on. Hubs also allow loops to be created using more than one type of connection media. For example, all Fibre Channel devices within buildings can use a short wave optical connection, and a long wave connection can be used between buildings. The disadvantage of hubs is in the area of performance. Only two devices on an arbitrated loop can communicate at the same time. As a result, the maximum average throughput to any one device is a fraction of the total bandwidth of the loop. Some of the advantages of hubs also apply to switches. Switches can connect up different devices using more than one type of connection media. In addition, switches allow a system to function with one or more ports, disconnected or turned off. Unlike hubs, switches (and the fabric in general) allow all devices access to the full bandwidth of the Fibre Channel system. Adding devices to a properly designed fabric does not affect the performance of the fabric in any way. Unfortunately, switches are more complex and costly than hubs.With hubs, a port is connected to the next port, forming a loop. With switches, a port is connected in a logical, or physical, crossbar to all other ports on the switch. As a result, all ports on a switch can connect to all other ports on a switch without any intervening ports. Switches also regenerate the data signal, which prevents cascading issues. Interconnecting switches is more complex than for hubs. With hubs, there is only one connection in between any two hubs. With switches, there are multiple connections needed in order to maintain the full bandwidth of the fabric. As with hubs, switches can have "zones" of allowed connections. (See "Zoning" on page C-12.) HP Fibre Channel libraries are designed to work with both hubs and switches equally, allowing you to use the topology that best meets your needs. Appendix C C- 11 Fibre Channel