IBM Tape Solutions for Storage Area Networks and FICON

Chapter 1. Storage area networks 13

Figure 1-9 Fibre Channel switched topology (cascaded switches)

1.4.3 Extended distance fabrics

Extending distance across a WAN is a big topic. It is covered only briefly here to show what

you can achieve, but not how you can achieve it. As stated earlier, the distance of a SAN can

be extended beyond the normal 10 km achieved with Fibre Channel longwave GBICs to about

100 km for synchronous transfers and to virtually unlimited distances for asynchronous

transfers over a WAN. In both cases, additional components are required to extend the fabric.

Synchronous transfer is limited by the speed of light in glass. A Fibre Channel frame in which

data is packaged spans just over 2 km of fibre. Therefore at 10 km, there can be almost five

frames in the pipeline. At 100 km, there can be almost 50 frames. The Fibre Channel

architecture includes a feature know as

buffer credits, where the sender is only allowed to

send as many frames as he has received buffer credits from the receiver.

To maintain performance at distances above 10 km, and fully use the potential bandwidth, a

switch can use multiple frame buffers. The buffers allow the switch to remember the data until

the data is successfully acknowledged. The switch uses these buffers together with a greatly

increased number of buffer credits for the port that is attached to the remote switch. The

number of buffer credits is normally controlled by a software feature in the switch’s operating

system. For example, in the IBM 2109 switch, the Extended Fabrics feature must be licensed.

In addition to extra frame buffers, there must also be a mechanism to physically send the light

over longer distances. Many products are available to do this. Two examples are the Finisar

Optilinx 2000 Optical Link Extender and the IBM 2029 Fibre Saver (see Figure 1-10).

Switched cascaded fabric

Any-to-any connectivity

Fibre Channel

Switch

Fibre Channel

Switch

Fibre Channel

Switch

Fibre Channel

Switch