Channels - IBM Mainframe

Perhaps the most interesting components of the 370 processors are the channels. The purpose of a channel is to provide a path between the processor and an I/O device. The above figure shows seven channels, numbered 0 through 5. As a result, there are eight different paths along which data can pass between the processor and I/O devices. Each channel can connect up to eight devices called control units that, as you will learn later in this chapter, connect to I/O devices. As a result, the processor can support up to 56 control units. Depending on the processor and the device, a control unit may be housed within the processor's cabinet, the I/O device's cabinet, or in its own cabinet.

Actually, a channel is itself a small computer: It executes I/O instructions called channel commands that control the operation of the I/O devices attached to it. As a result, the channel frees the processor to execute other instructions. Since channel processing overlaps CPU processing, overall system performance is improved.

The basic channel design of the System/370 requires that I/O devices be connected to channels using heavy copper cables that can be no longer than 400 feet in length. These channels use a parallel architecture, which means that the cable transmits all of the bits that make up a byte simultaneously. To do that, the cable must have a separate wire for each bit—sixteen in all (the channel sends two bytes simultaneously)—plus additional wires for control signals. The result is that parallel channel cable is both heavy and expensive.

In 1990, IBM announced a new channel architecture, called ESCON (for Enterprise System Connection), which is based on fiber optic rather than copper cable. Fiber optic cable is not only 80 times lighter than copper cable, but it is also 50 times less bulky. ESCON will allow many data centers to replace literally tons of unmanageable copper cable with neatly organized runs of fiber optic cable. Besides its reduced size and weight, ESCON provides two other advantages. First, it extends the 400-foot cable limit of standard channels to 26 miles. This lets installations locate disk devices on another floor or even in another building. Second, ESCON channels are nearly four times as fast as standard channels, transmitting data at 17MB per second rather than 4.5MB per second. This can reduce or eliminate the bottleneck encountered on many systems that use standard channels.

ESCON is still a new technology, one that most installations are not yet using. It will become more and more common with time, however, particularly as processor speeds and disk capacities continue to grow and the 400-foot, 4.5MB per second limitations of standard channels become more and more unbearable. In the new S/390 systems a new channel architecture—S/390 Fibre (Channel) Connectivity (FICON)—provides a new high-performance I/O channel, optimized for efficiency at high speed. FICON channel capacity for the S/390 G6 Server has been increased providing customers with improved performance and bandwidth over current I/O connectivity options.

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