Enterprise System Connectivity (ESCON) channel

ESCON replaces the previous S/370 parallel channel with the ESCON I/O interface, supporting additional media and interface protocols.

By replacing the previous bus and tag cables and their multiple data and control lines, ESCON provides half-duplex serial bit transmission. In contrast to the previous copper cables used in the parallel channel, ESCON utilizes fiber optic cables for data transmission.

Reminder: Half-duplex for ESCON is effectively a request-response format. Bi-directional communications are possible, but the synchronization of the ESCON I/O protocol limits communications to half-duplex.

An ESCON channel executes commands presented by the standard z/Architecture or ESA/390 I/O command set, and it manages its associated link interface (link level/device level) to control bit transmission and reception over the physical medium.

Figure 1 for an illustration of channel operation data flow. At the top layer, an application makes an I/O request using a macro or a supervisor call (SVC). This in turn causes a START SUBCHANNEL (SSCH), which moves the I/O request to the CSS. The SSCH includes a subsystem identifier (SSID) and operation-request block (ORB) as its operand for execution of the channel program.

Figure 1. ESCON channel operation flow

ESCON has a somewhat different topology for control unit and channel attachment compared to a parallel channel. ESCON control units can be connected:

Directly to an ESCON channel, which is called point-to-point, or
Dynamically switched through a device called the ESCON Director, which is called switched point-to-point

In order to accommodate parallel channel-attached control units and devices, the ESCON conversion mode allows communication from an ESCON channel to parallel channel-attached control units.

The maximum channel data rate of an ESCON channel is 17 MBps (megabytes per second) and the maximum unrepeated distance is 3 kilometers.