I/O buffering

During the XID exchange, each host informs the other about its read buffer size.

Figure 1. MPC activation flow

CTCA: Channel-to-channel adapter
PUNS: Physical unit services
SSCP: System services control point
TSC: Transmission subsystem component

Note: When MPC is used for an APPN host-to-host channel connection, the flows between the SSCP, PUNS, AND TSC components differ somewhat from those in this figure. The XID flows are the same.

1 PUNS queues an ACTLINK RUPE to the MPNCB PUPAB, causing TSC to get control.

2 TSC issues a DCM channel program on every device in the MPC group.

3 A single ACTLINK response is passed to PUNS and SSCP indicating that the MPC path ACTLINK initialization is completed.

4 PUNS XID0 is copied to all MPC subchannels so that they are being written to all MPC subchannels. In this case, subarea 2 XID0s are timed out and the other side is not ready to read.

5 Assume the operator activates the link in subarea 2. Subarea 2 assumes X-side protocols.

6 Subarea 1 initiates writing XID0s at all MPC subchannels and then completes.

7 The subarea with the lower number (subarea 1) responds first to the XID0 exchange by sending an XID7.

8 Subarea 2 checks the XID7 from subarea 1 and responds with its XID7. It also sends a CONTACTED RU to the SSCP.