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

In the blocking protocol, the total size of the single read buffer is communicated. The write buffer in the other host is allocated based on the size of the Read buffer.

In the nonblocking protocol, the total number of read buffers available, as well as the size of each buffer, is communicated. Each host must then allocate the write portion of its channel program to match the read portion of the other host.

CTCA

Channel-to-channel adapter

PUNS

Physical unit services

SSCP

System services control point

TSC

Transmission subsystem component

1 PUNS queues a RUPE to NCBPUPAB, causing TSC to get control.

2 TSC posts PUNS.

3 Subarea 4 assumes X-side protocols because it is initiating the XID exchange.

4 The XID exchange times out because the link in subarea 2 has not been activated. The CONTACT is issued in subarea 2, and upon attention, subarea 4 assumes Y-side protocols.

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

6 The subarea with the lower subarea number (subarea 2) responds first to the XID0 exchange by sending an XID7. (XID7 is an XID Format 2 with the contact option field set to 7.)

7 Subarea 4 checks the XID7 from subarea 2 and responds with its XID7. It also sends a CONTACTED RU to the SSCP.