IBM 3174 Introduction Manual page 29

CSCM also makes it possible to re-IPL the remote 3174 from the central site,
significantly reducing the need for manual intervention at the remote site.
Although not required, the 20MB Fixed Disk Drive feature is desirable for the 3174
controllers that will participate in central site change management. At central site
controllers, the 20MB Fixed Disk Drive improves usability by eliminating diskette
swapping when libraries for large networks are being maintained. For remote
controllers, it allows a backup copy of the current IML Control disk microcode to be
retained when migrating to a new microcode release.
Central Site
3174
Network Site
3174
Host
System
Network Site
3174
Network Site
3174
Network Site
3174
Figure 2-4. Electronic Distribution Using Central Site Change Management
2-14
IBM 3270 Information Display System 3174 Establishment Controller Introduction