Compatible Systems 5.4 - Manual (Page 227)

Chapter 14 - General 221
Physical T1 Configuration: WAN Dialog Box
Physical T1: WAN Configuration Dialog Box
To access this dialog box, select WAN/Physical Configuration from the
Device View.
Since many of the settings for a T1 line are dependent upon the service
provided by your ISP or telco, you may need to contact them to find out the
appropriate specifications. Unless otherwise noted, both ends of a T1 WAN
connection should have the same physical configuration settings.
> Clock Scheme
This set of radio buttons determines whether this interface will source clock
onto the T1 line (dry line operation) or accept clock from an external source
on a T1 line.