If you are creating a point to point E-1 line then you must have a clock source. If your E-1 line is provided to you by your carrier provider then most likely they will provide to you a clock source off the line. In this case use clock source line. However, if this line is an in house copper line with no intermediate equipment in the middle of it then one side needs to be the clock source (clock source internal) and the other side needs to be set for clock source line. Either way, you must understand how clocking is provided to you or how you need to implement it into your network. If any of your clock definitions are wrong then you will get errors on your data..Good Luck...