LTG its a siemens terminology, for Line Trunk Group.
DIU OR DSL: Digital interface Unit : is the interface where the actual E1s are terminated
Four DIU are connected to One LTG card.Siemens switch has a maximum of 256 LTG. that means u can have 256*4=1024 number of E1s terminated in a single switch.
I hope i have cleared u r doubt.