its 13 Mhz, basic diff between RRI and E1/T1 is with RRI – e1 dropping is logical 16 E1 supported to have physical dropping at site you need FIU 19 /IDU. Whereas E1T1 support physical dropping for 4 E1. generally at last site in chain you can use E1T1 else rri is better option. br//jignesh