thanks agha , Ur answer was really helpful , but this will lead me to another question playing in my mind for a long time…. what is the root of any E1 in huawei after it reach to the BSC , i mean i can see link magazines like RTN620 in the BSC with E1 cables and i don’t know where these E1’s go into…there is also cabinets in the BSC which contain a large amount of E1 cables and the OSN which have fibers and converting fiber to E1 cables connected to those came from BSC cabinets or whatever , and also when the BSC is near the CO(central office) it connected to it somehow to reach the MSC ,, i don’t no exactly what is going on