E1 is a primary rate in the PDH system.approximately 63 E1(2Mbps) make up an STM-I which is the 1st data rate for an SDH system.
basically in a GSM network, STM-1 155Mbps is used to interlink the BTS and BSC or BSC to MSC as the case may be.and act as backbone link for the network.
becourse of it high capacity rate,the full trunk/traffic could be transmitted through this medium to the switching center, and it can.
1. allow a simplified add and drop at any BTS or BSC.for any of it E1 it carrying.
2.since is an SDH stardard, new protocols, ATM,FRAME-RELAY,ETHERNET 100Mbps,etc can be transmitted over it
3.fully scallable and be administar at remote.
finally,the cost of equipment ar lesser compare to PDH system and not as bogus as PDH which entails each site demultiplexing.
at higher level of STM-4, 16, 64. fibre is the only medium, why STM-1 can take both medium radio and fibre