E1 is the European format for digital transmission which is defined by ITU, E1 services is utilizing for the PABX system which is able to communicate internally and externally in the offices. 

E1 is the European format for digital transmission which is defined by ITU, E1 services is utilizing for the PABX system which is able to communicate internally and externally in the offices. E1 is connecting technically two network based on different protocols:
1.    E1s connects two networks based on different protocols such as: PRI-R2 & No7 and so on
2.    Each protocol has different specifications and layers
3.    Generally for only voice traffic easily can choose any of them, but for more functionally connecting to support Voice, SMS No7 signaling protocol is preferred 
 Media Connectivity:
1.    Optical fiber 
2.    Microwave 
3.    Satellite