ITU-T standard for physical and logical traits of transmissions over digital circuits, including US 1.544Mbit/s and European 2.048Mbit/s (typically used to refer to the latter).
ITU-T Recommendation G.703, "Physical/Electrical Characteristics of Hierarchical Digital Interfaces".
ITU-T recommendation for the physical and electrical characteristics of hierarchical digital interfaces at rates up to 140 Mbit/s. http://www.itu.int/rec/recommendation.asp?type=folders&lang=e&parent=T-REC-G.703
ITU standard for E1 circuits (ISDN2) G.821 ITU standard for ISDN30
An ITU standard for the physical and electrical characteristics of various digital interfaces, including those at 64 kbps and 2.048 Mbps.
ITU-T electrical and mechanical specifications for connections between telephone company equipment and DTE using BNC connectors and operating at E1 data rates.
G.703 is a ITU-T standard for transmitting voice or data over digital carriers such as T1 and E1. G.703 provides the specifications for pulse code modulation (PCM).