ITU-T Rec. Q.553 (1988)
1 General
2 Characteristics of interfaces
2.1 Characteristics common to all 4-wire analogue interfaces
2.2 Characteristics of interface C11
2.3 Characteristics of interface C12
2.4 Characteristics of interface C13
3 Characteristics of half connections
3.1 Characteristics common to all 4-wire analogue interfaces
3.2 Characteristics for interface C11
3.3 Characteristics for interface C12
3.4 Characteristics for interface C13