ITU-T Q.553

This Recommendation provides characteristics for 4-wire analogue interfaces, input and output connections with 4-wire analogue interfaces and half-connections with 4-wire analogue interfaces in digital transit and combined local and transit exchanges in accordance with definitions given in ITU-T Rec. Q.551. This Recommendation is intended for switching connections that may be part of an international long-distance connection via 4-wire line circuits interconnected by 4-wire exchanges. Since 4-wire analogue interfaces on digital exchanges may connect with circuits which are used for both international and national traffic, the same values recommended for international connections may also be used for connections entirely within the national network.