Also known as the V interface.The ISDN point of reference for the point of interface at the network side of the connection between the line termination or loop termination and the exchange termination. In other words, it is the point between the circuit terminating equipment and the ISDN central office (CO).As the V interface exists only if the CO does not have embedded circuit terminating equipment, it is unusual in contemporary ISDN-compatible COs. See also CO and ISDN.