ABSTRACT
To understand the need for the isdn and the requirements for its implementation we must first understand the established services and the nature and requirements of the existing networks that are to be converted to the isdn. This preparatory treatment must necessarily be brief and must assume that the reader has considerable prior understanding of telecommunications. The proposal to introduce the isdn is predicated upon the existence of a digital network and the possibility of providing the digital interface at the user's premises. Local network line plant ranges from multipair cable in underground ducts in urban areas through duct cable plus overhead distribution in suburban environments to overhead cable using party-line, pair gain and carrier telephone systems in rural areas. Similar comments apply to the local exchange network with some additions. Within the trunk network ccitt requirements for overall transmission quality imply that the network must be four-wire, near-zero loss.
