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The subscriber loop multiplexer is a combined carrier and switching system for serving 80 main stations on 24 channels. The system has been designed for use in rural areas to handle growth and upgrading from multiparty service toward individual service. The system consists of a control terminal and up to six remote terminals located near the customers being served. The system is digital and uses a bit rate of 1.544 Mbit/s for the repeatered line interconnecting the terminals. The system has many maintenance provisions and alarms that have been economically achieved through digital integrated circuits. The test desk can initiate a series of tests on the voice frequency loop extending from the remote terminal to the customer. Delta modulation provides a simple and economic conversion between analog and digital forms. At the remote terminals each main station Served has an individual modem allowing equipment to be added as required to serve customers. The system utilizes 14 codes of beam-lead integrated circuits of which 10 are bipolar and 4 are insulated-gate field-effect transistor (IGFET). The IGFETs provide the adaptive delta modulation control and also perform the time division switching function at the remote terminals.