No. 1 ESS ADF: Magnetic Tape Subsystem

Message switching systems designed for nationwide use must provide a high degree of transmission reliability and protection against loss of messages. Extensive efforts have been made in the design of the No. 1 ESS A D F to insure these system objectives. Once the system has accepted a message from its originator, the A D F system has the sole responsibility for a correct and guaranteed delivery. Nevertheless, messages received at a user's terminal may be lost by an attendant, may be garbled because of transmission difficulties such as fades, distortion, and line hits, or may be lost as a result of a paper jam in the receiving terminal. To counter such occurrences, the A D F system has a message retrieval feature, whereby a missing or questionable message may be retrieved upon request. Magnetic tape is the vehicle used for long term message storage and retrieval in the A D F system. Considerations which led to the selection of magnetic tape as the message recording medium included storage volume, access speed, and frequency of access. A copy of every message handled by the A D F system is saved for days in this message file, called the permanent file. This represents an exceptionally large storage requirement be2915