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Development of the No. 1 Electronic Switching System, a stored program system, by Bell Telephone Laboratories has required considerable program debugging and system testing. The bulk of this effort is done in model telephone offices at the Bell Telephone Laboratories locations in Illinois and New Jersey. However, for some actual telephone offices that are the first of a kind, incorporating Hew program or hardware features, system testing is done at the office itself. To facilitate this remote testing, a means of providing an up-to-date copy of the program at the test site had to be provided. This paper describes a means for meeting this need by use of data transmission over telephone lines. The system uses unique terminal units to provide compatibility between magnetic tape units at laboratory locations and memory writing equipment at the test site. Transmission of error check bits with the data and retransmission of data to correct detected transmission errors is provided.