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The Teletype Model 33 contained an answer-back mechanism that was generally used in dial-up networks such as the TWX network. At the beginning of the message, the sending machine could transmit an enquiry character or WRU (Who aRe yoU) code, and the recipient machine would automatically initiate a response which was encoded in a rotating drum that could be programmed by breaking off tabs.[11] The answer-back drum in the recipient machine would rotate and send an unambiguous identifying code to the sender, so the sender could verify connection to the correct recipient. The WRU code could also be sent at the end of the message. A correct response would confirm that the connection had remained unbroken during the message transmission. Finally, the sending machine operator would press the disconnect button. Note that the receiving machine did not need operator intervention. Since messages were often sent across multiple time zones to their destination, it was common to send a message to a location where the receiving machine was operating in an office that was closed and unmanned overnight. (...) The communications module in the Model 33 was known as a Call Control Unit (CCU), and occupied the space to the right of the keyboard and printer. Various CCU types were available. Most operated on the Telefone network and included the relevant user controls; variants included rotary, DTMF ("Touch-Tone"), and card dialing. An acoustic coupler for a then-standard Telefone handset was also available. Another CCU type was called "Computer Control Private Line", which operated on a local 20 mA current loop (the standard serial protocol for computer terminals before the rise of RS-232). "Private Line" CCUs had a blank panel with no user controls or displays.