In the past, data center security was simpler to implement; in fact, there was a time when data center managers could see all the inputs and outputs to the mainframe in one or two rooms. However, this was when data was input via punched cards and output was recorded on tape or impact printers using green bar paper. A terminal had to be added as a logical unit and data center managers knew exactly who had access to the mainframe. Even when someone was logged in, managers always knew exactly what users were doing. Because computing resources were so precious, any abnormal behavior would have some effect on the environment. The SNA network was secured because all the devices on the network were defined. A physical survey of the data center was all that was needed to ensure it was secured.