Signal functions in the EIA232 standard can be subdivided into six categories. These categories are summarized below, after which each signal described. a) Signal ground and shield. b) Primary communications channel. This is used for data interchange, and includes flow control signals. c) Secondary communications channel. When implemented, this is used for control of the remote modem, requests for re transmission when errors occur, and governance over the setup of the primary channel. d) Modem status and control signals. These signals indicate modem status and provide intermediate checkpoints as the telephone voice channel is established. e) Transmitter and receiver timing signals. If a synchronous protocol is used, these signals provide timing information for the transmitter and receiver, which may operate at different baud rates. f) Channel test signals. Before data is exchanged, the channel may be tested for its integrity, and the baud rate automatically adjusted to the maximum rate that the channel can support.