Standards exist for DTE Control of DCE using Serial Data Interchange and Start-Stop framing (TIA/EIA-602, CCIlT V.25bis). When user data is not being transferred, DTE commands and DCE responses are delivered on the same data paths used for user data, such as V.24 circuits 103 and 104.

The above mentioned Standards use out-of-band mechanisms for control while user data is being transferred, such as V.24 circuits 108/2 and 109 for call control and status, and circuits 133 and 106 for flow control. All commands or status messages delivered on circuits 103 and 104 must be applied while user data transfer is suspended or terminated.

This Standard describes procedures for a DTE and DCE to exchange control and status using only the data transfer path. There is need for In-Band mechanism for DCE control, because Out-of-band mechanisms are not available on all DTE, due to interface restrictions or DTE system software.

These procedures depend on the integrity of the data transfer path. It is preferred that the DTE-DCE link should provide some means to prevent loss-of-data errors or other corruption of In- Band control information. The specification of means to prevent loss-of-data is beyond the scope of this standard.

These procedures must be time-independent because inter-character delay time cannot be preserved in DTE containing multi-tasking software, large character buffers or intermediate data links (e.g., Local Area Networks).

This standard defines adaptation of Basic Mode Control Procedures from ISO Standard 2111 for time-invariant In-Band signaling of the following functions:

a) Define a mechanism for in-band delivery of control and status information between DCE and DTE

b) Represent V.24 circuits using transparent In-Band Commands.

c) Poll the corresponding device for its V.24 circuit status.

Using these mechanisms, these In-Band Commands may be used to provide:

d) In-Band commands for Flow control (circuits 106 and 133)

e) Data stream termination (circuit 108/2 w/AT&D1 setting)

f) Terminate session (circuit 108/2 w/AT&D2 setting)

g) Report DCE hook status to DTE

h) Represent BREAK and Mark-Idle signals be character sequences.

i) encapsulate ordinary TIA-602 or TIA-615 formatted commands and responses in-Band.

Other uses of in-band Basic Mode control mechanisms are reserved for future study.

This standard also defines semantics for control of these mechanisms (section 8).