Flow Control Method - Paradyne COMSPHERE 3920PLUS Technical Reference Manual

Flow Control Method

The data transmitted from the DTE is sent to the DCE at a rate determined by the transmit clock
frequency. When SDC is active, this data is then compressed in the DCE and sent to the remote
DCE at the line rate less than the the rate between the DTE and DCE (usually twice, three, or four
times less). If the data sent by the DTE is not compressible, or bad line conditions cause
retransmissions between the two DCEs, then the transmitting DCE must indicate to the
transmitting DTE its temporary inability to accept supplementary data; this is flow control. Flow
control is also required in Buffer mode since the DTE rate and the line rate can be different. Three
methods are used to control the DTE flow:
• Hardware signal CTS (circuit 106/Pin 5).
• Transmit clock (circuit 114/Pin 15). If the transmit clock is clamped, then the DTE stops
• Natural flow control. Some DTEs can only send a limited number of information frames
3920-A2-GH30-40
sending data. This method only works when the transmit clock is provided by the modem
(transmit clock source is internal to the modem).
without any acknowledgement from the remote DTE. This number is the acknowledgment
window size. If the DCE buffers this number of DTE frames, it will not receive another
DTE frame. This is equivalent to the SyncFlowControl configuration option being set to
None.
October 1998
Synchronous Data Compression
H-5