14 November 1996 3980-A2-GZ40-20
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
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 more data; this is flow control. Flow
control is also required in Buffer mode since the DTE rate and line rate can be different. Three methods are used
to control the DTE flow:
•Hardware signal CTS (circuit 106).
•Transmit clock (circuit 114). If the transmit clock is clamped, then the DTE stops sending data. This
method only works when the transmit clock is provided by the modem.
•Natural flow control. Some DTEs can only send a limited number of information frames without any
acknowledgment 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.
SDC Negotiation
The use of SDC and the values of the associated parameters are negotiated at link establishment via a protocol.
The three associated parameters have the same meaning as those used for asynchronous data compression:
•Dictionary size parameter
•Maximum string length parameter
•Data compression request parameter
The SDC Negotiation configuration option determines how two modems connect when one or both modems have
SDC enabled. Two settings are available in the SDC Negotiation configuration option: LAPM_Buffer and
LAPM_Discon.
Table 5 shows the six possible SDC Negotiation configuration scenarios.