IBM 3708 Description page 55

XON/XOFF Pacing
XONjXOFF pacing (also called DCI/DC3 pacing) is the method the 3708 has
implemented for regulating data traffic with an asynchronous device. The 3708
allows XONjXOFF pacing to be configured individually for receive and transmit for
maximum flexibility. An additional pacing option, RECEIVE QUEUE PACING
THRESHOLD, is available when Transmit XONjXOFF pacing is configured that
allows the 3708 to provide pacing on the receive queue.
Receive XON/XOFF
Choices are: 1 = Yes or O=No.
Many downstream devices use XONjXOFF pacing to control data flow. This
configuration parameter informs the 3708 whether the downstream device will be
using XONjXOFF pacing to regulate the transmission of data from the 3708. If
receive pacing is configured, the 3708 monitors the data received from the
downstream device and suspend data transmission on the detection of an XOFF
(DC3 - X' 13 '). Data transmission is resumed on the receipt of XON (DCI -
X' 11'). When Receive XONjXOFF is configured, the hexadecimal values X'II'
and X' 13' are not available for data because they are interpreted as control
characters . .If X' 11' or X' 13' are to be sent as data, they must be encrypted.
The 3708 should respond to XOFF after 8 or fewer characters have been transmitted
for all device types.
Note: Most personal computers, printers, and some keyboard displays support
pacing. See IBM 3708 Network Conversion Unit Planning and Installation for a
listing of suggested XONjXOFF settings for terminals that the 3708 supports.
Transmit XON/XOFF
Choices are: 1 = Yes or O=No.
When Transmit XON/XOFF pacing is configured, the 3708 uses pacing to prevent
receive queue overrun.
The 3708's transmission of XON/XOFF is determined by the setting of the Receive
Queue Pacing Threshold.
If the receive queue threshold is set to 0 and the 3708 is operating in protocol
enveloping mode or processing 3708 logon screens, the 3708 sends XOFF at the
following times:
• Upstream busy is detected by the receipt of an RNR from the host. XON is
sent when an RR is received from the host indicating the busy condition has
cleared.
• The 3708 enters buffer slowdown when system buffer availability is scarce.
XON is sent when the buffer slowdown condition has been cleared.
If the receive queue threshold is set to a nonzero value, indicating that receive queue
pacing is to be performed, the 3708 sends XOFF as follows:
• When the receive queue is 50% or 75% full depending on the threshold selected.
XON is sent when the receive queue is 25% full.
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IBM 3708 Description