Available Characters By Receive Queue Size - IBM 3708 Description

If Transmit XON/XOFF pacing is configured, the 3708 always transmits XOFF
followed immediately by XON in the following instances:
• After each 3708 logon screen is sent
• After each screen or status is sent in protocol conversion mode
• After each screen or status is sent in control terminal mode.
These XON/XOFF transmissions are not used to pace the downstream device, but
are required for remote service.
Note: In order for a port to be used for remote service (Application of Microcode
patches), Transmit XON/XOFF must be set to I
=
ON and the Receive Queue
Pacing Threshold must
be
set to O.
Receive Queue Pacing Threshold
Choices are: O=None, I =50% Full, or 2=75% Full.
The receive queue pacing threshold is used by the 3708 to determine how full the
receive queue must be before pacing the downstream device. Receive queue pacing
can be defeated by selecting
o.
However, if the downstream service supports
XON/XOFF pacing, and this port is not used for remote service, it is recommended
that receive queue pacing be specified. Whether to select pacing when the queue is
50% full or 75% full depends on how fast the downstream device can stop
transmitting after receiving an XOFF, and how large the receive queue is. Only the
XOFF threshold can be selected, XON is sent when the receive queue is processed to
a point that it is only 25% full. The following chart shows the number of receive
queue character positions available when XOFF and XON is sent.
Threshold Selected
Receive
(#
of Characters Available in Receive Queue)
Queue
50 75
Size
XOFF XON XOFF XON
S
40
60
20 60
M
260 390
130 390
L
520 810 270 810
Figure 2-9. Available Characters by Receive Queue Size
For optimum performance, select a threshold of 75%. This percentage limits the
amount of pacing that must be done. A selection of 50% might be required when
small queue is specified and the downstream device cannot stop within 20 characters
after receiving XOFF. Another time that a 50% threshold may be required is for
personal computers performing file transfers. Some file transfer programs recognize
XOFF at block transfer boundaries. If this is the case, the receive queue size and
threshold must be configured so that the 3708 can complete the reception of a
complete block after transmitting XOFF. Otherwise, a receive queue overrun may
occur.
Chapter 2. 3708.0peration
2-15