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Pacing
The 3708 supports only outbound pacing. Outbound pacing is set in byte 9 of the
BIND command and is used as a tuning parameter for the system. Usage comments
are included in this discussion; however, control is under the user's discretion at
NCP or some equivalent definition time.
The pacing count (N) determines the number of normal flow request RUs that can
be accepted before a pacing response is required from the 3708 to allow the next
group of N RUs to continue. A special response designated as Isolated Pacing
Response (IPR) is used to return the pacing response. The 3708 indicates readiness
with a pacing response as soon as all the received data has been transmitted to the
downstream device. RUs may vary in length as specified in the BIND parameter.
Pacing (LU type 1 SCS)
The 3708 requires that outbound pacing be set to some nonzero value for LU type I
SCS BINDs and rejects an LU type I SCS BIND with outbound pacing set to O.
Outbound pacing is required to prevent the 3708 from going into buffer slowdown
when supporting LU type I SCS data streams. As the 3708 receives chains of data
from the host, it receives a piece of a chain or segment of data and then passes the
data to the printer. If the host can transmit new data faster than the 3708 can
transmit it to the printer, the 3708 is forced to store all of the unprocessed data in its
internal system buffer area. Without outbound pacing, the host can cause the 3708
to use up all of its available system buffers, driving the 3708 into buffer slowdown
mode, thus degrading the performance of the entire 3708, and resulting in loss of
data. The outbound pacing parameter is used to ensure that there is adequate space
in the 3708 so that buffer slowdown does not occur.
Pacing (LU type 2 and LU type 3 DSC)
For LU type 2, the 3708 generally operates faster than the link, and pacing is not
required for the controllers.
For LU type 3, requiring a definite response indicator when the WCC Start Print bit
is set is an effective alternative to pacing.
In telecommunication networks in which RUs are processed through more than one
SNA host, outbound pacing may be required for type 2 and 3 LUs to prevent data
traffic congestion in these SNA hosts.
Pacing for LU type 1 NTO Sessions
Outbound pacing mayor may not be set for LU type 1 NTO sessions. Generally, if
the device being supported is a printing device or a mechanical keyboard/printer,
outbound pacing should be set to some nonzero value. If the downstream device is a
keyboard/display, then outbound pacing may not be required.
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IBM 3708 Description
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