IBM 3708 Description page 289

As the 3708 processes each character from the receive queue, it echoes the character
back to the PC (if echoplexing is configured for the port) and places the character in
a system buffer. Each system buffer holds 256 data characters plus the information
necessary to envelop the data with SNA headers. When 256 characters are received,
the 3708 transmits the system buffer to the host as either the first or middle element
of an SNA chain.
The 3708 continues to process characters from. its receive queue until it recognizes a
defined LT A character or a configurable text timeout period is exceeded. At this
point, the last bit of data (including the LT A character, if received) are transmitted
to the host as the end element of the SNA chain. (If the text timer expires, or a
LT A character is received before the first system buffer is filled, the 3708 transmits
the system buffer as an "only in chain" element.)
Overflow of the Receive Queue
Depending upon the 3708's configuration, the speed at which file transfer is being
performed and the work load on the 3708, it is possible to overflow the 3708's
receive queue. Should this happen in protocol enveloping mode, the 3708 causes the
session to be lost and transmits an alert, A102 'RCV QUEUE OVERRUN' , to the
control terminal and SNA host.
To prevent an overrun of the receive queue, the 3708 can be configured to transmit
XON/XOFF pacing characters (TRANSMIT XON/XOFF = 1). When
TRANSMIT XON/XOFF = 1 is configured for a port, the user may also specify
the XOFF threshold with the RECEIVE QUEUE PACING THRESHOLD option.
The RECEIVE QUEUE PACING THRESHOLD allows the user to specify whether
the 3708 transmits an XOFF when its receive queue is 50% (RQPT= 1) or 75%
(RQPT=2) full. The 3708 transmits an XON
(X'll')
to the downstream device
when the receive queue is 25% full.
For XON/XOFF pacing to successfully prevent the receive queue overrun, the
downstream device must stop transmitting within a few character times after
receiving a XOFF from the 3708. If the downstream device does not do this or does
not recognize XON/XOFF characters (TRANSMIT XON /XOFF = 0), the 3708
allows the control terminal operator the ability to configure the size of the port's
receive queue. The control terminal operator may chose one of three receive queue
sizes (79, 519, and 1079 bytes) for each downstream port. The receive queue sizes
may be configured independently of any of the pacing options.
The 79 byte receive queue is the default and is recommended for those ports that will
be
handling normal interactive traffic. The larger receive queues are chosen at the
expense of 3708 system buffers. The constraint is:
(# of 519 byte ports)
+
2(# of 1079 byte ports)
+
2(# of ML U ports)
+
2(# of large screen ports) < 40 - 2A
where MLUs are the number of ports defined for device type 4
(display-with-printer), large screen ports are the ports that have 3270 Model
Emulation specified as 3, 4, or 5, and A is the number of ports not excluded.
Note: More system buffers are available and more larger receive queues can be
supported if unused ports are excluded by the control terminal operator.
If XON/XOFF'pacing is not supported, the customer's PC file transfer program
must transmit "lines" of data less than the configured receive queue size.
D-2
IBM 3708
Description