IBM 3708 Description page 293

Protocol Conversion
In protocol conversion mode, PCs performing file transfer must emulate one of the
3708's supported displays. These consist of several default terminals and up to six
user-defined terminals (UDTs).
Unlike in protocol enveloping mode, where file transfer is performed a "line" at a
time, the transmission of files in protocol conversion is based on "screens" of data.
Protocol Conversion Data Flow
As in protocol enveloping, when data is received from a downstream PC, it is placed
in a "receive queue". See Chapter 4, "Protocol Conversion Mode" for more
detailed information on data flow in protocol conversion mode. See
Appendix E, "Key Data Formats and Control Blocks" for a description of internal
3708 queues and buffers.
As the 3708 processes characters from the receive queue, it performs the protocol
conversion, echoes the characters back to the PC, and places the data in a device
buffer. The device buffer is used to emulate the terminal's screen (usually, 24 rows
by 80 columns, plus 80 bytes for a status line).
The 3708 continues to process characters from its receive queue until it recognizes an
AID key sequence (AID key sequences represent the PFI - PF24, PAOI - PA03,
Clear, and Enter functions). Upon processing an AID key sequence, the 3708
transfers any 3270 fields that have been modified into one or more system buffers
and transmits the updates to the SNA host.
Overflow of the Receive Queue
Encryption
As in protocol enveloping, it is possible to overflow the 3708's receive queue. Unlike
protocol enveloping, however, if this should happen, the session is not ended; rather,
the 3708 logically locks the PC's keyboard and either transmits an "OVRFLO"
status line message to the PC (if the status line is toggled on or the terminal supports
a 25th addressable line) or transmits a BEL character (if the status line is toggled
off). In any case, the PC's file transfer program must be able to handle unsolicited
status line (or BEL) notifications from the 3708.
To recover, the PC needs to transmit the RESET key sequence for the particular
terminalit's emulating and retransmit the data. Thus, instead of using XON/XOFF
pacing or the larger receive queue sizes, file transfer programs in protocol conversion
may elect to wait for an overflow condition to occur and then recover" using the last
character echoed by the 3708 to determine where their data stream was interrupted.
In protocol conversion mode, the 3708 emulates 3270 functions via certain control
character sequences. For example, on an IBM 3101 display, the keystroke sequence,
ESC 8, represents the function key PF8. To transmit control characters such as ESC
through the 3708, the customer's PC file transfer program must employ some
encryption scheme.
Binary data must always
be
encrypted.
D-6
IBM 3708 Description