IBM 3708 Description page 71

SDLe
Transmission Frames
Transmission between an SNA host and the 3708 takes place according to a
predefined frame format that consists of the following sequence of bytes:
Flag (F) Sequence
Secondary Station Address (A)
Control (C) Field
Information (I) Field
Frame Check Sequence (FCS)
Flag (F) Sequence
1 byte (X'7E')
1 byte
1 byte
Up to 256 bytes of message data, plus header
information
2 bytes
1 byte (X'7E')
When sending or receiving over an SDLC link, the 3708 supports modulo-8 mode;
that is, the 3708 can accept or transmit up to seven frames at a time.
Data link control for the 3708 is identical to that discussed in IBM Synchronous
Data Link Control General Information, GA27-3093, with the exception that the Set
Normal Response Mode command causes the 3708 to reset from an Activated
Physical Unit (ACTPU) to a Deactivated Physical Unit (DACTPU). All sessions
must be restarted by the sequence starting with ACTPU.
Normal Response Mode
The 3708 functions in two link operating modes: normal response mode (NRM) and
normal disconnect mode (NDM). The 3708 can initiate transmission only as a result
of receiving from the SNA host a frame that contains the P (poll) bit set to 1.
Single or multiple frames may be sent. The last frame (or a single frame)
transmitted by the 3708 in response to a command received with the P bit set to 1
must have the F (Final) bit set to 1. When the 3708 has completed a transmission, a
new transmission cannot be initiated until a subsequent frame is received from the
SNA host that contains the P bit set to 1. A response transmission initiated by the
3708, which requires acknowledgment from the SNA host, is repeated each time the
SNA host polls until the acknowledgment is received. There is no limit to the
number of transmissions. Responses that require acknowledgment from the
communications SNA host are information (I) frames, frame reject (FRMR). and
receive ready (RR) when transmitted with the F bit set to 0, to report clearing of a
busy condition.
Normal Disconnect Mode
When in NDM, the 3708 cannot accept or transmit I or supervisory (S) frames. Any
I or S frames received by the 3708 is ignored and responded to with a disconnect
mode (DM). Nonsequenced responses are not transmitted unless the 3708 is
solicited to reply. Invalid or nonimplemented commands received in NDM cause the
3708 to transmit a DM response at the next response opportunity. DM can be
retransmitted until a set normal response mode (SNRM) command is received.
Command reject conditions are not present in NDM.
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IBM 3708 Description