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Change Direction:
The 3708 uses a half-duplex, flip-flop (HDX-FF) mode to
transfer normal flow data in protocol conversion. Only one of the two LUs in the
session may send at a given time. The flip-flop protocol demands that, when one
LU is sending, the other must be prepared to receive. Therefore, the two states of
send and receive (RCV) must be active on each end of the session.
A bit in the request header, called the Change Direction (CD) indicator, is used to
keep the two end-point LUs in synchronization. Each time an LU accepts this CD
in a request, it means it is that LU's tum to send. Each time an LU sends the CD
in a request, that LU must then be prepared to receive. The 3708 always sends a
CD with an LIC or OIC in an FMD RH. Exceptions may occur following negative
responses.
Protocol enveloping LU type I (NTO) sessions may use a BIND specifying either
half-duplex flip-flop or half-duplex contention.
When in half-duplex flip-flop mode at session activation, one half-session is
designated HDX flip-flop bidder and the other is designated HDX flip-flop first
speaker. The 3708 is always the first speaker. Using Bracket protocol with
half-duplex flip-flop protocol requires a synchronization between the two
half-sessions. When between brackets, each half-session is in contention state: either
may send. The contention winner is always the first speaker. When not between
brackets, the half-sessions are subject to the protocol: the send issues normal flow
requests and the receiver issues responses. When the send completes its transmission
of normal flow requests, it transfers its control of sending to the other half session
by setting the Change Direction indicator on the last request sent.
When in half-duplex contention at session activation, one half session is designated
the contention winner and the other, the contention loser. Initially, both winner and
loser are in the contention state, and either one may independently begin sending
normal flow requests. Normal flow requests arriving at the loser, if it is sending are
queued. Normal flow requests arriving at the winner, if it is sending, may be
temporarily queued or may be rejected with an appropriate response. Valid normal
flow requests, arriving at a nonsending half session, place the half session in a
receiving state. The contention winner or loser reverts to contention state after
sending or receiving the last request of a chain. Upon reverting to the contention
state, a contention loser or a contention winner that queues received BIUs may
remove any requests and responses. Contention can be avoided through end user
protocols or by use of the Change Direction indicator.
Sessions operating in HDX use the following error recovery procedure. The
contention loser is responsible for the recovery. The contention loser half-session
assumes an HDX sending state at the appropriate moment after detecting an error
and initiates the sending of requests to attempt recovery. Correspondingly, the
contention winner half-session assumes an HDX receiving state and awaits recovery
requests from the contention loser.
If the 3708 is in flip-flop mode and receives an invalid frame, it sends a negative
response and remains in a receive state even if the invalid frame contains CD!. If
the 3708 is in contention mode, it sends a negative response and changes state if the
invalid frame contains CD!. It enters a send state only at the end of the chain or if
a CANCEL is received from the host.
Chapter 3. SDLC and SNA Communication
3-37
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