Envoy ACP/XF Application Programming Manual

Normal Response Mode (NRM) Protocols
EnvoyACP/XF Application Programming Manual–132179
3-10
Two-Way Alternate (TWA) Versus Two-Way
Simultaneous (TWS) Operation
Two-Way Alternate (TWA) Versus Two-Way Simultaneous (TWS)
Operation
Two-way alternate (TWA) operation allows frames to be sent in only one direction over
the line at any given time. Two-way simultaneous (TWS) operation allows frames to be
sent in both directions at the same time.
You can get maximum line usage by using full-duplex lines in TWS operation.
TWA
Each station in the network can transmit frames only when the station owns the P/F bit.
When the primary or supervisor station transmits a frame, this bit is called the poll
bit or P bit; when the secondary or tributary station transmits a frame, this bit is called
the final bit or F bit.
Ownership of the P/F bit originates with the primary or supervisor station. The
supervisor station can send up to the number of frames allowed by its window size. The
modifier L2WINDOW determines the window size; the window size defaults to 7. When
the primary station has no additional frames to send or when the number of frames sent
reaches the configured window size, the P/F bit passes to the secondary or tributary
station.
A tributary maintains ownership of the P/F bit until the tributary sends a frame with the
F bit set. The tributary station can send up to the number of frames allowed by its
window size before relinquishing control of the P/F bit to the supervisor.
TWS
The supervisor can transmit frames at any time, regardless of ownership of the P/F bit. A
tributary can transmit only when it has ownership of the P/F bit. The tributary can send
up to the number of frames allowed by its window size before returning the P/F bit to the
supervisor. As soon as the supervisor receives the P/F bit, the supervisor passes the P/F
bit to another tributary, permitting that tributary to send its frames.
Ownership of the P/F bit originates with the supervisor. The supervisor passes the P/F
bit to a tributary by sending a frame in which the P bit is set. The supervisor gives away
ownership of the P/F bit whenever it can. This practice provides the most efficient line
use. The tributary maintains ownership of the P/F bit until the tributary sends a frame
with the F bit set (at which time the supervisor immediately tries to give the P/F bit back
to a tributary).
Operation Line Type
Frames can be
sent... Ownership of P/F bit
TWA Half-
duplex
In only one
direction at a time
Supervisor can transmit only when it
owns the P/F bit.
Full-duplex In only one
direction at a time
Supervisor can transmit only when it
owns the P/F bit.
TWS Full-duplex In both directions
at same time
Supervisor can transmit without owning
the P/F bit.