V.42 Enhancements(VoiceView)
The following enhancements to previous implementations of V.42 in Hayes
modems are required.
Selective Reject
The modem receiver sends a SREJ frame to request retransmission of a
bad frame. This feature requires suitable buffering in the receiver. Buffering
of up to the frame window size is required.
Lost UA Tolerance
During a V.42 startup, the originator sends a SABME frame and expects
an answering UA frame before any data frames. The modem does not reject
the first (or any) data frames that are received after the modem has sent
its SABME frame. This lets the modem tolerate a missing UA frame (perhaps
due to line noise).
Send RR/poll on Idle
When the DTE stops delivering data to a sending modem, the modem normally
sends the last frame and stops. If this frame is not received at the other
end, that end times out and asks for the frame to be retransmitted. When
the last frame is sent, the modem follows it with a supervisory frame (usually
RR) in which the poll bit is set. This forces the receiver to respond and
indicate which frames have been received. If the response does not indicate
the last sent frame was received, the sending modem immediately repeats
the frame. This process speeds recognition of lost last frames.
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