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ZyXEL modem FAQ List v4.2, Nov 20 1995, Part 3 of 5 [Technical FAQs]
Section - T.9 What is the selective reject in V.42?

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Selective reject is an optional feature of the V.42 error correction protocol.
The v6.10 ROMs were the first ZyXEL ROMs that implemented selective reject.


Brent Mosbrook (brentm@zyxel.com) [ZyXEL USA] writes:
Selective reject is an optional feature of V.42 which allows you to get 
higher throughput on noisy lines.  The modem sends out blocks, and the 
remote side acknowledges after an agreed upon number of blocks (window 
size).  If one of these blocks is bad, then the modem will only send the 
bad one over again, rather than all the blocks (as is the case of other
modems).


The selective reject feature only works when both modems implement it.  Thus a
ZyXEL with 6.10 ROMs will not use selective reject connected to a ZyXEL with 
6.01 ROMs.  When in use, the initial connect string will look somewhat like 
"CONNECT 57600/ARQ/V42b/SREJ" (the exact string varies depending on the 
AT X[0-7] setting).


Chris Gray (cgra@btma74.nohost.nodomain) explains that selective reject is 
an optional part of the LAPM protocol used by V.42 modems.  A SREJ frame 
requests retransmission of one information frame, having the sequence number 
which is cited in the SREJ frame; after this transmission resumes _from 
where we left off_.  Contrast with ``ordinary'' REJ, which requests 
retransmission of _all_ information frames, starting with the one with the 
specified sequence number.  SREJ is more efficient when just one frame has 
got munged.


                      information(4)
        ------------------------------------------->
                     information(5)
        ------------------------------ (bad CRC)
                     information(6)
        ------------------------------------------->  uh uh, we've lost 5
                         SREJ(5)
        <-------------------------------------------
                     information(5)
        -------------------------------------------> that's better
                     information(7)
        ------------------------------------------->  

etc., as opposed to:

                     information(4)
        ------------------------------------------->
                     information(5)
        ------------------------------ (bad CRC)
                     information(6)
        ------------------------------------------->  uh uh, we've lost 5
                         REJ(5)
        <-------------------------------------------
                     information(5)
        -------------------------------------------> that's better
                     information(6)
        -------------------------------------------> yes I knew that 
                     information(7)
        ------------------------------------------->  


Note that SREJ causes some problems when handshaking with modems not 
supporting SREJ.  This is not a bug in ZyXEL modems.  Shutting off SREJ 
will avoid the handshake problem.

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