Top Document: ZyXEL modem FAQ List v4.2, Nov 20 1995, Part 3 of 5 [Technical FAQs] Previous Document: T.8 What is the meaning of the T401 and T402 timeouts seen in an "AT I2"? Next Document: T.10 What is the "Capture modem manufacturer information" feature? See reader questions & answers on this topic! - Help others by sharing your knowledge -------------------------------------------------------------------------------- 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. User Contributions:Top Document: ZyXEL modem FAQ List v4.2, Nov 20 1995, Part 3 of 5 [Technical FAQs] Previous Document: T.8 What is the meaning of the T401 and T402 timeouts seen in an "AT I2"? Next Document: T.10 What is the "Capture modem manufacturer information" feature? Part1 - Part2 - Part3 - Part4 - Part5 - Single Page [ Usenet FAQs | Web FAQs | Documents | RFC Index ] Send corrections/additions to the FAQ Maintainer: rwong@direct.ca (Robert Wong Jr.)
Last Update March 27 2014 @ 02:11 PM
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