james@bigtex.cactus.org (James Van Artsdalen) (10/17/88)
In <218@arnold.UUCP>, dave@arnold.UUCP (Dave Arnold) wrote: > Okay, somebody tell me why Zmodem is better than UUCP's 'g'? > Given a window size of 7, and large packet sizes, and 'g's small > 6 bytes header... What makes Zmodem better than 'g'? 1. Zmodem is a purely streaming protocol. There are limits to 'g' in terms of window size and packet size, both in the theoretical world where all implementations work with large packets, and in the real world where they don't. Over packet-switching networks with very long delays this is a major issue. In addition, Zmodem works well with half-duplex environments without needing spoofing, as the session can be completely asymetric (receiver silent during data reception) if there are no errors. 2. Zmodem transfers are recoverable. Should the phone line be dropped 3meg through GNU emacs on your 110bps link, you can call back and continue where you left off. 3. Zmodem has less overhead on the line and gets higher throughput rates. 'g' has a high overhead, though the net throughput is higher than with protocols like Xmodem that have lower overhead but aren't streaming. Zmodem also appears to take less overhead from the CPU, though this is strictly guesswork from looking at "ps -ef": I don't know if anyone has actually verified this one way or the other. 4. Zmodem is more robust in error detection than 'g'. Zmodem uses 32 bit CRC for error detection, whereas 'g' uses a simple 16 bit checksum. In addition, Zmodem dynamically adjusts the packet size based on the observed error rate: on noisy links the packet length will tend towards 32 bytes, but on error-free links, it tends towards 512 bytes with a maximum of 1K. I see uucp give up fairly often, whereas Zmodem is better about recovering from errors. 5. Zmodem source is public domain and readily available (and readable!). The protocol is well documented. -- James R. Van Artsdalen james@bigtex.cactus.org "Live Free or Die" Home: 512-346-2444 Work: 338-8789 9505 Arboretum Blvd Austin TX 78759
scott@cdp.UUCP (11/13/88)
Someone should take the zmodem code and turn it into a new UUCP protocol. -scott pyramid!cdp!scott