time@oxtrap.aa.ox.com (Tim Endres) (01/18/90)
Having watched both ZMODEM and "spoofed" XMODEM on a Telebit T2500, I am unsure as to the best protocol for high speed half duplex modems. Spoofed XMODEM got me about 1200 cps (characters per second) throughput, while being able to get 1400 in good cases. ZMODEM on the other hand rarely gets better than 1450. I beleive the maximum to be in the 1800 cps range (includes protocol overhead). I noticed that the problem with ZMODEM is in the interrupt it sends, which causes the Telebit to "turn around", and this is expensive, since the interrupt delay causes a backup of several sectors (or so it appears). When there are no interrupts, the thing does *very* well, reaching upto 1600 cps, but this almost never happens in normal operation. Unfortunately, experiments with other "streaming" protocols show other difficulties in synchronization, the very reason for XMODEM and ZMODEM. Does anyone out there know of a protocol that is better than ZMODEM for this particular application?