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"Round Trip Delay Time" includes the time for the last byte in a
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packet to propagate through the operating systems and network to the
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receiver, plus the time for the receiver's response to that packet
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to propagate back to the sender.
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The figures shown below are calculated for round trip delay times of
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40 milliseconds and 5 seconds. Shift registers in the two computers
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and a pair of 212 modems generate a round trip delay time on the
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order of 40 milliseconds. Operation with busy timesharing computers
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and networks can easily generate round trip delays of five seconds.
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Because the round trip delays cause visible interruptions of data
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transfer when using XMODEM protocol, the subjective effect of these
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delays is greatly exaggerated, especially when the user is paying
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for connect time.
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A 102400 byte binary file with randomly distributed codes is sent at
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1200 bps 8 data bits, 1 stop bit. The calculations assume no
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transmission errors. For each of the protocols, only the per file
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functions are considered. Processor and I/O overhead are not
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included. YM-k refers to YMODEM with 1024 byte data packets. YM-g
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refers to the YMODEM "g" option. ZMODEM uses 256 byte data
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subpackets for this example. SuperKermit uses maximum standard
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packet size, 8 bit transparent transmission, no run length
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compression. The 4 block WXMODEM window is too small to span the 5
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second delay in this example; the resulting thoughput degradation is
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ignored.
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For comparison, a straight "dump" of the file contents with no file
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management or error checking takes 853 seconds.
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TTTTAAAABBBBLLLLEEEE 2222.... Protocol Overhead Information
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(102400 byte binary file, 5 Second Round Trip)
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____________________________________________________________________________
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Protocol XMODEM YM-k YM-g ZMODEM SKermit WXMODEM
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________________________________________________________________________________________________________________________________________________________
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____________________________________________________________________________
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Protocol Round Trips 804 104 5 5 5 4
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____________________________________________________________________________
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Trip Time at 40ms 32s 4s 0 0 0 0
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____________________________________________________________________________
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Trip Time at 5s 4020s 520s 25s 25s 25 20
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________________________________________________________________________________________________________________________________________________________
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____________________________________________________________________________
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Overhead Characters 4803 603 503 3600 38280 8000
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________________________________________________________________________________________________________________________________________________________
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____________________________________________________________________________
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Line Turnarounds 1602 204 5 5 2560 1602
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Chapter 16 Rev 10-27-87 Typeset 10-27-87 36
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Chapter 16 ZMODEM Protocol 37
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_|____________________________________________|__________________|________________|______________|__________________|____________________|____________________|_
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_|______________________|_________|________|_______|_________|__________|__________|
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|Transfer Time at 0s | 893s | 858s | 857s| 883s | 1172s | 916s |
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_|______________________|_________|________|_______|_________|__________|__________|
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|Transfer Time at 40ms| 925s | 862s | 857s| 883s | 1172s | 916s |
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_|______________________|_________|________|_______|_________|__________|__________|
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|Transfer Time at 5s | 5766s | 1378s| 882s| 918s | 1197s | 936s |
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_|______________________|_________|________|_______|_________|__________|__________|
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FFFFiiiigggguuuurrrreeee 5555.... Transmission Time Comparison
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(102400 byte binary file, 5 Second Round Trip)
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************************************************** XMODEM
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************ YMODEM-K
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********** SuperKermit (Sliding Windows)
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******* ZMODEM 16kb Segmented Streaming
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******* ZMODEM Full Streaming
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******* YMODEM-G
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TTTTAAAABBBBLLLLEEEE 3333.... Local Timesharing Computer Download Performance
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__________________________________________________________________________
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| Command | Protocol| Time/HD| Time/FD| Throughput| Efficiency|
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_|________________________________|______________________|____________________|____________________|__________________________|__________________________|_
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_|________________|___________|__________|__________|_____________|_____________|
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|kermit -x | Kermit | 1:49 | 2:03 | 327 | 34% |
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_|________________|___________|__________|__________|_____________|_____________|
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|sz -Xa phones.t| XMODEM | 1:20 | 1:44 | 343 | 36% |
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_|________________|___________|__________|__________|_____________|_____________|
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|sz -a phones.t | ZMODEM | :39 | :48 | 915 | 95% |
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_|________________|___________|__________|__________|_____________|_____________|
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Times were measured downloading a 35721 character text file at 9600
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bps, from Santa Cruz SysV 2.1.2 Xenix on a 9 mHz IBM PC-AT to DOS
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2.1 on an IBM PC. Xenix was in multiuser mode but otherwise idle.
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