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