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a single space separates this field from the previous field.
This field is coded as a decimal number, and includes the
current file. This field is an estimate, and incorrect values
must not be allowed to cause loss of data. The receiver's use
of this field is optional.
NNNNuuuummmmbbbbeeeerrrr ooooffff BBBByyyytttteeeessss RRRReeeemmmmaaaaiiiinnnniiiinnnngggg Iff the number of bytes remaining is sent,
a single space separates this field from the previous field.
This field is coded as a decimal number, and includes the
current file. This field is an estimate, and incorrect values
must not be allowed to cause loss of data. The receiver's use
of this field is optional.
The file information is terminated by a null. If only the pathname
is sent, the pathname is terminated with ttttwwwwoooo nulls. The length of
the file information subpacket, including the trailing null, must
not exceed 1024 bytes; a typical length is less than 64 bytes.
14. PPPPEEEERRRRFFFFOOOORRRRMMMMAAAANNNNCCCCEEEE RRRREEEESSSSUUUULLLLTTTTSSSS
14.1 CCCCoooommmmppppaaaattttiiiibbbbiiiilllliiiittttyyyy
Extensive testing has demonstrated ZMODEM to be compatible with
satellite links, packet switched networks, microcomputers,
minicomputers, regular and error correcting buffered modems at 75 to
19200 bps. ZMODEM's economy of reverse channel bandwidth allows
modems that dynamically partition bandwidth between the two
directions to operate at optimal speeds.
14.2 TTTThhhhrrrroooouuuugggghhhhppppuuuutttt
Between two single task PC-XT computers sending a program image on
an in house Telenet link, SuperKermit provided 72 ch/sec throughput
at 1200 baud. YMODEM-k yielded 85 chars/sec, and ZMODEM provided
113 chars/sec. XMODEM was not measured, but would have been much
slower based on observed network propagation delays.
Recent tests downloading large binary files to an IBM PC (4.7 mHz
V20) running YAMK 16.30 with table driven 32 bit CRC calculation
yielded a throughput of 1870 cps on a 19200 bps direct connection.
Tests with TELEBIT TrailBlazer modems have shown transfer rates
approaching 1400 characters per second for long files. When files
are compressed, effective transfer rates of 2000 characters per
second are possible.
Chapter 14 Rev 10-27-87 Typeset 10-27-87 33
Chapter 14 ZMODEM Protocol 34
14.3 EEEErrrrrrrroooorrrr RRRReeeeccccoooovvvveeeerrrryyyy
Some tests of ZMODEM protocol error recovery performance have been
made. A PC-AT with SCO SYS V Xenix or DOS 3.1 was connected to a PC
with DOS 2.1 either directly at 9600 bps or with unbuffered dial-up
1200 bps modems. The ZMODEM software was configured to use 1024
byte data subpacket lengths above 2400 bps, 256 otherwise.
Because no time delays are necessary in normal file transfers, per
file negotiations are much faster than with YMODEM, the only
observed delay being the time required by the program(s) to update
logging files.
During a file transfer, a short line hit seen by the receiver
usually induces a CRC error. The interrupt sequence is usually seen
by the sender before the next data subpacket is completely sent, and
the resultant loss of data throughput averages about half a data
subpacket per line hit. At 1200 bps this is would be about .75
second lost per hit. At 10-5 error rate, this would degrade
throughput by about 9 per cent.
The throughput degradation increases with increasing channel delay,
as more data subpackets in transit through the channel are discarded
when an error is detected.
A longer noise burst that affects both the receiver and the sender's
reception of the interrupt sequence usually causes the sender to
remain silent until the receiver times out in 10 seconds. If the
round trip channel delay exceeds the receiver's 10 second timeout,
recovery from this type of error may become difficult.
Noise affecting only the sender is usually ignored, with one common
exception. Spurious XOFF characters generated by noise stop the
sender until the receiver times out and sends an interrupt sequence
which concludes with an XON.