ZMODEM原版英文协议说明文档

The ZMODEM Inter Application File Transfer Protocol

Chuck Forsberg

Omen Technology Inc

A overview of this document is available as ZMODEM.OV

(in ZMDMOV.ARC)

Omen Technology Incorporated

The High Reliability Software

17505-V Northwest Sauvie Island Road

Portland Oregon 97231

Chapter 0 ZMODEM Protocol 2

1. INTENDED AUDIENCE

This document is intended for telecommunications managers, systems

programmers, and others who choose and implement asynchronous file

transfer protocols over dial-up networks and related environments.

2. WHY DEVELOP ZMODEM?

Since its development half a decade ago, the Ward Christensen MODEM

protocol has enabled a wide variety of computer systems to interchange

data. There is hardly a communications program that doesn't at least

claim to support this protocol, now called XMODEM.

are not possible.

+ The short block length causes throughput to suffer when used with

timesharing systems, packet switched networks, satellite circuits,

and buffered (error correcting) modems.

+ The 8 bit checksum and unprotected supervison allow undetected errors

and disrupted file transfers.

+ Only one file can be sent per command. The file name has to be given

twice, first to the sending program and then again to the receiving

program.

+ The transmitted file accumulates as many as 127 bytes of garbage.

have proven satisfactory.

+ Lack of public domain documentation and example programs have kept

proprietary protocols such as Relay, Blast, and others tightly bound

to the fortunes of their suppliers. These protocols have not

benefited from public scrutiny of their design features.

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Chapter 2 ZMODEM Protocol 3

+ Link level protocols such as X.25, X.PC, and MNP do not manage

application to application file transfers.

Kermit's efficiency. Even with completely transparent channels,

Kermit control character quoting limits the efficiency of binary file

transfers to about 75 per cent.[1]

A number of submodes are used in various Kermit programs, including

different methods of transferring binary files. Two Kermit programs

will mysteriously fail to operate with each other if the user has not

correctly specified these submodes.

Kermit Sliding Windows ("SuperKermit") improves throughput over

networks at the cost of increased complexity. SuperKermit requires

full duplex communications and the ability to check for the presence

on high speed modems, from 1350 to 177 characters per second in one

test.

A number of extensions to the XMODEM protocol have been made to improve

performance and (in some cases) the user interface. They provide useful

improvements in some applications but not in others. XMODEM's unprotected

control messages compromise their reliability. Complex proprietary

techniques such as Cybernetic Data Recovery(TM)[2] improve reliability,

but are not universally available. Some of the XMODEM mutant protocols

have significant design flaws of their own.

+ XMODEM-k uses 1024 byte blocks to reduce the overhead from transmission

Chapter 2 ZMODEM Protocol 4

degrade performance. Some networks cannot transmit 1024 byte packets

without flow control, which is difficult to apply without impairing the

perfect transparency required by XMODEM. XMODEM-k adds garbage to

received files.

+ YMODEM sends the file name, file length, and creation date at the

beginning of each file, and allows optional 1024 byte blocks for

improved throughput. The handling of files that are not a multiple of

1024 or 128 bytes is awkward, especially if the file length is not

known in advance, or changes during transmission. The large number of

error recovery, YMODEM-g must be used hard wired or with a reliable

link level protocol. Successful application at high speed requires

cafeful attention to transparent flow control. When YMODEM-g detects a

CRC error, data transfers are aborted. YMODEM-g is easy to implement

because it closely resembles standard YMODEM.

+ WXMODEM, SEAlink, and MEGAlink have applied a subset of ZMODEM's

techniques to "Classic XMODEM" to improve upon their suppliers'

previous offerings. They provide good performance under ideal

conditions.

Another XMODEM "extension" is protocol cheating, such as Omen Technology's

between conflicting requirements:

3.1 Ease of Use

+ ZMODEM allows either program to initiate file transfers, passing

commands and/or modifiers to the other program.

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+ File names need be entered only once.

+ Menu selections are supported.

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Chapter 3 ZMODEM Protocol 5

+ Wild Card names may be used with batch transfers.

reliability, universality, and complexity according to the technology and

knowledge base available to their designers.

In the design of ZMODEM, three applications deserve special attention.

+ Network applications with significant delays (relative to character

transmission time) and low error rate

+ Timesharing and buffered modem applications with significant delays

and throughput that is quickly degraded by reverse channel traffic.

ZMODEM's economy of reverse channel bandwidth allows modems that

dynamically partition bandwidth between the two directions to operate

at optimal speeds. Special ZMODEM features allow simple, efficient