ZMODEM is a file transfer protocol developed by Chuck Forsberg in 1986, in a project funded by Telenet to improve file transfers on their X.25 network. In addition to dramatic performance improvements compared to longer protocols, ZMODEM also offers redirects that can be restarted, started automatically by the sender, expanded 32-bit CRCs, and control characters that support 8-bit net transfers, allowing them to be used on networks will not pass control characters. ZMODEM became very popular on bulletin board systems (BBS) in the early 1990s, replacing previous protocols such as XMODEM and YMODEM.
Video ZMODEM
Repair
The key enhancement in ZMODEM is the introduction of sliding window support to improve performance. Generally, the file transfer protocol breaks the file into a series of packets , and then sends them one-on-one to the recipient. Each packet is sent with checksum which can be used to determine whether the packet has been received correctly. If the packet is received correctly, the ACK message is sent and the sender then begins sending the next packet.
However, the phone system introduces a small delay known as latency that interferes with this process. Even if the receiver delivers an immediate ACK , delay in the phone line means that there will always be some time before the sender accepts it and sends the next packet. As the modem speed increases, this delay shows the larger and larger number of packets that can be sent during the delay, lowering channel efficiency .
The shift protocol window avoids this problem by allowing the sending machine to switch to the next packet without waiting for ACK . Instead, the recipient sends either ACK (or NAK if there is an error) along with the confirmed packet number. The sender can process this in his spare time, resubmit the package as needed. This effectively eliminates recognition latency when there are no errors, leaving only data transmission latency and error detection.
The performance of ZMODEM is greatly improved over previous general protocols that generally supersed even special protocols such as YMODEM-g, which included no error correction at all and instead relied on error-free links managed by modems. Although YMODEM-g feels faster, the lack of other features such as re-transfers makes it less attractive.
Auto-starting simplified management by enabling the sending machine to initiate the transfer. Previously the user must first request the file from the sender, put it into the "wait" status, then return to the local program and run the command to start the transfer. With automatic transfer, they only request files, the sender will then automatically trigger the transfer in the user program.
Restart is another addition, allowing transfers to be restarted after the line is broken or the user runs out of time and kicked.
Maps ZMODEM
Variations
A number of modified versions of ZMODEM appear. ZedZap is a ZMODEM variant with 8 kbyte blocks for better performance on high-speed modems. LeechZmodem is a naughty ZMODEM variant (between similar XMODEM and YMODEM derivatives) that deceive BBS download quotas. The ZMODEM-compatible extension with 32 kbyte and 64 kbyte block lengths was created by ADONTEC in 2002 and 2007 to improve performance on high-speed error-free connections such as ISDN or TCP/IP networks.
The most prominent ZMODEM implementation comes from Omen Technology, Chuck Forsberg, Inc. This includes DSZ (DOS Send ZMODEM), GSZ (Submit ZMODEM Graphics), and ubiquitous (l) rzsz for variants Unix.
In more recent times, the Synchronet developers have created a modern X/Y/ZMODEM implementation named SEXYZ , loosely based on the zmtx/zmrx package, which runs natively on Windows and Unix variants, supports long file names and faster and more reliable data transfers. The ZMODEM implementation of SEXYZ has also been included in the SyncTERM project. Synchronet, SEXYZ, and SyncTERM are open source, cross platform, BBS-centric projects.
Forsberg itself garnered a number of improvements to ZMODEM-90 . The first is MobyTurbo , which removes the citing controls to improve the performance further, by about 15%. Even on networks that "eat" control characters, ZMODEM-90 can be customized to quote only those characters that the network actually eats, as opposed to every possibility. Similar improvements allow ZMODEM-90 to work on a 7-bit network, whereas previous protocols (with Kermit exceptions) all require 8-bit to one degree or another. Finally, ZMODEM-90 includes a long-range encoding compression system to further improve performance on uncompressed files.
Limitations
- Some ZMODEM packages (eg ZACK, ZRPOS) embed the byte-offset in the transferred file as a 32-bit unsigned integer. This design limits the ZMODEM eligibility to only transfer files under 4GB.
- Although the protocol may allow it, the (l) rzsz reference implementation can not encode arbitrary non-control characters (eg '~') frequently used by TCP/IP connection programs such as telnet and ssh as client-side "escape" characters. Users should disable the terminal breakout feature to achieve reliable transfer of this type of link, e.g. ssh -e no user @ hostname .
References
- Inter Application File Transfer Protocol ZMODEM
- Synchronet External X/Y/Zmodem (SEXYZ) File Transfer Protocol Protocol
External links
- Evolution and Selection of File Transfer Protocol
Source of the article : Wikipedia
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