12/20/2023 0 Comments Syncterm xmodemXMODEM/64k is based on XMODEM/CRC and expects the 16-bit CRC. It will accept any supported value between 128 and 64k. The 64k option limits the block length to 64k. It also accepts requests for 8-bit checksum in order to support third party software that don't do 16-bit CRC. XMODEM/32k is based on XMODEM/CRC and expects the 16-bit CRC. It will accept any supported value between 128 and 32k. The XMODEM/32k limits the block length to 32k. Since the data transfer media is assumed error free increasing the block length will result to a lower number of blocks to transmit and thus less overhead which will improve the data throughput. DSL 1024/256).Ĭonnections through ISDN or TCP/IP can be considered as Error Free Channels 1 thus data loss is nearly impossible 2. On the internet DSL is widely used offering transmit speeds around 256Kbps (e.g. Intranet data connections are using the TCP/IP protocol with routers and switches at 10/100/1000Mbps. Digital communication via ISDN use 56Kbps/64Kbps/128Kbps. Today data connections are often far beyond 56Kbps.Data connections using serial interfaces often go from 230Kbps to 1Mbps. The 1k enhancement was a very usefull feature as the data communication speed was increasing beyond 14400bps or 19200bps. Using the Extended XMODEM the file does not „grow“ since the last short packet is recognized. And the XMODEM feature that many expected: It transmits the exact file size without padding the file with a number of EOF chars (ASCII 26)!.Receiver informs the sender about the file name to transmit (optional feature - Requesting a File).Transmitting file information (optional feature - Transmitting File Information).It can be easily extended to other packet length.It's backwards compatible to the standard XMODEM and XMODEM/CRC.It supports packets of 128, 512, 1K, 2K, 8K, 32K or 64K bytes in length.The features of this protocol can be summarized as follows: The Extended XMODEM specification, developed by ADONTEC, defines various buffer sizes in order to optimize the data throughput on what the data link has to offer. If the receiver does not know about it, the communication will fail. The XMODEM-1k implementation is driven by the sender and assumes that the receiver can handle it. The 1k feature described in the YMODEM protocol increased the block length of XMODEM/CRC to 1024 bytes. As computer power and communication speed was growing the throughput could get even better if the data blocks were bigger. The standard block length of 128 was acceptable for the low speed connections. So it's the receiver that decides which block check to be used by the sender.Īccording to the above, XMODEM/CRC differs from standard XMODEM only in the two byte CRC used instead of the 8-bit checksum and the SYN character. This was accomplished by using the character 'C' instead of the ( SYN character). The intention for XMODEM/CRC was to remain backwards compatible with XMODEM. To avoid this the protocol changed by John Mahr in order to be able to transmit a 16-bit CRC. Since the 8-bit checksum used was simple, errors within the data packet could go unnoticed. When the sender reads a message it re-send the data packet. If the calculated checksum was not equal to the one received the receiver transmits (ASCII 21) message instead. If the two checksum matched, the receiver replied with the (ASCII 6) message to inform the sender to transmit the next packet in sequence. When each data packet received, the packet's checksum was calculated and compared to the one received. The protocol is completely receiver controlled. With XMODEM a file were transferred one packet at a time. XMODEM was offering a block length of 128 bytes and a 8-bit checksum followed at the end of each block. Developed in 1977 by Ward Christensen, XMODEM was simple and easy to program and it became extremely popular. The XMODEM protocol is one of the oldest file transfer protocol invented to move files between computers. XMODEM Protocol, Serial Communication Library, XMODEM/CRC, XMODEM-1K, XMODEM/32K, XMODEM/64K XMODEM Protocol Feature Description
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