[嵌入式系统]xmodem,ymodem,zmodem的主要区别

根据modem所采用的文件传输协议：xmodem,ymodem,zmodem将其分类

xmodem:简单通用，传输信息单位是“包=128B”,传输速度慢，适合电话线路质量差的情况下用

ymodem:由XMODEM演变来，效率可靠性高，包=128*8B；一次传输可发送或接受几个文件

zmodem:于上两种不同，已连续的数据流发送数据，效率更高

XMODEM, YMODEM, and ZMODEM

This section deals with uploading and downloading files from you home computer over a modem. Two programs will be illustrated: sx sb sz
, and kermit
.

Sx rx
, sb rb
, and sz rz
implement the xmodem, ymodem, and zmodem file transfer protocols respectively.

XMODEM

Xmodemis one of the most widely used file transfer protocols. The
original Xmodem protocol uses 128-byte packets and a simple "checksum"
method of error detection. A later enhancement, Xmodem-CRC, uses a more
secure Cyclic Redundancy Check (CRC) method for error detection. Xmodem
protocol always attempts to use CRC first. If the sender does not
acknowledge the requests for CRC, the receiver shifts to the checksum
mode and continues its request for transmission.

Xmodem-1K

Xmodem 1K is essentially Xmodem CRC with 1K (1024 byte) packets.
On some systems and bulletin boards it may also be referred to as
Ymodem. Some communication software programs, most notably Procomm Plus
1.x, also list Xmodem-1K as Ymodem. Procomm Plus 2.0 no longer refers to
Xmodem-1K as Ymodem.

YMODEM

Ymodem is essentially Xmodem 1K that allows multiple batch file transfer. On some systems it is listed as Ymodem Batch.

Ymodem-g

Ymodem-g is a variant of Ymodem. It is designed to be used with
modems that support error control. This protocol does not provide
software error correction or recovery, but expects the modem to provide
the service. It is a streaming protocol that sends and receives 1K
packets in a continuous stream until instructed to stop. It does not
wait for positive acknowledgement after each block is sent, but rather
sends blocks in rapid succession. If any block is unsuccessfully
transferred, the entire transfer is canceled.

ZMODEM

Zmodem is generally the best protocol to use if the electronic
service you are calling supports it. Zmodem has two significant
features: it is extremely efficient and it provides crash recovery.

Like Ymodem-g, Zmodem does not wait for positive acknowledgement
after each block is sent, but rather sends blocks in rapid succession.
If a Zmodem transfer is canceled or interrupted for any reason, the
transfer can be resurrected later and the previously transferred
information need not be resent.