what is xmodem ?

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Oct 6th, 2008

what is xmodem ?

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scottmac Mon, 10/06/2008 - 08:51

It's a file transfer protocol that came out decades ago when modems rocked along at a whomping 300 baud.

It was developed in the CP/M days, and is a standardized version of "Modem 7" protocol created by Ward Christiansen(sp?) and Randy Seuss in Chicago ("Ward & Randy's Bulletin Board").

It was developed because the standard terminal equipment only used seven bit ASCII ... and data transfer (for files) required all eight bits.

Xmodem provides a way to send the data as block, each block is CRC'd and if it is found to be bad, it gets "nak"d and is resent in a future slot.

Kermit was developed later, and came up witha different way of passing binary data across a seven bit ASCII async line.

From Xmodem, we derived Xmodem1K (1024 byte blocks), Ymodem (batch) and Zmodem (remote features, better flow).

Xmodem is still a good way to get data across an ASYNC line ... Cisco uses it as a mechanism to get an image into a device with a trashed firmware.

Good Luck

Scott

garytayl Mon, 10/06/2008 - 09:03

It is just a file tranfer protocol. It is used a lot for Cisco devices when there is no IOS on the different devices therefore only console access is available. Under such conditions, xmodem is used to push the software (mostlikely IOS) in order to have "normal" access to the device once again.

Here is a link with a typical procedure:

http://www.cisco.com/en/US/products/hw/routers/ps259/products_tech_note09186a008015bfac.shtml

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