The DEC DDCMP Protocol
Digital Equipment Corporation (DEC) developed a character-oriented protocol called DDCMP (Digital Data Communications Message Protocol) in 1974. DDCMP links DEC computers and peripherals.
DDCMP, being the “new protocol on the block” incorporated many features that set it apart from IBM Bisync.
- DDCMP can run on asynchronous, synchronous, or even parallel transmission facilities.
- Modulo 255 message sequence numbers improved efficiency because there can be up to 256 outstanding frames (frames transmitted without an acknowledgement). This is important for transmission over facilities with long delays (e.g. satellite links).
- A 14-bit field is used as a counter for the number of bytes in the message. Therefore, overall efficiency is increased by using variable block sizes.
- Since DDCMP always uses a CRC-16 checksum, there is native support for transparent data formats. DDCMP uses the same CRC-16 algorithm as IBM Bisync.
- There are two CRC-16 checksums used, one for the header information, and one for the data content.
- True 8-bit station addressing results in being able to have more multipoint stations on a single line, compared to IBM Bisync.
General Frame Structure
The following diagram depicts the fields in a DDCMP message.