Digital Data Communications Message Protocol

DDCMP Overview

The Digital Data Communications Message Protocol (DDCMP) is a byte-oriented data communication protocol for use over full-duplex and half-duplex point-to-point and multi-drop links; principally using using synchronous or asynchronous serial lines, although other communication channels were also supported. It used CRCs to allow transmission errors to be detected; sequence numbers allow detection of completely lost messages. Retransmission is used to correct both.

It was originally designed by DEC for the DECnet Phase I network protocol suite, in 1974. It was retained in later versions of DECnet, with modifications and extensions.

From 1983 onwards DDCMP was step by step superseded by Ethernet which was superior in terms of speed and flexibility, but DDCMP is nevertheless retained in current VMS DECnet versions.

Synchronous DDCMP

There are Synchronous DDCMP controllers for several buses:

• UNIBUS
• QBUS
• VAXBI

Synchronous DDCMP Controller Capabilities

Bus	Controller	Point-to-Point V3.2	Point-to-Point V4.0	Tributary	Control	VMS Support
UNIBUS	DUP11	X (supported DDCMP versions yet unknown)		-	-	-
UNIBUS	DMC11	X	-	-	-	X
UNIBUS	DMR11	X	X	-	-	X
UNIBUS	DMP11	X	X	X	X	X
UNIBUS	DMF32	X	X	X	-	X
VAXBI	DMB32	X	X	X	-	X
VAXBI	DSB32	X (supported DDCMP versions yet unknown)		-	-	X
QBUS	DPV11	X (supported DDCMP versions yet unknown)		-	-	-
QBUS	DMV11	X	X	X	X	X
QBUS	DSV11	?	X	-	-	X

Synchronous DDCMP Configurations

There are two basic kinds of synchronous DDMCP configurations:

• Point-to-point
• Multipoint

Synchronous Point-to-Point DDCMP

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Point-to-point DDCMP connects exactly two DDCMP devices, either:

• A DMC11 and another arbitrary DDCMP device, using the older version (3.2) of DDCMP

or:

• Two arbitrary DDCMP devices (except for DMC11), using the newer version (4.0) of DDCMP

Synchronous Multipoint DDCMP

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With Multipoint DDCMP, exactly one Multipoint controller (e.g. a DMP11) acts as the control station of a DDCMP multipoint group.

Multiple other DDCMP Tributary controllers (e.g. DMP11, DMF32) are all connected to the control station.

All DDCMP controllers connected in this way constitute a DDCMP multipoint group.

Multidrop is an established synonym for Multipoint.

Synchronous DDCMP Line Protocols

Protocol	Description
DDCMP CONTROL	Specifies the line as a multipoint control Station. You can set multiple circuits for CONTROL lines. Each circuit must have a unique physical tributary address.
DDCMP DMC	Specifies that the line is in DMC emulator mode. DMC is similar to DDCMP POINT protocol, except that DMC uses an older version of DDCMP (Version 3.2). This protocol should be set for the local line when the remote line is a DMC.
DDCMP POINT	Specifies the line as one end of a point-to-point DDCMP connection. You may specify only one circuit per POINT line.
DDCMP TRIBUTARY	Specifies that the line is a multipoint tributary end of a DDCMP multipoint group. You may specify only one circuit per TRIBUTARY line.

External links

• 'DDCMP Specification Version 4.0', AA-D599A-TC (1 March 1978)
• Digital Data Communications Message Protocol (DDCMP) - Contains a good, detailed description of failure modes