Digital Data Communications Message Protocol (DDCMP)

Digital Data Communications Message Protocol (DDCMP) is a byte-oriented communication protocol and a synchronous data link layer protocol, devised by digital equipment corporation. A byte oriented protocol views the frames as a collection of bytes/characters.

Some of the popular byte oriented protocols are –

1. BISYNC (Binary synchronous communications protocol)
2. PPP (Point to point protocol)
3. DDCMP (Digital data communications message protocol)

Digital data communications message protocol (DDCMP) differs from BISYNC and PPP protocols, as it employs byte counting approach. Counting, here, refers to the number of bytes in the count field of the frame format.

DDCMP : Frame Format –

Figure – Frame format of DDCMP

• In the frame format, there are two synchronous fields(SYN) of 8 bits each (similar to synchronous fields in BISYNC protocol).
• A class field of 8 bits(similar to class field in PPP protocol).
• A header field of 42 bits.
• A count field of 14 bits, to count the number of bytes in the frame body.
• The body field is of variable length.
• The CRC (cyclic redundancy check) field of 16 bits, performs error detection, on the data.

DDCMP Drawback : Transmission error –
A Transmission error can affect any bit pattern in the frame. If it corrupts the count field, then the end of frame would not be correctly detected by the receiver. The frame structure can get cluttered.

Example –
Consider, we have three frames.

Figure – Example of frame format of DDCMP

• The count field is in every frame.
• Let us consider the frame, drawn green in the above figure be F3[Frame-3]. This is the frame created by the sender.
• Let us consider the frame, drawn red in the above figure be F2[Frame-2].
• Let us consider the frame, drawn blue in the above figure be F1[Frame-1].
• The sender, sends the information to the receiver, and the receiver reads the count field ‘5’ first and understands that, the next 5 bytes belong to one frame i.e., F1[Frame-1].
• Similarly. it reads the count field ‘4’ and ‘6’ respectively and understands that the next 4 and 6 bytes belong to two frames respectively.
• Unfortunately, when the transmission error occurs and say, affects F2[Frame-2]. The count field of F2 gets modified into ‘7’ from ‘4’. Hence, the receiver understands that the next 7 bytes after ‘7’ belongs to one frame i.e F2[Frame-2], but it is not the frame created by sender. When the receiver identifies this as F2[Frame-2], framing error occurs.

In above figure, Transmission error depicted in an example for DDCMP frame format.