Simplified Message Desk Interface

Simplified Message Desk Interface

Simplified Message Desk Interface (SMDI) is a protocol that defines the interface between a voice mail system and a phone system such as a PBX or Public Telephone Switch. It was developed by Bell Labs. It is used to provide the voice mail system the information it needs to process the call. Each time a call is sent to the voice mail a message is sent using SMDI over a serial interface that identifies the line, the type of call and Calling/Called party numbers. The SMDI protocol also specifies a method for turning Message Waiting Indication (MWI) on and off on individual telephone lines.

Some of the more recent voicemail systems transmit the SMDI protocol over a network TCP/IP session rather than a serial interface.

The current spec is maintained by Telcordia under document GR-283 (legacy document TR-NWT-000283). [ [ Telcordia Information SuperStore ] ]

Message Format


Carriage Return

Line Feed

MDXXX Message Desk. This is a 3 digit field (usually 001)

LLLL Logical Terminal Number (0001 - 4096)

T Reason Code (D equals Direct Call, A equals All Calls Forwarded, B equals Busy Call, U equals Unknown)

<0x20> Space

YYYY Calling Party Number

Carriage Return

Line Feed

<^Y> End

MWI Format

OP Operate Message Waiting Indicator (Turn On)


<0x20> Space

XXXX Extension Number

End of Transmission


RMV Operate Message Waiting Indicator (Turn Off)

<0x20> Space

XXXX Extension Number

End of Transmission


This is an example of a direct call from extension 3500. It was presented to the voice messaging system on LTN 0007 (port 7)



External links

* [ Cisco Call Manager Use of SMDI]
* [ Asterisk, an example open source PBX that supports SMDI]

ee also

* Session Initiation Protocol

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