Simplified Message Desk Interface


Simplified Message Desk Interface 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 with the information it needs to process the call. Each time a call is sent to the voice mail system, a message is sent using SMDI over a serial interface that identifies the line, the type of call, and the calling/called party numbers. The SMDI protocol also specifies a method for turning the message-waiting indicator on and off on individual telephones.
Some of the more recent voicemail systems transmit the SMDI protocol over a network TCP session rather than a serial interface.
The current requirements are provided in the Telcordia Technologies Generic Requirements document .
According to Telcordia, the Simplified Message Desk Interface
is an analog interface between the local stored program-controlled switching system and a message storage and retrieval system. Calls are distributed to the call answering points with normal call processing or using the multiline hunt group features. One or more MLHGs may be associated with the same set of SMDI data links. An identification scheme is assigned in the SPCS to identify each MLHG uniquely and each line in the MLHG, for which call information is provided over the SMDI data link. This identification information is provided to the MSR system, along with the call information, to help the MSR system correlate the call information with a call terminating on a particular line.

Message format

MDXXXLLLLTYYYYYYY<0x20>ZZZZZZZ<0x20><^Y>
Meaning of the individual segments:
Each of the numbers is a common fixed length for each link, such as 5, 7, or 10 digits, zero-padded on the left where necessary, and terminated with.
A typical use of the called party number is to select which voicemail greeting to play for the call.

MWI format

OP:MWI<0x20>XXXX!
Meaning of the individual segments:
RMV:MWI<0x20>XXXX!

Example

This is an example of a direct call from extension 3500. It was presented to the voice messaging system on LTN 0007. This would be someone calling from their extension to pick up their voicemail and thus would likely ask for sign-on credentials, such as a PIN.
MD0010007D<0x20>3500<0x20><^Y>