Maritime Mobile Service Identity

A Maritime Mobile Service Identity (MMSI) is effectively a maritime object's international maritime telephone number, a temporarily assigned UID issued by that object's current flag state (unlike an IMO number, which is a permanent global UID).

An MMSI comprises a series of nine digits, consisting of three Maritime Identification Digits (country-codes), concatenated with a specific identifier. Whenever an object is re-flagged, a new MMSI must be assigned.

A "maritime object" could be anything that requests an MMSI identifier.—e.g., a vessel, fixed offshore installation, mobile unit, maritime aircraft, coast station, etc. Communications may be routed to "individual objects", or to "groups of objects". A group call to objects can be based on an object's locale, owner/operator/fleet, type, etc. or combinations thereof.

MMSI are formed in such a way that the identity or part thereof can be used by telephone and telex subscribers connected to the general telecommunications network to call any of these objects automatically. Communications are sent in digital form over a radio frequency channel.