The Multi-Level Precedence and Preemption (MLPP) feature supports emergency communications for the military as part of the Defense Switched Network (DSN).
MLPP allows authorized users to
specify a precedence level when they make a call
preempt calls that have a lower precedence level.
Preemption allows important calls to take precedence over less important calls. Important calls that need immediate attention are identified by a continuous preemption warning tone. While on a call, if a caller hears a continuous preemption warning tone, the caller must hang up immediately, wait for an MLPP ring, and then answer the telephone (see MLPP User Features for details).
This functionality is supported for incoming and outgoing trunk calls on T1 ISDN PRI circuits, and for internal calls (calls between stations on the same switch).
The precedence level of a call determines whether or not it can be preempted by another call. The following precedence levels are supported:
FLASH OVERRIDE (highest precedence)
FLASH
IMMEDIATE
PRIORITY
ROUTINE (lowest precedence).
Calls that have a higher precedence level preempt calls that have a lower precedence level. So, for example, a call with a precedence level of FLASH can preempt a call with a precedence level of IMMEDIATE. You assign maximum precedence levels to directory numbers in the MLPP form. If you do not assign a precedence level to a directory number, the system assigns it with a precedence level of ROUTINE. If a user's directory number is assigned a precedence level of ROUTINE, the user cannot place calls at a higher precedence level; however, the user can receive calls of any precedence level.
You can assign telephone users to separate Service Domains. To preempt a call, a user must be in the same Service Domain as the user that is being preempted; however, users can place calls between different Service Domains. By default, all directory numbers are assigned to the same Service Domain which is normally programmed as the DSN Service Domain ("0"). Users that are not programmed with a Service Domain take on the default Service Domain of the system.
In addition, you can designate users as either preemptable or non-preemptable. Non-preemptable users can still assign precedence levels to calls.
MLPP trunk calls are restricted to TI ISDN PRI circuits on NSUs and embedded T1 cards that are programmed to support T1-619a signaling.
You do not program precedence levels or Service Domains for trunks. The MLPP feature assigns trunks with the precedence level and Service Domain of the call that is using the trunk. The system also supports calls to and from non-MLPP-capable trunks. These calls default to ROUTINE precedence level and cannot preempt or be preempted.
If the outgoing trunks are in a Route List, MLPP will select an idle or if necessary, pre-emptable trunk using the "Method 1" approach defined in the Generic Switching Center Requirements" (GSCR) specification.
If the precedence level of an outgoing trunk call is above ROUTINE, and there are no idle trunks currently in the selected MLPP trunk route, the system will attempt to find a circuit that has a lower precedence level call. If the system finds a circuit that has a lower precedence, it will attempt to preempt that call and use the circuit for the new call.
If the remote switch (the switch to which the system is connected) needs to make a call above ROUTINE to the system, it may pre-empt a call on a busy trunk and use that circuit for the new call. After the system seizes an incoming trunk, the far end communicates the precedence level of the call to the system via out-of-band signaling.
To make a call at a precedence level above ROUTINE, a user dials a two-digit access code comprised of an MLPP access digit (range of 2 to 9) followed by a precedence level digit (range of 0 to 4) or a service digit (range of 5 to 9). The MLPP access digit identifies the call as a precedence call; the precedence level digit identifies the actual precedence level (for example FLASH); the service digit allows users to dial out trunks that are not part of the DSN. If the chosen precedence level is less than or equal to the user's programmed maximum, the system allows the call to proceed. Otherwise, the system provides an error tone or announcement.
