Multilevel Precedence and Preemption

The device supports Multilevel Precedence and Preemption (MLPP) service. MLPP is a call priority scheme, which does the following:

■ Assigns a precedence level (priority level) to specific phone calls or messages.
■ Allows higher priority calls (precedence call) and messages to preempt lower priority calls and messages (i.e., terminates existing lower priority calls) that are recognized within a user-defined domain (MLPP domain ID). The domain specifies the collection of devices and resources that are associated with an MLPP subscriber. When an MLPP subscriber that belongs to a particular domain places a precedence call to another MLPP subscriber that belongs to the same domain, MLPP service can preempt the existing call that the called MLPP subscriber is on for a higher-precedence call. MLPP service availability doesn't apply across different domains.

MLPP is typically used in the military where, for example, high-ranking personnel can preempt active calls during network stress scenarios such as a national emergency or degraded network situations.

MLPP can be enabled for all calls, using the global parameter, CallPriorityMode, or for specific calls using the Tel Profile parameter, CallPriorityMode.

● The device provides MLPP interworking between SIP and ISDN (both directions).
● For Trunk Groups configured with call preemption, you must configure all to MLPP or all to Emergency.
● The global parameter must be set to the same value as that of the Tel Profile parameter; otherwise, the Tel Profile parameter is not applied.
● If you configure call preemption using the global parameter, and you subsequently add a new Tel Profile, the Tel Profile parameter 'Call Priority Mode' automatically acquires the same setting.
● If required, you can exclude the "resource-priority” tag from the SIP Require header in INVITE messages for Tel-to-IP calls when MLPP priority call handling is used. This is configured using the [RPRequired] parameter.
● For a complete list of the MLPP parameters, see MLPP and Emergency Call Parameters.

The Resource Priority value in the Resource-Priority SIP header can be any one of those listed in the table below.

A default MLPP call Precedence Level (configured by the SIPDefaultCallPriority parameter) is used if the incoming SIP INVITE or ISDN Setup message contains an invalid priority or Precedence Level value respectively.

For each MLPP call priority level, the Multiple Differentiated Services Code Points (DSCP) can be set to a value from 0 to 63.

MLPP Call Priority Levels (Precedence) and DSCP Configuration Parameters

MLPP Precedence Level

Precedence Level in Resource-Priority SIP Header

DSCP Configuration Parameter

0 (lowest)

routine

MLPPRoutineRTPDSCP

2

priority

MLPPPriorityRTPDSCP

4

immediate

MLPPImmediateRTPDSCP

6

flash

MLPPFlashRTPDSCP

8

flash-override

MLPPFlashOverRTPDSCP

9 (highest)

flash-override-override

MLPPFlashOverOverRTPDSCP

For digital interfaces:

The device automatically interworks the network identity digits (NI) in the ISDN Q.931 Precedence Information Element (IE) to the network domain subfield of the INVITE's Resource-Priority header, and vice versa. The SIP Resource-Priority header contains two fields, namespace and priority. The namespace is subdivided into two subfields, network-domain and precedence-domain. Below is an example of a Resource-Priority header whose network-domain subfield is "uc", r-priority field is "priority" (2), and precedence-domain subfield is "000000": 

Resource-Priority: uc-000000.2

The MLPP Q.931 Setup message contains the Precedence IE. The NI digits are presented by four nibbles found in octets 5 and 6. The device checks the NI digits according to the translation table of the Department of Defense (DoD) Unified Capabilities (UC) Requirements (UCR 2008, Changes 3) document, as shown below:

NI Digits in ISDN Precedence

Level IE

Network Domain in SIP Resource-Priority Header

0000

uc

0001

cuc

0002

dod

0003

nato

● If the received ISDN message contains NI digits that are not listed in the translation table, the device sets the network-domain to "uc" in the outgoing SIP message.
● If the received SIP message contains a network-domain value that is not listed in the translation table, the device sets the NI digits to "0000" in the outgoing ISDN message.
● If the received ISDN message doesn't contain a Precedence IE, you can configure the namespace value - dsn (default), dod, drsn, uc, or cuc - in the SIP Resource-Priority header of the outgoing INVITE message. This is done using the MLPPDefaultNamespace parameter. You can also configure up to 32 user-defined namespaces, using the table ini file parameter, ResourcePriorityNetworkDomains. Once defined, you need to set the MLPPDefaultNamespace parameter value to the desired table row index.

By default, the device maps the received Resource-Priority field of the SIP Resource-Priority header to the outgoing ISDN Precedence Level (priority level) field as follows:

■ If the network-domain field in the Resource-Priority header is "uc", then the device sets the Precedence Level field in the ISDN Precedence Level IE according to Table 5.3.2.12-4 (Mapping of RPH r-priority Field to ISDN Precedence Level Value):

Mapping of SIP Resource-Priority Header to ISDN Precedence Level for MLPP

MLPP Precedence Level

ISDN Precedence Level

SIP Resource-Priority Header Field

Routine

4

0

Priority

3

2

Immediate

2

4

Flash

1

6

Flash Override

0

8

■ If the network-domain field in the Resource-Priority header is any value other than "uc", then the device sets the Precedence Level field to "0 1 0 0" (i.e., "routine").

This can be modified using the EnableIp2TelInterworkingtable field of the ini file parameter, ResourcePriorityNetworkDomains.