ISUP and Local Number Portability

Local Number Portability (LNP) is the concept of having phone numbers that remain the same for the subscriber, regardless of whether the subscriber changes service providers or geographic location. Historically, phone numbers have been associated with a particular geographic region or a particular service provider. The actual use of LNP in the network exists today, but only to a small degree. It is being expanded in phases and will take some time before it is ubiquitous across all networks and locations. This section examines the different mechanisms used to provide portability services and how these mechanisms relate to setting up calls with ISUP.

Chapter 11, "Intelligent Networks (IN)" provides an overview of the various phases identified under the umbrella of Number Portability (NP), such as service provider portability and location portability. Some of the mechanisms used for NP employ Intelligent Network (IN) databases, so we cover NP in part both in the Chapter 11 and in this chapter.

When NP is implemented, numbers are transitioned from physical addresses that identify an exchange location to virtual addresses that identify a subscriber. A means of mapping must be used to derive a physical address in the network from the called number because the number no longer identifies a physical destination. The network in which the physical number existed before portability was introduced is called a donor network. Each time a number is ported and becomes a virtual address, the network has "donated" a number that previously belonged to that network. We use the term "donor" or "donor network" several times during the discussion of NP. The network in which the physical number now resides is called the recipient network.

Currently, four mechanisms are defined for implementing NP:

• All Call Query (ACQ)

• Query on Release (QOR)

• Dropback or Release to Pivot (RTP)

• Onward Routing (OR)

Each method has its merits in terms of resource efficiencies, maintainability, and competitive fairness among network operators, but those topics are outside of the scope of the book. The details of how each mechanism is implemented also vary from country to country. The following section provides a general understanding of NP and how it affects the ISUP call flow and messages.

All Call Query (ACQ)

ACQ sends an IN query to a centrally administered database to determine the call's physical address or routing address. Chapter 11 discusses the details of the IN query. The way the routing number returned by the query is used varies based on national standards. The following example illustrates how the routing number is used in North America.

The number returned from the database is a Location Routing Number (LRN) that identifies the exchange serving the called number. Each exchange in the network is assigned an LRN. The IAM sent after the database query is performed contains the LRN in the CdPN field. The call is routed on the CdPN using switching translations to reach the destination exchange. The IAM also includes a Generic Address Parameter (GAP) with the original dialed number (the virtual address). This allows the destination exchange to set up the call to the intended subscriber because the LRN can only identify the exchange. The Forward Call Indicators of the IAM include a Ported Number Translation Indicator (PNTI), which indicates that a query for the ported number has been performed.

Query On Release (QOR)

QOR routes the call from the originator to the donor network's ported number in the same manner used prior to NP. The donor network releases the call back with a cause value of Number Portability QOR number not found (ITU causes value 14, ANSI causes value 27 in the REL message). The originating network then performs a query to an NP database to determine what routing number to use in the IAM in order to reach the recipient network.

Dropback (Also Known as Release to Pivot)

Dropback, or Release to Pivot (RTP), routes the call to the ported number in the donor network, just like QOR. However, instead of having the originating network query for the number, the donor exchange provides the routing number for the ported number when it releases back to the originator.

Onward Routing (OR)

Onward Routing (OR) also routes the call to the donor network's ported number. It differs from QOR and RTP in that it does not release the call back to the originating network. Rather, it references an internal database to determine the new routing number that is associated with the ported number and uses the new number to route the call.

Using the QOR and RTP mechanisms, an IAM is sent and an REL received back from the donor network, therefore, requiring a subsequent call attempt. The ACQ and OR do not release back or require subsequent call attempts. The OR mechanism creates additional call legs because the call is being connected through the donor network rather than being directly set up to the recipient network.