13.11 Fixed/Mobile Number Portability
13.11.6 Routing in the Presence of Transit Network
Finally, we comment on the change in role for the network from which the call originates if there is at least a transit network between where the call originates and the recipient/donor network. This raises the following question: which network will trigger an NPDB query? Re- call that in the case of OR routing, the donor network is responsible for the NPDB query; thus, there is no change in this case. Thus, the question is really for the CD/RtP, QoR, and ACQ schemes. In the United States, the ACQ scheme is used and there is an industry agreed-upon (N−1)querying policy protocol to handle the NPDB query issue if there are transit networks along the call path. This means that the(N−1)network, or the network before the destina- tion network, on the path of a call from the actual origination to the destination, is required do to the NPDB query. Note that network here means call network, not the SS7 network. The- oretically, it is possible that the same SS7 network provider handles call signaling messaging routing for both the(N−1)network and the donor network.
The(N−1)policy is considered to be a good policy as it does not place the responsibility on the donor network when the ACQ scheme is used since the (N−1) network does all
TA B L E 13.6 Routing methods for number portability currently used in various countries (adapted from [236], [679], [691]).
Country Scheme Remark
Austria OR Routing prefix: 86XX where XX specifies recipient network Belgium ACQ Routing prefix: CXXXX where XXXX specifies the destination
TDM switch in the recipient network; use C00XX to specify the recipient network
Denmark ACQ Routing number is not communicated between providers; Na- ture of Address indicator is set; QoR possible through bilateral agreements
Finland ACQ Routing prefix: 1DXXY where XX specifies the recipient net- work, Y specifies service type
France OR Routing prefix: Z0XXX where XXX specifies the destination TDM switch in the recipient network
Germany ACQ Deutsche Telekom uses ACQ while other providers can select another scheme
Italy OR Routing prefix: C600XXXXXX where XXXXXX specifies the destination TDM switch in the recipient network
Japan OR The donor network performs SS7/IN lookup to obtain routing number
Netherlands QoR/ACQ Operators decide
Norway OR, ACQ OR is short-term, ACQ is long-term; QoR is optional; Nature of Address field used as indicator along with routing prefix Spain ACQ QoR used internally by Telefonica; Routing prefix: XXYYZZ
that specifies the recipient network along with Nature of Ad- dress field indicator
UK OR Routing prefix: 5XXXXX where XXXXX specifies the destina- tion TDM switch in the recipient network; in parts of the net- work, British Telecom also uses dropback scheme
US ACQ PNTI bit set in Forward Call indicator; location routing num- ber takes the place of Called Party Number field; the directory number is placed in Generic Address Parameter
the NPDB queries. Furthermore, the entire world numbering plan and routing architecture can still work globally without requiring fundamental changes to the current architecture.
We describe the possible scenarios below assuming the donor network is marked as a local exchange carrier, LEC-Z.
• A geographic local call: Consider a call that is local where LEC-Z operates that originates from a local exchange carrier, LEC-A. In this, the(N−1)network is LEC-A where the call originates and there is no transit network. Thus, LEC-A itself is required to do the NPDB query.
• A long-distance call that involves an IXC: Consider a call from a geographic area originat- ing from a local exchange carrier, LEC-B, to a ported number where the donor network
(“LEC-Z”) is in another geographic area where the call must traverse through an IXC. That is, the call path (before number portability) is LEC-B to IXC to LEC-Z. In this case, IXC is considered the(N−1)network and is required to do the NPDB query.
• An international call from outside the United States to the donor network, LEC-Z: The provider outside the United States would not be able to know which LEC is ported and which is not. In this case, the call is handed over from the other nation’s network provider to the IXC that provides service within the United States. If this carrier is the same carrier for long-distance service before reaching LEC-Z, then this carrier is required to do the NPDB query.
In all cases, since multiple switches are involved along the call path, a mechanism is still needed to ensure that a downstream switch does not perform an NPDB lookup if it has already been performed by an upstream switch. Thus, if a TDM switch performs an NPDB lookup to obtain an LRN, the ensuing IAM message that is sent out of this switch for this call is modified. This modification includes setting the PNTI bit in the Forward Call Indicator parameter in the IAM message; furthermore, the LRN code is placed in the Called Party Number parameter, and the called number is included in the Generic Address Parameter field. By inspecting the PNTI bit, a downstream TDM switch knows that the NPDB is already performed. This feature is also helpful in case (N−1)network fails to perform the NPDB query, for instance, due to a technical failure. Thus, the call setup IAM message without this modification will be routed all the way to the donor network’s supposed terminating switch;
that is, this will look like the onward routing case where it is left to the switch that would have been the terminating switch prior to number portability deployment to perform the NPDB query. Such default-routed calls are then the burden of the donor network to perform the query. Incidentally, in the United States, the Federal Communication Commission’s policy allows the donor network to charge the(N−1)network for this extra work and even allows it to “block default-routed calls, but only in special circumstances when failure to do so is likely to impair network reliability” [417].
In the case of the long-distance and international call scenarios above, there is another decision the IXC, the(N−1)network, is required to address. For instance, the call in the IXC’s part of the network usually traverses through multiple TDM switches. For instance, if dynamic call routing is used in the IXC’s network, there can be at most three TDM switches involved, the ingress switch, an intermediate (via) switch, and the egress switch. The decision partly depends on whether portability is local number portability or location portability. If it is local number portability, then the LRN will be adjacent to the donor network for the ported number that has been dialed; thus, in this, the egress switch in the IXC’s network is the appropriate switch to do the NPDB query and then identify the PoP for the network that serves this LRN. However, if it is for location portability, then we need to consider the possibility that the ported number might have moved anywhere in the entire country; thus, it would be more appropriate to do the NPDB query at the ingress switch, especially to avoid any crankback. Furthermore, with location portability, for an intracountry call, it might be more appropriate to do all call query (ACQ) at the originating LEC. We note that the idea of location portability is still in its infancy; further development is required to understand all its possible implications.