Background

Number portability (NP) is a generic service concept which provides a network capability to enable a subscriber to keep his/her telephone number the same with a change of network operator, location and service type, etc. There are three types of number portability services: service provider portability, location portability, and service portability [13]. With service provider portability, a subscriber may switch service provider without changing his/her telephone number. With location portability, a subscriber may change location without changing telephone number. With service portability, a subscriber may keep the same telephone number when changing telecommunications services, such as changing from fixed-lined telecommunications service to mobile service.

In most countries, location portability and service portability are not enforced, and only service provider portability is implemented. Service provider portability is considered essential for fair competition among operators, while location portability and service portability are typically treated as value-added services. Location mobility has been implemented in mobile system because whenever a subscriber moves into a mobile network, the visiting network updates the location information of the subscriber to the subscriber’s subscription network. Besides, the numbering plans of fixed- lined and mobile telecommunication services are different in most countries, where service portability is not available unless the numbering plan is modified.

Therefore, we focus our discussion on service provider portability.

Exchange

Fig. 2-1 The hierarchy of telecommunication networks

As illustrated in Fig. 2-1, a telecommunications network is a hierarchical architecture consisting of several layers of exchanges. Usually the North American Numbering Plan (NANP) in the format of NPA-NXX-XXXX is adopted as the naming mechanism of a fix- lined telecommunications system, where numbering plan area (NPA) is a non-geographic code or a service access code, N is a number between 2 and 9, and X is a number between 0 and 9. Following NANP, a local exchange serves the numbers from NPA-NXX-0000 to –9999. The communication region of a city may consist of several local exchanges. Once a number within a local exchange was ported to another network, the local exchange considers the whole set of numbers, which is called a number block, as ported numbers.

Conventionally, every operator has a unique numbering plan with respect to the national numbering plan and telecommunications policy. In fixed- lined telecommunications system, every telephone number indicates a physical location.

The routing of calls relies entirely on the network that originally issued the phone number, which is called the donor network of the telephone number. For routing, number portability relies on the capability of a switching network to route a ported number to the network that is currently serving the number. In mobile telecommunications system, a directory number (mobile telephone number) ind icates a subscriber, while the identification number uniquely identifies a mobile station (MS) in the mobile network. The donor network which first issued the directory number to a subscriber, and the subscription network which a subscriber registered to must track the location of the subscriber and his/her MS. In order to distinguish ported numbers and determine the destination network of them, number portability is a necessary network function to allow the switching network to route calls to the called parties.

NP implementation schemes can be classified into on-switch solutions and off-switch solutions [14]. In the fixed- lined network systems, for intercepting and routing ported calls efficiently, the on-switch solution always routes calls to the donor network and are then onward routes them to the destination network. Analogous to the on-switch solutions of the fixed- lined telecommunication system, signal relay function (SRF)-based solutions modify switches to support NP service in mobile network systems. SRF-based solutions founded on SS7 communication architecture enhance the switch functions and utilize the MAP (mobile application part) protocol to enable the translation of the dialed number and the destination address. Depending on the implementation, the translation can be performed in the donor network or the subscription network. The SRF is typically implemented on signaling transfer points (STP) in the SS7 communication model.

Origination switch

NRH

GMSC Subscription

GMSC Termination

MSC

HLR HLR

2

4

5

6 7

3

NPDB SRF

Origination network Termination network

Donor network Subscription network

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Fig. 2-2: SRF-based NP call routing

Methods of on-switch solutions implement routing knowledge on the switching center of service providers. When a caller dialed a call via an originating network, which a caller connects to, the originating network routes the call to the donor network by the prefix of the dialed number. The call is routed via the donor network to the destination network, hence the mapping of dialed numbers and the routing addresses of the gateway of the destination network or the destination address of the called party are maintained in the gateway switches of the donor network. The simplified NP call process is illustrated in Fig. 2-2. The origination network receives a call initiation request from a subscriber (step 1). It identifies the donor network of the called party by the prefix of the dialed number (MSISDN). The origination network issues an ISUP IAM message to the donor GMSC to initiate a call (step 2). The donor GMSC consults HLR and identifies the number was ported out (step 3), then it consults NPDB by MAP sending routing information message (step 4) to determine the routing number of the subscription network. The donor network forwards the IAM message to the subscription network (step 5), and the subscription GMSC queries HLR for the routing number (mobile station roaming number, MSRN) of the called party (step 6). The MSRN indicates the address of the termination switch, thus the subscription network can route the IAM message by the MSRN to the termination switch to set up the call (step 7).

Following this method, the operation logic of a switching center alters whenever a number is ported out or in. On-switch solutions confront the following problems: (1) The frequent alteration decreases the stability of communication services and increases the cost of system operation and maintenance, and (2) the growth of ported numbers leads to a large number of routing messages for number translation.

On the contrary, to prevent the alteration of switch networks, off-switch solutions use Intelligent Network (IN) to intercept and route ported calls without the participation of the donor network. [15]. NPDB that manage the mapping of ported numbers and corresponding routing information are involved in the process of call setup. Switch network queries NPDB to obtain the routing address of the dialed numbers that are marked in the switch as ported. NPDB can be established as an internal database that keeps only the NP information of numbers which are assigned from or subscribed to the network, or be centrally maintained by a neutral organization (Number Portability Administration Center, NPAC) that all ported numbers of every service provider are recorded. Fig. 2-3 represents the architecture of IN-based NP services in a switched circuit network (SCN, including ISDN and cellular networks) and IP telephony interoperable environment. In IN-based network architecture, the service data functions (SDF) return a routing number for a ported number that indicates an end-point or an end-user in a network. The service management system (SMS) manages the content of SDF and handles the data consistency between SDF and NPDB. For the query to the IN nodes, different protocols are used in different networks and by different operators. In the IP network, number gateways translate telephone numbers to IP addresses. The location servers (LS) work as SDF that maintain the location information of subscribers.

S M S

S C F

SSF S D F

N P D B

Numbering gateway

SS L S

S C N IP network

Signaling gateway

Fig. 2-3 IN-based number portability service architecture

As shown in Fig. 2-4, the origination network receives a call initiation request from a subscriber (step 1), it determines the dialed number indicates to a NP subscriber and issues an ISUP IAM to the donor network for initiating a call (step 2).

The donor network queries HLR and determines the number was ported out (step 3), and consults NPDB by an INAP initialDP for the routing number of the subscription network (step 4). The donor network forwards the IAM message to the subscription network (step 5). The subscription network queries HLR for the MSRN of the called party (step 6) to reach the termination network, and forwards the IAM message to the termination MSC to setup the call (step 7).

Origination switch

NRH

G M S C Subscription

G M S C

Fig. 2-4 IN-based solution of NP call routing

The IN-based solutions differ form the SRF-based solutions in the way to access NPDB. In the SRF-based solution only GMSC can query NPDB. But the IN-based solutions are implemented in the service control point (SCP). Every switch equipped with the IN protocol can access NPDB [15]. SRF-based solutions may centralized workload to and burden specific network entities, where IN-based solution mitigates that problem. By IN architecture, it is not necessary to re-test the function of switches when updating the routing information of new ported numbers. Off-switch solutions are wildly adopted for better flexibility and extensibility.

There are four off-switch schemes for supporting NP service: all call query (ACQ), query on release (QOR), call dropback (also known as return to pivot, RTP), and onward routing (OR) [16].

n All call query (ACQ): ACQ scheme requires a centralized database to keep information of all ported numbers of every service providers. As shown in Fig.

2-5(a), the originating network detects the dialed number is a ported number, and initiates a query to NPAC. NPAC returns the routing information of the dialed number to the originating network, then the originating network routes the call to the destination network to set up the call. The determination of ported numbers is performed in the origination network. In this method, the operation of the donor network was not affected by the operation of NP call setup. ACQ is the most efficient of using the network transmission facilities; therefore, it is widely adopted as NP solutions in many countries.

n Query on release (QoR): QoR scheme is a form of call re-routing method, which grounds on the release messages of the donor network that detect a dialed number was ported out. In Fig. 2-5(b), when the originating network detects that the dialed number is a ported number, it routes the call to the donor network by the prefix of the number. If the number is ported out, the donor network returns a release message to the originating network, then originating network queries NPAC for the routing information of the dialed number. When the originating

network receives the routing information, it re-routes the call to the destination network. The occupancy of transmission resources during the routing of calls may degrade the efficiency of source network.

n Call dropback: In Fig. 2-5(c), the originating network receives a call from the caller and routes the call to the donor network by the prefix of the number. The donor network determines the number was ported out, it returns the routing information of the number to the originating network and release the call. Then the origination network re-routes the call to the destination network. Following this method, the routing information is maintained separately in different donor networks of ported numbers, and the donor networks of every ported number need internal NPDB for recognizing whether the number is ported out.

n Onward routing (OR): As pictured in Fig. 2-5(d), the originating network routes the received call to the donor network of the dialed number. The donor network detects the number was ported out, and checks an internal NPDB for the routing information. Then the donor network routes the call to the destination network by the routing information. The scheme requires the setup of two physical call segments, one from the originating network to the donor network and the other from the donor network to the destination network. It is the least efficient in terms of using the network transmission facilities.

All call query

Fig. 2-5 Off-switch NP schemes

The four schemes exist in the NP solutions of different countries: UK, Finland, France, Germany, Span, Singapore, etc. [17][18][19] The considerations of which scheme to adopt include the network resources, the policy of addressing and routing, the impacts on the signaling system, and the interworking with other services. ACQ and call dropback are the two most popular solutions.

Providing NP service in fixed- lined telecommunication environment requires two procedures to determine and translate ported numbers in call setup process. First, whenever a number is dialed, the switching network needs to distinguish ported from non-ported numbers. Every call in FMC telecommunications environment can be set to a ported or non-ported number of a fixed- lined or mobile telecommunications subscriber. The switching network of a telecommunication system must determine the destination network of a dialed number to setup a call. While NP service broke the relation between dialed numbers and the corresponding subscription networks, the ported number marks of both fixed- lined and mobile telecommunication networks need to be maintained in every network in FMC environment to distinguish NP and non-NP call processes. The ported number marks in switches increase with the growing amount of NP users. When the amount of NP users grows to be enormous, the latency of this procedure will be long. Second, every call terminated to a ported number initiates NPDB queries. Moreover, not only ported numbers trigger NPDB queries. When a number was recorded as a ported number, the entire group of numbers was taken as ported numbers [15]. Every such NP and non-NP call brings about queries to NPDB. The databases search time is log2N in average for every call, where N is the number of NP users. As shown in Fig. 2-1, the hierarchy of telecommunications networks assembles the routing and signaling processing knowledge in the central switching centers. When the arrival rate of NPDB queries exceeds the threshold service rate of NPDB (the dot line in Fig. 1-2), the waiting time of NPDB queries increases rapidly because of the queuing delay (Fig. 1-2). The latency delay of NPDB searching will block other NPDB queries and cause the congestion of the transmission network, which results in prolonged response time of ported number translation. The prolonged call setup delay due to ported number translation leads to extra operation time and bandwidth consumption. Operators must bear the expenditure on extra consumed communication resources without bringing operators any revenue.

The process to set up a NP call consists of the following procedures: process and transmission of messages, determination of NP calls, NPDB queries for the translation of ported numbers, and seizure of transmission line. Let tlocal and tglobal represent the transmission time in local and in global network, tprocess and tseizure are the time required for the switching network to process messages and allocate transmission

line/channel to the call, tdetermination is the latency for distinguishing portable and non-ported numbers, and tNPDB-query presents the routing information query delay when consulting NPDB. The setup delay of a NP call can be presented as the following:

tcall-setup = tlocal + tglobal + tprocess + tseizure + tdeterminiation + tNPDB-query

Where ttranslation and tNPDB-query will increase when the amount of NP subscriber increases; however, tlocal, tglobal, tprocess, and tseisure will not vary with the amount of NP subscribers. Therefore, data search delay must be reduced to enhance the efficiency of NP service and to utilize telecommunication resources.

E[W}

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0 60 120 180 240 300 360 420 480 540

t

queries

Fig. 2-6 The bottleneck of NP service