23-01-2013, 10:19 AM
Mobile Number Portability
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Abstract
Mobile number portability allows a mobile subscriber to switch operators without
changing his/her phone number. This article describes and analyzes mobile number
portability routing mechanisms and their implementation costs. We first describe the
Signaling Relay Function (SRF)-based solution for call related and non-call related
routing. Then we describe the Intelligent Network (IN)-based solution for call related
routing. Cost recovery issues for number portability are discussed in this article from
a technical perspective. We note that rules for cost recovery also depend on business
and regulatory factors that vary from country to country.
INTRODUCTION
umber portability is a network function that
allows a subscriber to keep a “unique” telephone
number. Imposed by the National Regulatory
Authority and agreed upon among
different network operators, number portability is one of
three important mechanisms1 to enhance fair competition
among telecommunication operators and to improve customer
service quality. Three types of number portability have been
discussed: location portability, service portability, and operator
portability. With location portability, a subscriber may
move from one location to another without changing his/her
telephone number. This type of portability is already implied
in mobile phone service. With service portability, a subscriber
may keep the same telephone number when changing telecommunication
services. In the U.S., service portability between
fixed telephone service and mobile phone service is implementable
because both services follow the “NPA-NXX”
telephone number format. In Taiwan, the service
code “09” for mobile service is distinguished from area codes
of fixed telephone service. As a result, service portability cannot
be made available in Taiwan unless the numbering plan is
modified. With operator portability, a subscriber may switch
telecommunications operators without changing his/her telephone
number.
Number Portability for Mobile
Telecommunications Networks
Although most mobile operators are not enthusiastic about
implementing mobile number portability, they cannot avoid
the impact of fixed-network number portability. When a
mobile station (MS) originates a call to a ported number in
the fixed network, the originating mobile switching center
(MSC) needs to route the call to the correct destination by
using fixed-network number portability solutions. Alternatively,
the MSC may direct the call to a switch in the fixed network,
which then routes the call to the recipient switch. In this
case, the mobile operator should reimburse the fixed-network
operator for extra routing costs.
Before describing mobile number portability, we point out
that an MS is associated with two numbers: the directory
number and the identification number. In GSM, the mobile
station ISDN number (MSISDN) is the directory number,
which is dialed to reach the MS. In other words, MSISDN is
the telephone number of the MS. The international mobile
subscriber identification (IMSI) is a confidential number that
uniquely identifies an MS in the mobile network. The IMSI is
used to authenticate/identify the MS during mobile network
access such as location update and call origination, which is
hidden from the mobile user. When a mobile user switches
operators, a new MSISDN and IMSI pair is assigned to the
user. When mobile number portability is introduced, the
mobile user would keep the MSISDN (the ported number)
while being issued a new IMSI. In other words, IMSI shall
not be ported. When the ported number ceases to be an
active service number, the number is returned to the NRH
network. Note that lawful interception shall be possible on a
ported MSISDN.
Number Porting and Cost Recovery
When a number is ported from the donor operator to the
recipient network, the NPDBs of all operators in the portability
domain may need to be updated. Number porting is an offline
administrative process that can be performed centrally by
a neutral third party or distributed among the participating
mobile operators. This section describes the number porting
mechanisms, then discusses the cost recovery issues.
Number Porting Administration
We use the North America Number Portability Administration
Center (NPAC) and Hong Kong Central Ticketing System
(CTS) models to illustrate number porting administration
[12]. Mandated by regulators and service providers, North
America NPAC is administered by a neutral third party that
has a fix-term contract. When the contract expires, the new
NPAC is selected through open bidding, and the system ownership
is transferred to the new NPAC. The functions of the
NPAC include service provider data administration, subscription
data administration, audit administration, resource
accounting, billing and cost apportionment, and so on. The
NPAC is designed to support various types of number portability,
and is developed according to standardized functional
requirements and interface specifications that are maintained
in the public domain. Note that the NPAC supports the master
database and is not involved in individual call setups. Figure
8 illustrates the connectivity between the NPAC and a
mobile operator.