2. • Number portability is a network function that
allows a subscriber to keep a "unique"
telephone number. Number portability is one
of three important mechanisms to enhance
fair competition among telecommunication
operators and to improve customer service
quality. (The other two mechanisms for fair
competition are equal access and network
unbundling.)
• Three kinds of number portability are
discussed: location portability, service
portability, and operator portability.
3. • With location portability
– a subscriber may move from one location to another
location without changing his or 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 the telecommunication services. In the United
States, service portability between fixed telephone service
and mobile phone service is implementable because both
services follow the "NPA-NXX-XXXX" telephone number
format. In Taiwan, the service code "093" for mobile
service is distinguished from the area codes of fixed
telephone service. As a result, service portability is not
available in Taiwan unless the numbering plan is modified.
• With operator portability
– a subscriber may switch operators or service providers
without changing his or her telephone number.
4. • In most countries, location portability and
service portability are ignored, and only
operator portability is implemented in a fixed
network.
• The reason is twofold:
• First, operator portability is considered
essential for fair competition among
operators, while location portability and
service portability are typically treated as
value-added services;
• second, the implementation and operation
costs can be significantly reduced if we ignore
service portability and location portability,
5. Fixed-Network Number Portability
• Four basic solutions have been proposed to
support fixed-network number portability: call
forwarding, call drop-back, query-on-release,
and all-call-query.
• Call setup for these approaches is illustrated in
Figure 15.1. In the subfigures, dashed lines
represent signaling paths and solid lines
represent trunk setup paths.
• The call-forwarding call setup procedure is
illustrated in Figure 15.1(a); the steps are as
follows:
6.
7. • Step 1. The originating switch routes the call
to the donor switch according to the dialed
telephone number. In Figure 15.1(a), a solid
arrow between two switches represents a
trunk set up by using an SS7 ISUP Initial
Address Message (IAM).
• Step 2. If the dialed number has been ported,
the donor switch forwards the call to the
recipient switch. For a nonported number, the
recipient switch is the donor switch in this
step, and no forwarding action is performed.
• The call drop-back call setup procedure is
illustrated in Figure 15.1(b):
8.
9. • Step 1. The originating switch queries the donor switch to obtain the
routing information of the recipient switch. In Figure 15.1, dashed arrows
represent SS7 TCAP signaling message exchange without involving trunk
setup.
• Step 2. The originating switch sets up the trunk to the recipient switch
based on the obtained routing information.
• The query-on-release (QoR) call setup procedure is illustrated in Figure
15.1(c):
10. • Step 1. As in call forwarding, the originating
switch sets up the trunk to the donor switch
using an SS7 ISUP IAM message. If the dialed
number has been ported, the donor switch
replies with an SS7 ISUP Release Message (REL)
with the QoR cause value.
• Step 2. When the donor switch receives the REL
message, the trunk to the donor switch is
released. Since the QoR cause value indicates
that the called party number is ported, the
originating switch sends an SS7 TCAP message to
query the number portability database for the
routing address of the recipient switch.
• Step 3. The originating switch sets up the trunk to
the recipient switch based on the obtained
routing information.
11. • The all-call-query call setup procedure is illustrated in Figure
15.1(d):
• Step 1. The originating switch sends an SS7 TCAP message to
query the number portability database for the routing address
of the recipient switch.
• Step 2. The originating switch sets up the trunk to the
recipient switch based on the obtained routing information.
12. Number Portability for Mobile 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 one of the four
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 cost.
13. • Before we describe mobile number portability,
we point out that a mobile phone is associated
with two numbers: the directory number and the
identification number.
• In GSM, the MSISDN (mobile station ISDN
number) is the directory number, which is dialed
to reach the MS; in other words, MSISDN is the
telephone number of the MS.
• The IMSI (international mobile subscriber
identification) is a confidential number that
uniquely identifies an MS in the mobile network.
• IMSI is hidden from the mobile user, which is
used to authenticate/identify the MS during
location update and call origination
14. • 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 use keeps the MSISDN (the ported
number) while being issued a new IMSI.
• For mobile systems based on EIA/TIA IS-41, the
identification number and the directory number
are referred to as the mobile identification
number (MIN) and the mobile directory number
(MDN), respectively.
• The mobile operators typically assume that both
MIN and MDN have the same value, and so are
used interchangeably.
15. • The MIN/MDN is of the format NPA-NXX-XXXX.
The first six digits, NPA-NXX, identify the home
system of the MS.
• Without this home network identification,
roaming is not possible.
• The MDN is used as the calling-party number
parameters in signaling and billing records.
• If mobile number portability is introduced, the
MIN will be different from the MDN, in which
case, using the MIN as the calling party number
will result in misrouting in services such as
automatic callback and calling number/calling
name. Similarly, using MDN for location update
will result in errors when performing the
registration procedure.
16. • Thus, to support portability, separation of MIN and MDN is
required for the IS-41-based systems. This means that extra
costs are incurred to modify mobile software in the MSC,
home location register (HLR), and visitor location register
(VLR).
• The impact of number portability on mobile network is
considered in three aspects:
• Location update. The identification number (IMSI or MIN)
is used in the location update procedure. Since the
assignment of this number is not affected by the
introduction of number portability, location update is not
affected by portability-except that MIN/MDN separation is
required for the IS-41-based systems.
• Mobile call origination. As mentioned in the beginning of
this section, to originate a call to a ported number, the MSC
needs to be equipped with a routing mechanism.
• Mobile call termination. To deliver or terminate a call to a
ported mobile number, the standard mobile call
termination procedure is modified to accommodate the
portability mechanism.
17. Mobile Number Portability Mechanisms
• In mobile service, the network tracks the location
of every MS. The location information is stored in
two mobile databases, the HLR and the VLR.
• To deliver a call to an MS, the databases are
queried for routing information via the MSC
where the MS resides. Figure 15.3 illustrates a
simplified GSM call termination procedure where
the interaction between HLR and VLR is omitted.
• In Figure 15.3, the visited system (where the MS
resides) may be different from its home system.
Call termination to the MS must be routed to the
GMSC at the home system due to the following
restrictions:
18. • Restriction 1. The GMSC must be in the call path
for the provision of special features and services,
as well as for billing.
• Restriction 2. The originating switch does not
have the capability to query the HLR database,
which must be done by the GMSC.
• A tromboning effect may occur if both the calling
party and the called MS are in the same city (say,
Taipei) and the GMSC is located in a different city
(say, Tainan).
• The cost for tromboning can be highly significant,
especially for international roaming.
• To support mobile number portability, call
termination in Figure 15.3 is modified. We
describe four mobile number portability solutions
as follows.
19.
20. Signaling Relay Approach 1 (SRA 1)
• SRA 1 utilizes a signaling relay function (SRF)
mechanism to provide routing information for ported
numbers. The call setup procedure consists of the
following steps (shown in Figure 15.4):
• Step 1. The originating switch routes the incoming call
to the donor GMSC of the ported MS.
• Step 2. The donor GMSC sends the MSRN query
message to the SRF.
• Steps 3-5. The SRF determines the HLR of the
destination MS. If the MS is not ported, the original
HLR is queried. If the MS is ported, the recipient HLR is
queried.
• Step 6. Upon receipt of the MSRN, the donor GMSC
routes the call to the serving MSC.
• This approach violates restriction 1; that is, the GMSC
of the recipient network is not in the call path.
21.
22. All-Call-Query Approach 1 (ACQ 1)
• ACQ 1 utilizes the existing fixed-network, all-call-query
mechanism to route the calls to a ported MS (see
Figure 15.7).
• Steps 1 and 2. The originating switch queries the
mobile number portability database to obtain the IRN
of the recipient GMSC.
• Step 3. The originating switch sets up the trunk to the
recipient GMSC.
• Steps 4-6. The recipient GMSC routes the call according
to the standard GSM call termination procedure.
• Like the standard GSM call termination without
number portability, tromboning may occur if both
originating and serving MSCs are at the same location
while the recipient GMSC is located at a different place.
