AN ORGANIZATION-BASED CACHE MECHANISM FOR
SUPPORTING PCS NUMBER PORTABILITY SERVICE
C. W. Cheng
Department of Computer Science and Information Engineering, National Chiao Tung University, Hsin-Chu, Taiwan
C. G. Chung
Department of Computer Science and Information Engineering, Chang Gung University, Kwei-Shan Tao-Yuan,Taiwan
Keywords: Number portability, mobile communication, cache.
Abstract: The rapidly growth of PCS market promotes the number portability (NP) service becoming essential to
enhancing fair competition among operators. However, the existed NP solutions face the same problems of
huge NP database (NPDB), long delay time of portable number translation, and extra communication
resource consumption. Although researches pointed to caches can be utilized to alleviate the workload of
NPDB and to reduce NP call processing delay, it doesn’t fit for the mobile communication system. For large
organizations, members move among the working space or the subsidiaries, many mobile calls are made
between organization members. Furthermore, organizations usually have frequently contact targets, calls
made from an organization often have locality. If caches are applied to organization-based mobile
telecommunication components, the efficiency of NP service can be improved. In this paper, we propose an
operation model of applying organization-based caches to the public mobile communication system.
Besides, the time for NP call setup is investigated to state the feasibility and the effect of organization-based
caches in the mobile communication system.
1 INTRODUCTION
The burgeoning techniques and the maturity of the
business model attract more operators to join the
market of mobile communication. Users have more
opportunities to choose expedient service providers,
but they are tired of distributing the new number to
others every time they change service providers.
Hence, number portability (NP) service that allows
subscribers to keep numbers when changing service
providers is an important service to attract
subscribers and to enhance fair competition among
operators (Gans, King & Woodbridge, 2001).
The Intelligent Network (IN)-based solutions are
popularly adopted in present NP service networks.
Number portability databases (NPDB) which
maintain the mapping of portable numbers and the
corresponding routing information are necessary.
Every call to a portable number requires NPDB
queries for number translation. The large amount of
NP subscribers makes NPDB grows to be
cumbersome. The long delay time of NPDB queries
and the extra bandwidth be occupied during NP call
process become the most urgent performance issue
of NP service.
There were researches pointed that caches can
offload the effort of database to reduce the delay of
database queries. However, the position to
implement caches, the policy of cache management,
and the size of caches are all factors influencing the
benefits of caches in a communication system.
Caches on operator switching centers perform
poorly for the quantity of calls from a service area is
enormous and the dialed numbers are scattered. The
cache size must be large to accommodate the
numerous routing information to increase the cache
hit rate; but the cost to manage and update such a
large cache is expensive, and the search of cached
data is inefficient. On the other hand, caches on user
terminals confront the expensive cost for cache
update. Operators must broadcast the altered routing
information to all terminals that a lot of transmission
resources are consumed.
Studying the operation model and the hierarchy
of communication systems, we found that
80
W. Cheng C. and G. Chung C. (2005).
AN ORGANIZATION-BASED CACHE MECHANISM FOR SUPPORTING PCS NUMBER PORTABILITY SERVICE.
In Proceedings of the Second International Conference on e-Business and Telecommunication Networks, pages 80-87
DOI: 10.5220/0001417700800087
Copyright
c
SciTePress
organization-based (OGB) communications hold a
massive share of data- and telecommunication.
Organization members often connect others by
cellular phones on the move. Many mobile
communications happens between organization
members. Furthermore, organizations usually have
regular and frequent contact-targets, mobile calls
generated from an organization usually have locality.
For the properties of moderate amount of users and
the access locality of dialed numbers, OGB caches is
another approach for enhancing mobile NP service.
In this paper we introduce the IP-based iNetwork
(Cheng & Chung 2003) as an example to illustrate
the operation of OGB caches in mobile NP service,
and to demonstrate that OGB caches can efficiently
alleviate the traffic load of NPDB and remarkably
enhance the efficiency of mobile NP service.
The rest of this paper is organized as follow.
Section 2 gives an overview of related work. Section
3 and 4 introduce the operation model of OGB
communication systems and OGB caches. Section 5
investigates the performance of OGB caches in NP
service. A brief conclusion is given in section 5.
2 RELATED WORK
For providing mobile NP service, mobile
communication networks must be able to identify
portable numbers, and to obtain the routing
information of the number. NPDB which keeps the
mapping of portable numbers and routing
information is maintained in the operator network.
The approaches for providing mobile NP service
can be classified into SRF-based and IN-based.
Signal relay function (SRF)-based solutions enhance
the switch functions and utilize the MAP (mobile
application part) protocol to enable the portable
number translation. Depending on the
implementation, the translation can be performed in
the NRH (number range holder) network, which first
issued a telephone number, or in the subscription
network, which a subscriber registered to. The SRF
is typically implemented on signaling transfer points
(STP) in the SS7 communication model, and the
interrogation of NPDB and HLR are processed via
GMSC (gateway MSC).
The simplified NP call process is illustrated in
Figure 1. The origination network, which the calling
party connects to, receives a call initiation request
(step 1). It identifies the callee’s NRH network by
the prefix of the dialed number (MSISDN), and
issues an ISUP IAM message to the NRH GMSC to
initiate a call (step 2). The NRH GMSC consults
HLR and identifies the number was ported (step 3).
It queries NPDB by MAP sending routing
information message (step 4) to determine the
routing address of the callee’s subscription network,
and forwards the IAM message to the subscription
network (step 5). The subscription GMSC queries
HLR for the address (MSRN) of the termination
network (step 6), and routes the request by the
MSRN to set up the call (step 7).
Origination
switch
NRH
GMSC
Subscription
GMSC
Termination
MSC
HLR
HLR
2
4
5
6
7
3
NPDB
SRF
Origination network
Termination network
Number range holder
(NRH) network
Subscription network
1
8
Figure 1: SRF-based NP call routing
The IN-based solutions are implemented on the
service control point (SCP). 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 in the IN-based
solution every switch equipped with the IN protocol
can access NPDB. The call initiation processes of
IN-based and SRF-based systems are similar.
When a number was ported, the whole group of
numbers were taken as portable numbers. Every NP
call requires a NPDB query for the routing
information to reach the subscription network. When
the amount of users grows rapidly, the numerous
data of users incurs long NPDB query delay. The
considerable amount of queries burdens the load of
NPDB and HLR. NPDB searching becomes the
bottleneck of NP service. The bandwidth reserved
for the caller is occupied during the call setup
process, but operators rarely make profit on it.
The survey of Carpenter et al showed that caches
can offload a substantial amount of traffic at the
database, even with relatively small cache
(Carpenter et al. 2000). However, the effect of a
cache varies with the component and the hierarchy
the cache was applied. Caches can be implemented
to operator switching centers or to the user ends. In
the former case, a large cache is required to
accommodate the enormous dialed numbers and the
corresponding routing information. Mobile
subscribers move in and off a service realm of a
switching center, the numbers dialed by the
extensive amount of subscribers bring about almost
randomly access to the cached data, and result in a
poor cache hit rate. This approach confronts the
problem that either the cache size is too large to be
practical or the cache hit rate is poor to be
AN ORGANIZATION-BASED CACHE MECHANISM FOR SUPPORTING PCS NUMBER PORTABILITY SERVICE
81
3
efficacious. In the latter case, NP subscribers change
operators will cause the alteration of caches in all
terminals. Operators need to broadcast changed
routing information to all terminals, and the cost is
expensive; when subscribers move to other countries
or terminals were turned off, the altered information
may not be renewed to the terminal successfully,
thus the obsolete information may cause incorrect
routing of calls.
Studying the communication model and the
hierarchy of the mobile telecommunication system, a
large share of traffic load was generated from
organizations (e.g., enterprises, campuses,
government departments) in business hours. An
organization often has regularly or frequently
contact targets (e.g., agents of suppliers and business
associates, etc.), the external calls generated from an
organization have dialed-number locality. If the
routing information of the frequently dialed numbers
(FDN) is cached in OGB system, the translation of
portable numbers can be processed and routed
without querying NPDB. Therefore, the workload of
portable number translation is shared to OGB
network, the traffic load of NPDB is alleviated, and
the efficiency of NP service is improved in both the
global and the OGB communication systems.
3 ORGANIZATION -BASED
MOBILE COMMUNICATION
SYSTEM
Many mobile calls are set between organization
members. For organization members usually move
around the working space in business hours, and
colleagues often need to contact others on the move.
A local mobile communication system such as IEEE
802.11 based WLAN provides cost-free
communication service of an organization, but the
limited communication scope can not provide
service beyond the organization. Users must change
handsets and phone numbers when moving off the
scope. By contrast, a public mobile communication
system such as GSM enables global communication
service to users without limiting the communication
scope, but the high cost burdens to organizations and
the members.
There is a need of OGB mobile communication
systems which consist of the benefit of low-cost
local communication services and the convenience
of global mobility. The communications within an
organization can be processed and routed without
requiring exterior communication resources.
Organization members move off the service region
should not lose connection, and the communication
with users beyond the organization must be enabled
without the limitation of location. When setting a
call to an external user, the OGB system routes it to
the global communication system (e.g., GSM, PSTN)
according to the prefix of the dialed number (
Figure
2).
GMSCBSC
GSM
PSTN
Local
switching
center
Organization-based
network
Location
server
NPAC
NPDB
NPAC network
Figure 2: The concept of an organization-based
communication system
We had developed an IP-based OGB mobile
communication system—iNetwork—which fulfilled
the requirement of cost-free inter-organization
communication, and supported GSM-compliant
mobile service that guaranteed global mobility
beyond an organization (Cheng & Chung 2003). We
introduce iNetwork as an example to illustrate the
operation model of OGB caches, and investigate the
benefit when applying OGB caches to mobile NP
service.
3.1 Introduction of iNetwork
Based on the concept of providing user and terminal
mobility within and beyond organizations, iNetwork
was developed as a GSM-compliant OGB
communication system. By contract, an iNetwork
registers to a GSM operator as an add-on service.
GSM BTS is adopted as wireless access point, thus
users can utilize GSM-compatible terminals. The
mobile service of iNetwork follows GSM because it
is the most prevalent mobile system in the world,
and it possesses the capability of data services.
The register process is carried out when a user
moves to or turns on the terminal in iNetwork
service regions; only iNetwork subscribers are
allowed to access iNetwork resources. Intra- and
inter-iNetwork communications are processed
locally and transmitted through the IP network.
When moving off iNetwork service regions,
iNetwork subscribers register to the visiting GSM
operator to keep connection.
When an iNetwork registered to a GSM operator
as an add-on service or a business subscriber, the
subscribers of the iNetwork become the subscribers
of the GSM, and the numbering plan of the
iNetwork became a part of the GSM’s, the GSM can
ICETE 2005 - GLOBAL COMMUNICATION INFORMATION SYSTEMS AND SERVICES
82
determine a number as an iNetwork subscriber (by
IMSI). The most significant information to switch
and route flows between iNetwork and GSM is the
location information of subscribers. Both iNetwork
and GSM should provide a means for information
exchange.
3.2 iNetwork architecture
Figure 3 presents the iNetwork architecture. Except
the off-the-shelf GSM BTS, iNetwork components
are implemented as software components connected
to the IP-based network.
GSM
GSM MAP
ISUP
IP Network
GW
APNode
iNS
iNetwork
Internet
PSTN
SS7
IP
VLR
BTS/
BSC
MSC
Trans
BTS
LS
Meta
Server
HLR
Figure 3: iNetwork architecture
Every iNetwork includes at least an iNS to
provide switching and routing functions; a LS to
maintain user and location information; one or more
APNodes to handle requests from users, to collect
user information, and to manage network resources;
several BTS to support radio access to iNetwork
users, and they are bound with Trans that translate
protocols between the IP network and GSM system.
A MetaServer is an access interface residing in GSM
network that iNetwork can attain subscribers’
location information. iNetwork provides GSM MAP,
ISUP, and SIP interface to connect with GSM
network. SIP (Session Initiation Protocol) is adopted
as the signaling protocol of iNetwork system
administration, session control, and call handling.
The email-like addressing mechanism (i.e.,
user@host) integrates the communication models of
the IP network, PSTN, and GSM enable the
intercommunication of these systems. To consist
with ITU-T E.164 (Faltstrom, 2000) and the existed
switching systems, the user portion is a decimal digit
string that indicates a subscriber, host indicates an IP
address; aliases are allowed for easy memorizing.
3.3 Operation model of iNetwork
When an iNetwork user initiates a call, the request is
sent to an APNode. APNode forwards the request to
iNS. iNS consults LS by IMSI of the caller for user
authenticate. If the caller is not a registered user, the
call is bypassed or routed to the subscription GSM
network; otherwise, iNS consults LS by the dialed
number to determine whether it is an internal or
external call. In the case of internal call, iNS routes
the call to the callee directly through the IP network;
otherwise, the request is routed to cooperated
iNetwork or GSM according to the callee’s routing
information.
The cooperation models of iNetwork and GSM
are illustrated in the following.
3.3.1 The cooperation of iNetworks
To enlarge the service region, several iNetworks
cooperate as a communication community to share
communication resources to provide low-cost
mobile communication service. iNetworks of a
community share location information of subscribers
to identify and authenticate subscribers. The state
and location information of subscribers are informed
to iNetwork by location register process, which is
shown in Figure 4.
Visiting
iNetwork
A
Subscription
iNetwork
B
LS
LS
1
2
3
4
Alice
5
Inter ne t
Subscription
GSM_1
HLR
6
7
comm unity
Figure 4: iNetwork register process
GSM_1 is the subscription GSM of iNetwork_B.
User Alice subscribed to iNetwork_B and became a
subscriber of GSM_1. When Alice visits an
iNetwork, the latest location information is updated
to the subscription iNetwork_B by a
MAP_UPDATE_
LOCATION_AREA
message. The register information
consists of the IMSI and the MSRN (mobile station
roaming number, which indicates the address of the
termination switch where the MS resides) of Alice.
iNetwork_B keeps the information in local LS, and
forwards the register information to the subscription
GSM_1. By way of register process, the location
information of Alice is kept in both the subscription
iNetwork_B and GSM_1. According to the
maintained user and location information, iNetworks
can provide services of roaming, handoff, call setup
and termination services.
Intra-iNetwork-communications communications
are basically IP-based. Figure 5 illustrates the call
setup process in an iNetwork community. The
origination iNetwork-A determines the call is set up
to an iNetwork user by the prefix of the dialed
number. If iNetwork-A queries LS and confirms that
the callee resides in the local service region, it
forwards the call to the callee directly. If callee
wasn’t in iNetwork-A, the routing address of the
AN ORGANIZATION-BASED CACHE MECHANISM FOR SUPPORTING PCS NUMBER PORTABILITY SERVICE
83
5
subscription iNetwork-B is appended to the dialed
number (i.e., Alice@iNetwork-B), and a
SIP INVITE
message is issued to the subscription iNetwork-B.
iNetwork-B receives the message and queries LS for
the destination address (a MSRN) of the callee, then
forwards the request to the termination iNetwork-C
to set up the call.
Ben
Term ination
iN etw ork
C
Alice
6
Origination
iN etw ork
A
Subscription
iN etw ork
B
LS
LS
1
2
3 4
5
Figure 5: Call setup between iNetwork users
3.3.2 Interoperation with GSM
iNetwork users register to GSM network when
moving off iNetworks. The location information of
an iNetwork user is shared between the users
subscription iNetwork and the subscription GSM.
The interoperation between iNetwork and the
subscription GSM follows the SS7 signaling and
communication model. The protocol for exchanging
call handling messages is ISUP, routing information
inquires are based on GSM MAP, and the routing of
calling signals is conveyed by TCAP commands.
Every iNetwork subscriber has an iNetwork
address in the format of MSISDN@network. When
an iNetwork subscriber moves to a GSM, the
location information will be sent from the visiting
GSM to the subscription GSM. The subscription
GSM filters the information by the numbering plan
and updates the latest location information of the
subscriber to the subscription iNetwork via
MetaServer.
Origination
iNetw ork
A
Subscription
iNetwork
B
Ben
LS LS
1
2
3
4
5
+886912345678
(Alice)
Alice@ iN etwork_B
MSRN
Alice @ G SM _ 2
Subscription
GSM_1
GMSC
Term ination
GSM_2
sw itch
Alice
6 8
MSRN
Dest_addr
VLR
7
Figure 6: Call set up between iNetwork users who roam to
GSM networks
Figure 6 illustrates the process of setting a call to
a roaming iNetwork subscriber. Ben initiated a call
to Alice. The origination iNetwork-A queries LS to
determine the callee wasn’t in the local service
region, and routes the request to the subscription
iNetwork-B of Alice according to the dialed number.
iNetwork-B queries LS for the roaming information
of Alice and routes the request to the termination
GSM-2. If the GSM-2 is the subscription GSM of
iNetwork-B, iNetwork-B forwards the request to
GSM-2 directly; otherwise, iNetwork-B forwards
the request to it’s subscription GSM-1, then GSM-1
routes the request to the termination GSM-2 to set
up the call.
4 APPLYING CACHES TO
MOBILE NP SERVICE
Identifying portable numbers and determining the
corresponding destination addresses are the
foundation of NP service. The determination of
portable numbers and NPDB queries are
all-call-based, that prolong the time for NP call
process. Besides, the bandwidth reserved for the
caller is wasted during the long setup delay. As
specified in the above, caches can alleviate the
traffic load of NPDB and improve the efficiency of
data queries. For the dialed-number locality of OGB
networks, we purpose to apply caches to OGB
networks. Here we apply caches to iNetwork to
illustrate the operation and the benefit of OGB
caches.
4.1 Introduce OGB caches to mobile
NP service
Each cache entry in an OGB network includes the
MSISDN, the subscription assigned number IMSI,
and the routing address of the numbers subscription
network. iNetwork queries local cache for the
callee’s subscription network before issuing a call
setup request. A cache hit indicates the routing
information of a dialed number is confirmed, and a
time-consuming NPDB query is omitted.
The call initiation processes of iNetwork with
caches are classified to three types: intra-community,
inter-community, and inter-network-system calls.
The processes are illustrated in the following.
z Intra-iNetwork-community call
If a NP user subscribed as an iNetwork user,
iNetwork needs to mark the number as a portable
number, and identify the number as an iNetwork
subscriber. iNetwork queries caches before
consulting LS for the routing information of the
called party, thus the query of the NRH network can
be omitted.
Figure 7 illustrates an intra-community NP call
ICETE 2005 - GLOBAL COMMUNICATION INFORMATION SYSTEMS AND SERVICES
84
setup process. The origination iNetwork-A receives
a call initiation request (step 1) and determines the
dialed number is a portable number. iNetwork-A
queries LS first to identify if the callee resides in the
local service region. If not, iNetwork-A queries local
cache for the routing information by the callee’s
MSISDN (step 2). When cache hit, iNetwork-A
routes the request to the subscription network
directly; otherwise, it routes the request to the NRH
network by the prefix of the dialed number (step3).
The subscription network queries LS for the
termination network (step 4), and forwards the
request to the termination network (step 5). The
termination network pages the callee to set up the
call (step 6).
Ben
Term ination
iNetwork
C
Alice
Dest_addr
LS
6
Dest_addr
Alice@iNetwork_B
Origination
iNetwork
A
Subscription
iNetw ork
B
LS
LS
Cache
1
2
3
5
4
+886912345678
(Alice)
Alice@iNetwork_B
(Alice@ iN etwork_C )
MSRN
MSRN
Figure 7: NP call setup in an iNetwork community
z Inter-iNetwork-community call
Different iNetwork communities have no duty to
share information and communication resources.
The process of inter-community call setup is
illustrated in Figure 8.
Origination
iNetwork
A
Subscription
iNetwork
B
Termination
iNetwork
C
LS
Cache
LS
1
2
4
5
3
Ben
Alice
LS
+886912345678
(Alice)
Alice@iNetwork_B
Alice@iNetwork_B
(Alice@iNetwork_C)
MSRN
MSRN
Dest_addr
Dest_addr
6
Community 1
Community 2
Figure 8: NP call setup among different iNetwork
communities
The origination iNetwork-A determines the call
is set to a portable number (step 1), it queries LS for
identifying if the callee resides in the local service
region. If so, the call is set up to the callee directly;
otherwise, iNetwork-A queries cache for the callee’s
routing information (step 2) and routes the call to the
subscription network (step 3). But if it is a cache
miss, a NRH network query is issued to obtain the
routing address of the callee’s subscription network.
The subscription iNetwork-B queries LS to route the
request to the callee, the process is analogical to the
step 4 to 6 of intra-community call.
z Inter-network-system call
The connection of an iNetwork and the
non-subscription GSM networks often passes
through the subscription GSM network of the
iNetwork.
Ben
Origination
iNetwork
A
Subscription
iNetwork
B
LS
LS
1
2
3
4
5
Cache
+886912345678
(Alice)
Alice@iNetwork_B
Alice@iNetwork_B
(Alice@ GSM_2)
MSRN
MSRN
Subscription
GSM_1
GMSC
Termination
GSM_2
switch
Alice
6
7
MSRN
IMSI
Figure 9: NP call setup among iNetwork and GSM
Figure 9 elaborates the process of setting a call
from iNetwork-A to an iNetwork NP subscriber
Alice who roamed to a non-subscription GSM-2.
iNetwork-A determines the request is set to an
iNetwork NP user (step 1). It confirms the called
party does not reside in the local service region, then
queries cache for the callee’s subscription network
(step 2). If it is a cache miss, iNetwork-A queries the
callee’s NRH network for the callee’s subscription
network; otherwise, iNetwork-A forwards the
request to the callee’s subscription network
iNetwork-B (step 3). iNetwork-B queries LS and
finds the callee roamed to GSM_2, the callee’s
routing number (MSRN) is returned by the LS (step
4). According to the routing number, iNetwork-B
forwards the request to the termination GSM-2
through the subscription GSM-1 (step 5 to 7)
without querying HLR and VLR of the subscription
and the termination GSMs.
The benefit of OGB caches is to omit
unnecessary NPDB and HLR/VLR queries. Along
the increase of cache hit rate, the database queries
decrease when setting up calls. The locality of
contact targets of organizations causes small cache
size and better cache hit rate.
4.2 Issues of cache implementation
The policy of cache establishment and the
complexity of cache management influence the cost
and the benefit of caches. These issues are examined
in the following.
z Cache establishment
The data be cached should be as many as
AN ORGANIZATION-BASED CACHE MECHANISM FOR SUPPORTING PCS NUMBER PORTABILITY SERVICE
85
7
possible to enhance the cache hit rate, but the size of
a cache is restricted that the amount of data in a
cache is limited. In order to improve the hit rate of
caches to enhance the efficiency of the call setup
process, caches should be able to expose the
communication habits of users of the service region.
We propose to cache the most frequently dialed
numbers (FDN) which represent the cooperative
organizations, providers and suppliers, agents, etc. of
the organization, also include the families and
friends of every organization member. The
establishment and alteration of such a cache is
manually performed by the system administrator.
When a member joined an organization, the member
proposed a set of FDN to the system administrator.
The system administrator interrogates the contracted
GSM for the corresponding routing information of
the FDN. The interrogation is a batch process that
can be performed in off-time. When a contact target
changes subscription network, the contracted
operator notifies the registered iNetwork to renew
the altered routing information. The contact targets
of an organization have locality, and the change of
members is infrequent, the variation of FDN is
gentle.
z Cache update
iNetwork registers a profile of FDN to the
contracted GSM operator. When a NP subscriber
changes service provider, the effectiveness of the
change is postponed for couple hours to guarantee
the routing information consistency of NPDB and
the old and new subscription networks. iNetwork
queries contracted GSM to get the altered routing
information and update cached data periodically,
thus the cached data can be consistent with the
routing information in NPDB.
z Cache size
In an OGB network with m subscribers, every
subscriber has k FDN in average. The FDN set of an
organization member may overlap with that of the
others. Assume the overlap rate is r. Let U be the
universal set of all individual FDN set u
i
of member
i. The universal FDN set can be represented as
U =
L
UUU
+
=
+
==
=
m
kji0,kj,i,
k
u
j
u
i
u
m
ji0,j0,i
j
u
i
u
m
0i
i
u
+
()
m
m
uuu
L
21
1
)1(
The minimum cache size of the organization is
cache_size
min
=
1
1
1
)1(
=
i
m
i
i
kr
i
m
=
=
×
m
i
ii
r
i
m
k
1
11
)1( , where 0 < r < 1.
When the contact targets of every member is
scattered that r = 0, the maximum cache size is
cache_size
max
= m × k, where r = 0.
5 PERFORMANCE ANALYSIS
Mobile communication realizes the communication
on the move. A user may move across service areas
when a call is setting to him. To decrease the
overhead of miss-routing caused by obsolete routing
information, the call setup time shall be short. A
shorter call setup time saves time for callers without
waiting a long delay for the connection to be put
through, and means it saves the bandwidth which is
occupied by the caller during call setup; also the
communication resources for additional message
processing and transmitting is prevented. We study
the benefit of caches by comparing the NP call setup
time with or without organization-based caches.
5.1 Call setup time
The time required to initiate a call (denoted as t
c
) can
be represented as
t
c
=transmission delay + DB query delay + service
delay,
Let t
trans
, t
NPDB
, t
HLR
, t
LS
denote the transmission
delay, NPDB, HLR/VLR, iNetwork LS and cache
query delay, respectively. The size of LS is much
smaller than a HLR or a NPDB, the query delay t
LS
< t
HLR
, t
LS
< t
NPDB
, and t
HLR
< t
NPDB
. The processing
delay for user authentication, for codec, and for
message processing is assumed to be similar in GSM
and in iNetwork, and is denoted as t
s
.
Because t
LS
is small, the NP and non-NP call
setup time of intra-iNetwork and intra-community
are similar. However, the NP call setup time of
conventional GSM, inter-iNetwork-community, and
GSM with OGB cache are distinct.
z Setting a non-NP call in GSM requires
querying the subscription HLR for the
termination network, and querying VLR for the
location information of the called party :
t
c1
= t
trans conv
+ 2
×
t
HLR
+t
s
(1)
z Setting a NP call in GSM requires a NPDB query
for the subscription network of the called party:
t
c2
= t
trans conv
+ t
NPDB
+ 2
×
t
HLR
+t
s
(2)
The time for NPDB query and related message
processing makes t
c2
larger than t
c1
.
z Setting an inter-community NP call requires
querying origination LS and subscription LS:
t
c3
= t
trans iN
+2
×
t
LS
+ t
s
(3)
Since t
LS
is small, t
c3
is much less than t
c2
and t
c1
.
z Setting a NP call to a GSM user in FDN set, the
call is routed to the subscription GSM of the
dialed number:
t
c4
= t
trans conv
+ t
LS
+2
×
t
HLR
+ t
s
(4)
z Setting a NP call to a GSM user in FDN set, the
ICETE 2005 - GLOBAL COMMUNICATION INFORMATION SYSTEMS AND SERVICES
86
call is routed to the subscription network of the
dialed number directly:
t
c5
= t
trans conv
+ t
LS
+2
×
t
HLR
+t
s
(5)
Without querying NPDB for number translation,
t
c5
is much less than t
c2
.
call setup time
0
1000
2000
3000
4000
5000
6000
0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0. 8 0.9 1
p
t
(
msec
)
conventi onal GSM
iNetwork with cache
i Network communi t
y
Figure 10: The time for setting up a NP call from
iNetwork to a roaming GSM user
Figure 10 compares the call setup time of
initiating a NP call from iNetwork. Call setup delay
decreases when dialed numbers have locality and the
frequency of using FDN (p) increases, and the
bandwidth being occupied during the call setup
process is saved. In an organization with 10000
members, the average calling rate is 450 per hour.
Assume NPDB and HLR accesses cost 2 seconds. If
50% of the dialed numbers are FDM, for 8 hours
office time, the cache saves the organization 1
man-hour cost per day. There are 22 working days
per month, the cache will save 264 man-hour per
year for an organization.
8 hours
×
450 calls per hour
× 0.5 FDN utilization × 2
sec = 1 hour
6 CONCLUSION
In this article we stated the importance of NP service
in mobile telecommunication systems, and posed
that querying routing information from the large
NPDB prolonged NP call processing time. Besides,
Operators rarely made profit from the extra
consumed communication resources being occupied
during the NP call process time.
Follow from the property that OGB
communication has dialed number locality, we
introduced OGB caches to the mobile
communication system to share the load of portable
number translation to enhance the efficiency of NP
service. We also showed that OGB caches saved a
lot of communication cost for an organization.
In this paper we proposed an operation model of
OGB caches in the public mobile communication
environment. This implementation neither modified
the communication protocol nor changed the
operation logic of the conventional mobile
telecommunication networks.
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