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

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

 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

NPDB

SRF

Origination network

Termination network

Number range holder

(NRH) network

Subscription network

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

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

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

APNode

iNS

iNetwork

Internet

PSTN

SS7

VLR

BTS/

BSC

MSC

Trans

BTS

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

Subscription

iNetwork

Alice

Inter ne t

Subscription

GSM_1

HLR

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

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

Alice

Origination

iN etw ork

Subscription

iN etw ork

3 4

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 user’s

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

Subscription

iNetwork

Ben

LS LS

+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

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 number’s 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

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

Alice

Dest_addr

Alice@iNetwork_B

Origination

iNetwork

Subscription

iNetw ork

Cache

+886912345678

(Alice)

Alice@iNetwork_B

(Alice@ iN etwork_C )

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

Subscription

iNetwork

Termination

iNetwork

Cache

Ben

Alice

+886912345678

(Alice)

Alice@iNetwork_B

(Alice@iNetwork_C)

MSRN

Dest_addr

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

Subscription

iNetwork

Cache

+886912345678

(Alice)

Alice@iNetwork_B

(Alice@ GSM_2)

MSRN

Subscription

GSM_1

GMSC

Termination

GSM_2

switch

Alice

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

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

of member

i. The universal FDN set can be represented as

U =

UUU

≠≠=

∩∩+

≠==

∩−

⎟

⎠

⎞

⎜

⎝

⎛

⎟

⎠

⎞

⎜

⎝

⎛

kji0,kj,i,

ji0,j0,i

()

uuu ∩∩∩−

−

)1(

The minimum cache size of the organization is

cache_size

min

)1(

−

∑

⎟

⎠

⎞

⎜

⎝

⎛

−

∑

−−

⎟

⎠

⎞

⎜

⎝

⎛

−×

)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

) can

be represented as

=transmission delay + DB query delay + service

delay,

Let t

trans

, t

NPDB

, t

HLR

, t

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

< t

HLR

, t

< 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

Because t

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

trans conv

+ 2

HLR

(1)

z Setting a NP call in GSM requires a NPDB query

for the subscription network of the called party:

= t

trans conv

+ t

NPDB

+ 2

HLR

(2)

The time for NPDB query and related message

processing makes t

larger than t

z Setting an inter-community NP call requires

querying origination LS and subscription LS:

= t

trans iN

+ t

(3)

Since t

is small, t

is much less than t

and t

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

trans conv

+ t

HLR

+ t

(4)

z Setting a NP call to a GSM user in FDN set, the

ICETE 2005 - GLOBAL COMMUNICATION INFORMATION SYSTEMS AND SERVICES

call is routed to the subscription network of the

dialed number directly:

= t

trans conv

+ t

HLR

(5)

Without querying NPDB for number translation,

is much less than t

call setup time

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

(

msec

)

conventi onal GSM

iNetwork with cache

i Network communi t

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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AN ORGANIZATION-BASED CACHE MECHANISM FOR SUPPORTING PCS NUMBER PORTABILITY SERVICE