SERVICE INTEGRATION BETWEEN WIRELESS SYSTEMS:

A core-level approach to internetworking

Paulo Pinto, Luis Bernardo

Faculdade de Ciências e Tecnologia, Universidade Nova de Lisboa, P-2829-516 Caparica, Portugal

Pedro Sobral

Faculdade de Ciência e Tecnologia, Universidade Fernando Pessoa ,Porto, Portugal

Keywords: Interworking of wireless systems, wireless computing, handovers, mobility management

Abstract: The greater bandwidth provided by wireless LANs can be a precious asset to the wireless ubiquitous

computing if the integration with 3GPP systems is done at a certain level. This paper presents a proposal to

integrate wireless systems at core network level. Service integration becomes very powerful and easy. The

system is not so dependent on the critical latency of vertical handovers and the users feel a unique system

providing services. Little changes are required to the current 3GPP core network. Our architecture uses the

GPRS as the primary network and integrates WLANs as secondary networks, used on an availability basis.

Sessions on secondary networks survive disconnection periods contributing to a seamless service provision

to the user. The paper describes the overall architecture, the changes that are needed at the current 3GPP

core, and the operation of the secondary networks on the aspects of data routing and security associations.

Highlights about the application model are presented at the end.

1 INTRODUCTION

Cellular systems like Global System for Mobile

Communication/General Packet Radio Service

(GSM/GPRS), and its successor UMTS (Universal

Mobile Telecommunication System) already provide

IP-services in a ubiquitous mode. However, there are

obvious limitations on bandwidth due to their

coverage requirements. Wireless LANs (WLANs)

have been seen as a useful add-on to provide islands

of greater resources. Ways to integrate these systems

are being developed and a challenge discussed in

this paper is how the integration can be done to

allow new services to appear (e.g. exploring

mobility) and still support the existing ones.

Current proposals either envisage a complete

integration of WLANs in the cellular system’s

architecture (tightly coupled) or provide integration

in such a way that the systems interact poorly

(loosely coupled). The former solution does not

enlarge the type of services that can be designed

from the current cellular architecture framework and

the latter makes it difficult to provide a real sense of

service integration amongst the various access

networks.

This paper presents a solution based on an

integration performed at core network level amongst

the major components (such as the SGSN – Serving

GPRS Supporting Node). The system can be used in

different business scenarios and not only in a 3GPP

operator owned WLAN infrastructure. The paper

starts with a scenario of a service to justify why a

new approach to integration is viable. Our approach

maintains a user session regardless of the precise

Radio Access Network (RAN) used, claims that

vertical handovers (between RANs) are not needed,

and RAN switch can better be performed at core

network level (in the UMTS sense).

Each UE (user equipment) can maintain at least one

IP session over a specific RAN and can always use

the session, even when it is outside the coverage

area of that RAN (by using other active RANs).

Services can access core-level information to: (a)

improve the way they use the communication links

to the UE; and (b) handle and adapt to UE mobility

and connected periods (when the UE is inside the

coverage of a WLAN). In order to implement our

system only minor (software) modifications to the

current 3G core network need to be done.

127

Pinto P., Bernardo L. and Sobral P. (2004).

SERVICE INTEGRATION BETWEEN WIRELESS SYSTEMS: A core-level approach to internetworking.

In Proceedings of the First International Conference on E-Business and Telecommunication Networks, pages 127-134

DOI: 10.5220/0001384801270134

 SciTePress

2 ENVISAGED SCENARIO

Initial assumptions – First, the reality today is that

the cellular network is ubiquitous, covering 100% of

the populated areas. It is very unlikely that any other

radio network system will have such coverage. The

consequence is that any other network will have

dark areas, and supporting users in these networks

alone is not feasible. Second, our UEs are equipped

with two (or more) wireless interfaces working

simultaneously. Third, WLANs can be owned by

private organizations with agreements to the 3GPP

system operators or owned by the operators

themselves. Fourth, the security control provided by

the USIM smart cards and global roaming

agreements between 3GPP system operators form

the largest operational security system in the world

to date. AAA (Authentication, Authorization and

Accounting) procedures between 3GPP systems and

WLANs are on the verge of being approved (3GPP,

2003) and we assume them in our system.

Download service – Our service example extends

the infostation model presented in (Frenkiel, 2000)

with cellular network integration (the real subject of

the example – download of data – could be part of a

more sophisticated application).

A user is at home and uses the GPRS interface to

start a service to download some bulk data. In his

way to work, the system will try to use the WLAN

RAN (near semaphores, etc.) to deliver the data.

Eventually, all the data will be transferred.

In the rest of the paper, we consider GPRS as a

packet service in both 2.5G and 3G systems

standardized by 3GPP.

3 SYSTEM OVERVIEW

The capacity of radio cells will increase in the

future. Cells will be smaller than the current ones.

As stated in (Frodigh, 2001) we also agree that

extremely high rates will not be necessary

everywhere, but just in small hotspots. The question

is how to integrate these hotspots?

3.1 Hotspot Integration

One possibility is that they are part of the cellular

network as an ordinary cell. The network would

predict the user movement (using the cell

information) and could schedule the sending of large

bulks of data when a hotspot becomes available.

However, implementing such a facility at network

level can be rather complex (as there is not enough

relevant information). Moreover, unless applications

have knowledge of the differences in cells and adjust

to specific cell data rate conditions a user might

experience lack of bandwidth just because he

stepped out of the coverage.

Another possibility is that high bandwidth cells are

seen as special cells, not integrated in the cellular

system and having a special (direct) connection to a

packet data network. The user knows he is using a

different interface and stepping out of coverage is

easy to detect.

There are proposals for WLAN integration covering

both possibilities. The tightly coupling option

(Salkintzis, 2002) state that cells should be

integrated at a low level offering an interface

compatible with the 3GPP protocols. Besides the

drawbacks listed above there are still the following

disadvantages: (a) the WLAN must be owned by the

3GPP operator (to avoid strong exposure of core

network interfaces); (b) cell displacement and

configuration demands carefully engineered network

planning tools and WLAN integration becomes

difficult. Moreover, a great deal of control

procedures are based on configuration parameters

(CellID, UTRAN Registration Area (URA), Routing

Area (RA), etc.) and WLAN cells have to comply

with them; and (c) paging procedures and handovers

(including vertical handovers) have to be defined

and some technologies (e.g. IEEE 802.11) are not so

optimized to make them fast enough.

The loosely coupled option (Salkintzis, 2002 and

Buddhikot, 2003) assumes there is a WLAN

gateway on the WLAN network (with functionalities

of Foreign Agent, firewall, AAA relaying, and

billing) and the connection to the 3GPP core is via

GGSN (Gateway GPRS Support Node) (with a

Home Agent functionality). It only makes sense to

use this option with dual-mode UEs because a

vertical handover to WLANs would disconnect the

UE from all the functionality of the cellular

networks (paging, etc.). One advantage is that high-

speed traffic is never injected into the 3GPP core

network. A major disadvantage is the degree of

integration. WLAN networks are handled

independently and will be used on an availability

basis by the users, whom have to stay within the

same coverage. Any service provided by the 3GPP

(SMS (Short message Service), etc.) has to consider

the cellular system’s internet interface. Any

exploitation of the UE’s mobility (both in the

cellular system and inside the WLAN island) is

hidden by the mechanism of Mobile IP, for instance.

From the applications point of view, the UE is

stationary placed inside a big cloud called GPRS (or

WLAN). I.e. it has a stable IP address and any

mobility inside the 3GPP network is not seen from

the exterior.

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3GPP (3GPP, 2002) defined six scenarios of

increasing levels of integration between 3GPP

systems and WLANs. Scenario 3 addresses access to

3GPP PS services and includes access control and

charging. (3GPP, 2003) specifies how it should be

done. A loosely coupled approach was adopted but

the data routing aspects are still not fully agreed (the

specification covers mostly the access control and

charging).

Our proposal for hotspot integration is somewhere in

between the tightly and loosely options – it is at core

network level. It allows the use of WLANs as a

complement to the GPRS network. It is not fully

incompatible with the 3GPP effort, as it will be

described below.

3.2 Primary and Secondary Networks

In our system the GPRS network is the glue for all

the other RANs. It is the primary network having all

the control services (paging, etc.). All secondary

networks become simpler and can have control

services of their own not seen at core level (i.e. they

are simply internal optimizations). Almost all of the

works in internetworking assume that all these

features (including paging) exist in all networks and

are seen at core level. IDMP (Misra, 2001) is one of

the exceptions stating that they should be

customized. The most similar approach to ours was

taken by MIRAI (Wu, 2002). Their primary network

is a collection of BANs (Basic Access Network).

Each BAN contains the usual control services, and is

controlled by a CCN (Common Core Network)

manager. A user selects a RAN based on a list

provided by the BAN considering user location and

preferences. Although, the authors consider a long

list of issues to help the UE choose the RAN, some

too low level or “external” reasons (e.g. battery life)

can lead to unexpected choices from the

applications’ point of view. CCN handles micro-

mobility (possibly inter-RAN) and participates in

macro-mobility. The control features of the BAN are

very similar to the ones in UMTS. It could have

been implemented by the 3G system (as also stated

in (Wu, 2002)) but MIRAI authors decided to

implement a new radio interface.

In our system a WLAN RAN is a set of islands.

Each island is formed by a set of cells and is

controlled by an Island Manager (IM). Islands do not

cover the entire space (i.e. there will be dark areas).

All islands of a certain technology are seen by the

primary network as a Hotspot Network (HN) – a

secondary network.

A user is connected to the GPRS network and can

have other sessions simultaneously. Each HN

supports the notion of a session (i.e. IEEE 802.11

has one, HiperLan has another, etc.). Differently

from the other proposals is the fact that a session

survives the disconnection periods when the user is

moving in a dark area of a certain WLAN. For

instance, the user began a 802.11 session at the

airport, took a taxi to a hotel, and when he is in the

hotel, the same session is still on using the WLAN

infrastructure of the hotel (it is assumed that both

have agreements with 3GPP operators). On the way

from the airport to the hotel, if the user needs to be

contacted in the context of that session the primary

network is used.

Other works consider all RANs at the same level.

(Tönjes, 2002) defines a flow router at the core that

uses all RANs. This will lead to the existence of

control functions in all of them. If only one is chosen

to have these features the system will fall back to

ours. Moreover, with a monolithic core it would be

more difficult to add a new RAN.

4 ARCHITECTURE

Figure 1 shows the architecture for the data traffic

Figure 1: Data traffic in the hybrid networ

WLAN

distribution

network

WLAN radio

interface

SGSN

core

network

UTRAN radio

interface

HNAC

GGSN

GHSN

IMSI

HSS

Local

Route

SERVICE INTEGRATION BETWEEN WIRELESS SYSTEMS: A core-level approach to internetworking

129

(no access control, billing, etc.). The new

components are the HNAC (Hotspot Network Area

Controller) which controls one (or more) island, and

the GHSN (Gateway Hotspot network Support

Node) which is responsible for context management

and Internet access. The thicker lines belong to the

core but they are not present in the current 3G core.

All the high speed traffic goes through them not

overloading the current 3G infrastructure.

The 3GPP specification for scenario 3 (3GPP, 2003)

has a component that merges the HNAC and the

GHSN, called PDG (Packet Data Gateway). The

PDG is not connected to the SGSN (line A) as we

propose and all data integration between the systems

is done at IP level.

An UE has its identification at core level in the form

of an IMSI (International Mobile Subscriber

Identity) and the attachment procedure for GPRS is

the standard one (with temporary identifiers). In the

GPRS world an IP session can be established via the

GGSN (PDP context), having a routable address,

called IP

If the UE senses a WLAN to which it can perform a

connection establishment, it does so. From that time

on it can use the ‘local router’ to access the Internet

directly. An IP

address is used for that path

(whether this address is a care-of-address and

whether the local routing is performed at level 3 or

level 2 is irrelevant to this paper). If the WLAN has

roaming agreements with a 3GPP operator the UE

can perform an attachment procedure with the 3GPP

operator (3GPP, 2003). The attachment defines a

local identifier at core level for the UE in that

WLAN (possibly with temporary addresses, too). In

figure 1 an IP address was used as an example for

the local identifier (IPa). From this time on an UE

identified by the IMSI can be contacted via UTRAN

using the IMSI, or via WLAN using the IPa. It is

important to note that no assumption is made about

an all-IP technology in our system. It is sufficient

that it is IP-enabled. I.e. the UE communicates at IP

level with the core, but the core forwards packets to

the IM to be delivered to an UE with a specific local

identifier. The core does not assume any delivery

protocol in the island. The IM can use a layer 2

routing if it suits better. The important thing is that

IPa is stable. If the UE wants to use the Internet via

the WLAN it creates a PDP context (in similar

modes as to the GPRS case) and a routable address

is defined at GHSN. Every time there is an attach

update (in a different WLAN, for instance) a new

IPa is chosen but IP

remains the same.

is the main, fixed, UE address. IP

and IP

should be used on a temporary basis (e.g. client

applications). Therefore, reuse of addresses can be

made making the system scalable.

4.1 Overview of the Interactions

The HSS (Home Subscriber Server) has the

operational information about the UEs. Besides the

GPRS-related parameters that the HSS already has,

there is the information if an UE is HN attached, has

a session established and if it is currently inside a

WLAN coverage area (and the identification of the

HNAC responsible for it). SGSN and HNAC will go

to HSS to get updated information. The HSS also

provides authentication vectors, subscriber profiles,

and charging information.

The communication between the core (SGSN and

HNAC) and the UE can use any RAN. We will

describe two approaches: the first one, the smooth

transition, consists in keeping the GPRS almost as it

is with little add-ons. Any PS traffic will use

UTRAN but the HNAC can communicate directly

with the UE via WLAN, or can relay the traffic

through the SGSN to be delivered to the UE via

UTRAN (using the interface link A in figure 1. A

more concrete description is given below); the

second approach is more abrupt – both SGSN and

HNAC can convey their traffic through the other

component if they see some advantage. Currently, a

tunnel called GTP-U (GPRS Tunneling Protocol –

for User Plan) is established between the GGSN and

the serving RNC (Radio Network Controller). In our

second approach the tunnel goes as far as the SGSN

and a new tunnel is formed from there on. It is a

return to the original GPRS specification. Figure 2

shows the protocol stack at an UE. There is a

Connection Manager (CM) that manages the status

of both connections and offers a unique interface to

both RANs. The Delivery Service is a confirmed

service and switches to the UTRAN if it senses a

failure in the WLAN. If more than one RAN is

active the default one for each message is used. The

CM can signal the applications (or be queried by

them) about the current status of a specific

connection. With this information, applications can

L1/L2

Connection

Manager

(CM)

UTRAN

WLAN

Applicat. A Applicat. B

Figure 2: Protocol stack at a UE

Session Control

Delivery Service

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avoid using the link if the proper interface is not

active (transferring only urgent information, for

instance). The CM is able to contact each of the core

network components (SGSN or HNAC) either

directly or via the other RAN (for link maintenance

messages, etc.). The Session Control is responsible

for session survival when the UE is in dark areas.

For the smooth approach the following interactions

are needed: (a) permission by the SGSN to create a

session between the UE and the HNAC (using a

PDP context just as the GGSN uses them, with the

Session Management Protocol) (Kaaranen, 2001). A

GTP-U tunnel is created between the HNAC and the

UE (more precisely with the serving RNC); (b) an

event service from the SGSN to notify relevant

events – “UE availability”, “cell update”, “routing

area update”, “positive cell identification” and

“undefined cell identification”. It is important for

session management by the HNAC; and (c) mobility

management information by the SGSN (cell

identification if in GPRS state ready, or routing area

identification, otherwise) - it can be useful for the

HNAC. Suppose HNAC has a relation between cells

and WLAN islands topologies. It can force the

WLAN interface to switch off if no islands are

known in a certain routing area, for instance. It is

also important because HNAC change of

responsibility can happen when the UE performs a

routing area update.

For the abrupt approach the current GTP-U tunnels

have to be divided in two parts: one from the GGSN

to the SGSN; and another from the SGSN to the

RNC. The same will happen from GHSN to HNAC,

and from HNAC to IM. This separation allows the

second tunnels to be established either via the

default RAN, or via the other RAN.

4.2 Scope of Integration

In our system there is no need for vertical handovers

because the GPRS session is always on and the other

RANs are used as a complement. Communication to

the UE can use indistinguishably any available

RAN. A total switch of the communication from one

RAN to another is performed by the core

components, so no information is ever lost. In

systems with traditional vertical handoff, the

dominant factor is the time the UE takes to discover

that it has moved in/out of coverage (i.e. the cell has

to become active or inactive) (Steem, 1998). Using

RANs in a complementary form as we do, this time

is not so critical, and the GPRS can provide a

minimal bandwidth.

As the integration is performed at core level current

services can work with secondary RANs in a very

easy way. Figure 3 (taken from (3GPP, 2003))

shows how 3GPP plans to support SMS over

WLAN. A service specific gateway, IP-SM-GW,

must exist and offer an interface similar to an MSC

or an SGSN (interfaces E or Gd) to the GMSC/SMS-

IWMSC. The address of this gateway is returned by

the HSS in the “send routing information for short

message”. This gateway has a private database to

associate MSISDN to IP addresses. UEs in WLAN

have to specifically register and specifically

authenticate for SMS services and have secure

associations to the gateway. The gateway

communicates with the UE via Internet.

In our system (abrupt approach), the SMS service

could be provided without any modification. The

SGSN just gets the message and can use the HNAC

to convey the message to the UE, using the secure

associations that are already in place.

4.3 Application support at core level

HNACs have already the task of maintaining

sessions between appearances of the UE in WLAN

islands. A step further is their ability to work with

the applications in order to take advantage of the

mobility (and connect times) of the UEs to perform

the application task in a specific manner. This is not

the traditional approach in the Telecom world and

resembles more the activity of a middleware level

managing mobility.

In the Telecom world networks are seen as closed

systems that offer services. Services are carefully

specified procedures that use lower level procedures

called bearer services. The control procedures of the

network are seldom accessible from the exterior and

well protected from external components. It is

interesting to see that the same approach is being

planned for the introduction of Voice over IP (VoIP)

services (Lin, 2002) on top of GPRS. The RAN and

SM-SC

GMSC / SMS-IWMSC

IP-SM-GW

HSS/

(

HLR

)

WLAN UE

MSC

E’ or Gd’

SGSN

WLAN

AAA

Wx/(D’/Gr’)

PDG

SME

address

database

Figure 3: Support of SMS over WLAN

SERVICE INTEGRATION BETWEEN WIRELESS SYSTEMS: A core-level approach to internetworking

131

the GPRS network together are called the bearer

network. Through the Gm interface (which includes

radio, Iu, Gn, and Gi), the bearer network provides

bearers for signaling (control plane) and data (user

plane) between the UE and the IP Multimedia

Subsystem (IMS) (placed outside the core network).

The bearer network nodes (RAN, SGSN, and

GGSN) are not aware of the multimedia signaling

between the UE and the IMS.

The typical way to add new functionalities and

behaviors to networks is using frameworks such as

Intelligent Networks, or in case of UMTS, the

CAMEL (Customized Applications for Mobile

network Enhanced Logic), (Kaaranen, 2001).

However, the extensions are traditionally related

with the basic services and with inter mobile-

network interactions. For instance, personalization

of services, different control over switched circuits,

virtual home environments when roaming, etc.

In our system, HNACs can have a standard (and

protected) programming interface to be used by

third-party organizations to build services and

applications that take advantage of the information

gathered at the core (and not accessible today). The

end of this paper has an example of one.

5 HN OPERATION

The interaction between an island and its controlling

HNAC is performed by the Island Manager (IM).

The IM provides a stable identifier for the UE and

forwards packets between the UE and the HNAC. In

terms of security the 3GPP network can offer an

authentication service to the WLAN owner (WLAN

connection establishment is not covered by the

3GPP specification, obviously). It is important that

both networks rely very little on each other (not

disclosing authentication vectors, for instance).

Figure 4 shows the proposed setup. It consists of

two, almost similar, phases. The first provides

WLAN authentication assisted by the 3GPP

network, and the second provides 3GPP

authentication via WLAN. The UE senses a WLAN

and creates a provisional secure association with the

local router (LR) (1) (it is assumed that the AAA

functionality is inside the LR). Using this

association it sends a message to the LR to state its

willingness to authenticate. The LR triggers an

authentication process within the HNAC. The

HNAC gets authorization vectors from the HSS and

issues a challenge. The local router relays the

challenge and the corresponding response between

the HNAC and the UE (using, for instance EAP

Response/Identity) (2). The result of the

authentication is given to the LR by the HNAC

(EAP-Success/EAP-Failure) (3). At this moment the

LR knows the UE has the identity it claims it has.

The LR checks if the UE can use the WLAN, by

consulting a local database of users. If so, it creates a

definitive secure association (in the scope of

WLAN), provides the keying material to the UE for

local WLAN use, and informs the address of the IM.

The 3GPP could also approve a user not belonging

to the local WLAN community, in which case the

LR will tell the user that a local session cannot be

established but the IM address is given for a WLAN-

3GPP session.

If the UE has passed the first phase, it can now

start an authentication process with the 3GPP to

create a context there (4). The secure association is

created with the 3GPP without intervention, or

knowledge, of the WLAN. The attachment to the

3GPP network is covered in (3GPP, 2003) and uses

the EAP authentication procedure, providing enough

keying material for a secure tunnel to the PDG (or

HNAC in our case) through the IM. Once attached,

the HSS has the indication that the UE exists and a

session can be established (both by the UE and by

the 3GPP). A WLAN session can also be established

using the UTRAN interface (particularly useful in

dark areas). Each time a new island is entered a fast

update procedure must be done.

The concrete mobility management protocol used

inside the island and any mechanism to save power

or bandwidth are irrelevant and should not be seen

from the 3GPP. I.e., a micro-mobility move must not

change the local (IPa) address to maintain the secure

associations and the information in the core. Any

possible paging mechanism prior to the delivery of a

message is also hidden from the 3GPP. From the

core level point of view a packet is simply delivered.

Figure 5 shows the state diagram of the

interaction UE-3GPP. It is assumed that the UE is

always attached to the GPRS. Its idle state is the

Disconnected state – there is no operational WLAN

information about the UE in the core.

When the UE attaches and creates a PDP

context, the address IP

is defined, and the HSS has

information about its existence. An HNAC will be

Figure 4: Security flows for UEs

networ

3GPP

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responsible for it and the state changes to

Registered. In this state it is assumed that the UE is

always reachable via IP

Any communication to/from the UE is done in

the context of a session. A session represents extra

attention by the HNAC to the position of the UE

(using also the mechanisms provided by the SGSN

as stated above). The Session state is entered when

either the UE or the HNAC issue a Start of Session

message. The sub-state Connected means that the

UE is inside an island. The sub-state Searching is

entered when the UE is out of coverage. In the

Searching sub-state, the HNAC forces the UE to be

in active GPRS state to know its cell ID. It can

happen that the UE is in a cell that has no islands

nearby. In this case the HNAC can order the UE (via

UTRAN) to go to sub-state Waiting to save battery

power. When the UE moves to a cell where an island

exist it is told to change again to Searching (note

that these sub-states are simply optimizations and

can exist, or not).

Depending on the application it can be easy to know

when a session finishes, or not. If it is, an End of

Session message is sent. If it is not, a watchdog

mechanism based on inactivity triggers the sending

of the message, for instance.

6 APPLICATION MODEL

Applications may interact with external networks

using one of the three connections: UTRAN (IP

WLAN direct (IP

) or WLAN-HN (IP

). The

application models for the first two follow the

traditional Internet models – correspondent nodes

communicate with fixed remote IP addresses and

send data as soon as it is available. For the WLAN-

HN we can have a different approach that optimizes

the use of scattered hotspots over a ubiquitous 3GPP

network. With proper support at the core network,

these applications will be able to maintain sessions

independently of the hotspot availability, and

communicate the bulk part of the data only when the

UE is in the Connected sub-state.

Application functionality is divided between the

UE and the serving HNAC (figure 6). In the UE we

have a front-end component implementing the user

interface and interacting with the session control

entity and the associated lower services (which

behave as a middleware platform for the

applications). In the serving HNAC we have the

back-end component that cooperates with its peer on

the UE and maintains a stable interface with external

entities. By stable it is meant that any optimization

use of the air interface is hidden from the external

applications. These components work in the context

of the IP

session (Fig. 1), but can communicate with

each other using the WLAN RAN or the UTRAN.

They can work in an “Always Connected” mode,

using the SGSN each time the UE is in an HN dark

area, or, more interestingly, in “Hotspot Connected”

mode, communicating only urgent information via

SGSN while waiting for the UE to become

Connected again.

The middleware performs session and mobility

management providing applications with context

information. The middleware layer gathers UE

mobility information from the SGSN and network

status from probing its network interfaces. These

events are used by the middleware services to update

the execution environment parameters. Using this

information, applications are able to adapt their

behavior to different network conditions and

mobility scenarios.

For instance, if a user wants to download a news

summary stored in a web site, both components start

a new session, and the back-end component will start

to fetch the videos. If the UE gets out of coverage

the back-end component can store a portion of the

data waiting for the UE to pop up. Later, when the

UE enters into a hotspot, the information will be

forwarded to the front-end component. In the

meantime both modules can exchange control

information via the SGSN/UTRAN. This pre-

fetching feature optimizes UE connection time with

HN, avoiding fetching delays from exterior

networks. The back-end context has to be highly

mobile because it might have to change to another

HNAC pursuing the UE (dashed arrow in the

figure). Information can be stored either in the

HNAC or in a server close to the core network with

a guaranteed delay for access (avoiding copying

when the serving HNAC changes).

istere

Disconnected

Session

HN Attach

ocedu

HN Detach

ocedu

Connecte

Searching

Waiting

Start of

Session

End of

Session

Figure 5: UE-3GPP State Diagram

UE-3GPP State Diagram

SERVICE INTEGRATION BETWEEN WIRELESS SYSTEMS: A core-level approach to internetworking

133

7 CONCLUSIONS

The internetworking of wireless infrastructures

performed at core level with a pivot network seems a

simple and executable model. First, as most of the

control features already exist in the PLMN, they can

be absent in other networks. Second, because certain

details on secondary networks (such as micro-

mobility) are not managed at core level. Third,

because it defines an environment where new

features and services can be added to the core.

The addition of new modules at core level with

standard (and protected) programming interfaces can

open up new possibilities to explore terminal

mobility (a topic that is absent today).

Our solution does not impose relevant

requirements to the overall system: the architecture

does not need to be all-IP; there is no critical

dependence on vertical handovers; and, does not

create extra load to the current 3GPP core network.

Topics that are relevant for further work include the

algorithms to be used on top of the HNACs to

explore the mobility of UEs and their connection

periods, and the viability of service continuity using

this type of handovers.

REFERENCES

3GPP, 2003. Group Services and System Aspects; 3GPP

system to WLAN internetworking; System

Description (Release 6), TS 23.234.v.2.3.0, Nov.

Frenkiel, R., et al, 2000. The Infostations Challenge:

Balancing Cost and Ubiquity in Delivering Wireless

Data, IEEE Personal Communications, v.7, pp66-71,

n.2, April.

Frodigh, M., et al, 2001. Future-Generation Wireless

Networks, IEEE Personal Communications, v8, pp 10-

17, October.

Salkintzis, A., et al., 2002. WLAN-GPRS Integration for

Next-Generation Mobile Data Networks, IEEE

Wireless Communication, v.9, pp 112-124, October.

Buddhikot, M., et al., 2003. Design and Implementation of

a WLAN/CDMA2000 Interworking Architecture,

IEEE Comm. Mag., pp 90-100, November.

3GPP, 2002. Group Services and System Aspects;

Feasibility study on 3GPP system to WLAN

internetworking; (Release 6), TS 22.934.v.6.1.0, Dec.

Wu, G., 2002. MIRAI Architecture for Heterogeneous

Network, IEEE Comm. Mag., pp 126-134, February.

Tönjes, R., 2002. Flow-control for multi-access systems,

PIMRC’02, pp 535-539, September.

Steem, M., Katz, R., 1998. Vertical Handoffs in wireless

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Lin, Y., et al., 2002. An All-IP Approach for UMTS

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v.16, n.5, pp 8-19 September/October

Application

Front-End

Component

Session

Control, etc.

Application

Back-End

Component

Session

ntr

Application

Back-End

Component

Session

Control

etc.

SGSN

HNAC HNAC

Figure 6: Functional Blocks for HN session applications

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134