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A Distributed QoS Control Mechanism for Soft-QoS Provisioning in UMTS Multitier Systems

G. Araniti*, R. Agarwal**, A. Iera*, A. Molinaro***

* University “Mediterranea”, Dept. D.I.M.E.T., Reggio Calabria, ITALY - {aranitig, iera}@ing.unirc.it

** The Indian Institute of Technology, Kanpur, INDIA - raviag@iitk.ac.in *** University of Calabria, Dept. D.E.I.S., Cosenza, ITALY – molinaro@deis.unical.it

ABSTRACT This paper investigates the topic of QoS provisioning both at the application and network level

in an evolutionary scenario consisting of a UMTS multi-tier platform enhanced by a satellite coverage level and suitably thought for supporting multimedia traffic. In this “next-generation” scenario the main objective is the introduction of a distributed QoS control mechanism, with Middleware functionality, into the system and the evaluation of its effectiveness in achieving the major goal which it has been designed for: accomplishing a soft QOS paradigm which dynamically maximises the perceived QoS provided to the end user according to his/her needs. The highlighted objective has to be achieved by always coping with the highly variable constraints characterising the mobile environment under study. The foremost intended contribution of this paper is represented by the design of such a novel distributed mechanism exploiting algorithms able to accomplish a twofold functionality: dynamically optimising the QoS perceived by the user while always maximising the system efficiency. The assessment study presented in this paper testifies to the flexibility of the proposed algorithms and to their effectiveness in achieving the highlighted goals. To this aim suitable metrics are defined at different protocol levels and their output values are evaluated during a comprehensive simulation campaign under different conditions of load, traffic distribution and resource availability. Keywords: Middleware, Soft-QoS, UMTS, QoS Control Mechanism

I. INTRODUCTION

The introduction of multimedia applications into the mobile systems has opened a great

number of research issues, which are currently attracting most of the interest from both the industrial and the research telecommunications community. The highly attractive feature represented by the possibility of augmenting the communication means through multimedia, similarly to what is already feasible within a more mature multimedia fixed backbone, are counterbalanced by the many difficulties in effectively supporting multimedia applications in a mobile environment.

The mobile platforms have already entered a new era, characterised by the fast deployment of 3rd generation platforms, such as Universal Mobile Telecommunications System (UMTS), which have been suitable though to make available multimedia services at a controlled quality of service (QoS) to roaming users. Nevertheless, many research issues still remain open, which are driving the research community towards conceiving enhanced platforms based on the synergic interworking of different technologies and algorithms (integrated systems, terrestrial-satellite coverage systems, etc.). These efforts are mainly due to the most relevant limitation carried about by the advent of multimedia applications into mobile system: the rapid increase in the demands of network resources such as bandwidth, usually scarce and highly variable in a mobile environment. As a result of the fast convergence of services such as audio, video streaming, data and web, the effective handling of resources has become an increasingly important criterion in the design of new and improved technologies.

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The focus of the research is thus mainly concentrated on the attempts of guaranteeing the user always an optimum QoS, whilst adhering to the system constraints.

In highly heterogeneous scenarios, such as those envisaged for the next generation of multimedia systems, the idea of QoS need to be considered under a manifold point of view. It is clear enough that guaranteeing an acceptable QoS level could have different meanings depending on the protocol level this QoS has to be offered and on the kind of network segments the connections has to cross. Furthermore, the fast evolution towards the deployment of heterogeneous platforms consisting of different segments, such as terrestrially fixed as well as mobile sections augmented through the means of satellite links, has meant that the QoS is highly variable and only dynamically controllable. Nevertheless, it is the authors’ opinion that in spite of the difficulties in precisely defining what QoS is in multimedia heterogeneous scenarios, which are the QoS metrics to control at the different protocol levels, which is the best way to approach the problem of QoS guarantees, there is one certainty from which any valuable research should start. What matters eventually is the QoS perceived by the end user [1], and this is what is often referred to in literature as Perceptive QoS (P-QoS).

Approaching the QoS management problem by introducing application-level control schemes to account for the QoS perceived by the user (i.e. a “subjective” P-QoS) is not new in literature and its interest is rising fast. Nevertheless, as is often pointed out [2], approaching QoS control at the application level consists in merely setting certain high level parameters while the control of the specified QoS is left to mechanisms implemented at the lowest (network-to-physical) layers.

The problem is in effectively controlling the P-QoS derived from a QoS management architecture, which does not enable an efficient interaction between the mechanisms implemented at the Application layer and those operating at the lowest layers. In fact, a quantifiable degree of let’s say “objective QoS” can only be guaranteed by directly operating at the lowest layers of the system and requesting, monitoring and delivering QoS in terms of objective parameters such as bit rate, CLR (cell loss rate), mean delay or even BER (bit error rate), FER (frame error rate), SIR (signal-to-interference ratio), etc; this guaranteeing an easy QoS quantification and its precise dynamic control. However herein lies the risk for the developer in optimising certain low level algorithms, protocols and procedures to guarantee a quality of service but one which does not take into consideration the quality as is perceived by the user.

The research described in the present paper aims at contributing to the activity finalized to fill this gap between high-level and low-level QoS control mechanisms. The distributed mechanism proposed in fact has the interesting feature of basing its behaviour on a mapping of application level QoS indexes onto low level indexes and on the provisioning of a soft-QoS guarantee always matching the current resource availability at the air interface of the multilayered enhanced UMTS system. Through this mechanism, the user is supposed to be able to transmit multimedia traffic flows without service interruption, and always at the best perceived QoS currently available while roaming across different coverage areas and using different operators and different access modalities. The idea of a distributed mechanism operating like a Middleware between Application and Network layers and aligning the QoS control mechanisms of both protocol levels is a first step towards the achievement of the concept of Virtual Home Environment (VHE) [3], proposed within the 3rd and 4th generation mobile radio networks like UMTS [4].

The remaining part of the paper is organized as follows. In section II, we describe the general concept of our Distributed QoS Control Mechanism (DQCM), its functionality and the requirements imposed to the heterogeneous platform lying underneath. In section III, we specify the QoS mechanism and give emphasis to the way it can be deployed into a specific environment such as UMTS. In this we also present a brief description about the interaction between the mechanism and the functionality at the Application layer to provide the subscriber with an optimum P-QoS whilst adhering to the system constraints imposed by the lowest layers. Section IV gives a description of the reference performance evaluation environment and summarizes the most significant simulation results.

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II. THE DISTRIBUTED P-QOS CONTROL MECHANISM

II.1 Motivation and functionality The distributed QoS control mechanism (DQCM) foreseen in this paper has been conceived as

a functionality distributed across the different main components of the underlying mobile multimedia platform (user terminal, access node, etc.) and operating like a middleware at a protocol level between the application and the network layers. Before giving some details on its main functional modules and its implementation into an enhanced UMTS platform, we will briefly recall the main tasks it is expected to fulfil:

1. Optimising the P-QoS: as it has been stated earlier, user satisfaction is an essential

prerequisite in any emerging technology. The QoS control mechanism is thus requested to respect the user priorities by providing her/him with an optimised perceived QoS while adhering to the system constraints such as the inherent bandwidth variations of the wireless mobile links.

2. Implementing a Soft QoS paradigm: the mechanism has to refer to a soft Quality of Service paradigm, which means continuously interacting with the network layer for the negotiation and dynamic renegotiation of resources available to the user application whilst taking into account the user preferences (specified by means of soft Service Level Agreements, soft-SLA).

3. Providing a Virtual Home Environment: the VHE concept facilitates the full transparency of the offered services in a heterogeneous system, regardless of the network to which the user is currently connected. This means that the subscriber always obtains the required QoS she/he is paying for and thus the mechanism as to be conceived to be general purpose.

A complete functional description of our mechanism is given in [5], where it has been first

proposed. Here, we only provide a brief overview of the general framework, as the main focus is on its merging with a UMTS multimedia platform and the assessment of the achievable performance when different resource allocation mechanisms over the air interface are coupled with its functionality. The modular structure of our architecture is presented in Figure 1.

The main functions performed to adaptively control the QoS can be summarised in: monitoring of the network conditions; evaluation of the user profiles and relevant mapping into requirements expressed in terms of network metrics; application control; information exchange among the composing functional blocks.

In our framework, the monitoring of the network conditions is made at the sending and receiving ends (this means that both the uplink and downlink directions of the wireless system are monitored). Such an operation is necessary because at each location (sender and receiver locations) some information is available which could not be available at the other one (this is the case, for example, of two terminals communicating through the same multimedia application but accessing two different system segments). The functional module devoted to the network monitoring has a general-purpose nature, i.e. its behaviour is independent from the information the underlying network is able to supply to the upper layer. Therefore, some features are necessary to enable it to work efficiently even in adverse conditions, that is when the underlying network does not explicitly supply any information on the system congestion, neither when it enters a congestion phase nor when it recovers from this phase (an example is a native IP network without guarantees on the service quality). Differently, in the case in which a congestion phase is explicitly notified by the underlying network at its very beginning, or even in advance (as for Frame Relay networks), an adaptive QoS control procedure can be triggered to recover from this situation or to avoid it.

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Moreover, in the case in which also the end of a congestion phase is explicitly notified by the network, it is possible to recover to the original target QoS.

To ensure that the distribution/redistribution of resources is not executed by the mobile network in an arbitrary way, but the requirements of every multimedia application are fulfilled in any case, it is necessary to dynamically vary the transmission rates. To this aim, our mechanism is equipped with suitable functions to store and access the user profile data, containing information in terms of:

• type of demanded service (for example, phone, videophone, videoconference, video surveillance, and so on);

• price that the customer is willing to pay to obtain a certain level of quality of service. Another task of the distributed mechanism is the dynamic application control. It consists of

varying the transmission parameters (rate, number of frames per second, compression level and, consequently, quality level, type of coding, etc.) in such a way as to adapt it to the available resources and the customer preferences, in agreement with the outputs of the network monitoring and profile control activities.

Fig. 1: The Distributed QoS control architecture.

Variations in the bandwidth availability and burst error nature are typical features of the

wireless/mobile links. In our architecture the current status of the connection is, therefore, continuously monitored by the Network Detector, which exchanges control information with the under-lying network to provide indications of the transmission conditions. Then, the Network Detector makes the Network Control Module aware of the system status at all times. The latter module carries out the task of resource negotiation and renegotiation by interacting with the Network layer, at the same time taking into account the user preferences as is furnished to it by the User Profile Module. Clearly, the Network Control Module is the most crucial module of this innovative architecture, as it has to take decision relevant to the network resource handling and the rate adjustment. Furthermore, it transmits the control information to both the local and the remote Application Control Modules. The Application Control Module represents a set of functions directly interfacing the application. It performs actions aiming at modifying the application transmission

Network Detector

AApppplliiccaattiioonn CCoonnttrrooll AApppplliiccaattiioonn CCoonnttrrooll

Net Control

Net Control

Downlink Control

Uplink Control

Uplink Control

DownlinkControl

User Profile Control

User Profile Control

Network Layer

Application Layer

Sender Receiver

SSeennddeerr

RReecceeiivveerr

Network Detector

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parameters in accordance with the user priorities (provided by the User Profile Module) whilst adhering to the underlying system constraints. The User Profile Control Module is dedicated to user profile data management and furnishes the necessary data to the Network Control Module as well as the Application Control Module with the aim of optimising the user perceived QoS.

The Downlink/Uplink Detector Module has the task of monitoring the downlink/uplink channel. It must be able to realize that a variation occurred in the amount of resources available on such a channel and forward the appropriate control information to the Network Control Module. Subsequently, the correct parameters for the allocation to the necessary channels are communicated back to the Downlink/Uplink Detector Module, which performs the allocation by interfacing with the underlying system primitives.

II.3 Requirements Since we aim at proposing a general-purpose model, it needs to be independent from the type

of multimedia application and the network, therefore modularity is an essential feature. To exploit the QoS control mechanism for any multimedia application or network architecture it will be enough to foresee suitable interface modules, which are each time specified for the application/network we are interested into. The mechanism illustrated requests from the underlying platform the availability of QoS control mechanisms, which can be dynamically driven and controlled. On the fixed network side, like packet-switched IP, QoS control is enabled by the introduction of novel IntServ/RSVP [6] and DiffServ/RSVP-aggregate [7] models. On the mobile network side, numerous techniques for QoS differentiation at the different protocol level in 3rd generation UMTS mobile networks have already been studied [8].

The only burden introduced by the proposed mechanism is the extra load of signalling associated with dynamic QoS adaptation carried out between end users. However, this is a small price to pay considering the advantages associated with the idea of an end-to-end QoS according to the perceived quality at the Application layer, which the adopted algorithms accomplish. As will be shown in the later sections, this signalling can be appropriately minimised without adversely affect the overall performance.

As an example, in the present paper we will show the details relevant to a QoS control distributed infrastructure which has been equipped with the suitable interface modules for operating in an enhanced third-generation mobile system. In a later section we propose two different algorithms for the process of renegotiation implemented by the Network Control Module and do a comparative study of the results obtained on a UMTS network simulator

III. MERGING THE PROPOSED MECHANISM WITH UMTS III.1 Feasibility In a wireless multimedia scenario such as UMTS, the access points of the customer can vary

from session to session and even the network conditions could change during the progression of the same communication session. UMTS permits the user/application to negotiate bearer characteristics that are most appropriate for carrying information [4][8]. Moreover, UMTS allows for the radio bearer renegotiation during the progression of the connection. This renegotiation can be initiated either by the user/application or by the network. This is just the kind of provision required by our QoS control mechanism to carry out the dynamic QoS adaptation. It is in fact equipped with functionality for the maximisation of the P-QoS offered to the end user; thus, what is required is just enhancing its resource renegotiation algorithms in such a way as to interact with the QoS control mechanism and always accounting for the user satisfaction at the Application level.

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It is thus clear that the possibility in UMTS of performing a bearer renegotiation alone provides more than a sufficient motivation for the deployment of a QoS control mechanism such as that described above. Without such a provision for QoS management in the end terminals, the valuable characteristic of adaptive bearer control offered by UMTS could go wasted.

III.2 QOS Architecture in UMTS To enable the reader to better understand how the proposed functionality co-operates with the

QoS control protocols of UMTS, we give a brief description about the UMTS QoS architecture as specified in [8].

In UMTS, four QoS categories have been defined: Conversational, Streaming, Interactive and Background [8]. A list of bearer attributes such as Guaranteed Bit Rate, Max SDU, Transfer Delay etc. describes the service provided by the UMTS network to the users of the bearer service. Voice is always delivered over dedicated low delay data (LLD) bearers of the Conversational Class. All components of a multimedia call, such as audio, video, are also delivered over low delay and low delay variation bearers of the Conversational Class. Applications such as streaming multimedia with stringent limitations on delay variations are delivered over the Streaming Class, whereas bursty traffic with non real time delay requirements such as web browsing, network games and background downloads utilize the Background and Interactive Classes of traffic flow respectively. The QoS architecture of the UMTS-based reference network is that reported in the 3GPP documentation [8] and reported in Figure 2 for the sake of clarity in the presentation of our proposal.

Fig. 2: UMTS QoS architecture [8] The services delivered by the network are considered as end-to-end services, i.e. each one is

established between couples of Terminal Equipments (TE). The properties of the established service are implemented in terms of bearer services. The specifications of a bearer service include all the aspects tied to the provision of quality, such as control signalling, radio channel and resource management, etc. From Figure 2 it is evident that the traffic between the end terminals crosses different network segments. Therefore, it is delivered onto different bearer services. It is always true that each bearer service in a given layer of the QoS architecture offers its services to the upper layer and exploits the services offered by the lower layer. In our approach, the end-to-end service defined at the highest layer (that is the application one) makes use of a Middleware functionality obtained

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using a distributed QoS control mechanism to exploit the services offered by the underlying network.

UMTS to establish, modify and maintain a Bearer Service, with a specified value of QoS, foresees the use of functions, the QoS Management Functions, operating either in the Control Plane (Fig. 3) or in the User Plane [8]. As discussed above, in an end-to-end QOS adaptation scenario, the negotiation of bearer services must take place in every network entity constituting the link between the end users (Fig. 3) [8].

Transl. Transl.

Adm.Contr

RABManager

UMTS BSManager

UMTS BSManager

UMTS BSManager

Subscr.Control

Adm./Cap.Control

MT GatewayCN EDGEUTRAN

Ext.ServiceControl

LocalServiceControl

Iu BSManager

Radio BSManager

Iu NSManager

UTRAph. BS M

Radio BSManager

UTRAph. BS M

Local BSManager

Adm./Cap.Control

Adm./Cap.Control

Adm./Cap.Control

Iu BSManager

Iu NSManager

CN BSManager

Ext. BSManager

CN BSManager

service primitive interface

BB NSManager

BB NSManager

protocol interface

TE Ext.Netw.

Fig. 3: QoS management functions for UMTS bearer service in the control plane [8]

The Translation Function converts between the internal service primitives for UMTS bearer service control and the various protocols for service control of interfacing external network [8].

The UMTS BS Manager and RAB Manager co-ordinate the functions of the Control Plane for establishing, modifying and maintaining the service that they are responsible for [8]. The Admission/Capability Control maintains information of all available resources of a network entity and about all resources allocated to UMTS Bearer Services. It determines for each UMTS bearer service request or modification whether the required resource can be provided by the network entity and reserves the resource if allocated to a UMTS Bearer [8].

Our DQCM architecture has been designed to interact with the underlying resource allocation and management protocols of a generic wireless multimedia system. As an example, in the present paper we take as a reference for the performance study an underlying UMTS system. This means that the DQCM functionalities interact with the typical UMTS resource management protocol. More details about the way our generic QoS control mechanism framework for wireless multi-media systems can be exploited jointly with the resource management functionality of the UMTS system are given in the next Section.

III.3 The Renegotiation Process and Bearer Allocation As stated previously, UMTS allows for the possibility of negotiation of bearer characteristics.

In our QoS control architecture the bearer characteristic renegotiation process is executed by the Network Control Module. In this subsection we will describe the basic functional behaviour of the overall mechanism. Subsequently, in a following subsection we will propose two different algorithms for the process of renegotiation implemented by the Network Control Module and conduct a comparative study of the results obtained by means of a UMTS network simulator.

It clearly emerges that the UMTS architecture foresees functional blocks which are devoted to functions similar to those performed by blocks of our general QoS control architecture.

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By recalling the general functional scheme given in Figure 1, it becomes clear that the User Profile Control, the Network Control and the Uplink and Downlink Controller will directly interact with the UMTS Translation Function, the UMTS Bearer Service Manager (or RAB Bearer Service Manager) and the Admission/Capability Control respectively.

Besides the service class, a typical attribute utilized for admission control is the Guaranteed Bit Rate (kbps). In case of bearer reallocation the QoS control functionality implemented at the QoS layer within the two User Terminals must agree upon the same Guaranteed Bit Rate, this meaning that the signaling associated with the bearer renegotiation must be carried out end-to-end. This imposes the necessity of establishing suitable control channels between the terminal equipments. Two possible options are “in-band” and “out of band” signaling.

Let us describe, step by step, how the procedure for maximising the P-QOS offered to the end user at the Application layer behaves by referring at the Network Layer of a UMTS system.

1. In the most general scenario of a service such as Video Conference utilizing audio, video, and data bearers, the UMTS Network retrieves data from the database (User Profile) containing information about the user priorities, i.e. the importance the subscriber gives to each individual data streams.

2. By utilizing this information (which is constantly updated by implementing some user profiling techniques [9] in the User Profile Control Module) the UMTS Network initiates the negotiation process. This always aims at achieving the maximum QoS (related to the available overall Bit Rate) level possible in the network at any given time whilst also remaining within the soft QoS SLA agreed with the user.

3. When a user activates a new service application, the first entity invoked by the UMTS Network is the UMTS Translation Function. This function, placed in the MT, maps QoS attributes requested for a given application into service attributes typical of UMTS.

4. The translated attributes are sent to UMTS BS Manager, in the MT, to establish or modify a UMTS bearer service.

5. Every UMTS BS Manager in the MT interacts with its peer entity in the CN EDGE and the gateway and asks its associated Admission/Capability Control whether the entity can support the requested service, and whether the required service are available or not.

6. At the same time, the UMTS BS Manager, in the CN EDGE, verifies with its subscription control if the user has got the administrative rights for using the service required.

7. In case of success, the UMTS BS Manager, in the CN EDGE, contacts its Admission/Capability Control module to check the resources availability.

8. Then, UMTS BS Manager of the CN EDGE, translates the UMTS BS attributes into RAB service attributes, in the UTRAN. It also, similar to the other UMTS Managers, interrogates the Admission/Capacity Control inside the UTRAN.

9. Each the UMTS BS Manager translates its attributes into attributes for the lower layers thereby providing a certain QoS by using the services provided by the lower layers.

10. Even if one of these processes fails, the specific service request is refused. Otherwise, in every block of the network the resources are reserved, and a success message is sent to the users.

In case the requested QoS bearer resources are not available in the network, the UMTS Network starts a negotiation phase by degrading the bearer service, the user places least importance on. Such a soft-QoS paradigm is possible by the DQCM architecture acting as a negotiator between the network and the application. The relevant weights of each bearer service such as audio, video and data are obtained from the database which is the User Profile. Finally, if the minimum resource required by the application is not available in the network, the call is blocked or dropped.

Once the UMTS Network completes the bearer allocation phase, control information is provided to the Application Control Module of our overall mechanism. The Application Control Module has the task of modifying the perceptual level parameters of the application to best suite the user requirements. At the same time, these application level parameters must adhere to the

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constraints imposed by the network such as overall bandwidth, which depends on the characteristics of the negotiated bearer provided by the Network Control Module. In other words, the audio, data, and video high-level parameters are modified according to the resulting values of QoS attributes at the network level (such as the allocated bandwidth by the UMTS Network for each bearer service established). All the processes are performed in a way completely transparent to the user.

III.3.1 The Renegotiation Algorithm There are situations in which the agreed allocation of resources has to be checked against

changes in the resource availability. This happens for example, during the roaming of the user across the cellular network (handover event). Each time these changes happen, the proposed QoS control mechanism triggers a process of bearer renegotiation. Two possible algorithms for bearer renegotiation have been tested in this work, each one following a different policy.

Both of them are implemented in the Network Control Module which initiates the renegotiation process, and exploit for their QoS optimisation the availability of information from the User Profile Module.

Alg_1 • The Network Control Module chooses the maximum QoS (depending on the Bit Rate)

level requested by the application for starting the renegotiation process. This algorithm aims at offering maximum possible QoS available in the network to the end user whilst also maintaining a soft-QoS paradigm. In case the requested QoS bearer resources are not available in the network, the Network Control Module renegotiates by degrading the bearer service, the user places least importance on. The relevant weights of each bearer service such as audio, video and data are obtained from the database which is the User Profile Module. Finally if the minimum rate requested by the application is not available in the network, the call is blocked or dropped. This type of renegotiation is the same as that implemented the first time the call is accepted into the system and has the positive effect of allowing during handover a service already accepted with a given QoS to increase the QoS level experienced in the old cell if in the new cell there are more resources available.

Alg_2 • An alternative to the above mentioned algorithm is that the Network Control Module

chooses the current QoS (i.e. the corresponding current Bit Rate) level as a starting value for the new renegotiation process following the handover event. Again, in case the requested bearer resources are not available in the network (in the new cell), the Network Control Module renegotiates by degrading the bearer service, the user places least importance on (as obtained from the User Profile Module) thereby trying to provide her/him with an optimum quality whilst adhering to network constraints. The reason we choose as a target QoS always the QoS closest to the current QoS instead of that closest to the maximum requested QoS are the following:

o To minimize the variations of perceived QoS by the user. o To minimize variations in bandwidth in the network. o Intuitively, choice of the QoS closest to the current QoS should minimize

signalling between the end users. Figure 5 shows the flow diagram for both Soft-QoS algorithms, from the user’s equipment

point of view, when a handover event occurs. In both Soft-QoS cases, when a handover event occurs, the involved user asks for a given amount of resources. The “admission to new cell” phase verifies if these resources are available. If such is the case, then the user can be admitted into the new cell. Otherwise, a phase of Soft-QoS renegotiation starts, instead of dropping the call, aiming at assigning a smaller amount of resources to the connection. This amount cannot be smaller than a

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minimum, declared by the user at call set-up and able to satisfy a minimum QoS level acceptable to the user. This means that, for both Soft-QoS algorithms, the call is dropped only if at least the bandwidth corresponding to the “minimum” QoS requested by the user is not available.

Fig. 4: Alg_1 and Alg_2 Handover flow diagram.

In section IV, a comparative study of the two algorithms is presented. Simulation results in terms of blocking probability, dropping probability, amount of signalling in the network, network level QoS and P-QoS (in terms of MOS metrics) offered to the end user are provided.

III.4 Application Level Mapping and User Profiles During a call, if the characteristics of the underlying communication network change, resulting

in the reallocation of resources to the application, the role of the Application Control Module consists of dynamically adapting the application parameters according to the user preferences expressed by the means of soft-QoS user profiles, whilst conforming to the new resources assigned for the application.

In other words, the Application Control Module represents a set of functions mapping application specific parameters, such as resolution, bit rate, frame rate, colour depth, etc., to the parameters as obtained from the Network Control Module, such as overall bandwidth. To maximise the P-QoS of the user, the Application Control Module carries out the relevant mapping by taking into account the information on user priorities furnished to it by the User Profile Module.

The User Profile Module is a database containing application-specific user information files. A typical application-specific user file stores information on the priorities the user places on

Connected Mode

Handover Procedure

Request Max rate

Request Actual rate

Soft QoS Start new cell context

Dropped call

Alg_1 Alg_2

Admission to new cell

yes

no

R_req >= R_min

R_req < R_min

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individual streams such as audio, video and data (in case of a video conference application). These priorities are utilized by the Network Control Module during the bearer negotiation with the network. The user profile also stores information on the user priorities for certain application specific high-level parameters such as resolution, frame rate, colour depth, etc. These higher-level priorities are utilized by the Application Control Module in deciding how to subdivide the bandwidth allocated at the Network layer to a particular traffic stream among the high level parameters. Notice that we are using priorities automatically generated by some User Profiling algorithms implemented by our mechanism. The User Profile, used by the proposed mechanism, has been constructed by referring to the theory of User Modelling [9]. In particular, our techniques for constructing and handling the user profile are based on the assumption that, in a long period, user interests coincide with concepts stored in the information sources which she/he frequently accessed in the past [10,11]. A further, more interesting, characteristic of our approach consists in the exploitation of XML for storing and handling the User Profile. Our choice of exploiting XML is innovative also in the context of User Modelling and allows obtaining several advantages respect on traditional approach. Indeed, XML favours data exchange, this being a particularly interesting peculiarity when the involved information sources are highly heterogeneous. A further advantage of the XML choice is represented by the great “lightness”' of support data structures: although our User Profile is particularly rich, it is stored by means of a text file only. As a consequence, it is easily handled by a large variety of devices. The detailed description of the adopted technique is not the aim of the present paper. It is addressed in more details in [12]. It is worth noting that the subscriber is not required to have a detailed knowledge of the working attributes of an application. It is our objective to minimize the need for user intervention, at the same time providing her/him with the highest perceived QoS.

In this paper, it is not our aim to specify the mechanism used by the Application Control Module in the division of the network resource into application specific parameters as it is independent of the underlying heterogeneous network and is concerned only with the application. However for the sake of completeness, we outline a possible procedure utilized by the Application Control Module to provide the end user with an optimum P-QOS.

By using the application-specific user profile, the Application Control Module defines the following functions:

i

iN

ii ValueMax

AssignedValueiorityNN

QoSP)_(

)_(Pr)1(

21∑

=

⋅⋅+

=− (1)

( )nparameterparameterparameterfBandwidth ........,,........., 21= (2)

( ) ( ) ( )iii ValueMaxAssignedValueValueMin ___ ≤≤ (3)

where:

(Value_Assigned)i ! value assigned to application specific parameteri Min_Valuei ! minimum value permitted by the application Max_Valuei ! maximum value permitted by the application

i =1 …..N; N= total number of application-specific parameters

The Application Control Module invokes an inbuilt algorithm to maximise (1) subject to (2)

and (3). We are guaranteed a solution to the above problem since the function defined by (1) is maximised in an n-cubic volume (2), subject to the points lying on the curve represented by (3). A feedback control is also provided so that the user may wish to accept, renegotiate or reject the

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service. An alternative to the above procedure is specified in [12]. Here, the authors suggest a method based on the minimisation of the Euclidean distance between the permissible points and the most desired point in an n-dimensional normalized space.

IV SIMULATION ENVIRONMENT AND RESULTS In this section we will describe the results of a simulation campaign for the assessment of the

performance of the QoS control mechanism described above, when it is interfaced with a UMTS network simulator. The evaluation of user satisfaction is performed by using a videoconference application.

IV.1 The simulated environment In the proposed model the QoS is not fixed (static) and is not the same for all users but is

handled in an independent way for each of them. This kind of approach potentially implies a more efficient resource usage, and a less probable call blocking and handover failure.

To prove this we performed some early simulative studies by interfacing the proposed QoS control mechanism modules with the emulator of a multi-tier UMTS system.

The UMTS reference scenario adopted during our simulation campaign consists of repeated modules of 4 macrocells (900m cell radius), each one overlapping the area of 4 circular microcells (300m cell radius). Furthermore, we foresee the presence of a satellite spot-beam overlapping the whole area covered by the micro and macrocells. We assumed that each microcell has 4 neighboring microcells placed in the four cardinal directions; a user from each microcell moves by following one of these directions and roams freely across the whole area.

In order to avoid mutual interference between microcells and macrocells, the two layers are assumed to exploit different frequencies. We consider two classes of users: pedestrian and vehicular, respectively moving at 3 Km/h and 60 Km/h nominal speed. Users are originated and enter the network following a Poisson process. The user mobility within a cell is not modelled, the users freely roam and establish multimedia calls in the cells. The sojourn time of a user in a cell is exponentially distributed, its mean is given by the ratio between the radio coverage diameter of a cell and the user average velocity. Handover events model the user transition from cell to cell. They are distributed as a decreasing exponential with mean equal to the sojourn time. A perfect power control is assumed. The traffic generated by each user in the system can be of the following types:

• Traditional Telephone calls (PH), characterised by monomedia audio traffic; • Videoconference calls (VCF), characterised by voice, video and data traffics; • Web Browsing (Web), characterised by Web traffic. The different types of data streams utilize different radio bearers with characteristics which are

typical of the data stream. For example, the real time conversation stream of a videoconference is characterized by the fact that the end-to-end delay should be low and the limitation for delay variations is subject to human perception. Hence this type of traffic transmitted over the Dedicated Channel (DCH) would have bearer characteristics different from that of the Interactive and Background Classes, which do not have real time delay restrictions and utilize the Common Packet Channel (CPCH) and Downlink Shared Channel (DSCH) for data transmission.

Multibearer technique and multitier structure is well supported in UMTS and has been utilized in our simulations.

IV.2 Reference Multimedia Application and user MOS (Mean Opinion Score) The multimedia application used as a reference for our performance evaluation campaign is a

videoconference application. The choice of this particular application is motivated by the ease in the

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implementation of an Application Control module to be interfaced with it. This multimedia video-conference application is composed by several modules: audio, video, shared blackboard, sessions manager. This clear separation of the composing flows matches up with the concept of multibearer transmission assumed in our UMTS system simulator. Each traffic flow generated by the application can be independently handled and delivered over different bearer services of the underlying network (UMTS). This is a very important feature that we will strongly use to exploit and implement our soft-QOS paradigm.

To simplify the study, without loosing generality, we consider that the audio flow exploits few resources compared with the video one, and thus we focus our interest on the management of the video bit-rate variability indispensable to transmit a video flow. Furthermore, this application allows setting the kind of coding technique (chosen among different techniques), the number of frames per second, the image quality and other parameters that together determine the bit-rate value.

What we implemented is a specific interface (Application Programming Interface, API) within the Application Control that automatically modifies the graphical controls of the multimedia application, according to the information deriving from the underlying system and without any intervention of the customer.

This approach has several advantages because it can be extensible also to different multimedia applications, without any modifications. What will be necessary is the development of suitable interfaces for each Application of interest.

The User Profile Control, as already described above, has the task to do the mapping between the different qualitative levels perceived by the user and the relevant values of the transmission parameters and, therefore, the relevant amount of resources allocated in the network. This is made according to algorithms better specified and studied in [12]. The perceived quality levels (P-QoS) have been measured by averaging the opinions of a group of people (defining the Mean Opinion Score, MOS, standardized by ITU), which have contributed to obtain a set of typical profiles for the various types of services.

As for the Net Control, Downlink Control and Uplink Control, we decided to implement these functions in a single module. The implementation of these modules strongly depends on the underlying network, i.e. on the information coming from the underlying network layer interfaced with the proposed QoS control mechanism. Due to the absence of available API for UMTS, we implemented these functions in C++ within our UMTS system simulator.

Fig.5 shows the mapping between allocated bandwidth and resulting video MOS. It also shows the MOS corresponding to several speech coding rates taken from the 3GPP specifications [13].

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0

0,5

1

1,5

2

2,5

3

3,5

4

4,5

5

NVDCT - Video Speech

MO

S va

lues

384 Kbps

128 Kbps

64 Kbps

57,6 Kbps

32 Kbps

16 Kbps

8 Kbps

12,2kbps

10,2 kbps

7,95 kbps

6,7kbps

5,9kbps

4,75kbps

Fig. 5: Mean Opinion Score (MOS) for Video and speech. From the output of our study, it was decided to utilize NVDCT as the video coding technique.

For a videoconference application the data service occupies a minimal priority, so that the MOS levels considered are proportional to the bit rate provided. We have also derived a mapping between the video and audio MOS to the corresponding allocated bandwidth. It will be exploited by the QoS control mechanism to dynamically monitor the network QOS.

IV.3 Mapping from user MOS to network QOS and performance evaluation indexes As briefly mentioned above, we have derived a mapping from the MOS to the network

bandwidth utilized for the particular level of MOS score. The corresponding mapping for audio, video and data bearers is shown in Table I. All bit rates are in conformity with the rates specified for UMTS.

Speech (Conversational class)

MOS Value 4.01 4.06 3.91 3.77 3.72 3.50

Bit Rate 12,2kbps 10,2 kbps 7,95 kbps 6,7kbps 5,9kbps 4,75kbps

Video (Conversational class) MOS Value 4.42 4.12 3.76 3.42 3.24 3.20

Bit Rate 384 kbps 128 kbps 64 kbps 57.6 kbps 32 kbps 28.8 kbps

Data (Background class) MOS Value 5 4 3 2 1 0

Bit Rate 384 kbps 256kbps 128kbps 64kbps 32kbps 16kbps

Table I: MOS mapping onto required bit rate.

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The MOS values corresponding to each bearer are indicators of the user satisfaction associated with a service. As an indicator of the degree of satisfaction of the user we have also evaluated an Overall MOS. This parameter takes into consideration the priorities as were set by the user in the negotiation of bearer resources:

DDVVAAOverall MOSiorityMOSiorityMOSiorityMOS ⋅+⋅+⋅= PrPrPr

Indexes A, V and D refer to Audio, video and data respectively. At the same time as evaluating user satisfaction, we must also consider the utilization of

resources at the Network layer. A simple index indicating the efficiency of utilization of network resources is:

DDVVAAOverall QoSiorityQoSiorityQoSiorityQoS ⋅+⋅+⋅= PrPrPr

)__()__()__()__(

BandwidthVideoMinBandwidthVideoMaxBandwidthVideoMinBandwidthVideoAssignedQoSV −

−=

)__()__()__()__(

BandwidthAudioMinBandwidthAudioMaxBandwidthAudioMinBandwidthAudioAssignedQoSA −

−=

)__()__()__()__(

BandwidthDataMinBandwidthDataMaxBandwidthDataMinBandwidthDataAssignedQoSD −

−=

The QOS index is constrained to lie in the interval between 0 and 1. It is an indicator of the

utilization of bandwidth available in the network. Note that all bandwidth values utilized in the evaluation of QOS corresponds to the bandwidth allocated at the Network layer. This is an important criterion kept in mind for evaluation of the resources utilized by the QoS control mechanism. In this way we are able to map from the MOS values of a group of observers to the corresponding QOS values indicating the utilization of available resources.

For comparative study we have also introduced indexes of blocking and dropping probabilities in the simulator output. Finally we provide an indicator of the amount of signalling introduced in the network per bearer renegotiation request. This serves an indication of the load introduced by end-to-end signalling as a result of the renegotiation process of the QoS control mechanism.

IV.4 Performance Analysis

An extensive simulation campaign has been conducted to assess the effectiveness of our QoS

control approach based on the MOS evaluation. We focused our attention mainly on the videoconference traffic because it intrinsically introduces a sustained load into the system. The curves shown are plotted for different traffic profile conditions and different amounts of fast users in the system.

A first phase of our simulation study consists in analysing the system behaviour when varying the number of multimedia users and keeping the percentage of vehicular (fast) users fixed at 50%.

From Fig. 6 and 7, it can be easily observed how the soft-QoS approach of Alg_2 implemented through the DQCM functionality can drastically reduce the blocking and dropping probabilities for videoconference users. The different curves are drawn for different percentages of videoconference users in the system (ranging from 20 to 80%).

Whilst the Alg_2 approach leads to a significant reduction in the dropping and blocking probabilities, it also leads to a slight reduction in the Overall-QoS provided to the end user, as

- 16 -

illustrated in Fig. 8. The same is for the offered levels of MOS in Fig. 9. However, it can be seen that the offered levels of MOS fall within acceptable ranges. This is an expected result since Alg_1 always aims at providing the end user with the maximum MOS (and QoS) while maintaining a soft QoS paradigm.

0

0,05

0,1

0,15

0,2

0,25

0,1 0,2 0,4 1 2

CallS/sec

Blo

ckin

g Pr

obab

ility

Alg_1_vdc_80Alg_1_vdc_60Alg_1_vdc_40Alg_1_vdc_20Alg_2_vdc_80Alg_2_vdc_60Alg_2_vdc_40Alg_2_vdc_20

Fig. 6: Blocking Probability for Videoconference users.

0

0,01

0,02

0,03

0,04

0,05

0,06

0,07

0,08

0,09

0,1

0,1 0,2 0,4 1 2

Calls/sec

Dro

ppin

g Pr

obab

ility

Alg_1_vdc_80Alg_1_vdc_60Alg_1_vdc_40Alg_1_vdc_20Alg_2_vdc_80Alg_2_vdc_60Alg_2_vdc_40Alg_2_vdc_20

Fig. 7: Dropping Probability for Videoconference users

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0

0,1

0,2

0,3

0,4

0,5

0,6

0,7

0,8

0,9

1

0,1 0,2 0,4 1 2

Calls/sec

QoS

Ove

rall

Alg_1_vdc_20

Alg_2_vdc_20

Alg_1_vdc_80

Alg_2_vdc_80

Fig. 8: Overall QoS.

0

0,5

1

1,5

2

2,5

3

3,5

4

4,5

0,1 0,2 0,4 1 2

Calls/sec

MO

S O

vera

ll

Alg_1_vdc_20

Alg_2_vdc_20

Alg_1_vdc_80

Alg_2_vdc_80

Fig. 9: Overall MOS.

Finally, in Figure 10 we present an indicator of the average amount of end-to-end signalling

exchanges required to implement the soft-QoS paradigm during each handover event. The results obtained verify our intuitive idea that choosing a QoS level closest to the operating level (Alg_2) would minimize the signalling exchanges in the network. Moreover, the reductions in the MOS levels, as compared to Alg_1, are close enough to suggest that Alg_2 is a better implementation option.

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0

1

2

3

4

5

6

7

0,1 0,2 0,4 1 2

Calls/sec

Sign

allin

g pe

r han

dove

r req

.Alg_1_vdc_80Alg_1_vdc_60Alg_1_vdc_40Alg_1_vdc_20Alg_2_vdc_80Alg_2_vdc_60Alg_2_vdc_40Alg_2_vdc_20

Fig. 10: Average signalling load (number of signalling exchanges) per Handover Request.

Once the effectiveness of Alg_2 in increasing both the overall system load and the percentage

of videoconference users is assessed, it is interesting to investigate the performance of both algorithms proposed when varying the mobility profile of the users within the system.

Users exploiting Alg_2 during their handovers re-negotiate the bandwidth by starting from the value obtained in the old radio cell; if the user has a vehicular nature, it will obviously perform a higher number of handovers during its call progression than a pedestrian user does. This would likely imply a QoS (and MOS) degradation which increases as the percentage of fast user in the system increases as well. Curves in Figures 11 and 12 are drawn for different percentages of vehicular traffic in the system (20 and 80%) and confirm the expected behavior.

0

0,1

0,2

0,3

0,4

0,5

0,6

0,7

0,8

0,9

1

0,1 0,2 0,4 1 2

Calls/sec

QoS

Ove

rall

Alg_1_vh_20

Alg_2_vh_20

Alg_1_vh_80

Alg_2_vh_80

Fig. 11: Overall QoS.

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0

0,5

1

1,5

2

2,5

3

3,5

4

4,5

0,1 0,2 0,4 1 2

Calls/sec

MO

S O

vera

ll

Alg_1_vh_20

Alg_2_vh_20

Alg_1_vh_80

Alg_2_vh_80

Fig. 12: Overall MOS.

The foreseen slight worsening of the QoS perceived by the users corresponds to the manifest

reduction of the radio signalling consequent to the use of Alg_2, when different percentages (ranging from 20 to 80 %) of vehicular users load the system, as it clearly emerges from Figure 13.

0

1

2

3

4

5

6

7

0,1 0,2 0,4 1 2

Calls/sec

Sign

allin

g

Alg_1_vh_80

Alg_1_vh_60

Alg_1_vh_40

Alg_1_vh_20

Alg_2_vh_80

Alg_2_vh_60

Alg_2_vh_40

Alg_2_vh_20

Fig. 13: Signalling per Handover Request.

Figures 14 and 15 respectively illustrate the new call blocking probability and the handoff

dropping probability and show that Alg_2 outperforms Alg_1 in any case.

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0

0,05

0,1

0,15

0,2

0,25

0,1 0,2 0,4 1 2

Calls/sec

Blo

ckin

g Pr

obab

ility

Alg_1_vh_80

Alg_1_vh_60

Alg_1_vh_40

Alg_1_vh_20

Alg_2_vh_80

Alg_2_vh_60

Alg_2_vh_40

Alg_2_vh_20

Fig. 14: Blocking Probability.

0

0,01

0,02

0,03

0,04

0,05

0,06

0,07

0,08

0,09

0,1

0,1 0,2 0,4 1 2

Calls/sec

Dro

ppin

g Pr

obab

ility

Alg_1_vh_80

Alg_1_vh_60

Alg_1_vh_40

Alg_1_vh_20

Alg_2_vh_80

Alg_2_vh_60

Alg_2_vh_40

Alg_2_vh_20

Fig. 15: Dropping Probability

To complete the performance study, we have analysed the QoS trend when the percentage of

videoconference users is kept fixed and equal to 40% while the percentage of fast users varies from 20 to 80%. The traffic load in the system is also fixed and equal to 1 call per second. Both algorithms suffer from QoS degradation when the percentage of fast users increases. However, it is interesting noting in Figure 16 that the relative entity of QoS degradation is the same for both algorithms. This result further testifies to the effectiveness of Alg_2.

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0,45

0,475

0,5

0,525

0,55

0,575

0,6

0,625

0,65

20% 40% 60% 80%

% Vehicular Users

QoS

Ove

rall

Alg_1

Alg_2

Fig. 16: Overall QoS

VI. CONCLUSION

In this paper we focus on the design of an overall architecture for distributed QoS control in a multimedia multitier cellular environment. The DQCM implements a soft-QOS approach in an evolutionary heterogeneous system such as satellite-UMTS. The most promising aspect is that the DQCM functionality is based not only on maximising the resource availability in a highly varying network, such as a mobile radio network, but also on optimising the perceptual QOS offered to the end user. It has also been shown that by means of suitable algorithms for renegotiation of bearer services, the amount of signalling load in the network can be appropriately minimised without much degradation in the possible service levels. The effectiveness of the DQCM solution has been verified under different loading conditions and traffic distributions.

ACKNOWLEDGEMENT A special thanks to Dr. Carmelo Mallamace for his valuable contribution to MOS evaluation

activities.

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[7] IETF draft-ietf-issll-rsvp-aggr-02.txt, “Aggregation of RSVP for IPv4 and IPv6 reservations”, Mar. 2000.

[8] 3GPP, “Quality of Service (QoS) concept and architecture Technical Specification”, TS 23.107, 2002.

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[13] TR 26.075 3rd Generation Partnership Project; TSG-SA4: Codec; Performance Characterization of the AMR Speech Codec.