Tuan Quoc

Tien Anh Tran

Hang Le

Nguyen

WiMAX-based overlay conferencing service

Keywords: video conferencing service, distributed system, WiMAX, IMS

This paper proposes a new service architecture based on the interconnectivity between Subscriber Units (SUs) running on WiMAX network participating in an Application Layer Multicasting-based distributed multimedia conference. The novelty of the proposal is that it replaces the standard centralized architecture of IMS-based conference in order to utilize the intelligence and computational capacity of WiMAX Base stations. Thank to a distributed architecture, the expensive Media Resource Function Controller (MRFC) and Media Resource Function Processor (MRFP) can be completely omitted in this proposal. Experimental results show that the proposed architecture outperforms the previous system. Afterward, a prototype has been built to measure and prove the advanced performance of the proposed architecture.

I. Introduction

Video conference is the most complex type of service among variety of telecommunication services. It requires a guaranteed quality of service and a real-time interactivity. Nowadays, there are two main types of service architectures used by video conference services [START_REF] Le | Centralized and distributed architectures of scalable video conferencing services[END_REF]: Centralized architecture using Multipoint Control Unit (MCU) and distributed architecture using multicast mechanisms. The centralized architecture has advantages such as an easy management of admission control and a popular architecture which is currently being widely supported, but its main disadvantages are an exponential cost if the number of participants is increased, and a single point of failure. Because of the scalability and load balancing, there is another solution: distributed architecture. The mobile terminals are now equipped with a high computational capacity and a long battery life. New radio access technologies, such as LTE, WiMAX (in this research work, WiMAX network is used as a demonstration of the 4G infrastructure.), can enhance the mobile applications with a high throughput. Accordingly, these mobile terminals can participate into a distributed conference session together with desktop or laptop computers. The main problem is that the distributed video conferencing service and 4G network architecture have been designed with different requirements. The 4G network is based on the IP Multimedia Subsystem (IMS) to provide multimedia services including video conferencing service. However, the IMS-based video conference relies on a centralized service architecture or MCU [START_REF] Belqasmi | Design and implementation of advanced multimedia conferencing applications in the 3GPP IP multimedia subsystem[END_REF]. Hence, it has same problems with centralized video conference. For example, when the number of UEs increases, the centralized architecture will sustain a huge load for mixing and distributing video data. In this research work, we propose an IMS based WiMAX infrastructure which can seamlessly support the distributed video conferencing service. The main contributions of this research is that we propose an IMS-based architecture service without applying a central- ized MCU using the interconnectivity between the IMS-WiMAX platforn and WiMAX Subscriber Users (SU) participating in the distributed conference. The rest of this paper is organized as follows. First, we briefly describe the background knowledge including: IMSbased standard for video conferencing, WiMAX and interconnectivity between IMS and WiMAX, and the ALM architecture. Then, we present an IMS-based video conferencing service applying a distributed architecture with common operations such as: join/leave, pause/return, hand-over.

II. Background knowledge and related work

A. 3GPPs IMS-based Conferencing architecture

Figure 1 presents the 3GPP standard for the IMS-based conference architecture. In this standard, there are two principal protocols: Session Initiation Protocol (SIP) and Real-time Transfer Protocol (RTP) which are signaling and media transport protocols. The IMS architecture uses a media gateway (MGW) for transmitting and forwarding media data to UE. In this conference standard, two important components are applied: Media Resource Function Controller (MRFC) and Application Server (AS). MRFC controls a small element which is Media Resource Function Processor. MRFC provides all media related functions required by a conference service such as mixing, transcoding, trans-rating. It may also have the Floor Control Server(FCS). Since all media related functions are controlled by the MRFC, this architecture is centralized. To avoid the disadvantages of the IMS-based conferencing standard, several proposals have been made. Their common purpose is to support the distributed conferencing service on current centralized architecture of IMS-based conferencing service with WiMAX access network. F.Belqasmi et. al. [START_REF] Belqasmi | Design and implementation of advanced multimedia conferencing applications in the 3GPP IP multimedia subsystem[END_REF] proposed the FCS feature which is separated from MRFP. However, his solution did not change the centralized architecture of IMS-based conferencing standard. In [START_REF] Buono | A distributed IMS enabled conferencing architecture on top of a standard centralized conferencing framework [IP Multimedia Systems (IMS[END_REF], A.Buono et. al. proposed a distributed solution as an overlay network of centralized conferencing clouds. In addition, it has used a Content Distributed Network (CDN) with proxy servers to support distributed contents, but all participants still have to process all signaling and media loads. CDN servers are mainly proxy-based MCUs to connect some clients together using a centralized architecture and then connect other centralized groups together by creating an overlay network of those proxy servers. To conclude, the current solutions for IMS-based conferencing service either are centralized or have not fully utilized the capacity of the 4Gs infrastructure such as WiMAXs.

B. WiMAX and the interconnectivity between WiMAX and IMS

WiMAX is a broadband wireless access (BWA) technology for wireless metropolitan area networks. It was created by the WiMAX Forum, which was founded in June 2001 to promote the adoption of WiMAX compatible products and services [START_REF]WiMAX Forum Network Architecture -Architecture, detailed Protocols and Procedures -Policy and Charging Control[END_REF]. A WiMAX network usually contains the following network entities [START_REF]WiMAX Forum Network Architecture -Architecture, detailed Protocols and Procedures -Policy and Charging Control[END_REF]: ASN (Access Service Network), CSN (Connectivity Service Network), ASP (Access Service Provider) and MS/SS (Mobile station/Subscriber station). ASN is an entity which provides radio access to WiMAX subscriber, it also has some features such as: transferring of AAA authentication messages, authorization and session accounting for subscriber sessions, radio resource management. [START_REF]WiMAX Forum Network Architecture -Architecture, detailed Protocols and Procedures -Policy and Charging Control[END_REF], CSN provides IP connectivity services to WiMAX subscribers meaning that CSN supplies several functions such as: Internet access, AAA proxy and server, Policy and Admission Control based on user subscriptions profiles [START_REF]WiMAX Forum Network Architecture -Architecture, detailed Protocols and Procedures -Policy and Charging Control[END_REF] ASP is a business entity which provides applications or services [START_REF]WiMAX Forum Network Architecture -Architecture, detailed Protocols and Procedures -Policy and Charging Control[END_REF]. In this entity, the WiMAX Forum proposes two types of connetion to an application or a service: non-IMS application server and P-CSCF(IMS). However, it is not clear how to develop a non-IMS application server [START_REF]WiMAX Forum Network Architecture -Architecture, detailed Protocols and Procedures -Policy and Charging Control[END_REF]. Thus, we suppose that they use the IMS-based application server to provide services for WiMAX subscribers. WiMAX has two types of inter-connection with other wireless networks such as LTE(3GPP): loose couple and tight couple [START_REF] Xu | Interworking of Wimax and 3GPP networks based on IMS [IP Multimedia Systems (IMS) Infrastructure and Services[END_REF]. Figure 2 shows that loose couple interwork- ing re-uses the AAA-server of 3GPP network. The core network (SGSN, GGSN) does not transfer data stream. This type has an advance: independence of WiMAX network. On the other hand, Fig. 3 shows that tight couple uses RNC and core network (e.x : SGSN and GGSN). Therefore, there are some changes in its protocol, interface and service for interworking requirements. However, these two types of interworking provide the interconnectivity between WiMAX subscribers and the IMS platform or an IMS-based service. In fact, it is possible to provide services or applications for WiMAX subscribers via IMS.

III. IMS-based distributed video conferencing service for WiMAXs subscribers A. Main challenges

There are several challenges which are blocking IMSbased distributed video conferencing from providing services for WiMAXs subscribers. IMS is firstly a 3Gpps architecture for providing commercial services and applications regardless of the underlay network. On the other side, WiMAX is the standard proposed by the WiMAX Forum and is standardized to IEEE 802.16 [START_REF]WiMAX Forum Network Architecture -Architecture, detailed Protocols and Procedures -Policy and Charging Control[END_REF]. It is therefore called a Non-3GPP standard. Accordingly, it is difficult to connect between WiMAX and IMS, especially as providing IMS-based video conferencing service to WiMAXs subscriber. However, in a draft standard [START_REF]WiMAX Forum Network Architecture -Architecture, detailed Protocols and Procedures -Policy and Charging Control[END_REF], WiMAX Forum has presented a method to connect between WiMAX and IMS. Even though, IMS has proposed a standard for video conferencing service, it is still based on centralized architecture [START_REF] Belqasmi | Design and implementation of advanced multimedia conferencing applications in the 3GPP IP multimedia subsystem[END_REF]. Therefore, our research work manage to discard components of the centralized architecture such as MRFC, MRFP in order to avoid bottle-neck phenomenon and to reduce the entire system's CAPEX/OPEX. ALM works on application layer, it does not care which kind of access network technology is used by the terminal for participating into a conference session. To maintain ALM operations such as heart-beat, data-forwarding, all members use their resources, for instance: bandwidth, memory, computational processor. It is difficult to apply ALM in mobile terminals. Moreover, WiMAX is a mobile network in which components connect together using the same core network. WiMAX BSs are smart base stations, they are capable of computing and sharing loads [START_REF] Wu | The Design and Implementation of WiMAX Base Station MAC Based on Intel Network Processor[END_REF]. If these WiMAX BSs can represent UEs in an ALM overlay network to forward their data streams and to process signaling messages, UEs can participate into a distributed conferencing session as if they are participating in the conventional IMS-based centralized conference. Next, a distributed video conferencing service using ALM mechanism does not have any method for managing floor control (methods to handle conferences such as participants' joint/leave, pause/return, select and manage the active users). Thus, our architecture proposes a floor control to support ALM-based distributed conference. Another important limitation is that, a distributed video conferencing service working on WiMAX does not support QoS. On the other side, WiMAX owns a mechanism for supporting QoS and so does IMS [START_REF]WiMAX Forum Network Architecture -Architecture, detailed Protocols and Procedures -Policy and Charging Control[END_REF]. Therefore, a QoS converting mechanism between IMS and WiMAX is necessary to provide a QoS-guaranteed ALM conferencing service. Last but not least, our proposed architecture should support common mobile operations in a video conferencing session such as: join/leave, pause/return and hand-over.

B. Extended WiMAX base station and extended application server for WiMAX proposal for IMS-based distributed conferencing service

To construct an IMS-based distributed conferencing service for WiMAXs subscribers, we propose two extended components: IMS-based Extended Application Server and Extended WiMAX Base Station. Extended IMS-based provides the principal signaling video conferencing service, it also has floor control features for video conferencing session. Extended WiMAX BS (WiMAX xBS) is representative of some SUs that it is managing in the overlay network. It means that WiMAX xBS should understand the interface between ALM architecture and WiMAX-IMS architecture. Figure 4 shows a block diagram featuring an extended WiMAX base station. In general, this extended WiMAX base station is representative of several nodes that it is managing in the Overlay network. At first, when a WiMAX SU wants to participate into the conference, they sends their signaling over messages such as REGISTER or INVITE SIP messages to the signaling processor in WiMAX BS. Furthermore, these messages contain QoS and Layer registration information indicating that a WiMAX SU wants to receive the video stream. The signal- ing processor will handle this information to provide QoS and layer registration. Then QoS and layer registration demands the IMS core network to check the conditions of the WiMAXs SU such as: policies, bandwidth via Signaling processor. If WiMAXs SUs capacities are enough to receive scalable video, then QoS Layer notifies those requirements to the Overlay network via its Overlay Interface. Secondly, having received events in video conferencing session, the WiMAX SU sends these commands via SIP messages, the signaling processor sends commands to the event processor. Next, this event processor will handle all events in a video conference session. Thus, it may send requirements to the Overlay network via Overlay Interface. At the same time, WiMAX xBS will update SUs information to the Overlay SUs Repository such as: UEs IP, role of overlay network (source/relay/forwarding), status (idle/active), video layer registration. Figure 5 illustrates the block diagram feature of application server (AS). We propose this extended IMS-based Application Server (xAS). The principal features of xAS are floor control in video conferencing service and obtaining WiMAXs SUs information from the IMS-WiMAX core network without requesting information from WiMAXs SUs. First of all, when receiving an event from the video conferencing session, WiMAX xBS sends several signaling messages to xAS, and xAS receives these messages via its Signaling processor interface, processes events and then updates them to Video conferencing policy repository such as: conference ID, WiMAX BSs ID, status of conference (in process, paused or terminated). Thus, xAS can manage the floor control, for example: member joining, leaving, pausing and returning. On the other hand, because of the controlling process in Overlay network, all members have to transfer signaling messages. It is not practical for mobile terminals, because it takes too much of their resources such as: bandwidth, computing capacity, battery life. The xAS can obtain WiMAXs SU information using the core IMS-WiMAX network. After obtaining this information, xAS sends it to WiMAX xBS which is the representative of nodes it is managing. WiMAX xBS will convert the content and payload it to the signaling messages and send them to the Overlay network. This process helps saving mobile terminals resources by making use of WiMAX xBS capacity. Moreover, xAS assigns the policies for a video conferencing session such as: start time, du- SU wants to pause the service meaning that the WiMAXs SU does not want to receive or forward any video data nor signaling messages, but it still wants to return quickly to the ALM group afterwards. To turn into an idle status, WiMAXs SU sends the PAUSE(WiMAXs SU-SIP-Account) message to WiMAX xBS, xAS to update theirs status in the repositories. At the same time, WiMAX xBS sends a REQ-PAUSE (WiMAXs SU-ID) to the ALM group. The ALM group answers a REP-PAUSE(WiMAXs SU-ID) message to accept the idle status of the WiMAXs SU. After having confirmation from ALM, the IMS-WiMAX core network will change the reserved resource for WiMAXs SU to save network resource. Next, the ALM group stops sending video bit-stream and signaling messages (HEART BEAT message), so that the WiMAXs SU will be stored in the waiting list of the ALM group. After a pre-defined duration, if the WiMAXs SU does not return to video conference service, this ALM group will automatically remove this SU from the waiting list. If the WiMAX SU wants to return the video conferencing service in a period within the pre-defined duration, it may send a RETURN (WiMAXs SU-Sip-Account, WiMAXs The two of-the-same nodes are sending and receiving bitstreams as any other normal nodes in the ALM group. It will help the UE to maintain its conference service via both WiMAX BSs before the old connection is actually broken. The source WiMAX BS sends a LEAVE-REQ message to the ALM group and waits to receive the LEAVE-REP from the ALM group. Afterwards, the old UE-ID is removed from the ALM group and only one UE-ID will represent the UE in the conference.

C.4 HEARTBEAT OPERATION

Figure 9 shows the heartbeat handling process in the distributed conference. According to the fact that, many ALM algorithms have to depend on a heartbeat mechanism to maintain their group. If the UE has to directly respond to all heartbeats, it will soon run out of power and computational capacity. Meanwhile, all information regarding a WiMAXs SU availability is available in the core network; a simple interrogation operation triggered by the xAS can solve this problem. Therefore, the WiMAX xBS will periodically send a HEARTBEAT-QUERY message to the xAS asking for the availability of all WiMAXs SUs which are joining the distributed conference under its representative. When this message is received at the xAS, all WiMAXs SU-IDs are obtained from the Video Conference Repository. Next, an REQ-INTERROGATION (WiMAXs SU-IDs) message containing a list of all WiMAXs SU controlled by the originating WiMAX BS will be sent to the WiMAX-IMS Core network. The WiMAX-IMS core will send back the availability information of all requesting UE obtained from the network to the xAS for it to be forwarded to the WiMAX BS. The WiMAX BS will then send back the heartbeats of all the UEs it is representing to the ALM group to inform the group about their status. For floor control purposes, the ALM-based distributed conference will update the list of all peers participating in the conference from the ALM group to the xAS.

IV. Conclusion and Future Work

Our research proposes the new architecture for internetworking between WiMAX and IMS for a distributed video conferencing service. The advantage of the proposal is to omit the centralized components such as MRFC, MRFP from the standard IMS -based video conferencing architecture. Concurrently, it makes use of WiMAXs BS computational and robust capacity for distributed conferencing service and optimizing mobile network. From the proposal, the new architecture can reduce the signaling and data stream traffic handled by WiMAXs SU when they participate in the distributed video conferencing service.

Fig. 1 .

 1 Fig. 1. 3GPPs IMS-based centralized conference architecture.

Fig. 2 .

 2 Fig. 2. Loose couple of interworking.

Fig. 3 .

 3 Fig. 3. Tight couple of interworking.

Fig. 4 .

 4 Fig. 4. Block diagram features of WiMAX extended base station.

Fig. 5 .

 5 Fig. 5. Block diagram featuring an extended Application Server (xAS).

Fig. 6 .

 6 Fig. 6. Join/Leave of WiMAXs SU to/from distributed conference with QoS support.

Fig. 7 .

 7 Fig. 7. Pause/Return operations over distributed video conferencing service with QoS support.

Fig. 8 .

 8 Fig. 8. Hand-over operation when participating in a distributed conference with QoS support.

  Figure8presents a WiMAXs SU going through a hand-over process while participating in the distributed

Fig. 9 .

 9 Fig. 9. Hear-beat operation in a distributed conference.


Tien Anh Le

Hang Nguyen

Perception-based Application Layer Multicast Algorithm for scalable video conferencing

Keywords: application layer multicast, architecture analysis, video conference, service architecture

In this research work, a new application layer multicast algorithm using distributed service architecture and scalable video coding is proposed for scalable video conferencing services. The proposed algorithm considers the limitations of the human perception while participating in a video conference so as to minimize traffic that is not necessary for the communication session. A theoretical analysis was conducted to compare the total waiting time between a centralized and a distributed queues. The result shows that distributed service architecture can, by applying the newly proposed multicast algorithm, provide a much smaller delay than the centralized service architecture.

I. Introduction

Video communications is foreseen as the next popular digital communication method after voice communications [START_REF] Wilcox | Videoconferencing & interactive multimedia: the whole picture[END_REF]. Video conferencing, the most complex type of video communications, has been used for a number of years in business, and more recently, in everyday life. A very common conventional conferencing method is to use a Multipoint Control Unit (MCU) as the central point for receiving, transcoding, trans-rating, mixing, and then sending back to participants (centralized service architecture). However, the cost of conventional conferencing method increases exponentially with an increasing number of participants. Moreover, it cannot support satisfactorily users with different types of terminals due to scalability problems. In [START_REF] Eleftheriadis Alexandros | Multipoint videoconferencing with scalable video coding[END_REF], the centralized service architecture of conventional centralized conferencing services was combined with Scalable Video Coding (SVC) to provide more scalability. A new MCU architecture (Scalable Video Conferencing Server-SVCS) was proposed. Similar to the conventional centralized architecture, SVCS received SVC bit-streams from all participants. However, it directly routes bitstreams to their desired destinations without any mixing, or transcoding/rating. All of that workload is processed at the participants' terminals. To date, this is the most advanced method of centralized scalable video conferencing service and the one which is widely deployed because of its scalability and advanced performance compared with the conventional centralized methods. When more participants want to participate in a conference (e.g. for large events), the cost of centralized architecture increases sharply. Therefore, distributed architectures have been proposed. Naturally, when a multi-point communication service is to be provided on a distributed scheme, multicast is involved. IP-multicast [START_REF] Deering | Multicast routing in a datagram internetwork[END_REF] is currently the most efficient type of multicast. However, deployment issues are preventing it from being widely applied [START_REF] Diot | Deployment issues for the IP multicast service and architecture[END_REF]. In [START_REF] Luo | A Multiparty Videoconferencing System Over an Application-Level Multicast Protocol[END_REF], a distributed architecture was proposed for video confer-encing service based on an Application Layer Multicast (ALM) algorithm. However, that proposal did not consider the scalability problems associated with a variety of terminals. Moreover, the cost function used to construct the media distribution tree has not yet been investigated. Scalability is considered in [START_REF] Akkus | Peer-to-Peer Multipoint Video Conferencing using Layered Video[END_REF], [START_REF] Ponec | Multirate peer-to-peer video conferencing: A distributed approach using scalable coding[END_REF], and SVC was applied to support various types of participant terminals. Unfortunately, the proposed algorithms can only support a limited number of participants (e.g.10-15) since they do not have a mechanism to limit unnecessary traffic on the multicast tree.

To the best of our knowledge, none of the proposed distributed architecture systems has compared their performance with the conventional centralized architecture. Thus, no conclusion can be made on whether a particular distributed architecture is really better than the centralized architecture and therefore, if service users should change their current centralized conferencing system for a new distributed system. The first attempt to compare the two scalable conferencing architectures using its proposed simulation platform [START_REF] Anh | EvalSVC -an evaluation platform for scalable video coding transmission[END_REF] is in [START_REF] Le | Centralized and distributed architectures of scalable video conferencing services[END_REF]. Their results show that, even when using a much higher computational load, the MCU of a centralized architecture can only provide a bitrate that is similar to that of the distributed architecture at the trade-off of a much higher delay. The results are interesting but must be validated by theoretical analysis before they can be applied in more general conditions. In the research work presented here, we make the following contributions:

• An ALM algorithm is proposed in which the limitations of human perception when participating in a conference are considered with the objective of reducing the unnecessary traffic on the multicast tree,

• Theoretical queuing models for centralized and for the proposed distributed architectures are constructed and analyzed, and the total waiting time in each type of queue compared. The analysis has applied the real source model of SVC video published in [START_REF] Van Der Auwera | Traffic and quality characterization of the H.264/AVC scalable video coding extension[END_REF].

II. Perception-based Application Layer Multicast Algorithm for scalable video conferencing services

The proposed ALM algorithm is used to build an ALM media distribution tree for Scalable Video Coding contents. The distance between any pair of peers is calculated based on the multi-variable cost function (Equ.1) proposed in [START_REF] Le | Multivariable cost function for Application Layer Multicast routing[END_REF]:

u(x w , x d ) = x w κ w -x w . κ d x d -κ d (1)
In which κ w and κ d are the maximum available bandwidth and the minimum delay, respectively, that the network can provide to the service using all available resources, and x w , x d are the required bandwidth and required delay, respectively, from the SVC application. A cluster is a group of peers that have the shortest distance to each other. When a peer wants to join an ALM, it will first try to explore its nearest clusters using a cost function to measure the distance to reach the leader of each cluster. A cluster has the maximum size of k peers depending on the network conditions. A leader is the peer that has the minimum distances to all other peers in a cluster. All leaders together form a first layer and then use the same multi-variable cost function to calculate their distance from all other leaders at layer 1, to form clusters and to form the second layer. A leader will receive bit-streams from its cluster's peers and forward them to the other peers in its cluster and to the upper layer's leaders (each peer within a cluster will receive bit-streams from all other peers in the same cluster but not its own bit-stream). At the same time, layer-one leaders receive bit-streams from the upper layer's leaders and forwards them to its cluster's members.

The proposed ALM algorithm is applied when SVC (or other kinds of multilayer video coding) is used on the video conferencing service. The main advantage of SVC is that the video can be encoded into a base layer bit-stream and several enhancement layer bit-streams. In a video conferencing session, at any given time there are only one or a few active speakers (active speakers are the conference participants who are giving the speech or actively participating in the argument/ discussion). Active speakers can be found automatically by comparing the participants microphones' output power levels. One simple justification is that, if there are too many active speakers in a conference session, other participants will not have enough perception capacity to follow all of them. From the multicast tree's point of view, an Auto Active Speaker Detector (AASD) can easily reduce the unnecessary traffic for the entire distributed system. The AASD is a functional block placed at each peer to automatically detect whether the peer is an active speaker or not by comparing its output audio power with that of the others. The AASD will then notify its cluster's leader as to which peer is an active speaker so that the leader can update its active speaker database. All peers will send their base layer SVC stream (with its lower bit-rate) to its cluster's leader. In addition to sending their base video layer, active users also send their enhancement layers to the cluster's leader (with enhanced bitrates). Apparently, enhancement layer bit-streams from inactive but interested users may be desirable for some peers. In this case, those particular peers must inform their cluster's leader about which interested user(s) they want to receive enhancement video layers from. This information will then be transmitted to the interested users via the leader network. After receiving notification, the interested users will send an enhancement video layer to the group as if they are active speakers. A very important point to note is that each peer in the multicast group will contribute only a portion of its computational capacity to support the conference, according to the number of enhancement video layers required by the conference to maintain a steady-state of the multicast system (otherwise, there will be congestion for the peers). However, this portion of contributing computational capacity is flexible and should remain variable in situations where more enhancement video layers are required. Figure 1 demonstrates a use case of the proposed algorithm. The blue peers (participants j in each cluster) are the clusters' leaders. The red peer (participant 1 of cluster 1) is the active speaker, while the orange peers (participants 1 of cluster j and participant k of cluster log k N -the last cluster) are interested users.

The AASD placed at each user will detect whether or not that user is an active speaker in order to allow or block its enhancement video layers (red lines). Meanwhile, the base video layers (green lines) are always passed through the AASD to its cluster's leader. All cluster leaders then forms an upper multicast layer in order to transmit data among leaders from the first multicast layer. Similarly, the upper multicast layers are built until a general leader is found.

III. Theoretical analysis

In order to compare the queuing delay of the centralized and distributed architecture, two queuing models are constructed with the following notations:

• N: Total number of participating peers, • r ij : Transition probability of a packet from leaders of layer l or MCU to each peer,

• r ji : Transition probability of a packet from each peer to leaders of layer l or MCU,

• µ j , µ l , µ m : Service rates at each peer, leaders of layer l, and MCU, respectively,

• γ j , γ l , γ m : External arrival rates to each peer, leaders of layer l, and MCU, respectively,

• C 2 Bj , C 2 Bl , C 2
Bm : Squared coefficient of variation (SCV) of service distributions at each peer, leaders of layer l, and MCU, respectively,

• C 2 Oj , C 2 Ol , C 2
Om : SCV of external inter-arrival distributions to each peer, leaders of layer l, and MCU, respectively,

• λ i , λ l , λ m : Throughput at peers, leaders of layer l, and MCU, respectively,

• ρ i , ρ l , ρ m : Traffic intensity (traffic congestion) at each peer, at leaders of layer l, at MCU, respectively (ρ = λ µ ) According to [START_REF] Gross | Fundamentals of queueing theory[END_REF], the approximated waiting time of each service architecture (G/G/1 queue of General distribution of inter-arrival time, General distribution of service time, 1 parallel server) is calculated by:

W q ≈ ρ 1 -ρ C 2 A + C 2 B 2 1 µ g(ρ, C 2 A , C 2 B ) (2)
where: 2) has a nice "product form" of three terms: (i) a traffic-intensity factor; (ii) a variability factor; and (iii) a time-scale factor (each packet requires 1 µ [unit time] of service). For calculating (2), we must obtain C 2 A from (4):

g(ρ, C 2 A , C 2 B ) =    exp - 2(1 -ρ) 3ρ . (1 -C 2 A ) 2 C 2 A + C 2 B , if C 2 A < 1 1, if C 2 A ≥ 1 (3) Formula (
C 2 Ai = γ i λ i C 2 Oi + k j=1 λ j r ji λ i r ji ρ 2 j C 2 Bj + (1 -ρ 2 j )C 2 Aj + 1 -r ji (4
) In which, λ i is calculated by:

λ i = γ i + k j=1 λ j .r ji (5)
Since the end-to-end delay of each architecture is proportional to the queuing time, if we succeed in building queuing models and then calculate the approximated waiting time for the two architectures, end-to-end delay can be compared based on these results.

A. Centralized architecture

Figure 2 shows the queuing model for the MCU-based architecture. Here, all N peers are generating a media stream with a mean data rate of γ j [Mbps]. Each peer sends this encoded video to the MCU at a transition probability of r ji = 1 (e.g. peer's output is all sent to the common MCU). The MCU then routes back N media streams to N participating peers, each contains data from (N-1) other peers (assuming that each peer will not receive its own stream). Since the transition probability r ij is defined by the probability at which a packet who completes service at node i transits to node j, we have:

r ij = 1 N (N -1) (6)
From ( 5), we have:

λ m = γ m + N.λ j λ j = γ j + λ m .r ij ( 7
)
Since there is no external input other than from participating peers to the MCU then γ m = 0. Because all peers are generating aggregated SVC bit-streams, then they seem to "receive" external inputs at the same rate of γ j = γ a [Mbps] in which γ a is the mean bit-rate of the aggregated SVC bit-stream. According to [START_REF] Van Der Auwera | Traffic and quality characterization of the H.264/AVC scalable video coding extension[END_REF], the mean rate of an aggregated (base QCIF + enhancement CIF) spatial traffic is γ a = 0.383Mbps, the mean rate of an enhancement layer's spatial traffic is γ e = 0.231Mbps, and the mean rate of a base layer's spatial traffic is γ b = 0.152Mbps. From the above values, by replacing (6) into the system of equations ( 7) and solve it, we have:

       λ m = N (N -1) 2 (N -2) γ a λ j = (2N -3) (N -2) γ a (8)
The traffic intensity at the MCU is ρ m = λm µm (assuming that only one server is used as the MCU). In order for the queue at the MCU to be in steady-state conditions, we must have ρ m < 1 or µ m > λ m :

µ m > N (N -1) 2 N -2 .γ a (9)
Assuming that we have designed a MCU to support of up to N max participants, then the maximum throughput to be managed at the MCU is:

M m = ( N max (N max -1) 2 N max -2 + 1)λ a (10)
We have ρ m = λm

Mm

Similarly, the traffic intensity at each peer is ρ j = λj µj . Assuming that all peers have a similar computational capacity, the service rates at all peers are equal (µ j = µ p ). Let:

M j = ( 2N max -3 N max -2 + 1)γ a ( 11
)
We have ρ j = λj Mj . SCV values are defined by ratio of the standard deviation to the mean value (SCV = V ar E 2 ). From the statistical data reported in [START_REF] Van Der Auwera | Traffic and quality characterization of the H.264/AVC scalable video coding extension[END_REF], we obtained the SCV value of the aggregated traffic of the spatial scalability bit-streams arriving at each peer: C 2 Oj = 46[kbit]. We have: Using the values of γ m , λ m , λ j , ρ 2 j , ρ 2 m , r ij and ( 12), the approximated waiting times at the MCU is W q m. Then, we obtain the total waiting time of the centralized architecture:

C 2 Om = 0; C 2 Oj = 46[kbit] C 2 Bm = 10[kbit]; C 2 Bj = 30[kbit] (12)
W qc ≈ N W qm (13)

B. Distributed architecture

Figure 3 shows the queuing model of a random cluster on layer l in the ALM-based architecture. Each cluster has k peers, one of them is elected to be the cluster's leader. All peer is generating its base layer bit-stream at a mean data rate of γ j = γ b [Mbps] and then sending that baselayer to the leader at the transition probability of r ji = 1. Some of them (n 1 ≤ k) are also sending an enhancement layer to the leader (according to the layer registration information). All of the enhancement layers form an external arrival rate of γ l1 = n 1 .γ e [Mbps] to the cluster's leader in which γ e is the arrival data rate created by the enhancement layers (γ p = γ b + pγ e ). Here, we use a mean value of p to represent the percentage that an enhancement video layer is sent from a peer (0 < p < 1). The leader is also receiving another external data rate of γ l2 [Mbps] from the leader on a higher layer. The leader then distributes the data to k members of its cluster at the transition probability of r ij . In layer l, a leader will receive k bit-streams from its cluster, each with a data rate of (

γ j = k (l-1) γ p )[Mbps]. We have γ l = (N -k l )γ p + γ p (k -3)k l . γ l = N γ p + γ p (k -3)k l γ j = k (l-1) γ p (14)
Since the transition probability r ij is defined by the probability at which a packet who completes service at node i transits to node j, we have:

r ij = 1 k(k -1) + k = 1 k 2 (15)
From ( 5), we have:

λ l = γ l + kλ j λ j = γ j + λ l r ij (16)
Replace ( 14) and ( 15) into ( 16) and solve the equation system, we have:

   λ l = kγp k-1 N + γpk(k-2)k l k-1 λ j = γ p k(k -1) N + (2k -3)γ p k(k -1) k l (17)
The traffic intensity at a leader of layer l is ρ l = λ l µ l . In order for the queue at the leader to be in steady-state conditions, we must have ρ l < 1 or µ l > λ l , with (17), we have:

µ l > kγ p k -1 N + γ p k(k -2)k l k -1 (18)
Assume that the top leader has been designed to support the maximum throughput of maxλ and the system can support of up to N max participants, at the top leader, k l = N , therefore we have:

M l = (k + 1)N max γ p , (19)
Thus, we have

ρ l = λ l M l . C 2 Ol = 16[kbit]; C 2 Oj = 16[kbit] C 2 Bl = 30[kbit]; C 2 Bj = 30[kbit] (20)
According to (4), remembering that r ji = 1, we have:

       C 2 Al = γ l λ l C 2 Ol + k λj λ l ρ 2 j C 2 Bj + k λj (1-ρ 2 j ) λ l C 2 Aj C 2 Aj = γj λj C 2 Oj + λ l rij λj (r ij ρ 2 l C 2 Bl + 1 -r ij ) + ... ... + λ l r 2 ij λj (1 -ρ 2 l )C 2 Al ( 21
)
Let:

       A 1 = γ l λ l C 2 Ol + k λj λ l ρ 2 j C 2 Bj B 1 = k λj (1-ρ 2 j ) λ l ; B 2 = λ l .r 2 ij λj (1 -ρ 2 l ) A 2 = γj λj C 2 Oj + λ l .rij λj (r ij ρ 2 l C 2 Bl + 1 -r ij ) (22)
From ( 21) and ( 22), we have:

C 2 Al = A 1 + A 2 .B 1 1 -(B 1 .B 2 ) ( 23
)
Replace the values of ( 14), ρ j , ρ l , (15) and ( 20) into (22) to obtain A 1 , A 2 , B 1 , B 2 , and then use them to calculate (23) Replace ( 23) into (2) we have W ql . Thus, the total waiting time in the distributed architecture is:

W qd ≈ k lmax l=1 W ql

IV. Result Analysis

Figure 4 shows the comparison between the total waiting time for centralized [START_REF] Kuehn | Approximate analysis of general queuing networks by decomposition[END_REF] and distributed (24) queues:

• Total waiting time in a centralized queue,

• Total waiting time in a distributed queue when the clusters' sizes are k = 3, k = 5, and k = 7, respectively. In the centralized architecture, we assume that the MCU can support of up to N max participants at the same time, all participant is sending its base video layer. A fracture of p peers will send their enhancement video layers. Meanwhile in the distributed architecture, all peers are sending both base layers and each leader can support at least k peers in its cluster. From the result in Fig. 4 we can find that, the total waiting time at the MCU increases rapidly with the increasing number of participants. Meanwhile the total waiting time of the distributed architecture increases linearly but at a much lower speed. When the cluster size increases, the total waiting time of the distributed architecture increases. There is an interesting conclusions which we can withdraw after analyzing the results in Fig. 4: When the number of peers in a cluster (k) increases (from k = 3 to k = 7), the total waiting time in the distributed queue increases. Therefore, for a certain number of participants, it is recommended to use a smaller cluster size to maintain a lower total waiting time.

V. Related work

In [START_REF] Kuehn | Approximate analysis of general queuing networks by decomposition[END_REF], Kuehn has applied an approximate method for building the analysis model of general queuing networks. The queuing network is of the open network type, having N single server queuing stations with arbitrary interconnections. In [START_REF] Banerjee | Scalable application layer multicast[END_REF], NICE has been proposed as an ALM algorithm in which the idea of clustering and layering has been built. However, from the best of our knowledge, none of the conventional research has either considered the perception limitation of all participants in order to reduce unnecessary traffic or built an analysis model for evaluating the delay in a distributed conference.

VI. Conclusion and future work

In this research work, a new Application Layer Multicast algorithm considering the limitation of human's perception has been proposed. The newly proposed algorithm can effectively reduce the total traffic load of the scalable video conferencing service. Analysis models have been built and compared for centralized and distributed architectures using queuing theory. The theoretical analysis result has shown a great advantage of the distributed architecture against the centralized architecture especially when the total number of conferencing participants increases. The analysis model has also proposed a method for calculating the necessary computational capacity of the MCU and leader nodes in the proposed ALM algorithm in order to provide a sufficient service rate for obtaining a steady-state in each service architecture. Our future work will consider the delay of the underlay network, the possibilities of congestion that may happen when more participants are involved into the two concerning architectures, and the ability of controlling the number of enhancement video layers in the centralized architecture while building the analysis model.

VII. Acknowledgement

The research work is partly supported by Poseidon, a French national project on the evaluation of next generation wireless networks and advanced multimedia services over these infra-structures.

Fig. 1 .

 1 Fig. 1. Perception-based ALM for scalable video conferencing services

Fig. 2 .

 2 Fig. 2. Queuing model of the MCU-based video conference service architecture

Fig. 3 .

 3 Fig. 3. Queuing model of the ALM-based video conference service architecture in cluster of the l th layer

Fig. 4 .

 4 Fig. 4. Comparison of queuing waiting time between centralized (13) and distributed (24) architectures.


V Passi

A Lecestre

C Krzeminski

G Larrieu

E Dubois

J.-P Raskin

A single layer hydrogen silsesquioxane (HSQ) based lift-off process for germanium and platinum

Keywords: electron beam lithography, hydrogen silsesquioxane (HSQ), lift-off technique, platinum deposition, germanium deposition, tetramethylammonium hydroxide (TMAH)

Primarily used as etch mask, single layer hydrogen silsesquioxane has never been investigated for lift-off technique. In this article, we propose a new technique where a single layer of hydrogen silsesquioxane, a negative tone electron beam resist, is used to make lift-off of germanium and platinum. Removal of exposed hydrogen silsesquioxane is tested for various concentrations of hydrofluoric acid. Ultrasonic agitation is also used to reduce the formation of flakes due to accumulation of matter (evaporated metal in our case) along the sidewalls of the liftoff narrow slots. Results demonstrate potential in applying the hydrogen silsesquioxane as a negative tone lift-off resist to pattern nanometer scale features into germanium and platinum layers.

I. Introduction

Lift-off is a relatively simple and easy method for making metal patterns on substrate, especially for metals such as platinum, tantalum, and nickel, which are difficult to etch by conventional methods that involve wet chemical or dry reactive ion etching. So far, the lift-off technique has been applied to positive tone electron beam (e-beam) resists such as poly methyl methacrylate (PMMA) [START_REF] Cumming | 3 nm NiCr wire made with ebeam lithography and PMMA resist[END_REF]- [START_REF] Carlberg | Lift-off process for nanoimprint lithography[END_REF] and ZEP 520-12 [START_REF] Conway | E-beam process for ZEP-520A to be used for Metal lift-off and RIE[END_REF]. A single layer positive tone e-beam resist process is the simplest to implement and involves only one lithography step. However, the drawbacks of this approach are; the positive tone resists are relatively more expensive than negative tone [START_REF] Zhang | Literature research report for lift-off method[END_REF] and the resist removal step that can result in rough sidewall profile of the metal. During deposition, metal film could be deposited on the vertical sidewall of the resist due to insufficient undercut and continue to adhere to the substrate following resist removal. This sidewall may peel-off during subsequent processing steps, resulting in shorts and particulate matter and/or it may collapse and interfere with the ensuing processing steps. Thus, a sufficient undercut would help in avoiding rough sidewall profile of metal. Fig. 1a shows the ideal undercut profile for a lift-off using single layer positive tone resist. There are two methods to obtain an undercut; count on proximity effect, and use a bilayer resist process [START_REF] Abellan | EBL Bi-layer resist scheme for CdTe/Si submicron structures for lift-off processing[END_REF]. The proximity effect determination is a complex procedure depending on numerous parameters such as exposure dose, electron energy, substrate material, resist thickness, development time, development temperature, etc., to name a few. The bilayer process is a relatively easier method that helps in formation of an undercut resulting in a reliable lift-off. A bilayer process consists of two resist layers of different molecular weight spin coated one over the other. During exposure, the bottom layer of low molecular weight being more sensitive to electrons is exposed more than the top layer of higher molecular weight, which is relatively less sensitive. During development, the bottom layer is developed at a faster rate than the top layer resulting in formation of a structure similar to an undercut Fig. 1b. This process however is more complex and of higher cost than a single layer process. In this article, we present a lift-off process using a single layer negative tone e-beam resist, hydrogen silsesquioxane (HSQ) for germanium and platinum.

II. Lift-off process using positive and negative resists to pattern narrow openings in metal

In order to fabricate narrow lines of materials, which are difficult to etch by either wet chemical or reactive ion etching, lift-off technique is used. The two main advantages of lift-off process are: i) It does not require etch selectivity between the film to pattern and the substrate since no etching is required; ii) Any material or stack of materials can be used as long as they can be evaporated at relatively low temperature (temperature at the wafer surface should be less than the glass transition temperature of the resist to avoid resist overflow).

In order to create narrow openings in metal using lift-off process, a negative tone resist is more beneficial than a positive tone resist in terms of time and cost of exposure. The process steps to obtain narrow openings in metal using positive tone resist are explained briefly. Positive resist layer is spin coated on the substrate. Exposure is done in regions where the metal is to be deposited, followed by development. This results in resist layer remaining in the region where narrow openings in metal are desired. Deposition of metal is then done followed by removal of resist using remover. These steps are illustrated in Fig. 2. As can be seen above this step requires exposure of large areas thereby increasing the time and cost of exposure.

To obtain the same using negative tone resist the process steps are similar to the process steps using positive resist Fig. 2, but the significant advantage being that here the area to be exposed is much less, thereby comparatively reducing the time and cost.

In this article, negative tone resist hydrogen silsesquioxane (HSQ) layer is exposed and developed, followed by deposition of germanium/platinum. Finally, lift-off is performed to obtain narrow openings.

A. Hydrogen silsesquioxane (HSQ) as a negative tone lift-off resist

Hydrogen silsesquioxane commonly known as HSQ or Flowable Oxide (FOx), is used as a low-k dielectric for back-end metal interlayer isolation [START_REF] Larrieu | Schottky-barrier source/drain MOSFETs on ultrathin SOI body with a tungsten metallic midgap gate[END_REF]- [START_REF] Liu | Improvement on Intrinsic Electrical Properties of Low-k hydrogen silsesquioxane/copper Interconnects employing deuterium plasma treatment[END_REF] and has excellent planarization and gap filling capabilities [START_REF] Cornu-Freleux | Spacer-First Damascene-Gate FinFET Architecture Featuring Stringer-Free Integration[END_REF]- [START_REF] Penaud | Transformation of hydrogen silsesquioxane properties with RIE plasma treatment for advanced multiple-gate MOSFETs[END_REF]. We use negative tone electron beam resist HSQ for our application owing to the following advantages that it offers: (i) HSQ can be applied using standard spin coating techniques, (ii) presents a high resolution [START_REF] Namatsu | Fabrication of Si single-electron transistors with precise dimensions by electron-beam nanolithography[END_REF]- [START_REF] Word | Nanometer-period gratings in hydrogen silsesquioxane fabricated by electron beam lithography[END_REF], (iii) it features good resistance to plasma etching and can be used directly as an etch mask for pattern transfer, (iv) when exposed with e-beam or ultraviolet it approaches the structure of SiO x [START_REF] Cornu-Freleux | Spacer-First Damascene-Gate FinFET Architecture Featuring Stringer-Free Integration[END_REF], [START_REF] Namatsu | Three dimensional siloxane resist for the formation of nanopatterns with minimum linewidth fluctuations[END_REF], (v) its thickness is uniform and well-controlled and (vi) it presents a very low defect density. HSQ-based lift-off process requires the use of hydrofluoric acid (HF) for the removal of the exposed patterns. One disadvantage associated to this is the incompatibility with an underlying layer composed of silicon-dioxide. However, since the etch rate of HSQ in HF is much larger (~ 800 nm/min in HF-1%) than that of e.g. plasma enhanced chemical vapor deposited (PECVD) or thermal SiO 2 , etching depth of SiO 2 can be controlled by varying the etch time and HF-based solution concentration. Another disadvantage is that HF attacks some metals [START_REF] Williams | Etch rates for Micromachining Processing -Part II[END_REF]. In our experiments, we do not face that issue since germanium and platinum layers are considered.

B. Patterning of germanium and platinum

Germanium, being a group IV semiconductor, shares many similar properties with silicon and is compatible with silicon microfabrication. Germanium is foreseen as an interesting layer for building Micro-Electro-Mechanical Systems (MEMS) in post CMOS process [START_REF] Li | Germanium as a Versatile Material for Low-Temperature Micromachining[END_REF], due to its unique complimentary characteristics such as low temperature physical vapour deposition in the amorphous state, excellent etch selectivity to materials that are commonly used in silicon micromachining, such as silicon-dioxide, silicon nitride and aluminium. Being resistant to HF, nanometer scale features can be defined by lift-off using HSQ-based electron beam lithography.

Moreover, the excess of germanium can be easily removed using preheated hydrogen peroxide solution (at 50°C) without causing damage to silicon or silicon-dioxide, and the deposition of germanium at low temperature (less than 150°C) does not cause any resist burning.

Germanium can also be used as an implantation mask owing to its stopping power to define buried implanted layers. The change of electrical properties of buried layers by implantation is of interest when reduction of the ohmic contact is desired [START_REF] Terrill | A new method for preventing CMOS latch-up[END_REF]- [START_REF] Vellvehi | Design considerations for 6.5 kV IGBT devices[END_REF]. The change of chemical properties of buried layers by implantation is also of interest to create nanometer scale selfaligned gates for a planar double gate MOSFET by tuning the etch rate of implanted oxide in vapour phase hydrofluoric acid [START_REF] Dominguez | Etching rate modification in silicon-dioxide by ion implantation and rapid thermal annealing[END_REF]- [START_REF] Charavel | Etch rate modification of silicon-dioxide by ion damage[END_REF]. It is worth noting that metal like platinum can be patterned as demonstrated hereafter by a lift-off process of HSQ. This last remark is of special interest as platinum is a widely used metal layer for building micro and nanometer scale metallic lines or nanometer spaced interdigitated electrodes in the field of biosensors [START_REF] Fasching | Cantilever tip probes arrays for simultaneous SECM and AFM analysis[END_REF]- [START_REF] Tang | Direct Protein Detection with a Nano-Interdigitated Gate MOSFET[END_REF], for instance.

III. Experiments

Figure 3 shows the process flow of lift-off using HSQ for germanium/platinum. The starting substrate is a p-type bulk silicon wafer. The process begins with a standard cleaning step followed by HF-1% dip to remove chemical oxide followed by de-ionized (DI) water rinse. Dry oxidation is performed at 1100°C to obtain a 200 nm-thick silicon-dioxide layer. Flowable Oxide (FOx-16) is subsequently spin-coated at a speed of 1000 rpm for 60 s, baked at 90°C for 4 minutes resulting in a thickness of 500 nm (as measured by ellipsometry). Electron beam exposure is performed using a VISTEC EBPG 5000+ beam writer at 50 keV to obtain 0.1, 0.3, 0.5, 0.7, and 1 µm-wide isolated lines. Resist is developed in tetramethyl ammonium hydroxide (TMAH) -25% solution for 1 minute followed by DI water rinse for 2 minutes. Scanning electron microscope imaging is then performed to determine the width of the patterned structures. A 300 nm-thick germanium is deposited by evaporation. For platinum process, 50 nm-thick platinum is deposited by evaporation. Lift-off is done by dipping the sample in HF solution. Various concentrations of HF solutions and various etching times are tested to optimize the etching of HSQ in order to obtain good quality lift-off results.

IV. Results and discussion

Cross section of HSQ lines of varying widths after exposure and development are shown in Fig. 4-a through 4-d. Dose determination is done by varying exposure dose, from 1500 µC/cm² to 3000 µC/cm² in steps of 100 µC/cm², to obtain lines widths from 100 nm to 1 µm. Line widths are measured using Scanning Electron Microscopy (SEM) image for each dose value. Finally, the dose value of 2500 µC/cm² has been selected since it gives the desired dimension for each width. Fig. 5-a and 5-b show the under etching of silicon-dioxide resulting from a lift-off in concentrated HF (50%) for 3 minutes and 30 s, respectively. Alternatively, Fig. 5-c and5-d show the cross section after lift-off in a diluted HF solution (1%) for time of 15 and 10 minutes, respectively.

The underetch observed in Fig. 5-c and5-d is further reduced by reducing the lift-off time to 2 minutes. In addition, the flakes on the sidewalls are removed with the help of ultrasonic agitation. One way of reducing this effect is to decrease the thickness of the deposited germanium layer. For a platinum based process, top-view of HSQ lines after exposure and development for various widths are shown in Fig. 7-a through 7-d. The spacing between the lines is varied from 300 nm to 2 µm. Fig. 8-a through 8-d show narrow openings in platinum from 100 nm down to 25 nm after HSQ lift-off. The use of ultrasonic agitation results in better sidewall definition by reducing the metal flakes as is also observed for germanium. By tuning the HF concentration and the duration of lift-off, the unintentional etching of the silicon-dioxide layer lying on top of the silicon substrate is controlled.

V. Conclusion

We have demonstrated a new process to perform a lift-off using single layer negative tone resist, hydrogen silsesquioxane (HSQ), for germanium and platinum.

The required HSQ thickness of 500 nm is obtained with the spin coat parameters; speed 1000 rpm for 60 s, bake temperature 90°C for 4 minutes. HSQ widths of 0.1, 0.3, 0.5, 0.7, and 1 µm are achieved with an exposure dose of 2500 µC/cm². Different concentrations and etching duration of hydrofluoric acid (HF) to remove the exposed HSQ are tested. Influence of the ultrasonic agitation on etched profile geometry and debris presence is evaluated. Best lift-off results are obtained when 1% of HF for 2 minutes with presence of ultrasound agitation is used. Narrow openings of 100 nm for germanium lift-off case and 25 nm for the platinum lift-off case are defined by e-beam lithography without any proximity correction. Openings of 100 nm width with spacings from 300 nm to 1 µm.

Fig. 5 -

 5 Fig. 5-e and 5-f exemplify this optimized process for lift-off of HSQ. The presence of the slope in

Fig. 6 -

 6 Fig.6-a through 6-d show the cross section SEM image of openings with varying width after lift-

Figure 1 :

 1 Figure 1: Undercut profile for lift-off using (a) single layer and (b) bilayer positive tone resist.

Figure 2 :

 2 Figure 2: Comparison of positive and negative tone process for obtaining narrow openings in

Figure 3 :

 3 Figure 3: Process flow of HSQ-based lift-off for germanium/platinum.

Figure 4 :

 4 Figure 4: Cross section SEM images of HSQ lines with a width of (a) 100, (b) 300, (c) 500, and

Figure 5 :

 5 Figure 5: Cross section SEM images showing lift-off of 300 nm-thick Ge layer in various wet

Figure 6 :

 6 Figure 6: Cross section SEM images after lift-off of HSQ with 300 nm-thick germanium using

Figure 7 :

 7 Figure 7: Top view SEM images of HSQ narrow lines of various widths and spaced from 300 nm

Figure 8 :

 8 Figure 8: Top view SEM images of openings in platinum for different spacing: (a) Openings of

Figure 1 :Figure 2 :Figure 3 :Figure 4 :Figure 5 :

 12345 Figure 1: Undercut profile for lift-off using (a) single layer and (b) bilayer positive tone resist.

Figure 6 :Figure 7 :Figure 8 :

 678 Figure 6: Cross section SEM images after lift-off of HSQ with 300 nm-thick germanium using


Synthetic Voice Forgery in the Forensic Context: a short tutorial Guillaume GALOU IRCGN 1 bld. Théophile SUEUR 93110 ROSNY-SOUS-BOIS, FRANCE Gérard CHOLLET CNRS-LTCI Télécom Paristech, dép. TSI 37-39 rue Dareau 75014 PARIS Abstract-Technical voice forgery in the forensic area has led to several studies, mainly dealing with voice conversion. In the last decade, latests developments around voice synthesis have reached satisfactory intelligibility and quality levels. Moreover several web-based or standalone apps can be used for TTS 1 . Nowadays, HMM-based synthetic voices can be built to fool biometric systems. Several authors reported FARs 2 as high as 70 to 80% when TTS voices where used. Nevertheless, the forensic context is quite different since the human ear might be able to detect a synthetic voice, thus leading to a case discarding. We used the MARY TTS platform in order to produce a speakerdependent synthetic target voice sample. Given a single synthetic trial, our very preliminary work is to show how feasible and successful such an attack is. Further work is needed to build other voices and provide accurate statistics. Our aim is to confirm a criminal opportunity and to show that synthetic voice detection might become mandatory in a near future.

Index Terms-TTS, speech synthesis, forensic, speaker recognition, voice comparison, HMM, HTS, MARY Text-to-Speech, voice forgery, voice disguise.

I. INTRODUCTION

V OICE forgery is not only a threat for biometrics, but also a threat for identity inference in the forensic context. The performance of commercial forensic speaker verification systems is usually assessed through a standard evaluation protocol like NIST SRE. This is not fully satisfactory from a forensic point of view since impostors attacks are random [START_REF] Genoud | Speech pre-processing against intentional imposture in speaker recognition[END_REF]; robustness to intentional impersonation is not assessed in this case. Whatever the technique, we think that voice forgery is absolutely necessary to be considered when measuring performance of a forensic system, and that natural/transformed/synthesized classifiers front-ends are required.

Facing Forensic Automatic Speaker Recognition (FASR) systems, the aim of a forger is to "provide" either:

• samples from the target (using unit selection synthesis for example) ; • spectral target-shaped samples (voice transformation or concatenation synthesis). Keeping in mind that prosodic features are very difficult to forge while preserving quality, most commercial FASR systems do not take into account any prosodic information to 1 Text-to-Speech 2 False Acceptance (or Alarm) Rate model a speaker. But, unlike "voice vaults" applications, forensic speaker recognition can be considered as semi-automated. The forger must also fool the human ear so that:

• any transformation/synthesis cannot be detected;

• perceptual proximity to target must be ensured. Thus quality and intelligibility are as important as spectral proximity to target. Moreover, considering technical and intentional voice forgery only, good quality and proximity require data samples from the target. Most studies deal with voice conversion [START_REF] Stylianou | Continuous probabilistic transform for voice conversion[END_REF], [START_REF] Qin Jin | Is voice transformation a threat to speaker identification?[END_REF], [START_REF] Farrús | Speaker Recognition Robustness to Voice Conversion[END_REF]. In such a technique, the aim is to estimate a source-to-target acoustic features space transform. Despite a very useful realtime conversion capability, the major drawback is the need of a parallel corpus for training. Such a corpus might not be easy to build and must ensure a full feature space coverage.

As an alternative, unit selection voice synthesis can be used. Although it allows best speech quality and proximity, it is very unlikely since a huge target database is needed. Concatenation synthesis, has also been studied [START_REF] Stylianou | Applying the harmonic plus noise model in concatenative speech synthesis[END_REF], [START_REF] Perrot | Voice disguise and automatic detection: review and perspectives[END_REF].

We believe that Hidden Markov Model synthesis [START_REF] Masuko | HMM-Based speech synthesis and its applications[END_REF], as a special case of concatenation synthesis, is a good candidate for several reasons:

• satisfactory quality vs. affordable target database;

• several toolkits, even web-based TTS, are available;

• prosody can be modeled;

• speaker-adaptation voice modeling is supported (will be discussed further). Of course, the quality of HMM-based TTS is not perceptually perfect, but it can be considered as satisfactory on short utterances with little background noise.

In our work, we used MARY TTS [START_REF] Schröder | The German Text-to-Speech Synthesis System MARY: A Tool for Research, Development and Teaching[END_REF]. It provides a general and handy framework that could be used for criminal purposes in a near future.

II. HMM-BASED FORGERY AND AUTOMATIC SPEAKER RECOGNITION: PREVIOUS WORK

Advances in HMM-based speech synthesis made possible smooth and natural sounding speech. However, until recently, modeling a target speaker required a large amount of data. Now, state-of-the-art HMM synthesis allows model adaptation of speaker-independent models using a small target database [START_REF] De Leon | Detection of synthetic speech for the problem of imposture[END_REF]. In other words, a first speaker-independent training ensures a broad phonetic coverage and model building whereas only a small speaker-dependent (target) is needed afterwards [START_REF] Oller | Synthesis using speaker adaptation from speech recognition db[END_REF]. Direct speaker-dependent is possible, but phonetically balanced sentences are required. This is the most straightforward strategy. Nevertheless, it is difficult to build such a database using open-source data (web, media, ...).

The authors of [START_REF] Satoh | A robust speaker verification system against imposture using an hmm-based speech synthesis system[END_REF] developed a text-dependent speaker recognition system. A simple GMM-based approach was used for speaker verification. Given a baseline 0.5 % FAR at EER 3 with natural speech, the system reached 86.3% FAR with synthetic speech at the natural case EER threshold. The EER reached 27% with synthesized voices.

Among the very first works on the subject (1999), the authors of [START_REF] Masuko | On the security of hmm-based speaker verification systems against imposture using synthetic speech[END_REF] studied a HMM-based text-dependent speaker verification system. From a baseline 0% FAR at EER, the FAR reached between 65% and over 80%, depending on different conditions.

In [START_REF] Pl De Leon | Evaluation of the vulnerability of speaker verification to synthetic speech[END_REF][3], a more recent work, the FAR of GMM-UBM system raised from 0.4% (natural speech) to 90% (synthetic speech at natural EER threshold).

Interest in synthetic voice forgery has grown due to major advances in TTS systems. The results above show that the next big challenge is to provide good quality at an affordable data collection cost. Adaptation allowed drastic training data requirement reduction.

III. HMM-BASED SYNTHESIS

The HMM Speech Synthesis System, H Triple S (HTS) is widely used. Many voices were created in different languages [START_REF] Yamagishi | Thousands of Voices for HMM-Based Speech Synthesis-Analysis and Application of TTS Systems Built on Various ASR Corpora. Audio, Speech, and Language Processing[END_REF]. This system is highly flexible and is promising because its ability to model different styles of speech [START_REF] Zen | The HMM-based speech synthesis system (HTS) version 2.0[END_REF]. As shown in figure1, the system is divided into training and synthesis part. The training part is very similar to that used in speech recognition HMM systems (HTS is a patch of HTK). Spectral and source features are used to model context-dependent HMMs; spectrum, excitation and durations are unified in a common HMM framework. Input parameters can be derived from different analysis schemes: MFCCs, PLPs, STRAIGHT, 3 Equal Error Rate HNM,... Context features are much more complex than in standard speech recognition (mainly based on triphones: single backward and forward phone contexts). To reach a better prosodic modelling, the different dependencies are segments, syllables, words, utterances, sentences, and part-of-speech. Thus, it is necessary to have a phonetizer and a tokenizer. These Natural Language Processing (NLP) tools are not provided with the system.

Waveforms can be synthesized using mel log spectral approximation (MLSA) filter.

As mentioned above, models can be adapted from a previous training. This is a key feature, reducing the needed amount of data for training. In most cases, voice characteristics, speaking styles, or even emotions can be adapted.

IV. IMPACT ON A COMMERCIAL FASR SYSTEM

In our experiment, we used the MARY Text-to-Speech system, an open-source, multilingual Text-to-Speech Synthesis platform written in Java [START_REF] Schröder | The German Text-to-Speech Synthesis System MARY: A Tool for Research, Development and Teaching[END_REF]. It was originally developed as a collaborative project of DFKI's Language Technology lab and the Institute of Phonetics at Saarland University and is now being maintained by DFKI.

The reasons for such a choice are: For the time being, the latest release does not support upto-date HTS capability. Especially, speaker adaptation is not supported yet. For the training, sentences from the PolyVar corpus [START_REF] Chollet | Swiss french polyphone and polyvar: telephone speech databases to model inter-and intra-speaker variability[END_REF] were chosen and recorded in an anechoic chamber. Tokenization and phonetization was realized using LIA PHON [START_REF] Béchet | Lia phon: un système complet de phonétisation de textes[END_REF]. We built a 80-sec unquestioned sample from the recordings. Then both natural and synthetic questioned samples were built (40 sec). 10 listeners were chosen to evaluate both intelligibility and naturalness of the synthetic voice. Mean opinion scores were 4.15/5 and 2.75/5.

The Even if these results are not statiscally significant to conclude about the robustness of the system, results in table I are not surprising, even for a state-of-the-art FASR tool. More surprising is the fact that the LR is even larger with the synthetic sample.

V. CONCLUSIONS Unsurprisingly, we confirmed that HMM-based voice forgery is able to fool a state-of-the-art FASR system. Because of its flexibility and its speaker-adaptation capability, HMM synthesis is a good candidate for criminal purposes. High-level applications are already available. Thus, it is quite easy to build either "anonymous" or target voices with small corpora. As a consequence, synthetic voice detection becomes mandatory in the forensic context. Further work is needed in order to produce statiscally significant results.

Figure 1 .

 1 Figure 1. HTS overview (from [18])

•

  french support in Mary TTS has recently been achieved for the first time (french NLP tools and voice) [16]; • Mary TTS provides a handy user-interface and is based on HTS for HMM-based voice training and synthesis; • Mary architecture is client-server based, thus allowing straightforward and flexible TTS.

  FASR system is BATVOX 3.0. Speaker verification results and samples characteristics are given in table I.

	question recording	SNR	LR
	natural	25,6 dB	1735
	synthetic	43,9 dB	73000
	Table I
	SPEAKER VERIFICATION RESULTS AND SAMPLES CHARACTERISTICS


Vanessa Garcia Larsen

Marta Luczynska

Marta Luczynzka

PhD Marek L Kowalski

MSc Helena Voutilainen

Matti Alhstrom

PhD Tari Haahtela

PhD Vanessa Garcia-Larsen
email: v.garcialarsen@imperial.ac.uk

Marta Luczynska Bsc

Matti Ahlström Bsc

MD Elina Toskala

PhD Angelina Bockelbrink

PhD Hae-Hyuk Lee

PhD Emilia Vassilopoulou

MD Nikolaos G Papadopoulos

MSc Renata Ramalho

MD André Moreira

MD Luís Delgado

MD María G Castel-Branco

PhD Philip C Calder

PhD Caroline E Childs

MSc Ioannis Bakolis

PhD Richard Hooper

MD Peter G Burney

Use of a common food frequency questionnaire (FFQ) to assess dietary patterns and their relation to allergy and asthma in Europe: Pilot study of the GA2LEN FFQ

Keywords: FFQ, validation, Ga 2 len, asthma, fatty acids, diet

à la diffusion de documents scientifiques de niveau recherche, publiés ou non, émanant des établissements d'enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.

Introduction

Diets are complex mixtures of foods providing a wide variety of nutrients. The most common methods for assessing food and nutrient intakes are through recall of all foods eaten over a fixed period, most commonly 24 hours, or by estimates of the frequency with which specified foods are eaten over a longer period, usually days to a year (Food Frequency Questionnaires (FFQ)) (Subar, 2004). From a list of foods eaten and information on the nutrient composition of foods listed in food composition tables, individual intake of nutrients can be estimated. Nutrients can also be measured in biological samples such as blood and urine and such data may be more precisely related to nutrient intakes than information gained from dietary recall or the use of FFQs.

However blood concentrations of nutrients may not represent intake as they are also influenced by digestion, absorption, metabolism and excretion [START_REF] Jenab | Biomarkers in nutritional epidemiology: applications, needs and new horizons[END_REF].

Traditionally, diets have been assessed as individual foods and nutrients, but more recently there has been a trend to look at dietary patterns instead (Michels et al, 2005). It has been argued that this may better represent the complexity of different diets and allows for the possibility that interactions among different foods may be of importance.

Internationally comparable data on the types of dietary patterns and their association with nutrition-related chronic diseases are hindered by the heterogeneity of the dietary surveys. Attempts have been made in few studies (Tunstall-Pedoe et al 2000;Kuulasmaa et al 2000;Slimani et al 2002) to make international comparisons between dietary habits in relation to chronic diseases.

The Epidemiological Prospective Study in Cancer (EPIC) has reported dietary patterns in adults and elderly individuals in 10 European countries (Touvier et al 2009;Masala et al 2007;Bamia et al 2007) using locally designed FFQs. Comparisons of dietary patterns were usually studied with a single 24 hour recall questionnaire, standardised for all participant countries. To date no data are available on dietary patterns across Europe using a common FFQ.

The Global Allergy and Asthma Network of Excellence (GA 2 LEN) survey includes a nutritional component designed to study the relation between diet and allergy and asthma in adults across Europe. The GA 2 LEN FFQ pilot study developed and tested a method for determination of dietary intake patterns in Europe, targeting allergic disease and asthma.

The aim of the pilot was to evaluate whether the FFQ would yield measurements with a sufficient level of accuracy between very different countries; and to assess the validity and reproducibility of the FFQ.

Methods

Population and study design

7 centres participating as partners or collaborators in the GA 2 LEN Network were invited to take part in the study, of which 5 centres took part: Poland (Lodz), Germany (Berlin), Portugal (Porto), Greece (Athens) and Finland (Helsinki).

Each centre invited 40 males and females aged 21 to 75 years to complete the FFQ and to provide a fasting blood sample. Exclusion criteria were minimal in order to allow for a sample as representative as possible of the general population: having a severe gastrointestinal disease or mental illness, having changed diet in the past 12 months or being pregnant. Individuals were selected from hospital or university centres through posted invitations, and from direct recruitment from patient clinics. Selected participants answered the FFQ on two separate occasions within a time frame of 3 months and provided a blood sample to assess blood levels of fatty acids after answering the first FFQ.

Food frequency questionnaire (FFQ)

The GA 2 LEN FFQ was designed to include a wide range of foods and nutrients that could be representative of the average intake in national populations. Foods were also included to allow the investigation of potential associations between intake of specific food items or nutrients and allergic diseases across Europe.

In order to create a representative dietary questionnaire for most countries in Europe, information was collected from national dietary surveys that were used as part of the EPIC study (Slimani et al 2002) and the FFQ used in the European Community Respiratory Health Survey II (ECRHS II). After combining items that had a very similar nutritional composition, a final list of 200 foods was included.

Classification into food groups

Food groups were classified using the European Food Group (EFG) classification system (Brussaard et al 2002). This is an international classification based on The European Food Consumption Survey Method (EFCOSUM) designed to "define a method for the monitoring of food consumption in nationally representative samples of all age-sex categories in Europe in a comparable way".

The EFG classification seems sufficiently extensive and clear to allow international comparisons as well as between national nutritional surveys (Ireland et al 2002) but it has not been implemented in standard international nutritional studies until now. The GA 2 LEN FFQ was therefore designed with 32 food sections. To improve the accuracy in the collection on data on fat, animal and vegetable spreads were asked separately.

Following the example in the Dutch EPIC FFQ, we included various percentage of fat to ascertain the different types of spreads. Local brand names were also included wherever possible in order to facilitate their identification by the participants. Similarly, a broad range of oils was included in the FFQ, and an additional section specifically asked for types of oils or fats used for cooking.

Translation of the FFQ

The standard operational procedure for translation of documents as advised by the World Health Organisation was used.

Nutritional composition and portion sizes

The GA 2 LEN FFQ was comprised of 7 options to report frequency of food consumption, namely 2+ per day; once per day; 5-6 per week; 2-4 per week; once per week; 1-3 times per week; and rarely or never. Dietary intake reported in the FFQ was registered in standard portion sizes following the Food Standard Agency Food Portion Sizes Guidelines (Food Standard Agency 2006). The GA 2 LEN FFQ includes these recommended portion sizes across all the food sections. Nutritional composition was calculated using the 6 th Edition of the McCance and Widdowson's Composition of Foods (McCance & Widdowson 2006).

Biomarkers of fatty acid (FA) intake: Determination of fatty acid composition of plasma phosphatidylcholine

Samples were analysed at the University of Southampton. Blood (2 ml) was taken into lithium heparin, and centrifuged to separate cells from plasma (3000 rpm for 10 min in a standard bench top centrifuge). Plasma was stored at minus 70 o C until shipping on dry ice. The fatty acid composition of plasma phosphatidylcholine (PC), the major circulating phospholipid, was determined using gas chromatography. In brief, total lipid was extracted from plasma using chloroform/methanol (2:1 vol/vol). PC was isolated from the total lipid extract using aminopropylsilica solid phase extraction columns; PC was eluted with chloroform/methanol (3:2 vol/vol). Fatty acid methyl esters were prepared from the isolated PC by incubation with methanol containing 2% sulphuric acid for 2 hr at 50 o C.

Fatty acid methyl esters were extracted using hexane and then separated on a Hewlett Packard 6890 gas chromatograph. Running conditions were as described elsewhere (Thies et al 2001). The fatty acid methyl esters were identified by comparison with standards run previously and data are expressed as % of total fatty acids present in the PC fraction.

Statistical analyses

Agreement between nutrient intakes in FFQ 1 and FFQ 2 was assessed using the intra-class correlation coefficient (ICC). Nutrient intakes were log transformed to make their distributions less skewed (a small constant was added to each intake so that zero values could be transformed). Agreement was also calculated using the log transformed nutrient density (nutrient intake in milligrams divided by total energy intake (TEI) in kcal) as a way of adjusting for energy intake. Dietary intake of total saturated fatty acids (SFAs), monounsaturated fatty acids (MUFAs), polyunsaturated fatty acids (PUFAs), n-3 PUFAs, n-6 PUFAs and trans fatty acids (TFAs) was validated against the corresponding levels in plasma PC using the Pearson correlation coefficient. Strength and direction of association between intake of specific foods rich in fatty acids with individual and total fatty acids in plasma PC was measured as the Pearson correlation coefficient. Statistical analyses were performed using Stata 10.1

Ethics

Each participant gave written informed consent and ethical approval was obtained by each participant centre.

Results

A total of 177 participants took part in the study. Mean age was 36.7 years (range 21 to 75) (Table 1). The distribution of dietary intakes in the participating countries is described in Table 2, which includes data for those that completed the FFQ on both occasions (FFQ t1 and FFQ t2 ) (n=162). In Greece, Portugal, Poland and Germany, the TEI was higher for FFQ t1 than FFQ t2 , and macro-nutrient intakes were typically higher for FFQ t1 than FFQ t2 , while in Finland the reverse was true. Intake of micro-nutrients was also slightly higher in FFQ t1 than FFQ t2 (data not shown).

Repeatability of the FFQ at the nutrient level

The ICCs for TEI as well as macro-and micro-nutrient intakes for FFQ t1 compared to FFQ t2 are shown in Table 3. ICCs for macronutrients in all participants (i.e. all countries) ranged from 0.65 (n-3 fatty acids) to 0.78 (proteins and TFAs). ICCs for micronutrients ranged from 0.70 (Retinol) to 0.80 (vitamin D). Analyses within individual countries showed ICCs in a similar range. Agreements were generally lower after energy adjustment (using nutrient densities).

A Bland Altman plot (on a log scale) of n-3 fatty acid intake is shown in Figure 1. The 95% limits of agreement for the ratio of intakes in FFQ t2 and FFQ t2 were 0.41 to 2.33

Validation against plasma PC fatty acids

Total dietary intake (FFQ t2 ) of SFAs, MUFAs, PUFAs, n-6 PUFAs and n-3 PUFAs was compared against plasma PC levels of the corresponding fatty acids. Table 4 shows the correlations between fatty acid estimates from FFQ t2 and plasma PC levels of those fatty acids for all participants (all countries). Total dietary PUFAs had a correlation of 0.11 with total plasma PC PUFAs across all participants (all countries) and ranged between -0.22 and 0.16 within individual countries.

Dietary n-6 PUFAs showed negative correlations with plasma PC n-6 PUFAs in all countries. Dietary n-3 PUFAs showed a correlation of 0.40 with the corresponding plasma PC level in the whole sample and ranged between 0.16 and 0.45 across countries.

Good correlations were also observed between dietary n-3 PUFA intake and several individual n-3 PUFAs in plasma (data not shown). Table 5 shows the correlation between estimated dietary intakes of n-6 and n-3 PUFAs and plasma PC levels of individual fatty acids of potential interest for asthma and allergy. Dietary n-3 PUFAs and plasma PC EPA showed a correlation of 0.31 in the total sample, ranging from 0.21 to 0.43 across countries. Dietary n-3 PUFAs showed a correlation of 0.54 with plasma PC DHA in the total sample which ranged between 0.19 to 0.54 across countries. The correlations for dietary SFAs and MUFAs and the corresponding plasma PC fatty acids showed generally poor correlations in all participant countries.

Discussion

The objective of this pilot study was to assess the repeatability and validity of a FFQ designed to collect typical dietary intake information using a common instrument in different countries across Europe. The study aimed specifically at validating the intake of n-3 fatty acids, as these have a potential role in asthma and allergic diseases [START_REF] Kremmyda | Atopy Risk in Infants and Children in Relation to Early Exposure to Fish, Oily Fish, or Long-Chain Omega-3 Fatty Acids: A Systematic Review[END_REF]. We found a good level of repeatability in the FFQ, although we found evidence that intakes from FFQ t1 were systematically and significantly higher than intakes from FFQ t2 in most centres and in the total sample. There was generally poorer agreement after adjusting for TEI. Nevertheless the 95% limits of agreement were still fairly wide: n-3 fatty acid intake estimated from FFQ t1 could still be half or double the intake estimated from FFQ t2 . Intake of major food groups such as meats, cereals, vegetables, fruits, and fish showed a high repeatability (data not shown).

We included a number of food items that were only relevant to one country but not to others. This was intended to ascertain local consumption as well as general intake of foods. The average TEI reported from the FFQ indicates that the questionnaire is able to capture general nutritional intake within the usual range of energy consumed by adults.

The assignment of standard portion sizes was preferred in order to facilitate the interpretation and completion of the FFQ. In large epidemiological studies that involve many different geographical and cultural areas a questionnaire should be kept simple (Noethlings et al 2003). Studies looking at the effect of including portion size questions have shown small changes in estimated nutrient intakes (Block 1990). Tjonneland and colleagues reported that correlation coefficients for food groups and nutrients changed only slightly, indicating that little extra information could be obtained by additional questions about portion size (Tjonneland 1992). Cade et al observed that correlation coefficients of nutrient intakes tended to be higher in subjects who reported their own portion sizes compared to standard portion size specified in the questionnaire (Cade et al 2004). Bearing in mind the objective of our FFQ which is to ascertain food intake and variations in dietary habits across countries, we opted to include standard food portion sizes.

We used a nutrient density approach to adjust for energy intake, rather than looking at residuals from a regression adjusting for energy intake. Both approaches are in common use, and a high degree of correlation between intakes adjusted by these two methods (Willet & Stampfer 1986). Calculating nutrient density meant that we were using the same adjustment for each FFQ assessment, and thus comparing the same quantity each time.

A major element of interest in the design of the FFQ was to include a representative variety of foods rich in n-3 and n-6 fatty acids because of their potential roles in allergic disease risk (Sala-Vila et al 2008). Measurement of plasma levels of fatty acids is considered a good biomarker of the intake of some fatty acids (Ashley et al 1996;Hodson et al 2008). Although all diet-related biomarkers carry a measurement error and may be poor indicators of intake due to variations in digestion, absorption and metabolic changes after consumption, it is suggested that PUFA status in biological samples including plasma is a useful marker of their regular dietary intake (Arab et al 2003; Hodson et al 2008).

The GA 2 LEN FFQ includes food groups with separate questions for the varieties of foods with a high content of n-3 and n-6 fatty acids. These include fish, shellfish, eggs and egg based dishes, margarine, meats and cereals (Meyer et al 2003). Our study found a correlation between dietary intake of n-3 PUFAs and plasma PC levels of n-3 PUFAs of 0.40 for all participants and a range between 0.16 and 0.45 across countries. Similar ICCs were observed between dietary n-3 PUFAs and plasma PC DHA in all the participants.

Our results are in agreement with those reported by other studies validating dietary n-3 PUFA estimated intakes from FFQs with corresponding plasma levels (Ma et al 1995;McNaughton et al 2007;[START_REF] Sullivan | Biomarker validation of a long-chain omega-3 polyunsaturated fatty acid food frequency questionnaire[END_REF]Woods et al 2002). In the GA 2 LEN FFQ fatty fish showed the best correlation with plasma DHA and EPA in analyses per country and with the total sample (data not shown). Correlations between lean fish and plasma fatty acids were less strong, while shellfish and other seafood showed poor correlations.

In our study, correlations between dietary intake of SFAs and MUFAs and plasma levels of these fatty acids were poor. This is likely to be explained by the capacity for endogenous synthesis of SFAs and MUFAs. It is also plausible that differences might be due in part to the fact that plasma reflects more recently intake of fatty acids than other tissues such as red blood cells and adipose tissue (Hodson et al 2008).

The overall aim of the GA 2 LEN FFQ is the assessment of the relation between dietary habits, food intake and nutrient intakes with asthma and allergy in adults across Europe, using a single common dietary instrument. The pilot study indicates that the FFQ can be answered without major difficulties by individuals from very different countries and cultures. We chose two Mediterranean countries (Greece and Portugal), a central European country (Germany), an Eastern European country (Poland) and a Nordic country (Finland) in an attempt to represent the diversity of the people that will participate in the survey where the FFQ will be used.

There is little evidence so far accumulated in international comparisons of dietary patterns in relation to chronic disease, and none using a common dietary questionnaire or a common food classification system that facilitates comparisons. With few exceptions (Bamia et al 2003), most of the data published on comparisons of dietary patterns across the participant countries in EPIC have been analysed using a standardised 24 hour recall questionnaire (Slimani et al 2002), which gave a generally accurate measurement of intake in the short term (Kaaks 1996).

The correlations between foods rich in fatty acids or total dietary n-3 and n-6 PUFAs with plasma PC levels of AA, EPA, DHA and total plasma PC levels of n-3 and n-6

PUFAs found in this study indicate that the FFQ captures moderately well dietary intake of these fatty acids. We are aware of the limitation that using a country specific Table in this validation study with several European countries might not always reflect nutrient composition relevant to each of these countries. An advantage of using the British Table is that it is one of the most comprehensive Food Composition Tables and is being used as a reference method by countries where national Tables are less complete. We are currently working with the EuroFir Network to use the updated nutritional composition databases from all the available tables in Europe to analyse the nutritional information obtained from the GA 2 LEN follow-up.

In summary, the GA 2 LEN FFQ is an appropriate tool to estimate dietary intake of a varied range of foods across Europe regardless of cultural and idiomatic differences. The FFQ is a reasonably good indicator of the intake of n-3 fatty acids, but seems not to be useful for estimating intakes of other fatty acids. Total n-6 PUFAs (g) 9.7 (6.9 -13.4) 9.5 (6.4 -13.3) 7.9 (6.5 -11.9) 7.8

(5.7 -9.5) 10.2 (7.7-15.4) 12.3 (8.9-15.0)

Total n-3 PUFAs (g)

1.3

(1.0 -1.8)

1.3 (0.9 -1.9)

1.0 (0.9 -1.5) 1.0 (0.8 -1.4)

1.8

(1.3-2.4)

1.9

(1.5-2.4) SFAs (g)

Food

  Standard Agency Food. Portion Sizes Guidelines, 3 rd Edition London 2006 Ireland J, van Erp-Baart AM, Charrondiere UR, Moller A, Smithers G, Trichopoulou A, et al. Selection of a food classification system and a food composition database for future food consumption surveys 2002. Eur J Clin Nutr 56, S33-45 Thies F, Nebe-von-Caron G, Powell JR, Yaqoob P, Newsholme EA, Calder PC. Dietary supplementation with eicosapentaenoic acid, but not with other long chain n-3 or n-6 polyunsaturated fatty acids, decreases natural killer cell activity in healthy subjects aged > 55 years 2001. Am J Clin Nutr 73, 539-48 Tjonneland A, Haraldsdóttir J, Overvad K, Stripp C, Ewertz M, Jensen OM. Influence of individually estimated portion size data on the validity of a semi-quantitative food frequency questionnaire 1992. Int J Epidemiol 21, 770-7 Touvier M, Niravong M, Volatier JL, Lafay L, Lioret S, Clavel-Chapelon F, Boutron-Ruault MC. Dietary patterns associated with vitamin/mineral supplement use and smoking among women of the E3N-EPIC cohort 2009. Eur J Clin Nutr 63, 39-47 Tunstall-Pedoe H Vanuzzo D Hobbs M Mahonen M Cepaitis Z Kuulasmaa K. Estimation of contribution of changes in coronary care to improving survival, event rates, and coronary heart disease mortality across the WHO MONICA Project populations 2000. Lancet 355, 688 -700 Varraso R, Kauffmann F, Leynaert B, Le Moual N, Boutron-Ruault MC, Clavel-Chapelon F, et al. Dietary patterns and asthma in the E3N study 2009. Eur Respir J 33, 33-41 Vioque J, Weinbrenner T, Castelló A, Asensio L, Garcia de la Hera M. Intake of fruits and vegetables in relation to 10-year weight gain among Spanish adults 2008. Obesity 16, 664-70 Willet W, Stampfer MJ. Total energy intake: implications for epidemiological analyses.Am J Epidemiol 1986; 124: 17-27 Woods RK, Stoney RM, Ireland PD, Bailey MJ, Raven JM, Thien FC, et al.A valid food frequency questionnaire for measuring dietary fish intake 2002. Asia Pac J Clin Nutr 11, 56-61 20

Table 1

 1 General characteristics of the participants

	Centre (n)	Age (years)	Males
		Mean (SD)	(%)
	All (177)	36.7 (13.2)	35.0
	Greece (39)	35.7 (7.0)	28.2
	Portugal (40)	38.0 (14.3)	25.0
	Poland (42)	42.0 (14.5)	47.6
	Germany (31)	34.9 (10.6)	26.1
	Finland (25)	29.8 (12.3)	44.0

Table 2

 2 Daily intake of macronutrients and selected micronutrients by country in participants who responded to the FFQ in both occasions (Median (IQR))

	Nutrient	All countries (n=162)	Greece (n=24)	Portugal (n=40)
		FFQ t1	FFQ t2	FFQ t1	FFQ t2	FFQ t1	FFQ t2
	TEI (kcal)	2463.2	2457.5	2092.0	1772.8	3225.2	3177.3
		(1921.7 -3169.1)	(1680.9 -3154.4)	(1629.3 -2420.0)	(1375.2 -2160.5)	(2292.5-3683.1)	(2289.0-3674.2)
	Protein (g)	103.0	98.9	83.5	70.6	143.8	144.9
		(77.5 -138.5)	(67.2 -137.7)	(64.8 -100.3)	(55.5 -90.0)	(114.9-165.7)	(112.6-171.1)
	Total fat (g)	117.7	115.7	95.7	83.3	137.0	142.8
		(87.4 -154.5)	(78.6 -152.4)	(66.9 -110.5)	(58.0 -97.4)	(95.0-181.0)	(108.7-166.7)

Table 2

 2 (continued) Intake of macro and micro nutrients by country (Median (IQR))

	Nutrient	Poland (n=42)		Germany (n=31)	Finland (n=25)
	FFQ t1	FFQ t2	FFQ t1	FFQ t2	FFQ t1	FFQ t2
	TEI (kcal)

Table 3

 3 Intra class correlation coefficients (ICCs) for log-transformed nutrient intake (1) and log-transformed nutrient density (2) in FFQ 1 and FFQ 2

	Nutrient	All countries (n=152)	Greece (n=25)	Portugal (n=40) Poland (n=42) Germany (n=31)	Finland (n=25)
		1	2	1	2	1	2	1	2	1	2	1	2
	TEI	0.76	---	0.76	---	0.79	---	0.87	---	0.89	---	0.86	---
	Protein	0.78	0.65	0.78	0.47	0.74	0.51	0.84	0.43	0.86	0.72	0.84	0.75
	Total fat	0.73	0.56	0.73	0.61	0.74	0.29	0.86	0.70	0.81	0.38	0.82	0.46
	Total n-6 PUFAs	0.65	0.53	0.65	0.72	0.67	0.39	0.69	0.48	0.56	0.15	0.84	0.74
	Total n-3 PUFAs	0.75	0.60	0.75	0.63	0.71	0.44	0.69	0.45	0.73	0.46	0.84	0.81
	SFAs	0.70	0.70	0.70	0.40	0.70	0.33	0.86	0.83	0.80	0.70	0.81	0.80
	MUFAs	0.72	0.52	0.72	0.67	0.71	0.38	0.84	0.59	0.78	0.47	0.80	0.40
	PUFAs	0.73	0.61	0.73	0.61	0.73	0.52	0.79	0.54	0.74	0.45	0.85	0.68
	Cholesterol	0.79	0.72	0.79	0.54	0.77	0.53	0.84	0.59	0.83	0.73	0.90	0.90
	Carbohydrates	0.74	0.64	0.74	0.67	0.78	0.34	0.85	0.66	0.89	0.74	0.81	0.70
	Total sugars	0.71	0.67	0.71	0.57	0.73	0.66	0.81	0.82	0.77	0.57	0.84	0.72
	Sodium	0.75	0.65	0.75	0.41	0.74	0.45	0.89	0.73	0.88	0.69	0.81	0.68
	Potassium	0.77	0.80	0.77	0.74	0.73	0.65	0.80	0.78	0.90	0.72	0.87	0.75
	Calcium	0.75	0.78	0.75	0.50	0.74	0.71	0.83	0.73	0.87	0.88	0.89	0.88

Acknowledgments

This GA²LEN study is supported by EU Framework programme for research; contract No FOOD-CT-2004-506378. We are indebted to the researchers who provided us with the EPIC questionnaires: Dr. Kim Overvad, Professor Anne Tjønneland, and Jytte Fogh-Larsen (Denmark); Dr. Francoise Clavel-Chapelon and Dr Maryvonne Niravong (France); Professor Antonia Trichopoulou (Greece); Professor Paolo Vineis, Dr Simonetta Salvini, and Dr Domenico Palli (Italy); Åsa Ågren (Sweden); Dr. Caroline van Rossum, Dr. Jet Smit, Dr. Jeanne de Vries, Dr. Bas Bueno de Mesquita (The Netherlands); Dr Nick Wareham (UK); Dr Joachim Heinrich (Germany); Dr Eiliv Lund and Guri Skeie (Norway); and to the ECRHS Steering Committee for providing a copy of the ECRHS II FFQ.

Conflict of interest

All authors declare no conflict of interest. Geometric mean of intakes