Specification of a Collision-Free CSMA MAC Protocol for Wireless LANs: the CANlike protocol

Transcription

1 Specificaion of a Collision-Free CSMA MAC Proocol for Wireless LANs: he CANlike proocol Guy Juanole, Xuan Hung Nguyen, Gérard Mouney To cie his version: Guy Juanole, Xuan Hung Nguyen, Gérard Mouney. Specificaion of a Collision-Free CSMA MAC Proocol for Wireless LANs: he CANlike proocol. SaCoNeT The 4h Inernaional Conference on Smar Communicaions in Nework Technologies, Jun 2013, Paris, France. pp.5, <hal > HAL Id: hal hps://hal.archives-ouveres.fr/hal Submied on 27 Apr 2013 HAL is a muli-disciplinary open access archive for he deposi and disseminaion of scienific research documens, wheher hey are published or no. The documens may come from eaching and research insiuions in France or abroad, or from public or privae research ceners. L archive ouvere pluridisciplinaire HAL, es desinée au dépô e à la diffusion de documens scienifiques de niveau recherche, publiés ou non, émanan des éablissemens d enseignemen e de recherche français ou érangers, des laboraoires publics ou privés.

2 Specificaion of a Collision-Free CSMA MAC Proocol for Wireless LANs: he CANlike proocol Guy Juanole 1,2, Xuan Hung Nguyen 1,2, Gérard Mouney 1,2 1 CNRS, LAAS, 7 avenue du Colonel Roche, F Toulouse, FRANCE 2 Univ de Toulouse, UPS, LAAS, F Toulouse, FRANCE juanole@laas.fr, xhnguyen@laas.fr, mouney@laas.fr Absrac Collision-free Medium Access Conrol (MAC) proocols based on Carrier Sense Muliple Access (CSMA) and prioriies associaed o he frames are ineresing soluions in considering real-ime raffic in a wireless conex. We have already presened such a proocol using prioriies represened by he BlackBurs echnique [1]. The goal of his paper is, a firs and mainly, o specify anoher such proocol, named CANlike, which is an adapaion of he wired CAN bus proocol o he wireless conex, and hen o show is ineres for he implemenaion of process conrol applicaions hrough a wireless nework. I. INTRODUCTION Wireless neworks and, more paricularly, Wireless LANs are more and more used oday in he indusrial area where we have real-ime applicaions which require Qualiy of Service (QoS) guaranees. In his conex, he MAC proocols which implemen he scheduling of he frames on a shared radio channel have an essenial role. Two main ypes of MAC proocols are TDMA (Time Division Muliple Access) and CSMA (Carrier Sense Muliple Access). CSMA is a oally disribued procedure whereas TDMA requires some cenralized schemes. Then, CSMA is more flexible han TDMA wih respec o changes in a nework (adding or wihdrawing saions and/or applicaions). Furhermore, CSMA is more suiable for sporadic raffic. In his work, we consider MAC proocols of he ype CSMA for single-channel and single-hop WLANs (i.e. each node is in he ransmission range of he oher nodes, we do no have he hidden erminal and exposed erminal problems). The main MAC proocol used in WLANs and based on CSMA is IEEE DCF [2] (DIFS, Backoff, CW (Conenion Window)) which does no suppor packe prioriies and raffic differeniaions. Anoher main one supporing packe prioriies is IEEE e EDCA [3] (AIFSs, Backoff, CW) which allows raffic differeniaions (by means of differen AIFSs which expresses prioriies represened by differen imeous). Some ohers in which he prioriies are defined based on differen sizes of Iner frame spaces, CW and Backoff are [4]. However, he big drawback of hese proocols is ha collisions can always occur due o he asynchronism beween he ransmission needs and he random behavior of he Backoff mechanism. Obaining collision-free CSMA MAC proocols and he QoS guaranees is possible by associaing prioriies represened by messages preceding he frames. The firs approach is o use he BlackBurs echnique [5], [6]. The idea is o le conending nodes send firs jamming signals (called BlackBurs (BB) messages) of lengh according o he prioriy. The node ha has he longes signal (i.e. he highes prioriy) wins he compeiion and hen sends is frame. The drawback of his echnique is ha if we have a grea prioriy number, he jamming signals wile very long and give imporan delays. ThesecondapproachisoadapheMACproocolofhewired CAN bus [7] (he prioriy of he frame is expressed by he ID field which precedes he daa field) o he wireless conex [8]. This proocol is named CANlike. This paper is concerned by an exhausive presenaion of his proocol. This paper includes he following secions: he secion 2 concerns preliminaries which are necessary o well undersand and specify he complee problemaic; he secion 3 presens he specificaion of he CANlike proocol; he secion 4 concerns he conclusion. II. PRELIMINARIES We presen knowledges concerning he physical (PHY) and MAC layers which are absoluely essenial for he specificaion of he CANlike proocol. A. Wireless ransceiver In a wireless conex (conrarily o he wired conex), a ransceiver canno simulaneously send and receive on a channel and has hree saes: Transmier, Receiver, Sleeper. Here we do no consider he sae sleeper which is used for consideraions of energy economy. Two ime aribues characerize he ransceiver behavior: he channel Sensing Time τ ST and he Turnaround Time τ. τ ST allows he ransceiver o es he channel sae (busy or idle): i is busy or idle depending on he deeced energy on τ ST which is higher or lower han a prefixed hreshold (his represens he Clear Channel Assessmen (CCA)). τ is he ime o go from he receiver (ransmier) sae o he ransmier (receiver) sae. During a CCA, if he channel is deeced busy, he ransceiver sill says in he receiver sae in order o coninue lisening o he channel; on he oher hand, if he channel is deeced idle, he ransceiver (afer a τ ) goes in he ransmier sae which allows he MAC eniy o send a frame. Afer he frame ransmission, he ransceiver comes back o he receiver sae. B. Concep of Ambiguiy Time Window We inroduced his concep [1] in order o quaniaively characerize he ambiguiy in he CSMA conex of he expression Channel deeced idle a an insan in a MAC

3 MAC eniy (node i) TR of i channel deeced idle for i τ ST τ + τ PT τst τ ST TR (Transmission Reques) : Reques o send a daa Fig. 1. Ambiguiy Time Window. MAC eniy (node j) TR of j channel deeced idle for j +τ + τ eniy. This expression is ambiguous because i only expresses a local view whereas he channel is geographically disribued. For example, a MAC eniy i sees he channel idle a he insan bu jus before, anoher one has jus sen a frame and his frame has no arrived a i ye, a collision can occur when i sends is frame. So a local view can be differen from a global one which can creae collision siuaions. The concep of Ambiguiy Time Window represens he maximal duraion which is possible beween he decision o send a frame by a MAC eniy and he ineviabiliy of he occurrence of a collision on his frame. The quaniaive characerizaion depends on he ransceiver parameers (τ ST, τ ) and he maximum propagaion ime beween wo nodes (call τ PT his Propagaion Time). The Ambiguiy Time Window is represened on Fig. 1. The MAC eniy i receives a Transmission Reques (TR) from he upper layer a he insan ( τ ST ) o send a daa and hen makes a CCA during τ ST. We suppose ha he channel is deecedidleaheinsan(= τ ST +τ ST )andhenhemac eniy i decides o send is frame. Is ransceiver urns o he Transmi sae (during τ ), and hen he frame is ransmied and arrives a he level of he mos remoe MAC eniy j a he insan ( + τ + τ PT ). Suppose ha he MAC eniy j is jus, a his insan, finishing o make a CCA (sared by a TR a he insan ( + τ + τ PT τ ST )) and as he channel has been deeced idle during τ ST, i decides o send a frame a he insan ( + τ + τ PT ). So we have, a his insan, a siuaion of an ineviable fuure physical collision. Acually, he beginning of he arrival of he frame coming from he MAC eniy i coincides wih he insan where he ransceiver of he MAC eniy j urns around o go in he ransmier sae (duraion τ ). During his ime τ, he MAC eniy j is blind. Then a he end of τ, he MAC eniy j will sar o send is frame and we will hen have he physical collision. The duraion (τ + τ PT ) is he lengh of he Ambiguiy Time Window. C. On he prioriies associaed o he frames The prioriies of he frames are exraced from he values of an idenifier (ID) field. We can consider wo ypes of prioriies [9] (saic prioriies, hybrid prioriies) bu here we only consider saic prioriies i.e. each flow has a unique prioriy (specified ou of line) and all he frames of his flow have his prioriy. D. Concep of ournamen The ournamen consiss in he comparison of he prioriies of he conending frames. This allows o ransform a siuaion which would have been a collision siuaion (if we have he PT sric CSMA mechanism i.e. only based on he scheme Lisen before Send ) ino a winner-loser(s) siuaion. There will be only one winner who has he highes prioriy among he conending frames. The winner can send is frame afer he ournamen while he losers have o wai unil he end of he frame ransmission of he winner and resar he ournamen. The good funcioning of a ournamen is dependen of he duraion of he ID field. I is he consideraion of he consrain done by he Ambiguiy Time Window which allows o specify correcly hese duraions. A. Principle III. CANLIKE PROTOCOL As inroduced, CANlike is an adapaion of he MAC proocol of he wired bus CAN o he wireless nework. In he wired bus CAN, MAC eniies can send bis and lisen o he channel simulaneously. Each MAC eniy has a unique ID (idenifier) field placed a he beginning of he frame. The ID represens he prioriy and allows o do he channel access ournamen. The ournamen is done by a comparison bi by bi of he same rank among he IDs of he frames rying o access he channel. In one bi-by-bi comparison, a bi 0 which is a dominan bi overwries a bi 1 which is a recessive bi. The MAC eniy which has he highes prioriy wile he only one winner afer he ournamen and i will send is frame. In he wireless conex, he bus CAN proocol canno be direcly implemened wih wireless ransceivers since he ransceivers canno ransmi and receive simulaneously in he same channel, so we consider he proposal, which has been done in [8]: one slo ime (duraion) is provided for each ID bi, a dominan bi consiss in he sending of a carrier pulse during is duraion while a recessive bi consiss in he sensing/lisening of he channel during is duraion. So, in each MAC eniy, he ournamen on each bi has he following characerisics: The MAC eniy has a dominan bi: i sends a carrier pulse on he channel and a he end of he sending, i wins by definiion he ournamen relaed o his bi and hen coninues he ournamen on he nex bi. The MAC eniy has a recessive bi: eiher i senses a carrier pulse, hen i loses he ournamen relaed o his bi and sops he ournamen phase, or i senses nohing (ha means ha here is no dominan bi sen by anoher MAC eniy) and hen i can coninue he ournamen on he nex bi. B. Main poins o consider Necessiy of a synchronizaion phase: The sar of he ournamen by a MAC eniy (he sending of he firs ID bi i.e. he MSB bi) mus be preceded by he sending of a synchronizaion (SYN) signal which is an energy pulse (carrier pulse like a dominan bi). The role of he SYN signal is o announce o he oher MAC eniies he arrival of he ID of his MAC eniy and hen o provide for hem a ime reference for he analysis of his ID.

4 MAC Eniy i l s g MAC Eniy j l s MAC Eniy i MAC Eniy j MAC Eniy i MAC Eniy j sending + τ 1 + ( τ + τ ) + τ PT ( τ ) + τpt propagaion + + ( τ + τpt) propagaion + propagaion 3 1 =+ τ +l s ; 3 =+(τ + τ PT )+l s + τ PT Fig. 2. CANlike: SYN signals. : SYN signal : dominan bi τ : recessive bi τ + Fig. 3. CANlike: Evaluaion of. : dominan bi : recessive bi + τ + 2 PT τ (a). Case 1 (b). Case 2 + 2τ + τ + l PT b A MAC eniy, which has deeced a SYN signal wihou iself having sen before a SYN signal, do no paricipae o he ournamen. Necessiy of a guard ime: Having in mind he ime inerval defined by he Ambiguiy Time Window, several MAC eniies can send he SYN signals which wile overlapping. Consequenly, in each MAC eniy among hese MAC eniies, he end of he SYN signal sending can be overaken by he end of he SYN signal arrivals. Considering such siuaion, a MAC eniy canno send he firs ID bi immediaely afer he SYN signal sending. We need o have a guard ime following he SYN signal and hen we send he firs ID bi. The guard ime guaranees ha afer his ime, we have a clean (idle) channel i.e. here is no more residue of he SYN aciviy. We also have he overlap beween he ID bis of differen rank, hen we need o add a guard ime a he end of each ID bi. Necessiy of a channel observaion ime: We need an observaion ime (noed TOBS1) before he sar of a ournamen. The role of his ime is o ensure ha he channel is globally idle and here is neiher a ournamen nor a ransmission in progress. C. The sages of CANlike We can now precise he global ournamen. All he conending nodes lisen o he channel during TOBS1. If he channel is deeced idle, all he conending nodes send a SYN signal and hen do he ournamen by comparing heir ID bis from he MSB bi o he LSB bi. The only winner is he one who did no lose on any bi during he ournamen. The winner will send is frame while he losers will wai unil he end of he frame ransmission of he winner before o ry o do a new ournamen. D. Specificaion of CANlike parameers We have o specify he following parameers: he duraion of he SYN signal (noed l s ), he duraion of he guard ime (noed g ), he lengh of he ID bi (noed ) and TOBS1. l s duraion: The SYN signal mus be deeced by a receiver MAC eniy. Then: l s τ ST (1) g value: Suppose ha a MAC eniy i sends he SYN signal. The duraion g is he bigges difference beween he end of he SYN signal sending and he end of he propagaion of a SYN signal of anoher MAC eniy j. In order o specify he value g, we consider he Ambiguiy Time Window concep wih he mos consrained scenario which is represened on Fig. 2: he MAC eniy i decides o send is SYN signal a he insan while he MAC eniy j decides o send is SYN signal a he insan +(τ + τ PT ) i.e. he laes wih respec o he one sen by i. We can see (Fig. 2) ha we do no have an overaking in he MAC eniy j because i sends laer and ha we have an overaking in he MAC eniy i (because i sen earlier). This overaking (= 3 1 ) defines he value of g. Then we have: g =2τ PT + τ (2) Remark: As g > τ, we consider ha, during he guard ime, a MAC eniy makes he urnaround if necessary, which depends on he firs ID bi.if he 1s ID bi is a dominan one, i is no necessary (as he MAC eniy was in he ransmi sae for he sending of he SYN signal); if i is a recessive one, we do he urnaround. duraion: We analyze he ournamen (afer he SYN phase) beween he firs ID bi of he MAC eniy i (his bi is a dominan bi) and he firs ID bi of he MAC eniy j (his bi is a recessive bi) by considering, always in he conex of he Ambiguiy Time Window, he following scenarios: The MAC eniy j sars lisening o he channel a he ime and ends he lisening sae a he ime +. TheMACeniyisarshesendingofhecarrierpulse eiher (case 1) a he ime (τ + τ PT ) i.e. a he earlies (Fig. 3.a), or (case 2) a he ime +(τ + τ PT ) i.e. a he laes (Fig. 3.b). For he case 1 (Fig. 3.a), he carrier pulse sen by i arrives a j a (τ + τ PT )+τ PT = τ and lass ill τ +. This arrival mus be deeced by j (i.e. j sees his arrival during a leas one τ ST ), he condiion is: τ + + τ ST (i.e. an overlap a he beginning of he lisening sae) which gives: τ + τ ST (3) For he case 2 (Fig. 3.b), he carrier pulse sen by i arrives a j a + (τ + τ PT ) + τ PT = + τ +2τ PT and lass ill + τ +2τ PT +. The MAC eniy j mus deec his carrier pulse arrival (i.e. j sees his arrival during a leas one τ ST ), he condiion is: + τ +2τ PT + τ ST + (i.e. an overlap a he end of he lisening sae) which gives: 2τ PT + τ + τ ST (4)

5 A B TOBS1 TOBS1 SYN SYN g 1 g 0 g 0 g g 1 g 0 g 1 Ambiguiy Time Window : dominan bi : recessive bi Fig. 4. Example of a ime diagram of CANlike. Considering he consrains (3) and (4), we need 2τ PT + τ + τ ST. We ake here: daa =2τ PT + τ + τ ST (5) Noe ha, in he case 2, he end of he arrival of he carrier pulse is higher han +. The difference is 2τ PT +τ which is equal o g (2). We hus need a g added a he end of each ID bi in order o have a clean sysem (when he ournamen beween he bis of a given rank sars, here is no hing on he channel from he ournamen beween he bis of he previous rank). We also consider ha during g, a MAC eniy does he urnaround if necessary, which depends on he nex ID bi. If nex ID bi and he curren one are idenical, i is no necessary; if hey are differen, we have o do he urnaround. TOBS1 duraion: TOBS1 mus be higher han he maximum duraion during a ournamen where he channel is idle. This exreme case is when we have a channel access of only one MAC eniy which has all recessive ID bis. Considering he ID field of n bis, he channel wile idle during n( + g ), hus TOBS1 >n( + g ). We ake: TOBS1 = (n+1)( + g ). E. Summary TOBS1 = (n+1)(4τ PT +2τ + τ ST ) (6) We presen in Tab. I he values of he parameers which characerize he CANlike proocol. Concerning he duraion of he SYN signal l s, we only indicaed is consrain in (1) (l s > τ ST ). We consider ha i is no necessary o disinguish is duraion from he ID bi duraion and hen we ake he same value (l s = ). =l s 2τ PT + τ + τ ST g (for and l s ) 2τ PT + τ TOBS1 (n+1)(4τ PT +2τ + τ ST ) TABLE I. CANLIKE PARAMETERS. From hese parameers, we can deduce he ime for accessing he channel (called τ a ) by he winner. τ a composes of he observaion phase, he SYN phase and he ournamen phase: τ a = TOBS1+(l s + g )+n( + g ) τ a = 2(n+1)(4τ PT +2τ + τ ST ) (7) F. Example of a ime diagram of CANlike We presen an example of he ournamen of CANlike on Fig. 4. We consider an ID field of 3 bis and he ournamen of 2 nodes A and B which have he prioriies (1; 0; 0) and (1; 0; 1) respecively. Thus A has a higher prioriy han B. The Inpu reference r C3 Conroller Conroller-Acuaor flow u Sensor-Conroller flow N E T W O R K C2 C1 D A Z O H Acuaor Sensor u() Process o conrol (DA: Digial Analog Converer; AD: Analog Digial Converer; ZOH: Zero Order Hold) A D h Oupu y() Fig. 5. Implemenaion of a process conrol applicaion hrough a nework. node B sars he ournamen laer (of one Ambiguiy Time Window) han he node A. A he 1s ID bi, he wo nodes, which have recessive bis, find he channel idle; a he 2nd ID bi, he wo nodes have dominan bis so hey coninue he nex bi; a he las ID bi, A has a dominan bi so i is he winner by definiion while B, which has a recessive bi, finds he channeusy and hen B sops he ournamen. A hen sends is daa. IV. CONCLUSION In his paper, we have ried o do an exhausive and pedagogical presenaion of he specificaion process of he CANlike proocol. We have shown how he main elemens of he ournamen (synchronizaion signal, bis of he ID field, guard ime, T OBS1) depend on he physical parameers (τ ST,τ,τ PT ) and on he concep of Ambiguiy ime window. We wan also o show in his conclusion he ineres of his proocol, in considering real-ime raffic and in comparison wih a proocol used very ofen in he wireless conex (IEEE DCF). In his goal, we have, a firs, considered an example of a process conrol applicaion he characerisics of which are: he process o conrol has he ransfer funcion [10] G(s) = 1000 s(s+1) and he conroller is a Proporional Derivaive (PD) conroller (ransfer funcion: K(1+T d s)) in order o have a phase margin of 45 which imposes K =0.7291; T d = s. The inpu reference r() is a uniy posiion sep and he feedback is made by aking direcly he oupu y(). The performance of his applicaion i.e. he Qualiy of Conrol (QoC) is evaluaed by means of a cos funcion ITSE (Inegral of Time weighed Square Error) noed J wih J = R (r() y()) 2 d. We call J 0 he value which is obained wihou he nework. Then we have considered he implemenaion of such four Process conrol applicaions (P i wih i = {1,2,3,4}) hrough a nework where he MAC proocol is eiher IEEE DCF proocol or he CANlike proocol. The analysis of he implemenaion has been done by using he ool TrueTime [11] and by considering, for he frame forma he frame of IEEE DCF (for he CANlike proocol we add also an ID field of 8 bis). The scheme of he implemenaion of a process conrol applicaion hrough a nework is shown on Fig. 5. We have wo frame flows: he sensor-conroller flow (noed f sc ) and he conroller-acuaor flow (noed f ca ). Then considering he implemenaion of he four applicaions hrough he nework, we have eigh flows which share he nerwork and compee for

6 400 J/J0 % P1 P2 P3 P4 IEEE DCF Fig. 6. Graphic represenaion of he QoC ( J/J 0 %). P1 P2 P3 P4 CAN like is use: f sci, f cai, wih i = {1,2,3,4}. Accouning for he delays which will affec he frames of hese flows, he performances obained for he four applicaions implemened hrough a nework wile obviously less good han for he applicaion implemened wihou he nework and will also depend on he proocol (IEEE DCF, CANlike). Concerning CANlike, we consider he following scheme of he prioriies for he frames of he flows f sc and f ca : prio f ca1 > prio f ca2 > prio f ca3 > prio f ca4 > prio f sc1 > prio f sc2 > prio f sc3 > prio f sc4 which means ha we consider: imporance P 1 > imporance P 2 > imporance P 3 > imporance P 4. The comparison beween he performances obained, by he implemenaion of he four process conrol applicaions, wih he wo proocols, IEEE DCF and CANlike, is made wih he performance crieria J J 0 J 0 % = J J 0 % which shows he deviaion of he QoC in comparison wih he resul obained (J 0 ) when he implemenaion is done wihou nework (his comparison shows he influence of he delays induced by he proocols) The higher he value J J 0 % is, he more degraded he QoC is. The Fig 6 shows he resuls which have been obained. Concerning he IEEE DCF we did 20 simulaions and we have represened he mean value by a simple line and he maximum gap beween he resuls of he 20 simulaions wih a doed underline. We see ha his proocol induces big gaps beween he performances, which can be obained for each applicaion, and hen i canno guaranee a performance (i.e. we have random performances). On he oher hand, we see ha he CANlike proocol guaranees performances which obviously depend on he prioriy associaed o he flows of he process conrol applicaions (higher is he prioriy, beer is he performance). We have here deerminisic performances. ACKNOWLEDGMENT This work has been parly funded by he projec OSEO AMIC-TCP (Archiecure Muliplexage Informaique Communicaion pour Transpor en Commun de Personnes). We wan o express here our sincere hanks. REFERENCES [1] H. Nguyen Xuan, G. Juanole, G. Mouney, and C. Calmees, Wireless communicaion neworks and process conrol applicaions: sudying he influence of MAC proocols, in Proc. IEEE Globecom 2010, Workshop SaCoNAS, Miami, Forida, USA, Dec [2] B. Crow, W. I., K. L.G., and S. P.T., IEEE Wireless Local Area Neworks, IEEE Communicaions Magazine, vol. 35, pp , Sepember [3] e 2005, IEEE Sandard for Informaion Technology - Telecommunicaions and Informaion Exchange Beween Sysems - Local and Meropolian Area Neworks - Specific Requiremens Par 11: Wireless LAN Medium Access Conrol (MAC) and Physical Layer (PHY) Specificaions Amendmen 8: Medium Access Conrol (MAC) Qualiy of Service Enhancemens, [4] D.-J. Deng and R.-S. Chang, A Prioriy Scheme for IEEE DCF Access Mehod, IEICE Trans Commun (Ins Elecron Inf Commun Eng), vol. E82-B, no. 1, pp , [5] J. Sobrinho and A. Krishnakumar, Real-ime raffic over he IEEE Medium Access Conrol Layer, Bell Labs Technical Journal, vol. 10, no. 1, pp , [6] A. Pal, A. Dogan, and F. Özgüner, MAC Layer Proocols for Real- Time Traffic in Ad-Hoc Wireless Neworks, in Proceedings of he 2002 Inernaional Conference on Parallel Processing (ICPP 02). Washingon, DC, USA: IEEE Compuer Sociey, 2002, pp [7] G. Bosch, Can specificaion 2.0, [online]. Available: [8] N. Pereira, B. Andersson, and E. Tovar, WiDom: A Dominance Proocol for Wireless Medium Access, IEEE Transacions on Indusrial Informaics, vol. 3, no. 2, pp , May [9] H. Nguyen Xuan, Réseaux de communicaion e applicaions de conrôle-commande, Ph.D. disseraion, Univ de Toulouse, INSA, LAAS, Toulouse, France, Dec [10] K. J. Åsröm and B. Wienmark, Compuer-conrolled sysems: Theory and design, 3rd ed. Prenice Hall, [11] M. Ohlin, D. Henriksson, and A. Cervin, TrueTime 1.5 Reference Manual, Lund Insiue of Technology, Sweden, Jan 2007.