MODEL OF I UB INTERAFCE IN THE UMTS NETWORK

Transcription

1 POZNAN UNIVERSITY OF TECHNOLOGY ACADEMIC JOURNALS Macej STASIAK* Janusz WIEWIÓRA** Potr ZWIERZYKOWSKI* 2007 Poznańske Warsztaty Telekomunkacyjne Poznań 6-7 grudna 2007 MODEL OF I UB INTERAFCE IN THE UMTS NETWORK The paper presents a new analytcal method blockng probablty determnaton n Iub nterface n the UMTS network. In our consderaton we use a modfed model of fullavalablty group wth mult-rate traffc as a model of the Iub nterface. The proposed scheme s applcable cost-effectve Iub resource management n 3G moble networks and can be easly appled to network capacty calculatons. Keywords: UMTS, Iub, blockng probablty 1. INTRODUCTION Unversal Moble Telecommuncaton System (UMTS) usng WCDMA rado nterface s one of the standards proposed thrd generaton cellular technologes (3G). Accordng to the 3GPP (ang. 3rd Generaton Partnershp Project) recommendatons, 3G systems should nclude servces wth crcut swtchng and packet swtchng, transmt data at a speed of up to 2 Mbt/s, and ensure access to multmeda servces [1]. The dmensonng process the UMTS system should make t possble to determne such a capacty of ndvdual elements of the system that wll secure wth the assumed load of the system a pre-defned level of GoS (Grade of Servce). Wth dmensonng the UMTS system the most characterstc constrants are: rado nterface and Iub nterface. When the rado nterface s a constrant, then, n order to ncrease the capacty, access techonolgy should be changed or subsequent branches of the system should be added (another NodeB). If, however, the constrant on the capacty of the system results from the capacty of Iub nterface, then a decson to add other statons (nodes) can be fnancally unfounded havng ts roots n ncomplete or ncorrect analyss of the system. Ths means that n any analyss of the system, a model that corresponds to Iub nterface should be routnely ncluded. Due to the possblty of resource allocaton dfferent traffc classes, the capacty determnaton of WCDMA rado nterface s much more complex than n the case of GSM systems. The capacty of the WCDMA nterface s lmted by the ncrease n nterference whch s caused by the users servced by other cells of the * Poznań Unversty of Technology, Char of Communcaton and Computer Networks ** Polska Telefona Cyfrowa Sp. z o.o. PWT POZNAŃ 6-7 GRUDNIA /9

2 Macej Stasak, Janusz Wewóra, Potr Zwerzykowsk system who make use of the same frequency channel, as well as by the users makng use of the adjacent rado channels and by the multpath propagaton occurrng n the rado channel. To ensure an approprate level of servce n UMTS, t s thus necessary to lmt the nterference by decreasng the number of actve users or the allocated resources employed to servce them. Several papers have been devoted to traffc modellng n cellular systems wth WCDMA rado nterface [2-8]. To date, however, no Iub models that take the dynamc resouce allocaton dfferent servces nto account have been consdered smultaneously by any author. Ths artcle presents a blockng probablty determnaton method a cellular system wth Iub nterface and dynamc resource allocaton scheme. The artcle has been dvded nto fve sectons. Secton 2 dscusses basc dependences descrbng Iub nterface n UMTS network. Secton 3 presents an analytcal model appled to blockng probablty determnaton statc and dynamc resource allocaton dfferent traffc classes. The followng secton ncludes the results obtaned n the study of the system. The fnal secton sums up the dscusson. 2. Iub INTERACE IN UMTS NETWORK Let us consder the structure of an UMTS network presented n Fg.1. The network conssts of 4 functonal blocks desgnated respectvely: User Equpment, UTRAN - UMTS Rado Access Network, CN - Core Network and external networks. Fg.1. Elements of the UMTS network structure 1 1 The followng notaton has been adopted n Fg. 1: USIM-UMTS subscrber dentty module, ME-Moble Equpment, RNC Rado Network Controller, MSC/VLR Moble Swtchng Centre/Vstor Locaton Regster, HLR-Home Locaton Regster, SGSN-Servng GPRS Support Node, GMSC-Gateway MSC, GGSN-Gateway GPRS Support Node, Uu-rado nterface, Iub-nterface connectng Node B and RNC, Iur-nterface connected wth RNC, Iu- nterface connectng RNC and MSC (IuCS) or wth SGSN (IuPS). PWT POZNAŃ 6-7 GRUDNIA /9

3 Model of Iub Interface n the UMTS network Fg.2. Two ways of organzaton of the Iub nterface n the UMTS network a) dvson of the nterface nto two dedcated groups b) nterface dvdes R99 and HSPA resources dynamcally In the dmensonng process the UMTS network, an approprate dmensonng of the connectons n the access part (UTRAN) has partcular sgnfcance,.e. the rado nterface between the user and the NodeB and the Iub connectons between the NodeB and the RNC (Rado Network Controller). The ssues pertanng to rado nterface dmensonng are wdely dscussed n the subject lterature, example n earler works of the authors, whereas those dealng wth dmensonng of Iub nterface have not been rased so far. Fgure 2 shows two ways of the organzaton of the Iub nterface. It s assumed that separate dedcated groups are desgned to servce R99 traffc (Release 99) [14] and HSPA (Hgh-Speed Packet Access) [13] (Fg. 2a) or that the capacty of the Iub nterface makes just one group and the resources that are unused by R99 traffc are assgned HSPA traffc transmsson (Fg. 2b). The fgure also shows exemplary classes of servces that are part of traffc desgnated ether as HSPA or R99. Preselected parameters of the servces are presented n Table 1. Tabela 1. Exemplary servces wth constrants n ATM layer (PS-non real tme) No Servce R DL peak rate (kbps) DL overhead R UL peak rate (kbps) UL overhead 1 AMR % % 2 CS % 64 25% 3 PS 384/ % % 4 HSPA Varous (max. 7,2 Mbps) 30% Varous (max. 7,2 Mbps) 30% PWT POZNAŃ 6-7 GRUDNIA /9

4 Macej Stasak, Janusz Wewóra, Potr Zwerzykowsk 3. MODEL OF THE SYSTEM The Iub nterface n UMTS network can be treated as the full-avalablty group (FAG) wth mult-rate traffc. Let us assume that the total capacty of the Iub s equal to V BBUs. The nterface s offered M ndependent classes of Posson traffc streams havng the ntenstes: λ 1, λ 2,..., λ M. The class call requres t BBUs to set up a connecton. The holdng tme calls of partcular classes has an exponental dstrbuton wth the parameters: µ 1, µ 2,..., µ M. Thus, the mean traffc offered to the system by the class traffc stream s equal to: a λ / µ =. (1) The demanded resources n the group servcng partcular classes can be treated as a call demandng an nteger number of the so-called BBUs (Basc Bandwdth Unts) [9]. The value of BBU,.e. t PJP, s calculated as the greatest common dvsor of all traffc classes offered to the system: t PJP = GCD ( R 1,..., R M ), (2) where R s the amount of resources demanded by class call n kbps. Both ways of Iub organsaton presented n Fg. 2 can be descrbed by multdmensonal Markov process whch can be expressed by one-dmentonal Markov chan. Occupancy dstrbuton n FAG can by descrbed by Kaufman- Roberts recurson [10, 12]: n P( n) a t = M = 1 P( n t ), (3) where P(n) s probablty of state n BBUs beng busy, and t s the number of BBUs requred by a class call: t = R / t. (4) The total capacty of the Iub nterface s also expressed n BBUs: PJP V = V Iub / t PJP, (5) where V Iub s the physcal capacty of Iub nterface n kbps. The dagram n Fg. 2 corresponds to (3) the system wth two call streams (M=2, t 1 =1, t 2 =2). The y (n) symbol denotes reverse transton rates of a class PWT POZNAŃ 6-7 GRUDNIA /9

5 Model of Iub Interface n the UMTS network a 2t 2 a 2 t 2 n 1 a 1 t 1 y ( n) 1 t 1 n a 1 1t 1 a 1 t n +1 n + 2 y 1( n + 1) t 1 y 1( n + 2) t1 y 2 ( n + 1) t 2 y 2 ( n + 2) t2 Fg.3. Fragment of a dagram of the one-dmensonal Markov chan n a mult-rate system (M=2, t 1 =1, t 2 =2) servce stream outgong from state n. These transton rates a class stream are equal to the average number of the class calls servced n state n. Based upon [10], the reverse transton rate class calls n (n+t ) state s equal to: ap( n) / P( n + t ) y ( n + t ) = 0 n + t n + t V, > V. Ths parameter determnes the average number of class calls servced n state n [9, 10]. The value of y (n), n a gven state of the group, ms the bass of the method of occupancy dstrbuton calculaton n the group presented n Fg. 4. Fgure 2a shows the organsaton of the Iub nterface accordng to whch t s dvded nto two dedcated groups servcng ndependently R99 and HSPA traffc. The analyss of such a system coresponds to ndependent analyss of two FAGs servcng mult-rate traffc. In each case t s possble then to determne, after determnng the occupancy dstrbuton P(n), the blockng probablty B class stream on the bass of the mula: B = V n= V t + 1 (6) P( n). (7) Fgure 2b shows a more complex case n whch HSPA traffc can use resources dedcated to R99 traffc. Ths takes place when R99 traffc does not entrely make use of the allocated resources and occupes at least G BBUs, where G < V. Such a case can be nterpreted as a dynamc lmtaton of resources R99 traffc classes whch s acommpaned by not lmted HSPA traffc. In order to determne the occupancy state of the group n whch, there s dynamc lmtaton of resources, we ntroduce the parameter G(n), defned n the followng way: M G( n) = = 1 y ( n) t S, where S s a set of constraned traffc classes ( example R99 traffc classes). The parameter G(n) determnes the averange number of BBUs beng busy by calls of selected (constraned) classes, n the state n. (8) PWT POZNAŃ 6-7 GRUDNIA /9

6 Macej Stasak, Janusz Wewóra, Potr Zwerzykowsk In the proposed model we search such a constraned state n, n whch the number of BBUs beng busy by calls of constraned classes meets the condton: G ( n) = G. (9) Fndng such a state of occupancy n (desgned later as N) determnes a possblty of lmtng access to the resources of the system traffc classes that belong to the set S. We assume that n all states older than n only those classes whch have no constrant are servced (Fg. 4). a2t2σ 2 ( n 2) a1t1 σ1( n 2) n 2 n 1 y1 ( n 1) t1 a1t1 σ 1( n 1) a1t1 σ1( n) n n + 1 y1 ( n) t1 y 1 ( n + 1) t1 y2 ( n) t2 G ( n) = G Fg.4. Fragment of a dagram of the modfed one-dmensonal Markov chan n a mult-rate system (M=2, t 1 =1, t 2 =2, G(n)=G, S={2}) The modfcaton of the servced process shown n Fg. 4 results n a transmaton n the occupancy dstrbuton P(n) nto the generalsed Kaufman-Roberts dstrbuton: n P( n) a t σ ( n t ) P( n t ) = M = 1, (10) where σ (n) s state-passage-probablty between adjecent states of the process. In the system shown n Fg. 4, the parametr σ (n) can be determned n the followng way: 1 σ ( n) = 0 1 S S S and and and n N, N > n, each n. In the determnaton of the blockng probablty of calls of ndvdual traffc classes servced n the system shown n Fg. 4, one has to take nto consderaton the dfferences n the avalablty of the group dfferent traffc classes. Theree we get: (11) PWT POZNAŃ 6-7 GRUDNIA /9

7 Model of Iub Interface n the UMTS network V n= N + 1 B = t V n= N t + 1 P ( n) P ( n) S, S. (12) On the bass of the above consderatons, the algorthm of blockng probablty calculatons n the Iub may be wrtten as follows: 1. Calculaton of offered traffc load a of class (Eq. (1)). 2. Determnaton of the value of t PJP as the greatest common dvsor (Eq. (2)) 3. Determnaton of the value of t as the nteger number of demanded resources by class calls (Eq. (4)) 4. Determnaton of state probabltes P(n) n the FAG (Fg. 3, Eq. (3)). 5. Calculaton of reverse transton rates y (n) (Eq. (6)). 6. Determnaton of state N n whch condton (9) s fulflled. 7. Determnaton of the occupancy dstrbuton P(n) n the modfed Markov chan (Eq. (10)). 8. In the modfed dstrbuton (10) we check f, a gven N, the condton (9) s met. If the condton s not fulflled, then we adopt N=N±1 and proceed to step Determnaton of blockng probabltes B class calls (Eq. (12)). 4. NUMERICAL EXAMPLES The proposed analytcal model of Iub nterface s approxmate one. Thus, the results of the analytcal calculatons of the Iub have been compared wth the results of the smulaton experments. The study carred out users demandng a set of servces (Tab. 1) and t was assumed that: a call of partcular servces demanded t 1 =1680, t 2 =8000, t 3 =49152 and t 4 = BBUs n the uplnk, a physcal capacty of Iub n the uplnk s equal to V Iub = 4 (R99)+ 7,2 x 1,3 =13,36 Mbps, a capacty of Iub n BBUs n the uplnk s equal to V= BBUs, one BBUs s equal to 0.001, the lmtaton G release R99 s equal to 4 Mbps ( BBUs) the servces were demanded n equal proportons a 1 t 1 :a 2 :t 2 :a 3 t 3 :a 4 t 4 =3:1:1:5. Fgure 5 shows the results obtaned traffc classes presented n Tab. 1. All the presented results show the robustness of the proposed method blockng probablty calculaton. In each case, regardless of the offered traffc load, the results are charactersed by far accuracy. PWT POZNAŃ 6-7 GRUDNIA /9

8 Macej Stasak, Janusz Wewóra, Potr Zwerzykowsk 1,E+00 blockng probablty 1,E-01 s-class-1 s-class-2 s-class-3 s-class-4 c-class-1 c-class-2 c-class-3 c-class-4 traffc load 0,35 0,50 0,65 0,80 0,95 1,10 1,25 1,40 1,55 Fg. 5 Blockng probablty traffc classes presented n Tab. 1 (G=4 Mbps and V Iub =13,36 Mbps) The results of the smulatons are shown n the charts n the m of marks wth 95% confdence ntervals calculated after the t-student dstrbuton. 95% confdence ntervals of the smulaton are almost ncluded wthn the marks plotted n the fgures. 5. CONCLUSIONS The dmensonng process the UMTS system should am at determnng such a capacty of the elements of the system that wll allow wth the predefned load of the system to ensure the assumed level of GoS (Grade of Servce). In the dmensonng of the UMTS system the most characterstc constrants are: rado nterface and the Iub nterface. The paper presents a new calculaton method blockng probablty determnaton traffc offered n the Iub nterface. In our consderatons, we use a modfed model of the full-avalablty group wth mult-rate traffc as a model of the nterface. The calculatons are valdated by a smulaton. The proposed method can be easly appled to 3G network capacty calculatons. PWT POZNAŃ 6-7 GRUDNIA /9

9 Model of Iub Interface n the UMTS network REFERENCES [1] Wesołowsk K: Moble Communcaton Systems, John Wley and Sons, [2] Staehle D. and Mader A.: An analytc approxmaton of the uplnk capacty n a UMTS network wth heterogeneous traffc, 18 th Internatonal Teletraffc Congress ITC 18, Berln, 2003, pp [3] Stasak M., Wśnewsk A., Zwerzykowsk P.: Blockng probablty calculaton n the uplnk drecton cellular systems wth WCDMA rado nterface, 3 rd Polsh-German Teletraffc Symposum, Dresden, Germany, Sept. 2004, pp [4] Głąbowsk M., Stasak M., Wśnewsk A., Zwerzykowsk P.: Uplnk blockng probablty calculaton cellular systems wth WCDMA rado nterface and fnte source populaton, 2 nd Internatonal Workng Conference on Permance Modellng and Evaluaton of Heterogenous Networks, Ilkley, UK, June 2004, pp. 80/1-80/10. [5] Stasak M., Wśnewsk A., Zwerzykowsk P.: Uplnk blokng probablty a cell wth WCDMA rado nterface and dfferently loaded neghbourng cells, Servce Assurance wth Partal and Intermttent Resources Conference SAPIR, Lsbon, 2005, pp [6] Głąbowsk M., Stasak M., Wśnewsk A., Zwerzykowsk P.: Uplnk blockng probablty calculaton cellular systems wth WCDMA rado nterface, fnte source populaton and dfferently loaded neghbourng cells, n Asa-Pacfc Conference on Communcatons, Perth, Western Australa, October 2005 (the best paper award). [7] Stasak M., Wśnewsk A., Zwerzykowsk P., Prawdopodobeństwo blokady dla łącza "w górę" w systemach komórkowych z nterfejsem WCDMA, Zeszyty Naukowe Wydzału Elektronk, Telekomunkacj Inmatyk Poltechnk Gdańskej, 1(1), [8] Stasak M., Wśnewsk A., Zwerzykowsk P., Uplnk and Downlnk Blockng Probablty Calculaton Cellular Systems wth WCDMA Rado Interface and Fnte Source Populaton, Polsh Teletraffc Symposum (PST 2007), Zakopane, s [9] Broadband Network Teletraffc, Fnal Report of Acton COST 242, Sprnger, [10] J.S. Kaufman, Blockng n a shared resource envronment, IEEE Trans. on Comm., vol. COM-29, nr 10, 1981, s [11] J.W. Roberts, A servce system wth heterogeneous user requrements, North Holland Pub. Co., Amsterdam, [12] M. Stasak, M. Głąbowsk, A smple approxmaton of the lnk model wth reservaton by a one-dmensonal Markov chan, Journal of Permance Evaluaton 41 (2 3) (2000) [13] H.Holma, A. Toskala, HSDPA/HSUPA UMTS, Hgh Speed Rado Access Moble Communcatons, John Wley & Sons, [14] H.Holma, A. Toskala, WCDMA UMTS, Rado Access For Thrd Generaton Moble Communcaton, Thrd Edton, John Wley & Sons, PWT POZNAŃ 6-7 GRUDNIA /9