Cost Analysis of the MBMS Multicast Mode of UMTS

Cost Analyss of the MBMS Multcast Mode of UMTS Antonos Alexou, Chrstos Bouras, Evangelos Rekkas Research Academc Computer Technology Insttute, Greece and Computer Engneerng and Informatcs Dept., Unv. of Patras, Greece N. Kazantzak str, GR 26500, Patras, Greece Tel:+30260960375 Fax:+30260960358 Emal: alexua@ct.gr, bouras@ct.gr, rekkas@ct.gr Correspondng author: Prof. Chrstos Bouras Abstract Along wth the wdespread deployment of the thrd generaton cellular networks, the fast-mprovng capabltes of the moble devces, content and servce provders are ncreasngly nterested n supportng multcast communcatons over wreless networks and n partcular over Unversal Moble Telecommuncatons System (UMTS). To ths drecton, the thrd Generaton Partnershp Project (3GPP) s currently standardzng the Multmeda Broadcast/Multcast Servce (MBMS) framework of UMTS. In ths paper, we present an overvew of the MBMS multcast mode of UMTS. We analytcally present the multcast mode of the MBMS and analyze ts performance n terms of packet delvery cost under varous network topologes, cell types and multcast users dstrbutons. Furthermore, for the evaluaton of the scheme, we consder dfferent transport channels for the transmsson of the data over the UTRAN nterfaces.. Introducton UMTS consttutes the thrd generaton of cellular wreless networks whch ams to provde hgh-speed data access along wth real tme voce calls. Although UMTS networks offer hgh capacty, the expected demand wll certanly overcome the avalable resources. The 3GPP realzed the need for broadcastng and multcastng n UMTS and proposed some enhancements on the UMTS Release 6 archtecture that led to the defnton of the MBMS framework. MBMS s a pont-to-multpont servce whch allows the networks resources to be shared [9]. Several multcast mechansms for UMTS have been proposed n the lterature. In [], the authors dscuss the use of commonly deployed IP multcast protocols n UMTS networks. However, n [2] the authors do not adopt the use of IP multcast protocols for multcast routng n UMTS. The scheme presented n [2], can be mplemented wthn the exstng network nodes wth only trval changes to the standard locaton update and packet-forwardng

2 procedures. Furthermore, n [3] a multcast mechansm for crcut-swtched GSM and UMTS networks s outlned. Fnally, the MBMS framework of UMTS s currently beng standardzed by the 3GPP [5]. In ths paper, we present an overvew of the MBMS multcast mode of UMTS. We analytcally present the multcast mode of the MBMS and analyze ts performance n terms of packet delvery cost under varous network topologes, cell types and multcast users dstrbutons. Furthermore, for the evaluaton of the scheme, we consder dfferent transport channels for the transmsson of the data over the UTRAN nterfaces. 2. Overvew of the UMTS n the Packet Swtched Doman UMTS network s splt n two man domans: the User Equpment (UE) doman and the Publc Land Moble Network (PLMN) doman. The UE doman conssts of the equpment employed by the user to access the UMTS servces. The PLMN doman conssts of two land-based nfrastructures: the Core Network (CN) and the UMTS Terrestral Rado-Access Network (UTRAN) (Fgure ). The CN s responsble for swtchng/routng voce and data connectons, whle the UTRAN handles all rado-related functonaltes. The CN s logcally dvded nto two servce domans: the Crcut-Swtched (CS) servce doman and the Packet-Swtched (PS) servce doman [8], [9]. The PS porton of the CN n UMTS conssts of two knds of General Packet Rado Servce (GPRS) Support Nodes (GSNs), namely Gateway GSN (GGSN) and Servng GSN (SGSN) (Fgure ). SGSN s the centerpece of the PS doman. It provdes routng functonalty nteracts wth databases (lke Home Locaton Regster (HLR)) and manages many Rado Network Controllers (RNCs). SGSN s connected to GGSN va the Gn nterface and to RNCs va the Iu nterface. GGSN provdes the nterconnecton of UMTS network (through the Broadcast Multcast Servce Center) wth other Packet Data Networks (PDNs) lke the Internet. [9]. Fgure. UMTS and MBMS Archtecture UTRAN conssts of two knds of nodes: the frst s the RNC and the second s the Node B. Node B consttutes the base staton and provdes rado coverage to one or more cells (Fgure ). Node B s connected to the User Equpment (UE) va the Uu nterface (based on the W-CDMA technology) and to the RNC va the Iub nterface. One RNC wth

3 all the connected to t Node Bs s called Rado Network Subsystem (RNS). In the UMTS PS doman, the cells are grouped nto Routng Areas (RAs), whle the cells n a RA are further grouped nto UTRAN Regstraton Areas (URAs). The moblty-management actvtes for a UE are characterzed by two fnte state machnes: the Moblty Management (MM) and the Rado Resource Control (RRC). The Packet MM (PMM) state machne for the UMTS PS doman s executed between the SGSN and the UE for CN-level trackng, whle the RRC state machne s executed between the UTRAN and the UE for UTRAN-level trackng. After the UE s attached to the PS servce doman, the PMM state machne s n one of the two states: PMM dle and PMM connected. In the RRC state machne, there are three states: RRC dle mode, RRC cell-connected mode, and RRC URA connected mode [7]. 3GPP s currently standardzng the Multmeda Broadcast/Multcast Servce. Actually, the MBMS s an IP datacast type of servce, whch can be offered va exstng GSM and UMTS cellular networks. It conssts of a MBMS bearer servce and a MBMS user servce. The latter represents applcatons, whch offer for example multmeda content to the users, whle the MBMS bearer servce provdes methods for user authorzaton, chargng and Qualty of Servce mprovement to prevent unauthorzed recepton. The major modfcaton n the exstng GPRS platform s the addton of a new entty called Broadcast Multcast - Servce Center (BM-SC). Fgure presents the archtecture of the MBMS. The BM-SC communcates wth the exstng UMTS GSM networks and the external Publc Data Networks [4], [5]. 3. Descrpton of the MBMS Multcast Mode In ths secton we present an overvew of the multcast mode of the MBMS framework. Fgure 2 shows a subset of a UMTS network. In ths archtecture, there are two SGSNs connected to a GGSN, four RNCs, and twelve Node Bs. Furthermore, eleven members of a multcast group are located n sx cells. The BM-SC acts as the nterface towards external sources of traffc [5]. In the presented analyss, we assume that a data stream that comes from an external PDN through BM-SC, must be delvered to the eleven UEs as llustrated n Fgure 2. The analyss presented n the followng paragraphs, covers the forwardng mechansm of the data packets between the BM-SC and the UEs (Fgure 2). Regardng the transmsson of the packets over the Iub and Uu nterfaces, t may be performed on common (ex. Forward Access Channel - FACH) or dedcated (Dedcated Channel - DCH) channels. As presented n [0], the transport channel that the 3GPP decded to use as the man transport channel for pont-tomultpont MBMS data transmsson s the FACH wth turbo codng and QPSK modulaton at a constant

4 transmsson power. DCH s a pont-to-pont channel and hence, t suffers from the neffcences of requrng multple DCHs to carry the data to a group of users. However, DCH can employ fast closed-loop power control and soft handover mechansms and generally s a hghly relable channel [9], []. Fgure 2. Packet delvery n UMTS Wth multcast, the packets are forwarded to those Node Bs that have multcast users. Therefore, n Fgure 2, the Node Bs 2, 3, 5, 7, 8, 9 receve the multcast packets ssued by the BM-SC. We brefly summarze the fve steps occurred for the delvery of the multcast packets. Frstly, the BM-SC receves a multcast packet and forwards t to the GGSN that has regstered to receve the multcast traffc. Then, the GGSN receves the multcast packet and by queryng ts multcast routng lsts, t determnes whch SGSCs have multcast users resdng n ther respectve servce areas. In Fgure 2, the GGSN duplcates the multcast packet and forwards t to the SGSN and the SGSN2 [3]. Then, both destnaton SGSNs receve the multcast packets and, havng quered ther multcast routng lsts, determne whch RNCs are to receve the multcast packets. The destnaton RNCs receve the multcast packet and send t to the Node Bs that have establshed the approprate rado bearers for the multcast applcaton. In Fgure 2, these are Node B2, B3, B5, B7, B8, B9. The multcast users receve the multcast packets on the approprate rado bearers, ether by pont-to-pont channels transmtted to ndvdual users separately or by common channels transmtted to all members n the cell. In our analyss, we consder transport channels such as FACH and DCH [3]. 4. Cost Analyss of the MBMS Multcast Mode 4. General assumptons We consder a subset of a UMTS network consstng of a sngle GGSN and N SGSN SGSN nodes connected to the GGSN. Furthermore, each SGSN manages a number of N ra RAs. Each RA conssts of a number of N rnc RNC nodes, whle each RNC node manages a number of N ura URAs. Fnally, each URA conssts of N nodeb cells. The total number of RAs, RNCs, URAs and cells are:

5 NRA= NSGSN Nra () NRNC = NSGSN Nra Nrnc (2) NURA = NSGSN Nra Nrnc Nura (3) NNODEB = NSGSN Nra Nrnc Nura Nnodeb (4) The total transmsson cost for packet delveres ncludng pagng s consdered as the performance metrc. Furthermore, the cost for pagng s dfferentated from the cost for packet delveres. We make a further dstncton between the processng costs at nodes and the transmsson costs on lnks, both for pagng and packet delveres. As presented n [6] and analyzed n [2], we assume that there s a cost assocated wth each lnk and each node of the network, both for pagng and packet delveres. For the analyss, we apply the followng notatons: D gs D sr D rb D DCH D FACH S sr S rb S a p gm p sm p rm p b a s a r a b Tx cost of packet delvery between GGSN and SGSN Tx cost of packet delvery between SGSN and RNC Tx cost of packet delvery between RNC and Node B Tx cost of packet delvery over Uu wth DCHs Tx cost of packet delvery over Uu wth FACHs Tx cost of pagng between SGSN and RNC Tx cost of pagng between RNC and Node B Tx cost of pagng over the ar Processng cost of multcast packet delvery at GGSN Processng cost of multcast packet delvery at SGSN Processng cost of multcast packet delvery at RNC Processng cost of packet delvery at Node B Processng cost of pagng at SGSN Processng cost of pagng at RNC Processng cost of pagng at Node B The total number of the multcast UEs n the network s denoted by N UE. For the cost analyss, we defne the total packets per multcast sesson as N p. Snce network operators wll typcally deploy an IP backbone network between the GGSN, SGSN and RNC, the lnks between these nodes wll consst of more than one hop. Addtonally, the dstance between the RNC and Node B conssts of a sngle hop (l rb = ). In the presented analyss we assume that the dstance between GGSN and SGSN s l gs hops, whle the dstance between the SGSN and RNC s l sr hops. We assume that the probablty that a UE s n PMM detached state s P DET, the probablty that a UE s n PMM dle/rrc dle state s P RA, the probablty that a UE s n PMM connected/rrc URA connected state s P URA, and fnally the probablty that a UE s n PMM connected/rrc cell-connected state s P cell. In the remander of ths secton, we descrbe a method that models the multcast user dstrbuton n the network. In partcular, we present a probablstc method that calculates the number of multcast users n the network (N UE ), the number of SGSNs that serve multcast users (n SGSN ), the number of RNCs that serve multcast users (n RNC ) and fnally the number of Node Bs that serve multcast members (n NODEB ). As ntroduced n [3] and analyzed n [2], we classfy the RAs nto L RA categores. For L RA there are N

6 RAs of class. Therefore, the total number of RAs wthn the network s N RA LRA = N. Suppose that the dstrbuton of the multcast users among the classes of RAs follows the Posson dstrbuton wth = λ θ ( RA ) = where L RA. In general, the probablty that k exactly multcast users resde n the RAs of class s calculated from the followng equaton: (, θ ) p k θ ( θ ) k e = (5) k! Thus, the probablty none of the RAs of class serves multcast users s p( 0, θ ) e θ that the probablty at least one multcast user s served by the RAs of class s ( θ ) N Snce every class conssts of ( ) ( ) RA e θ ( RA ) =, whch n turn means p = p 0, = e θ. N RAs, the total number of the RAs n the class, that serve multcast users s. Thus, the total number of the RAs of every class that serve multcast users s: LRA nra = N e θ (6) = ( ) ( ) RA where θ represents the number of multcast users for the N RAs of class. ( RA ) If there are n RA RAs that are servng multcast users, the probablty that an SGSN does not have any such RA s: NRA Nra NRA /, nra NRA Nra psgsn = nra nra (7), othewse 0 Based on eqn (7), the total number of SGSNs that are servng multcast users can be calculated as follows: ( ) n = N p SGSN SGSN SGSN. The total number of multcast users n the network s: L RA UE = N = N θ (8) where θ s the number of multcast users n a RA of class. As n [2], we assume that all RNCs wthn a servce area of class have the same multcast populaton dstrbuton densty as n the RA case. Based on a unform densty dstrbuton wthn a sngle RA, the multcast populaton of an RNC wthn the servce area of a class RA s θ ( RNC ) θ = Nrnc. The total number of RNCs of class s

7 N = N Nrnc. ( RNC ) Assumng that the number of RA categores s equal to the number of RNC categores (L RNC =L RA ), the total number of RNCs that serve multcast users s: RNC LRNC ( RNC ) (9) = ( ) ( RNC n = N e θ The same are appled to the cells wthn the servce area of an RNC. The average number of multcast users for a ( sngle cell of class s B ) ( RNC) θ θ ( N N ) =. ura nodeb ) The number of Node Bs belongng to class s N = N N N ( B) ( RNC) ura nodeb. Assumng that the number of the RNC categores s equal to the number of the Node B categores (LRNC=L NODEB ), the total number of Node Bs that serve multcast users s: NODEB LNODEB ( ) ( ( B ) B ) = n = N e θ (0) 4.2 Cost Analyss of the Multcast Mode In the multcast scheme, the multcast group management s performed at the BM-SC, GGSN, SGSN and RNC and multcast tunnels are establshed over the Gn and Iu nterfaces. It s obvous that the cost of a sngle packet delvery to a multcast user depends on ts MM and RRC state. If the multcast member s n PMM connected/rrc cell-connected state, then there s no need for any pagng procedure nether from the SGSN nor from the servng RNC. In ths case, the packet delvery cost s derved from eqn(). It has to be mentoned that ths quantty does not nclude the cost for the transmsson of the packets over the Iub and Uu nterfaces snce ths cost depends frstly on the number of multcast users and secondly on the transport channel used for data transmsson. Ccell = pgm + Dgs + psm + Dsr + prm () If the multcast member s n PMM connected/rrc URA connected state, then the RNC must frst page all the cells wthn the URA n whch moble users resde and then proceeds to the data transfer. After the subscrber receves the pagng message from the RNC, t returns to the RNC ts cell ID. The cost for pagng such a multcast member s: ( ) C = N S + a + S + S + a + S +a (2) URA nodeb rb b a a b rb r

8 If the multcast member s n PMM dle/rrc dle state, the SGSN only stores the dentty of the RA n whch the user s located. Therefore, all cells n the RA must be paged. The cost for pagng such a multcast member s: ( ) ( ) ( ) C = N S + a + N N N S + a + S + S + a + S + a + S + a (3) RA rnc sr r rnc ura nodeb rb b a a b rb r sr s After the pagng procedure, the RNC stores the locaton of any UE at a cell level. In multcast, the SGSN and the RNC forward a sngle copy of each multcast packet to those RNCs or Node Bs respectvely that are servng multcast users. After the correct multcast packet recepton at the Node Bs that serve multcast users, the Node Bs transmt the multcast packets to the multcast users va common or dedcated transport channels. The total cost for the multcast scheme s derved from the followng equaton where n SGSN, n RNC, n NODEB represent the number of SGSNs, RNCs, Node Bs respectvely that serve multcast users. ( ) ( ) ( ) _ Ms = p + n D + p + n D + p + Y N + P C + P C N = D + D gm SGSN gs sm RNC sr rm p RA RA URA URA UE packet delvery pagng (4) where ( ) ( ), n NODEB Drb + pb + DFACH, f channel = FACH Y = NUE Drb + pb + DDCH f channel = DCH ( ) ( ) ( ) D = p n D p n D p Y + + + + + N D = P C + P C N packet _ delvery gm SGSN gs sm RNC sr rm p pagng RA RA URA URA UE Parameter Y represents the multcast cost for the transmsson of the multcast data over the Iub and Uu nterfaces. Ths cost depends manly on the dstrbuton of the multcast group wthn the UMTS network and secondly on the transport channel that s used. In case we use the FACH as transport channel, each multcast packet send once over the Iub nterface and then the packet s transmtted to the UEs that served by the correspondng Node B. However, n case we use DCHs for the transmsson of the multcast packets over the Iub each packet s replcated over the Iub as many tmes as the number of multcast users that the correspondng Node B serves. 5. Evaluaton of the MBMS Multcast Mode In ths secton we present some evaluaton results regardng the MBMS multcast mode performance under dfferent cell confguratons, dfferent user dstrbutons and fnally dfferent transport channels for the transmsson of the multcast data over the UTRAN nterfaces. Therefore, we assume a general network topology, wth N SGSN =0, N ra =0, N rnc =0, N ura =5 and N nodeb =5. 5. Evaluatons Parameters The packet transmsson cost (D xx ) n any segment of the UMTS network depends on two parameters: the number of hops between the edge nodes of ths network segment and the capacty of the lnk of the network segment. Ths

9 means that D gs = l gs /k gs, D sr =l sr /k sr and D rb =l rb /k rb. Parameter k xx represents the profle of the correspondng lnk between two UMTS network nodes. More specfcally, n the hgh capacty lnks at the CN, the values of k xx are greater than the correspondng values n the low capacty lnks at UTRAN. For the cost analyss and wthout loss of generalty, we assume that the dstance between the GGSN and SGSN s 8 hops, the dstance between SGSN and RNC s 4 hops and the dstance between RNC and Node B s hop. The above parameters as well as the values of the k xx are presented n detal n Table. Regardng the transmsson cost of pagng (S xx ) n the segments of the UMTS network, t s calculated n a smlar way as the packet transmsson cost (D xx ). More specfcally, S xx s a fracton of the calculated transmsson cost (D xx ) and n our case we assume that t s three tmes smaller than D xx. Lnk Lnk Capacty factor (k) Number of hops (l) Transmsson cost (D) GGSN-SGSN k gs = 0.8 l gs = 8 D gs = 0 SGSN-RNC k sr = 0.7 l sr = 4 D sr = 4/0.7 RNC Node B k rb = 0.5 l rb = D rb = 2 Table. Chosen values for the calculaton of transmsson costs n the lnks As we can observe from the equatons of the prevous secton, the costs of the schemes depend on a number of other parameters. Thus, we have to estmate the value of these parameters. The chosen values of the parameters are presented n Table 2. S sr S rb S a p gm p sm p rm p b a s a r a b P RA P URA P cell 4/2. 2/3 4/3 2 2 2 0.6 0.2 0. Table 2. Chosen parameters values Regardng the transmsson over the Uu, two dfferent transport channels are examned: the DCH and the FACH. The fundamental parameter that defnes the transmsson cost over the ar (D DCH, and D FACH for DCH and FACH respectvely) s the amount of Node B s transmsson power that must be allocated for these two transport channels. More specfcally, a FACH channel essentally transmts at a fxed power level snce fast power control s not supported n ths channel. A FACH channel must be receved by all UEs throughout the cell. Consequently, the fxed power should be hgh enough to ensure the requested QoS n the whole coverage area of the cell and ndependently of the locaton of UEs. Ths means that the parameter D FACH represents the cost of usng a FACH channel to serve all the multcast users resdng n a specfc cell. The total downlnk transmsson power allocated for DCHs s varable and manly depends on the number of UEs, ther locatons throughout the cell, the requred bt rate of the MBMS servce and the experenced sgnal qualty (E b /N 0 ) for each user. Eqn(5) calculates the total Node B s transmsson power requred for the transmsson of the data to n users n a specfc cell [2]. The total Node B s transmsson power s the sum of the Node B s power

0 allocated to each DCH user n the cell. Ths means that the parameter D DCH represents the cost of usng a sngle DCH channel to serve one multcast user n a specfc cell. P P = b n N b, 0 T = PT = n = p P + n = ( PN + x) L W + p E ( ) R W E ( b ) R N 0 b, + p p, (5) where P s the base staton total transmtted power, P s the power devoted to the th user, P s the power T T P devoted to common control channels L p, s the path loss, Rb, the th user transmsson rate, W the bandwdth, P N the background nose, p s the orthogonalty factor and x s the ntercell nterference observed by the th user gven as a functon of the transmtted power by the neghborng cells P Tj, j =, K and the path loss from ths user to the th j cell L j. More specfcally: x K Tj = (6) j= P L j At ths pont, we have to menton that snce the nodes that are responsble for the forwardng of the multcast packets are the GGSN, SGSN and the RNC, we consder a lower packet processng cost n the Node B than the correspondng costs n the GGSN, SGSN and RNC snce some overhead s needed n the above mentoned three nodes n order to mantan the routng lsts requred for the packet forwardng n the multcast scheme. Furthermore, we have chosen approprately the probabltes P RA, P URA and P cell More specfcally, the probablty that a UE s n PMM dle/rrc dle state s P RA =0.6. The probablty that a UE s n PMM connected/rrc URA connected state s P URA =0.2 and the probablty that a UE s n PMM connected/rrc cell-connected state s P cell =0.. Addtonally, there s a probablty that the UE s not reachable by the network and we consder t to be 0.. It s true that the performance of the three schemes depends manly, on the confguraton of the UMTS network that s under nvestgaton. In our analyss, we assume that we have two classes of RAs. A class = RA has multcast user populaton of θ = /δ and a class =2 RA has a multcast user populaton of θ 2 = δ. If δ >>, the class = RA has a small multcast user populaton and the class =2 RA has a large multcast user populaton. Let α be the proporton of the class = RAs and (-α) be the proporton of the class =2 RAs [3]. Thus, the number of class =

RAs s N =αnra and the number of class =2 RAs s N =(-α)nra. Each RA of class {,2} s n turn subdvded nto N rnc RNCs of the same class and smlarly, each RNC of class {,2} s subdvded nto 2 N ura N nodeb Node Bs of the same class. Take nto consderaton the above mentoned parameters, eqn(8) can be transformed to eqn(7). It s obvous from eqn(7) that as α decreases and δ ncreases the number of multcast users ncreases rapdly. 2 α NUE = N θ = N θ+ N2 θ2 = NRA + δ = δ αδ (7) For the cost analyss of the MBMS multcast mode, we consder the cases of urban macrocell (hexagonal, 3-sector cells, 000m ste-to-ste dstance) and urban mcrocell (Manhattan grd wth 360m base staton spacng) envronments wth Vehcular A and Pedestran A multpath channel models respectvely. Moreover, an 64kbps MBMS servce s assumed. The basc smulaton parameters are presented n Table 3 [4], [5], [6]. Parameters Macro Cell Mcro Cell BS Max Tx Power 43dBm 33dBm Common channel power 30dBm 20dBm orthogonalty factor 0.5 0. Downlnk Eb/N 0, 5dB 6.5dB Other-to-own cell nterference rato 0.65 0.4 Multpath channel Vehcular A (3km/h) Pedestran A (3km/h) Propagaton model Okumura Hata Walfsch-Ikegam FACH Tx Power (no STTD, 95% coverage) 7.6 W(38% of BS Tx Power) 0.36 W (8% of BS Tx Power) Table 3. Smulaton Parameters In our analyss, we calculate n each cell of the network topology the Node B s power n the case of usng DCHs or FACH. Then, by comparng these power values wth the total avalable Node B s transmsson power of any cell, we select the approprate values for the D DCH, and D FACH. Obvously the values D DCH, and D FACH are proportonal to the percentage of the Node B s transmsson power allocated to DCH or FACH n any cell. The D DCH, and D FACH values are then used n eqn(4) that calculates the total telecommuncaton cost of the MBMS multcast mode. At ths pont t should be remnded that the D FACH represents the cost of usng a FACH channel to serve all the multcast users resdng n a specfc cell whle the D DCH represents the cost of usng a sngle DCH to transmt the multcast data to a multcast user of the network. Thus, the total cost of usng DCHs to transmt the multcast data n a cell s the sum of the D DCH values n the correspondng cell. Furthermore, we assume that the mnmum value that the total D DCH, per cell and the D FACH could take s the value of 0 snce ths value s the cost of the data transmsson n the wred lnk between the GGSN and the SGSN and generally the transmsson cost n a wred lnk s assumed to be

2 lower than the transmsson cost n a wreless lnk. 5.2 Results In Fgure 3 the total costs for the multcast mode usng dfferent transport channels and cell envronments n functon of α are presented. From these plots we can see that the costs decrease as α ncreases. Ths occurs because as α ncreases the number of RAs wth no multcast users ncreases and hence the multcast users are located n a small number of RAs. (a) (b) Fgure 3. Total cost n functon of α wth (a) δ = 300, (b) δ = 3000 More specfcally, n Fgure 3a the cost n case we use DCHs s smaller than the cost n case we use a FACH channel both n macro and mcro envronments. Ths occurs because the small value of δ, results to a reduced number of UEs n the network and hence the DCH s more effcent for the data transmsson n terms of total cost. The opposte occurs n Fgure 3b where the value of δ s ncreased, whch means that the number of UEs s also ncreased. Therefore, the use of DCHs s neffcent for the transmsson of the data over the Iub and Uu nterfaces whle the FACH s the most sutable transport channel n terms of total cost. In Fgure 4, the total costs usng dfferent transport channels and cell envronments n functon of δ are presented. We choose a small value for the parameter α because the multcast mode becomes effcent when there s an ncreased densty of UEs n the network. Therefore, a value of α=0. s chosen whch means that there are many RAs n the network wth a great number of multcast users n these. From Fgure 4, t s clear that as parameter δ ncreases (whch means that the number of multcast users ncreases), the total cost for all cases ncreases too. However, the ncrease n total cost for DCHs s greater than that of FACH due to the fact that a DCH s a pont-to-pont channel and strongly depends on the number of multcast users.

3 Fgure 4. Total cost n functon of δ, α = 0. More specfcally, n Fgure 4, we observe that for small values of δ, the total cost usng DCHs s small because there s a small number of UEs n the network, whle for bgger values of δ, whch mples bgger number of UEs, the total cost usng DCHs overcomes the cost of usng FACH. Thus, for small values of δ the use of DCHs s more effcent whle for bgger values of δ, the use of FACH s more approprate. The swtchng pont between multple DCHs and a sngle FACH, n terms of total transmsson cost, s 4 UEs (or δ=000) for a macro cell and 2 UEs (or δ=500) for a mcro cell as shown n Fgure 4. Ths means that, n the case of a macro cell, for 4 UEs and above a FACH should be used, whle for less than 4 UEs the use of multple DCHs s the more effcent choce. (a) (b) Fgure 5. Swtchng pont for (a) macro cell, (b) mcro cell In Fgure 5a, the total Node B s transmsson power for a macro cell when usng multple DCHs and a sngle FACH s presented. Smlarly, n Fgure 5b the same power profles are presented but for the case of a mcro cell. From Fgure 5a, t s obvous that for a macro cell, by takng nto account only the Node B s transmsson power, the swtchng pont between DCH and FACH channels s 7 UEs per cell, whle from Fgure 5b the swtchng pont for a

4 mcro cell s 4 UEs. However, as shown prevously n Fgure 4, these swtchng ponts are reduced to 4 UEs and 2 UEs for macro and mcro cells respectvely, when takng nto account the total transmsson cost and not just the Node B s transmsson power. Ths reducton s caused by the addtonal cost ntroduced by the Iub nterface, representng the transmsson cost of packet delvery between RNC and Node B. Recall from eqn(4) that computes the total cost of the multcast scheme, the parameter Y represents the multcast cost for the transmsson of the multcast data over the Iub and Uu nterfaces. The cost added from Iub s not neglgble and depends on the lnk capacty whch s, however, operator dependent. For the smulatons presented above, the lnk capacty factor was set to k rb = 0.5. For greater values of k rb, the swtchng ponts converge to the values presented n Fgure 5. From the above observaton, t s clear that the selecton of an approprate rado bearer for the multcast data transmsson s dramatcally affected by the cost added by the Iub nterface. The Node B s transmsson power should not be the only crteron for the selecton of a transport channel, but the total transmsson cost (ncludng the Iub cost) should always be taken nto account. 6. Conclusons and Future Work In ths paper we presented an overvew of the MBMS multcast mode of UMTS. We nvestgated the performance of the multcast mode of the MBMS n terms of packet delvery cost through an analytc theoretcal model and by smulatons based on ths model. The nvestgatons were made assumng varous network topologes, cell envronments and multcast users dstrbutons. In addton, we examned the DCH and FACH transport channels n terms of data transmsson cost over the Iub and Uu nterfaces. The step that follows ths work s to examne the total transmsson cost of HS-DSCH ntroduced n the Release 5 of UMTS as the transport channel for the transmsson of the MBMS data n Iub and Uu nterfaces. Addtonally, an algorthm for the optmzaton of the multcast data transmsson over the Iub could be developed. References [] Hauge M, Kure O. Multcast n 3G networks: Employment of exstng IP multcast protocols n UMTS. 5th ACM Internatonal Workshop on Wreless Moble Multmeda. 2002. 96 03. [2] Rummler R, Chung Y, Aghvam H. Modelng and Analyss of an Effcent Multcast Mechansm for UMTS. IEEE Transactons on Vehcular Technology 2005; 54(). 350-365. [3] Ln Y. A multcast mechansm for moble networks. IEEE Communcaton Letters 200; 5(). 450 452.

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