A Multicast Approach for UMTS: A Performance Study.

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F. Boavida et al. (Eds.): NETWORKING 2006, LNCS 3976, pp. 1086 – 1091, 2006.
© IFIP International Federation for Information Processing 2006
A Multicast Approach for UMTS: A Performance Study
Antonios Alexiou, Dimitrios Antonellis, and Christos Bouras
Research Academic Computer Technology Institute, N. Kazantzaki str,
26500 Patras, Greece and
Computer Engineering and Informatics Department,
University of Patras, 26500 Patras, Greece
alexiua@cti.gr, antonel@ceid.upatras.gr, bouras@cti.gr
Abstract. In this paper, a multicast scheme for UMTS which only requires in-
significant modifications in the current UMTS network infrastructure is ana-
lyzed. We analytically present the multicast routing mechanism behind our
scheme as well as the multicast group management functionality of it. Further-
more, we present an evaluation of our scheme in terms of its performance. The
critical parameters for the evaluation of the scheme are the number of multicast
users within the multicast group, the amount of data sent to the multicast users,
the density of the multicast users within the cells and the type of transport
channel used for the transmission of the multicast data over the air.
1 Introduction
UMTS constitutes the third generation of cellular wireless networks which aims to
provide high-speed data access along with real time voice calls. Wireless data is one
of the major boosters of wireless communications and one of the main motivations of
the next generation standards [7]. The multicast transmission of real time multimedia
data is an important component of many current and future emerging Internet applica-
tions, such as videoconference, distance learning and video distribution. It offers
efficient multidestination delivery, since data is transmitted in an optimal manner with
minimal packet duplication [8].
Compared with multicast routing in the Internet, mobile networks such as UMTS
pose a very different set of challenges for multicast. First, multicast receivers are
nonstationary and consequently, may change their point of attachment to the network
at any given time. Second, mobile networks are generally based on a well-defined tree
topology, with the nonstationary multicast receivers being located at the leaves of the
network tree. It is therefore not appropriate to apply conventional IP multicast routing
mechanisms in UMTS.
Several multicast mechanisms for UMTS have been proposed in the literature. In
[1], the authors discuss the use of commonly deployed IP multicast protocols in
UMTS networks. However, in [2] the authors do not adopt the use of IP multicast
protocols for multicast routing in UMTS and present an alternative solution. The
scheme presented in [2] can be implemented within the existing network nodes
with only trivial changes to the standard location update and packet-forwarding

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A Multicast Approach for UMTS: A Performance Study 1087
procedures. Furthermore in [3], a multicast mechanism for circuit-switched GSM and
UMTS networks is outlined while in [4] an end to end multicast mechanism for soft-
ware upgrades in UMTS is analyzed. Additionally, the 3rd Generation Partnership
Project (3GPP) is currently standardizing the Multimedia Broadcast/Multicast Service
(MBMS) [5], [9].
In this paper, we analytically present a multicast scheme for UMTS with the rout-
ing mechanism behind the scheme. Additionally, the multicast group management
functionality of our mechanism and the performance of the scheme are analyzed. The
critical parameters for the evaluation of the scheme are the number of multicast users
within the multicast group, the amount of data sent to the multicast users, the density
of the multicast users within the cells and the type of transport channel used for the
transmission of the multicast data over the air.
2 A Multicast Approach for UMTS
In this section we present an overview of a multicast scheme for UMTS. More spe-
cifically, the way that the multicast packets are delivered to a group of mobile users is
presented in detail. Additionally, we analyze the packet forwarding and routing
mechanism behind the multicast scheme as well as the multicast group management
functionality of the scheme.
Fig. 1 shows a subset of a UMTS network consisting of eleven multicast users lo-
cated in six cells. The BM-SC acts as the interface towards external sources of traffic
[5]. In the presented analysis, we assume that a data stream coming from an external
PDN through BM-SC, must be delivered to these UEs as illustrated in Fig. 1. For the
efficient packet forwarding mechanism, every node of the network (except the UEs)
maintains a routing list. In these lists of each node, we record the nodes of the next
level that the messages for every multicast group should be forwarded.
With multicast, the packets are finally forwarded to those Node Bs that serve mul-
ticast users. Therefore, in Fig. 1, the Nodes B2, B3, B5, B7, B8, B9 will receive the
multicast packets issued by the BM-SC. We briefly summarize the steps occurred for
the delivery of the multicast packets. Before the transmission of the multicast data, the
routing lists of the nodes must be filled with useful information. This procedure can
be initialized either from the UEs or from the BM-SC (ex. software upgrades). In the
former case, consider a UE that decides to become a member of a multicast service.
Thus, it sends an appropriate message to the BM-SC requesting this service. Then,
every node located in the path between this UE and the BM-SC, when it receives the
message from the UE, updates its routing list and forwards the message to the next
node. In the second case, the BM-SC initializes this procedure and since it does not
have any information regarding the location of the multicast members, a paging pro-
cedure at RA and URA level is necessary for the updating of the routing lists.
Consider that the BM-SC receives a multicast packet and forwards it to the GGSN
that has registered to receive the multicast traffic. Then, the GGSN receives the multi-
cast packet and by querying its routing list, it determines which downstream SGSCs
have multicast users residing in their respective service areas. In Fig. 1, the GGSN
duplicates the packet and forwards it to the SGSN1 and the SGSN2. After both desti-
nation SGSNs have received the multicast packet and having queried their routing list,

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1088 A. Alexiou, D. Antonellis, and C. Bouras

Fig. 1. Packet delivery in UMTS
they determine which RNCs must receive the multicast packet. The destination RNCs
receive the multicast packet and send it to the Node Bs that have established the ap-
propriate radio bearers for the multicast application. In Fig. 1, these are Node B2, B3,
B5, B7, B8, B9. The transmission of the packets over Uu interface, may be performed
on dedicated (DCH) or shared transport channels (ex. High Speed Downlink Shared
Channel – HS-DSCH) [7].
3 Evaluation of the Multicast Scheme
In this section we present an evaluation, in terms of the telecommunication costs, of
the multicast scheme. In particular, we consider a subset of a UMTS network consist-
ing of a single GGSN and NSGSN SGSN nodes connected to the GGSN. Furthermore,
each SGSN manages a number of Nra RAs. Each RA consists of a number of Nrnc
RNC nodes, while each RNC node manages a number of Nura URAs. Finally, each
URA consists of Nnodeb cells. The total number of RNCs and cells are:
RNCSGSN rarnc
NNNN
=⋅⋅

(1)
NODEBSGSN rarnc uranodeb
NNNNNN
=⋅⋅⋅⋅

(2)
The total transmission cost for packet deliveries is considered as the performance
metric. We make a further distinction between processing costs at nodes and trans-
mission costs on links. Similar to [6], there is a cost associated with each link and
each node of the network for the packet deliveries. We apply the following notations:
Dgs
Dsr
Drb
DDCH
DHS-DSCH
pg
ps
pr
pb
Transmission cost of packet delivery between GGSN and SGSG
Transmission cost of packet delivery between SGSN and RNC
Transmission cost of packet delivery between RNC and Node B
Transmission cost of packet delivery over the air with DCHs
Transmission cost of packet delivery over the air with HS-DSCH
Processing cost of packet delivery at GGSN
Processing cost of packet delivery at SGSN
Processing cost of packet delivery at RNC
Processing cost of packet delivery at Node B

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A Multicast Approach for UMTS: A Performance Study 1089
The total number of the multicast UEs in the network is denoted by NUE. For the
cost analysis, we define the total packets per multicast session as Np. Furthermore,
network operators will typically deploy an IP backbone network between the GGSN,
SGSN and RNC. Therefore, the links between these nodes will consist of more than
one hop. Additionally, the distance between the RNC and Node B consists of a single
hop (lrb = 1). In the presented analysis we assume that the distance between GGSN
and SGSN is lgs hops, while the distance between the SGSN and RNC is lsr hops.
In multicast, the SGSN and the RNC forward a single copy of each multicast
packet to those RNCs or Node Bs respectively that serve multicast users. After the
correct multicast packet reception at the Node Bs that serve multicast users, the Node
Bs transmit the multicast packets to the multicast users via Dedicated or High Speed
Shared Transport Channels. The total cost for the multicast scheme is derived from
the following equation where nSGSN, nRNC and nNODEB represent the number of SGSNs,
RNCs and Node Bs respectively serving multicast users. The parameter X represents
the multicast cost for the transmission of the multicast data over the air.
(
,
,
HS DSCHNODEB
Dn ifchannel
−
⋅=
⎩
)
()()
SgSGSN
n
gssRNCsrrNODEBrbbp
DCHUE
pDpnDpnDpN
MX
DN ifchannelDCH
HS
X
DSCH
=++++
=
++
⎡
⎣
⎤
⎦
+
⋅
⎧
=⎨
−

(3)

(4)
Having analyzed the costs of the multicast scheme, we try to evaluate the cost in
function of a number of parameters. The first parameter is the number of the total
packets per multicast session (Np) and the second one is the number of the multicast
users (NUE). We assume a more general network configuration than that illustrated in
Fig. 1, with NSGSN =10, Nra =10, Nrnc =5, Nura =5 and Nnodeb =5.
As we can observe from the equations in the previous section, the cost of the
scheme depends on a number of other parameters. Thus, we have to estimate the
value of these parameters appropriately, taking into consideration the relations be-
tween them. The chosen values of the parameters are presented in Table 1.
Table 1. Chosen parameters’ values
Dgs Dsr Drb
36 18
pg
1
ps
1
pr
1
pb
1
DDCH DHS-DSCH
5
lgs
6
lsr
3
lrb
1 6 3
In our analysis, the values for the transmission costs of the packet delivery over the
air with each of the two transport channels are different. More specifically, the trans-
mission cost over the air with DCHs, is greater than the cost of the packet delivery
over the air with HS-DSCH. Therefore, we define the following probabilities for the
calculation of the number of the UMTS nodes that serve multicast users:
PSGSN : The probability that an SGSN serve multicast users
The probability that an RNC (served by an SGSN with multicast users),
serves multicast users
The probability that a Nobe B (served by an RNC with multicast users),
serves multicast users
PRNC:
PNODEB:

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1090 A. Alexiou, D. Antonellis, and C. Bouras
For the cost analysis, we assume that PSGSN=0.4, PRNC=0.3 and PNODEB=0.4. Con-
sequently, the number of the SGSNs, the RNCs and the Node Bs that serve multicast
users are nSGSN = NSGSN PSGSN = 4, nRNC = NRNC PSGSN PRNC = 60 and nNODEB = NNODEB
PSGSN PRNC PNODEB = 600 respectively.
Fig. 2a presents the cost of the multicast scheme in function of the Np for different
transport channels (DCH and HS-DSCH) used for the transmission of the multicast
data over the air. The y-axis presents the total cost of the multicast scheme, while the
x-axis shows the total packets per multicast session. Regarding the use of DCHs, in
Fig. 2a, we have calculated the costs for three different values of the number of multi-
cast users, indicating that the multicast cost increases rapidly when the amount of the
multicast data increases. Furthermore, for a given Np, the multicast cost increases as
the members of the multicast group increase. This occurs because the greater the
number of multicast users is, the greater the number of DCHs needed for the trans-
mission of the multicast data over the air. Additionally, eqn (3) shows that in case of
HS-DSCH, the cost of the multicast scheme depends only on the number of packets
per multicast session. This can be shown in Fig. 2a where we can observe that the
greater the Np is, the greater the multicast cost becomes.
Another interesting parameter is the PNODEB. Assuming that NUE=1500, Np=500,
we can calculate the cost for the multicast scheme, for the transport channels we use.
Fig. 2b presents the cost of the multicast scheme in function of PNODEB for different
transport channels. It is obvious from Fig. 2b that the cost of the multicast scheme is
decreased as PNODEB converges to zero. This means that the greater the number of


a) b)
Fig. 2. Costs of the multicast scheme against Np and PNODEB for different transport channels

a) b)
Fig. 3. Costs of the multicast scheme against NUE using different transport channels