Side_1_360

The GGSN is the node connecting the UMTS

network to external IP networks. It contains

routing information to reach attached users and

interfaces various control nodes, e.g. to support

authentication and find the location of mobile

users. The routing information is used to tunnel

data packets to the MS’s current point of attach-

ment, i.e. the SGSN. The GGSN is also respon-

sible for allocating IP addresses to the terminals.

The SGSNs and GGSNs of an operator are con-

nected together by an IP network. Packets are

tunnelled between those nodes using the GPRS

tunnelling protocol (GTP) which runs over the

IP protocol layer. DiffServ is used to provide

QoS differentiation within this IP network. The

HSS is connected to the SGSNs by an SS7 net-

work.

2.2 UTRAN

The access network in UMTS, the UTRAN

(UMTS Terrestrial Radio Access Network)

[8–21], is the new revolutionary part from

GSM/2+ supporting a data-rate of up to 2 Mb/s.

This data rate is achieved in UMTS by deploying

W-CDMA(Wideband-Code Division Multiple

Access) technology deploying the 2.2 – 2.4 GHz

frequency band. With this new radio technology

all services share a common radio resource

deploying spread spectrum technology with a

“usage on demand” approach. This is spectrum

efficient but challenging regarding support of

QoS. Since it is a common resource the band-

width for each user may vary. Apart from the

number of simultaneous users the data rate is

dependent on the users’ distance from a base

transceiver, the velocity of the users as well as

different air disturbances as random noise,

weather condition, etc.

The UTRAN (Figure 2-2) is comprised of two

distinct nodes, the Radio Network Controller

(RNC) and the Node B. The Node B is the radio

transceiver station in UMTS. Each node B may

have several radio transceivers and each RNC

may control up to 64 Node Bs. The RNC is in

charge of the radio resources and allocates radio

channels to different types of traffic according to

their demand of resources and service require-

ments.

The physical transport varies across the different

interfaces. Across the Iub interface (ref. Figure

2-2) AAL2/ATM is deployed to transport radio

frames between Node B and RNC. No distinc-

tion is made between data or real time traffic

across the Iub interface. Since there is no differ-

entiation of traffic types across the Iub interface

only one ATM QoS class may be deployed. At

the RNC the radio frames are reassembled and

transported to the SGSN. In handover cases

where a mobile moves out of reach from the

serving RNC over to another RNC’s control

domain traffic is routed across the Iur interface.

This interface therefore ties two RNCs together

and thereby avoids including the Core network

in these handover cases. This optimises the QoS

since only a redirection handling is needed to

maintain the connection. The “old” RNC func-

tions as the anchor RNC and still controls the

session/call. Also across the Iur interface AAL2/

ATM is used for transport of both data and real

time traffic. Across the Iu interface, i.e. the inter-

face between the RNC and the MSC and the

SGSN, data traffic is transported on IP over

AAL5/ATM. Circuit switched traffic is trans-

ported directly on AAL2/ATM to/from the

3G-MSC.

As seen, the UTRAN heavily deploys ATM as

the underlying transport both for packet data and

circuit switched data. Several arguments have

been put forward regarding the use of ATM in

the RAN. People claiming that the bandwidth in

the RAN would be scarce argue that ATM is the

most flexible and best suited technology to sup-

port service differentiation and optimal usage of

bandwidth (at least on the Iub). They also argue

that by specifying ATM under IP, it puts the

operators in a position to choose to use ATM

QoS mechanisms or IP based QoS. Today ATM

is used in access networks, but it is foreseen that

with IP based QoS mechanisms ATM may be

obsolete in a couple of years. Much work is

therefore conducted in 3GPP to look into different

possibilities for usage of IP based protocols in

the RAN to try to substitute the ATM protocol

stacks and still support the QoS requirements.

2.3 The IP Multimedia Subsystem

The IP Multimedia Subsystem [3] gives better

support for value added services such as multi-

media, multimedia messaging, global text tele-

phony, push services, etc. Many new functional

entities compose this system. Most of them are

related to call control and service control for

multimedia sessions.

Figure 2-2 The UTRAN
reference architecture

Node B

Node B

Node B

Node B

RNC

RNC

3G SGSN

3G MSC

Iub Iu

Iur