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Access Stratum


User Radio Access Core
Equipment Network Network

Uu Iu

Figure 6.6 UMTS general architecture

have different quality of service (QoS) characteristics, such as different reliabilities. A
common example of this is that different classes of bits with different bit error rates can
be realized as different RAB sub¬‚ows. RAB sub¬‚ows are established and released at
the time the RAB is established and released, and are delivered together over the same
transport bearer.
A radio link is de¬ned as a logical association between a single UE and a single
UTRAN access point, such as an RNC. It is physically comprised of one or more radio
bearers and should not be confused with a RAB.
Looking within the UTRAN, the general architecture model is as shown in Figure 6.7(a).
Now shown are the Node B or BTS and RNC components, and their respective internal
interfaces. The UTRAN is subdivided into blocks referred to as radio network subsystems
(RNS), where each RNS consists of one controlling RNC (CRNC) and all the BTSs under
its control. Unique to UMTS is the interface between RNSs, the Iur interface, which plays
a key role in handover procedures. The interface between the BTS and the RNC is the
Iub interface.
All of the ˜I™ interfaces “ Iu, Iur and Iub “ currently1 use ATM as a transport layer. In
the context of ATM, the BTS is seen as a host accessing an ATM network, within which
the RNC is an ATM switch. Therefore, the Iub is a user-to-network interface (UNI),
whereas the Iu and Iur interfaces are considered to be network-to-network interfaces
(NNI), as illustrated in Figure 6.7(b).
This distinction is because the BTS to RNC link is a point-to-point connection in that
a BTS or RNC will only communicate with the RNC or BTS directly connected to it,
and will not require communication beyond that element to another network element.

1
UMTS release 5 provisions for the use of IP as a RAN transport protocol.
272 UNIVERSAL MOBILE TELECOMMUNICATIONS SYSTEM



Radio Network Subsystem (RNS)



BTS

RNC

BTS
Iub


Iur Core
Network

BTS
UNI
RNC RNC
BTS

BTS Core
Iub NNI Network

Radio Network Subsystem (RNS)
RNC
UMTS Terrestrial Radio Access
Network (UTRAN) Iu
(a) (b)

Figure 6.7 UTRAN architecture

For each user connection to the core network, there is only one RNC, which maintains
the link between the UE and core network domain, as highlighted in Figure 6.8. This
RNC is referred to as the serving RNC (SRNC). That SRNC plus the BTSs under its
control is then referred to as the SRNS. This is a logical de¬nition with reference to that
UE only. In an RNS, the RNC that controls a BTS is known as the controlling RNC
(CRNC). This is with reference to the BTS, cells under its control and all the common
and shared channels within.
As the UE moves, it may perform a soft or hard handover to another cell. In the case
of a soft handover, the SRNC will activate the new connection to the new BTS. Should
the new BTS be under the control of another RNC, the SRNC will also alert this new
RNC to activate a connection along the Iur interface. The UE now has two links, one
directly to the SRNC, and the second through the new RNC along the Iur interface. In this
case, this new RNC is logically referred to as a drift RNC (DRNC) (Figure 6.8). It is not
involved in any processing of the call and merely relays it to the SRNC for connection
to the core. In summary, SRNC and DRNC are usually associated with the UE and the
CRNC is associated with the BTS. Since these are logical functions it is normal practice
that a single RNC is capable of dealing with all these functions.
A situation may arise where a UE is connected to a BTS for which the SRNC is not
the CRNC for that BTS. In that situation, the network may invoke the serving RNC
relocation procedure to move the core network connection. This process is described in
Section 6.19.
6.5 UMTS QOS CLASSES 273




SRNS



BTS

SRNC


BTS
Iub

Core
Iur
Network

UE

BTS

DRNC


BTS
Iub


DRNS

Iu

Figure 6.8 Serving and drift RNC for soft handover


6.5 UMTS QOS CLASSES
QoS has been de¬ned by the ITU-T as ˜the collective effect of service performance,
which determines the degree of satisfaction of a user of a service™. QoS is associated
with the user experience; the user is not concerned with how a service is provided
but only whether or not they are satis¬ed with that service. So, from a user™s point
of view the QoS is a subjective matter and if the network does not perform adequately,
the user may decide not to use a particular service or look around for another mobile
network operator offering the same service with a better QoS. From the mobile net-
work operator™s point of view, this QoS requires technical analysis where the operator
has to overcome certain implementation challenges within a cost constraint. 3G is the
convergence of circuit switched networks such as GSM and the IP packet networks.
A brief description detailing the state of QoS in these two different types of network
now follows.
For UMTS, four different QoS classes are de¬ned as well as an expected standard set
of bit rates. These are summarized in Tables 6.1 and 6.2.
274 UNIVERSAL MOBILE TELECOMMUNICATIONS SYSTEM


Table 6.1 UMTS QoS classes
Name Delay Buffering Mode Bit rate
Conversational Minimal ¬xed None Symmetric Guaranteed
Streaming Minimal variable Allowed Asymmetric Guaranteed
Interactive Moderate variable Allowed Asymmetric Not guaranteed
Background Large variable Allowed Asymmetric Not guaranteed


Table 6.2 UMTS bit rates
Bit rate Comment Coverage type
144 kbps (basic) Peak rate for packet transfer Rural/suburban, fast moving
vehicles, outdoor
384 kbps (extended) Peak rate for packet transfer Urban, moving vehicles, outdoor
2 Mbps (hotspot) Peak rate for packet transfer Urban centre, walking speeds,
indoor


For real-time traf¬c, only the conversational and streaming classes will be relevant
since the remaining classes provide little QoS in terms of delay and bandwidth provision.
GSM networks have traditionally measured QoS in a network™s busy hour call blocking
probability, call drop rates, call setup delay and voice quality. Cost constraints limit the
number of cells and the amount of transmission links to the base station controllers (BSCs)
and BTSs. Once the call is registered, the user and mobile device are authenticated and
checked to see if they are authorized to make a particular call. Once these checks have
been completed successfully then the call goes through. This is not instantaneous and
may take 5 or 10 seconds before the ringing tone is heard. Once the call goes through,
the parties have a time slot for themselves over the air interface, a TDM slot between
the BTS and the transcoder and rate adaptation unit (TRAU) and usually a 64 kbps link
through the MSC and across the external network. There is little need for buffering in the
network. If a user service on the mobile device wishes to send data more quickly then it
is buffered in the mobile device. Thus there is little jitter and delays are comparatively
constant. The resources were reserved at call setup and are used only by the called and
calling parties.
The 3G network has a variety of services to offer, each of which requires its own
distinct QoS provisions. A multimedia call may consist of voice, video and whiteboard
connections, each of which requires its own QoS.
Within the scope of 3G, QoS is a requirement for many end-to-end communications.
It can be seen in Figure 6.9, that for this end-to-end QoS, the QoS has to be guaranteed
across a number of different areas. For the voice call, the PSTN or ISDN will guarantee
a certain quality of service since at call setup the resources are provided and set aside for
this call for the duration. However, it must also be noted that this call setup does require
time, especially for an international call to a mobile device. This can have an adverse
effect on the user perception of the quality of a call. One aspect of QoS which is a simple
concept is that of delay. Once the call is established over the ISDN, the delay will be
almost constant at approximately 20 ms, which is virtually unnoticeable by the calling
6.5 UMTS QOS CLASSES 275



Voice call over the circuit switched network
PSTN/ISDN/
CSPDN
RAN Core Network Telephone

Circuit Core
Internet

WebSite
Packet Core
UE
RNC
WBTS
Private IP
network
HTTP request via the packet switched network
Telephone
Voice over IP call via the packet switched network

Figure 6.9 QoS considerations

parties. The end-to-end delay, however, takes into consideration:
Delay over ISDN + Delay over core network + Delay over RAN + Delay over air
+ Processing time in mobile device, and others + Buffering = Total delay
Although on the surface, delay is one of the more simple aspects of QoS to quantify, it
is important to also take into consideration retransmissions (if data is retransmitted). The
air interface works at the speed of light and thus on initial inspection seems to have little
effect on the overall delay. However, this is where most of the interference takes place
and there may be a number of retransmissions. How long should one wait for a packet?
If the overall delay of the connection is more than 300 ms then for a voice call, this will
most likely be noticed by the user.
Examining an IP network, it can be seen that the delay for browsing a web page is not
so important. A user will already tolerate delays of the order of 10 seconds or more for
a web page over a current dial-up connection. With the higher data rates available with
UTMS, it is anticipated that a page could be downloaded in about 4 seconds. However,
the design goal is to reduce this to 1 or 2 seconds.
QoS is provided in the UMTS network via bearer services. Figure 6.10 shows how the
end-to-end service depends on the services below it. The UTRAN consists of the BTS
and the RNC, the core network (CN) edge device will be the 3G MSC for circuit switched
and SGSN for packet switched and the CN gateway will be the gateway-MSC for circuit
switched and the GGSN for the packet switched network.
It can be seen for end-to-end QoS, there is a requirement that both the UMTS network
and external bearer services provide QoS. In fact, if a user is working on a laptop which
is connected to the mobile device then QoS is also needed between these devices (known
as the local bearer service).
The RAB requires that there is a guarantee of service over the WCDMA air interface
and also across the UTRAN. The air interface QoS for a particular mobile device is
related to the power it transmits compared to the surrounding noise. The RNC provides
the bearer control over the UTRAN.
276 UNIVERSAL MOBILE TELECOMMUNICATIONS SYSTEM


End to End Bearer Service

UMTS Bearer Service External Bearer Service

Radio Access Bearer Service CN Bearer Service

Radio Bearer Service Iu Bearer Service Backbone Bearer Service

WCDMA Bearer UTRA Bearer Service Physical Bearer Service BBone Physical Bearer Service
Service
Circuit Core


ISDN
Iu-CS
Iub
3G MSC G-MSC
Iu-PS

Internet

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