Side_1_360

set-up to a specific radio bearer is not part of the

standards and is left for the implementation and

deployment. We illustrate in Table 5 one simple

algorithm taken from [22]. The Iu bearer is set-

up after the radio bearer has been established.

3.6 QoS and Handovers

3.6.1 UMTS Handovers

Most wireless systems need mechanisms to sup-

port situations where radio connections allocated

to mobiles moved out of reach of the radio

transceiver they initiated the communication ses-

sion from. To be able to keep the session alive it

has to be taken over by another radio transceiver.

This switching of radio transceiver is denoted

handover. In UMTS there are two fundamentally

different ways of conducting handovers – hard

handover and soft handover. With hard handover

the communication session is broken when a

mobile leaves a radio transceiver and established

again at the new radio transceiver. This switch-

ing of radio transceiver introduces a slight drop

of connection and possible drop of packets. The

user may hear it as a clip in a speech conversa-

tion. In UMTS this handover mode is compara-

ble with that of GSM today regarding QoS. The

radio system of UMTS however is especially

constructed to perform soft handover with macro

diversity. With soft handover the mobile can

send or receive on up to several radio channels

simultaneously thereby increasing the QoS per-

formance, however at the cost of used band-

width. Figure 3-7 shows a mobile receiving data

from two Node Bs, connected to the same RNC,

simultaneously.

This handover mechanism enhances the QoS,

especially for voice and real-time services, since

the session will always be on. The RNC receives

the data from both Node Bs and based on differ-

ent QoS criteria chooses the best one to forward

to the remote host. This mechanism is denoted

macro diversity. Macro diversity may be used

in both uplink and downlink. In uplink it is the

RNC that chooses the best traffic stream, in

downlink it is the mobile itself that selects the

best traffic stream. Figure 3-7 illustrates a soft

handover case.

The handover procedures are transparent to the

Core network except in the case of a terminal

moving from one RNC to another. In that case

the Core network may have to re-establish the

bearers to the new RNC. This procedure called

SRNC relocation first establishes bearers on the

new path before the switching is accomplished.

It uses the make before break concept allowing

faster switching of routes and thus less disrup-

tion.

3.6.2 Handover to Legacy Systems

UMTS was designed to allow smooth deploy-

ment. A typical scenario for the first years of

UMTS deployment is that UMTS covers a lim-

ited number of cities while GPRS/GSM is nation

wide. In that context users would like to seam-

lessly roam between the two accesses. Handover

between UMTS and GPRS/GSM is thus an

important feature.

To allow easy interworking the provisioning of

QoS in GPRS was aligned with UMTS for the

release 99 of GPRS. GPRS supports only two

QoS classes, the Background class and the Inter-

active class. In case of handover between UMTS

and GPRS QoS renegotiation may occur. This

allows the user/application to decide whether it

can accept lower Quality of Service. If it cannot

the session will be broken.

Figure 3-7 Soft handover in
UMTS

BS A BS B

UTRAN

Network

BS A channel

information

Transmission

channel and

Toffset

Measure Toffset

Handover

command and Toffset

Toffset

PCCCH

frame

PDCH/PCCH

frame