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846 WIRELESSINTERNETconsequently reduced leasing costs. A radio frame is
a short data segment coded/decoded and transmit-
ted/received by the base station. These radio frames must
be delivered from RNC to BS and vice versa in a timely
fashion with limited delay. Otherwise, the BS or RNC
will discard them. Due to the time constraints on the de-
livery of radio frames the majority of the traffic in an
IP-based RAN can be considered to be real-time traf-
fic. Seen from the architecture of a RAN and the nature
of the transported data, the IP-based RAN has differ-
ent characteristics when compared to traditional IP-
networks. Typically, the wireline transmission in a radio-
break access network contains a relatively high volume
of leased lines. The fact that thousands of radio BSs are
spread over a wide geographical area and are in general
situated at large distances from the backbone typically
results in high cost for the transmission links. Further,
the majority of the traffic transported on the wireline
transmission links used by the RAN is radio frames.
This means that the traffic is very sensitive to delays
and delay variation (jitter). Deploying resource manage-
ment schemes in this environment is therefore essential.
The introduction of IP-based transport in the RAN in-
dicates that an IP QoS-capable domain will have to be
managed in the radio access network. Currently, DiffServ
(Blake et al., 1998) as a scalable IP QoS architecture is the
favorite one to be used in an IP-based RAN. The scalabil-
ity is achieved by offering services on an aggregate basis
rather than per flow and by forcing as much of the per-flow
state as possible to the edges of the network, that is, to the
edge nodes. In order to allow for dynamic resource man-
agement in DiffServ, an extension, called RMD (resource
management in DiffServ), has been proposed (Heijenk,
Karagiannis, Rexhepi, & Westberg, 2001). RMD extends
the DiffServ architecture with new reservation concepts
and features, such that the IP-based RAN resource man-
agement requirements are met.
These trends will lead to a cellular network architec-
ture, where all nodes are interconnected using an QoS-
enabled IP-based transport network. On top of this IP
protocol, protocols for cellular specific functions, related
to mobility and radio, will be running. Further, an end-
to-end IP protocol will run on top of this to enable end-
users to use IP-based services, and to connect to other
end-users.
Currently, IP-based services over cellular networks are
mainly best-effort type of services, both interactive, such
as Web browsing, and background, such as e-mail down-
loading. For the near future, IP-based services might be
extended with more streaming type of services for audio
and video. A situation where all services, including con-
versational services, in a cellular network are IP-based is
somewhat further away. Prerequisites for such a situation
are very efficient header compression techniques, and a
migration to a cellular network architecture where also
signaling is IP-based, e.g., based on SIP. Efforts in these
directions are being made in both research and standard-
ization.
Cellular networks provide wide-area coverage for
mobile users at moderate data rates. Besides cellular
networks, other wireless systems are gaining popularity,
in particular WLAN and short-range technologies. WLANsystems provide wireless ethernet extension to notebooks.
WLANs based on the IEEE 802.11b standard (Institute of
Electrical and Electronics Engineers, 1999) and operating
in the 2.4-GHz ISM band are widely available in the mar-
ket. They offer data rates up to 11 Mbps and have a range
of 50 to 300 m. New products operating in the 2.4-GHz
ISM and 5-GHz band offering data rates up to 54 Mbps
are starting to appear on the market. These systems
have been primarily designed for nomadic applications
and, consequently, their support for mobility is very
poor.
Short-range technologies, e.g., Bluetooth, have a more
limited coverage, support lower data rates, and consume
less power than WLANs (Haartsen, 1998). Operating in
the ISM band, these technologies originally designed to
replace the cabling connecting peripherals and other de-
vices are also suitable for inexpensive communication be-
tween portable devices.
The above-mentioned technologies have been opti-
mized for different applications. Even as they evolve, it is
not expected that they will be replaced by a common mul-
tipurpose technology in the future. On the contrary, it is
expected that the next generation wireless systems will in-
tegrate different and complementary technologies. Wire-
less devices able to operate with different radio interfaces
will access the communication facilities using the “always
best connected” paradigm. This means that a wireless de-
vice that has the choice will use that radio interface that
it deems most appropriate for its purposes, e.g., highest
performance or lowest cost.
In the future, a person might use multiple personal de-
vices, such as laptop, phone, and organizer, that are mu-
tually interconnected, forming a PAN (see Figure 5) using,
for example, Bluetooth technology. These devices will all
have one or more wireless interfaces. At a certain mo-
ment, the person might use his laptop and be connected to
his company network via a wireless LAN interface. When
moving out of the office, he may want to stay connected
using a Bluetooth link between his laptop and phone,
where his phone will act as an intermediate hop, via a
UMTS link to the fixed network. In this scenario, the PAN
acts as a moving network with multiple interfaces, which
moves along different wireless APs to the fixed network,
and which may merge with another moving network, e.g.,
the network in a vehicle.
The challenge in these scenarios is to achieveseam-
less integration, meaning that from the point of view of
the application, switching from one network technology
to the other is imperceptible and the level of security is
maintained. This requires measures to be taken at differ-
ent levels of the protocol stacks. Suitable techniques for
supporting mobility and smooth transitions between dif-
ferent technologies and systems as well as in between pri-
vate and public networks are under study (Wireless World
Research Forum, 2001).
Even as wide deployment of 3G has been experienc-
ing delays, 4G wireless technology is in an active research
stage. 4G is intended to provide much of what 3G had orig-
inally envisioned, i.e., a broadband cellular service provid-
ing high-speed capacity at low cost, along with IP-based
applications and services. Data rates of up to 20 Mbps are
targeted, even as the MS moves at up to 200 km/h, and