Technical Aspects of a Wireless Infrastructure

Technical Aspects of a Wireless Infrastructure
Next, we consider important issues that must be addressed in the design and operation
of the wireless network infrastructure. These issues are often considered under the
heading network engineering, as they are issues concerning the design and operation
of the network as a whole.
Network Deployment Planning. In the preceding section we briefly discussed traffic
engineering as a key element in network planning. With a traffic model, both temporal
and geographic, as a starting point, the network engineer can begin to plan the layout
of the access points or cell sites that will carry the wireless traffic to and from the
fixed wired backbone network. This aspect of network planning is typically performed
with the aid of signal coverage prediction models.
Signal coverage prediction models, usually based on a combination of radio-wave
propagation theory and experimental measurements, provide the designer with a means
of estimating the optimum placement of access points or cell sites for covering the
intended area of user terminals with acceptable signal quality. A tutorial description
of signal coverage in a wireless network with multiple access points or cell sites will
typically illustrate signal coverage with a diagram of abutting hexagons or perhaps
circles with some overlapping coverage areas. Those are both highly idealized descriptions
that do not accurately represent the real world of wireless signal propagation and coverage. Even in a relatively benign office layout, planning for a WLAN installation
must take account of the types and locations of office furniture and equipment,
office partitions, walls, doorways, and so on, all of which can affect signal coverage.
In a factory setting, even more complex situations might be encountered, with various
metal surfaces, manufacturing machinery, and so on, all affecting signal propagation
throughout the building. In the case of cellular telephone networks covering large
service areas, signal coverage prediction models must take account of a wide range
of factors, including terrain type (flat, hilly, mountainous), land use (rural, suburban
development, city high-rise, urban canyon), and special situations such as roadways on
high bridges and over-water propagation paths.
Some of the more sophisticated cellular network planning tools are elaborate software
packages that incorporate time-varying traffic models, population distributions,
cellular antenna types, optional propagation models, and call-handoff models in order
to make accurate estimates of received signal quality for customers situated in various
sectors of a service region. Even very sophisticated network planning tools can provide
only an estimate of network performance, and a network engineer may well also
conduct drive tests in selected regions of the service area in order to verify or refine
computer-generated performance estimates.
Mobility and Location Management. An important requirement that users will place
on wireless networks is mobility, freedom for the wireless user to maintain a reliable
wireless connection while moving about an area that is relevant to the application.
In the case of a WLAN system, users may want the capability to move their wireless
terminals to different locations in an office building, factory, or campus without having
to reregister with the network. Here, users are not likely to move about rapidly, and
the problem is a relatively simple one. However, in the case of WANs such as cellular
telephone networks, mobility is the raison d’etre of the technology and is the principal
differentiator between traditional wired telephone networks and wireless networks.
Users expect to be able to move about freely on foot, by automobile, or even traveling
on trains, while enjoying seamless connectivity for their wireless communication. They
also expect to be able to migrate from one cellular company’s coverage region to
another’s, placing and receiving calls reliably in any region.
In traditional wired telephone networks, the subscriber’s telephone is always wired
to the same central office (CO) switch, and the network directs every incoming call
to the subscriber’s line using his or her telephone number. Outgoing calls are always
made through the same local CO to which the subscriber is permanently connected.
However, in a cellular network, the cell site to which the user connects when receiving
a call depends on the user’s physical location at that moment. In order for a subscriber
to receive a call, the network must determine the cell in which the user is currently
located. This is the essence of the location management problem, and this problem
has been solved in cellular networks by designing location awareness into both the
wireless and wired portions of a wireless network infrastructure.
An important facet of location management is call handoff, the process in which a
user’s call connection is transferred seamlessly from one serving cell to another as the
user moves about the service area. This comes under the heading of what is known
as mobility management in cellular systems. This is accomplished by a combination
of signal strength measurements made in the releasing and the receiving cells, and
coordination of frequency channels in the two cells, typically done under the control of a mobile switching center (MSC). Once again, this calls for a considerable amount
of complexity in the design of the wired and wireless segments of the wireless network
infrastructure.
Related to call handoff is the process of roaming, by which a user who has subscribed
to particular services in his or her home area can travel to another service provider’s
region and use the same services. This feature greatly enhances the value of a wireless
service to a subscriber by lessening geographic restrictions on his or her access to
services. Roaming capabilities in cellular networks have been achieved by cooperation
among service providers and among manufacturers, largely in the venue of standards
bodies. Roaming requires the adoption of a standardized air interface, standardization of
phone-type identification, and cooperation among the operators for transfer of location
data between home and visited networks. Cooperation is also required in administrative
areas such as transfer of calling charges and subscription information.
Radio Resource and Power Management. A characteristic of any wireless network
is that it must operate within a strictly defined spectrum allocation. Radio spectrum
is a limited resource, and regulatory agencies set specific spectrum allocations for
different services. For example, a cellular network operator has a license for 25 MHz
of bandwidth, 12.5 MHz for each direction of full-duplex communication. With a
typical cellular reuse factor of 7, about 3.6 MHz of bandwidth is available for twoway
traffic in each cell, and this bandwidth must be shared among active users in the
cell. The bandwidth available is far less than what would be required if all subscribers
in the network were to demand call connections simultaneously. This is in marked
contrast to a wired telephone network, in which we may always add new subscribers
to the network by installing additional local loops. (To be sure, there is an issue in
equipping a wired telephone network in sizing the central office switches and the longhaul
switches to ensure enough connections through the switches to meet expected call
demand.) Thus, to ensure efficient sharing of the allocated spectrum, RF channels must
be assigned and released dynamically, on a per-call basis.
Furthermore, directing a call from the wired network to a specific mobile subscriber
is not a trivial procedure. Cellular networks reserve a portion of the allocated
bandwidth for control channels, which are utilized in establishing and managing call
connections. Paging messages are transmitted from cell sites, and a paging/response
protocol is used to let the network determine which cell is currently the best one by
which to reach the called subscriber. Only when this location determination has been
made is an RF channel assigned for the call. All of these functions of assigning and
managing the limited number of available RF channels come under the heading of
radio resource management.
Another important element of radio resource management is power management. A
cellular network is designed to operate under interference-limited conditions. That is,
the dominant source of signal degradation in the network is interference from other
active users of the network. With frequency reuse, the signal power radiated from a
given cell is held to a sufficiently low level that the same subset of frequencies can be
used simultaneously in another cell a reuse-separation distance away. In some cellular
networks, power control is performed in both the base stations and the mobile phones.
Power control at both ends of the wireless link helps to hold radiated power to a
level sufficient to maintain good-quality communication without unduly increasing the
overall interference level in the network. Security. Although the use of wireless communications relieves the user of the wired
tether to the public telephone network, with the enormous advantage of freedom of
movement, the wireless medium also makes the user’s communications vulnerable to
eavesdropping and even fraudulent intrusion. In fact, when standards were being developed
for digital wireless networks, a major benefit recognized for digital transmission
was the facility it provided for the implementation of authentication and encryption
techniques. All of the digital wireless interoperability standards have included procedures
for authentication of users entering the network. With respect to the privacy
problem, WLANs utilize spread-spectrum transmission, which has an inherent resistance
to casual eavesdropping. Cellular networks based on CDMA are also using spread
spectrum, providing inherently private transmission. In other cellular networks, such as
GSM, encryption is provided in some operators’ networks as a selectable feature. For
communications that are particularly sensitive, some users may employ applicationlayer
end-to-end encryption and use a wireless data service to carry encrypted traffic
across the network. In this case, the user does not rely on the wireless network to
provide security or privacy.