Wireless WAN Systems
Most wireless WANs are cellular based, but some make use of
space. Take a closer look at both of these.
Cellular-Based Wireless WANs
As shown in Figure 7-6,
a cellular system consists of cell towers, concentrators, voice switches, and
data gateways. The cell tower receives signals from user devices and transmits
information back to the user. The voice switch connects the user device to
another wireless, or wired, user through the telephone distribution system. This
part of the system supports customary phones calls between users.
The component that makes the system a wireless WAN is the data
gateway. In this case, the gateway is able to interface with data protocols in a
way that makes it possible for users to surf the Internet, send and receive
e-mails, and utilize corporate applications.
Text messaging is a popular application of cellular-based
wireless WANs. Users converse by typing in short text messages and sending them
to other users, similar to instant-messenger applications available for PCs.
With smaller wireless WAN devices, however, it's important that users can save
canned messages such as, "I'm traveling today and I'll call you later," which
can be sent at the press of single button. Some wireless WAN devices also
capture digital pictures and video that is sent across the network.
First-Generation Cellular
When mobile phones first became available, wireless
communications used only analog signals. This initial cell phone system is known
as first-generation cellular (1G cellular). When someone speaks
through a 1G system, his voice is sent using frequency modulation (FM), which
merely changes the frequency of carrier wave according to the audio signal. 1G
systems make use of a limited number of channels that use FSK to send control
signals necessary to set up and maintain the calls.
1G systems work well for voice phone calls, despite occasional
crackles and pops, but they are not sufficient for sending computer data. As
with the voice, analog signals must represent data. Users must interface PCs to
the cellular system using a modem that converts the digital signals from the
computing device into an analog form (such as FSK or PSK) that is suitable for
transmission through a small, 4-KHz voice channel. This results in slow 20- to
30-kbps data rates.
1G systems also lack capacity to support an authentication and
encryption mechanism. The digital FSK control channel only has enough capacity
to support telephone calls. There is not enough room for sending usernames and
passwords to an authentication service or coordinating encryption processes.
It's quite apparent that 1G cellular was designed to carry voice, not data.
1G systems at one time covered most of the U.S. Today, however,
they exist only in areas having low population density, where it's not feasible
to upgrade the infrastructure to newer digital systems.
Second-Generation Cellular
Not too long ago, digital cellular became available, allowing
both the voice and control channels to make use of digital signing. The first
phase of this totally digital system is referred to as second-generation
cellular (2G cellular). Most of the
telecommunications operators today have 2G systems, with various enhancements
occurring periodically.
The use of digital signaling for the voice channels allows for
more efficient modulation. This makes it possible to support more phone calls
and data over a lower frequency spectrum. In fact, 2G systems enable enhanced
services—such as short messaging, authentication, and phone software updates—to
be accessed wirelessly.
Enhanced versions of 2G systems (sometimes referred to as 2.5G)
include even better modulation, which increases data rates and spectrum
efficiency. For example, the General Packet Radio Service (GPRS) offers
high-speed data rates over a global system for mobile communications (GSM)
network. Maximum data rates over GPRS are 171.2 kbps. The use of GPRS, however,
requires a specialized mobile phone. Also, the Enhanced Data Rate for Global
Evolution (EDGE) enhances GSM using 8-level PSK, where each transmitted symbol
represents 3 data bits. This results in a maximum data rate of 474 kbps.
Third-Generation Cellular
Many of the telecommunications operators are now beginning to
offer what's known as third-generation cellular (3G
cellular), with even better support for data communications. The
Universal Mobile Telecommunications System (UMTS) is capable of 2-Mbps data
rates for in-building implementations, up to 384 kbps in urban areas, and 144
kbps in rural areas. As a result, 3G is able to support multimedia
applications.
There has been considerable argument in the wireless industry
on whether 3G will replace 802.11 (Wi-Fi) wireless LAN technology. With higher
data rates for indoor use, 3G is an alternative to wireless LANs. 802.11
continues, however, to have performance upgrades that significantly exceed 3G.
For example, the 802.11a standard specifies data rates of 54 Mbps, which is much
higher than 3G. Also, wireless LANs are much less expensive to deploy.
Wireless LANs, however, are not practical for providing
coverage over wide areas. There would be too much infrastructure. 3G makes use
of existing cell tower sites and distribution systems. Expenses of modifying 1G
and 2G cellular systems to 3G are still high, but it's the most feasible method
for providing wireless networking over wide areas.
Thus, both 3G and wireless LAN systems complement each other.
This has prompted standards groups and manufacturers to find ways to seamlessly
integrate 3G and wireless LANs. In fact, mobile phones and PDAs are available
today that implement both technologies. With this capability, a user can roam
outside the range of a wireless LAN and automatically associate with a cellular
system. The problem is that standards that define this form of roaming are not
yet available, which requires the user to carefully choose service providers
that support the phone or PDA of choice.
|
One of the most common services for wireless WANs is short
message service (SMS), which is a text messaging system capable of sending a
couple hundred characters at a time. SMS is a wireless form of the familiar
instant messaging that is available from many of the ISPs. The following are
additional applications of SMS for use with wireless WANs:
-
Content delivery— SMS enables
the efficient delivery of updates for user devices. For example, a user can
download new ring tones and fancy backgrounds to her phone over SMS. In
addition, SMS makes it possible for users to query databases and news feeds. For
example, you can keep up with the latest breaking news by receiving instant
updates through SMS.
-
Alerts— Most operators offer a
variety of alerts, such as voicemail waiting, sports scores, current stock
prices, and other reminders. This allows you to receive updates when something
that you define as important occurs.
-
Interaction— Some television
shows allow interactivity among viewers and hosts through SMS. This enables
participation of the audience, which dramatically increases the viewing audience
size
-
Application integration— It's
feasible for developers to integrate SMS into many corporate applications. For
example, a company can have a sales management system that enables sales
representatives to track customers and products. A company can usually easily
add SMS to these applications. For example, the addition of an alert mechanism
would be beneficial. Sales representatives would be notified that a product has
gone on sale.
Many web sites use Wireless Markup Language (WML) to transform
regular web pages into a format that is more easily read on a small device, such
as a PDA or cell phone. WML also reduces the graphics on the page to compensate
for the slower data rates of wireless WAN technologies. |
Space-Based Wireless WANs
In addition to the land-based cellular systems, the use of
space-based systems provides a means for networking users over wide areas.
Satellites
The use of satellites for broadcasting television and other
communications has been around for several decades. Not until recently, however,
did satellite systems provide users with connections to the Internet. (See Figure 7-7.) Data rates are appreciable,
with up to 1.5-Mbps downloads.
Some satellite systems support two-way exchange of data,
allowing a user to send data up to the satellite (and vice versa). For example,
a user's mobile device can transmit a web page request up to the satellite, and
the satellite retransmits it down to the appropriate Earth station. The Earth
station then sends the web page through the satellite and back down to the user.
Other satellite systems, however, only support a downlink. A user's device must
request the web page through another network, such as a telephone link, and the
satellite broadcasts the page to the user.
By incorporating active radio repeaters in man-made,
Earth-orbiting satellites, it is possible to provide broadcast and
point-to-point communications over large areas of the Earth's surface. The
broadcast capability of the satellite repeater is unique and, by suitable
selection of satellite antenna patterns, it can be arranged to cover a
well-defined area.
Satellites are located at various points in the geostationary
orbit depending on the system mission requirement. To obtain global coverage, a
minimum of three satellites is required. To obtain reasonably constant RF signal
levels, however, four satellites are employed. This also provides some freedom
in positioning.
With satellite communications, favorable frequencies are used:
power efficiency, minimal propagation distortions, and minimal susceptibility to
noise and interference. Unfortunately, terrestrial systems tend to favor these
frequencies as well. Space is an international domain, and the International
Telecommunications Union (ITU) controls satellite frequencies.
The band of frequencies between 450 MHz and 20 GHz is the most
suitable for an Earth-space-Earth radio link. It is not practical to establish
links to an Earth terminal located in a climatic region of heavy rainfall at
frequencies higher than 20 GHz if consistent availability is expected.
For all operating bands, the lowest-frequency spectrum is used
for the downlink because it has the most severe power constraints. Lower
frequencies are less sensitive to free-space attenuation when compared to the
higher-uplink frequencies. Losses are easier to overcome in the uplink with the
higher transmit power available at the Earth station.
The satellite acts as a signal repeater. Signals sent to it on
the uplink are rebroadcast back to Earth on the downlink. The device that
handles this action is referred as a transponder. The satellite transponder is
analogous to a repeater in a terrestrial communications link; it must receive,
amplify, and retransmit signals from Earth terminals. A satellite transponder is
capable of acting as a transponder for one or more RF communications links.
Low-altitude satellites, which can have circular, polar, or
inclined orbits, have orbital periods of fewer than 24 hours. Therefore, they
appear to move when seen from the Earth's surface. These orbits are useful for
surveillance purposes, and can be used to provide communications at extreme
north and south latitudes.
One type of special interest to public data communications is
the geostationary orbit. A satellite in such an orbit has a 24-hour period at an
altitude of 22,300 miles and remains over a fixed location on the equator. As a
result, the satellite appears motionless to an observer on Earth.
Actually, the satellite does not remain truly fixed. Even if
the orbit were perfectly circular and at precisely the right altitude, natural
phenomena (because of low-level lunar and planetary-gravitational fields and
solar-radiation pressure) introduce slight drifts in the orbit. This slow and
minor drift is corrected from time-to-time by small onboard thrusters activated
by ground stations.
Because of the long RF path involved (approximately 22,300
statute miles from an Earth terminal to a satellite in geostationary orbit), a
transmission delay of approximately 100 ms is experienced between an Earth
terminal and the satellite. This results in an approximate Earth-to-Earth-delay
of 200 ms. This causes the system to be inefficient for use with protocols, such
as 802.11, that require a response after each packet of information is
transmitted before transmitting the next packet. In fact, most networking
protocols do not work efficiently over satellite links because the protocols
expect timely acknowledgments from the destination.
Meteor Burst Communications
Billions of tiny microscopic meteors enter the Earth's
atmosphere. In fact, meteors fall often throughout the day over all parts of the
world. As these meteors penetrate the atmosphere, at a high altitude, they
ionize into a gas. This gas is seen as a shooting star, which is an uncommon and
large meteor as compared to most others.
Known as a poor man's satellite system, meteor burst
communications (see Figure 7-8) bounce RF
signals off meteor trails. This enables a long-haul (1,500 mile) wireless- data
transmission link without the expense of launching and maintaining a
satellite.
Meteor burst communications direct a 40 to 50 MHz radio wave—
modulated with a data signal— at the ionized gas. The radio signal then reflects
off the gas and is directed back to Earth. The availability of meteor trails is
good but they are present only often enough to rely on 300 to 2400 bps. This is
extremely slow, even compared to telephone modems.
However, the cost of deploying meteor burst equipment is so low
compared to satellite systems that low-performance applications, such as
telemetry, are feasible. Meteor burst, for example, works well for transmitting
snow levels from remote mountainous areas to monitoring centers.
Sections
