Wireless Broadband and Other Fixed-Wireless Systems
networkcomputing.com
By Peter Rysavy Our appetite for bandwidth is insatiable. And
now, just as wireline modems are topping out at 56 Kbps and
ISDN service is finally available in most locations, new
technologies, such as DSL (Digital Subscriber Line) and cable
modems that offer transmission speeds of megabits per second,
are beginning field trials. Meanwhile, old standbys, such as
corporate T1 connections at 1.54 Mbps, are being upgraded by
many companies to T3 fiber connections. But as quickly as
LECs (local exchange carriers) and competitive access
providers lay new fiber, many companies are finding
high-bandwidth connections difficult to obtain or prohibitively
expensive. Wireless has always been an alternative for
high-speed connections, but never has the range of choices been
as great nor the rate of innovation as rapid. This chapter delves
into the world of wireless broadband and other fixed-wireless
connections that deliver data rates from T1 to 155 Mbps. These
wireless connections serve the same function as a wireline--
interconnecting private networks, bypassing a local exchange
carrier or connecting to the Internet.
In our first chapter on wireless networks
(http://www.networkcomputing.com/netdesign/wireless1.html),
we examined wide-area wireless networks, covering data over
PCS (personal communications systems), packet data networks
and Metricom Ricochet. In our second chapter, we surveyed
wireless LANs
(http://www.networkcomputing.com/netdesign/wlan1.html).
Both chapters concentrated on mobile computer
communications. This chapter focuses on communications that
are fixed and at higher data rates. A simple form of such a
system might involve a private microwave point-to-point
connection; a more complex system might involve a carrier that
has deployed a complete network using sophisticated
point-to-multipoint hubs. A LEO (low-earth-orbiting) system of
satellites would be even more complex. There are as many
variations in high-speed wireless systems as there are variations
in wireline systems.
Fixed-wireless systems have a long history. Point-to-point
microwave connections have long been used for voice and data
communications, generally in backhaul networks operated by
phone companies, cable TV companies, utilities, railways,
paging companies and government agencies, and will continue
to be an important part of the communications infrastructure.
Frequencies used range from 1 GHz to 40 GHz. But technology
has continued to advance, allowing higher frequencies, and
thus
smaller antennas, to be used, resulting in lower costs and
easier-to-deploy systems for private use and for a whole new
generation of carriers that are planning to use wireless access as
their last mile of communication. The terms wireless broadband
and broadband wireless are not used consistently, but generally
both apply to carrier-based services in which multiple data
streams are multiplexed onto a single radio-carrier signal. Some
vendors also use the terms to refer to privately deployed
networks.
(A backhaul connection is a company's internal infrastructure
connection. For example, a phone company's backhaul might
be from one central office to another.)
The goal of this chapter is to show how fixed-wireless systems
are no longer a communications tool restricted to large or
specialized organizations. They are available to almost any size
company in a variety of ways for a variety of purposes. You1ll
find that you have a wide range of choices, including whether
to
use licensed or unlicensed spectrum, whether to deploy a
private network or use a carrier network, and whether to use a
terrestrial network or a satellite network. In some cases, you
may not even know that your service provider is using wireless
technology. This chapter discusses the options available, how
the various technologies work and how to go about
implementing a fixed-wireless solution.
Wireless Broadband and Other
Fixed-Wireless Systems
Broad Fundamentals
Before delving into the types of fixed-wireless systems and how
to
use them, let1s look at some of the fundamental concepts and
issues involved. These include:
Fixed-Wireless Applications | Wireline versus Wireless |
Private
versus Carrier
Unique Aspects of Wireless | Radio Spectrum
Fixed-Wireless Applications
Fixed-wireless systems can be used for almost anything that a
cable
is used for, whether the cable is a T1 circuit, a cable television
cable, an Ethernet cable or a fiber optic cable. Fixed-wireless
systems are designed so that they emulate cable connections,
and
they use the same type of interfaces and protocols, such as T1,
frame relay, Ethernet and ATM. For this chapter, we1ll assume
that your application is data, and we emphasize wireless
systems
designed for data communications. Keep in mind that
fixed-wireless systems are also used for voice communications
as
well as for carrying television programming. But most new
development in fixed-wireless systems is data-centric, such as
for
Internet access, or is flexible in supporting both voice and data
communications. Fixed-wireless systems match cable-based
systems for all important parameters, including delay, bit-error
rate (1 in 100 million or better) and throughput (1 Mbps to 155
Mbps). Consequently any application that operates over a cable
should be able to operate over a fixed-wireless system. The
only
exception is communication involving geosynchronous
satellites
where delays can exceed a quarter of a second.
Wireline versus Wireless
In some cases, a fixed-wireless system is the only wireless
option.
So you must decide if a fixed-wireless connection is practical
and
if it is competitive with available wireline connections. Today
fewer than 10 percent of buildings have fiber to them, and only
about 50 percent are close enough to a central office (12,000
feet
or 3.5 km) to take advantage of DSL technology. Thus in many
cases, a wireless connection could be the only option for
high-speed communications. This is especially true in more
remote
areas. In some areas, the only option for communications will
be
by satellite.
When both wireless and wireline options exist, the potential
reasons to consider wireless include lower costs, faster
deployment, greater flexibility and better reliability.
Unfortunately
costs and deployment have to be evaluated on a case-by-case
basis
and actual costs will depend on the particular circumstances. In
developed countries, a wireless system won't be more reliable
than
wireline options, but in developing countries wireless
communications may be much more reliable.
Private versus Carrier
A fundamental distinction is whether you deploy your own
wireless
connection or whether it is supplied by a wireless carrier. In the
past, most fixed-wireless connections were private, but with
new
spectrum licenses a number of companies are deploying or
planning to deploy networks in most major metropolitan areas.
Don't think of these companies as "wireless" companies,
though.
Think of them as CLECs or ISPs that happen to be using
wireless
technology. Instead of running fiber to a building, they are
using
wireless links that may be less expensive to deploy than fiber.
Once one of these companies, Winstar
(http://www.winstar.com)
and Advanced Radio Telecom (http://www.art-net.net) are two
examples, has a wireless connection to a building, often to the
roof, they can then market their communications services to the
tenants of the building. These services look like standard
networking services, and interfaces include frame relay, ATM,
T1,
and Ethernet using the same connectors, such as BNC and
RJ-45.
These services also can include value-added services, such as
network management, Internet access, Web hosting and e-mail.
The fact that these carriers use wireless technology will be
transparent. (See the section How To Use a Wireless Carrier for
a
list of some tough questions you can ask to ensure dependable
service. For instance, terrestrial wireless carriers only have
regional licenses so you should ask whether the carrier can
provide
service to all of your locations.)
Figure: wireless broadband providing last mile of connectivity
to
buildings not having fiber connections.
Satellites are another form of carrier service,
offering broadband services using
geosynchronous satellites today and higher
capacity LEO satellites in the near future.
Unique Aspects of Wireless
Fortunately, you do not need to be a radio
engineer to take advantage of fixed-wireless
communications. But it is helpful to
understand what makes wireless different from other forms of
communication, particularly when qualifying and specifying
service parameters.
Wireless communications offers tremendous flexibility and
ever-improving performance, but it does have some limitations.
First and foremost, wireless uses radio spectrum, a finite
resource.
This limits the number of wireless users and the amount of
spectrum available to any user at any moment in time. The
amount
of spectrum available equates almost directly to data
bandwidth,
with 1 Hz of spectrum typically yielding between 1 bps and 4
bps
of throughput depending on various factors, such as the type of
modulation used and environmental factors. The amount of
spectrum actually available varies from radio band to radio
band,
but suffice it to say that fiber optic cable offers far greater
overall
capacity. Despite this capacity limitation, wireless offers more
than sufficient bandwidth for many applications. But it is
important to know the capacity of a particular wireless system
in
order to understand how it can satisfy your requirements if they
should expand in the future. Another limitation is that
fixed-wireless systems operate at frequencies that almost
always
require line of sight and that are restricted to distances that vary
from a few miles to tens of miles. It is no mystery why
microwave
dishes are located at tops of towers, hills and buildings. Unlike
cellular and other mobile wireless systems, fixed-wireless
systems
use fixed antennas with narrowly focused beams. A 3 degree to
4
degree beam is not uncommon. And unlike cellular systems, in
which base stations communicate with dozens of mobile
stations,
broadband systems usually operate in a point-to-point manner,
though a number of point-to-multipoint systems are in
development.
Very few standards exist for fixed wireless systems, and you
will
need to purchase equipment from the same vendor for both
sides of
the connection to ensure interoperability.
Radio Spectrum
Fixed-wireless systems use frequencies allocated for such use
from
about 900 MHz to 40 GHz. The number of different bands can
be
bewildering, with multiple frequency bands assigned for private
use and multiple bands assigned for carrier use. In addition,
some
bands are designated for licensed use while others can be used
without a license.
Should you care what frequency you use? Yes, but only in a
general sense. Higher frequencies have some advantages over
lower frequencies, but also suffer some drawbacks. The
principle
advantage of higher frequencies is that there is more spectrum
available for broadband applications. Most higher bandwidth
systems use frequencies above 10 GHz. Antennas at these
frequencies are smaller due to the smaller wavelengths, making
systems easier to deploy. But with higher frequency,
components
demand more sophisticated technology, so systems cost more.
Also, propagation distance for reliable communications
decreases
and the signal is more susceptible to weather conditions like
rain
and fog. Higher frequency systems, those above about 30 GHz,
are
sometimes referred to as millimeter wave because the
wavelength
of these signals is on the order of 1 millimeter.
Both private and carrier systems have a choice of using licensed
or
unlicensed spectrum. The main advantage of unlicensed
spectrum
is being able to deploy a system without applying for a license
from the FCC (or equivalent body in other countries). The
disadvantage is that you could experience or cause interference,
though the type of technology used in these frequencies
minimizes
this possibility.
The principal frequencies of interest in this chapter are:
ú 900 Hz, 2.4 GHz and 5.8 GHz: unlicensed systems using
spread-spectrum techniques
ú 2.5 GHz: licensed to carriers for MMDS (Multichannel
Multipoint Distribution System)
ú 5 GHz: new unlicensed band referred to as UNII (Unlicensed
National Information Infrastructure) band
ú 23 GHz: commonly used for microwave LAN systems
ú 28 GHz: licensed to carriers for LMDS (Local Multipoint
Distribution Service)
ú 38/39 GHz: licensed to carriers for general purpose
communications services
A good resource for additional information about frequency
allocations is a book called "The Spectrum Guide" by Bennett
Kobb. See newsignals.com for details.
networkcomputing.com
networkcomputing.com
networkcomputing.com |
