There are two basic types of wireless
services: fixed, which delivers services to a fixed location, such as
a building, and mobile, which reaches a moving target. Fixed wireless
can be further subdivided into terrestrial and satellite services, though
one mobile satellite service is also discussed. In terms of the ability
to transmit data reliably, fixed wireless is far ahead of mobile.
The figure shows the radio
frequency (RF) spectrum and the areas that are used for transmitting
Some of these technologies have been around
for awhile and others are just emerging. Both microwave and infrared/laser
frequencies have been used in point-to-point topologies for several
years. These technologies require a line-of-sight between the two endpoints
of the wireless link and, depending on the specific service, may need
extremely precise alignment, as well.
Earlier fixed wireless technologies exploited
the lower end of the radio frequency spectrum for data communications
because, when the power is boosted enough, these lower-frequency signals
can travel a long distance and can even penetrate buildings, as does
Following are descriptions of some of the
many emerging and existing fixed wireless technologies:
Local Multipoint Distribution Service (LMDS):
LMDS is a new fixed wireless technology that shows
considerable promise. LMDS uses microwave (actually, milimiter-wave)
signals to send voice, video, and data at 1 Gbps or more over frequency
bands ranging from 27.5 GHz to 31 GHz. Transmission is within small
cells roughly three miles in diameter. Hewlett-Packard has predicted
throughputs as fast as 1.5 Gbps downstream, with upstream rates as
high as 200 Mbps.
LMDS can provide multi-services because a single
device can send and receive multiple signals simultaneously, each
with its own carrier frequency. LMDS can be engineered to provide
99.999 percent availability. And, analysts expect LMDS services to
cost 50 to 75 percent less than traditional leased-line services.
The FCC first auctioned LMDS licenses in March 1998.
The FCC excluded ILECs and cable television companies from participating
in order to promote entrepreneurial opportunities. The FCC was to
reopen bidding on the remaining licenses in April 1999. 1999 is predicted
to be the year when license holders detail their service plans and
How it Works: LMDS uses
the "very high frequency" Ka band, but over a much shorter distance
than the early wireless offerings. (The Ka band is above the UHF band
and below the far infrared region.)
Fortunately, because of the combination of the very
high frequencies and the small cell sizes (cells are spaced three
to six miles apart), LMDS signals can be transmitted and received
with only a small, six-inch square antenna, but it must be in line-of-sight
of a cell. The graphic
Repeaters or reflectors can be used to spread a strong
signal into shadow areas, thereby increasing coverage. Adding overlapping
cells can bring total coverage to almost 85 percent of all homes in
Cellsize is also influenced by the amount of local
rainfall. Because LMDS signals are microwaves, they are attenuated
by water (called "rain fade"). Leaves, trees, and branches can also
cause signal loss. Because it uses frequency modulation (FM) rather
than the amplitude modulation (AM) used by most cable television providers,
LMDS can generate signals with 10 times higher signal quality than
Multipoint Multichannel distribution Service (MMDS):MMDS
networks operate in the 2.5 to 2.7 GHz range. Unlike LMDS, it is a
fixed wireless technology that is available today.
Licenses were distributed about 15 years ago to be
used to offer asymmetric wireless cable television services. That
never happened and the FCC has now authorized two-way traffic in the
MMDS range in a handful of cities, with approval expected this year
for about 500 additional cities.
Winstar and Teligent:
Two companies, Winstar and Teligent, are currently offering wireless
services in the 24-GHz and 38-GHz bands today using proprietary technologies.
Teligent also plans to offer services in the LMDS range. Both of these
approaches use point-to-multipoint toplogies.
Wavespan: Wavespan of
Mountain View, CA sells a wireless Ethernet bridge.
Return To Top
High-speed microwave using geosynchrous satellites
has been available for quite some time. Now, however, there are many
new choices appearing. Some of the newer satellite wireless services
include the following:
DirectPC: This offering
from the DirectTV satellite television providers uses a standard telephone
line and modem for the uplink, a 12 Mbps connection between the DirectPC
network operations center and the satellite, and a connection of up
to 400 Kbps to deliver data from the satellite to the user. Television
service can use the same dish.Two-way service will be offered beginning
sometime in 2001.
Starband: A two-way data service that is available
now from Starband, owned by Gilat Satellite Networks. Television service
from The Dish can be received using the same dish.
The figure illustrates how a
one-way satellite service works.
Spaceway: Hughes plans
to offer faster speeds than land-based networks and at a cost 20-30%
less. The initial U.S. network will use two geosynchronous satellites
operating in the Ka-band with an in-orbit spare. Once this is operational,
Hughes will add low-Earth orbit (LEO) satellites to expand the system's
reach worldwide. The low orbits avoid the long signal delay normally
experience with geosyncronous satellites (up to 5 seconds round-trip).
Astrolink: By mid-2002,
Lockheed Martin plans to use four (later nine) geosynchronous satellites
operating in the Ka-band to deliver data rates from 416 Kbps (65-cm
dish) to 10.4 Mbps (1.8-m dish). Up to 100 gateways will connect Astrolink
to terrestrial networks worldwide.
Teledesic: Craig McCaw
plans to use LEO satellites to create a global, broadband "Internet-in-the-sky"
service. It will use small, low-power terminals and antennas which
will mount flat on a rooftop and provide two-way connections with
up to 64Mbps on the downlink and up to 2 Mbps on the uplink. Higher-speed
terminals will offer upwards of 64 Mbps each way. The service will
operate in a portion of the Ka-band, using 288 satellites, divided
into 12 planes of 24 satellites each. For efficient spectrum use,
frequence will be allocated dynamically and reused many times within
each satellite footprint.
Cyberstar: Loral Space
and Communications Ltd. will use the Ku-band to initially provide
IP multicast services at speeds up to 29 Mbps. Within two years, Loral
expects to deliver two-way communication services to businesses and
SkyBridge: Alcatel will
offer Ku-band service using 40 LEO satellites (expanding to 80). The
service will provide global connectivity with downstream speeds in
multiples of 20 Mbps and upstream speeds in multiple of 2 Mbps. SkyBridge
will use an innovative re-use scheme to avoid interference problems
with geostationary satellite systems and terrestrial services also
using the Ku-band. Traffic management and routing will be done in
the ground stations, so satellites will not need direct links between
Return To Top
Though there is a lot happening in this area, not
many good solutions, in terms of bandwidth, reliability, and response
time, have yet appeared. Mobil computing has always faced a big obstacle:
As wireless users move around, their connections fade in and out: some
applications can shrug this off, while others may become unstable or
even damage data.
Another problem to contend with is latency, which ranges
from .5 second to more than 5 seconds, round trip. This delay is such
that many end-users, after staring at their screens for what seems a
long time, might assume the application or machine has hung and either
terminate the application or reboot their PC.
Wireless middleware is starting to appear which reduces
both the amount of information that travels wireless and the number
of messages that must be exchanged. In some cases, it can queue messages
when end-users are outside a covereage area. Of course, wireless middleware
also adds cost and complexity to the solution. These problems can also
be at least partially alleviated by proper configuration.
Growth in mobile wireless is likely to be primarily
in metropolitan areas, since vendors delivering services aren't likely
to build antennas in areas where there aren't enough customers to make
the upfront expenditures pay off. Already there are new "packages" being
offered in metropolitan areas that include all long distance, roaming,
voicemail, paging, and other features, all for one fixed monthly price,
often with no activation or cancellation charges either. One area that
has seen successful application of mobile wireless technology is in
the situation where drivers are dispatched from a central office to
various locations within an area. Data transmissions tend to be small
and session durations short.
Some of the current and emerging mobile wireless technologies
include the following:
Analog Cellular: No
other wireless coverage can match the analog cellular network. With
the right PC Card cellular modem, cable, and cellphone, any end-user
can dial an ISP and communicate at 9.6 Kbps. Compatibility between
modems and cellphones, however, has been a problem and carriers have
been inconsistent in deployment of gateways between the cellular and
Bellsouth Wireless Data & Ardis: As
the two oldest wireless packet-data networks, these networks reach
over 90 percent of the U.S. population. Though both use proprietary
protocols, IP gateways are available. However, they use volume-based
pricing, low effective throughput (4 Kbps for Bellsouth and 2.4 or
9.6 Kbps for Ardis), and trip latencies of three seconds or higher.
Cellular Digital Packet Data (CDPD): CDPD uses
idle channels of existing, but CDPD-ugraded, cellular voice networks
to transmit data at an effective throughput of about 10 Kbps. CDPD
uses a full voice channel, but it can move your connection from one
channel to another to avoid congesting voice communications. Each
mobile end-stations has a fixed IP address. Service is available in
most major cities.
Data-over-digital PCS Solutions: In
1999, 14.4 Kbps Internet access should become available for both Global
System for Mobile communication (GSM) and Code-Division Multiple Access
(CDMA) services. These services are still circuit switched, but are
faster because there is no analog modem. Beginning late in 2000, CDMA,
GSM, and Interim Standard 136 (IS-136) will all start offering packet
data service at from 64 to 384 Kbps. Extensive roaming agreements
still need to be worked out and availability may be limited to highly
populated areas for quite some time.
Ricochet: Metricom offers Ricochet, a 28.8
Kbps wireless IP service in the San Francisco Bay area, Seattle, and
Washington, D.C. Expansion to other cities and a 128 Kbps offering
are both planned.
Universal Mobil Telecommunications System (UMTS):
British Telecom with NEC and Nortel Networks began
testing third-generation wireless communications services using UMTS,
which promises data rates to 2 Mbps for stationary users, 384 Kbps
for pedestrians, and 144 Kbps for users in cars or trains.
High Data Rate (HDR) Architecture: Qualcomm,
with Cisco and U S West Wireless, is conducting trials of its IP-based
HDR, which provides wireless Internet access at 1.8 Mbps or greater.
Wireless Application Protocol (WAP):
A number of large wireless providers have banded together
as the WAP forum. The group is developing standards that make Web
content readily available to mobile wireless devices like smartphones.
IEEE 802.11: Released
in 1997, this standard, currently supports data rates of 1 and 2 Mbps
over three physical interfaces: direct-sequence spread spectrum (DSSS),
frequency-hopping spread spectrum (FHSS), and infrared. Both DSSS
and FHSS operate in the unlicensed 2.4 GHz band. 802.11 specifies
a contention access method, CSMA/CA.
Direct-sequence provides slightly higher throughput
and range, and better overall interoperability between current products.
Frequency-hopping offers superior protection from interference and
better overall performance when there are numerous users in a limited
802.11b uses on DSSS, but achieves up to 11 Mbps,
a significant improvement over the original standard.. A 20-30 Mbps
version which will operate in the 5 GHz range is also being worked
on, but not expected to appear until after 2000.
Wireless LANs: Although
it is unusual for sites to deploy strictly wireless LANs, it could
become more popular as the technology improves. Wireless LANs offer
limited mobility at best, with a maximum range measured in hundreds
of feet. Nevertheless, it can be highly benficial where mobility is
an important component of the business process. RadioLAN has a proprietary
product that operates in the 5 GHz range and delivers 10 Mbps, but
only up to about 100 feet.
Mobile IP: A companion
standard to wireless LAN connectivity is RFC 2002, IP Mobility Support,
which allows hosts with a fixed IP address to connect to any IP subnet
and immediately be reachable from the Internet. Mobile IP is an extension
to IP and is also being integrated into IPv6. There are also extensions
to Internet Control Message Protocol (ICMP).
Iridium: Iridium launched its LEO service
in 1998, offering handheld satellite communications from almost anywhere,
but at very high rates. Iridium went bankrupt by 2000. The phones
and the service was simply too expensive to win enough customers to
keep it going.
Other: There are other specifications and
proprietary implementations being developed both in the U.S. and Europe.
There is even a standard for wireless home networks is being worked