Issues for Consideration in Off-board Infrastructure
ISSUES FOR CONSIDERATION IN
OFF-BOARD INFRASTRUCTURE
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Prepared by:
WAEA Internet Working Group (IWG):
Off-Board Infrastructure Ad Hoc
Adopted by:
WAEA Technology Committee,
August 2, 2001
©2001 World Airline Entertainment Association. All Rights Reserved.
The World Airline Entertainment Association (WAEA) is the author and creator of this specification for the purpose of
copyright and other laws in all countries throughout the world. The WAEA copyright notice must be included in all
reproductions, whether in whole or in part, and may not be deleted or attributed to others. The WAEA hereby grants to its
members and their suppliers a limited license to reproduce this specification for their own use, provided it is not sold.
Others should obtain permission to reproduce this specification from WAEA Headquarters, Attn: Executive Director,
401 N. Michigan Ave., Chicago, Illinois 60611-4267 USA; (312) 245-1034 voice, (312) 321-1080 facsimile.
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TABLE OF CONTENTS
1
1.1
1.2
1.3
2
2.1
2.2
2.3
2.4
2.5
2.6
2.7
3
3.1
3.2
3.3
3.4
3.5
4
4.1
4.2
4.3
4.4
5
6
Terrestrial Links Supporting Mobile Communications ..........................................3
Introduction ...........................................................................................................................3
Issue One........................................................................................... ....................................3
Issue Two ..............................................................................................................................3
Satellite Systems: Issues ......................................................................................................3
Introduction ...........................................................................................................................3
Issue One: Licensing,Background .........................................................................................4
Issue Two: Latency Delays ........................................................................................ .............5
Issue Three: Advantages/Disadvantages .......................................................................5
Issue Four: Capacity ..........................................................................................................6
Issue Five: Communications Requirements ..................................................................6
Issue Six: Security ..............................................................................................................7
Terrestrial Links Supporting Fixed Communications: Issues ........................................8
Introduction ..........................................................................................................................8
Issue One: Availability ..........................................................................................................8
Issue Two: Capacity ..............................................................................................................8
Issue Three: Traffic Estimates and Bandwidth .....................................................................8
Issue Four: Cost .....................................................................................................................9
General Issues. .....................................................................................................................9
Issue One: ISP's .....................................................................................................................9
Issue Two: "Roaming" Capability .......................................................................................10
Issue Three: Passengers' Expectations.................................................................................10
Issue Four: Connectivity.....................................................................................................10
Note 1: Explanation of FHSS and DSSS .........................................................................10
Appendix ...........................................................................................................................12
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Note: issues are grouped either by the type of link or into a “general” category.
1
Terrestrial Links Supporting Mobile Communications: Issues
1.1 INTRODUCTION
Typically, terrestrially based systems carrying communications to/from the aircraft
cabin (as opposed to the cockpit) are based on allocations of Ultra High Frequency
channels. By the close of 2000, three such systems were known to be operational,
all in North America: AT&T’s Claircom division, GTE Airfone – both of which use
frequencies assigned for the North American Telephone Service (849-851/894-896
MHz); and AirCell, which uses cellular frequencies (824-849/869-894 MHz). A
fourth system, JetPhone, which had operated primarily in Europe, had ceased
operations.
1.2 ISSUE ONE: WILL THERE BE SUFFICIENT BANDWIDTH TO SUSTAIN GROWTH OF
COMMUNICATIONS IN THE UHF BAND?
Tentative answer: The FCC has allocated a total of 4 MHz for the NATS
services in the US (2 MHz up, 2 MHz down). Different providers may
“channelize” their allocations differently. AirCell shares frequencies with cellular
systems and has approximately 832 channels, each 32 KHz wide. Additional
allocations in the US appear unlikely at this time.
1.3
ISSUE TWO: WILL THE BANDWIDTH OF EXISTING TERRESTRIAL SERVICES SUSTAIN ONLY
BASIC VOICE AND LOW-RATE DATA OR CAN THE EXISTING SYSTEMS INCREASE BANDWIDTH TO
SUPPORT OTHER SERVICES SUCH AS INTERNET ACCESS?
Tentative answer: The North American UHF based systems are limited in the
bandwidth they can supply, due to the limited allocations made to them. They
support data communications at rates varying from 2.4 kilobits per second to 9.6
kbps. Increased rates may be possible if several channels are grouped together,
but “wide band” services on these frequencies would not be attainable under the
present regulatory structure.
2 Satellite Systems: Issues
2.1 INTRODUCTION
Communications satellite systems available at present and in the near term (next
five years) to support communications to/from the cabin of an aircraft include both
geosynchronous and low-earth orbiting global systems (having worldwide or near
worldwide coverage) and regional/national systems. Satellites in use or being
considered to support aeronautical communications operate at one (or more) of
several frequencies: L-band, Ku-band, S-band, and Ka-band. Where issues noted
below are common to most or all of these systems, it is so noted; where an issue
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pertains exclusively or primarily to one type of system, it is also noted.
2.2 ISSUE ONE: (APPLIES TO ALL SATCOM SYSTEMS) – LICENSING, BACKGROUND
In general, all electronic systems that transmit (whether by RF, satcom or by other
means) are required in most countries of the world to be licensed for transmission
by the relevant authorities (e.g., the FCC in the US). Similarly, aircraft themselves
require radio licenses to transmit. If the State in which a radio license is issued is a
signatory to the ICAO Convention Civil Aviation, which most are, the license has
validity in all other ICAO signatory states for ICAO approved and standardized
systems. Licensing criteria are essentially technical, i.e., the radio station on the
aircraft must meet requirements for controlling spurious emissions, etc. Volume of
traffic carried, and whether communications are initiated from the aircraft to the
ground or the reverse are irrelevant. Aircraft operating with these systems must
have appropriate radio licenses. Issues of possible intra-system or inter-system
interference are generally reviewed at the licensing authority level where
constraints, if any, are imposed. For example, satellites operating at the same
frequencies are routinely “spaced” a certain number of degrees apart in the orbital
arc to reduce the potential for interference. (For a detailed description of the
process for worldwide frequency allocations, definitions of terms, and the FCC
process for licensing, see the FCC Web site: www.fcc.gov/oet/spectrum). Two
frequently asked questions surrounding licensing are as follows:
a. While portions of the L-band spectrum have been allocated for mobile
communications and, specifically, for aeronautical communications, other bands,
e.g., Ku-band, today do not have a worldwide allocation for transmissions
to/from aircraft. How, therefore, can aircraft be assured that the system they
may intend to use is properly licensed to support aeronautical communications?
Tentative answer: A new mobile system must be designed to operate in bands
where AMSS (aeronautical mobile satellite service) is authorized, or, service
providers may identify spectrum, e.g., Ku-band, that will support the proposed
operation/service and then initiate rulemakings and/or regulatory processes
necessary to allow the operation of AMSS. Boeing, for example, has applied to
the ITU for an allocation for Ku-band to be used for the return link (aircraft to
satellite). Pending the grant of such an application, systems using Ku-band
satellites could possibly be given “experimental licenses” rather than “operational
licenses”. ARINC has applied to the FCC for a similar authorization to use Kuband on a secondary basis. It must be shown that the intended use does not
interfere with the primary service.
b. Country Sovereignty: all countries reserve the right to license use of any
communications satellites providing service over their “territory”. Will this mean
that some satellite systems will not be able to provide communications services
in some areas of the world?
Tentative answer: This has been the case in the past and will likely continue to
be the case. Individual satellite operators will need to apply to radiate over
sovereign territories.
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2.3 ISSUE TWO: HOW WILL “LATENCY DELAYS” AFFECT THE QUALITY AND RELIABILITY OF
SATELLITE BASED TRANSMISSIONS?
Tentative answer: The delay introduced by a satellite is only one element of
possible delay in message delivery. There is no specific metric that can be used to
demonstrate delay introduced by, e.g., a terrestrial link. A typical delay on a
complete geosynchronous satellite transmission, i.e., from the originating point to
the receiving point, is approximately one quarter of one second. Delays on
satellites at lower earth orbits (LEO satellites) are less. Such delays are most
noticeable in voice transmissions where the effect may be noticed as a form of
“echo”, which can be disorienting to the user. This problem has been successfully
mitigated for telephony users by the insertion of echo cancellers and careful
attention to line impedance matching.
For digital transmissions, in particular for packetized formats compatible with the
Internet, transmission line delay only affects the “hand shake” protocol used for call
initiation. Satellite based digital communications systems carrying e-mail traffic
have become routine for LEO constellation satellites. For higher orbiting satellites
(e.g., GEO’s) that impose longer delays in the transmission path, the “hand shake”
protocols will need to use data packet interleaving techniques to keep protocol
overhead down. Work is proceeding from those companies intending to transmit
packetized data on GEO systems, and in certain cases, e.g., with Inmarsat’s IPDS
service, solutions are available.
2.4 ISSUE THREE: WHAT ARE THE ADVANTAGES/DISADVANTAGES OF VARIOUS TYPES OF
SATELLITE SYSTEMS?
Tentative answer: All geosynchronous satellites operate in such a way as to
provide coverage of a “fixed” portion of the earth, excluding the polar regions. A
single geosynchronous satellite can cover approximately one-third of the earth;
therefore, a system of at least three geosynchronous satellites strategically located
can provide worldwide coverage, including over oceans, although this concept
presupposes that the satellites are positioned in such as way as to have minimal
overlap.
The existing Inmarsat system is a good example of a global
geosynchronous system. By contrast, low-earth orbiting (LEO) or medium-earth
orbiting (MEO) satellites can provide global coverage, including for polar regions,
provided enough satellites are operating in multiple planes to provide coverage.
New Iridium is a good example of such a system. Some LEO/MEO systems do not
provide global coverage because each satellite needs to “see” a ground station at
all times, and in the ocean regions there are no “ground stations”. An example of
this is the existing Globalstar system. LEO/MEO systems may, however, have
inter-satellite links (as does New Iridium), alleviating the problem of oceanic
coverage.
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2.5 ISSUE FOUR: CAPACITY:
WILL THERE BE ENOUGH CAPACITY FOR SUCH APPLICATIONS AS
INTERNET ROAMING IF MOST AIR TRANSPORT AIRCRAFT ADOPT SUCH AN APPLICATION, AND
ESPECIALLY IF THERE ARE ONLY ONE OR TWO SATELLITE AVAILABLE TO SUPPORT SUCH
APPLICATIONS FOR ALL USER AIRCRAFT?
Tentative answer: Capacity is partially dependent on the frequency bands in which
satellites operate since certain frequency allocations limit the number of channels
that can be used. Capacity is also affected in terms of the way in which the operator
of a given satellite has chosen to “channelize” that particular satellite (for example,
in a system with limited data rates per channel, it may be possible to “merge”
several channels to achieve higher data rates per channel, but at a cost). Capacity
is also a function of the effective radiated power of a satellite. In general, satellites
providing at least 4.8 kbps per channel can supply very good voice quality and lowspeed data that would, for example, be adequate for transferring of short email
messages. Satellites offering between 9.6 and 64 kbps could support higher speed
services, e.g., high-quality voice, faxes, and longer email messaging. Satellites
offering hundreds of kilobits per second could support compressed video, moderate
to large files transfers, and like applications. Finally, availability of bandwidth on a
given satellite system can be “dynamic” if, for example, frequencies are assigned on
a demand basis, e.g., only for the duration of a specific transmission, versus
distributed or leased/shared bandwidth whereby a given customer controls a fixed
amount of bandwidth at all times, either exclusively or on a shared basis. Demand
assigned systems can generally accommodate greater fluctuations in traffic up to
the point where the satellite system is virtually saturated regardless of changes in
demand. It is generally assumed that the greater capacity will be required in the
ground-to satellite-to aircraft direction for applications such as compressed video
transfer and Internet browsing.
2.6 ISSUE FIVE: COMMUNICATIONS REQUIREMENT
Communications via any satellite system require four fundamental elements: a
“ground station” that “looks at” the satellite and from which the transmissions
originate to the satellite and return from the satellite; the satellite; receiving
equipment on the mobile unit (aircraft in this case) that includes an antenna and
avionics (electronics) that can receive and send signals; and (usually) a terrestrial or
second satellite link to the ground station to carry information beyond the ground
station, e.g., through the PSTN or through a dedicated network. In the case of
satellite system with inter-satellite links (e.g., Iridium), it is also possible to transfer
information from satellite to satellite, alleviating the need for each satellite to “see” a
ground station at all times. Satellites without inter-satellite links must be able to
“see” a ground station to receive or transmit. (For example, a satellite system may
show a “coverage” map that shows their satellites radiating over large geographic
areas but without ground stations to communicate with the satellites, there is no
service.) The frequencies at which communications satellites operate also affect the
nature and reliability of transmissions. Generally, satellites operating at lower
frequencies (below 10000 MHz; i.e., in the L, S, and C-bands) do not have their
signals attenuated by rain or other atmospheric conditions. Once transmissions
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move into higher frequencies (e.g., Ku and Ka bands), attenuation or loss of signal
may occur in extremely inclement weather, especially heavy rain. Therefore, some
satellite operators deem it advisable to have redundant ground earth stations to
reach the same satellite. Generally, it would not be expected that satellites
operating at these higher frequencies would be approved to support safety-of-flight
services but could still be useful for non-safety communications services, e.g.,
entertainment in the cabin.
2.7 ISSUE SIX: SECURITY: HOW SECURE CAN COMMUNICATIONS BE, ESPECIALLY IF THERE IS
EMAIL AND INTERNET BROWSING AVAILABLE?
Tentative answer:
The issue of “security” occurs at several levels of
communications. At the top level, or level of transmission of basic information
(whether in the form of data or voice), the question is, can the information be easily
intercepted? With satellite communications, the digitization of data that takes place
in order to transfer information does, in and of itself, afford a certain level of
security. An “average person” would not be easily able to replicate a satellite earth
station, complete with transceiving antenna, to be able to intercept satellite-based
communications. A higher level of transmission security can be achieved with
encryption both on and off the aircraft, in other words, at both the transmitting and
receiving ends – this is often done for satellite-based military communications. A
second “level” of security concerns address the ability, for example, of email users
to break through corporate “fire walls”. At present, it appears that several
companies planning to offer email are working on this issue with no specific
resolution yet announced. There is also a level of security concerns about voice
calls to the aircraft; some airlines do not want unlimited or unfiltered calls reaching
passengers or crew. This issue can be easily dealt with through the establishment
of protocols for ground-to-air calls that require callers on the ground to have access
to special code or PIN numbers in order to place the calls. Also, multi-stage dialling
is possible, whereby the call originator has to go through several computercontrolled “gates” where he/she must know the right codes in order to reach
someone on an aircraft in flight. This solution can be made available even when
there is true “roaming” of cellular calls up to aircraft. Finally, there is a level of
security that has to do with “addressing” of messages, i.e., can we be sure the
message from the aircraft gets to the right destination and vice versa? This
concern can be resolved through message heading protocols that are independent
of the medium of delivery (e.g., the satellite).
3 Terrestrial Links Supporting Fixed Communications: Issues
3.1 INTRODUCTION
Planning and implementation of “gatelink” systems is now taking place to support
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large files transfers to and from aircraft while they are parked at gates. Utilization of
gatelink technology requires a server on board an aircraft, which may (or may not)
be shared with other applications, e.g., airborne email services. Gatelink current
solutions are based on IEEE 802.11 standards that either make use of frequency
hopping spread spectrum (FHSS) or direct sequence spread spectrum (DSSS) in
the unlicensed ISM band (2.4 GHz). All mobile clients (airborne as well as groundbased) in a given location share this frequency band. (See Note 1 at the conclusion
of this paper for a fuller explanation of FHSS and DSSS).
3.2 ISSUE ONE:
SOON?
AVAILABILITY:
WHICH AIRPORTS WILL HAVE GATELINK AVAILABLE AND HOW
Tentative answer: While several experiments with gatelink technology have taken
place and a few airlines have issued RFP’s for the provision of gatelink at their most
frequented airports, gatelink is unlikely to be available as a pervasive technology,
i.e., supporting most gates at most major airports, for several years. This could
change as the air transport industry and/or airports perceive it as extremely valuable
and implement it more quickly.
3.3 ISSUE TWO:
CAPACITY: HOW MUCH CAPACITY WILL GATELINK OFFER?
Tentative answer: Gatelink has the advantage of being able to transfer on the
order of 2 Mbts of data in one burst, depending on the actual transmission
technology chosen. For example, DSSS can provide a higher data rate, e.g., 11
Mbps, than can FHSS (approximately 2 Mbps). There is uncertainty, however, as to
whether gatelink would be an efficient medium for the transfer of, e.g.,
entertainment programming (movies) or would be most efficient for medium to large
files, e.g., crew handbooks, post-flight reports, email attachments held until the
conclusion of a flight, Web pages stored for accessing by passengers and crew in
flight.
3.4 ISSUE THREE:
TRAFFIC ESTIMATES AND BANDWIDTH SHARING AT AIRPORTS: IS THERE
GOING TO BE ENOUGH SPECTRUC AT EVERY AIRPORT FOR MULTIPLE LAN’S (LOCAL AREA
NETWORKS), INCLUDING GATELINK, TO SATISFY DEMAND?
Tentative answer: Given the state of development of gatelink, it is difficult to
estimate traffic for a “typical” airline. Much will depend on whether FHSS or DSSS
is chosen and on the specific applications an airline seeks to support via gatelink
technology and the number of airports at which these applications are needed.
Airlines restricting their use of gatelink-based communications to pre-flight loading
of manifests and off-loading of post-flight reports could, for example, be expected to
use only a few megabytes per month and per aircraft, while others using gatelink for
multiple applications that include loading of in-flight entertainment might use in the
tens of megabytes of data transfer per aircraft “session” at an airport. There is no
general answer as to whether or not frequency congestion will be a problem at a
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given airport. The number of users, traffic patterns and choice of spread spectrum
technology (FHSS or DSSS) will influence how the spectrum is being used.
Analyses can be made via RF surveys of the sites. It should also be noted that the
2.4 GHz band, in which gatelink systems primarily operate, is an ISM – industrial,
scientific and medical applications – band. Many of the uses in this band are
unlicensed and therefore could cause interference to gatelink operations. Such
interference is being experienced between unlicensed systems today as the
wireless consumer applications proliferate. For the most part, operations in this
band are unlicensed and unprotected. They can cause interference to each other
and one unlicensed system does not have any rights over any other unlicensed
system.
3.5 ISSUE FOUR:
COST:
HOW ARE GATELINK COSTS LIKELY TO COMPARE WITH OTHER
TRANSMISSION MEDIA COSTS?
Tentative answer: Until gatelink systems are at least operational in a few airports
and the vendors/service providers have been clearly established, it is difficult to
estimate what user charges may look like or how they would compare to delivery
prices via other media, such as communications satellites. As with many new
technologies or platforms, it can probably be assumed that costs initially will be
higher (due to development expenses and the size of the market) and will come
down over time, with greater deployment of gatelink, greater usage of its
capabilities, and maturing of technology.
4 General Issues
4.1 ISSUE ONE: WILL PASSENGERS HAVE THE ABILITY TO CONNECT TO THEIR PREFERRED
ISP’S FROM AN AIRCRAFT?
Tentative answer: This appears to be a commercial question at present. Some of
the companies offering email services, whether via terrestrial or satellite links, have
announced that passengers will be able to connect with “any” ISP; other providers
of airborne email may themselves be ISP’s or limit passengers’ access to other
ISP’s. Issues of penetrating corporate “firewalls” are being dealt with by the
individual companies that have announced they will provide or support airborne
email and Internet services.
WILL PASSENGERS BE ABLE TO HAVE “ROAMING” CAPABILITY WITH THEIR
VOICE AND DATA CAPABLE CELLULAR PHONES AND PDA’S WHILE INFLIGHT AND IN THE AIRPORT?
4.2 ISSUE TWO:
Tentative answer: Several service providers are arranging for the capability for
passengers to send and receive calls and messages while inflight, using their
cellular phones and PDA’s, although the final step in this process will require some
type of wireless LAN on the aircraft. Many airports are expected to install LANS
within the airport that will support wireless calling so that the passenger may be able
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to move from the aircraft to the airport seamlessly.
WHAT ARE PASSENGERS’ EXPECTACTIONS AND WILL SOME, MOST OR ALL
OF THE COMMUNICATIONS SYSTEMS BEING DISCUSSED LIKELY MEET THESE EXPECTATIONS?
4.3 ISSUE THREE:
Tentative answer: Many passengers expect communications of the same
nature, ease and price that they use on the ground. They do not understand and
probably cannot be expected to understand the relatively greater complexity and
cost of getting communications to and from flying aircraft. The three major
constraints in the airborne environment today are: channel capacity limitations of
existing transmission systems; costs; reliability of a lesser degree than with
systems operating on the ground due to the fact that even with the most reliable
satellite system, for example, a plane can bank steeply and lose a signal
temporarily. Passenger expectations probably need to be managed, e.g., limiting
airborne services to transmissions of short or moderate length while using
gatelink-type technologies for transfers of greater amounts of information.
ISSUE FOUR: CONNECTIVITY: WHATEVER THE MEDIUM OF TRANSMISSION OR “BEARER
SYSTEM” AN AIRCRAFT
4.4
Whatever the medium of transmission or “bearer system” an aircraft uses –
whether UHF based, satcom, or gatelink – all the relevant “pieces” of the
particular bearer system chain need to be in place for communications to take
place. For example, with airborne email, there must be a way for passengers
(and crew) to get power for their lap-tops, transmit messages through the aircraft
(via a wired or wireless system) to the airborne server that, in turn, is connected
to the avionics that work through the UHF or satcom (or other) antenna to reach
the appropriate satellite or ground-based tower. And, finally, there must be a
path to and from the final destination for messages, e.g., the ISP. In most of the
commercial arrangements available to aircraft today, there may be a single
service provider who can make (or assist in making) all of these “pieces”
available and connected, but no single company will own/operate all the pieces.
Therefore, it is incumbent on airlines wishing to purchase such services to
become knowledgeable about which companies offer which of these “pieces”,
who provides what part(s) of the interconnection, and how pricing is set.
5 Note 1: Explanation of FHSS and DSSS
In frequency hoping spread spectrum (FHSS), the signal hops from channel to
channel (within the 2.4 GHz band) in a pseudo-random fashion. Depending upon
geographical location and associate regulatory requirements, multiple hop
patterns are available for use where each hop pattern consists of multiple hop
sequences, e.g., North American and most of Europe have 78 hop patterns, and
each hop pattern has 70 channels for hopping in a pseudo-random fashion.
Each access point in a given location is configured to make use of a different hop
pattern and all mobile clients follow the hop pattern of the access point with
which they are associated. The low contention between access points is a result
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of the utilization of different hop patterns and hopping within that pattern
continuously.
Direct sequence spread spectrum (DSSS) is a more popular option as it provides
for higher data rates but works on static channels. The number of DS channels
in the 2.4 GHz band is also dependent on geographical location and associated
regulatory requirements, e.g., North America has 11 channels (most of Europe
has 13 channels) but a location may have only three channels. If RF cells of
access points are non-overlapping, they may all be assigned a single channel to
cover the whole airport, but if RF cells of two access points overlap, they are
assigned two different channels to reduce contention. Mobile clients make use of
the channel assigned to the access point of their association.
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6 APPENDIX
GLOSSARY OF TERMS
Used in WAEA Internet Working Group Off-Board “Issues” Paper
GHz – Gigahertz: radio wave frequency of 1 billion cycles per second
kHz – Kilohertz: radio wave frequency of 1000 cycles per second
MHz – Megahertz: radio wave frequency of 1 million cycles per second
kbps – Kilobits per second (1000 bits per second)
Mbps – Megabits per second (1 million bits per second)
VHF – Very High Frequency: 30 MHz to 300 MHz
UHF – Ultra High Frequency: 300 MHz to 1000 MHz
L-band – frequency designation for band from 1 GHz to 2 GHz
S-band – frequency designation for band from 2 GHz to 4 GHz
C-band – frequency designation for band from 4GHz to 8 GHz
Ku-band – frequency designation for band from 12 GHz to 18 GHz
Ka-band – frequency designation for band from 18 GHz to 27 GHz
AMSS – Aeronautical Mobile Satellite Service (designated in L-band)
DSSS – direct sequence spread spectrum
FCC – Federal Communications Commission
FHSS – frequency hopping spread spectrum
GEO – geosynchronous, referring to satellites orbiting at 22,300 miles above earth
LEO – low earth orbit, referring to satellites in orbits close to earth
MEO – medium earth orbit, referring to satellites in orbits between LEO and GEO
ICAO – International Civil Aviation Organization
IEEE – Institute for Electrical and Electronics Engineering
ISP – Internet Service Provider
ITU – International Telecommunications Union
LAN – local area network
PDA – personal digital assistant
PIN – personal identification number
PSTN – public switched telecommunications network
RF – radio frequency
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