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I GENERAL QUESTIONS
Q 1.1 What is AMPS?
A 1.1 The Advanced Mobile Phone System (AMPS) is the analog system
used
in the United States for cellular telephones. It uses Frequency
Modulation
(FM). AMPS uses the FDMA air interface. The channels are 30Khz wide.
The
frequency band for AMPS is 824Mhz to 849Mhz and 869Mhz to 894Mhz.
Q1.2 What is GSM?
A1.2 GSM was established in 1982 as a European standard for digital
wireless communications. The first system was on line in 1991. GSM is
now used in many countries around the world. The original name was
Groupe Spˇcial Mobile, but now the most accepted name is Global System
for Mobile Communications. GSM uses a combination of FDMA and TDMA.
FDMA: The 25Mhz-wide band is divided into 124 frequencies of 200Khz
each.
One or more of those frequencies are assigned to each base station.
TDMA: Each of those frequencies uses 8 time slots.
Q 1.3 What is TDMA?
A 1.3 Time Division Multiple Access is an air interface that allows
mobile
stations to use the same frequency, but are separated by time slots.
Q 1.4 What is CDMA?
A 1.4 Code Division Multiple Access is an air interface that allows
mobile
stations to use the same frequency, but are separated by unique
digital
codes. CDMA uses spread spectrum techniques. The standard (IS-95) was
published by the Telecommunications Industry Association (TIA) in
July,
1993 and revised to IS-95A in May, 1995. The channels are
approximately 1.25Mhz wide.
Q 1.4a What is the difference between TDMA and CDMA?
A 1.4a (Credit: Steve Punter)
The first thing you have to learn is that "TDMA" is used frequently as
a
direct reference to IS-136. This is an ambiguous reference, since
there
are actually three technologies that presently use a form of Time
Division
Multiple Access. Be careful when talking with people who use TDMA in
this way.
Of the four competing technologies, IS-136 (Cantel), GSM (Fido), and
iDEN
(Mike) use their own versions of TDMA. IS-95 (Bell Mobility & Clearnet
PCS)
is an implementation of CDMA.
US Carriers:
IS-136: AT&T, Cingular (in former Bell South, Southwestern Bell,
Cellular
One markets)
GSM: T-Mobile USA (formerly VoiceStream), Cingular
(in former BellSouth Mobility DCS and Pacific Bell Wireless markets)
IS-95: Sprint PCS, Verizon (formerly Bell Atlantic, Airtouch, GTE, and
Primeco), Alltel, and Qwest Wireless
iDen: Nextel
Chad Z. Hower points out that GSM should also include BellSouth
Mobility DCS
and Aerial.
As you have already guessed, the main difference between TDMA and CDMA
is
how they share the spectrum. TDMA does so by assigning each user on a
channel a different "slot" in time. IS-136 and iDEN have 3 slots per
channel, while GSM has 8 slots per channel. Note that a GSM channel is
much
wider than an IS-136 or iDEN channel. Each user "takes turns"
transmitting
and receiving in their designated time slot.
CDMA on the other hand has everyone on the channel transmitting and
receiving AT THE SAME TIME. Conventional wisdom would lead you to
believe
this is impossible, but it works because of a modulation technique
known as
Spread Spectrum. A CDMA channel is very wide (1.25 Mhz actually). Each
user
is only transmitting a small amount of data, which would normally fit
well
into a much smaller (narrow) channel. What the transmitter does is to
take
this small number of bits and spread it out over the entire
1.25Mhz-wide
channel. It does so using an encoding technique that makes each user's
bits
interfere very little with everyone else's. The receiver's job is to
"unspread" the bits and extract only the data from the desired user.
Because this wide channel can accommodate many different people using
it at
the same time, the channel is said to have "soft capacity", since the
upper
limit on the number of users is determined strictly by how much
interference
a subscriber is willing to endure. Up to a certain point, the level of
inter-user interference is well within the capability of the voice
coding
algorithm to correct. In a TDMA system, the capacity is "hard coded"
by the
number of slots allocated to a channel.
CDMA also offers a unique feature known as "soft handoff". TDMA
systems use
what is known as a "hard handoff", since the connection with one site
must
be broken and then a new connection created. CDMA systems keep your
call
connected through 3 (the spec allows up to 6, but that isn't used)
different
sites. Your phone can pick and choose between these 3 sites at any
time, and
can in fact combine the signals from two or more sites at the same
time.
It would take volumes to go into the necessary detail required to
explain
the true differences. I therefore recommend you buy a book called
"Wireless
Personal Communications Systems" by David J. Goodman. It's an
Addison-Wesley
publication, and its ISBN number is 0-201-63470-8. This book is a
marvelous
source of information about the various technologies (excluding iDEN)
used
in North America.
Q 1.5 What is PCS?
A 1.5 The Federal Communications Commission (FCC) opened a new
frequency
band for digital phones and named the new service Personal
Communication
Services (PCS). The band is generally referred to as the 1900Mhz
(1.9Ghz)
band. The band is from 1850MHz to 1990MHz.
Q 1.6 What is an ESN?
A 1.6 The Electronic Serial Number is a 32-bit number that is stored
in the
phone. It is used, along with other numbers, to identify your phone.
Q 1.7 What is meant by dual-mode?
A 1.7 A dual-mode phone operates on two modes. Usually this refers
to
phones that use digital and analog, however it also pertains to phones
that
operate on AMPS and N-AMPS.
Q 1.8 What is a dual-band phone?
A 1.8 A dual-band phone operates on the cellular band (800Mhz) and
the
PCS band (1900Mhz).
[Questions 1.9, 1.10, and 1.11 deleted]
Q 1.12 What is GSM1900?
A 1.12 It is the GSM system used in the United States on the PCS
band.
Q 1.13 What is a SIM card?
A 1.13 The SIM (Subscriber Identity Module) card is a "smart card"
used by
a subscriber to GSM. The card contains information about the user and
can
be plugged into another GSM phone. This allows the user to use
another GSM
phone as if it was his/her own phone.
Q 1.14 What are the potential health hazards to using a portable
wireless
phone?
A 1.14 The 'short' answer is that there is no known health hazards
associated with the proper use of portable wireless phones. For more
technical information on this subject go to http://www.fcc.gov/oet/.
The Office of Engineering and Technology (OET) has a number of
bulletins
on the subject. The two that you want to start with are OET56 and
OET65C.
OET56 gives a lot of other references.
Q 1.15 What is IRIDIUM? What is GlobalStar?
A 1.15 The IRIDIUM and GlobalStar systems will use low earth orbit
satellites for wireless communication. A special handset will allow
the user
to communicate via ground based cellular or by satellite. For the
latest
information see:
http://www.iridium.com
http://www.globalstar.com
Q. 1.16 What is EVRC?
A. 1.16 [Credit: Bill Walker]
EVRC is the Enhanced Variable Rate Coder. It is an 8 kbps vocoder
that's supposed to sound about as good as the current 13 kbps vocoder.
Thus, you can have the same voice quality while improving the capacity
of the system (which can handle more 8 kbps calls than 13 kbps calls.
Q 1.17 What is 3G (3GP)?
A 1.17 The short answer is:
3G is a generic term covering a range of future wireless network
technologies, including cdma2000, UMTS, GPRS, WCDMA, and EDGE.
3G combines high-speed mobile access with Internet Protocol (IP) based
services. This doesn't just mean fast mobile connection to the World
Wide
Web - by liberating us from slow connections, cumbersome equipment and
immovable access points, 3G will enable new ways to communicate,
access
information, conduct business and learn.
For a more complete answer go to:
http://www.ericsson.com/technology/3G.shtml
To learn more about the very latest technologies in the wireless
arena, go to
http://www.ericsson.com/technologies.shtml
II TECHNICAL QUESTIONS
Q 2.1 In CDMA what is meant by "slot cycle index"?
A 2.1 [Credit: Bill Walker]
On the CDMA Paging Channel (this is the shared channel that all phones
listen to for incoming calls and other control info), time is divided
into
"slots". To conserve power, phones that are currently idle only "wake
up"
and listen for messages on the Paging Channel during their assigned
slots. The slot cycle index determines how often the phone's slot
comes
around. If the slot cycle index is 0, the phone wakes up every 1.28
seconds. If it's one, it wakes up every 2.56 seconds. If it's two,
it's
every 5.12 seconds, and so on. The larger the setting, the more power
is
conserved, but the longer it takes to page the phone for an incoming
call. You can imagine that if it were set to 10.24 seconds, few
callers
would hang on the line long enough for you to answer your phone.
There are two settings that govern this. One is the "preferred slot
cycle
index" in the phone, and can be set via service programming. The
other is
the "maximum slot cycle index" set in the base station and broadcast
in
overhead messages on the Paging Channel. The phone has to use
whichever
number is smaller.
Q 2.2 What are the frequencies of the 800MHz and 1900MHZ channels?
A 2.2 The receive (RX) and transmit (TX) frequencies for the mobile
units are:
CELLULAR
CHANNEL RX TX
1 870.030 825.030
2 870.060 825.060
3 870.090 825.090
.
.
799 893.970 848.970
800-990 no voice channels
991 869.040 824.040
992 869.070 824.070
.
.
1023 870.000 825.000
PCS
CHANNEL RX TX
1 1930.050 1850.050
2 1930.100 1850.100
3 1930.150 1850.150
.
.
1200 1990.000 1910.000
III MANUFACTURER SPECIFIC QUESTIONS
Q 3.8 Which digital phone should I buy?
A 3.8 (Credit: Michael Geary)
Which phone is "better" will depend more on the service provided by a
given carrier than the actual piece of equipment you hold in your
hand.
So backing up a bit, here is the way to answer your question:
First, investigate the various wireless carriers in your area. Look up
their price plans, get feedback from customers about their coverage
and reliability, and then finally see what kind of phones they offer.
Then, and only then, it starts to make sense to think about which
model of phone. You may well find the decision has already been made
for you--most digital carriers sell only certain phones that work with
their service.
IV WIRELESS CARRIER SPECIFIC QUESTIONS
Q 4.1 Which wireless carrier should I sign up with?
A 4.1 See A 3.8.
V HELPFUL URLs
Q 5.1 Where can I get more information about CDMA?
A 5.1 Qualcomm, Inc.: http://www.qualcomm.com
CDMA Development Group: http://www.cdg.org
Q 5.3 What are the URLs for the major wireless phone manufacturers?
A 5.3 Qualcomm: http://www.qualcomm.com
Motorola: http://direct.motorola.com/hellomoto/index.html
Nokia: http://www.nokiausa.com
Sony Ericsson: http://www.sonyericsson.com/us/
Samsung: http://www.samsungusa.com
Kyocera Wireless Corp.: http://www.kyocera-wireless.com
Audiovox: http://www.audiovox.com
LG: http://www.lgmobilephones.com/
Q 5.4 Where can I get more information about GSM?
A 5.4 http://kbs.cs.tu-berlin.de/~jutta/gsm/js-intro.html
http://www.gsmworld.com
A 5.4a (Special Thanks to: George Czajkowski)
GSM Memorandum of Understanding Association(official GSM page,
includes history of GSM):
http://www.gsmworld.com
Overview of the Global System for Mobile Communications (GSM):
http://ccnga.uwaterloo.ca/~jscouria/GSM/
Cellular GSM Phones:
http://www.geocities.com/CapeCanaveral/2042/nokia.html
GSM all over the world (links to GSM related sites):
http://www.webring.org/cgi-bin/webring?ring=gsm;list
GSM North America:
http://www.communicreate.com/gsmnew.htm
GSMag International:
http://www.gsmag.com/
Q 5.6 What other helpful URLs are available?
A 5.6
Steve Punter's page. Contains lots of interesting info. Check it
out!
http://www.arcx.com/sites
Comparisons of phones and services - national
http://www.wirelessdimension.com (new name: point.com but URL still
works)
http://www.myrateplan.com
Cellular Phone Information For Beginners
http://web.idirect.com/~aaa/p0000003.htm
Cellular, PCS and Wireless Phone Service Information
http://www.wirelessadvisor.com/
Another FAQ (mostly concerning CDMA):
http://denbeste.nu/cdmafaq/
A 5.6a More URLs (Credit: Bob Smith)
Web Links on wireless information & FAQ's
http://www.berkana.com/tower.php3 - Locator of towers near you.
http://www.arcx.com/sites/ - Canadian info, but good FAQ's
http://www.point.com/ - Source to compare services, phones & coverage
areas.
http://www.letstalk.com - Same
http://www.mybestplan.com/ - Same
http://www.mcw.edu/gcrc/cop/cell-phone-health-FAQ/toc.html
http://www.sierrawireless.com/
Sprint PCS Phone Links
http://www.sprintpcs.com/learn/help_sitemap.html
http://www.sprintpcs.com/wireless/capable_phones.html
Wireless Internet URL's
http://www.datalink.net/home.htm
http://mobile.msn.com/
http://sprintpcs.my.yahoo.com/
http://www.hz.com/
http://www.messaging.sprintpcs.com/sms/
http://www.squeezenet.com
http://www.mailandnews.com/
http://www.HerMobile.com -- provides email aliases for wireless phones
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