GSM uses two main techniques, TDMA AND Frequency-Hopping. This, combined with a third feature that doesn’t transmit “quiet times” — points where there is no speech, combines to produce between a 15x-20x increase in utilization. TDMA combined with the “quiet time” cancellation is called ETDMA. There are lots of different flavors of GSM, though, that implement different combinations of the above three pieces, so not every GSM provider realizes the same 15x-20x increase.
GSM is cool because a GSM phone hops all over the spectrum in a pattern. Therefore if a particular frequency is overcrowded, weak, or malfunctioning, the problem is solved on the next frequency hop (usually one second later). This makes GSM highly survivable (wish they used it in the Bay Area) and really, really cheap.
GSM is of course widely deployed in Europe. This large installed base of fairly standardized carriers is why there are several iterations of GSM and why GSM gets so many new cool features sooner (like SMS).
-Ian.
At 11:26 AM 18/05/00 -0700, Darren Gibbons wrote:>In Canada, we have Clearnet and Fido. Clearnet is CDMA, but Fido is GSM,
>which if I understand correctly is based on a varient of TDMA.
>
>Ian, do you know if GSM has the same issues as TDMA when it comes to
>handling data? On an unrelated note, Fido supports SMS (short message
>service) which is very handy — you can send short text messages to and from
>other Fido phones, something that is not possible (to my knowledge) via any
>other service.
>
>Telus just launched their digital data service, including WAP.
>http://www.telusmobility.com/bc/wireless/wireless.htm
>
>
>Darren.
>
>
>
> > —–Original Message—–
> > From: Ian Andrew Bell [mailto:despot [at] ianbell [dot] com]
> > Sent: Thursday, May 18, 2000 11:18 AM
> > To: Mark Schneider
> > Cc: foib [at] ianbell [dot] com
> > Subject: Re: @F: RE: AT&T Slammed on Wireless Data
> >
> >
> > Oops. Let me clarify: CDM = 20x, TDM = 3x.
> >
> > AT&T and Rogers/Cantel use TDMA.
> > Sprint, Telus, and most others use CDMA.
> >
> > -Ian.
> >
> > At 11:04 AM 18/05/00 -0700, Ian Andrew Bell wrote:
> >
> > >Oh. Sorry.
> > >
> > >TDMA: Time Division Multiple Access 3x Frequency Re-use
> > >CDMA: Code Division Multiple Access 20x Frequency Re-use
> > >
> > >Refers, basically, to how cellular carriers squeeze multiple phones onto
> > >one “circuit” (which is actually a frequency/modulation). The difference
> > >is essentially describes how the economics of the business are
> > mitigated by
> > >technology.
> > >
> > >TDM essentially slices up the analog signal using Pulse Code Modulation
> > >from a bunch of phones and interleaves them with one another,
> > and transmits
> > >the signal from each as barely perceptible chopped up bits of sound (as
> > >though you were talking to someone through a desk fan) expressed as Zeros
> > >and Ones.
> > >
> > >CDM very simply encodes and then compresses (this is the key) the voice
> > >using a digital CODEC right on the phone and sends the signal over the
> > >network as Zeros and Ones. This increases the number of phones per
> > >frequency by 3x.
> > >
> > >Thus CDM is more natively “digital” and better-suited to handling bursty
> > >data traffic like internet. And because there’s less bandwidth used on
> > >each frequency by any given handset, you get more bang for the
> > buck. This
> > >increases the number of phones per frequency by 20x.
> > >
> > >It’s basically like the difference between a CD and a DVD. Why
> > can you get
> > >so much more data onto the same physical media with DVD? Compression!
> > >
> > >-Ian.
> > >
> > >
> > >
> > >At 10:30 AM 18/05/00 -0700, you wrote:
> > > >Ian wrote:
> > > >
> > > >”CDMA carriers will now be able to dance circles around the
> > TDMA guys until
> > > >the
> > > >TDMA guys implement WAP, which is more expensive on TDMA than
> > on CDMA (for
> > > >reasons which should be obvious).”
> > > >
> > > >Why?
> > > >
> > > >—–Original Message—–
> > > >From: Ian Andrew Bell [mailto:ian [at] cafe [dot] net]
> > > >Sent: Wednesday, May 17, 2000 9:34 PM
> > > >To: foib [at] ianbell [dot] com
> > > >Subject: @F: AT&T Slammed on Wireless Data
> > > >
> > > >
> > > >
> > > >AT&T launched PocketNet a few days ago, which is CDPD-based.
> > Sprint has
> > > >been offering free WAP access for months now (I have a Sprint
> > mobile now in
> > > >the US) for free in some packages, with unrestricted web access.
> > > >
> > > >You can even surf to https://ianbell.com on your phone and it parses
> > > >quite well into WAP!
> > > >
> > > >This Gartner report slams AT&T, and shows how companies that
> > made the jump
> > > >to digital networks early (and implemented crappy TDMA) are
> > now going to
> > > >pay the price for not really thinking through the notion of
> > > >”Digital”. Because of the greater efficiencies and reduced costs, CDMA
> > > >carriers will now be able to dance circles around the TDMA
> > guys until the
> > > >TDMA guys implement WAP, which is more expensive on TDMA than
> > on CDMA (for
> > > >reasons which should be obvious).
> > > >
> > > >-Ian.
> > > >
> > > >—=—
> > > >Wednesday May 17 06:00 PM EDT
> > > >Commentary: AT&T PocketNet–when “free” is still too expensive
> > > >By Gartner Viewpoint, CNET News.com
> > > >See news story: AT&T Wireless offers free phone-based Net access
> > > >By Robert Egan, Gartner Analyst
> > > >
> > > >As a competitive response to Sprint, AT&T’s effort falls short
> > in several
> > > >ways.
> > > >
> > > >First, PocketNet is a far cry from the Sprint service today,
> > or from other
> > > >competitive wireless Internet services. For one, it limits people to 40
> > > >selected sites (out of more than 100,000 wireless-friendly
> > sites) unless
> > > >they want to pay extra fees. Through an untested business plan, this
> > > >”sticky” strategy may bring advertising and other revenue to
> > AT&T and its
> > > >business partners, but it needlessly restricts customer choice
> > in a service
> > > >that should be highly personalized.
> > > >
> > > >The “free” service includes access only to these selected sites and the
> > > >customer’s “personal Web page.” In addition to wider Web
> > access, email and
> > > >fax service will cost customers from $6.99 to $14.99 over and
> > above their
> > > >regular airtime and other wireless charges. (To be clear about the term
> > > >”free,” AT&T does charge for airtime while Internet services
> > are used, as
> > > >do Sprint and other wireless providers.)
> > > >
> > > >AT&T has been unable to attract equipment suppliers to build
> > phones for its
> > > >offering, so customers have only two models to choose from, whereas
> > > >Sprint’s Internet service is supported on many more phones.
> > This is in part
> > > >a penance AT&T is paying for its decision to use TDMA (time division
> > > >multiple access) technology, which is unsuited to data transmission,
> > > >instead of the more modern, robust technology used by Sprint.
> > > >
> > > >The same constraint limits AT&T to markets that support the
> > CDPD (cellular
> > > >digital packet data) protocol, which covers only about half the United
> > > >States.
> > > >Therefore, the sheer numbers tip the balance toward Sprint:
> > > >
> > > >* Sprint’s more modern data protocols are supported by almost twice
> > > > as many points of presence as AT&T’s.
> > > >* Sprint offers 10 times the number of handset models that support
> > > > its data services.
> > > >* Sprint customers can access 3,000 times as many Web sites for the
> > > > same (“free”) price.
> > > >
> > > >Gartner predicts that AT&T will not be able to fully benefit from the
> > > >ongoing rapid expansion of wireless data services until it
> > begins to more
> > > >accurately meet its customers’ needs and modernizes its underlying
> > > >technology, which will probably take until 2002.
> > > >
> > > >Entire contents, Copyright © 2000 Gartner Group, Inc. All
> > rights reserved.
> > > >The information contained herein represents Gartner’s initial
> > commentary
> > > >and analysis and has been obtained from sources believed to be
> > reliable.
> > > >Positions taken are subject to change as more information
> > becomes available
> > > >and further analysis is undertaken. Gartner disclaims all
> > warranties as to
> > > >the accuracy, completeness or adequacy of the information.
> > Gartner shall
> > > >have no liability for errors, omissions or inadequacies in the
> > information
> > > >contained herein or for interpretations thereof.
> > > >