Received: by bu-cs.BU.EDU (5.58/4.7) id AA12868; Fri, 27 Jan 89 01:54:23 EST Message-Id: <8901270654.AA12868@bu-cs.BU.EDU> Date: Fri, 27 Jan 89 1:22:21 EST From: The Moderator Reply-To: TELECOM@bu-cs.BU.EDU Subject: TELECOM Digest V9 #31 To: TELECOM@bu-cs.bu.edu TELECOM Digest Fri, 27 Jan 89 1:22:21 EST Volume 9 : Issue 31 Today's Topics: Re: Cellular Setup International Calling Cards Re: ATT commerical Re: ATT commerical Re: Nuisance phone calls ---------------------------------------------------------------------- From: tim@Athena.UUCP (Tim Dawson) To: telecom@bu-cs.bu.edu Subject: Re: Cellular Setup Date: 25 Jan 89 17:32:12 GMT >X-TELECOM-Digest: volume 9, issue 24, message 4 > >Question: How is phase shifting actually involved in communications between the mobile unit and the switching office ? > >Question: Is it possible to access cellular setup channels and place fraudulent call with a ham radio? > >Thanks for your help .. > >Perry > >Reply here on this newsgroup or e-mail to boottrax@csd4.milw.wisc.edu (arpanet) To answer your questions as best as possible: 1) The "Phase Shifting" you refer to is in all probability referring to the modulation of the RF going from the mobile to the cell site. (I forget the actual emission designators) and is similar to FM. Typically communications from the cell site to the cellular switching office are via T-1 pcm carrier systems. 2) Extremly improbable. For the why, first let me describe the scenario of a modbile to land call set up. a) User enters phone number and hits send. b) Mobile listens to data stream on signalling channel, and checks busy/idle bits to see if another mobile has channel in use. If idle, mobile sends request containing mobile Electronic Serial Number (manufactured into the radio), the mobiles phone number, and the called number. c) System receives request and sends data burst back to mobile confirming that request is received, and assigning a voice channel. d) Mobile changes frequency to voice channel, verifies SAT (sub audible tone used to verify that mobile has reached correct channel) and returns same SAT to cell site. Mobile also verifies DCC (Digital Color Code - like SAT but in digital domain) to confirm channel. Mobile unmutes audio and call setup proceeds through switch. At this point, all progress tones, etc heard from the mobile are coming from the land office, not the mobile switch. e) Call is now in progress. While call is up, Cell sites constantly are scanning mobile signal strength. If dips below threshhold for a certain (variable from system to system) number of scans, a handoff request is made. Adjacent cells scan the mobile, and if signal is ABOVE threshold, the system initiates handoff. A request is sent digitally to the mobile to mute audio, and change to the new frequency (also sent). The mobile mutes, changes frequency, verifies SAT and DCC on the new channel and unmutes (all in about 50 ms or so, typically). This handoff is generally inaudible to the user, but is what makes using cellular with modems a pain - no audio/data can be sent during this handoff. f) For call termination, mobile sends disconnect request to switch, and all facilities are idled. As can be seen, this is not a trivial process. The primary problem with trying to defraud a Mobile system is that you have to know a valid mobiles Electronic Serial Number/Mobile Number Combination or the system will deny service. You also have to be ablo to transmit and receive 9600 baud FSK (to the best of my memory - my spec isn't handy) to the system in order to determine what voice channel assignment has been made. And you have to do it FAST! Most all call setup items described above must occur within very closely difined time windows, or the system will fail the call. Also, as soon as the guy who gets stuck with the bill bitches, they will most likely change his mobile number, or start tracing the calls and can determine who is the fraudulent user based on who is being called quite easily. This is one of the big plusses of cellular telephony - if somebody steals a phone, their ESN can be denied nationally, and they can't use it. It is not impossible to change ESN in a phone, but is extremely difficult since it is manufactured physically into the unit, and is not generally documented by the manufacturer is public domain documnets for security reasons. So what you would end up doing is basically redesigning a cellular mobile, and seriously doubt whether many people have the skill and knowledge to even come close to being able to do so. Also, with the security provisions in cellular systems, even if you could manage the hardware, the system software would still make it highly unlikely that you could use it. -- ================================================================================ Tim Dawson (...!killer!mcsd!Athena!tim) Motorola Computer Systems, Dallas, TX. "The opinions expressed above do not relect those of my employer - often even I cannot figure out what I am talking about." ------------------------------ To: ukc!comp-dcom-telecom From: jpp@slxsys.specialix.co.uk (John Pettitt) Subject: International Calling Cards Date: 24 Jan 89 10:34:07 GMT Following on from the recent postings on use of calling cards and the problems encountered please spare a thought for the poor users of non US charge cards. I have British Telecom phone credit card, which in the uk works very well and has some nice features (3 wrong pin attempts cancels the card!) but in the USA it's a pain. There are two ways of using a BT card to call england from the US. 1) Call the operator, say you want to call a UK number and bill it to a uk credit card, expalain the yes uk cards do work, yes I know the number starts 44M and calling cards dont start that way, get supervisor, explain again, get international operator, recite card number (13 digits), number to call (8 digits) and sometimes get connected. or 2) Call 1-800-4455667 and talk to a BT operator in london, recite over poor quality transatlatic line, card number, number to call, and number of phone you are using, repeat until operator gets all numbers correct. or 3) Pay hotels rip-off phone rate and just dial the number . . . John Pettitt Specialix International (Solution is to get calling card number for US office, now all I have to do is sort out the internal accounting . . . .) -- John Pettitt, Specialix, Giggs Hill Rd, Thames Ditton, Surrey, U.K., KT7 0TR {backbone}!mcvax!ukc!pyrltd!slxsys!jpp jpp@slxsys.specialix.co.uk Tel: +44-1-398-9422 Fax: +44-1-398-7122 Telex: 918110 SPECIX G >>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>><<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<< ------------------------------ To: telecom@bu-cs.bu.edu From: palmer@hsi.UUCP (Mike Palmer) Subject: Re: ATT commerical Date: 18 Jan 89 14:58:17 GMT In article <9300027@m.cs.uiuc.edu> kadie@m.cs.uiuc.edu writes: > >ATT is running a new set of commericals (I guess they got >tired of the "slice of death" series). > >In one of the new commerials the guy says something to the effect >"Yea, we cost more on the first minute, but how often do you make > one minute phone calls?" > >Do the rates really converge? Or are they trying to help people >rationalize sticking with a more expensive service. About a year and a half ago, the phone company sent me a notice about having to choose my long distance carier along with all the option in my area and their phone numbers. I proceeded to call each one and ask about their rates. everyone of them asked me for a long distance number that I called often and then quoted rates. All the non-AT&T companies also gave comparisons with AT&T for 2-4 minute calls. As I remember, AT&T's rates were something like 15 cents for the first minute and 9 cents for each additional minute. All the other companies were 12 cents for the first minute and 10 cents for each additional minute. A little quick math and the realization that most of my calls were of the 10 minute variety and my long distance carrier became AT&T. > >- Carl -- ======= Mike Palmer {uunet,noao,yale}!hsi!palmer ======= ======= Health Systems International palmer@hsi.com ======= ------------------------------ To: telecom@bu-cs.bu.edu.UUCP From: john@zygot.UUCP (John Higdon) Subject: Re: ATT commerical Date: Wed, Jan 25, 1988 In article <7303@ihlpf.ATT.COM>, jnl@ihlpf.ATT.COM (John N. Le) writes: > Even if AT&T rates are higher in some cases, so what. If you like > to pay less, it's your choice, but I wish you luck if you try to demand > from your LD company the same high quality service that AT&T are doing > for yearssssss. But now it's 1989, and the old stand-by reliable hissy analog connections still provided by AT&T are a little passe. Occasionally, I get a digital transcontinental connection from AT&T, but it's a small percentage of the time and only to major metro areas. Sprint on the other hand, seems to provide fast, reliable digital connections virtually everywhere. I'd use them if their rates were higher, but they're *lower*. -- John Higdon john@zygot ..sun!{apple|cohesive|pacbell}!zygot!john ------------------------------ To: telecom@bu-cs.bu.edu From: glee@cognos.uucp (Godfrey Lee) Subject: Re: Nuisance phone calls Date: 23 Jan 89 16:26:47 GMT >>I believe the time has come to do something about nuisance phone calls. >I found after I installed an answering machine, that junk callers >usually hang up when they realize they got an answering >machine. Problem is that there are more and more automated phone solicitations. With these you don't get the satisfaction of hanging up on them, and if you have an answering machine, you get junk filling up your tape! I do sense some consumer rebellion on this though, I got a few of them about a year ago, but lately have gotten none, what is the situation in the rest of Canada and in the U.S.? -- Godfrey Lee P.O. Box 9707 Cognos Incorporated 3755 Riverside Dr. VOICE: (613) 738-1338 x3802 FAX: (613) 738-0002 Ottawa, Ontario UUCP: uunet!mitel!sce!cognos!glee CANADA K1G 3Z4 ------------------------------ End of TELECOM Digest *********************