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(Page 6) Cellular Telephone Basics continued . . .
Pages: Getting a Call -- The Process
Okay, your phone's now registered with your local system. Let's say you get a call. It's the F.B.I., asking you to turn yourself in. You laugh and hang up. As you speed to Mexico you marvel at the technology involved. What happened? Your phone recognized its mobile number on the paging channel. Remember, that's always the forward control channel or path except in a CDMA system. The mobile responded by sending its identifying information again to the MTSO, along with a message confirming that it received the page. The system responded by sending a voice channel assignment to the cell you were in. The cell site's transceiver got this information and began setting things up. It first informed the mobile about the new channel, say, channel 10 in cell number 8. It then generated a supervisory audio tone or SAT on the forward voice frequency. What's that?
The SAT, Dial Tone, and Blank and Burst
An SAT is a high pitched, inaudible tone that helps the system distinguish between callers on the same channel but in different cells. The mobile tunes to its assigned channel and it looks for the right supervisory audio tone. Upon hearing it, the mobile throws the tone back to the cell site on its reverse voice channel. What engineers call transpond, the automatic relaying of a signal. We now have a loop going between the cell site and the phone. No SAT or the wrong SAT means no good.
AMPS generates the supervisory audio tone at three different non-radio frequencies. SAT 0 is at 5970 Hz, SAT 1 is at6000 Hz, and SAT 2 is at 6030 Hz. Using different frequencies makes sure that the mobile is using the right channel assignment. It's not enough to get a tone on the right forward and reverse path -- the mobile must connect to the right channel and the right SAT. Two steps. This tone is transmitted continuously during a call. You don't hear it since it's filtered during transmission. The mobile, in fact, drops a call after five seconds if it loses or has the wrong the SAT. [Much more on the SAT and co-channel interference] The all digital GSM and PCS systems, by comparison, drops the call like AMPS but then automatically tries to re-connect on another channel that may not be suffering the same interference.
The cell site unmutes the forward voice channel if the SAT gets returned, causing the mobile to take the mute off the reverse voice channel. Your phone then produces a ring for you to hear. This is unlike a landline telephone in which ringing gets produced at a central office or switch. To digress briefly, dial tone is not present on AMPS phones, although E.F. Johnson phones produced land line type dial tone within the unit. [See dial tone.]
Can't keep track of these steps? Check out the call processing diagram Enough about the SAT. I mentioned another tone that's generated by the mobile phone itself. It's called the signaling tone or ST. Don't confuse it with the SAT. You need the supervisory audio tone first. The ST comes in after that; it's necessary to complete the call. The mobile produces the ST, compared to the SAT which the cell site originates. It's a 10 kHz audio tone. The mobile starts transmitting this signal back to the cell on the forward voice path once it gets an alerting message. Your phone stops transmitting it once you pick up the handset or otherwise go off hook to answer the ring. Cell folks might call this confirmation of alert. The system knows that you've picked up the phone when the ST stops.
Thanks to Dwayne Rosenburgh N3BJM for corrections on the SAT and ST AMPS uses signaling tones of different lengths to indicate three other things. Cleardown or termination means hanging up, going on hook, or terminating a call. The phone sends a signaling tone of 1.8 seconds when that happens. 400 ms. of ST means a hookflash. Hookflash requests additional services during a conversation in some areas. Confirmation of handover request is another arcane cell term. The ST gets sent for 50 ms. before your call is handed from one cell to another. Along with the SAT. That assures a smooth handoff from one cell to another. The MTSO assigns a new channel, checks for the right SAT and listens for a signaling tone when a handover occurs. Complicated but effective and all happening in less than a second. [See SIT]
Okay, we're now on the line with someone. Maybe you! How does the mobile communicate with the base station, now that a conversation is in progress? Yes, there is a control frequency but the mobile can only transmit on one frequency at a time. So what happens? The secret is a straightforward process known as blank and burst. As Mark van der Hoek puts it,
"Once a call is up on a voice channel, all signaling is done on the voice channel via a scheme known as "Blank and Burst". When the site needs to send an order to the mobile, such as hand off, power up, or power down, it mutes the SAT on the voice channel. This is filtered at the mobile so that the customer never hears it. When the SAT is muted, the phone mutes the audio path, thus the "blank", and the site sends a "burst" of data. The process takes a fraction of a second and is scarcely noticeable to the customer. Again, it's more noticeable on a Motorola system than on Ericsson or Lucent. You can sometimes hear the 'bzzt' of the data burst."
Blank and burst is similiar to the way many telco payphones signal. Let's say you're making a long distance call. The operator or the automated coin toll service computer asks you for $1.35 for the first three minutes. And maybe another dollar during the conversation. The payphone will mute or blank out the voice channel when you deposit the coins. That's so it can burst the tones of the different denominations to the operator or ACTS. These days you won't often hear those tones. And all done through blank and burst. Now let's get back to cellular.
D. Origination -- Making a call
Making a mobile call uses many steps that help receive a call. The same basic process. Punch out the number that you want to call. Press the send button. Your mobile transmits that telephone number, along with a request for service signal, and all the information used to register a call to the cell site. The mobile transmits this information on the strongest reverse control channel. The MTSO checks out this info and assigns a voice channel. It communicates that assignment to the mobile on the forward control channel. The cell site opens a voice channel and transmits a SAT on it. The mobile detects the SAT and locks on, transmitting it back to the cell site. The MTSO detects this confirmation and sends the mobile a message in return. This could be several things. It might be a busy signal, ringback or whatever tone was delivered to the switch. Making a call, however, involves far more problems and resources than an incoming call does.
Making a call and getting a call from your cellular phone should be equally easy. It isn't, but not for technical reasons, that is setting up and carrying a call. Rather, originating a call from a mobile presents fraud issues for the user and the carrier. Especially when you are out of your local area. Incoming calls don't present a risk to the carrier. Someone on the other end is paying for them. The carrier, however, is responsible for the cost of fraudulent calls originating in its system. Most systems shut down roaming or do an operator intercept rather than allow a questionable call. I've had close friends asked for their credit card numbers by operators to place a call. [See cloning comments]
Can you imagine giving a credit card number or a calling card number over the air? You're now making calls at a payphone, just like the good old days. Cellular One has shut down roaming "privileges" altogether in New York City, Washington and Miami at different times. But you can go through their operator and pay three times the cost of a normal call if you like. So what's going on? Why the problem with some outgoing calls? We first have to look at some more terms and procedures. We need to see what happens with call processing at the switch and network level. This is the exciting world of precall validation.
Notes
[Dial tone] During the start of your call a "No Service" lamp or display instead tells you if coverage isn't available If coverage is available you punch in your numbers and get a response back from the system. Imagine dialing your landline phone without taking the receiver of the hook. If you could dial like that, where would be the for dial tone? (back to text)
[Much more on the SAT and co-channel interference] The supervisory audio tone distinguishes between co-channel interferrors, an intimidatingly named but important to know problem in cellular radio. Co-channel interferrors are cellular customers using the same channel set in different cells who unknowingly interfere with each other. We know all about frequency reuse and that radio engineers carefully assign channels in each cell to minimize interference. But what happens when they do? Let's see how AMPS uses the SAT in practice and how it handles the interference problem.
Mark van der Hoek describes two people, a businessman using his cell phone in the city, and a hiker on top of a mountain overlooking the city. The businessman's call is going well. But now the hiker decides to use his phone to tell his friends he has climbed the summit. (Or as we American climbers say, "bagged the peak.")
From the climber's position he can see all of the city and consequently the entire area under cellular coverage. Since radio waves travel in nearly a straight line at high frequencies, it's possible his call could be taken by nearly any cell. Like the one the businessman is now using. This is not what radio engineers plan on, since the nearest cell site usually handles a call, in fact, Mark points out they don't want people using cell phones on an airplane! "Knock it off, turkey! Can't you see you're confusing the poor cell sites?"
If the hiker's mobile is told by the cell site first setting up his call to go channel 656, SAT 0, but his radio tunes now to a different cell with channel 656, SAT 1, instead, a fade timer in the mobile shuts down its transmitter after five seconds. In that way an existing call in the cell is not disrupted.
If the mobile gets the right channel and SAT but in a different cell than intended, FM capture occurs, where the stronger call on the frequency will displace, at least temporarily, the weaker call. Both callers now hear each other's conversation. A multiple SAT condition is the same as no SAT, so the fade timer starts on both calls. If the correct SAT does not resume before the fade timer expires, both calls are terminated
Mark puts it simply, "Remember, the only thing a mobile can do with SAT is detect it and transpond it. Either it gets what it was told to expect, and transponds it, or it doesn't get what it was told to expect, in which case it starts the fade timer. If the fade timer expires, the mobile's transmitter is shut down and the call is over." (back to text)
- [SIT] "A large supplier and a carrier I worked for went round and round on this. If their system did not detect hand-off confirmation, it tore down the call. Even if it got to the next site successfully. Their reasoning was that, if the mobile was in such a poor radio frequency environment that 50 ms of ST could not be detected, the call is in bad shape and should be torn down. We disagreed. We said, "Let the customer decide. If it's a lousy call, they'll hang up. If it's a good call, we want it to stay up!" Just because a mobile on channel 423 is in trouble doesn't mean that it will be when it hands off to channel 742 in another cell! In fact, a hand-off may happen just in time to save a call that is going south. Why?"
"Well, just because there is interference on channel 423 doesn't mean that there is on 742! Or what if the hand-off dragged? That is, for whatever reason the call did not hand off at approximately half way between the cells. (Lot's of reasons that could happen.) So the path to the serving site is stretched thiiiiin, almost to the point of dropping the call. But the hand-off, almost by definition in this case, will be to a site that is very close. That ought to be a good thing, you'd think. Well, the system supplier predicted Gloom, Doom, and Massive Dropped Calls if we changed it. We insisted, and things worked much better. Hand-off failures and dropped calls did not increase, and perceived service was much better. For this and a number of other reasons I have long suspected that their system did not do a good job of detecting ST . . ." [back to text][Clone comments] "You could make more clear that this is due to validation and fraud issues, not to the mechanics of setting up the call, since this is pretty much the same for originations and terminations."
"By the way, at AirTouch we took a big bite out of fraudulent calls when we stopped automatically giving every customer international dialing capability. We gave it to any legitimate customer who asked for it, but the default was no international dialing. So the cloners would rarely get a MIN/ESN combo that would allow them to make calls to Colombia to make those 'arrangements'. Yes, the drug traffic was a huge part of the cloning problem. We had some folks who worked a lot with law enforcement, particularly the DEA. Another large part of it was the creeps who would sell calls to South America on the street corners of L.A. Illegal immigrants would line up to make calls home on this cloned phone."
"Actually, even though it's an inconvenience, being cloned can be fun if you are an engineer working for the carrier. You can do all kinds of fun things with the cloner. Like seeing where they are making their calls and informing the police. Like hotlining the phone so that ALL calls go straight to customer service. It would have been fun to hotline them to INS, but INS wouldn't have liked that."<grin> (back to text)
