Mobile Phones - The Basics

Getting a Call -- The Process

Okay, your mobile 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 phone 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 mobile 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 phone 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. 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 mobile 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.

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 phone 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.

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 mobile 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.

Okay, we're now on the line with someone. Maybe you! How does the mobile communicate with the base station, know that a conversation is in progress? Yes, there is a control frequency but the mobile phone 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 phone 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 phone 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 mobile 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.

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.

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