Mobile Phones - The Basics

VII. AMPS Call Processing

Let's look at how cellular uses data channels and voice channels. Keep in mind the big picture while we discuss this. A call gets set up on a control channel and another channel actually carries the conversation. The whole process begins with registration. It's what happens when you first turn on a phone but before you punch in a number and hit the send button. It only takes a few hundred milliseconds. Registration lets the local system know that a phone is active, in a particular area, and that the mobile can now take incoming calls. What cell folks call pages. If the mobile is roaming outside its home area its home system gets notified. Registration begins when you turn on your phone.

Mobile phones run a self diagnostic when they're powered up. Once completed they act like a scanning radio. Searching through their list of forward control channels, they pick one with the strongest signal, the nearest cell or sector usually providing that. Just to be sure, the mobile phone re-scans and camps on the strongest one. Not making a call but still on? The mobile phone re-scans every seven seconds or when signal strength drops before a pre-determined level. After selecting a channel the phone then identifies itself on the reverse control path. The mobile sends its phone number, its electronic serial number, and its home system ID. Among other things. The cell site relays this information to the mobile telecommunications switching office. The MTSO, in turn, communicates with different databases, switching centers and software programs.

The local system registers the mobile phone if everything checks out. Mr. Mobile can now take incoming calls since the system is aware that it is in use. The mobile then monitors paging channels while it idles. It starts this scanning with the initial paging channel or IPCH. That's usually channel 333 for the non-wireline carrier and 334 for the wireline carrier. The mobile is programmed with this information and 21 channels to scan when your carrier programs your phone's directory number, the MIN, or mobile identification number. Again, the paging channel or path is another word for the forward control channel. It carries data and is transmitted by the cell site. A mobile first responds to a page on the reverse control channel of the cell it is in. The MTSO then assigns yet another channel for the conversation. But I am getting ahead of myself. Let's finish registration.

Registration is an ongoing process. Moving from one service area to another causes registration to begin again. Just waiting ten or fifteen minutes does the same thing. It's an automatic activity of the system. It updates the status of the waiting phone to let the system know what's going on. The cell site can initiate registration on its own by sending a signal to the mobile. That forces the unit to transmit and identify itself. Registration also takes place just before you call. Again, the whole process takes only a few hundred milliseconds.

AMPS uses frequency shift keying to send data. Just like a modem. Data's sent in binary. 0's and 1's. 0's go on one frequency and 1's go on another. They alternate back and forth in rapid succession. Don't be confused by the mention of additional frequencies. Frequency shift keying uses the existing carrier wave. The data rides 8kHz above and below, say, 879.990 MHz. Read up on the earliest kinds of modems and FSK and you'll understand the way AMPS sends digital information. Data gets sent at 10 kbps or 10,000 bits per second from the cell site. That's fairly slow to begin with but fast enough to do the job. Since cellular uses radio waves to communicate, of course, signals are subject to the vagaries of the radio band. Things such as billboards, trucks, and underpasses, can deflect a cellular call. So the system repeats each part of each digital message five times. That slows things considerably. Add in the time for encoding and decoding the digital stream and the actual transfer rate can fall to as low as 1200 bps.

Remember, too, that an analog wave carries this digital information, just like most modems. It's not completely accurate, therefore, to call AMPS an analog system. AMPS is actually a hybrid system, combining both digital and analog signals.

A quick word about 'slots'. Slots hold individual call information within the frame, remember? In this case we have one frame of information containing six slots. Two slots make up one voice circuit in TDMA. Like slots 1 and 4, 2 and 5, or 3 and 6. The data rate is 48.6 Kbits/s, less than a 56K modem, with each slot transmitting 324 bits in 6.67 ms. How is this rate determined? By the number of samples taken, when speech is first converted to digital. Remember Pulse Amplitude Modulation? Let's look at what's contained in just one slot of half a frame in digital cellular.

Okay, here are the actual bits, arranged in their containers the slots. All numbers above refer to the amount of bits. Note that data fields and channels change depending on the direction or the path that occurs at the time, that is, a link to the mobile from the base station, or a call from the mobile to the base station. Here are the abbreviations:

G: Guard time. Keeps one time slot or data burst separate from the others. R: Ramp time. Lets the transmitter go from a quiet state to full power. DATA: The data bits of the actual conversation. DVCC: Digital verification color code. Data field that keeps the mobile on frequency. RSVD: Reserved. SACCH: Slow associated control channel. Where system control information goes. SYNC: Time synchronization signal.

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