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Now that we've looked at frames and time slots, let's look more closely at channels. They have many definitions. Borrowing heavily from the good folks at Webopedia, a channel is a "communications path between two computers or devices." Most commonly a channel describes a pair of radio frequencies, one to receive on and one to transmit. They link the mobile to the nearest base station. 879.360 Mhz might be a transmit frequency and 834.360Mhz might be the receive frequency. Those paired radio frequencies make up a channel. Find out more by skipping ahead.
In a digital discussion, however, a channel is also a communications path within a data stream. A specified place in that train of 1s and 0s going back and forth between the mobile and the computerized base station transceiver. In IS-54, now IS-136, voice traffic is digitized and put within the digital traffic channel as you see below.
Different data channels in a bit stream go beyond the base station to a mobile telephone switch and out to the greater telephone network at large. These bits convey voice, signaling, and administrative information. It's fascinating: if you talk to another digital phone user on your mobile then the entire conversation has gone digital from one end of the telephone system to the other. Let's look again at the D-AMPS digital traffic channel. It carries data, voice, and some signaling:
The Digital Traffic Channel in Digital-AMPS)
A conversation's data bits makes up the DATA field. Six slots make up a complete IS-54 frame. DATA in slots 1 and 4, 2 and 5, and 3 and 6 make up a voice circuit. DVCC stands for digital verification color code, arcane terminology for a unique 8-bit code value assigned to each cell. The DVCC acts like a digital marker, similar to the supervisory audio tone in AMPS, keeping a mobile on frequency.
G means guard time, the period between each time slot. As you might guess, RSVD stands for reserved. SYNC represents synchronization, a critical TDMA data field. Each slot in every frame must be synchronized against all others and a master clock for everything to work.
(1) How the Digital Traffic Channel Works
Let's see how these strange terms and abbreviations come together by describing handoffs -- what happens when you go from one cell to another. Again, this is an AMPS discussion. If you want call processing in GSM you should download Levine's GSM/PCS .pdf file. First things first. As we'll see in call processing, the mobile idles on the analog control channel or ACC waiting for a call. That's a radio channel, usually the first in a cell's set of frequencies.
Click here for my GSM call processing article
Once a call comes in the mobile switches to a different pair of frequencies; a voice radio channel which the system carrier has made analog or digital. This pair carries the call. If an IS-54 signal is detected it gets assigned a digital traffic channel if one is available. The mobile stays there for the call, returning to the ACC only after the conversation is done. The fast associated channel or FACCH performs handoffs during the call, with no need for the mobile to go back to the control channel. As shown above the fast associated channel is embedded within the digital traffic channel. The DTC is in turn carried on a radio channel. Got it?
The slow associated control channel or SACCH does not perform handoffs but conveys things like signal strength information to the base station. The SACCH runs together with the slot's voice traffic. It's called an associated channel since it is "associated" with the slot that carries the voice. In other words, signaling and voice traffic smoothly together.
The fast associated control channel or FACCH, on the other hand, runs in a blank and burst mode. It transmits during handovers or when the slow associated channel can't send information quickly enough.. Like when entering a tunnel or possibly when a large truck gets in front of you. At that point the data link might be broken so the FACCH acts quickly. As an engineer puts it, "The FACCH overrides the voice payload, degrading speech quality to convey control information." This keeps Mr. Mobile linked to the base station.
All of this goes on while retaining a backward compatibility with analog phone service or AMPS. Don't have digital service in your area? No problem. Your IS-136 phone will still work, just in analog mode and without the fancy features. Speaking of features, IS-136 is now the standard TDMA cellular technology. It adds a digital control channel to the bit stream., enabling features that IS-54 doesn't have, and presenting true competition for Personal Communication Services. So let's keep discussing channels.