<-- Last topic: Digital principles Next topic: Channels -->

XI. Frames, slots, and channels

Frames, slots, and channels organize digital information. They're key to understanding cellular radio. And discussing them gets really complicated. So let's back up, review, and then look at the earliest method for organizing digital information: Morse code.

 

We saw in the last page how information gets converted from sound waves to binary numbers or bits. It's done by pulse code modulation or some other scheme. This binary information or code is then sent by electricity or light wave, with electricity or light turned on and off to represent the code. 10101111, for example, is the binary number for 175. Turning on and off the signal source in the above sequence represents the code.

 

Early digital wireless used a similar method with the telegraph. Instead of binary, though, they used Morse code. Landline telegraphs used a key to make or break an electrical circuit, a battery to produce power, a single line joining one telegraph station to another, and an electromagnetic receiver or sounder that upon being turned on and off, produced a clicking noise.

A telegraph key tap broke the circuit momentarily, transmitting a short pulse to a distant sounder, interpreted by an operator as a dot. A more lengthy break produced a dash. To illustrate and compare, sending the number 175 in American Morse Code requires 11 pulses, three more than in binary code. Here's the drill: dot, dash, dash, dot; dash, dash, dot, dot; dash, dash, dash. Now that's complicated! But how do we get to wireless?

 

Let's say you build a telegraph or buy one. You power it with, say, two six volt lantern batteries. Now run a line away from the unit -- any length of insulated wire will do. Strip a foot or two of insulation off. Put the exposed wire into the air. Tap the key. Congratulations. You've just sent a digital signal. An inch or two. The line acts as an antenna, conducting electrical energy. And instead of using a wire to connect to a distant receiver, you've used electromagnetic waves, silently passing energy and the information it carries across the atmosphere.

 

Transmitting binary or digital information today is, of course, much more complicated and faster than sending Morse code. And you need a radio transmitter, not just a piece of wire, to get your signal into the practical radio spectrum. But transmission still involves sending code, represented by turning energy on and off, and radio waves to send it. And as American Morse code was a logical, cohesive plan to send signals, much more complicated and useful arrangements have been devised.

We know that 1s and 0s make up binary messages. An almost unending stream of them, millions of them really, parade back and forth between mobiles and base stations. Keeping that information flowing without interruption or error means keeping that data organized. Engineers build elaborate data structures to do that, digital formats to house those 1s and 0s: frames, slots, and channels. Frames hold slots which in turn hold channels. These elements all act together. We'll discuss these in turn. Here's the heiarchary again:

Frames

Slots

Channels

A frame is an all inclusive data package. A sequence of bits makes up a frame. Bit stands for binary digit, 0s and 1s that represent electrical impulses. (Go back to the previous discussion if this seems unclear.) A frame can be long or short, depending on the complexity of its task and the amount of information it carries. A frame carries conversation or data as well as information about the frame itself. More specifically, a frame contains three things:

1. Control information, such as a frame's length, its destination, and its origin;

2. The payload or content, the actual call or data;

3. A error checking routine, known as "error detection and correction bits." These help keep the data stream intact while the mobile moves about.

Slots hold individual call information within the frame, that is, the multiplexed pieces of each conversation as well as signaling and control data. With TDMA, used in IS-136, and most GSM systems, each user occupies a radio frequency for a predetermined amount of time in an assigned time slot. Calls are combined or multiplexed into a digital stream by the base station. It assigns these chopped up bits and pieces into an efficient order by putting that information into the right time slots at the right time. Most TDMA based systems use two slots out of a possible six.

Multiplexing combines several different calls into one coherent digital stream. 

 

Channels handle call processing, the actual mechanics of a call. Don't confuse these data channels with radio channels. Two radio frequencies make a cellular radio channel. One frequency to transmit on and one to receive. In digital working, however, we call a channel a dedicated time slot within a data or bit stream. We'll go over this again soon. A channel sends particular messages. Things like pages, for when a mobile is called, or origination requests, when a mobile is first turned on and asks for service.

We'll discuss frames, slots and channels further by looking at representative diagrams of different digital communication systems. I'm not trying to depict every digital cellular or PCS format. I'm trying instead to give you enough terms and ideas that so that you can understand the basics and so that you can go further.

1. Frames

Generic frame with time slots

In the diagram above we look at the basis of time division multiplexing. As we've discussed, TDMA or time division multiple access, places several calls on a single frequency. It does so by separating the conversations in time. Its purpose is to expand a system's carrying capacity while still using the same numbers of frequencies. In the exaggerated example above, imagine that a single part of three digitized and compressed conversations are put into each frame as time goes on.

In IS-136 each radio channel is 30 KHz wide, just as with conventional cellular. Frequencies and control channels are the same, in fact, the whole system is compatible with AMPS, since, at least with IS-136, call setup is done using the AMPS protocol. The difference is that voice traffic is digitized, compressed, and multiplexed to save space or bandwidth. This is true with all digital schemes and bears repeating. With digital, voice traffic is digitized, compressed, and multiplexed to use as little bandwidth as possible.

2. Slots

IS-54B, IS-136 frame with time slots

TDMA puts each time segment into 6 slots. Two slots make up one voice circuit. 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? If not, go back. Let's look at what's contained in just one slot of half a frame in digital cellular.

IS-54B time slot structure (Part of the digital traffic channel)

All numbers refer to the amount of bits. Note that data fields and channels change depending on direction. 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. Full explanations on next page. 

<-- Last topic: Digital principles Next topic: Channels -->

 

Text link to CellTowerInfo.com