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TUCoPS :: Phreaking General Information :: digitals.txt

Semi-Digital and Digital Systems




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The most commonly used digital cellular system in America uses
TDMA or time division multiple access . Call set up is the same
as for AMPS, it's just that the conversation gets digitized 
and multiplexed. A conversation gets passed to TDMA once the 
call gets going. TDMA based systems and most TDMA phones can 
handle AMPS calls as well. TDMA's chief benefit comes from 
increasing call capacity -- a channel can carry three 
conversations instead of just one. But, you say, so can NAMPS,
Motorola's analog system that we looked at last issue. What's
the big deal?

NAMPS can carry the same number of calls as most TDMA systems.
NAMPS though, has the same fading problems as normal AMPS, it
lacks the error correction that digital systems provide and it
isn't sophisticated enough to handle encryption or advanced 
services. Things such as calling number identification, 
extension phone service and messaging. In addition, you can't
monitor a TDMA conversation as easily as an analog call. So, 
there are other reasons than call capacity to move to a 
different technology. Many people ascribe benefits to TDMA 
because it is a digital system. Yes and no. Advanced features
depend on digital but conserving bandwidth does not. How's 
that? Three conversations get handled on a single frequency.
Call capacity increases. But is that a virtue of digital? No,
it is a virtue of multiplexing. A digital signal does not 
automatically mean less bandwidth, in fact, it means more. 
Multiplexing means transmitting two or more conversations on 
the same frequency at once. In this case, small parts of three
conversations get sent simultaneously. This is not the same as
NAMPS, which splits the frequency band into three discrete 
sub-frequencies of 10khz apiece. TDMA uses the whole frequency 
to transmit while NAMPS does not. NAMPS does not involve 
multiplexing. And besides, TDMA is a hybrid system, combining
both analog and digital components. It must be since it uses 
the AMPS protocol to set up calls. Despite what the marketing 
boys say, only CDMA or code division multiple access is a fully
digital system. More on CDMA later. Let's look at some TDMA 
basics first.

We see that going digital doesn't mean anything special. A 
multiplexed digital signal is what is key. Each frequency gets
divided into six repeating time slots or frames. Two slots in 
each frame get assigned for each call. An empty slot serves as 
a guard space. This may sound esoteric but it is not. Time 
division multiplexing is a proven technology. It's the basis 
for T1, still the backbone of digital transmission in this 
country. Using this method, a T1 line can carry 24 separate 
phone lines into your house or business with just an extra 
twisted pair. Demultiplexing those conversations is no more 
difficult than adding the right circuit board to a PC. TDMA 
is a little different than TDM but it does have a long history
in satellite working.

What is important to understand is that the system synchronizes
each mobile with a master clock when a phone initiates or 
receives a call. It assigns a specific time slot for that call
to use during the conversation. Think of a circus carousel 
and three groups of kids waiting for a ride. The horses 
represent a time slot. Let's say there are eight horses on the
carousel. Each group of kids gets told to jump on a different
colored horse when it comes around. One group rides a red horse,
one rides a white one and the other one rides a black horse. 
They ride the carousel until they get off at a designated point.
Now, if our kids were orderly, you'd see three lines of children
descending on the carousel with one line of kids moving away. In
the case of TDMA, one revolution of the ride might represent one
frame. This precisely synchronized system keeps everyone's call 
in order. This synchronization continues throughout the call. 
Timing information is in every frame. Any digital scheme, 
though, is no circus. The actual complexity of these systems 
is daunting. I invite you to read further if you are interested. 

There are variations of TDMA. The only one that I am aware of in
America is E-TDMA. It's operated in Mobile, Alabama by Bell 
South. Hughes Network Systems developed E-TDMA or Enhanced TDMA.
It runs on their equipment. Hughes developed much of their 
expertise in this area with satellites. E-TDMA seems to be a 
dynamic system. Slots get assigned a frame position as needed. 
Let's say that you are listening to your wife or a girlfriend. 
She's doing all the talking because you've forgotten her 
birthday. Again. Your transmit path is open but it's not doing
much. As I understand it, "digital speech interpolation" or DSI
stuffs the frame that your call would normally use with other 
bits from other calls. In other words, it fills in the quiet 
spaces in your call with other information. DSI kicks in when 
your signal level drops to a pre-determined level. Call capacity
gets increased over normal TDMA. This trick had been limited 
before to very high density telephone trunks passing traffic 
between toll offices. Their system also uses half rate vocoders,
advanced speech compression equipment that can double the amount
of calls carried.

Code Division Multiple Access has many variants as well. 
InterDigital, for example, produces a broadband CDMA system 
called B-CDMA that is different from Qualcomm's narrowband CDMA 
system. For this article, however, I'll just mention a few things.
I give references at the end of the article for those going 
further. A CDMA system assigns a specific digital code to each 
user or mobile on the system. It then encodes each bit of 
information transmitted from each user. These codes are so 
specific that dozens of users can transmit simultaneously on 
the same frequency without interference to each other. They are
so specific that there is no need for adjacent cell sites to 
use different frequencies as in AMPS and TDMA. Every cell site 
can transmit on every frequency available to the wireline or 
non-wireline carrier. CDMA, is also much less prone to 
interference than AMPS or TDMA. That's because the specificity 
of the coded signals helps a CDMA system treat other radio 
signals and interference as irrelevant noise. Some of the 
details of CDMA are also interesting.

Qualcomm's CDMA system uses some very advanced speech 
compression techniques, in particular, a variable rate vocoder.
Phil Karn, one of the principal engineers has written that it 
"operates at data rates of 1200, 2400, 4800 and 9600 bps. When a 
user talks, the 9600 bps data rate is generally used. When the user
stops talking, the vocoder generally idles at 1200 bps so you still
hear background noise; the phone doesn't just 'go dead'. The 
vocoder works with 20 millisecond frames, so each frame can be 3,
6, 12 or 24 bytes long, including overhead. The rate can be 
changed arbitrarily from frame to frame under control of the 
vocoder." This is really sophisticated technology. 




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