If you were Registered and logged in, you could reply and use other advanced thread options
|
Posted by Dave Garland on September 6, 2008, 12:35 am
It was a dark and stormy night when hancock4@bbs.cpcn.com wrote:
>Today many public safety units are converting to digital radios from
>analog. However, there have been many newspaper reports of digital
>radios failing in critical situations due to dead spots, apparently a
>digital signal is harder to receive than the older analog signals.
An analog signal degrades gracefully. As reception becomes worse and
worse, it just becomes noisier and more staticky up to the point where
you can't make out what they're saying any more. There are various
tricks that can be used to reduce noise. And when things are bad, words
can be repeated or spelled out.
A digital signal either works, or it doesn't. So reception is good,
then it craps out completely. (People with over-the-air reception of
digital TV will notice this.)
>I'm not sure what the advtg of digital over analog is in public safety
>applications (saving bandwidth?).
Yes. And that it can easily be used for things other than voice, and
can be easily encrypted to prevent eavesdropping.
>But one capabiltiy lacking today is
>optional common channels between police, fire, and rescue which often
>work on isolated networks.
That's not a digital problem, that's a design or administrative one. In
fact, that situation is probably easier to deal with today than it was
in the past, where the radios were crystal controlled and could only
operate on one (or a very few) channels.
Dave
|
|
Posted by Tor-Einar Jarnbjo on September 6, 2008, 12:49 pm
Dave Garland schrieb:
> A digital signal either works, or it doesn't. So reception is good,
> then it craps out completely. (People with over-the-air reception of
> digital TV will notice this.)
That is not always the case and for digital cell phone networks in most
cases wrong. Even the first digital GSM networks (early 90s) used
different levels of error correction for parts of each data packet,
making it possible for a receiver with bad reception still to decode the
most important parts of the data, resulting in lower voice quality. Even
if the reception is so poor, that some data cannot be completely
restored by the error correction algorithms, the receiver usually still
tries to decode the erroneous data. Depending on the actual codec, this
causes different problems, like e.g. blocking artifacts in the digital
tv picture.
The AMR voice codec used in more recent UMTS networks, even allows the
network and cell phone to dynamically switch between eight different
levels of error correction during a conversation to find the optimal
point on a scale between an error-prone/high effective bitrate/high
voice quality signal and a robust, but low voice quality signal.
At some point, the reception is of course too poor for the receiver to
do anything useful at all with the signal, but the steps leading there
are far more complex than an either/or decision.
Tor
|
|
Posted by harold@hallikainen.com on September 6, 2008, 5:32 pm
I remember RCC! In the late 1970s and early 1980s, I had a GE mobile
radio in my 1971 VW van. As I recall, it was either all vacuum tube,
or maybe just tube finals. I think it had a transistorized power
supply to generate the high voltage. My RCC number was 248-0663. I
think the 248 identified the radio common carrier, which was RCS in
San Luis Obispo. It was owned by Jerry Peterson, who just died
recently. I also had a tone only pager from them and used their
answering service. They had a bunch of operators on cord boards. The
answering service used bridge connections off the cable pair going to
our shop. I don't think any sort of call forwarding was available at
the time. They also operated the marine VHF mobile telephone channel.
My father lived on a boat. When he came into the area and tried to
call me from the boat, the operators knew exactly where I was. That
impressed him! My RCC radio did not have selective calling, so I got
to listen in on all the other conversations as I drove around. Jerry
sold RCS as cellular telephones started to appear. He told me that he
knew he could not compete with them and did not think he could get
into the cellular business. The answering service is still running,
but moved from the old location. All those telephone lines still show
up in the old building. They put several call concentrators in there
to route those lines to their new office where operators sat in front
of computers (not cord boards!). I worked on that call concentrator
maybe 20 years ago, so I have no idea if it's still there.
0663 Clear!
|
|
Posted by on September 6, 2008, 11:33 pm
> That is not always the case and for digital cell phone networks in most
> cases wrong. Even the first digital GSM networks (early 90s) used
> different levels of error correction for parts of each data packet,
> making it possible for a receiver with bad reception still to decode the
> most important parts of the data, resulting in lower voice quality.
A few years ago I was on a train with my by then old analog cell
phone. Suddenly, everyone else talking on the train lost their signal
(they're all going hello and staring at their phone). My phone
continued to work.
Now that I'm digital I find more dead spots and cutouts than I had
with analog, though very recently reliability seems to have improved.
|
|
Posted by Tor-Einar Jarnbjo on September 8, 2008, 11:10 am
hancock4@bbs.cpcn.com schrieb:
> A few years ago I was on a train with my by then old analog cell
> phone. Suddenly, everyone else talking on the train lost their signal
> (they're all going hello and staring at their phone). My phone
> continued to work.
The reason for that was probably that your cell tower was in a different
location, the analogue network operated with higher transmission power
or that the lower radio frequencies used by AMPS propagate more easily
through buildings than the higher frequencies used by the newer digital
networks. It is unlikely that the different transmission technology
(analogue or digital) caused the behaviour you observed.
> Now that I'm digital I find more dead spots and cutouts than I had
> with analog, though very recently reliability seems to have improved.
I suppose that this was the case with every new mobile network
technology. Early AMPS users probably complained about worse coverage
compared to IMTS as well. The main reason is the smaller cell size and
reduced transmission power used on purpose to increase network capacity.
Recently, high power digital networks, operating on lower frequencies
have been installed in several countries to achieve better rural
coverage, e.g. ice.net operating a CDMA based network at 450MHz in the
Scandinavian countries. With relatively low network capacity, such
solutions cannot however target the mainstream user.
Tor
|
Page 3 of 5 < 1 2 3 > last >>
| Similar Threads | Posted | | Re: 1965 mobile phone on "Get Smart" [telecom] | September 6, 2008, 11:39 pm |
| Smart Phone Wars: Apple vs. RIM vs. ...the Android Operating System? [Telecom] | April 5, 2008, 1:31 pm |
| Motorola Makes TV Shows Mobile -- From DVR to Mobile Phone and | April 6, 2006, 9:13 am |
| Re: size a major consideration in mobile phone sets [Telecom] | April 30, 2009, 11:25 pm |
| Federal Court to Rule on Privacy of Mobile Phone Location Data [Telecom] | February 8, 2010, 12:11 am |
| "Get Smart" TV show coming as movie [telecom] | May 25, 2008, 11:08 pm |
| Trading Nude Photos Via Mobile Phone Now Part of Teen Dating, Experts Say [Telecom] | April 14, 2008, 11:15 pm |
| Mobile Phone TV Set For Primetime | May 9, 2005, 12:54 pm |
| New Mobile Phone forum | November 15, 2005, 7:20 am |
| Ever Want to Throw Away Your Mobile Phone? | August 28, 2006, 11:00 pm |
|
|
Yahoo! 
Google 
Windows Live 
del.icio.us 
digg 
Netscape 
XML site map
>analog. However, there have been many newspaper reports of digital
>radios failing in critical situations due to dead spots, apparently a
>digital signal is harder to receive than the older analog signals.