I was slightly mistaken about the access method to the phone line.
He talks about how he DOSEN'T use a DAA chipset. Instead he uses a
isolation tranformer and some optocouplers. He claims that it meets
the Bell 202 standard. It's still really simple.
In summary it is a 2400 baud modem where the Scenix chip handles the
DTMF generation, DTMF detection and all the FSK mod/demod in
software.
Scenix's apnote have several articles on using the chip in telephony
apps:
After thinking about the topic a little it's clear that for one phone
line you could add all kinds of neat stuff for cheap. The author
claims his modem costs $7 in parts.
However I am not sure that it's going to scale up well. The article
said that doing the system dosen't quite use all the horsepower of a
50Mhz part. When you start talking multiple extensions, multiple
phone lines I think you would need mutiple processors and it's going
to get hairy fast. I think I am beginning to see why these systems
are fairly expensive now.
If we really want to be serious about this then the next step would
be to put together a list of hardware features that the card would
have to support and on what scale. With that we should be able to
get a fairly good estimate if its going to be worth the effort.
--
Richard A. Smith Bitworks, Inc.
rsmith@bitworks.com 501.521.3908
Sr. Design Engineer http://www.bitworks.com
Posted: Tue Jan 11, 2000 5:54 pm Post subject: [Asterisk] Low Budget system
How different is it if you use the DAA? That sounds like fewer components
than what they propose here.
Ideally here's what I'd like to see for a basic Asterisk channel:
Required:
* RING detection
* 16-Bit signed linear, little endian DAC/ADC
* Off hook, on hook
Preferable:
* Dialtone detection
* BUSY detection
* DTMF tone detection
* DTMF tone transmission
* Echo Cancellation
* Some kind of compression (G.723.1, GSM, whatever)
Don't go with a serial port -- too slow, and too much CPU overhead,
particularly for large #'s of ports. Instead, I would suggest grafting a
PCI front end on it and using DMA or programmed I/O. By doing so, we
could put multiple interfaces on a single card and support, say, 4 phone
lines on a single card.
Posted: Tue Jan 11, 2000 6:18 pm Post subject: [Asterisk] Low Budget system
This (first putting together a list of features we want our hardware to
have) is an excellent suggestion. Let me start it off (since I am an
unemployed, inexperienced, college student) by suggesting some obvious
features:
Ports:
Analog in from Telco
Analog out to extension
Analog in from telco:
o Ring detection
o DTMF detection
o Caller ID
Analog out to extension:
o Ring generator
o Dial tone generator
o DTMF detection
o Caller ID
A good experement would be to first make a simple two in, two out piece of
hardware (with expandibility in mind) so that we could engineer in other
ports at a later date.
Comments more than welcome (and please feel free to let me know where I've
said something idiotic) !
James
--
"I know not with what weapons World War III will be fought, but World
War IV will be fought with sticks and stones." -- Albert Einstein
On Tue, 11 Jan 2000, Richard A. Smith wrote:
Quote:
Ok I found the article in Circut Cellar. Actually this article is
also available on-line from Circu
Cellar.
I was slightly mistaken about the access method to the phone line.
He talks about how he DOSEN'T use a DAA chipset. Instead he uses a
isolation tranformer and some optocouplers. He claims that it meets
the Bell 202 standard. It's still really simple.
In summary it is a 2400 baud modem where the Scenix chip handles the
DTMF generation, DTMF detection and all the FSK mod/demod in
software.
Scenix's apnote have several articles on using the chip in telephony
apps:
After thinking about the topic a little it's clear that for one phone
line you could add all kinds of neat stuff for cheap. The author
claims his modem costs $7 in parts.
However I am not sure that it's going to scale up well. The article
said that doing the system dosen't quite use all the horsepower of a
50Mhz part. When you start talking multiple extensions, multiple
phone lines I think you would need mutiple processors and it's going
to get hairy fast. I think I am beginning to see why these systems
are fairly expensive now.
If we really want to be serious about this then the next step would
be to put together a list of hardware features that the card would
have to support and on what scale. With that we should be able to
get a fairly good estimate if its going to be worth the effort.
--
Richard A. Smith Bitworks, Inc.
rsmith@bitworks.com 501.521.3908
Sr. Design Engineer http://www.bitworks.com
Posted: Tue Jan 11, 2000 7:01 pm Post subject: [Asterisk] Low Budget system
Hello all - I'm new here. I have some experience with telephony, but
alas only from the software end.
Mark Spencer wrote:
Quote:
Ideally here's what I'd like to see for a basic Asterisk channel:
Required:
* RING detection
* 16-Bit signed linear, little endian DAC/ADC
* Off hook, on hook
Out of curiousity, why 16 bit samples? 8 bit at 8khz is pretty much the
standard, I believe. More resolution won't help signal quality, given
the fact that it's already been quantized by the phone network. I'm not
sure there's any point to an audiophile-grade phone card, when you've
got less than 4khz bandwidth anyway :)
Quote:
Preferable:
* Dialtone detection
* BUSY detection
* DTMF tone detection
* DTMF tone transmission
* Echo Cancellation
* Some kind of compression (G.723.1, GSM, whatever)
This is quite a bit of work, here...
I've worked with one of the commercial product lines, ranging from 8 to
96 ports (loop start and T1) per board, and they use a number of DSPs
for the above, and a coprocessor to handle the dispatching of tasks and
communication with the host. It's fairly complex. I suspect the only
way to make a really cost-effective homebrew board would be to offload
as much of this as possible onto the host. Even then, it's certainly
non-trivial...
Just some thoughts, I don't mean to be discouraging.
Posted: Tue Jan 11, 2000 7:50 pm Post subject: [Asterisk] Low Budget system
Quote:
communication with the host. It's fairly complex. I suspect the only
way to make a really cost-effective homebrew board would be to offload
as much of this as possible onto the host. Even then, it's certainly
non-trivial...
That can work, but if you do the audio compression on the host
then you get an increased latency that can add enough that it
makes the call quality suffer.
Greg
/********************************************************************
Greg Herlein Quicknet Technologies, Inc.
Member of Technical Staff 415-864-5225 x541
gherlein@quicknet.nethttp://www.quicknet.net
*********************************************************************/
Posted: Tue Jan 11, 2000 7:53 pm Post subject: [Asterisk] Low Budget system
Quote:
What sampling frequency? 8Khz a assume.
Exactly.
Quote:
Ok I will assume 2 lines, 2 extensions for now. I haven't really
jumped into the asterisk architecture yet. How is the connection
between the Telco line and the extension made? Via hardware or
software? Hardware implies a crosspoint switch. Software would
imply som sort of multiplexed A/D D/A front in for each line and
letting the host shuttle the data between them. At 8KHz this
shouldn't impose too much load on the host for a small number of
lines. Actually it might make more sense to make the board do this
with out host intervention.
First I'm talking about the line side. That's probably the easier part
(FXO). Doing the FXS (like the PhoneJack by quicknet) I think is probably
more complicated. Bridging calls will generally need to go through the
host, but that shouldn't be too tough.
Quote:
What about USB? The 2.3 kernels support USB don't they? By the time
Posted: Tue Jan 11, 2000 8:05 pm Post subject: [Asterisk] Low Budget system
On Tue, 11 Jan 2000 11:54:07 -0600 (EST), Mark Spencer wrote:
Quote:
How different is it if you use the DAA? That sounds like fewer components
than what they propose here.
I will have to do some more research.. DAA stands for Digital Access
Arrangement so it might not really be what we need since our app
would be almost entirely analog.
Quote:
Ideally here's what I'd like to see for a basic Asterisk channel:
Quote:
* RING detection
* 16-Bit signed linear, little endian DAC/ADC
What sampling frequency? 8Khz a assume.
Quote:
* Off hook, on hook
Ok I will assume 2 lines, 2 extensions for now. I haven't really
jumped into the asterisk architecture yet. How is the connection
between the Telco line and the extension made? Via hardware or
software? Hardware implies a crosspoint switch. Software would
imply som sort of multiplexed A/D D/A front in for each line and
letting the host shuttle the data between them. At 8KHz this
shouldn't impose too much load on the host for a small number of
lines. Actually it might make more sense to make the board do this
with out host intervention.
Quote:
Preferable:
* Dialtone detection
* BUSY detection
* DTMF tone detection
* DTMF tone transmission
Shouldn't much of a problem. Looks like the same subsystem could do
all the above.
Quote:
* Echo Cancellation
* Some kind of compression (G.723.1, GSM, whatever)
Ugh.. Probally some problems here. Unless the compression is really
simple. If we really wanted to include the ablility to do this on
low budget I don't think a microcontroller will cut it. Even at
100Mhz I think you will reach limits quickly. This suggests a
DSP to me. Texas Insturments makes a $10 (Qty 1000) floating point
DSP that sounds like it would be perfect for this. TI used to make a
$100 developent kit to play with.
Quote:
Don't go with a serial port -- too slow, and too much CPU overhead,
particularly for large #'s of ports. Instead, I would suggest grafting a
PCI front end on it and using DMA or programmed I/O. By doing so, we
PCI involves a moderate hit for the ASIC needed to interface to the
PCI bus. I will have to go find out what we used on our last PCI
product and how much it was.
What about USB? The 2.3 kernels support USB don't they? By the time
we get this done 2.4 should be released.
Posted: Tue Jan 11, 2000 8:09 pm Post subject: [Asterisk] Low Budget system
On Tue, 11 Jan 2000 14:01:00 -0500, Kevin Day wrote:
Quote:
communication with the host. It's fairly complex. I suspect the only
way to make a really cost-effective homebrew board would be to offload
as much of this as possible onto the host. Even then, it's certainly
non-trivial...
Just some thoughts, I don't mean to be discouraging.
Keep those thoughts comming. This is the kind of info we need to
determine if this is really something worth tackeling.
Posted: Wed Jan 12, 2000 6:48 pm Post subject: [Asterisk] Low Budget system
On Tue, 11 Jan 2000 13:53:10 -0600 (EST), Mark Spencer wrote:
Quote:
First I'm talking about the line side. That's probably the easier part
(FXO). Doing the FXS (like the PhoneJack by quicknet) I think is probably
more complicated. Bridging calls will generally need to go through the
host, but that shouldn't be too tough.
Forgive my ignorance.. But I don't know all the lingo yet. Whats a
FXO and FXS and what exactly do they do?
Quote:
> What about USB? The 2.3 kernels support USB don't they? By the time
Depends on how expensive USB would be.
Depends on what we think we need. Overall USB is pretty cheap.
$5-10 for an interface chip. It also might be more expandable on the
small scale since you can have something like 128 devices on a USB
link.
I did some research last night. One of the links off the linmodems
page to a company called Ambient which has some chips that will most
of the previous listed requirements and extras. (Including the
compression and echo canceling) AND thier web page says that they are
Linux friendly. The page claims that the PCI Host assisted Modem
(HaM) chipset will have a linux driver soon.
But following the link in the press release brings up the pre-release
data sheet for the Serial, and ISA version of the chip not the PCI.
Hopefully it's just a mixup.
I also found out that you must use a DAA (Data Access Arrangement)
that meets FCC Part 68 or it will have to have FCC approval.
Approvals run ~$2000 per approval.
The price for DAA's I saw referenced was about $25.
I haven't found much info on dealing with the stations though.
Anyone have some pointers to info for interfaceing with various
stations? I know most key systems use custom formats are any of
these documented anywhere? I guess one could just use regular phones
and DTMF codes.
Posted: Wed Jan 12, 2000 7:14 pm Post subject: [Asterisk] Low Budget system
Quote:
>First I'm talking about the line side. That's probably the easier part
>(FXO). Doing the FXS (like the PhoneJack by quicknet) I think is probably
>more complicated. Bridging calls will generally need to go through the
>host, but that shouldn't be too tough.
Forgive my ignorance.. But I don't know all the lingo yet. Whats a
FXO and FXS and what exactly do they do?
FXO - Foreign Exchange Office. The "phone" side...accepts a ringing
signal.
FXS - Foreign Exchange Station. The "line" side...generates the ring
signal.
Personally...I've always thought these backwards, but I guess it depends
on your point of view.
Quote:
I haven't found much info on dealing with the stations though.
Anyone have some pointers to info for interfaceing with various
stations? I know most key systems use custom formats are any of
these documented anywhere? I guess one could just use regular phones
and DTMF codes.
I've never found any interface information...most of the phone-to-switch
communication in commercial PBXs is proprietary and pretty well guarded.
A $7 phone from walmart is probably the easiest way of doing it.
Trap a hookflash from the phone, then you can transfer or put someone on
hold.
Posted: Wed Jan 12, 2000 8:28 pm Post subject: [Asterisk] Low Budget system
On Wed, 12 Jan 2000 14:14:12 -0500 (EST), James Sharp wrote:
Quote:
FXO - Foreign Exchange Office. The "phone" side...accepts a ringing
signal.
FXS - Foreign Exchange Station. The "line" side...generates the ring
signal.
Personally...I've always thought these backwards, but I guess it depends
on your point of view.
Thanks.. Yeah they seem backwards to me as well considering the use
of the word "station" in key systems as meaning the actual phone
device.
Quote:
I've never found any interface information...most of the phone-to-switch
communication in commercial PBXs is proprietary and pretty well guarded.
A $7 phone from walmart is probably the easiest way of doing it.
Trap a hookflash from the phone, then you can transfer or put someone on
hold.
Yeah.. That's probally what I will have to do first round. It's a
shame thought not to be able to use the 20 or so stations that we got
at an auction.
It can't be that complicated I guess I will have to end up reverse
engineering it from what we have.
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