Batboard

Hello all...

Some of you may remember the thread about building 4wire links using Cisco routers - which work great for moving analog voice. I know that v.24 is a serial protocol, I know that you require a physical modem added to an Astro device (Quantar, AstroTac RX, Astrotac 3000, etc. What I am trying to figure out is how to send the v.24 data over routers. In looking in to using Astro Modems it does not look like G711ULAW will end up working as it introduces too much time which does not allow the modem's to SYNC. Cisco also does not seem to support the modem signaling which allows the router to sync up with the modem on each end and pass the data stream as digital between the ports. In reading the documentation it looks like it adds 200ms to the time and that's just too much coupled with internet latency.

From talking with a couple of friends of mine on the networking side, it's been suggested to use the CISCO WIC-2T interface which supports Synchronous Serial, then using some configuration in the cisco software to forward the HDLC frames from one router to another. I've got the configuration examples to do this, but I'm also seeing discussion that the links that Cisco provides are not very good. From doing more reading it seems to be that they are not setup for 9600 - they are setup for higher baud rates. This means they wait for the frames to "fill" before sending them, increasing the time.

I don't really care if it takes 3-4 seconds for a frame to go from location A to location B since those sites could be 300 miles away. The subscriber is not going to know. The end point equipment does care which of course causes some issues.

Is anyone actually using Cisco equipment to do what I am talking about doing? Does anyone have any configuration examples or maybe even a contact or two at Cisco that has proposed this sort of solution?

I also could use a bit of a lesson (e.g. does a quantar want to be a DCE or DTE type device?, What do the UDS modems provide? etc? There really is not a ton of documentation - if someone has a system planner which says here are the 3-4 ways you can use this protocol to link that would be even cooler.) I'd also love to set up and bench this experiment - what's the "cheapest" set of used M equipment that I should look at acquiring to run the experiment?)

Thank you in advance.....

Alex

You may have already read my posts here https://www.p25.ca/threads/1856-Explori ... -interface and here http://batboard.batlabs.com/viewtopic.php?f=10&t=104496

While these don't reference the Cisco situation they may be a start.

Put simply the V.24 Astro interface is a 9600 bps synchronous circuit (no star or stop bits like normal serial) which uses a transmit and receive clock signal to identify the centre of each bit, with the bytes identified using unique framing characters ($7E), and a set of rules to ‘bit stuff’ if the data stream happens to contain $7E characters in a way that allows the data to be recovered intact.

On top of this physical layer is a layer two protocol called HDLC which provides mechanism somewhat similar to IP for detecting errors in packets and requesting re-transmission.

The Quantar is a child of its time when leased circuits and 9600 bps were state of the art. It is easy to forget that the Quantar is over 20 years old, IP just wasn’t a big deal back then and Americans didn’t txt on their phones…

The Quantar V.24 daughter board is almost impossible to find. If you have two Quantars side by side they can be easily linked at TTL level which saves the problem of find the daughter cards. Alternative it is not hard to home brew the V.24 interface using your favorite RS-232 interface chips. The European V.24 and American RS-232 recommendations are essentially identical and describe the electrical characteristics of the interface the formats. The physical connections like the original 25 pin connection are in other recommendations. You'll need to provide three drivers and three receivers to implement the interface, data, clock, and control in each direction. It’s easily done on a bit of perf board, if you have the patience, and mounted on the WL card. The connections to the J300 WL board connector and the pins on the Quantar V.24 Astro connector you need to emulate are in those posts.

I would completely forget about trying to use Astro modems over anything other than top grade telco leased circuits. These modems used complex V.29 QAM modulation and needed the best audio analog circuits to work reliably, any sort of VOIP just isn’t going to cut it as they’re designed for the human voice and not tones with complex modulation. If you need to run mixed mode, then that’s another set of issues.

To get the CISCO working I believe you need the feature CEoIP – circuit emulation over IP. Older CISCO products talk about HDLC but they work the other way around, they carry IP over HDLC WAN circuits, you need to carry HDLC over an IP WAN. Different story and different hardware needed. The problem with CEoIP is, as you’ve noted, that today 9600 is so slow that some adapters won’t even go that low and the internal buffers are too big, introducing delays

I’ve been working on a project to do all this with simple hardware but it is still a ways off for the moment. But it is on the ‘to do’ list for this year. Recently another board member has shown interest and I hope this moves things along.

This probably covered things you know already but I thought it worth putting it all in one place.

Go over to www.P25ca.org we are linking stations using Motorola UDS v.3225 Modems and OBI-100 Voip boxes. Next is to get a Astro-Tac so we can connect to it.