steele@EE.ECN.PURDUE.EDU (Richard Steele) (12/07/89)
Thanks to everyone who responded to my question about crest factors; the subject, though now I see a rather simple one in concept, has been bothering me for a while. Now, for another one: Here at Pudue U., we now have access to something called data over voice (DOV) units. These are small boxes, about the size of a regular modem, that gives the user a 9600 bps asynchronous data line to the University computer. Not only is the bugger faster than a modem, _but it leaves the phone free for regular use_. We can still receive and make calls with the DOV working with no obvious distortion on the line. Thus, in addition to having a reasonably fast connection (and I used to think a 2400 baud modem was speedy!), I don't have to take the wrath of my roomates for hogging the phone line all night long. Question: How would something like this work? The phone company _does_ need to make some changes to their central switch (or something like that -- although I'm a EE, telephony is definately _not_ my specialty. :-) ) So, what magic is involved here? Thanks again to all who respond... Richard A. Steele Purdue University
couric@rm1.UUCP (Carl Couric) (12/08/89)
The phone line that can do both voice and data transmittion is a part of the new ISDN. Here at RM, we have been working on this for quite some time. Your twisted pair to your house/area contains digital only signals. The box splits the signals at 64KB up, to produce your two lines. A digital one (easy convertion!) and a voice signal of 3Khz bandwidth. I am awaiting for our local PBX (846-XXXX) to go ISDN world wide soon. I live close enough to get onto this PBX and no longer have to DIAL in but have a 9600 direct connect to work. The best as you have found, I still can get and make phone calls. What a country? Carl -- ------------------------------------------------------------------------------- R. Carlton Couric, Racal-Milgo, (305) 846-6706 and NO COLLECT CALLS! VAX VMS Cluster Manager -KB4RLS- (Even if its your daughter!) -------------------------------------------------------------------------------
henry@utzoo.uucp (Henry Spencer) (12/09/89)
In article <8912061817.AA28752@en.ecn.purdue.edu> steele@en.ecn.purdue.edu (Richard Steele) writes: >Here at Pudue U., we now have access to something called data over voice >(DOV) units. These are small boxes, about the size of a regular modem, >that gives the user a 9600 bps asynchronous data line to the University >computer. Not only is the bugger faster than a modem, _but it leaves >the phone free for regular use_... >Question: How would something like this work? The phone company _does_ >need to make some changes to their central switch (or something like that... The wires connecting your phone to the phone company have lots more bandwidth than voice can use, as witness folks who are using similar wiring (admittedly over shorter distances) to carry 10Mb/s Ethernet signals. The bandwidth limitations on normal voice calls generally come in the switching gear and long-haul connections, not in the local wiring. So *if* your phone company (or your university, if it has its own phone system) is willing to put a box on the other end of those wires, and connect that box to something, it's not difficult for something on your end to use some of that bandwidth to talk to said box digitally. With a bit of care in design, it can avoid interfering with the miniscule fraction of the bandwidth normally used for voice. Presto, data and voice on the same wires. The hard part is convincing the phone company to put that box on your wires, and put matching boxes on the wires of the places you'd like to talk to, and connect all the boxes together, and not charge you two arms and a leg for it. -- 1233 EST, Dec 7, 1972: | Henry Spencer at U of Toronto Zoology last ship sails for the Moon. | uunet!attcan!utzoo!henry henry@zoo.toronto.edu
davidc@pyr.gatech.EDU (David Carter) (12/09/89)
In article <8912061817.AA28752@en.ecn.purdue.edu> steele@en.ecn.purdue.edu (Richard Steele) writes: >Here at Pudue U., we now have access to something called data over voice >(DOV) units. Here at Georgia Tech, I thought the service was called Voice Over Data. >Question: How would something like this work? The phone company _does_ >need to make some changes to their central switch... >So, what magic is involved here? The box contains both a 9600 bps modem and low-pass and high-pass filters. A similar device is installed at the CO. I was also under the impression that you must be reasonably close to your CO for this to work. The service uses Frequency Domain Multiplexing -- it gives your phone a band from 0 to 4KHz. Everything higher is filtered out so you don't hear the modem noise. Otherwise, your phone is working the way it always did, on an analog line. Similarly, the modem is kept from hearing everything below some frequency (probably a little bit higher than 4KHz). That's all there is to it. If you could listen to the phone line before it goes into the box, you'd hear both your voice calls and the modem noise. There is probably just a bit more circuitry to enable you to still get incoming ring without blowing up the modem, and to get a dialtone when you pick up the phone. But I don't know the details. I've only heard about the service here (available to certain dorm residents); I haven't actually used it. I think users had to place a very short phone call to the CO (maybe a *nn or #nn type thing) in order to activate the modem each time it is used. David Carter davidc@pyr.gatech.edu
davidc@pyr.gatech.EDU (David Carter) (12/09/89)
I forgot to mention -- there are also high-capacity line(s) from the CO to the university's computer (like 56K bps) on which several of the 9600 bps connections are multiplexed. I know that the system here at Georgia Tech is =not= ISDN. ISDN provides for the data to go to other CO's (or even other cities), a feature that isn't needed in our case. The Voice Over Data hardware is also much less expensive. David Carter davidc@pyr.gatech.edu