karn@thumper.bellcore.com (Phil R. Karn) (03/05/88)
> Has anybody had experience in transmitting PCM encoded voice over Ethernet? > Is it practical to use Ethernet for such an application? Does anybody know > of attempts to implement this? Yes, we've been doing it here at Bellcore for years. It began back at Bell Labs before the AT&T divestiture. We currently have several dozen "packet phones". Externally they look just like ordinary office-style desk phones, except that there's an Ethernet transceiver cable coming off the back instead of an ordinary telephone cable. Inside is a 68000 single-board computer with RAM and boot ROM, a standard telephone CODEC chip, and DMA. When you pick up the receiver, you hear dial tone. This is coming from the local 68000 processor. When you push buttons on the dial, you hear touch-tones; these are also generated locally for your benefit. When you finish dialing, a call-request packet is sent out on the net to the called phone; an ack packet from the remote phone causes your local phone to generate standard audible ringing until the remote phone sends another packet indicating that it has been answered. When you talk, you generate a constant-size packet stream at a 8 kilobyte/sec rate (standard, uncompressed u-law PCM voice); when you stop talking, a threshold algorithm stops the packet generation. We currently run with a packet size somewhere around 160 bytes; this represents a compromise between voice delay and efficient use of the network. The processor also contains TCP/IP software. There's an RS-232 jack on the back panel for a dumb terminal so you can talk and hack at the same time. It also tells you who's calling. Two years ago a summer student added some nifty voice storage features. If you don't answer your phone, a standard FTP session is set up with a local Sun file server and a voice message can be recorded. You get your messages by hitting a few keys on the dial; this retrieves your voice mail, again using vanilla FTP. Now that we have Ethernet bridges with T-1 lines between the various Bellcore locations, we can carry our phones around with us to the different locations and plug them in. Phil
asher@TONTO.CC.ROCHESTER.EDU (Samuel Asher) (03/10/88)
That sounds like a wonderful use of communication lines. Is there any work under way to bring this kind of service (ie digital packets plus voice). into the home. I worked at a company a few years back where that was our goal (we called them digital phones). The hope was that the Divestiture would make such an advance possible, however, I have seen none of it. There were also some technical difficulties to be overcome (such as easily adding extension phones) and inertia problems (such as the huge investment in analog phones). - Sam Asher asher@cc.rochester.edu
budden@tetra.NOSC.MIL (Ray A. Buddenberg) (03/11/88)
With the advent of FDDI and 100 mbit capacities, there is some increase in interest in digital voice. The real problem has to do with latency in the network -- if you aren't down to a couple milliseconds, things don't come out very practically. I know a couple companies that are working on integrating voice along with data (and video) on FDDI based fiber nets toped with lightweight transport protocols. The applications are shipboard LANs rather than household though. You aren't yet in the area where you can compete with the phone yet. Rex Buddenberg