There are several ground rules to consider when applying compressed voice equipment in the real world. These are mostly due to the way compressed voice products convert analog voice to a digital format and the bandwidth required for transporting that digitally encoded voice signal. If they are ignored, the system will either work poorly or not at all. When the rules are met, compressed voice products can provide reasonable quality voice communications quite economically.

1. There is no free bandwidth ride. There must be sufficient bandwidth between the two multiplexers to transmit the data. For example, a SR-VM may have up to four voice channels. If they are configured to use 9.6 Kbps data rate each, then the aggregate data rate over the network is is 38.4 PLUS some amount for multiplexer overhead. This will not work over a 33.6 Kbps modem connection! Similarly, If one is to transmit voice compressed to 9.6 Kbps (including overhead) along with data from two terminals running at 19.2Kbps each, there is a total bandwidth requirement of 47 Kbps (19.2 + 19.2 + 9.6) plus a little for multiplexer overhead. This will not work using a 38.4 Kbps communications path, unless the data circuits can be "throttled" back using flow control.… in which case they aren’t really running at 19.2 Kbps, but are simply "connected to the multiplexer" at 19.2 Kbps. In the SR-VM multiplexer, the voice channels have priority over the data channels, so the multiplexers will attempt to control the incoming data to make it fit within the available bandwidth.
2. Compression methods are optimized for voice transmission. A normal telephone connection is converted to data format at the standard rate of 64 Kbps. The SR-VM multipexers use industry standard compression methods to reduce this rate to a much lower value.… toll quality voice is achieved at about 7.2 Kbps, and resonable voice quality only requires 5.333 Kbps. This "bit rate compression" is accomplished by electronically sampling the voice signal and making use of known voice characteristics to lower the amount of data required at the other side of the link to reconstruct the voice signal. This works quite well for voice. But, it does not work for other analog signals such as remote control tones and modem tones (See the touch-tone and fax exception below). For example, there are often pauses between spoken words. This dead time is filtered out and those pauses are transmitted quite efficiently. A solid tone, such as a 2 Khz remote control system command tone has no pauses and will not be transmitted successfully. In fact, it won’t even get through at all.
3. Exceptions for Rule #2… Some tones will get through by "cheating". There are two exceptions to the above rule. Touch tones, those tones generated by
   pressing the buttons on a telephone, are necessary for most phones to be useful. So, touch-tones are converted to digital format at the receiving unit, transmitted to the remote unit in a digital format, then reconstructed from that digital format. Since there are only 16 touch-tones, and you can’t talk on the voice line while pressing the buttons, this works quite well. So Touch-tones are transmitted just fine with the SR-VM. The second "exception" is for fax tones. Group three fax tones are also treated as a special case and detected at the receiving multiplexer, transmitted digitally to the other end, and then reconstructed. Again, since these are "recognized signals", this works. One other tone, a 1004 hz test tone, is also transferred through the system well. This is a commonly used test in the telephone industry, and is included so craftsmen can perform line level tests on the network.
4. Modem tones will NOT pass through a SRVM. The tones used by modems do not fit the voice algorythms used to compress voice sounds. These units will not pass modem tones. You can not use a telephone modem through this system… it simply will not work. Think about it; if the modem tones can be compressed for a 33.6 Kbps modem to work through our 6.4 Kbps compression method; wouldn’t the modems already have that built-in and raise their speed by a factor of 5? We can cheat some physical laws by intelligently applying methods to side-step the rules, but we simply can’t out and out break them.
   
5. The SR-VM will pass data. If one wants to pass data through the system, simply use the SR-08-VM and one or more of the data channels to pass the digital data instead of trying to pump modem tones through the voice channel. For example, instead of a PC with a modem at a remote site connecting to the ISP through a voice channel of the SRVM; place the PC at the remote site and connect its RS-232 COM: port to a data port on the SRVM, then place the modem at the host site where the telephone line is terminated. That way, only data is transmitted through the SRVM, and the modem tones are only between the ISP and the host location… not all the way through the SRVM system.

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