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<TITLE>Communicating with other network voice applications</TITLE>
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	<PARAM NAME="Keyword" VALUE="protocol">
	<PARAM NAME="Keyword" VALUE="RTP protocol">
	<PARAM NAME="Keyword" VALUE="VAT protocol">
	<PARAM NAME="Keyword" VALUE="Real-Time Protocol (RTP)">
	<PARAM NAME="Keyword" VALUE="Visual Audio Tool (VAT)">
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<H2><A NAME="protocol"></A>Communicating with other network voice programs</H2>

<P>Note: sending voice over the Internet is complicated enough
even when the same application is used on both ends.   <i>Speak
Freely</i> supports the emerging standard protocol, RTP, and will
eventually transition to using it as the default.  Since the
standard is so new, no program, <i>Speak Freely</i> included, can be
absolutely guaranteed to support it completely correctly.  So
unless you are an experienced <i>Speak Freely</i> user who needs to
communicate with users of other programs which support RTP or
VAT protocols, you're well advised to ignore this section.  As
long as you're talking to another <i>Speak Freely</i> user, <i>Speak
Freely</i>'s own protocol (selected by default) will give you
better performance and more secure encryption than are
available with the standard interchange protocols.  Further,
many of the widely-publicised commercial Internet voice
programs have their own proprietary protocols and do not yet
support RTP.  Do not assume you can communicate with a user of
such a program unless its vendor explicitly certifies it to be
RTP compliant.</P>

<P>As voice communication over the Internet moves from the
exotic to the everyday, standards are emerging which allow
users of a variety of Internet voice programs to communicate
with one another.  As users demand the ability to speak to one
another regardless of which program they're using, vendors will
be forced to conform to these standards.</P>

<P><i>Speak Freely</i> supports the Internet Real-Time Transport
Protocol (RTP) (RFC 1899) and RTP Profile for Audio and Video
Conferences with Minimal Control (RFC 1890), as issued in
January of 1996.  See the <A HREF="bookshelf.htm">Bookshelf</A>
for complete citations of these documents, including where you
can download them on the Internet.  In addition to RTP, <i>Speak
Freely</i> also supports the protocol used by the Lawrence Berkeley
Laboratory Visual Audio Tool (VAT), a widely used Unix
conferencing program.  (Starting with Version 4, VAT supports
RTP in addition to its native protocol, so there's no reason to
use the more limited VAT protocol except when communicating
with an earlier version of VAT, or a VAT-compatible program
which does not yet support RTP.)</P>

<P>When you receive sound from a remote site, <i>Speak Freely</i>
automatically detects the protocol the user is transmitting and
displays this in the connection window (unless a <A
HREF="face.htm">face image</A> is displayed, but since only
<i>Speak Freely</i>'s native protocol supports face images, if a face
appears you know you're connected to another copy of <i>Speak Freely</i>).</P>

<P>To transmit to the user of an RTP or VAT compatible
application, you must select the appropriate protocol with the
Options/Protocol menu item--<i>Speak Freely</i> does not automatically
transmit in the protocol it's receiving, so if you are
establishing a new connection to a site, you have to choose a
protocol that site understands.  (Since <i>Speak Freely</i>
understands all three protocols, you can communicate with <i>Speak
Freely</i> users regardless of which protocol you've chosen.)</P>

<P>Due to design differences in the various protocols, the
following restrictions apply when using RTP and VAT
protocols:</P>

<p>
<ul>
<li>Simple and LPC-10 <a href="compresswhy.htm">compression modes</a> cannot 
be selected.  RTP and VAT protocols do include these forms of compression.

<p>
<li>The only <a href="encryptwhy.htm">encryption option</a> available is 
<a href="encryptdes.htm">DES</a>.  The other forms of encryption provided by
<i>Speak Freely</i> are not presently a part of the specification of the
other protocols.

<p>
<li>VAT protocol programs do not all use the same 
transformation of the key string into the DES key, so DES encryption may or may 
not work in VAT protocol, depending on which program you're talking to.  It's best 
to use RTP protocol if at all possible.

<p>
<li>The <a href="encryptdes.htm">DES encryption</a> specified for RTP and VAT 
protocols includes the initial and final bit permutations, which most cryptographers 
believe serve only to deter software implementations.  They do this quite 
effectively--DES encryption in RTP and VAT protocol require a substantially faster 
CPU to perform in real time than the permutation-free DES used by <i>Speak Freely</i>'s 
own protocol.

<p>
<li>RTP and VAT use smaller packets than <i>Speak Freely</i>. 
This,  coupled with the relatively poor real-time response of
Windows, may result in  "choppier" audio than with <i>Speak
Freely</i>'s own protocol.  The stronger the  compression mode
you've selected (GSM and LPC are the strongest), the less the 
small packets will degrade the audio quality.  A <a
href="workarounds.htm">workaround</a> allows  you to transmit larger
packets in RTP protocol, but this is not guaranteed to work 
with all RTP applications.

<p>
<li>RTP and VAT encrypt the entire transmitted packet, as 
opposed to including an in-the-clear prefix containing control
information of no use  to an eavesdropper as <i>Speak
Freely</i> does.  This has the consequence that if  somebody is
sending you encrypted packets in RTP or VAT protocol, there's
no  way to determine the identity of the sender or to discern
what protocol they're  sending.
</ul>

<h3>Payload Types Supported</h3>

<P>The "payload type" of a real-time protocol packet refers to
the encoding and compression modes used to represent the data
within it.  All applications are not required to support all
payload types, though support of a minimum subset is
encouraged.  <i>Speak Freely</i> supports all the payload types of VAT
protocol and the following RTP payload types, using the
nomenclature of RFC 1890.</P>

<p>
<blockquote>
<b>        DVI4, GSM, L16, LPC, PCMA, PCMU</b>
</blockquote>

<P>To transmit in the protocol RTP and VAT refer to as DVI,
select ADPCM Compression on the Options menu.  PCMA is
supported on receive only--since PCMU provides equivalent
compression and fidelity and is one of the recommended minimum
subset payload types, there is no need to implement PCMA
transmission.</P>

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