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<div class="div">
<h1>TFO (Tandem Free Operation)</h1>

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<h2>General</h2>

<p>The speech quality of GSM has become more and more important
to operators. Quality is essential in gaining competitive advantage. Nokia
has been evaluating the TFO feature for improving the quality in mobile-to-mobile
calls has been implemented.</p>


<p>In the GSM network, a tandem connection has been created
in a mobile-to-mobile call as in this case as two TRAUs (Transcoding and Rate
Adaptation Units) make unnecessary decoding and encoding in the network.</p>


<a name="apr1641241403" shape="rect"></a>
<div>
<img alt="urn:mars:dn0470158:1:en:global:tiff:data:data" border="0" src="NED?action=retrieve&amp;identifier=dn0470158&amp;edition=1&amp;language=en&amp;coverage=global&amp;encoding=gif&amp;component=data&amp;item=data" /><p class="figure-caption">Figure: MS to MS call, traditional connection</p>
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<p>So far, this has not been a major problem as the proportion
of mobile-to-mobile calls has been rather small and the degradation to the
speech quality of full rate coding caused by tandeming is not very dramatic.
However, the number of mobiles, as well as the amount of MS to MS calls, is
increasing rapidly and the half rate coding with somewhat weaker tandeming
properties is being introduced to the networks. This results in the degradation
in speech quality of these calls becoming more noticeable to the end users.</p>


<p>Improving the quality of mobile-to-mobile calls is being
standardised in ETSI and the work item is called Tandem Free Operation (TFO)
of Speech Codecs.</p>
</div>


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<div class="topic">
<h2>Tandem
Free Operation</h2>

<p>The primary objective of TFO is to improve the speech quality
in mobile-to-mobile calls by avoiding double transcoding. By using TFO, speech
is transported in its compressed form across the network(s), which minimises
the number of transcodings that are done. Subjective tests performed by ETSI
show significant gain in the subjective quality for all speech codecs (FR,
EFR, and HR).</p>


<a name="apr1641242254" shape="rect"></a>
<div>
<img alt="urn:mars:dn0470161:1:en:global:tiff:data:data" border="0" src="NED?action=retrieve&amp;identifier=dn0470161&amp;edition=1&amp;language=en&amp;coverage=global&amp;encoding=gif&amp;component=data&amp;item=data" /><p class="figure-caption">Figure: MS to MS call, basic principle of TFO</p>
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<p>Tandem free should apply to mobile-to-mobile calls independently,
regardless of whether the mobiles are located within the same PLMN network
or in different networks. Tandem free can be established if the same speech
codec – for example, EFR - is used on both sides. It therefore applies to
all existing codecs (FR, HR, and EFR) if the same codec is used on both sides. </p>
</div>


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<div class="topic">
<h2>The
establishment phase and the in-band signalling required</h2>

<p>When a call is started, there are several things that are
not known to the system, although they are essential in order to decide whether
TFO should be applied or not. First of all, the call should naturally be a
mobile-to-mobile call if TFO is to be applied. Furthermore, both transcoders
involved should be TFO-capable and the same codec should be used on both sides,
that is, codec negotiation is not supported in the Nokia Solution. In addition,
the connection between the two transcoders should be transparent.</p>


<p>Therefore, an establishment protocol takes place between
the two transcoders in order to verify these issues. This happens before switching
to the TFO mode while still transmitting the normal PCM speech between the
two transcoders. The establishment phase hence requires an in-band-signalling
channel obtained by performing some bit-stealing on the PCM speech. The bit-stealing
is performed in such a way that the degradation of the speech is inaudible;
establishing TFO will be attempted on all calls whether MS to MS or MS to
fixed phone.</p>


<p>The signalling channel is obtained by stealing the least
significant bit of every 16th PCM sample with a pseudo-random pattern. This
guarantees that the signalling cannot be heard. </p>
</div>


<a name="mar1841248813" shape="rect"></a>
<div class="topic">
<h2>TFO
mode</h2>

<p>In the TFO mode, the compressed speech is transmitted in
a format close to the TRAU frame used today in the Abis interface. The format
is called the TFO speech frame and requires one or two bits per PCM sample
depending on the type of codec (HR for the one-bit case and FR and EFR for
the two-bit case). The compressed speech will be mapped onto the 2 LSB (Least
Significant Bit) of each PCM sample or the LSB depending on the number of
bits required (see the figure below). Therefore there is always some PCM speech
information available in the upper bits. The codec negotiation mechanism will
not be implemented, and TFO operation will start only if both codecs used
by the BSS are identical.</p>


<p>The handling of equipment that could violate the transparency
of the TFO link between TRAUs, like stand-alone acoustic echo cancellers,
is also standardised. In other words, the manufacturers of this equipment
can update it to support TFO transparency. This equipment is called In Path
Equipment (IPE). If IPEs are not updated, the TFO transparency should be guaranteed
by transmission planning in order to make TFO possible.</p>


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<img alt="urn:mars:dn0470173:1:en:global:tiff:data:data" border="0" src="NED?action=retrieve&amp;identifier=dn0470173&amp;edition=1&amp;language=en&amp;coverage=global&amp;encoding=gif&amp;component=data&amp;item=data" /><p class="figure-caption">Figure: TFO mode</p>
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<p>It is possible to implement TFO by changing only the transcoder
software, and there are no modifications to other network elements. The feature
supports all the previous functions of the network and prevents tandem coding
in all the mobile-to-mobile calls when the speech codecs at both ends are
the same, thus improving the speech quality. The only requirements are an
end-to-end digital connection and TRAUs that support the method. A non-digital
connection or non-compatible TRAU results only in a normal-quality call as
the tandem prevention method cannot be used.</p>


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<dl class="note-notification">
<dt class="note-caption">Note</dt>
<dd>TRAU-features that use PCM-coded speech cannot be used
during TFO-calls. These features are AEC, noise cancellation, fixed gain,
and adaptive gain.  Also the Nokia solution does not include the speech codec
negotiation between HR, FR, and EFR. TFO is available for TCSM2 PCM types
A (FR&amp;EFR&amp;144) and B (HR).</dd>
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