tfo&trfo technique

1. WirelessMoves: Deep Inside the Network: TFO and TrFO http://mobilesociety.typepad.com/mobile_life/2006/11/i_was_not_sure_.html
WirelessMoves
Thoughts on the evolution of wireless networks and mobile web 2.0
Deep Inside the Network: TFO and TrFO
I was not sure if I should post my “TFO and TrFO introduction” here as there are probably only a handful
of people on the globe who are interested in this topic. Also, some prior knowledge is required on how
GSM and UMTS networks handle circuit switched calls and how things change with the introduction of
3GPP Release 4 BICN functionality. A good intro on this topic can for example be found in my book (take
a look on the left). Most of my blog entries require no such in-depth prior knowledge but this one
definitely does. I decided to do it anyway as I didn’t find any other usable overview on the web.
Tandem Free Operation (TFO) and Transcoding Free Operation (TrFO) Overview
Voice calls from mobile phones today always involve several voice transcoding steps in GSM and UMTS Release 99
networks. This is done mainly to transport a voice call, which uses sophisticated compression in the radio network,
through the core network with the standard G.711 PCM (Pulse Code Modulated) codec over 64 kbit/s circuit switched
links. Furthermore, transcoding a voice data stream to the 64 kbit/s G.711 PCM codec is necessary in order to call fixed
line phones. Another advantage of voice transcoding in the network is to be able to process the voice data stream and to
compensate for unwanted effects such as echo in the speech path.
With the introduction of the Bearer Independent Core Network functionality in the 3GPP Release 4 standards, core
network connections are based on high bandwidth ATM or IP links rather than on 64 kbit/s TDM links. In such networks
it is therefore possible to transmit a voice data stream in the core network with other codecs than 64 kbit/s G.711. This
also has the additional benefit of removing transcoders from the speech path which reduces speech path delay and
improves speech quality to some degree as the speech signal is no longer degraded by the transcoding process.
Furthermore, not having to convert to the narrow voice band G.711 codec allows the use of AMR Wideband (AMR-WB)
codecs. These codecs are not backwards compatible to G.711 without loosing their much better audio quality which they
achieve by encoding twice the frequency range as previous Full Rate, Enhanced Full Rate, AMR and G.711 codecs. A
general downside of not using a transcoder in the network is that it is no longer possible to process the voice data stream
to remove effects such as echo.
Transcoder Free Operation (TrFO)
In UMTS Bearer Independent Core Networks (BICN), PCM links which require the use of the G.711 codec are no longer
used. Instead, connections to RNCs in the radio network and connections to other Media Gateways are based on ATM
and IP. Therefore, the MSC servers involved in a call can establish a bearer path without activating voice transcoders in
the media gateways between two mobile phones. For a mobile to landline call the connection remains transcoder free up
to the media gateway which connects the wireless network to the fixed network. Here, the transcoder is used to convert
the compressed audio signal to G.711.
In mobile to mobile calls, both handsets report their codec capabilities to the MSC servers involved in the call as part of
the signaling before the bearer path is established. Thus, an AMR-WB codec is automatically used if supported by both
terminals. This greatly improves the voice quality of the call. If not supported on both ends, a standard narrow band
AMR codec will be used for the connection instead. This is called Transcoder Free Operation (TrFO) and the out of band
transcoder control performed by MSC servers is described in 3GPP TS 23.153.
While TrFO works well while both ends of the connection remain in the UMTS network, it has to be deactivated if one of
the two parties is handed over to a GSM network during the connection. This happens, for example, if the subscriber
roams out of the UMTS coverage area or in case the UMTS coverage is temporarily weakened by obstacles while GSM
network coverage remains acceptable. This can happen for a number of reasons like because of different locations of the
UMTS and GSM base station sites or due to better GSM in-house coverage. If AMR-WB was used for the call,
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2. WirelessMoves: Deep Inside the Network: TFO and TrFO http://mobilesociety.typepad.com/mobile_life/2006/11/i_was_not_sure_.html
deactivating TrFO also requires a fallback to AMR which results in an abrupt voice quality degradation of the call in case
the GSM network does not support Tandem Free Operation which is described next (see also 3GPP 23.153, chapter 6.5).
To communicate with services in the networks such as a voice mail system, or a prepaid top up voice server, DTMF (Dual
Tone Multiple Frequency) tones are used to send passwords and commands to the system. If a call is established from a
UMTS terminal, DTMF tones are sent as out of band signaling messages to the MSC Server. The MSC Server is then
responsible to forward the message. If the service platform which analyzes the DTMF tones is BICN compatible, the tone
is delivered as a message. If the service platform is still using PCM links and the G.711 codec, the DTMF tone is inserted
into the voice connection at the Media Gateway at border to the PCM network. If a call is established from an external
PCM network to a service in BICN network, DTMF tones are not sent as messages but inside the voice bearer. In this
case, 3GPP 23.153, 5.9 requires the gateway at the border of the BICN network to discover the inband DTMF tone,
remove it from the voice stream and send a DTMF signaling message to its MSC Server which then forwards the DTMF
message out of the voice band to the destination.
Tandem Free Operation (TFO)
For GSM networks, TrFO can not be used even if the core network uses BICN media gateways and MSC-servers. This is
because the A-Interface, which connects the media gateway to the BSC/transcoding unit is based on PCM. Thus, voice
calls from GSM radio networks always arrive at the media gateway as a PCM G.711 64 kbit/s circuit switched stream.
Unlike in UMTS, where the transcoding is a functionality of the media gateway, GSM uses a dedicated transcoding unit
between the BSC and the media gateway. This is necessary in order not to change the radio network architecture when
the core network is upgraded to BICN. As a consequence, a mobile to mobile GSM call will always have two PCM links in
the connection, one to and from each transcoder and thus a GSM call is always established using the G.711 codec. The
two transcoders of the connection are also called a tandem. In order to remove the transcoder tandem from such a
connection, in band codec negotiation between the two transcoders can be attempted once the call is established. Instead
of sending signaling messages via the MSC-servers through the network, the two transcoder units send their information
embedded in the G.711 link. This is done by using a special bit pattern as part of the voice stream which is recognized by
the other end as a transcoding control message and not as a part of the voice signal. If both transcoder units support at
least a single common codec like AMR 12.2 or an AMR-WB codec, the 64 kbit/s G.711 connection is used to tunnel the
compressed and encoded voice stream. As the audio stream is compressed most of the bits of the transparent 64 kbit/s
stream are not used. This effectively removes the transcoder tandem from the voice connection and is thus called
Tandem Free Operation (see 3GPP TS 23.053 and 3GPP TS 28.062). While audio quality and delay times can be
improved this way, no bandwidth savings can be achieved in the core network unlike with TrFO.
Some events in the network can temporarily interrupt TFO and the speech path in the core network automatically falls
back to standard G.711 encoding. This is the case for handovers for example in which the call is handed over to a cell
which is controlled by a different TRAU. This is the case for an inter-BSC handover for example. Once the handover is
complete, TFO has to be negotiated again between the two TRAUs (see 3GPP TS 23.053, 6.5.7). TFO is also temporarily
interrupted when the MSC inserts DTMF tones or announcements. As the MSC is not aware of TFO it overwrites the
compressed speech information in the circuit connection. This is detected by the terminating transcoder and transcoding
elements are automatically reinserted into the speech path. Especially for AMR-WB, TFO has to be started again as soon
as possible in order to avoid speech quality degradation.
Conference bridges are another network feature which automatically disable TFO. This is because standard conference
bridges mix G.711 voice signals from several parties to produce a combined signal which also overwrites the TFO data
stream. This again forces the TRAUs to introduce a transcoder in the speech path again. While this has little impact on
narrow band speech calls from a voice quality perspective, it is not possible to use AMR-WB when a conference bridge is
inserted unless the conference bridge also supports TFO and the AMR-WB codec.
There we go, if you have made it up to here, please consider leaving a comment :-)
November 23, 2006 | Permalink
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3. WirelessMoves: Deep Inside the Network: TFO and TrFO http://mobilesociety.typepad.com/mobile_life/2006/11/i_was_not_sure_.html
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Comments
Greetings; we are just about to introduce MSC servers and Media Gateways in our network and your artilce (blog) has helped answer
quite a few grey areas. Thanks. Live long and prosper.
Posted by: Farooq Sheri | December 12, 2006 at 11:00 AM
Hi Martin, this was a very useful read about TFO and TrFO. Please feel free to post other documents that expand on how the media
gateways are changing as we move away from circuit switched networks.
Posted by: Nate Braynt | December 13, 2006 at 07:51 PM
This is a very succinct yet informative write-up on TFO/TrFO. Thanks!
Between this and your other article on AMR-WB, there is some ambiguity about whether calls that use AMR-WB are amenable to TFO
operation. Can you shed some light on this?
Best regards
Chetan
Posted by: Chetan Vinchhi | January 10, 2007 at 11:56 AM
Hello Chetan,
For AMR-WB you need TrFO if the call runs over UMTS and TFO if the call runs over GSM.
Best regards,
Martin
Posted by: Martin | January 14, 2007 at 09:40 PM
Hello
if an operator want to define AMR codec on BSC ,,
and want to know the percentage of subscribers devices supporting this.. what is the distinguishing in handset ???
Posted by: amin | January 29, 2007 at 10:36 AM
Hey,
the BSC, or rather the MSC, can see if a mobile is capable of AMR or not by analyzing the Vocoder Preference List which should be part
of the Bearer Capability Information Element for example in a SETUP message. The mobile is AMR capable if the list includes Full Rate
Codec 3 and Half Rate Codec 3.
Martin
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4. WirelessMoves: Deep Inside the Network: TFO and TrFO http://mobilesociety.typepad.com/mobile_life/2006/11/i_was_not_sure_.html
Posted by: Martin | January 30, 2007 at 12:06 PM
Good explanation of a tricky subject, thanks Martin. You might consider what happens when a mobile changes AMR codecs mid-call,
e.g. does it renegotiate end-to-end ?
Posted by: simon | March 19, 2007 at 10:05 PM
Hi,
In TrFO call how what is the measure of Voice Qulaity on BSC as BSC is only shipping frames it recieves from mobiles to MG on packet
network. Are there standard practices to measure Voice Qulaity on TrFO call.
Mustafa
Posted by: Mustafa | May 04, 2007 at 07:05 PM
Hello Mustafa,
I haven't been involved in Voice Quality testing before. One of the companies I know that is doing this is Polystar for example. In case
you are interested, go to their website and check them out.
Cheers,
Martin
Posted by: Martin | May 09, 2007 at 09:18 PM
Excellent review, thanks.
Posted by: L L L | May 27, 2007 at 10:58 AM
Is TFrO and TFO mainly a NW funcationality? As far as UE end, if UE can support AMR rate change dynamically then it can work with
TFrO/TFO enabled NW?
Posted by: Sunny | June 06, 2007 at 04:34 PM
Hi Sunny,
thanks for the comment. TFrO and TFO are network functionalities. But what they do is to allow AMR capable mobile to communicate
directly instead of with transcoders in the network. That means that if a mobile wants to change the codec it will not tell the network but
directly the mobile on the other side.
There are still some questions open for me on this topic like for example the following: In GSM, the network can decide to put a
connection on a half rate air interface channel. This means not all codecs will work anymore. So I wonder how the mobile on the far side
knows that it can no longer use certain codecs because the other mobile is on a half rate channel. So if anybody knows the answer to this
question, please leave a note.
Cheers,
Martin
Posted by: Martin | June 07, 2007 at 02:13 AM
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5. WirelessMoves: Deep Inside the Network: TFO and TrFO http://mobilesociety.typepad.com/mobile_life/2006/11/i_was_not_sure_.html
Very good Review. Thanks
Posted by: Jagan | June 15, 2007 at 06:25 AM
Hi Martin. Thanks for this explanation. I have a simple question. Enabling TrFo in a network, could impact terminals? How does a
terminal behaves? Thanks and regards.
Posted by: Eva | July 13, 2007 at 02:25 PM
Eva,
in theory it should be transparent to mobiles. However, I wonder how mobile B knows that mobile A is only on a half rate channel and
can thus not support the higher AMR codecs. Haven't had the time to dig that deep.
Cheers,
Martin
Posted by: Martin | July 18, 2007 at 04:07 PM
Thanks, very good and helpfull posts (especially about TrFO/TFO, which most of the normal people do not read :) ) .
Posted by: Boris | September 12, 2007 at 11:06 AM
The comments to this entry are closed.
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