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SPA 122 - Loopback capabilities


I am looking at ways to test audio quality when a customer reports audio issue. The PBX can echo audio and I would like to see if the ATA could do the same so that I could originate a tone from the PBX to an ATA in loopback mode so I can listen to it and check the quality I recieve.

Ideally would be to have a command (like sending a NOTIFY to the unit) sent to the ATA to put the FXS in loopback. Or have a some physical cable we could ask our customer to plug on PHONE1 and PHONE2 so I could call one FXS and answer that call on the second line ...

Anyone has ideas about that?



Dan Lukes

Yes, Linksys devices are "loopback call" capable. They can be either loopback stream source as well as loopback deflector. So you can initiate loopback call to end-user's ATA device with no user intervention. The called device will transmits the audio packets that it receives back to the transmitter/receiver instead of transmitting the data sampled on attached analog telephone. Unfortunatelly, the loopback call feature is not documented by Cisco (as far as I know). On the other side, it seems that RFC 6849 has been created with the Linksys's implementation in the mind and Cisco's employee is co-author of such RFC. So it may be relevant and may help to understand the feature details despite Linksys implementation may deviate from it a lot.

Following informations are known to me.

No, such feature is not triggered by a NOTIFY message. The loopback call is negotiated in SDP. See catched SDP bellow:

Standard call

Loopback call

(source/media mode)

m=audio 16532 RTP/AVP 8 101

a=rtpmap:8 PCMA/8000

a=rtpmap:101 telephone-event/8000

a=fmtp:101 0-15



m=audio 16530 RTP/AVP 8

a=rtpmap:8 PCMA/8000




m=audio 16530 RTP/AVP 113


a=rtpmap:113 PCMA/8000

In packet media loopback type, the loopback:rtp-pkt-loopback is used instead of loopback:rtp-media-loopback

In mirror media loopback mode, the loopback-mirror:8 is used instead of loopback-source:8 and m=...113 media descriptor is not present at all.

The id 113 is id of RTP-Start-Loopback Dynamic Payload as configured in phone setup (113 is default value). The id 8 in loopback-source/loopback-mirror is id of RTP-Start-Loopback Codec as configured in phone setup (here 8 = PCMA).

Following setup options are related to loopback call feature:

Media Loopback Code

The star code used for enabling media loopback on the phone.

The default i *03.

Accept Media Loopback Request

Controls how to handle incoming requests for loopback operation. Choices are: Never, Automatic, and Manual,


  • never — never accepts loopback calls; reply 486 to the caller
  • automatic — automatically accepts the call without ringing
  • manual — rings the phone first, and the call must be picked up manually before loopback starts

The default is Automatic.

Media Loopback Mode

The loopback mode to assume locally when making call to request media loopback. Choices are: Source and Mirror.

Default is Source.

Note that if the ATA device answers the call, the mode is determined by the caller.

Media Loopback Type

The loopback type to use when making call to request media loopback operation. Choices are Media and Packet.

Default is Media.

Note that if the ATA device answers the call, then the loopback type is determined by the caller (the ATA device always picks the first loopback type in the offer if it contains multiple types.)

ENCAP RTP Dynamic Payload

The dynamic payload value (96 – 127) used for the encapsulating RTP packets when offering the SDP to loopback packets. This setting is used if the SPA is the offerer of the SDP. Otherwise, the value is decided by the peer.

The default value is 112.

RTP-Start-Loopback Dynamic Payload

The dynamic payload value (96 – 127) used by the mirror in the self-generated RTP packets before receiving any RTP packets from the source. This setting is used only when the SPA is acting as the loopback source. Otherwise, the value is decided by the peer.

This value must be different from any of the dynamic payload values that might be used by the source and the mirror (including the encaprtp payload type). This is necessary so that the source can easily tell when the mirror has switched from sending self-generated RTP packets to sending loopback packets.

The default value is 113.

RTP-Start-Loopback Codec

The actual codec corresponding to RTP-Start-Loopback Dynamic Payload, whose codec name is used in the rtpmap attribute for the for the mirror self-generated RTP audio stream, prior to receiving any RTP packets from the source.