T.140 to Baudot Relay

The T.140 to Baudot Relay feature uses the ESBC's transcoding resources to relay T.140 text messages to Baudot tones and vice versa. The T.140 Protocol is used for multimedia text conversation over IP and is designed as a replacement for TDD devices. Baudot tones are a common protocol in the US in Telecommunications Device for the Deaf (TDD). Details of the protocol implementation are available in TIA/EIA-825-A. The T.140-Baudot relay is a regulatory requirement, and is specified for both emergency and non-emergency traffic. This feature is not available on virtual platforms; it is only available on Acme Packet hardware platforms.

T.140 to Baudot transcoding entails that one call leg is provisioned with a Baudot-capable codec, and the other call leg accepts T.140 in the SDP. Additionally, the T.140 side includes an audio stream. Once this scenario is established, the call may begin as audio-to-audio. At any point forward, T.140 may be received on one call leg or Baudot tones may be received on the other call leg. Each text indication will be transcoded to its compliment on the other side of the call. When T.140 <-> Baudot tone transcoding is active, the existing audio stream is preempted.

The system's transcoding hardware detects baudot tones in the incoming audio stream and generates T.140 packets on an outbound text stream. In the reverse direction T.140 packets will be detected on the text stream and Baudot tones will be generated on the appropriate outgoing audio stream.

T.140 to Baudot relay is invoked when an outbound codec-policy removes any text "m=" line from the egressing offer. The processing also removes all non-Baudot-tone capable codecs from the egress SDP offer. If at this point no Baudot-capable codecs remain in the SDP, the call is torn down.

Baudot Tones capable codecs:

PCMA
PCMU
EVRC
EVRC0
EVRC1
EVRCB
EVRCB0
EVRCB1

Codec Policy Configuration for T.140 to Baudot Relay

To support T.140 to Baudot relay, configure the allow-codecs parameter in the codec-policy with text:no. This value causes the ESBC to strip any "m=text" occurrence in the outbound INVITE and enable T.140 to Baudot transcoding. When SDP passes through the ESBC and a text "m=" line is removed on the egress side of the call, then T.140-baudot relay/transcoding will be invoked for that call.

Unlike other transcodable codecs, T.140 is not valid in add-codecs-on-egress parameter.

Limitations

The following scenarios are not supported:

Configuration of T.140 in add-codes-on-egress
Hairpin calls (T.140 - Baudot Relay - T.140)
Lawful Intercept
SRTP for T.140

T.140 to Baudot Relay Examples

This section includes two examples; access-side call initiation and core-side call initiation. The following codec-policy is applied to the Core-side realm.

ACMEPACKET (codec-policy)#
name T140-to-tone
allow text: no *
add-codecs-on-egress PCMU

Figure 14-1 Call Initiated from the Access-side

This diagram shows the baseline implementation of T.140 to Baudot tones. The UE sends an INVITE into the core, via the ESBC. Codec-policy indicates ESBCto remove all text codecs (including T.140), it also indicates to add PCMU. All non-Baudot codecs are removed from the egress invite - AMR in this case. The core confirms PCMU in a 200 OK. The ESBC forwards the 200 OK to the UE confirming the initially-offered codecs (AMR and T.140).

The call is set up so that the ESBC will transcode audio between AMR and PCMU, and seamlessly relay T.140 to and from Baudot tones.

Figure 14-2 Call Initiated from the Core with T.140 reINVITE

In this example, the call is initiated from the core. The ESBC transcodes between PCMU and AMR via expected means. Then, in the same dialog, the access side then sends a reINVITE adding T.140 and AMR. Codec policy dictates to accept T.140 on the UE-side and strip the text codec and all non-Baudot capable codecs from the core side. Additionally, the ESBC is ready for T.140 / Baudot relay (transcoding). Thus when the UE begins sending T.140, they are transcoded into tones within the PCMU stream.