Cisco Text Relay

Cisco text relay provides the functional equivalent of fax and modem relay for text telephones. The Baudot tones used by text devices are decoded and passed as characters across the IP network before being played back as Baudot signals once again on the far side.

Cisco text relay is a proprietary solution that leverages portions of different specifications to implement a viable method for transporting text over IP. Table 5-5 details the multiple specifications that Cisco text relay uses.

Table 5-5 Specifications Used by Cisco Text Relay

Specification

Description

ITU-T V.151

Procedure for the end-to-end connection of public switched telephone network

(PSTN) text telephones using text relay over IP.

ITU-T T.140

Specifies a simple text protocol for conversing between text devices.

IETF RFC 4351

Describes how to transport real-time text session contents based on ITU-T

T.140 in RTP packets.

IETF RFC 2198

Specifies an RTP payload format for encoding redundant audio data. RFC 4351

details how RFC 2198 can also be used with redundant text data.

From an implementation perspective, Cisco text relay is much closer to a feature such as Dual Tone Multi Frequency (DTMF) relay than modem or fax relay. DTMF relay does not have a switchover procedure but instead only relays DTMF digits when they are detected in the voice media stream. Cisco text relay operates in the same fashion, only activating when a Baudot text character is detected in the voice media stream.

With modem and fax relay, the original voice media stream is replaced by a special relay protocol after the triggering of a switchover using NSEs or the signaling protocol stack. This does not happen with Cisco text relay because an explicit switchover does not occur and the voice media stream remains intact for the call duration.

Because Cisco text relay does not create a new media session, it must use the existing voice media stream, including the same UDP ports. So, how does Cisco text relay distinguish itself from the actual voice packets already using these established IP sockets?

Cisco text relay implements a unique RTP payload type (PT) to coexist within the same media stream as the RTP voice packets. The voice packets will always use a standard RTP PT indicating the voice codec negotiated for that connection. Cisco text relay implements a dynamic RTP PT, which is set to 119 by default. Figure 5-19 illustrates voice and text relay packets within the same RTP media stream.

Figure 5-19 Cisco Text Relay over an Existing RTP Voice Stream

Figure 5-19 Cisco Text Relay over an Existing RTP Voice Stream

Of course, for many text telephone conversations, there will not be any voice exchanged over the connection unless Hearing Carry Over (HCO) or Voice Carry Over (VCO) is being used. In these situations, even though the call is initially created as a voice call, practically all the packets across the media stream will be Cisco text relay. The concepts of HCO and VCO were discussed previously in the sections "HCO (Hearing Carry Over)" and "VCO (Voice Carry Over)" in Chapter 3, "How Text Telephony Works."

The packet format implemented by Cisco text relay is derived from RFC 4351. This specification mandates an RTP encapsulation with ITU-T T.140 encoding for the text characters in the RTP payload. However, because Cisco text relay forces redundancy to be enabled, the RTP payload must be able to handle redundancy. Figure 5-20 illustrates the RTP portion of a Cisco text relay packet when redundancy is set to a value of 1.

Figure 5-20 Cisco Text Relay Packet Format with a Single Level of Redundancy

1 byte_I_1 byte_i_2 bytes

V P X CC

M Payload Type

Sequence Number

Timestamp

Synchronization Source (SSRC) Identifier

1

T.140 PT

Timestamp Offset of "R" "R" Block Length

0

T.140 PT

"R" T.140 Block Counter

T.140 "R" Data*

"P" T.140 Block Counter

T.140 "P" Data*

*RFC 4351 shows the T.140 data fields as 8 bytes long but Cisco text relay Baudot characters are transported as 1 byte ASCII

NOTE You can find additional information concerning the RTP header and its contents in the section "Passthrough Fundamentals" in Chapter 4.

The shaded portion of Figure 5-20 highlights the RTP payload, and the unshaded portion indicates the RTP header. The RTP payload supporting redundant text characters is defined in RFC 4351. Elements of RFC 2198 are used by RFC 4351 to address the actual packet formatting of these redundant characters. Table 5-6 defines the RTP payload fields used in transporting text characters with redundancy.

Table 5-6 Cisco Text Relay Redundancy Frame Field Definitions

Cisco Text Relay

Redundancy Field

Definition

T.140 PT

T.140 payload type, set to 119 for Cisco text relay.

Timestamp Offset

Offset of redundant block relative to the timestamp given in the RTP

header.

Block Length

Length of data block not including the header.

T.140 Block Counter

16-bit counter used to detect lost blocks and to avoid duplication of

blocks. Separate block counters are used for R (redundant) characters

and P (primary) characters.

T.140 "R" Data

R or Redundant Data field repeats a previously transmitted primary

character in 1-byte ASCII format.

T.140 "P" Data

P or Primary Data field is a new character being transmitted for the

first time as 1-byte ASCII.

Cisco text relay defaults to a redundancy value of 2. Adding redundancy to the RTP payload increases the packet size, but the redundant data "piggybacks" along with the primary data whenever possible so that the bandwidth increase is minimized.

Because Cisco text relay does not require a switchover using NSEs or the voice signaling protocol stack, there is not a switchover call flow as diagrammed for fax and modem relay. Cisco text relay packets are simply generated whenever a Baudot character is detected in the voice stream. Figure 5-21 illustrates Baudot characters being handled by Cisco text relay while voice packets pass concurrently along the same connection.

Figure 5-21 Transport of Text Characters Using Cisco Text Relay

Sending Text Telephone

Figure 5-21 Transport of Text Characters Using Cisco Text Relay

Sending Text Telephone

Text user types in "BYE". Characters sent using Cisco text relay with an RTP payload type of 119 within voice stream.

PT-119, Cisco text relay Character "B"

Answering Text Telephone

Text user types in "BYE". Characters sent using Cisco text relay with an RTP payload type of 119 within voice stream.

PT-119, Cisco text relay Character "B"

Voice

PT-119, Cisco text relay Character "Y"

PT-119, Cisco text relay Character "E"

Voice Voice

PT-119, Cisco text relay Character "C"

PT-119, Cisco text relay Character "Y"

Voice

PT-119, Cisco text relay Character "A" Voice Voice

Answering Text Telephone

Text user types in "CYA". Characters sent using Cisco text relay with an RTP payload type of 119 within voice stream.

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