The ITU-T, considering
(a) that text telephones place special operational needs on the use of DCEs.
(b) that for historical reasons, many existing text telephones do not use V-series modulation.
(c) there is a desire to have all future GSTN text telephones employ V-series modulation.
(d) to provide a migration path from the diverse installed base it will be necessary to provide interworking with existing text telephones
(e) to provide for interworking the DCE will need to convert the 5-bit character code or Recommendation Q.23 (DTMF) character set used by some existing text telephones into the 7-bit code set as given in Recommendation T.50.
(f) that such character conversion in the DCE be undertaken solely to enable interworking with existing text telephones and to impose no constraints on character sets used in future text telephones.
(g) that new technology is being developed that could be used to provide additional text telephone modes.
recommends the procedure below.
This Recommendation specifies features to be incorporated in DCEs intended for use in, or communicating with, text telephones primarily used by the deaf and hard of hearing. One of the goals of this Recommendation is to provide a platform on which a future universal text telephone could be built. To accommodate this goal, procedures for interworking with identified existing text telephones are provided in Section 5 of this Recommendation. In addition, this Recommendation has the goal of identifying ways in which the multi-media Recommendations could be used to support new modes of operation or create new multi-mode text telephone devices. To accommodate this additional goal, Section 6 of this Recommendation, identifies some possible uses of this new technology to support text telephony and additionally specifies requirements for multi-mode text telephone devices.
To provide for maximum flexibility it is envisaged that any of the text telephone modes of operation specified in this Recommendation will be invoked on an as required basis using the commands specified in ITU-T Recommendation V.25ter or some equivalent mechanism.
It provides for:
The following ITU-T Recommendations and other references contain provisions which, through reference in this text, constitute provisions of this Recommendation. At the time of publication, the editions indicated were valid. All Recommendations and other referenced Standards are subject to revision; all users of this Recommendation are therefore encouraged to investigate the possibility of applying the most recent editions of the Recommendations and other references listed below. A list of currently valid ITU-T Recommendations is regularly published.
ITU-T(CCITT) Recommendation Q.23: 1988, Technical features of push-button telephone sets.
ITU-T(CCITT) Recommendation T.50: 1992, International Reference Alphabet (IRA) (Formerly International Alphabet No. 5 or IA5) - Information technology - 7-bit coded character set for information interchange
ITU-T(CCITT) Recommendation V.8 1994, Procedures for starting sessions of data transmission over the general switched telephone network.
ITU-T Draft Recommendation V.8bis: (in progress), Procedures for the identification and selection of common modes of operation between data circuit terminating equipment (DCE) and between data terminal equipment (DTE) over the general switched telephone network and on leased point-to-point telephone-type circuits.
ITU-T(CCITT) Recommendation V.21: 1988, 300 bits per second duplex modem standardized for use in the general switched telephone network.
ITU-T (CCITT) Recommendation V.23: 1988, 600/1200 bits per second modem standardized for use in the general switched telephone network.
ITU-T (CCITT) Recommendation V.25: 1988, Automatic answering equipment and/or parallel automatic calling equipment on the general switched telephone network including procedures for disabling of echo control devices in both manually and automatically established calls.
ITU-T Recommendation V.25ter: 1995, Serial automatic dialing and control.
ITU-T Draft Recommendation V.61 1996, A simultaneous voice plus data modem, operating at a voice + data signaling rate of 4800 bit/s, with optional automatic switching to data-only signaling rates of up to 14,400 bit/s, for use on the general switched telephone network and on leased point-to-point 2-wire telephone-type circuits.
ISO 10646-1 1993, Universal multiple-octet coded character set (UCS) Part1: Architecture and multi-lingual plane.
ISO 6429 1992, Information Technology-Control functions for coded character sets.
For the purposes of this Recommendation the following definitions apply.
Carrierless mode: A mode for communication, where signals are only present on the connection only when data is being exchanged (e.g. in response to the pressing of a key on a keyboard).
Carrier mode: A mode for communication, where continuous signals (i.e., carriers) are present on the connection irrespective of whether data is being exchanged or not.
CI A signal transmitted from the call DCE to indicate the general communications function, consisting of a repetitive sequence of bits at 300 bit/s, modulating Recommendation V.21(1), the low-band channel defined in Recommendation V.21. The cadence of this shall be bursts of 4 CI sequences separated by 2 s of silence. The CI sequence shall be coded in octets as defined below. The 30-bit CI sequence in left-to-right order of transmission is given by:
(1 1111 1111 1) (0) 0000 0000 (1) (0) 1000 0010 (1) where brackets enclose start and stop bits.
Master mode: The mode used by the device, in an asymmetric connection, that normally is assumed to have initiated the connection. In the Annex E case this role is usually assigned to the terminal that sends in 75 bits/s and receives in 1200 bit/s.
Multi-mode Text Telephone A device which incorporates simultaneous voice and data in addition to conforming to Sections 4 and 5 of this Recommendation.
Slave mode: The mode used by the device, in an asymmetric connection, that it is usually assumed to have answered a call. In the Annex E case this role is usually assigned to the terminal that sends in 1200 bits/s and receives in 75 bit/s.
Text telephone: A device incorporating text telephony functions.
Text telephone mode: The operational mode when two devices are interconnected to provide text telephone communications.
Text telephony: A telecommunications capability which supports real time text conversation on communication networks.
TXP A signal transmitted to allow early termination of answer tone, and also to confirm V.18 capability in the answering device. It consists of a repetitive sequence of bits at 300 bit/s modulating V.21(1) if transmitted from the call DCE, or modulating V.21(2) if transmitted from the answer DCE. The 40-bit TXP sequence in left-to-right order of transmission is given by:
(1 1111 1111 1) (0) 0010 1011 (1) (0) 0001 1011 (1) (0) 0000 1010 (1) where brackets enclose start and stop bits.
V.18 mode: The operational mode when two devices conforming to this Recommendation are interconnected to provide text telephone capability. Except as specified in 6.1 below, the modulation in this mode shall be in accordance with ITU-T Recommendation V.21.
V.18 text telephone: A communications device conforming to the requirements of this Recommendation.
The DCE, when configured to support text telephone mode shall:
NOTE: Because of the subjective nature of this indication the operational thresholds of this circuit are left to the discretion of the implementors however, means should be provided to prevent the presence of CI signal specified in this Recommendation from interfering with the indication of call progress signals.
This section specifies procedures for connecting in text telephone mode. This includes procedures for establishing communications between two V.18 text telephones as well as procedures for establishing communications between a V.18 text telephone and the pre-existing text telephone devices specified in Annexes A through F of this Recommendation. Although it is envisaged that for most connections the user will have a priori knowledge of the type of device being called and will preset the DCE to the correct mode, automatic procedures are provided for both originating and answering. These procedures provided for automoding and, where required for interworking, modulation and protocol conversion.
These procedures are based on the assumption that the DCE has been placed in the V.18 mode with the CI specified in this Recommendation and the called party is known to be equipped with a text telephone. The procedure is defined below, and represented in Figure 1/V.18.
After connecting to line, the DCE shall transmit no signal for 1 s, and then transmit the CI signal as specified in this Recommendation with the ON/OFF cadence defined in 3. Detection of incoming signals shall take place during OFF periods in the signal CI, as indicated in Figure 1/V.18. After transmitting 4 of the CI sequences defined in this Recommendation(see 3), the DCE shall condition its receivers to detect the following signals during the 2-s OFF period in signal CI (see Figure 1/V.18):
Note: care should be taken in the design of 980 and 1180 Hz detectors to prevent incorrect triggering by echoes of transmitted CI signals.
If any of the above signals are detected the DCE shall stop transmitting CI. No disconnect timers shall be started.
5.1.2 If ANS is detected the DCE shall stop transmitting the CI signal, transmit no signal for 0.5 s, and then initiate the transmission of signal TXP in V.21(1) mode. The DCE shall then monitor for 900 Hz, 1 650/1 850 Hz ,1 300 Hz, 390 Hz, 450 Hz and loss of ANS.
5.1.2.1 When the DCE detects the absence of ANS, it shall stop transmission of signal TXP after completion of the current TXP sequence and continue to monitor for 1 650Hz, 1 850 Hz, 1 300, 390 Hz and 450 Hz.
5.1.2.2. If the DCE detects TXP it shall connect as V.18, i.e. Recommendation V.21 with the operational characteristics given in section 4.
5.1.2.3 If the DCE detects 1 650 or 1 850 Hz for 0.2 s it shall connect as per Annex F.
5.1.2.4 If the DCE detects 1 300 for 1.7 s it shall connect as per Annex E in master mode. If 390 Hz is detected for 3 s, then the DCE shall connect as per Annex E in slave mode (i.e. transmit on the higher speed channel).
5.1.3 If the DCE detects 2025 Hz for 0.5 s it shall connect as Annex D.
5.1.4 If 1 650 Hz or 1 850 Hz for 0.2 s, are detected the DCE shall connect as per Annex F.
5.1.5 If 1 300 Hz is detected for 1.7 s the DCE connect as per Annex E in master mode. (i.e. transmitting on the 75 bit/s channel).
5.1.6 If 390 Hz or 450 Hz is detected for 3 s the DCE shall initiate a connection as per Annex E in slave mode.
5.1.7 If a sequence of 1 400 Hz or 1 800 Hz FSK signals (i.e. valid 5-bit characters) are detected the DCE shall analyze the bit duration and connect in the appropriate signalling rate as per Annex A.
5.1.8 If Dual Tone Multi-Frequency (DTMF) signals are detected the DCE shall connect in the DTMF mode using the character conversion specified in Section 5.4 and the operational characteristics specified in Annex B.
5.1.9 If 980 Hz or 1 180 Hz signals are detected the DCE shall start a 2 sec timer (Tr) and attempt to determine the data signalling rate of the sequence.
5.1.9.1 If the data signalling rate is 110 bit/s the DCE shall connect as per Annex C.
5.1.9.2 If timer Tr expires the DCE shall connect as per Annex F in answer mode.
5.2.1 When in the answer mode the DCE shall connect to the line, and condition its receiver to detect :
The 3s timer Ta shall be started. No disconnect timers shall be started. The procedures are defined below, and are provided in Figures 2a and 2b/V.18 as an aid to the reader.
5.2.2 If signal CI coded for text telephone is detected, the DCE shall initiate transmission answer tone ANS as defined in Recommendation V.25, monitor for signal TXP and start a 3 s timer (Tt).
5.2.2.1 If signal TXP is detected, the DCE shall transmit no signal for 75ñ5ms, transmit 3 TXP sequences in V.21(2) mode, and then proceed as V.18 (i.e., Recommendation V.21 with the operational requirements specified in Section 4 of this Recommendation).
5.2.2.2 If Tt expires the DCE shall return to Monitor A.
5.2.3 If 2 100 Hz is detected the DCE shall continue to monitor for 980 Hz, 1 300 Hz or 1 650 Hz.
5.2.3.1 If 980 Hz is detected for 0.4 s the DCE shall connect as per Annex F in answer mode.
5.2.3.2 If 1 300 Hz is detected for 1.7 s the DCE shall connect as per Annex E in master mode.
5.2.3.3 If 1 650 Hz is detected for 0.4 s the DCE shall connect as per Annex F in the calling mode.
5.2.4 If 980 Hz is detected for 5 ms the DCE shall start a 2.7 s timer Te and monitor for 1650 Hz and 1180 Hz.
5.2.4.1 If 1650 Hz is detected for 0.4 s the DCE shall connect as per Annex F in the calling mode.
5.2.4.2 If 1s of no signal is detected the DCE shall connect as per Annex C.
5.2.4.3 If either 1 180 Hz is detected for 5 ms or timer Te expires, the DCE shall start 2 s timer Tr and attempt to determine the data signalling rate of the data sequence.
5.2.4.3.1 If the data signalling rate is 110 bit/s the DCE shall connect as per Annex C.
5.2.4.3.2 If the data signalling rate is 300 bit/s or timer Tr expires the DCE connect as per Annex F.
5.2.5 If a sequence of 1 400Hz and 1 800 Hz FSK signals (i.e. valid 5-bit characters) are detected the DCE shall analyze the bit duration and connect in the appropriate signalling rate as per Annex A.
5.2.6 If Dual Tone Multi-Frequency (DTMF) signals are detected the DCE shall connect in the DTMF mode using the character conversion specified in Section 5.4 and the operational characteristics specified in Annex B.
5.2.7 If 1 070 Hz or 1270 Hz is detected for 0.4s the DCE shall connect as per Annex D in answer mode..
5.2.8 If 2025 Hz or 2225 Hz is detected for 1 s the DCE shall connect as per Annex D in the calling mode.
5.2.9 If Ta expires the DCE shall enter probing state, sending signals intended to stimulate the calling text telephone or its user to respond. The DCE shall start a 1 s timer and send an initial probing sequence that shall consist of transmission of carrierless mode sequence consisting of the character "-" sent in 5-bit mode with 47.6 bit/s, in EDT mode and in DTMF mode in rapid sequence and continue to monitor . If the timer expires the DCE shall proceed either per 5.2.9.1 or per 5.2.9.2. depending on the most likely scenario preset by the user. (see Appendix 1)
5.2.9.1 The DCE shall transmit a buffered message and start variable Timer Tm (default 3 sec) to allow for a response from the caller. If Tm expires and no response is received the DCE shall proceed to next appropriate probe (e.g., ANS, carrier, or the next carrierless mode).
Note: The DCE shall have a stored, user changeable, default answer message (e.g. Hello, GA). Although the primary use of this stored message is to stimulate a response from a carrierless text telephone it may also be optionally sent after a connection is established with a continuos carrier based text telephone.
5.2.9.2 The DCE shall transmit ANS, for 0.5 s, then remain silent for 75+/- 5 ms and then transmit, for the duration of variable timer Tc (default 6 secs) depending on the user setting, 1300 Hz carrier , 1650 Hz or 2025 Hz . The DCE shall monitor for all appropriate signals while transmitting one of the above carriers
5.2.9.2.1 If 980 Hz is detected for 300 ms the DCE shall connect as Annex F
5.2.9.2.2 If 1270 Hz is detected for 300 ms the DCE shall connect as Annex D.
5.2.9.2.3 If 390 Hz is detected for 3 s the DCE shall connect as per Annex E in the slave mode.
5.2.10 If Tc expires the DCE shall proceed to the next appropriate probe (e.g. next carrier mode, or next carrierless mode).
The capability for simultaneous voice and data (SVD) provided by ITU-T Recommendations H.324, V.61 and V.70 can be used to support expanded modes of text telephone without need of any special modifications. When this capability is added to a device that supports the provisions of Sections 4 and 5 of this Recommendation the device shall be considered a V.18 multi-mode text telephone device. In this case, Recommendation V.8bis procedures shall be used, whenever possible, for the exchange and negotiation of capabilities as well as provide the means for switching between supported text telephone modes and between text telephone mode and voice.
6.1 Simultaneous voice and text telephony (SVT)
When SVD capability is added, to a basic V.18 text telephone, connections between hearing people and oral deaf or hard-of-hearing people are facilitated In these cases, after the SVD capability is established, the deaf or hard of hearing user communicates orally over the audio channel and the hearing user responds using text telephony over the data channel.
6.2 Simultaneous Data and Text Telephony
The audio channel provided by quadrature audio data modulation (QADM) SVD DCEs (e.g., V.61) can, in many cases, support V.18 text telephony. In this case the V.18 devices could be connected to the audio input of such devices and the text telephone connection would be established, in accordance with provisions of section 5 of this Recommendation after the SVD connection is established. In this case, however, the SVD devices are not considered to be text telephone devices and therefore would not need to conform to the provisions of this Recommendation.
Annex A -- 5 bit Operational Mode
A.1 Mode of operation
The communication channel is half-duplex with no channel turnaround. . Carrier is transmitted 10 ms before the first character is transmitted. The receiver shall be disabled for 300 ms when a character is transmitted to mitigate false detection of echoes (in non-V.18 devices carrier is 1s after the last character to provide this same function).
A.2 Modulation
The modulation is frequency shift keyed modulation (i.e. no carrier is present when a character is not being transmitted) using 1 400 Hz ( ( 56 Hz) for a binary 1 and 1 800 Hz ( ( 72 Hz) for a binary 0. The frequency deviation is + 80 Hz. A bit duration of either 20 or 22.00 0.40 ms is used providing either a nominal data signalling rate of 50 or 45.45 bits/s respectively.
A.3 Probe
The probe in answer mode shall be at a 47.6 bit/s data signalling rate.
A.4 Character conversion
The initial condition of the converter shall be the Letters (LTRS) mode, therefore the DCE shall send the LTRS character (11111) to the line prior to transmitting the first translated character. The receiver decoding shall also start up in the LTRS mode. Additionally, the DCE shall send the appropriate mode character (i.e., LTRS or FIGS) every 72 characters.
The 5-bit codes supported are given in Tables A-1 and A-2. Each character shall consist of the 5-bit sequence given in the tables preceded by a one start bit and followed by a minimum of one and one half stop bits.
The DCE shall convert the 5-bit coded characters received from the line to the appropriate 7-bit coded characters and transfer them to the DTE on circuit 104 (or its equivalent).
The DCE shall convert the 7-bit coded characters received from the DTE on Circuit 103 (or its equivalent) to the appropriate 5-bit coded characters (see Annex A) and transmit to the line.
Annex B -- DTMF Operational Mode
B.1 Mode of operation
The communications channel is half-duplex. The receiver is disabled for 300 ms when a character is transmitted to mitigate false detection of echoes.
B.2 Character conversion
The Recommendation Q.23 (DTMF) characters supported are given in Tables B-1 and B-2. Each character shall consist of the appropriate code sequence given in the table.
The DCE shall convert the DTMF characters received from the line to their equivalent T.50 coded characters and transfer them to the DTE on circuit 104 (or its equivalent) per Table B-1.
The DCE shall convert the Recommendation T.50 coded characters received from the DTE on Circuit 103 (or its equivalent) to the appropriate DTMF characters and transmit to the line per Table B-2.
Annex C -- EDT Operational Mode
C.1 Mode of operation
The communications channel is half-duplex. Carrier is transmitted 10 ms before the first character is transmitted. The receiver shall be disabled for 300 ms when a character is transmitted to mitigate false detection of echoes (in non-V.18 devices carrier is 1s after the last character to provide this same function).
C.2 Modulation
The modulation is frequency shift keyed modulation using Recommendation V.21(1) frequencies. The data signalling rate is 110 bits/s.
C.3 Characters in the EDT mode
The EDT must use the following character structure. The 7-bit T.50 coded character shall be proceeded by one (1) START bit and shall be followed by one EVEN PARITY bit, and 2 STOP bits.
Annex D -- Bell 103 mode
D.1 Mode of operation
The communication circuit for data transmission is a duplex circuit whereby data transmission in both directions simultaneously is possible at 300 bit/s or less. The frequency of the ANS used by this DCE is 2 225 Hz.
D.2 Modulation
The modulation is a binary modulation obtained by frequency shift, resulting in a modulation rate being equal to the data signalling rate.
For channel No. 1, the nominal mean frequency is 1 170 Hz, For channel No. 2, it is 2 125 Hz.
In each channel, the higher characteristic frequency (FA) corresponds to a binary 1. (i.e. channel No. 1 (FA = 1 270 Hz and Fz = 1 070 Hz); channel No. 2 (FA = 2 225 Hz and Fz = 2 025 Hz)). The allowable frequency deviation is ñ 100 Hz
Annex E -- Minitel mode
E.1 Mode of operation
The modulation is asymmetric duplex conforming to ITU-T Recommendation V.23 with a 1200 bit/s forward channel and the 75 bit/s backward channel active. Minitels intended for text telephone use can act as either master or slave, with role selection automatically being done at connection establishment. The carrier connection procedure assigns the role of "master mode " to the DCE which transmits at the 75 bit/s data signalling rate and the role of "slave mode" to the DCE that transmits at the 1200 bit/s data signalling rate. In Minitel operation the slave echoes received characters and uses local echo to view transmitted characters.
Any implementation of the Minitel must follow the profile 2 of the CEPT Videotext Recommendation
E.2 Character code and framing
When used in text telephone mode the basic C0, G0 and G2 character sets shall be supported. The characters are sent in asynchronous mode, 7 bit characters framed by one even parity bit, one start bit and one stop bit. (receive parity is ignored).
E.3 Control
A repertoire of control sequences is defined for Minitel in accordance with Profile 2 of the CEPT Videotex protocol. A subset is required for text telephone usage. After connection, the slave takes the initiative to set the terminals in a mode suitable for text telephony by the following control sequences.
Slave sends
Master responds
Reset (1B,39,7F)
Initialization (13, 5E)
Request scroll up mode (1B, 3A,69, 43)
Acknowledge scroll mode (1B,3A,73,46)
Clear screen (0C)
E.4 Error Control
The slave can request initiation of Error Control Procedure (1B, 3A, 69, 44) to which the master responds with Acknowledge error control (1B, 3A, 73, 44). The slave does not initiate any other transmission while waiting for the Acknowledge error control message. When the Error Control Procedure is active, and the master detects errors, it can request retransmission. When in error control mode, the slave sends characters blocked in groups of 15 characters followed by a CRC character and a validation character with all bits set to 0. Erroneous blocks are discarded. Partially filled blocks are filled with NUL characters if no characters are entered for 16 ms. The slave can also request deactivation of the error control procedure.
E.5 Interworking with Normal (non-text telephone) Minitels
Connection to a Minitel not intended for text telephone use requires manual intervention on the part of the user. In this scenario the calling V.18 device must be configured to connect in the answer (slave) mode
Annex F/V.18 -- V.21 Text Telephone mode
F.1 Mode of operation
The communication connection is 300 bit/s duplex.
F.2 Modulation
The modulation is frequency shift keyed modulation using continuous carriers according to Recommendation V.21 frequencies.
F.3 Channel selection
Existing text telephone devices use several different ways to select the mode of operation (i.e., originate or answer). The following is a list on known methods used for resolution of mode assignments:
1. The DCE starts in answer mode and then toggles at random intervals (0.6-2.4 s) between the originate and answer modes until a carrier connection is established.
2. The DCE uses stored information and chooses it's mode of operation depending on whether the device has most recently dialed or detected a ring.
In other cases, where no form of resolution is provided, the assignment of the mode of operation relies on the users selecting different modes at each end by prior agreement.
F.3 Character code and framing
Characters shall be coded in 7 bit national character sets according to Recommendation T.50. Characters and framed by one start bit, 7 bit data, with one parity bit and one stop bit. Devices should be designed to accept one or two stop bits.
F.4 Presentation Control
Transmitted characters are viewed through the use of local echo. Erasure of the last character is requested by BS (0/7). New line is requested by CR LF. Local word wrapping is used at end of line, and does not cause CR LF to be sent to the line.
F.5 Usage conventions
Most existing devices have only one common window for display of both directions of transmission, therefore a indicator is used to indicate when a user is through typing. The most commonly used indicators for this purpose are the "*" (e.g., in the Nordic countries) and the character string "GA" (e.g., in the UK).
Appendix 1/V.18 -- Representative ordering of automoding
The following orderings of automoding are suggested starting points for development of probing sequences for the specified countries:
Australia, Ireland
send 5 bit code buffered message,
send V.21 carrier
send V.23 carrier
send EDT code buffered message
send DTMF buffered message
send Annex D carrier
Germany, Switzerland, Italy
send EDT code buffered message,
send V.21 carrier
send V.23 carrier
send 5-bit code buffered message
send DTMF buffered message
send Annex D carrier
Netherlands
send DTMF buffered message,
send V.21 carrier
send V.23 carrier
send 5 bit code buffered message
send EDT buffered message
send Annex D carrier
Scandinavian Countries
send V.21 carrier
send DTMF buffered message
send 5 bit code buffered message
send EDT code buffered message
send V.23 carrier
send Annex D carrier
UK
send V.21 carrier
send 5 bit code buffered message
send V.23 carrier
send EDT code buffered message
send DTMF buffered message
send Annex D carrier
USA
send 5 bit code buffered message,
send Annex D carrier
send V.21 carrier
send V.23 carrier
send EDT code buffered message
send DTMF buffered message
Appendix 2/V.18 -- Guidelines for software designers and device manufacturers
Character set and control
Characters shall be sent in asynchronous mode, with 8 bits of data 1 start bit and 1 stop bit with no parity bit. Characters shall not be echoed by the receiving device. The sending device shall provide for display of transmitted characters. The default character set shall conform to the two octet version, level 1 of ISO 10646. Devices shall support procedures for negotiation of other character sets. The following basic set of presentation control functions from ISO 6429 shall be supported:
The escape sequence to designate a character set shall follow the ISO International register of coded character sets to be used with escape sequences. ESC 2/5 2/15 4/0 shall be the code use to enter UCS-2 level 1.
When the 16 bit UCS character set is being used ISO 6429 control functions shall be padded with 00 characters as specified in ISO 10646-1.
Visual presentation
It is recommended that sent and received characters be presented in separate areas of the display or at a minimum be assigned distinctive fonts to allow for easy determination of the conversation flow by the user.
Line status display
Visual indications on the status of the connection should be visually presented, including call progress information as well as the status of Circuit 135, line energy present