V18 Specifications

Many people have asked me (Karen) if it's possible to connect to their friend's TTY using their PC and modem. The general answer is: NO. There is no software package that will magically transform your normal modem into a TTY-modem, but that's what this V.18 standard is all about -- making sure future generations of modems will support TTY encoding. But no software package can turn a generic modem into a TTY modem, you *MUST* buy a TTY-modem.

Several companies will sell you a TTY modem and if you have an IBM PC Windows compatible machine, there are some nice TTY-modem answering machine terminal programs available that will turn your TTY-modem into an answering machine and high powered TTY terminal. Very nice. Contact Harris Communications or one of the other vendors on my vendor's page for more information.

It is rumored that some of the new v.34 (28.8Kbps) modems from Motorola and other companies will have V.18 (tty) support. So far, it just seems a rumor, but we should apply pressure on these companies to accomodate Deaf/HOH/Speech Imparired people.

"It's not only good business... it's the law.

--- Karen Nakamura, October 27, 1995

Click to go back up to Karen Nakamura's Deaf Resources.

(not available in text-mode) acesses since August 8, 1995.

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** THERE ARE TWO PARTS: IMPLICATIONS & RECOMMENDATION **
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What are the implications of v.18?

v.18 is the first telecommunication standard that has recognized
the technological ghettoization of TTY users.  It is a
breakthrough to have a standards group address this issue, and a
first step toward universal design for accessibility of
telecommunication products.  It will spawn new products, both in
the modem and TTY markets.

However,  v.18 is not a requirement that all new modems be
TTY-compatible.  It is a "reference" standard that tells industry
how to achieve compatibility.  In the U.S., standards are
voluntary.  We are hopeful, however, that modem manufacturers
will want to improve the marketability of their modems, here and
in Europe, by including TTY compatibility.

For example, government purchasers of modems may require v.18, as
a way of enhancing their compliance with the Americans with
Disabilities Act.  Telephone Relay Services and 911 centers will
benefit from v.18 modems.  And businesses that are purchasing
modems will see v.18 as a way to get a TTY for ADA compliance.

v.18, if implemented in U.S. modems, will first appear in
high-end, high-speed modems.  Specifically, v.18 would be
expected to appear in new v.34 modems.  Lower-speed modems, which
are typically not the focus of new design and development, will
probably not carry v.18.  

Modem manufacturers that include v.18 will mark their products as
such.  This means that a buyer can check a modem's box to
determine whether TTY compatibility is built- in.  

If v.18 is successful for allowing communication with a wide
variety of TTYs, it should generate a new type of TTY (as well as
computer modem) that can handle international calls.

It's important to note that v.18 has yet to be tested with
real-world products.  There will probably be bugs to be ironed
out.  Gallaudet hopes to take an active role in testing and
cooperating with European centers testing v.18.

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Recommendation v.18 Final Version - Sept 20, 1994

Operational and interworking requirements for modems operating in
the text telephone mode

Background


The ITU-T, considering

(a) that text telephones place special operational needs on the
use of modems.

(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.

recommends the procedure below.


1  Scope


This Recommendation defines features to be incorporated in modems
intended for use in, or communicating with, text telephones
primarily used by the deaf or hard of hearing.  It is the goal of
this Recommendation to provide a platform on which a future
universal text telephone could be built. To accommodate this
procedures for interworking with most existing text telephones
are provided in this Recommendation.

To provide for maximum flexibility it is envisaged that text
telephone mode of operation will be invoked on an as required
basis in response to commands issued by the DTE.

It provides for:

Calling identification signals using  the procedures specified in
Recommendation V.8

No disconnection on loss of carrier

Procedures for interoperation with:

     (1) Existing text telephones using a 5-bit code

     (2) Existing text telephones that use V. series modulation
modes,

     (3) Existing DTMF-based text telephones, and 

     (4) Existing European Deaf Telephone (EDT) text telephones,
and

     (5)Others as identified 


2  References

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.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.8 1994, Procedures for starting
sessions of data transmission over the general switched telephone
network.

3  Definitions

For the purposes of this Recommendation the following definitions
apply.

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 signal when  used in this Recommendation shall be bursts
of 4 CI sequences separated by 2 seconds of silence.  The CI
sequence when used with this Recommendation shall be coded in
octets as defined in Tables 1, 2 and 3/V.8. CI shall be coded to
indicate the function 'text telephone'.  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.

TXP   A signal transmitted to allow early termination of answer
tone, and also to confirm V.18 capability. 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.

4  Operational requirements

4.1  To allow for easy movement between data and voice modes the
modem shall not initiate a disconnect.

4.2  To facilitate transfers, on a established connection,
between text telephones using different modes of operation (i.e.,
5-bit, T.50 CODE, DTMF), the modem shall have the capability to
be configured by the user such that whenever transmission has
ceased  for 2 s (e.g. a call transfer) the modem shall re-assume
the initial interworking state, re-initiate the calling id signal
and activate the appropriate detectors (see Section 5 ).

4.3  The modem shall implement the CI signal coded as specified
in Recommendation V.8 for text telephones.

4.4  The modem shall provide call progress indications to the
DTE.  These signals shall include, but not be limited to: BUSY,
RINGING, CARRIER, LOSS OF CARRIER and CONNECT(x) where x
indicates the mode of connection (e.g., V.18, EDT, etc.)

4.5  The modem shall implement Circuit 135  Received energy
present (or its equivalent). Because of the subjective nature of
this indication the operational thresholds of this circuit are
left to the discretion of the implementors.  To prevent confusion
with call progress tones during call establishment, the modem
shall provide filtering of the signals specified in
Recommendation V.8 to the detector associated with this circuit.

5    Interworking

This section provides procedures for interworking with equipment
operating in accordance with Recommendations V.18, V.21, V.23,
EDT, 5 - bit or DTMF based text telephones. Although it is
envisaged that for most connections the user will have a priori
knowledge of the type of terminal being called and will preset
the modem 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.

5.1  Automoding Originating

These procedures are based on the assumption that the modem has
been placed in the V.18 mode with the CI specified in V.8 for
text telephones 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


5.1.1  After connecting to line and dialing the number, the modem
shall transmit no signal for 1 second, and then transmit the CI
signal as specified in V.8 for text telephones 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
Recommendation V.8 for text telephones (see 3), the modem shall
condition its receivers to detect the following signals during
the 2-second OFF period in signal CI (see Figure 1/V.18):

TXP
2100 Hz (ANS)
2225 Hz
1300 Hz
1650/1850 Hz
1400/1800 Hz
DTMF tones
980/1180 Hz(note)
390 Hz

     Note: care should be taken in the design of 980/1180 Hz
detectors to prevent incorrect triggering by echoes of
transmitted CI signals.

If any of the above are detected the modem shall stop
transmitting CI. No disconnect timers shall be started.

5.1.2  If ANS is detected the modem shall stop transmitting the
CI signal, transmit no signal for 0.5 seconds, and then initiate
the transmission of signal TXP in V.21(1) mode. The modem shall
then monitor for 1650/1850 Hz, 1300 Hz and 390 Hz.

5.1.2.1  When the modem detects the absence of ANS, it shall stop
transmission of signal TXP after completion of the current TXP
sequence and continue to monitor for 1650/1850 Hz, 1300 and 390
Hz.

5.1.2.2.  If the modem detects TXP it shall connect as V.18, i.e.
Recommendation V.21 with the operational characteristics given in
section 3.

5.1.2.3  If the modem detects 1650/1850 Hz for  0.2 second it
shall connect as per Recommendation V.21.

5.1.2.4  If the modem detects 1300/2100 Hz for 1 second it shall
connect as per Recommendation V.23.  If 390 Hz is detected for 1
second, then the modem shall connect as per Recommendation V.23
reverse mode (i.e. transmit on the higher speed channel).

5.1.3  If the modem detects 2225 Hz it shall connect as Annex D.

5.1.4  If 1650/1850 Hz for 0.2 second, are detected the modem
shall connect as per Recommendation V.21.

5.1.5  If 1300 Hz is detected for 1 second the modem connect as
per Recommendation V.23.

5.1.6  If a sequence of 1400 Hz and 1800 Hz FSK signals (i.e.
valid 5-bit characters)are detected the modem shall connect in
the 5-bit mode using the character conversion specified in
Section 5.2 and the operational characteristics specified in
Annex A.

5.1.7  If Dual Tone Multi-Frequency (DTMF) signals are detected
the modem shall connect in the DTMF mode using the character
conversion specified in Section 5.4 and the operational
characteristics specified in Annex B.

5.1.8  If 980 /1180 Hz signals are detected the modem shall start
a 2 sec timer Tr and attempt to determine the data signalling
rate of the sequence.

5.1.8.1  If the data signalling rate is 110 bit/s the modem shall
connect in the EDT mode using the operational characteristics
specified in Annex C.

5.1.8.2  If the timer expires  the modem shall return to the
monitor A state.

5.2  Automoding Answering

5.2.1  When in the automatic answer mode the modem shall answer
the incoming call, and condition its receiver to detect :
390/450 Hz
1300 Hz
1400/1800 Hz
DTMF tones
980/1180 Hz
signal CI
2100 Hz
1070/1270 Hz 

The 3s timer Ta shall be started. No disconnect timers shall be
started.  The procedures are defined below, and represented in
Figure 2a and Figure 2b/V.18. The order of the procedures given
below, following the detection period for CI, is given for
illustration only as the automoding should be ordered by the user
on the basis of a  "most likely to occur" scenario. (see Appendix
1)

5.2.2  If signal CI coded for text telephone is detected, the
modem shall transmit answer tone ANS as defined in Recommendation
V.25, and monitor for signal TXP.

5.2.2.1  If signal TXP is detected, the modem shall transmit no
signal for 755ms, 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).

5.2.3  If 2100 Hz is detected the modem shall continue to monitor
for 980 Hz, 1300 Hz or 1650 Hz.

5.2.3.1  If 980 Hz is detected the modem shall connect as per
Recommendation V.21.

5.2.3.2  If 1300 Hz is detected the modem shall connect as per
Recommendation V.23.

5.2.3.3  If 1650 Hz is detected the modem shall connect as per
Recommendation V.21 reversed (in the calling mode).

5.2.4  If 980 Hz is detected the modem shall start 2.7s timer Te
and monitor for 1650 Hz and 1180 Hz.

5.2.4.1  If 1650 Hz is detected the modem shall connect as per
Recommendation V.21 reversed (in the calling mode).

5.2.4.2  If 1s of no signal is detected the modem shall connect
as per Annex C.

5.2.4.3  If either 1180 Hz is detected or timer Te expires, the
modem shall start 1 sec 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 modem
shall connect in the EDT mode using the operational
characteristics specified in  Annex C

5.2.4.3.2  If the data signalling rate is 300 bit/s or timer Tr
expires  the modem shall continue as per Recommendation V.21.

5.2.5  If a sequence of  1400 /1800 Hz FSK signals (i.e. valid
5-bit characters) are detected the modem shall connect in the
5-bit mode using the character conversion specified in Section
5.3 and the operational characteristics specified in Annex A.

5.2.6  If Dual Tone Multi-Frequency (DTMF) signals are detected
the modem 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 1070/1270 Hz is detected the modem shall connect as per
Annex D.

5.2.8  If Ta expires the modem shall proceed either per  5.2.8.1
or per  5.2.8.2. depending on the most likely scenario
determined by the user.

5.2.8.1  The modem shall transmit the 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 modem
shall proceed to next appropriate probe (e.g., ANS, carrier, or
the next carrierless mode).

  Note: The modem 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.

5.2.8.2   The modem shall transmit ANS, set 2 sec timer Tb and
monitor the line for 980 Hz, 1270 Hz and 1300 Hz.

5.2.8.2.1  If 980 Hz is detected the modem shall connect as V.21

5.2.8.2.2  If 1270 Hz is detected the modem shall connect as
Annex D.

5.2.8.2.3  If 1300 Hz is detected the modem shall connect as
V.23.

5.2.9  If timer Tb expires, the modem shall  remain silent for
75+/- 5 ms then transmit, for the duration of variable timer Tc
(default 2 secs) depending on the user setting 1300 Hz carrier,
1650 Hz or 2125 Hz . The modem shall monitor for the appropriate
response while transmitting one of the above carriers.

5.2.10  If Tc expires the modem shall proceed to the next
appropriate probe (e.g. next carrier mode, or next carrierless
mode).

5.3  Character conversion in 5-bit mode

The initial condition of the converter shall be the Letters
(LTRS) mode, therefore the modem 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 modem shall send the appropriate mode
character (i.e., LTRS or FIGS) every 72 characters.

5.3.1  The 5-bit codes supported are given in Tables B-1 and
Tables B-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.

5.3.2  The modem 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) .

5.3.3  The modem 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.

5.4  Character conversion in DTMF mode

5.4.1  The Recommendation Q.23 (DTMF) characters supported are
given in Tables C-1 and C-2.  Each character shall consist of the
appropriate code sequence given in the table.

5.4.2  The modem 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 C-1.

5.4.3  The modem 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 C-2.

5.5  Character conversion in the EDT mode

Although no character conversion is required 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 A
5 bit Operational Mode

A.1  Mode of operation
The communication channel is half-duplex with no channel
turnaround.  The receiver shall be disabled for 300 ms when a
character is transmitted. 

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 for a binary 1 and 1 800 Hz for a binary 0.   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.




Annex B
DTMF Operational Mode

B.1  Mode of operation
The communication channel is half-duplex.



Annex C
EDT Operational Mode

C.1  Mode of operation
The communications channel is half-duplex with no channel
turnaround.  The receiver is disabled for 300 ms when a character
is transmitted. 
C.2  Modulation 
The modulation is frequency shift keyed modulation (i.e. carrier
is 10 ms before a character is transmitted and removed 1s after
the last character) using Recommendation V.21(1) frequencies. 
The data signalling rate is 110 bits/s. 


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 modem is 2225 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 1170 Hz, For
channel No. 2, it is 2125 Hz.

The frequency deviation is  100 Hz.  In each channel, the higher
characteristic frequency (FA) corresponds to a binary 0.  (i.e.
channel No. 1 (FA = 1270 Hz and Fz = 1070 Hz);  channel No. 2
(FA = 2225 Hz and Fz = 2025 Hz)).


Appendix 1/V.18
Representative ordering of automoding

The following orderings of automoding are suggested for the
specified countries:

USA

CI detection period 
send 5 bit code buffered message,
send ANS
send Annex D carrier
send V.21 carrier
send V.23 carrier
send EDT code buffered message
send DTMF buffered message


UK/Scandinavian Countries

CI detection period
send ANS
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


Australia, Ireland

CI detection period
send 5 bit code buffered message,
send ANS
send V.21 carrier
send V.23 carrier
send EDT code buffered message
send DTMF buffered message
send Annex D carrier


Germany, Switzerland

CI detection period
send EDT code buffered message,
send ANS
send V.21 carrier
send V.23 carrier
send 5-bit code buffered message
send DTMF buffered message
send Annex D carrier

Netherlands

CI detection period
send DTMF buffered message,
send ANS
send V.21 carrier
send V.23 carrier
send 5 bit  code buffered message
send EDT buffered message
send Annex D carrier


Annex 2
Guidelines For Text Telephones Used With Recommendation V.18

Character set and control

The recommended initial character format when two V.18 modems are
interconnected is 8 bits data and no parity bit.  The initial
character set provided should be in accordance with ISO 8859-1.
There should also be provision made for changing the character
set using the control procedures specified in  ISO 6429.

The following minimal set of presentation control functions
should be supported: carriage return, line feed, back space,
select character.

The erasure of characters should be controlled by the sending
party only.

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 users.

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.


V18-F1.GIF
V18-F2A.GIF
V18-F2B.GIF
V18-TA1.GIF
V18-TA2.GIF
V18-F2A.GIF