Call Progress Tones

Dual frequency call progress tones can be observed by singling out two strongest frequencies in a sample and comparing them to the table above.

Special Information Tones (SIT)

Special Information Tones (SIT) consist of three tones. Information can be encoded using the length (short - 276 ms, long = 380 ms) and low/high frequency of each tone. 3rd tone has the same frequency (1776.7 Hz) for low and high and always played the longest.

Using the table above, you can put together a three tone combination for Vacant SIT:

954.2 Hz (380ms) -> 1370.6 Hz (276ms) -> 1776.7 Hz (380ms)

SIT tones can be detected by observing the last 1776.7 Hz tone for the duration of 380 ms.

T.30 Automatic Answering Sequence

CED Answer Tone

For a period of at least 0.2 s after it is connected to line, it shall transmit no signal. After this period, it shall transmit the Called terminal identification (CED) answer tone, a continuous 2100 Hz ± 15 Hz tone for a duration of not less than 2.6 s and not more than 4.0 s. The terminal delays for a period of 75 ± 20 ms after transmitting the CED tone before transmitting further signals.

CNG Calling Tone

To indicate a calling non-speech terminal or that the terminal is in the transmit mode, it shall transmit a continuous 1100 Hz ± 38 Hz tone for a duration of 0.5 second ON and 3 seconds OFF ± 15%.

Preamble

Coded control procedures should be transmitted in a synchronous mode on the general switched telephone network at 300 bits per second ±0.01% utilizing the characteristics of V.21 channel No. 2 modulation system. Transmission of signals utilizing the modulation system of V.21 channel. No. 2 should be followed by a delay of 75 ± 20 ms before the signalling, utilizing a different modulation system, commences.

The tonal signal will have one of the following formats:

• 1650 Hz (±6 Hz) ON for 1.5 s and OFF for 3 s (timing tolerance ±15%)
• 1850 Hz (±6 Hz) ON for 1.5 s and OFF for 3 s (timing tolerance ±15%)
• 1650 Hz (±6 Hz) ON for 1.5 s immediately followed by 1850 Hz ON for 0.75 s followed by silence for 3 s (timing tolerance ±15%).

Carrier Frequency

v.17

Modem intended only for use in high speed facsimile applications. The modem has the following principal characteristics:

• Provision for half duplex operation at data signalling rates of:
• * 14 400 bit/s synchronous,
• * 12 000 bit/s synchronous,
• * 9600 bit/s synchronous,
• * 7200 bit/s synchronous;
• Quadrature amplitude modulation with synchronous line transmission at 2400 symbols per second.
• Inclusion of data scramblers, adaptive equalizers and eight-state trellis coding.
• Two sequences for training and synchronization: long train and resync.

The channel carrier frequency is 1800 ± 1 Hz. The receiver must be able to operate with received frequency offsets of up to ± 7 Hz.

v.21

• Data transmission may take place at low data signalling rates on telephone calls set up on switched telephone circuits (or on leased telephone circuits).
• 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 modulation is a binary modulation obtained by frequency shift, resulting in a modulation rate being equal to the data signalling rate.
• The nominal characteristic frequencies;
• * channel No. 1 (FA = 1180 Hz and Fz = 980 Hz)
• * channel No. 2 (FA = 1850 Hz and Fz= 1650 Hz)

v.27

The principal characteristics for this recommended modem for transmitting data at 4800 bits per second on leased circuits are as follows:

• it is capable of operating in a full-duplex mode or half-duplex mode;
• differential eight-phase modulation with synchronous mode of operation;
• possibility of a backward (supervisory) channel at modulation rates up to 75 bauds in each direction of transmission, the use of these channels being optional;
• inclusion of a manually adjustable equalizer.

The carrier frequency is to be 1800 ± 1 Hz. No separate pilot frequencies are provided.