t38_core.h

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/*
 * SpanDSP - a series of DSP components for telephony
 *
 * t38_core.h - An implementation of T.38, less the packet exchange part
 *
 * Written by Steve Underwood <steveu@coppice.org>
 *
 * Copyright (C) 2005 Steve Underwood
 *
 * All rights reserved.
 *
 * This program is free software; you can redistribute it and/or modify
 * it under the terms of the GNU Lesser General Public License version 2.1,
 * as published by the Free Software Foundation.
 *
 * This program is distributed in the hope that it will be useful,
 * but WITHOUT ANY WARRANTY; without even the implied warranty of
 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
 * GNU Lesser General Public License for more details.
 *
 * You should have received a copy of the GNU Lesser General Public
 * License along with this program; if not, write to the Free Software
 * Foundation, Inc., 675 Mass Ave, Cambridge, MA 02139, USA.
 *
 * $Id: t38_core.h,v 1.29 2008/09/04 14:40:05 steveu Exp $
 */

/*! \file */

#if !defined(_SPANDSP_T38_CORE_H_)
#define _SPANDSP_T38_CORE_H_

/*! \page t38_core_page T.38 real time FAX over IP message handling
There are two ITU recommendations which address sending FAXes over IP networks. T.37 specifies a
method of encapsulating FAX images in e-mails, and transporting them to the recipient (an e-mail
box, or another FAX machine) in a store-and-forward manner. T.38 defines a protocol for
transmitting a FAX across an IP network in real time. The core T.38 modules implements the basic
message handling for the T.38, real time, FAX over IP (FoIP) protocol.

The T.38 protocol can operate between:
    - Internet-aware FAX terminals, which connect directly to an IP network. The T.38 terminal module
      extends this module to provide a complete T.38 terminal.
    - FAX gateways, which allow traditional PSTN FAX terminals to communicate through the Internet.
      The T.38 gateway module extends this module to provide a T.38 gateway.
    - A combination of terminals and gateways.

T.38 is the only standardised protocol which exists for real-time FoIP. Reliably transporting a
FAX between PSTN FAX terminals, through an IP network, requires use of the T.38 protocol at FAX
gateways. VoIP connections are not robust for modem use, including FAX modem use. Most use low
bit rate codecs, which cannot convey the modem signals accurately. Even when high bit rate
codecs are used, VoIP connections suffer dropouts and timing adjustments, which modems cannot
tolerate. In a LAN environment the dropout rate may be very low, but the timing adjustments which
occur in VoIP connections still make modem operation unreliable. T.38 FAX gateways deal with the
delays, timing jitter, and packet loss experienced in packet networks, and isolate the PSTN FAX
terminals from these as far as possible. In addition, by sending FAXes as image data, rather than
digitised audio, they reduce the required bandwidth of the IP network.

\section t38_core_page_sec_1 What does it do?

\section t38_core_page_sec_2 How does it work?

Timing differences and jitter between two T.38 entities can be a serious problem, if one of those
entities is a PSTN gateway.

Flow control for non-ECM image data takes advantage of several features of the T.30 specification.
First, an unspecified number of 0xFF octets may be sent at the start of transmission. This means we
can add endless extra 0xFF bytes at this point, without breaking the T.30 spec. In practice, we
cannot add too many, or we will affect the timing tolerance of the T.30 protocol by delaying the
response at the end of each image. Secondly, just before an end of line (EOL) marker we can pad
with zero bits. Again, the number is limited only by need to avoid upsetting the timing of the
step following the non-ECM data.
*/

/*! T.38 indicator types */
enum t30_indicator_types_e
{
    T38_IND_NO_SIGNAL = 0,
    T38_IND_CNG,
    T38_IND_CED,
    T38_IND_V21_PREAMBLE,
    T38_IND_V27TER_2400_TRAINING,
    T38_IND_V27TER_4800_TRAINING,
    T38_IND_V29_7200_TRAINING,
    T38_IND_V29_9600_TRAINING,
    T38_IND_V17_7200_SHORT_TRAINING,
    T38_IND_V17_7200_LONG_TRAINING,
    T38_IND_V17_9600_SHORT_TRAINING,
    T38_IND_V17_9600_LONG_TRAINING,
    T38_IND_V17_12000_SHORT_TRAINING,
    T38_IND_V17_12000_LONG_TRAINING,
    T38_IND_V17_14400_SHORT_TRAINING,
    T38_IND_V17_14400_LONG_TRAINING,
    T38_IND_V8_ANSAM,
    T38_IND_V8_SIGNAL,
    T38_IND_V34_CNTL_CHANNEL_1200,
    T38_IND_V34_PRI_CHANNEL,
    T38_IND_V34_CC_RETRAIN,
    T38_IND_V33_12000_TRAINING,
    T38_IND_V33_14400_TRAINING
};

/*! T.38 data types */
enum t38_data_types_e
{
    T38_DATA_NONE = -1,
    T38_DATA_V21 = 0,
    T38_DATA_V27TER_2400,
    T38_DATA_V27TER_4800,
    T38_DATA_V29_7200,
    T38_DATA_V29_9600,
    T38_DATA_V17_7200,
    T38_DATA_V17_9600,
    T38_DATA_V17_12000,
    T38_DATA_V17_14400,
    T38_DATA_V8,
    T38_DATA_V34_PRI_RATE,
    T38_DATA_V34_CC_1200,
    T38_DATA_V34_PRI_CH,
    T38_DATA_V33_12000,
    T38_DATA_V33_14400
};

/*! T.38 data field types */
enum t38_field_types_e
{
    T38_FIELD_HDLC_DATA = 0,
    T38_FIELD_HDLC_SIG_END,
    T38_FIELD_HDLC_FCS_OK,
    T38_FIELD_HDLC_FCS_BAD,
    T38_FIELD_HDLC_FCS_OK_SIG_END,
    T38_FIELD_HDLC_FCS_BAD_SIG_END,
    T38_FIELD_T4_NON_ECM_DATA,
    T38_FIELD_T4_NON_ECM_SIG_END,
    T38_FIELD_CM_MESSAGE,
    T38_FIELD_JM_MESSAGE,
    T38_FIELD_CI_MESSAGE,
    T38_FIELD_V34RATE
};

/*! T.38 field classes */
enum t38_field_classes_e
{
    T38_FIELD_CLASS_NONE = 0,
    T38_FIELD_CLASS_HDLC,
    T38_FIELD_CLASS_NON_ECM
};

/*! T.38 message types */
enum t38_message_types_e
{
    T38_TYPE_OF_MSG_T30_INDICATOR = 0,
    T38_TYPE_OF_MSG_T30_DATA
};

/*! T.38 transport types */
enum t38_transport_types_e
{
    T38_TRANSPORT_UDPTL = 0,
    T38_TRANSPORT_RTP,
    T38_TRANSPORT_TCP
};

#define T38_RX_BUF_LEN  2048
#define T38_TX_BUF_LEN  16384

/*! T.38 data field */
typedef struct
{
    int field_type;
    const uint8_t *field;
    int field_len;
} t38_data_field_t;

typedef struct t38_core_state_s t38_core_state_t;

typedef int (t38_tx_packet_handler_t)(t38_core_state_t *s, void *user_data, const uint8_t *buf, int len, int count);

typedef int (t38_rx_indicator_handler_t)(t38_core_state_t *s, void *user_data, int indicator);
typedef int (t38_rx_data_handler_t)(t38_core_state_t *s, void *user_data, int data_type, int field_type, const uint8_t *buf, int len);
typedef int (t38_rx_missing_handler_t)(t38_core_state_t *s, void *user_data, int rx_seq_no, int expected_seq_no);

/*!
    Core T.38 state, common to all modes of T.38.
*/
struct t38_core_state_s
{
    /*! Handler routine to transmit IFP packets generated by the T.38 protocol engine */
    t38_tx_packet_handler_t *tx_packet_handler;
    /*! An opaque pointer passed to tx_packet_handler */
    void *tx_packet_user_data;

    /*! Handler routine to process received indicator packets */
    t38_rx_indicator_handler_t *rx_indicator_handler;
    /*! Handler routine to process received data packets */
    t38_rx_data_handler_t *rx_data_handler;
    /*! Handler routine to process the missing packet condition */
    t38_rx_missing_handler_t *rx_missing_handler;
    /*! An opaque pointer passed to any of the above receive handling routines */
    void *rx_user_data;

    /*! NOTE - Bandwidth reduction shall only be done on suitable Phase C data, i.e., MH, MR
        and - in the case of transcoding to JBIG - MMR. MMR and JBIG require reliable data
        transport such as that provided by TCP. When transcoding is selected, it shall be
        applied to every suitable page in a call. */

    /*! Method 1: Local generation of TCF (required for use with TCP).
        Method 2: Transfer of TCF is required for use with UDP (UDPTL or RTP).
        Method 2 is not recommended for use with TCP. */
    int data_rate_management_method;
    
    /*! The emitting gateway may indicate a preference for either UDP/UDPTL, or
        UDP/RTP, or TCP for transport of T.38 IFP Packets. The receiving device
        selects the transport protocol. */
    int data_transport_protocol;

    /*! Indicates the capability to remove and insert fill bits in Phase C, non-ECM
        data to reduce bandwidth in the packet network. */
    int fill_bit_removal;

    /*! Indicates the ability to convert to/from MMR from/to the line format to
        improve the compression of the data, and reduce the bandwidth, in the
        packet network. */
    int mmr_transcoding;

    /*! Indicates the ability to convert to/from JBIG to reduce bandwidth. */
    int jbig_transcoding;

    /*! For UDP (UDPTL or RTP) modes, this option indicates the maximum
        number of octets that can be stored on the remote device before an overflow
        condition occurs. It is the responsibility of the transmitting application to
        limit the transfer rate to prevent an overflow. The negotiated data rate
        should be used to determine the rate at which data is being removed from
        the buffer. */
    int max_buffer_size;

    /*! This option indicates the maximum size of a UDPTL packet or the
        maximum size of the payload within an RTP packet that can be accepted by
        the remote device. */
    int max_datagram_size;

    /*! This is the version number of ITU-T Rec. T.38. New versions shall be
        compatible with previous versions. */
    int t38_version;

    /*! The fastest data rate supported by the T.38 channel. */
    int fastest_image_data_rate;

    /*! \brief The number of times an indicator packet will be sent. Numbers greater than one
               will increase reliability for UDP transmission. Zero is valid, to suppress all
               indicator packets for TCP transmission. */
    int indicator_tx_count;

    /*! \brief The number of times a data packet which does not end transmission will be sent.
               Numbers greater than one will increase reliability for UDP transmission. Zero
               is not valid. */
    int data_tx_count;

    /*! \brief The number of times a data packet which ends transmission will be sent. Numbers
               greater than one will increase reliability for UDP transmission. Zero is not valid. */
    int data_end_tx_count;

    /*! TRUE if IFP packet sequence numbers are relevant. For some transports, like TPKT
        over TCP they are not relevent. */
    int check_sequence_numbers;

    /*! The sequence number for the next packet to be transmitted */
    int tx_seq_no;
    /*! The sequence number expected in the next received packet */
    int rx_expected_seq_no;

    /*! The current receive indicator - i.e. the last indicator received */
    int current_rx_indicator;
    /*! The current receive data type - i.e. the last data type received */
    int current_rx_data_type;
    /*! The current receive field type - i.e. the last field_type received */
    int current_rx_field_type;
    /*! The current transmit indicator - i.e. the last indicator transmitted */
    int current_tx_indicator;
    /*! The bit rate for V.34 operation */
    int v34_rate;

    /*! A count of missing receive packets. This count might not be accurate if the
        received packet numbers jump wildly. */
    int missing_packets;

    logging_state_t logging;
};

#if defined(__cplusplus)
extern "C"
{
#endif

/*! \brief Convert the code for an indicator to a short text name.
    \param indicator The type of indicator.
    \return A pointer to a short text name for the indicator. */
const char *t38_indicator_to_str(int indicator);

/*! \brief Convert the code for a type of data to a short text name.
    \param data_type The data type.
    \return A pointer to a short text name for the data type. */
const char *t38_data_type_to_str(int data_type);

/*! \brief Convert the code for a type of data field to a short text name.
    \param field_type The field type.
    \return A pointer to a short text name for the field type. */
const char *t38_field_type_to_str(int field_type);

/*! \brief Convert the code for a CM profile code to text description.
    \param profile The profile code from a CM message.
    \return A pointer to a short text description of the profile. */
const char *t38_cm_profile_to_str(int profile);

/*! \brief Convert a JM message code to text description.
    \param data The data field of the message.
    \param len The length of the data field.
    \return A pointer to a short text description of the profile. */
const char *t38_jm_to_str(const uint8_t *data, int len);

/*! \brief Convert a V34rate message to an actual bit rate.
    \param data The data field of the message.
    \param len The length of the data field.
    \return The bit rate, or -1 for a bad message. */
int t38_v34rate_to_bps(const uint8_t *data, int len);

/*! \brief Send an indicator packet
    \param s The T.38 context.
    \param indicator The indicator to send.
    \param count The number of copies of the packet to send.
    \return ??? */
int t38_core_send_indicator(t38_core_state_t *s, int indicator, int count);

/*! \brief Send a data packet
    \param s The T.38 context.
    \param data_type The packet's data type.
    \param field_type The packet's field type.
    \param field The message data content for the packet.
    \param field_len The length of the message data, in bytes.
    \param count The number of copies of the packet to send.
    \return ??? */
int t38_core_send_data(t38_core_state_t *s, int data_type, int field_type, const uint8_t field[], int field_len, int count);

/*! \brief Send a data packet
    \param s The T.38 context.
    \param data_type The packet's data type.
    \param field The list of fields.
    \param fields The number of fields in the list.
    \param count The number of copies of the packet to send.
    \return ??? */
int t38_core_send_data_multi_field(t38_core_state_t *s, int data_type, const t38_data_field_t field[], int fields, int count);

/*! \brief Process a received T.38 IFP packet.
    \param s The T.38 context.
    \param buf The packet contents.
    \param len The length of the packet contents.
    \param seq_no The packet sequence number.
    \return 0 for OK, else -1. */
int t38_core_rx_ifp_packet(t38_core_state_t *s, const uint8_t *buf, int len, uint16_t seq_no);

/*! Set the method to be used for data rate management, as per the T.38 spec.
    \param s The T.38 context.
    \param method 1 for pass TCF across the T.38 link, 2 for handle TCF locally.
*/
void t38_set_data_rate_management_method(t38_core_state_t *s, int method);

/*! Set the data transport protocol.
    \param s The T.38 context.
    \param data_transport_protocol UDPTL, RTP or TPKT.
*/
void t38_set_data_transport_protocol(t38_core_state_t *s, int data_transport_protocol);

/*! Set the non-ECM fill bit removal mode.
    \param s The T.38 context.
    \param fill_bit_removal TRUE to remove fill bits across the T.38 link, else FALSE.
*/
void t38_set_fill_bit_removal(t38_core_state_t *s, int fill_bit_removal);

/*! Set the MMR transcoding mode.
    \param s The T.38 context.
    \param mmr_transcoding TRUE to transcode to MMR across the T.38 link, else FALSE.
*/
void t38_set_mmr_transcoding(t38_core_state_t *s, int mmr_transcoding);

/*! Set the JBIG transcoding mode.
    \param s The T.38 context.
    \param jbig_transcoding TRUE to transcode to JBIG across the T.38 link, else FALSE.
*/
void t38_set_jbig_transcoding(t38_core_state_t *s, int jbig_transcoding);

void t38_set_max_buffer_size(t38_core_state_t *s, int max_buffer_size);

void t38_set_max_datagram_size(t38_core_state_t *s, int max_datagram_size);

int t38_get_fastest_image_data_rate(t38_core_state_t *s);

/*! Set the T.38 version to be emulated.
    \param s The T.38 context.
    \param t38_version Version number, as in the T.38 spec.
*/
void t38_set_t38_version(t38_core_state_t *s, int t38_version);

/* Set the sequence number handling option.
    \param s The T.38 context.
    \param check TRUE to check sequence numbers, and handle gaps reasonably. FALSE
           for no sequence number processing (e.g. for TPKT over TCP transport).
*/
void t38_set_sequence_number_handling(t38_core_state_t *s, int check);

t38_core_state_t *t38_core_init(t38_core_state_t *s,
                                t38_rx_indicator_handler_t *rx_indicator_handler,
                                t38_rx_data_handler_t *rx_data_handler,
                                t38_rx_missing_handler_t *rx_missing_handler,
                                void *rx_user_data,
                                t38_tx_packet_handler_t tx_packet_handler,
                                void *tx_packet_user_data);

#if defined(__cplusplus)
}
#endif

#endif
/*- End of file ------------------------------------------------------------*/