RFC 2793 (rfc2793) - Page 3 of 10

RTP Payload for Text Conversation

RFC 2793           RTP Payload for Text Conversation            May 2000


   characters regardless of the number of octets per character. It also
   implies that any composite character sequence (CCS) SHOULD be placed
   within one block.

   The T140blocks MAY be transmitted redundantly according to the
   payload format defined in RFC 2198 [3].  In that case, the RTP header
   is followed by one or more redundant data block headers, the same
   number of redundant data fields carrying T140blocks from previous
   packets, and finally the new (primary) T140block for this packet.

2.1 RTP packet header

   Each RTP packet starts with a fixed RTP header. The following fields
   of the RTP fixed header are used for T.140 text streams:

   Payload Type (PT): The assignment of an RTP payload type is specific
     to the RTP profile under which this payload format is used.  For
     profiles which use dynamic payload type number assignment, this
     payload format is identified by the name "T140" (see section 6).
     If redundancy is used per RFC 2198, the Payload Type MUST indicate
     that payload format ("RED").

   Sequence number:  The Sequence Number MUST be increased by one for
     each new transmitted packet. It is used for detection of packet
     loss and packets out of order, and can be used in the process of
     retrieval of redundant text, reordering of text and marking missing
     text.

   Timestamp: The RTP Timestamp encodes the approximate instance of
     entry of the primary text in the packet. A clock frequency of 1000
     Hz MUST be used. Sequential packets MUST NOT use the same
     timestamp. Since packets do not represent any constant duration,
     the timestamp cannot be used to directly infer packet losses.

2.2 Additional headers

   There are no additional headers defined specific to this payload
   format.

   When redundant transmission of the data according to RFC 2198 is
   desired, the RTP header is followed by one or more redundant data
   block headers, one for each redundant data block to be included.
   Each of these headers provides the timestamp offset and length of the
   corresponding data block plus a payload type number indicating this
   payload format ("T140").






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