Figure 17.3 RTP packet example for text/t140with one redundant blockThe text blocks may contain one or more text characters that are UTF-8
encoded. No delimiters are transmitted between the characters in a text block.
The components of the RTP packet for text conversation in the example,
shown from the bottom up, are:
■■ T.140 encoded data: A primary data block “P” carrying the latest
entered characters and a redundant data block “R” to have for correc-
tion of possible packet loss or errors on poor links.
■■ Payload type number for each block of text tied to the text/T140
payload in the SIP session startup (T140 PT).
■■ The “R” block length.
■■ Timestamp offset of “R” from the current packet.
■■ RTP Synchronization Source (SSRC) identifier.
■■ Time stamp for “P.” The clock for texttimestamps is 1,000 Hz, while
the clock for audio/t140is 8,000 Hz for compatibility with PSTN
gateways.
■■ Sequence number of the text block. Can be used to detect missing pack-
ets and packets out of order.
■■ Payload type number tied to the redundant textpayload type in the
SIP session setup (“RED” PT).
■■ M (marker) bit is set to one in the first packet in a sequence following a
silent period.
The other RTP fields at the start of the packet are as defined in RFC 3550:
■■ (CC) is a 4-bit CSRC count of the CSRC (contributing source) identifiers.
■■ X is an extension bit as defined in RFC 3550.0123456789012345678901 001P23
23456789 1
V=2 XMCC=0 “RED” PT sequence number
timestamp of primary encoding 1,000 Hz
SSRC identifier
timestamp offset of “R”
“R” T140 encoded redundant data“P” T140 encoded primary data“R” block length
T140 PT(^1) T140 PT
0
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