■■ (P) is one bit to indicate padding of the payload.
■■ (V) is a 2 bit indicator of the version for RTP.An SDP example that describes RTP text transport using port 11000 and hav-
ing two levels of redundancy is shown here:
m=text 11000 RTP/AVP 98 100
a=rtpmap:98 t140/1000
a=rtpmap:100 red/1000
a=fmtp:100 98/98/98Performance Metrics for ToIP
The average speed for typing is considered here to be about 10 characters per
second. Using 300 milliseconds as the average time between transmissions
and considering three character payloads, the maximum average bit rate is
2,000 bits/s. For adequate QoS, the delay must not be greater than 500 ms, so
as to support true interactive text conversation. The delay requirement for QoS
for ToIP is, therefore, slightly less stringent than for voice, but not much.
The combination of ToIP with voice and with video for sign language is both
very powerful and a natural application for even the lowest-cost PCs. The
combination is also called Total Conversation. Such an application is shown in
Figure 17.4.
Figure 17.4 The Total Conversion application
Courtesy of Omnitor AB
Accessibility for the Disabled 293