Technical data

TOS for H.323 B-6
Appendix B: VoIP Service
SBX IP Programming & Operations Guide
ADMIN PROGRAMMING
Enable Normal/Fast Mode
TOS for H.323
In the IP Datagram, 8 bits are reserved for the Service type. These bits can be further broken
into five sub-fields as shown below.
The three precedence bits specify the datagram precedence varying from 0 to 7. This allows
the senders to indicate the importance of each datagram.
The TOS provides a mechanism that can allow control information precedence over data. The
other 3 bits represent the following:
• D - requests low delay
• T - requests high throughput
• R - requests high reliability
The precedence bits (0,1,2) are significant in having an effect on load. So, they are varied to try
to monitor the changes in the RTT (Round Trip Time) for varying loads.
At low loads, the traffic is not significant enough for the TOS to have any effect on the ping as
the packets will get through anyway. Similarly at high loads, the TOS will not have an effect on
the ping statistics, but instead in cases where there's too much traffic, there is a significant
amount of packet loss, and round trip times are elongated regardless of the precedence set.
Somewhere in between these two extremes a window is expected in which the TOS actually
has an effect on the RTT/packet loss (that is, high RTT/Packet loss at TOS=0, and low
RTT/Packet loss at TOS=224).
H.323 (Fast) Mode -- selects H.323 Mode. PGM 340+ FLEX 18 + 1
(Value) + [HOLD/SAVE]
VALUES --
0 = Normal
1 = Fast
01234567
PRECEDENCE D T R UNUSED
Release 3.5
June 2010