Class Of Service; Ip Type Of Service And Differentiated Services - 3Com 3C10402B Administrator's Manual

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PPENDIX ONNE

Class of Service

IP Type of Service and
Differentiated
Services
X H.323 G
TIONS ATEWAY
Extension Dial Plans
PBX systems can use different dial plans. You must consider dial plan
differences when setting up calls between systems. Dial plans differ in
their use of leading digits, number of digits, and excluded numbers. For
more information, see
Extension Delay
Call setup times for digital connections, compared to analog connections,
are instantaneous so there is no need to include a delay between the
IP address and an appended extension.
Incoming H.323 calls to a system usually go directly to the
Automated Attendant. Although the Auto Attendant can respond with
voice instructions, the call does not have to wait until the end of the voice
instruction to respond. The Auto Attendant accepts extensions whether
they are entered manually or as part of a speed dial number.
The use of an H.323 gateway affects the Class of Service assignments
that are applied to extensions because:
H.323 calls use IP addresses instead of the familiar numbers that are
■
used for public switched network calls (different dial plan).
The cost of an H.323 call is distance-independent, so you do not need
■
to limit long distance calling for cost reasons.
External Call Control
Users of ConneXtions-equipped systems can place H.323 calls to other
H.323 systems anywhere in the world without having to pay long
distance charges. Since there are no long distance charges for H.323
calls, there is no need to restrict them for cost reasons.
The header of each IP packet contains an 8-bit Type of Service (TOS) field
that indicates the precedence (relative importance) of the packet. Routers
then examine the TOS field and give precedence to packets with a higher
TOS setting.
Although your telephone system supports prioritization using the TOS
field, this facility is not supported for H.323 calls. However, for H.323 calls
over the WAN, routers can prioritize voice traffic using alternative means.
For example, during installation, you can select a range of UDP or TCP
port addresses to help with router setup.
Chapter 11.