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VoIP gateway IP400 version 5.01
VoIP and heavily loaded WAN links
If voice data is transmitted over heavily loaded, narrowband WAN links, the voice
quality can suffer, if the links can no longer ensure adequate transmission quality
(see Table 11 on page page 77 and Table 12 on page page 79).
Prioritisation of voice data on the WAN links can help here. This can usually be
achieved by the routers used.
Direct use can be made of the “Prioritisation of H.323 voice data” function, if it is
supported by your router.
If your router is able to use the IP
type of service
(TOS) field for prioritisation,
you can make use of this function. Your gateway sets the TOS field to
0x10
for
all IP packets that it transmits. You can change this value as required in the con-
figuration applet in the
TOS Value
field in the
IP Interfaces
area.
If this is not the case, you can use the function “Prioritisation according to source/
destination address”, if available. In this way, data packets from and to the gate-
way are prioritised. This in effect corresponds to the prioritisation of voice data as
above.
In any case, the maximum size of packets transmitted over the WAN link (often
referred to as
MTU Size
) should be restricted to a value smaller than 800 bytes.
This ensures that, in spite of the prioritisation of voice data, larger data packets
do not block the line for an extended period of time during transmission.
Some routers are able to prioritise but are unable to interrupt the transmission of
larger packets once it has started. This can result in poor quality in spite of prior-
itisation. In such a case, check whether this interruption can be separately
enabled. Some routers refer to this function, somewhat confusingly, as
inter-
leaving
.
Tip
You can enter hexadecimal, octal or decimal values; the entries
0x10
,
020
and
16
are all equivalent. Bear in mind that the same value should be set in
the TOS field for all devices.