Specifications

TD 92326GB
2009-03-10 / Ver. E
Installation and Operation Manual
VoIP Gateway
140
The values specified here are approximate values, as determining the bandwidth exactly
depends upon a number of factors.
Please note however that the specified values only apply to one direction. The overall
values for a call without “Silence compression” are thus twice as high.
The bandwidth of communications media are usually specified per direction. An ISDN
connection uses 64 kbit/s per direction, the data in the table can thus be compared
intuitively with the familiar bandwidths.
Another way of saving bandwidth is by not transmitting any data during pauses in speech.
Considerable bandwidth can be saved in this way, since only one party usually speaks at a
time during a conversation. This function is referred to as “Silence compression” and can
usually be activated without any loss of quality.
Absolute silence at one end would cause some irritation at the active end, since users
often assume that the connection is faulty if they do not hear anything from the remote
end. To avoid this situation, an artificial background noise referred to as “comfort noise” is
introduced at this end. Information is exchanged at regular intervals in order to match the
volume of these simulated background noises to the actual background noises at the
currently silent end. These so called “comfort noise updates” still require considerably less
bandwidth than the bandwidth saved by “silence compression”. “Silence compression”
and “Send comfort noise updates” should therefore be activated together and only
deactivated if compatibility problems arise involving third party devices.
19.2 Registering the VoIP Gateway with another Gatekeeper
If the VoIP Gateway (or the gatekeeper in it) has to log on to another gatekeeper as in the
scenario illustrated in figure 123 on page 135, this can be done using a gateway definition
in “Register as gateway” mode. In most cases, this is the correct mode. Use the “Register
as endpoint” mode, if the other gatekeeper only allows the registration of a VoIP end
point. On the other hand, the behaviour is identical in both modes, if the external
gatekeeper is an Ascom VoIP Gateway.
• To register with a gatekeeper, in the area “GK” under “GW1” to “GW12”, set up a
definition in “Register as gateway”, or “Register as endpoint” mode.
• You can leave the “Gatekeeper Address” field empty, if the gatekeeper is to be found
using Gatekeeper Discovery (see section 19.1.1 Gatekeeper Discovery on page 135).
Otherwise, enter the IP address of the gatekeeper there.
• If the gatekeeper operates with a gatekeeper ID (see section 19.1.2 The Gatekeeper
Identifier (ID) on page 135), enter it into the “Gatekeeper Identifier” field.
• Define the H.323 name required to identify yourself with the gatekeeper. It usually
makes the most sense if the gateway only registers with an H.323 name and not with an
E.164 address (i.e. with a telephone number). This is obligatory with some gatekeepers
though. Look therefore at the documentation for the gatekeeper where you want to
register.
• Define the H.323 protocol options for communication with the gatekeeper (see section
19.1.3 H.323 Interop Tweaks on page 136).
Tip: The effective bandwidth required can vary according to conditions in the given
environment. On the one hand, routers used in the transmission link can apply
special compression techniques (RTP header compression) and thus reduce the
required bandwidth. On the other hand, voice channels being switched off during
pauses in speech also results in reduced bandwidth requirement. The values
specified in the table represent the most unfavourable values for transmission over
long-distance routes (PPP).