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IP-based trunks
Issue 6 January 2008 133
IP-based trunks
In circuit switched networks, trunks provide the means to interconnect PBXs with each other
and to the PSTN. Connection to the public network allows PBX station users to call and be
called by terminals that are not part of the PBX private network. An analogous arrangement
exists in packet-switched IP networks.
H.323 trunks connect H.323 systems or gateways over IP networks, similar to circuit-switched
tie trunks. Similarly, SIP trunks connect SIP systems or gateways over IP networks.
A set of Communication Manager switches can each be attached to an IP network, and voice
and fax calls can flow between them in the usual manner except that the call signaling and
audio/fax streams are carried over the IP network. The signaling is carried through the C-LAN
circuit packs, and the audio and fax streams are carried between switches through the Media
Processor circuit packs.
The benefits of using IP trunks include:
● Reducing long distance voice and fax expenses
● Facilitating global communications
● Providing a fully functional network with data and voice
● Converging and optimizing networks by using the available network resources
IP trunk calls can be compressed to save network bandwidth. Repeated compression and
decompression (transcoding) results in a loss of data at each stage and degrades the final
quality of the signal. The maximum recommended number of compression cycles on a call is
three. Normal corporate voice calls or fax calls typically go through fewer than three
compression cycles.
IP (H.323 and SIP) trunks can also connect to other vendors' compliant PBXs.
TTY
US
Default, 45.45 Baudot, interoperates with previous releases
UK
50 Baudot
pass-thru
similar to Fax pass-thru
off
system ignores TTY tones, call remains in administered
codec
Table 16: Fax, Modem, and TTYoIP options (continued)
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