Nortel VoIP Gateway, Configuration Manual

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Norstar VoIP Gateway Configuration Guide
Chapter 2
IP Telephony overview
This section provides a brief overview of the “Key IP telephony concepts” . It also provides a
“Prerequisites checklist” to help you set up your IP Telephony network.
Key IP telephony concepts
In traditional telephony, the voice path between two telephones is circuit switched. This means that
the analog or digital connection between the two telephones is dedicated to the call. The voice
quality is usually excellent, since there is no other signal to interfere.
In IP telephony, the VoIP Gateway encodes the speech of the call into small data packets called
frames. The system sends the frames across the IP network to the other VoIP Gateway, where the
frames are decoded and sent to the receiving telephone. If some of the frames get lost while in
transit, or are delayed too long, the receiving telephone experiences poor voice quality. On a
properly-configured network, voice quality should be consistent for all IP calls.
The following sections describe some of the components for IP telephony:
• “VoIP trunks” on page 21
• “Gatekeepers” on page 22
• “Codecs” on page 22
• “Jitter Buffer” on page 23
• “QoS routing” on page 24
VoIP trunks
VoIP trunks allow voice signals to travel across IP networks. The VoIP Gateway converts the voice
signal into IP packets, which are then transmitted through the IP network to a gateway on the
remote system. The device at the other end reassembles the packets into a voice signal. Business
Communications Manager and Meridian 1 IPT are devices that can use the H.323 protocol trunks
which the VoIP Gateway supports.
VoIP trunks and analog/digital telephones
While analog and digital telephones cannot be connected to the VoIP Gateway system with an IP
connection, they can make and receive calls to and from other systems through VoIP trunks. Calls
received through the VoIP trunks to system telephones are received through the LAN or WAN and
are translated within the VoIP Gateway to voice channels.