
22 Chapter 1 Introduction
P0937663 03.1
The Business Communications Manager 2.5 Programming Operations Guide has a data section
that describes the internet protocols and data settings that the Business Communications Manager
requires or is compatible with. Ensure that this connection is correctly set up and working before
you attempt to deploy any remote IP devices.
LAN
A Local Area Network (LAN) is a communications network that serves users within a confined
geographical area. For Business Communications Manager 2.5, a LAN is any IP network
connected to a LAN card on the Business Communications Manager 2.5 system. Often, the LAN
can include a router that forms a connection to the Internet. A Business Communications Manager
can have up to two LAN connections.
Public Switched Telephone Network
The Public Switched Telephone Network (PSTN) can play an important role in IP telephony
communications. In many installations, the PSTN forms a fallback route. If a call across a VoIP
trunk does not have adequate voice quality, the call can be routed across the PSTN instead, either
on public lines or on a dedicated ISDN connection between the two systems. The Business
Communications Manager also serves as a gateway to the PSTN for all voice traffic on the system.
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, voice quality between IP telephones can vary significantly from call to call and
time of day. When two IP telephones are on a call, each IP telephone encodes the speech at the
handset microphone into small data packets called frames. The system sends the frames across the
IP network to the other telephone, where the frames are decoded and played at the handset
receiver. If some of the frames get lost while in transit, or are delayed too long, the receiving
telephone experiences poor voice quality.
Codecs
The algorithm used to compress and decompress voice is embedded in a software entity called a
codec (COde-DECode).
Two popular Codecs are G.711 and G.729. The G.711 Codec samples voice at 64 kilobits per
second (kbps) while G.729 samples at a far lower rate of 8 kbps.
Voice quality is better when using a G.711 CODEC, but more network bandwidth is used to
exchange the voice frames between the telephones.
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