Appendix D PRI Technical Notes
ViewStation FX/VS 4000 User’s Guide 186 www.polycom.com
Channel Service Unit
In North America only, the network cable for the T1 PRI network
interface module can be connected to an external device called a
Channel Service Unit (CSU). The CSU ensures that the PRI is
isolated from the actual network. The CSU regenerates the signals
from the PRI network interface module on the actual network cable.
If the PRI network interface module loses power or is disconnected,
the CSU ensures that a signal continues to be transmitted back to
the network. This prevents the line from being deactivated. A CSU
is normally only needed when the network cable for the T1 PRI
network interface module is connected directly to a telephone
company switch, not when the network cable is connected to a PBX.
If you use an external CSU, you must specify the following
information on the PRI Setup screen (System Info>Admin
Setup>Video Network>IMUX>PRI Network>PRI Setup):
• In the CSU field, select External.
• In the Line Buildout field, select the length of the cable that
connects the PRI network interface module to the CSU.
You can also configure the T1 PRI network interface module to act
as a CSU. To do this, the external power supply must be plugged in.
To use the T1 PRI network interface module as an internal CSU, you
must specify the following information on the PRI Setup screen
(System Info>Admin Setup>Video Network>IMUX>PRI
Network>PRI Setup):
• In the CSU field, select Internal.
• In the Line Buildout field, select an appropriate dB setting. The
telephone company usually determines the dB value by
measuring the characteristics of the line. If an external CSU was
previously used, use the setting selected for that CSU.
Otherwise, start with 0.
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