TD 92326GB
2009-03-10 / Ver. E
Installation and Operation Manual
VoIP Gateway
154
22.3 The TE and NT modes
TE (terminal equipment) mode means here that the interface is operating like a normal
piece of ISDN terminal equipment which means that:
• layers 2 and 3 of the ISDN protocol are configured as terminal equipment.
• the connection lines are used accordingly and the VoIP Gateway synchronises itself to
the network clock (clock slave).
NT (network termination) mode on the other hand, means that the interface operates like
an ISDN network termination (NTBA Network Termination Basic Access) which means that:
• layers 2 and 3 of the ISDN protocol are configured as a network.
• the connection lines are crossed accordingly and the VoIP Gateway provides the clock
(clock master).
The Gateway does not have a stable enough clock to run in Free Mode, therefore it
requires a sync source from the PBX in order to maintain SYNC on the PRI. If there is no
clock source, the PBX will complain about excessive slips, leading to a disruption of service
on the PRI. Normally, this disruption only occurs for a few seconds, but some PBX systems
will shut it down waiting for midnight, counter decrementation, manual intervention, etc.
Since the PBX is receiving clock from the Central Office, and we are attached to the PBX,
we should always receive clock from the PBX.
The Gateway provides us the ability to make changes to the clocking, but it is relevant to
the type of connection as well.
The PRI settings has a section called NT Mode (Network Termination Mode). If this is
checked, the Gateway acts as a Network side (Master) for clocking and for D-Channel
setup. If it is unchecked the gateway acts as a User side (Slave) for clocking and D-Channel
setup.
Normally, the Master side sends clock, and the User side receives clock. However, some
PBX systems (Meridian for example), can only support User side for 5ESS protocol.
Therefore, we have the ability to set the gateway as Network for D-Channel, but Slave for
Clocking. The Clock Mode setting allows this.
Clocking and NT Gateway settings
NT Mode = Checked (We are Network/Master)
Gateway Clocking
Clock Mode = Derived from NT Mode
Sending
Clock Mode = Slave
Receiving
Clock Mode = Master
Sending
NT Mode = Unchecked (we are User/Slave)
Clock Mode = Derived from NT Mode
Receiving
Clock Mode = Slave
Receiving
Clock Mode = Master
Sending
Changing these settings may result in a reversal of the TX/RX leads as well. So if you
make a change on a working PRI, it may require you to check the Swap TX/RX box as
well. Changing the NT mode or Clock Mode requires a gateway reset, swapping TX/RX
does not.