User guide

Point-to-Point Connection Technology 155

PBX Networking

L1 Clock

To enable PBXs in the ISDN network to communicate with each

other, they must be “clock-aligned”. The L1 protocol master sets

the clock for layer 1, and the L1 protocol slave adopts (synchro-

nises to) this clock.

When planning a PBX networking scheme, you must make sure

that the L1 clock propagates from a master via a number of PBXs.

Example: propagation of the L1 clock

If more than one port with the setting L1 Type = “Slave” is con-

figured on an Forum 523/524 and the setting L1 sync possible

has been activated, then one of the ports is automatically defined

as the L1 clock source. The Forum 523/524 will automatically

switch the clock source to another port configured as an L1 clock

source (if a line fails, for example).

Please note: Reciprocal or circular application of the L1 clock is

not allowed.

Example: In the above case you could reverse the L1 slave/mas-

ter setting for the connection between PBX 1 and PBX 3. However,

if you then activate the setting L1 sync possible for the port of

PBX 1, this may cause parts of the PBX network to stop function-

ing temporarily.

When applying the L1 clock of trunk lines, you can assume that

the public network is “clock-aligned”. So, in the above example,

you can connect additional trunk lines to one of the PBXs.

PBX 3

PBX 2

Point-to-point

connection

Trunk line

Network

operator

PBX1