PBX Networking Types of Point-to-Point Connections143If more than one port with the setting L1 Type = “Slave” is configured on anOpenCom 510 and the setting L1 sync possible has been activated, then one ofthe ports is automatically defined as the L1 clock source. The OpenCom 510 willautomatically switch the clock source to another port configured as an L1 clocksource (if a line fails, for example).LED 3 (top right) of an interface card will light up if one of the interfaces on thecard has been selected as a clock source. If an S2M interface of the type MT+S2M1-1 is installed, it will be selected as the preferred clock source.Please note: Reciprocal or circular application of the L1 clock is not allo-wed.Example: In the above case you could reverse the L1 slave/master setting for theconnection between PBX 1 and PBX 3. However, if you then activate the settingL1 sync possible for the port of PBX 1, this may cause parts of the PBX network tostop functioning temporarily.When applying the L1 clock of trunk lines, you can assume that the public networkis “clock-aligned”. So, in the above example, you can connect additional trunk linesto one of the PBXs.13.2 Types of Point-to-Point ConnectionsThere are different types of connection available for an point-to-point connectionbetween two PBXs, depending on the distance between them.13.2.1 Direct ConnectionThis type of ISDN point-to-point connection joins the two systems directly to eachother using a crossover twisted-pair cable. An S 0 connection can be used for dis-tances up to 1,000 metres, while an S 2M connection can span up to 250 meters.Normally one PBX is the protocol master for all three layers, and the other PBX isthe protocol slave for all three layers.Direct connectionPBX 1L1 masterL2 masterL3 masterPBX 2L1 slaveL2 slaveL3 slave
