PABX stands for Private Automatic Branch Exchange.Through it different users can establish communication.A PABX can be connected as a stand-alone private network or directly to its own network or using a set of gateway services.Gateways can be simple or complex depending on the device or network interface.PABX is primarily associated with star topology.Each extension is connected to the PABX via twisted copper wire.There are different types of PABX, but most digital systems connect users through a PABX with a bandwidth of 64 kbps.ISDNPABX 144 kbps page exists.