Practical Considerations
As HDLC can operate successfully in quite poor (errored) data environments, DPNSS will work over a 2Mbs link running without proper synchonisation (plesiochronously) and over poor quality connections (including badly terminated connectors). When setting up PBXs to run a DPNSS connection one end must be defined as the primary or 'A' end. This is a protocol requirement and has nothing to do with link synchronisation. However, such badly synchronised links were frowned upon because of the problems associated with sending faxes and/or other modem based communication which were not specifically identified within the protocol.
Read more about this topic: Digital Private Network Signalling System
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