Preventing Overload and Verifying Link Stability in an Implementation of the DPNSS Layer 2 Protocol

The Digital Private Network Switching System (DPNSS) layer 2 protocol specification does not explicitly indicate methods of layer 2 overload control or link stability verification. This paper describes how AT&T's 5ESS-PRX(TM) Switch has implemented these features. Since the protocol recommends 64 frame retransmissions before resetting a link, the key to preventing and controlling overload is to explicitly ignore some retransmissions of incoming frames. This strategy reliably handles the range of frame transmission strategies that are consistent with the DPNSS protocol. Periodic transmission of specialized layer 2 test frames over the DPNSS link assures link stability.