The TN771DP performs maintenance functions. These functions include packet bus reconfiguration that allows diagnosis and correction of recoverable packet bus failures before the link access procedure on the D-channel (LAPD) links fail. LAPD is a link-layer protocol on the ISDN-BRI and ISDN-PRI data link layer (level 2). LAPD provides data transfer between 2 devices and error and flow control on multiple logical links. It recovers packet bus failures involving up to 3 malfunctioning leads (1 or 2 data or parity leads and 1 control lead) by swapping spare leads with the malfunctioning leads. Other maintenance functions include ISDN-PRI testing that originates and terminates loopback tests on ISDN facilities. It provides bit and block error rate information indicating ISDN facility quality.
Avaya Lucent Definity ECS Maintenance R6 Vol I and II.pdf