This Recommendation "B-ISDN operation and maintenance principles and functions" describes the set of functions required to operate and maintain the physical layer and ATM layer aspects of the B‑ISDN for permanent, semi-permanent, reserved and on-demand virtual connections.

The functions of the layers above the ATM layer are not considered.

For a detailed description of the maintenance mechanisms and functions at the physical layer, the appropriate ITU-T Recommendations need to be considered depending on the transmission system in use (e.g. SDH, PDH).

For maintenance purposes F4 and F5 flows are defined at the ATM layer covering the VP and VC level, respectively. Both flows are bidirectional and follow the same physical route as the user‑data cells, thus constituting an in-band maintenance flow.

Besides the vertical subdivision into F4 and F5 levels a "horizontal" partition also exists: both flows can either cover the entire virtual connection (end-to-end flow) or only parts of the virtual connection (segment flow).

Dedicated OAM cells with pre-assigned VCI and PTI values are used to implement the F4 and F5 flows at segment or end-to-end levels.

This Recommendation specifies the purpose and the realization (using different OAM cell types) of the following functions:

1)          Fault management, using AIS, RDI, CC and LB cells.

2)          Performance management, using FPM and BR cells.

3)          Activation/deactivation of PM and/or CC, using activation/deactivation cells.

4)          System management cells for use by end-systems only.

In addition, the coding of the APS cell used for providing ATM protection switching is specified.