OAM FUNCTION AND SIGNALLING

OAM Levels in the B-ISDN

#)OAM functions in the network are performed on five OAM hierarchical levels associated with the ATM and physical layers of the protocol reference model.

#)The functions result in corresponding bidirectional information flows F1, F2, F3, F4, and F5 referred to as OAM flows.

#)Not all of these need to be present.

#)The OAM functions of a missing level are performed at the next higher level.

#)The levels are as given in Table below:

#)functions related to OAM levels are independent from the OAM functions of other layers and have to be provided at each layer.

#)Each layer, where OAM functions are required, is able to carry out its own processing to obtain quality and status information. OAM functions are performed by the layer management.

#)These results may be provided to the plane management or to the adjacent higher layer.

#)Higher-layer functions are not necessary to support the OAM of the lower layer.

Mechanisms to Provide OAM Flows

1)Physical Layer Mechanisms

#)The physical layer contains the three lowest OAM levels as outlined in the table.

#)The allocation of the OAM flows is as follows:

•    F1: regenerator section level
•    F2: digital section level
•    F3: transmission path level

#)The mechanisms to provide OAM functions and to generate OAM flows F1, F2, and F3 depend on the format of the transmission system as well as on the supervision functions contained in B-NT1 and B-NT2 for the section crossing the T reference point.

#)There are three types of transmission for customer B access: SDH-based, cell-based, and PDH-based.

SDH-Based Transmission Systems

#)Flows F1 and F2 are carried on bytes in the section overhead SOH ; flow F3 is carried in the path overhead POH of the transmission frame.

Cell-Based Transmission Systems

#)Such transmission systems may use an interface structure.

#)Flows F1 and F3 are carried through maintenance cells for the physical layer using a specific pattern in the header for F1 and F3.

#)F2 flows are not provided, but the associated functions are supported by F3 flows.

#)These cells are not passed to the ATM layer.

#)The occurrence of a PLOAM cell is determined by the requirements of the supported OAM functions.

#)For each (type F1 and F3) of PL-OAM cell, maximum spacing is applied.

#)If maximum spacing is exceeded, loss of OAM flow LOM will occur.

PDH Based Transmission Systems

#)These systems may only be used on the network side of the B-NT1.

#)Specific means to monitor the section perfomance e.g., violation code counting CRC, etc. are specified for these systems.

#)The capability to carry OAM information other than bit-oriented messages is very limited.

ATM Layer Mechanism: F4 and F5 Flows

#)The ATM layer contains the two highest OAM levels shown in Table.

#)These are:
•    F4: virtual path level
•    F5: virtual channel level

#)These OAM flows are provided by cells dedicated to ATM layer OAM functions for both virtual channel connections VCC and virtual path connections VPC . I

#)n addition, such cells are usable for communication within the same layers of the management plane.

F4 Flow Mechanism

#)The F4 flow is bidirectional. OAM cells for the F4 flow have the same VPI value as the user cells of the VPC and are identified by one or more preassigned VCI values.

#)The same preassigned VCI value shall be used for both directions of the F4 flow.

#)The OAM cells for both directions of the F4 flow must follow the same physical route so that any connecting points supporting that connection can correlate the fault and performance information from both directions.

#)There are two kinds of F4 flows, which can simultaneously exist in a VPC.

#)These are as follows:

End-to-End F4 Flow:

#)This flow, identified by a standardized VCI, is used for end-to-end VPC operations communications.

Segment F4 Flow:

#)This flow, identified by a standardized VCI is used for communicating operations information within the bounds of one VPC link or multiple interconnected VPC links, where all of the links are under the control of one administration or organiza-tion.

#)Such a concatenation of VPC links is called a VPC segment.

#)A VPC segment can be extended beyond the control of one administration by mutual agreement.

#)F4 flows must be terminated only at the endpoints of a VPC or at the connecting points terminating a VPC segment. Intermediate points i.e., connecting points along the VPC or along the VPC segment may monitor OAM cells passing through them and insert new OAM cells, but they cannot terminate the OAM flow.

#)The F4 flow will be initiated at or after connection setup.

#)The administration/organization that controls the insertion of OAM cellsfor operations and maintenance of a VPC segment must ensure that such OAM cells are extracted before they leave the span of control of that administration/organization.

F5 Flow Mechanism

#)The F5 flow is bidirectional.

#)OAM cells for the F5 flow have the same VCI/VPI values as the user cells of the VCC and are identified by the payload type identifier PTI .

#)The same PTI value shall be used for both directions of the F5 flow.

#)The OAM cells for both directions of the F5 flow must follow the same physical route so that any connecting points supporting that connection can correlate the fault and performance information from both directions.

#)There are two kinds of F5 flows, which can simultaneously exist in a VCC.

#)These are as follows:

End-to-End F5 Flow:

#)This flow, identified by a standardized PTI, is used for end-to-end VCC operations communications.

#)Segment F5 Flow:

#)This flow, identified by a standardized PTI, is used for communicating operations information within the bounds of one VCC link or multiple interconnected VCC links, where all of the links are under the control of one administration or organiza-tion.

#)Such a concatenation of VCC links is called a VCC segment. A VCC segment can be extended beyond the control of one administration by mutual agreement.

#)F5 flows must be terminated only at the endpoints of a VCC or at the connecting points terminating a VCC segment. Intermediate points i.e., connecting points along the VCC or along the VCC segment may monitor OAM cells passing through them and insert new OAM cells, but they cannot terminate the OAM flow.

#)F5 flow will be initiated at or after connection setup.

#)The administration/organization that controls the insertion of OAM cells for operations and maintenance of a VCC segment must ensure that such OAM cells are extracted before they leave the span of control of that administration/organization.

OAM Functions of the Physical Layer

#)OAM Functions: Two types of OAM functions need to be distinguished:

1. OAM functions supported solely by the flows F1, F2, and F3.
•    Dedicated to detection and indication of unavailability state
•    Requiring ‘‘real time’’ failure information transport toward the affected endpoints for system protection

. OAM functions with regard to the system management
•    Dedicated to performance monitoring and reporting, or for localization of failed equipment
•    May be supported by the flows F1 to F3 or by other means

#)OAM Functions Supported Solely by the Flows F1 to F3.

#)Tables below gives an overview of the OAM functions and the related OAM flows.

#)It also lists the different failures to be detected together with the failure indications for the SDH-based physical layer for the cell-based physical layer.

OAM Functions of the ATM Layer

#)The F4 flow relates to the virtual path and the F5 flow relates to the virtual channel.

#)In both cases the fault management functions consist of monitoring of the path/channel for availability and other, overall performance monitoring

#)A path or channel is either not available or has degraded performance.

VP-AIS and VP-FERF Alarms:

#)The VP-AIS (virtual path-alarm indication signal) and the VP-FERF (virtual path-far-end reporting failure)alarms are used for identifying and reporting VPC virtual path connection failures.

VP-AIS:

#) VP-AIS cells are generated and sent downstream to all affected VPCs from the VPC connecting point e.g., ATM cross-connect which detects the VPC failure.

#)VP-AIS results from failure indications from the physical layer as shown in Tables.

VP-AIS Cell Generation Condition:

#)VP-AIS cells are generated and transmitted as soon as possible after failure indication, and transmitted periodically during the failure condition in order to indicate VPC unavailability.

#)The generation frequency of VP-AIS cells is nominally one cell per second and is the same for each VPC concerned. VP-AIS cell generation is stopped as soon as the failure indications are removed.

VP-AIS Cell Detection Condition:

#)VP-AIS cells are detected at the VPC endpoint and VP-AIS status is declared after the reception of one VP-AIS cell.

#)VP connecting points may monitor the VP-AIS cells.

VP-AIS Release Condition:

#)The VP-AIS state is removed under either of the following conditions:
•    Absence of VP-AIS cell for nominally 3 s
•    Receipt of one valid cell user cell or continuity check cell

VP-FERF:

#)VP-FERF is sent to the far-end from a VPC endpoint as soon as it has declared a VP-AIS state or detected VPC failure.

VP-FERF Cell Generation Condition:

#)VP-FERF cells are generated and transmitted periodically during the failure condition in order to indicate VPC unavailability. Generation frequency of VP-FERF cells is nominally one cell per second and shall be the same for all VPCs concerned.

#)VP-FERF cell generation shall be stopped as soon as the failure indications are removed.
VP-FERF Cell Detection Condition: VP-FERF cells are detected at the VPC endpoint and VP-FERF state is declared after the reception of one VP-FERF cell. VPC connecting points may monitor the VP-FERF cells.

VP-FERF Release Condition:

#) The VP-FERF state is removed when no VP-FERF cell is received during a nominally 3-s period.

#)VPC Continuity Check. The continuity check cell is sent downstream by a VPC endpoint when no user cell has been sent for a period of t, where Ts

#)This mechanism can also be applied to test continuity across a VPC segment.

#)The need for supporting this mechanism for all VPCs simultaneously is for further study.

VP Performance Management Functions

#)Performance monitoring of a VPC or VPC segment is performed by inserting monitoring cells at the ends of the VPC or VPC segment, respectively.

#)In the procedure supporting this function, forward error detection information e.g., the error detection code is communicated by the endpoints using the forward outgoing F4 flow.

#)The performance monitoring results, on the other hand, are received on the reverse incoming F4 flow.

#)Note that when monitoring VPCs that are entirely within one span of control or when monitoring VPC segments, the monitoring result may be reported using the reverse F4 flow or via some other means e.g., TMS telecommunications management network

#)Performance monitoring is done by monitoring blocks of user cells.

#)A performance monitoring cell insertion request is initiated after every N user cells.

#)The monitoring cell is inserted at the first free cell location after the request.

#)The block size N may have the values 128, 256, 512, and 1024.

#)These are nominal block size values, and the actual size of the monitored cell block may vary.

#)The cell block size may vary up to a maximum margin of 50% of the value of N for end-to-end performance monitoring.

#)However, for end-to-end performance monitoring, the monitoring cell must be inserted into the user cell stream no more than N/2 user cells after an insertion request has been initiated.

#)The actual monitoring block size averages out to approximately N cells.

#)To eliminate forced insertions when monitoring VPC segment performance, the actual monitoring block size may be extended until a free cell is available after the insertion request.

#)However, in this case, the actual monitoring block size may not average out to N cells.

#)Forced insertion at the segment level remains as an option.

OAM Functions for the VCC F5 Flows

#)VCC F5 functions are similar to the VPC F4 functions.

ATM Layer OAM Cell Format

#)The ATM layer OAM cells contain fields common to all types of OAM cells, as well as specific fields for each specific type of OAM cell. See Table above.

#)The coding for unused common and specific fields is as follows:
•    Unused OAM cell information field octets are coded 0110 1010 (6AH)
•    Unused cell information field bits (incomplete octets) are coded all zeros

Common OAM Cell Fields

#)All ATM layer OAM cells have the following five common fields:

1. Header. For F4 flow identification, two pre assigned VCIs are used to distinguish OAM cells for VPCs and VPC segments. For F5 flow identification, two PTI values are used to distinguish OAM cells for VCCs and VCC segments.

2. OAM Cell Type (4 bits) . This field indicates the type of management function performed by this cell, e.g., fault management, performance management, and activation/deactivation.

3. OAM Function Type (4 bits) . This field indicates the actual function performed by this cell within the management type indicated by the OAM Cell Type field.

4. Reserved Field for Future Use (6 bits). Default value coded all zero.

5. Error Detection Code EDC (10 bits) . This field carries a CRC-10 error detection code computed over the information field of the OAM cell.