Definitions of Managed Objects for Synchronous Optical Network (SONET) Linear Automatic Protection Switching (APS) Architectures
Extracted elements (28)
The MIB separates APS group configuration (apsConfigTable) from channel configuration (apsChanConfigTable), requiring channels to be created first and the group activated second. This staged creation model ensures that by the time a group is set active, all channel entries can be validated for consecutive numbering and architectural consistency, returning inconsistentValue atomically on any error.
The restriction that agents need not process Set PDUs affecting multiple conflicting control objects is an intentional simplification. It avoids requiring agents to implement complex transactional conflict-resolution logic for PDUs containing varbinds that are mutually inconsistent, such as simultaneously activating a group while deactivating one of its channels.
apsConfigExtraTraffic may need to be disabled to interwork with other SONET network elements that do not support extra traffic on the protection channel. Extra traffic is only relevant to the 1:n architecture and must be disabled for 1+1.
A row in apsChanConfigTable MAY NOT be created, deleted, set to notInService, or otherwise modified if the associated APS group (matching apsConfigName) has apsConfigRowStatus equal to active. Modifications are permitted when the group status is notInService.
An agent is not required to process SNMP Set Requests that affect multiple control objects within this MIB. This eliminates the possibility that a single Set PDU contains conflicting varbinds (e.g., simultaneously activating an apsConfigEntry while deactivating an associated apsChanConfigEntry).
apsConfigExtraTraffic MUST be set to disabled if the architecture is 1+1. This object controls whether extra traffic is transferred on the protection channel in a 1:n architecture and may need to be disabled to interwork with network elements that do not support extra traffic.
Because onePlusOneCompatible and onePlusOneOptimized modes necessitate bidirectional switching, apsConfigDirection MUST be set to bidirectional whenever either of these modes is selected.
Before an APS group is made active (apsConfigRowStatus set to active), the set of channels MUST begin with channel number 0 (for all architectures except onePlusOneOptimized) or channel number 1 (for onePlusOneOptimized) and MUST consist of consecutive channel numbers not exceeding 14. Failure to meet this constraint causes apsConfigRowStatus to return inconsistentValue.
Since switching is revertive with the 1:n architecture, apsConfigRevert MUST be set to revertive if apsConfigMode is set to oneToN. The 1+1 architecture optionally supports revertive switching.
The value of apsChanConfigIfIndex MUST be unique among all instances; a particular SONET LTE (ifType sonet(39)) can only be configured in one APS group. This object cannot be changed while the associated APS group is active.
A mode mismatch (apsStatusCurrent bit 'modeMismatch') occurs when there is a conflict between the local mode and received K2 information: K2 bit 5 (architecture) and K2 bits 6–8 (uni/bidirectional mode). This check applies to modes other than 1+1 unidirectional.
A Protection Switch Byte Failure (PSBF) is indicated in apsStatusCurrent bit 'psbf' and counted by apsStatusPSBFs. It occurs when either an inconsistent APS byte (no three consecutive identical K1 bytes in the last 12 frames) or an invalid code (unused or contextually inappropriate K1 code, or invalid channel number, in three consecutive frames) is detected.
apsConfigSdBerThreshold (range 5–9, default 5) and apsConfigSfBerThreshold (range 3–5, default 3) define Signal Degrade and Signal Failure BER thresholds respectively. The negated value is used as the exponent of 10; e.g., value 5 means BER threshold of 10^-5. Both may be modified while the group is active.
apsConfigWaitToRestore specifies the Wait-to-Restore period in seconds (range 0–720, default 300). After a condition requiring automatic protection switching is cleared, this period must elapse before reversion to the working line, to avoid rapid switch oscillations. GR-253-CORE specifies a 5–12 minute range; G.783 also permits shorter intervals.
Five SNMP notification types are defined: apsEventSwitchover (sent when apsChanStatusSwitchovers increments), apsEventModeMismatch (apsStatusModeMismatches increments), apsEventChannelMismatch (apsStatusChannelMismatches increments), apsEventPSBF (apsStatusPSBFs increments), and apsEventFEPLF (apsStatusFEPLFs increments). Each is selectively enabled via the apsNotificationEnable BITS object.
The apsChanStatusTable (augments apsChanConfigEntry) tracks per-channel statistics: apsChanStatusSignalDegrades (BER exceeds apsConfigSdBerThreshold), apsChanStatusSignalFailures (loss of signal/frame, AIS-L, or BER exceeds apsConfigSfBerThreshold), apsChanStatusSwitchovers, apsChanStatusLastSwitchover, and apsChanStatusSwitchoverSeconds (cumulative Protection Switching Duration, only valid when revertive switching is enabled).
The apsCommandTable (indexed by group name and channel number, exists only while apsConfigEntry is active) provides apsCommandSwitch (ApsSwitchCommand) and apsCommandControl (ApsControlCommand) objects. Reading apsCommandSwitch returns the last command written since initialization, not necessarily the currently active request; the K1/K2 status objects must be read to determine current APS state.
The apsConfigMode object specifies the APS architecture: onePlusOne (permanently bridges working to protection), oneToN (one protection channel for up to n working channels), onePlusOneCompatible (1+1 bidirectional compatible with ITU-T G.783 Annex A), and onePlusOneOptimized (1+1 bidirectional optimized per ITU-T G.783 Annex B). Default is onePlusOne.
The apsConfigTable supports creation, modification, and deletion of linear APS group entries indexed by a text group name (1–32 characters). Key configuration objects include apsConfigMode, apsConfigRevert, apsConfigDirection, apsConfigExtraTraffic, apsConfigSdBerThreshold, apsConfigSfBerThreshold, apsConfigWaitToRestore, and apsConfigRowStatus. Most configuration objects cannot be modified while apsConfigRowStatus is active.
The ApsControlCommand textual convention applies only to 1:n architecture LTEs and provides three values: noCmd (read-only sentinel), lockoutWorkingChannel (prevents a specific working channel from switching to the protection line), and clearLockoutWorkingChannel. Specifying the protection line for lockout or clear operations returns an inconsistentValue error.
The apsMapTable lists all SONET LTE interfaces (ifType sonet(39)) on the system, providing a mapping from ifIndex to APS group name (apsMapGroupName) and channel number (apsMapChanNumber, range -1..14, where -1 means not in any group and 0 means null/protection channel). Entries are automatically updated as side effects of apsChanConfigTable operations and cannot be directly created or deleted.
The apsStatusTable (augments apsConfigEntry) provides per-group status: apsStatusK1K2Rcv and apsStatusK1K2Trans carry the current received and transmitted K1/K2 bytes on the protection channel. apsStatusCurrent is a BITS object indicating active conditions: modeMismatch, channelMismatch, psbf, feplf, and extraTraffic.
The ApsSwitchCommand textual convention defines the set of APS switch commands: noCmd (read-only sentinel), clear, lockoutOfProtection, forcedSwitchWorkToProtect, forcedSwitchProtectToWork, manualSwitchWorkToProtect, manualSwitchProtectToWork, and exercise. If an equal or higher priority request is already in effect when a switch command is issued, an inconsistentValue error is returned.
The APS-MIB module is registered under the SNMP transmission subtree at OID { transmission 49 }, assigned MODULE-IDENTITY apsMIB. The MIB subtree is divided into apsMIBObjects (1), apsMIBNotifications (2), and apsMIBConformance (3).
Deployment of SNMP versions prior to SNMPv3 is not recommended. Implementers should use SNMPv3 with full cryptographic mechanisms (authentication and privacy). SNMPv3 access controls should restrict GET/SET rights to only those principals with legitimate authority, since older SNMP versions lack per-principal access control even on otherwise secure networks.
Several read-write and read-create MIB objects can disrupt APS operations if set with malicious intent: apsCommandSwitch, apsCommandControl, apsConfigRowStatus, apsConfigMode, apsConfigRevert, apsConfigDirection, apsConfigExtraTraffic, apsConfigSdBerThreshold, apsConfigSfBerThreshold, apsConfigWaitToRestore, apsChanConfigRowStatus, apsChanConfigIfIndex, apsChanConfigPriority, and apsNotificationEnable. SET operations in a non-secure environment without proper protection can negatively affect network operations.
The apsChanStatusCurrent BITS object defines per-channel states: lockedOut (channel prevented from switching to protection, or for null channel, no working channel may switch), sd (Signal Degrade active), sf (Signal Failure active), switched (channel currently on protection line), and wtr (Wait-to-Restore timer active). These states are not mutually exclusive.
The ApsK1K2 textual convention encodes the SONET K1 and K2 APS protocol bytes as a 2-octet OCTET STRING. K1 bits 1–4 encode the request type (e.g., 1111=Lockout of Protection, 1110=Forced Switch, 1000=Manual Switch, 0000=No Request); K1 bits 5–8 encode the associated channel (0=Null, 1–14=working, 15=extra traffic). K2 bits 1–4 encode the channel, bit 5 encodes architecture (0=1+1, 1=1:n), and bits 6–8 encode mode (100=unidirectional, 101=bidirectional, 110=RDI-L, 111=AIS-L).