Managed Objects of Ethernet Passive Optical Networks (EPON)
Extracted elements (28)
FEC (RS(255,239,8)) is placed as a separate layer between PCS and PMA — not within the MAC — to enhance optical link budget without changing the MAC. This enables greater split ratios and longer reach. The standard notes this code is similar to ITU-T G.975. The FEC layer is optional.
MPCP timestamps are OLT-set and network-synchronized to enable TDM scheduling of uplink transmissions. RTT is measured per ONU and subtracted from GATE grant timing to compensate for propagation delay, which can reach 200 usec (25 Kbytes at gigabit rate) for a 20 km fiber run.
Point-to-Point Emulation (P2PE) partitions the shared downstream broadcast medium into per-ONU virtual private paths using LLID tags. This solves both subscriber privacy (dedicated traffic is not shared) and IEEE 802.1D bridge rule compliance (peer-to-peer ONU traffic would otherwise have the same source and destination port, requiring discard).
The broadcast virtual EPON interface (LLID=0xffff) is added at the OLT to allow single-copy broadcast transmission to all ONUs, preserving the inherent bandwidth efficiency of the shared PON medium. This interface has no corresponding bridge port mapping.
The MIB assigns a separate ifIndex to each OLT virtual link (LLID) rather than modeling the P2MP port as a single interface. The justification is that virtual links are sufficiently distinct — separate per-link error counters, different RTTs, different remote MAC addresses — making per-LLID management more useful and consistent with how IF-MIB, MAU-MIB, and Ether-like MIB already model them.
Each EPON physical interface and each OLT virtual LLID interface MUST have corresponding rows in both the IF-MIB (RFC2863) and Ethernet-like Interfaces MIB (RFC3635) tables. All OLT virtual interfaces share the same ifPhysAddress (the OLT physical MAC address) because there is only one physical MAC.
OLT virtual interfaces are stacked onto the physical OLT interface via the IF-MIB ifStackTable and ifInvStackTable (ifStackHigherLayer=virtual_ifIndex, ifStackLowerLayer=physical_ifIndex). The broadcast virtual interface is also stacked onto the physical interface.
Section 5 provides an explicit mapping table between DOT3-EPON-MIB objects and IEEE 802.3ah clause 30 managed attribute identifiers (e.g., dot3MpcpRegistrationState ↔ aMPCPRegistrationState at 30.3.5.1.6), enabling implementors to verify compliance with the IEEE standard.
When the EFM OAM MIB (RFC4878) is co-implemented, each OAM instance is indexed by the LLID virtual link's ifIndex. This is non-obvious because OAM is defined for point-to-point links, but EPON's P2PE abstraction makes each virtual link appear as a distinct point-to-point interface.
Bridge MIB modules (RFC4188 and RFC1525) MAY be implemented when the EPON MIB is implemented for an OLT, with dot1dBasePortIfIndex values corresponding to LLID-based virtual interface ifIndex values.
Implementing the DOT3-EPON-MIB module MUST require implementation of the MAU-MIB module (RFC4836), which defines EPON-specific MAU types for the EPON physical interface.
The EFM OAM MIB module (RFC4878) MAY be implemented alongside DOT3-EPON-MIB, with OAM instances indexed by virtual link ifIndex (one per LLID) since OAM is defined for point-to-point links.
The dot3EponFecTable provides per-virtual-link FEC management objects: dot3EponFecAbility, dot3EponFecMode, dot3EponFecCorrectedBlocks, dot3EponFecUncorrectableBlocks, and dot3EponFecPCSCodingViolation.
The dot3MpcpAdminState object (read-write TruthValue, default false) enables or disables the Multi-Point MAC Control sublayer. Changing this value can interrupt service for all users connected to the respective EPON interface.
The dot3MpcpControlTable provides per-virtual-link MPCP control and status objects indexed by ifIndex. Key objects include dot3MpcpMode (olt/onu), dot3MpcpRegistrationState, dot3MpcpLinkID, dot3MpcpRemoteMACAddress, dot3MpcpRoundTripTime, and dot3MpcpAdminState.
The dot3MpcpRegistrationState object reflects three states: unregistered(1), registering(2), and registered(3). At the OLT, each virtual interface has a distinct value; rows for virtual link ifIndexes are created on ONU registration and deleted on ONU deregistration.
The dot3MpcpRoundTripTime object reports per-link MPCP RTT in 16ns Time Quanta units (Unsigned32, range 0..0xffff TQ). The OLT holds a distinct RTT per virtual interface reflecting the physical distance of each ONU; the standard allows up to 20 km (approximately 200 usec RTT).
The dot3MpcpStatTable provides per-virtual-link MPCP statistics counters indexed by ifIndex, including transmitted/received counts for GATE, Register, RegisterRequest, RegisterACK, and REPORT message types.
The dot3OmpEmulationTable and dot3OmpEmulationStatTable provide per-virtual-link P2P Emulation management objects, including SLD errors, CRC8 errors, and LLID counters (GoodLLID, BadLLID, ONU/OLT PONcast LLID).
The extended package group includes dot3ExtPkgControlTable (per-link EPON control), dot3ExtPkgQueueTable (per-link, per-queue ONU queue status for MPCP REPORT), dot3ExtPkgQueueSetsTable (per-link, per-queue, per-queue-set), and dot3ExtPkgOptIfTable (optical interface control).
The DOT3-EPON-MIB module is assigned OID { mib-2 155 } by IANA. This is the primary IANA registration created by this document.
Several read-write objects in the DOT3-EPON-MIB (notably dot3MpcpAdminState) can disrupt EPON service if set by unauthorized parties. SNMP security mechanisms (SNMPv3 with authentication and privacy) should be used to restrict write access to authorized managers only.
The downstream EPON link is a physical broadcast medium; all ONUs receive all OLT transmissions. The LLID-based P2PE filtering provides logical separation but not cryptographic confidentiality — a compromised ONU could observe traffic intended for other ONUs.
The MPCP auto-discovery message sequence is: OLT sends Discovery GATE → ONU sends Register Request → OLT sends Register (assigning LLID) → OLT sends GATE → ONU sends Register ACK. Higher-layer protocols may additionally authenticate the ONU before it participates in the network.
The MPCP ONU registration state machine has three states: unregistered → registering (on receipt of Discovery GATE) → registered (after REGISTER message assigns LLID and REGISTER_ACK is exchanged). A random backoff mechanism handles collision among multiple ONUs attempting registration simultaneously.
MPCP messages are MAC Control frames using Ethertype 0x8808 and are not forwarded beyond the MAC. Each MPCP message carries a timestamp (set by the OLT, synchronized network-wide) used for TDM scheduling and RTT measurement.
The FEC-protected Ethernet frame format is: S_FEC delimiter | Preamble/SFD | Ethernet Frame | FCS | T_FEC delimiter | Parity bytes | T_FEC delimiter. The RS(255,239,8) code is used, improving BER from 1E-4 to 1E-12. FEC is optional and adds a fixed delay in both transmit and receive paths.
The LLID field is a 2-byte value (15-bit link ID plus one broadcast bit) embedded in the Ethernet preamble, marking destination port downstream and source port upstream. The broadcast LLID has value 0xffff, limiting virtual links per PON to 32768.