Ethernet in the First Mile

Frequently Asked Questions


EPON Related Questions

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1. What is a Passive Optical Network?

A Passive Optical Network (PON) is a network in which a plurarility of stations are connected to each other by means of a fiber-optical network that does not contain any active equipment; the different fiber segments to which the stations are attached, are coupled together by means of a passive (not electrically powered) optical coupler or splitter; this is a device that splits optical power from one fiber into multiple fibers and vice versa.

Adapted from: Michael Beck, Ethernet in the First Mile, copyright © 2005 McGraw-Hill Professional.

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2. What is the purpose of the LLID in EPON? What is the equivalent in GPON?

If the OLT (the central node of the EPON) were to be implemented as a PHY with a single MAC attached to it, this would cause serious trouble for any bridge (=switch) to which the MAC were connected. Consider a MAC frame coming into the bridge from an ONU (a subscriber node) via an EPON port. The bridge associates the source address of the frame with the port on which it came in, i.e. the EPON port. When at a later time another MAC frame comes in from an ONU, this time destined for the MAC address previously learned, it will not be transmitted back to the EPON port, because the bridge assumes that the frame was already received by all the stations on the attached “broadcast” LAN. However, this is not the case; upstream transmissions are not received by other ONUs. Standard bridging has no way of accomodating an attached LAN that behaves as a broadcast LAN in downstream and as a point-to-point LAN in upstream.

The solution to this problem was designed in close cooperation with Working Group 802.1. Instead of one single MAC, the OLT would have a different dedicated MAC for every ONU attached to the EPON. As a result, higher layers can consider the EPON as a collection of logical point-to-point links. From the individual MACs down to the OLT PHY, the logical point-to-point links share a common GMII; hence, a way to identify data frames for/from the different ONUs is required. The Logical Link identifier (LLID) was created for this purpose. The LLID is carried along by the frame in the bytes of its preamble.

In GPON, the other main standard for high-speed Passive Optical Networks (ITU-T Recommendation G.984), the different flows are distinguished by means of “port-IDs”. Although GPON does not specify the interconnection with a bridge, the underlying concept is similar to what is described above for EPON. Note however that in a GPON, a single ONU may have several port-IDs associated with it, such that it can keep data flows intended for different user interfaces apart without looking at the destination MAC addresses of the frames.

First two paragraphs adapted from: Michael Beck, Ethernet in the First Mile, copyright © 2005 McGraw-Hill Professional.


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