Research and implementation of LLID-TAG filtering database in EPON

Abstract

Recent years, data traffic is increasing at an unprecedented rate. Digital Subscriber Line (DSL) and cable modem (CM) networks will not afford the huge bandwidth consuming in the near future. Ethernet Passive Optical Network (EPON) is an inexpensive, simple, scalable, and capable of delivering bundled voice, data and video services to an end-user over a single network. EPON, which is a typical point to multi point topology, has become an attractive technology to both vendors and carriers. To ensure compatibility with the IEEE 802 architecture, Logical Link Identifier (LLID) tag is introduced in EPON to achieve point to point emulation. Usually one LLID tag associates with one stream or one ONU (Optical Network Unit) which is remote device of EPON. In order to simplify operations, LLID tag is added as part of preamble in Optical Line Terminal (OLT) Reconciliation Sublayer (RS) and is checked at ONU side. It’s urgent to have a powerful mechanism to achieve dynamic LLID filtering. Just in the downstream, each packet must have its own LLID according to the Destination Address field before being sent; in the upstream, LLID is learned based on the Source Address Field of the packet. Although address filtering is very popular in the L2 switch chips, it can’t be applied to EPON LLID filtering directly, owing to EPON is neither a point to point network nor a broadcast network. In this paper, the main principles and features of Logical Link Identifier (LLID) tagging system are firstly explained, and then the process of LLID both in OLT and in ONU side is described. What’s more, a novel method to realize dynamic LLID filtering is discussed in detail, involving in LLID scanning, LLID learning, LLID aging, and so on. An erasable Content Addressable Memory (CAM) structure is used to meet this demand. Not only the architecture of the concrete circuits, but the related timing sequences, operation process, service arbitration and aging state transition diagram are presented. Note that this mechanism has been verified in the actual EPON system and worked very well. At last, some conclusion remarks about using LLID filtering to enhance the performance of EPON is given.