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A new media access control protocol guaranteeing fairness among users in Ethernet-based pas

A New Media Access Control Protocol Guaranteeing Fairness Among Users in Ethernet-based Passive Optical

Networks

Fu-Tai An, Hopil Bae, Yu-Li Hsueh, Kyeong Soo Kim, Matthew S. Rogge, and Leonid G. Kazovsky Optical Communication Research Laboratory, Stanford University (http://www.wendangku.net/doc/71791ea10029bd64783e2cab.html)

Phone: 1.650.725.1110; e-mail: ftan@http://www.wendangku.net/doc/71791ea10029bd64783e2cab.html

Abstract: We propose a new EPON MAC protocol guaranteeing fairness among users by

allocating excess bandwidth proportional to their subscription rates. The novelty of the protocol is

in the use of scalable per-subscription-rate-queuing with round-robin scheduling and packet

reclassification at ONU.

?2000 Optical Society of America

OCIS codes: (060.4250) Networks

1. Introduction

Low cost and minimal protocol overhead make Ethernet-based Passive Optical Networks (EPONs) a promising candidate for next-generation broadband access networks. In implementing EPON systems, one of key challenges is designing Media Access Control (MAC) protocol to achieve network efficiency and meet various Quality of Service (QoS) requirements.

For efficient use of upstream bandwidth, EPON MAC protocol resorts to Dynamic Bandwidth Allocation (DBA) scheme [1]. The current DBA specification, based on per-priority queuing with reporting of queue lengths at Optical Network Units (ONUs) via GATE and REPORT messages, can differentiate service priorities among different types of traffic, but does not guarantee fairness among multiple users served by the same ONU [2]. In this case the bandwidth allocation to lower-priority traffic, especially for Best Effort (BE) traffic without any connection parameters, simply depends on the queue length reported. Hence, malicious users are able to gain more bandwidth than others by over-flooding the priority queues, which cause severe unfairness among those users served by the same ONT.

In this paper we propose and evaluate the performance of a new MAC protocol that solves the fairness issue in EPON. The proposed MAC protocol divides bandwidth for lower-priority traffic among users proportional to their subscription rates, preventing over-flooding from malicious users from affecting the performance of other users. 2. Proposed MAC Protocol and ONU Architecture

The current EPON DBA scheme is based on per-priority-queueing at ONU (up to 8 priority queues), and each queue contains packets from all the users served by the ONU. When the ONU sends a REPORT message for grant requests to the OLT, queue length is the only information that is conveyed. Therefore, the OLT has no idea about statistics of each user’s traffic. It grants the ONU a slot size that is proportional to the ONU’s queue length only. Hence, users of an ONU could gain more bandwidth by over-populating the priority queues.

To provide better fairness among multiple users at an ONU, in the proposed MAC protocol we assume that the OLT knows each user’s subscription rate. The OLT grants the associated bandwidth to ONUs, and ONUs then distribute the bandwidth to each user according to the subscription rate as follows:

1. When all the users in the network send traffic at rates equal to or more than their subscription rates and there is

no surplus bandwidth available, the OLT distributes bandwidth according to users’ subscription rates.

2. When actual traffic rates from some users are less than their subscription rates, any surplus bandwidth is

allocated to all other users proportional to their subscription rates.

3. After steps 1 & 2, if the OLT still finds any surplus bandwidth available, not used by some users, it allocates the

unused bandwidth again to users (as in step 2) who have packets to send in the queues.

The OLT aggregates the bandwidth belonging to users of an ONU and sends a grant reply that indicates the slot size for the ONU. The ONU divides the slot according to subscription rates and queue length for each user. For users of