Quality of Recovery (QoR) of Access Networks Based on PON Piotr Cholda, Andrzej Jajszczyk, Miroslaw Kantor Department of Telecommunications, AGH University of Science and Technology 30-059 Krak´ ow, Poland, Telephone: (+48 12) 634–28–52, Fax: (+48 12) 634–23–72 E-mails:{cholda,jajszczyk,kantor}@kt.agh.edu.pl Abstract— The paper deals with the resources avail- ability, recovery time and redundancy of the fibre-based Passive Optical Networks. It also takes into account ar- chitectures of the power supply system. The availability models for recovery architectures are presented. The overall quality of recovery calculation is performed. On this basis, different schemes are compared and the rules which of them to use are given. I. I NTRODUCTION One of the Next Generation Internet (NGI) objectives is to offer multi-service/multimedia applications with high bandwidth connectivity not only in core networks but also in the access part. Offered services have to as- sure the Quality of Service which is strongly influenced by resilience parameters. A large variety of recovery procedures and architectures leveraging the level of the reliability has been developed in recent years. A today’s client of access networks is not offered a large set of differentiated recovery methods. Although there are some methods of this kind, they are not widely deployed. On the other hand, access networks’ operators lack frameworks which suggest how to compare different recovery architectures and how to select the most appro- priate to particular groups of clients. There are some frameworks proposed for core networks, e.g., [1], but there are almost none existing for access networks. In this paper we try to fill the gap. The organization of the paper is as follows. Section II presents architectures proposed for optical access networks by ITU-T. Topologies introducing fiber fault tolerance as well as different power supply solutions are shortly described. Section III describes the original method of resilience differentiation, called Quality of Recovery (QoR) methodology. Although this methodol- ogy was first proposed for core networks, in this paper it is adapted to cover access networks. Section IV is devoted to the comparison of different optical access network architectures aiming at leveraging the level of the resilience. The comparison is based on the QoR methodology. Finally, Section V contains a numerical example which shows the usability of the proposed methodology. II. OPTICAL ACCESS NETWORKS :ARCHITECTURES AND SERVICES In Passive Optical Network (PON) architectures, an Optical Line Termination module (OLT) is connected with multiple Optical Network Units (ONUs) by a passive Optical Distribution Network (ODN) [2]. To establish such point to multi-point connectivity, a passive optical splitter is used. Generally, the OLT is located in a central office and delivers the interface between the access network and the service node. As the requirements for service bandwidth increase in the broadband access network, it becomes more and more important to protect against access network equip- ment failures. In [3] four types of network protection architectures are introduced. These schemes are shown in Fig. 1. Protection architectures of the PON network aim at enhancing the reliability of access networks. In the type A protection scheme (Fig. 1(a)) only the common optical fibre is protected. Since the switching is only applied to the optical fibres, no switching protocol is required for OLT/ONU in this scheme. In such a case, a fibre switch and PON TC (Transmission Con- vergence) protocols, such as ranging (upstream delay control), are controlled independently. The TC layer performs the re-ranging procedure after failure detection and optical switch recovery. The repeated procedure of determining upstream delay, i.e. re-ranging, is necessary as the distance between OLT and ONUs changed because the spare fibre length is significantly different from the working one. In this case, the signal loss or even cell loss is unavoidable in the switching period. Type B protection shown in Fig. 1(b) duplicates equipment between the OLT and the first splitter. The splitter has