120 International Journal on Advances in Telecommunications, vol 5 no 3 & 4, year 2012, http://www.iariajournals.org/telecommunications/ 2012, © Copyright by authors, Published under agreement with IARIA - www.iaria.org Blocking Performance of Multi-rate OCDMA PONs with QoS Guarantee John S. Vardakas * , Ioannis D. Moscholios † , Michael D. Logothetis * , and Vassilios G. Stylianakis * * WCL, Dept. of Electrical and Computer Engineering University of Patras, Patras, 265 04, Greece, Emails: {jvardakas, m-logo, stylian}@wcl.ee.upatras.gr † Dept. of Telecommunications Science and Technology University of Peloponnese, 221 00, Tripolis, Greece, Email: idm@uop.gr Abstract—In this paper, we propose a new teletraffic model for the calculation of blocking probabilities in an Optical Code Division Multiple Access (OCDMA) Passive Optical Network (PON) supporting multiple service-classes of Poisson traffic. OCDMA is a promising candidate of PON configuration for the provision of moderate security communications with large dedicated bandwidth to each end user. The PON accommodates multiple service-classes that are differentiated by either different data-rates or different Quality of Service levels. Parameters related to the additive noise, multiple access interference and user activity are incorporated into our analysis. Based on a two- dimensional Markov chain, we propose a recursive formula for the calculation of the number of in-service codewords, when the OCDMA PON guarantees Quality of Service (QoS), or not. To evaluate the proposed model, the analytical results are compared with simulation results to reveal that the model’s accuracy is quite satisfactory. Keywords-Passive Optical Network; Optical Code Division Mul- tiple Access; Multiple Access Interference; Blocking Probability; Quality of Service; Parallel Mapping. I. I NTRODUCTION The exponential growth of the Internet traffic volume and popularity of broadband applications have accelerated the demand for higher data rates. In backbone networks the capacities have been significantly increased, mainly due to the utilization of the Wavelength Division Multiplexing (WDM) with hundreds of channels in each optical fiber. On the other hand, current solutions in the access domain, such as the Digital Subscriber Line (DSL), are inadequate to deal with the growing bandwidth-hungry applications. To break the bottleneck between the access networks and the ultrahigh- speed backbone networks, high capacity and cost effective access solutions are required. The Passive Optical Network (PON) has received a tremendous attention from both aca- demic [1] and industrial [2] communities, mainly due to the low operational cost, the enormous bandwidth offering and the absence of active components between the central office and the customer’s premises. Over the years, several standards for PONs have been evolved, in the form of the G.983 ITU-T recommendations, which include Asynchronous Transfer Mode PONs (ATM- PONs) and Broadband PONs (BPONs) [3], or in the form of IEEE 802.3ah for the Ethernet PON (EPON) [4], etc. These PONs are based on a Time Division Multiple Access (TDMA) scheme and they typically use a 1550 nm wavelength for downstream and a 1310 nm for upstream [5]. While these TDMA-PONs employ two wavelengths for the upstream and downstream direction, respectively, the WDM-PON utilizes multiple wavelengths, so that two wavelengths are allocated to each user for down/upstream transmissions. A different approach for the provision of multiple access in PONs is the Optical Code Division Multiple Access (OCDMA). In contrast to the other multiple access schemes, OCDMA can multiplex a number of channels on the same wavelength and on the same time-slot [6]. In addition, OCDMA offers full asynchronous transmission, soft capacity on demand, low latency access, simple network control and better security against unauthorized access [7]. In OCDMA, each communication channel is distinguished by a specific optical code. At the receiver each data is multi- plied by a unique code sequence either in the time domain [8], or in the wavelength domain [9], or in a combination of both (simultaneously) [10]. The decoder receives the sum of all en- coded signals from different transmitters and recovers the data from a specific encoder, by using the same optical code. All the remaining signals appear as noise to the specific receiver; this noise is known as multiple access interference (MAI) and is the key degrading factor of the network’s performance. Apart from MAI, other forms of additive noise deteriorate the network performance, such as beat noise, shot noise, thermal noise and fiber-link noise, and worth considering them in performance analysis [11]. Service differentiation in OCDMA networks can be per- formed by considering either different data-rates or different QoS levels for the supported service-classes. For the provi- sion of data-rate differentiation several solutions have been investigated. A simple approach is based on the utilization of multi-length codes [12]; however, under multi-length coding, short-length codes introduce significant interference over long- length codes, while high error probability emerges for high rate users. Optical fast-frequency hopping has been also proposed for multi-rate OCDMA networks [13]. This technique is based on multiple wavelengths, which requires multi-wavelength