1 Optical Receivers with Multiple Front-Ends for Low Latency Optical Slot Switching Rings Bogdan Uscumlic * , Yvan Pointurier * , Annie Gravey † , Philippe Gravey † , and Michel Morvan † * Alcatel-Lucent Bell Labs, Nozay, France (email: firstname.lastname@alcatel-lucent.com) † Institut Mines T´ el´ ecom, T´ el´ ecom Bretagne, Brest, France (email: firstname.lastname@telecom-bretagne.eu) Abstract—We propose a new receiver architecture for coherent detection in slotted optical packet switching rings with elastic (rate adaptive) optical transponders. Such rings are a candidate solution for future datacenter and metropolitan networks. The new receiver can detect more than a single packet per time slot and consequently has higher flexibility (translating into higher supported capacity, or, equivalently, lower end-to-end latency, or a combination or both), at the cost of a moderate increase in the transponder complexity and energy consumption (less than 10%). We apply network planning and traffic engineering simulation tools (which we validate on small examples using theoretical models) to quantify the increase in network capacity and latency reduction that can be achieved thanks to the use of the new receivers. Index Terms—Optical packet switching, metropolitan rings, coherent detection, computer simulation, network planning, per- formance evaluation. I. Introduction Optical packet switching leverages high bandwidth effi- ciency, network flexibility and low energy consumption for future metropolitan and inter- and intra-datacenter intercon- nection networks (for a survey on optical packet switching see, e.g. [1]). In this paper, we focus on a particular technology employing optical packet switching that is called Optical Slot Switching (OSS) rings where packet duration is fixed [2]. These rings were widely studied, from the point of view of the optical transmission technology, e.g. see [3], [4], and from the point of view of achievable network performance [1], [5], [6]. Many works have shown the advantages of OSS rings over the optical circuit switching technologies. In particular, in [7] it is shown that optical traffic grooming allows to minimize the number of wavelengths used in the ring and to benefit from the capacity increase thanks to statistical multiplexing. It is also shown that the gain in terms of insertion latency is much more important than the penalty due to the extraction latency, when the non-coherent fixed receivers are employed. In this paper, we propose an enhancement of the coherent receiver used in OSS rings, in order to increase network capacity and decrease network latency. More precisely, we propose to duplicate the hardware parts that allow the re- ception of several optical slots simultaneously. Hence, the proposed receiver has a greater (optical) receiving capacity but the same (electrical) client-side capacity compared with a standard receiver. The new hardware in the proposed receiver increases the transponder power consumption only minimally (less than 10%). Compared with the standard receiver, the proposed receiver significantly improves the network capacity and the insertion latency of its nodes, thanks to the receiver’s capability to extract several slots from the network at the same time. This paper quantifies the gain in network capacity and latency that are made possible by this transponder. Although the proposed receiver introduces the additional latency at slot extraction, the end-to-end latency savings due to the increased flexibility of the new receivers largely offset the extraction queueing delay, as will be seen further. The remainder of the paper is organized as follows. In the Section II, we describe the architecture of an OSS ring, focusing first on standard transponders (Section II-A) and then on our proposed transponder (Section II-B). We validate the simulator tool in Section III. Detailed simulation study is presented in Section IV. Finally, the concluding remarks are given in the last section. II. Optical Slot Switching (OSS) Ring The physical topology of an OSS network is a ring consist- ing of two parallel fibers, used to connect the network nodes. The two fibers are usually used in counter-rotating directions, in a time slotted manner. One of the fibers can be used to carry backup traffic for fast failure recovery. In this paper, for the sake of simplicity, we consider unidirectional OSS rings. Optical packets are used for the transport of encapsulated client data and are sent on different wavelength channels of wavelength division multiplexing (WDM) signal comb. A dedicated, possibly low-datarate (e.g., 10 Gb/s) wavelength control channel carries the headers (including slot source and destination information) of all synchronous slots on a separate wavelength, which is extracted, processed and re- inserted at every node. Each node consists of an electronic aggregation layer that encapsulates client frames into fixed- duration optical slots, one or several transponders (TRX) that effectively receive and transmit slots from/on the fiber medium, the aforementioned TRX for the control channel, and a “slot blocker” that can erase the received optical slots in order to make room for new inserted slots (see [3] for a possible implementation of the slot blocker). The TRXs are able to change the symbol rate and consequently their transmission rate. The transmitting side of a transponder is noted with TX and the receiving side with RX. The supported datarates of the transponders are ≥100 Gbit/s.