To Overcome the Scalability Limitation of Passive Optical Interconnects in Datacentres

Abstract

Abstract:We propose to add optical amplifier(s) to passive optical interconnect (POI) at top-of-rack in datacentres and validate this approach by introducing impairment constraints into POIs design. It is shown that one amplifier can improve scalability by a factor of 16. IntroductionThe volume of datacentre traffic is ever-growing, causing a serious bottleneck in terms of bandwidth and energy consumption. To address this problem, optical communication and switching techniques are introduced in datacentres. However, commodity switches are still widely used at the top-of-rack (ToR). They consume a lot of energy and also limit capacity upgrading. Therefore, a coupler-based passive optical interconnect (POI) has been proposed [1-3] to replace the commodity switch at ToR, leading to an overall reduction of energy consumption of switching equipment in datacentres by a factor of 10. The gain in energy consumption increases when a higher data rate on a per-server basis is considered [2]. Apart from the advantages of capacity and energy consumption, the coupler-based ToR interconnect can also offer high reliability and cost efficiency [4] thanks to its passive manner. However, the design of existing couplerbased POI architectures [1-4] does not consider the physical layer impairments which may significantly affect the quality of the signals. If the received signal power fails to reach the receiver sensitivity threshold, the transmitted data cannot be detected. As a result, the reserved resources are wasted. Therefore, it is of key importance to take the physical layer impairments into account and analyze their impact on the feasibility of the POI architectures. In this paper, we focus on the intra-rack communications. A thorough analysis of physical layer impairments is carried out considering the coupler-based POI at ToR in order to quantify its scalability. It has been shown that with introduction of an optical amplifier to increase the optical power budget, the interconnect size can be increased by a factor of more than 10, making it possible to meet the scalability requirement at ToR [5]. Passive optical interconnect architectureThe authors of [2,4] proposed a POI architecture based on an Nx2 coupler, as shown in Fig. 1(a). It takes advantage of the high capacity of the dense wavelength division multiplexing (DWDM) technique and the broadcast-and-select nature of optical couplers to handle the bursty and multicast traffic in datacentres. In this approach, the optical network interfaces (ONIs) are dedicated to each server and connected to the side of the coupler with N ports. The transmitted signals pass through the coupler and are selected by a wavelength selective switch (WSS) according to whether the destination is inside or outside the rack. For the intra-rack traffic (see the red arrow in Fig. 1), the signals are looped back via an isolator (ISO) to the coupler and broadcast to all the servers. The optical tunable filter (OTF) at the receiver selects the wavelength of interests and the signal is detected by a p...

Scalability analysis methodology for passive optical interconnects in data center networks using PAM

“…The upper bound of the scalability of such POI architectures can be obtained through the comparison between the link loss and system power budget which can be obtained by theoretical analysis of the transmission performance [36], [37], simulation [38], and experiment works [39].…”