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: Network and Node Architecture for Elastic Optical Networks

To fulfill our ever-increasing bandwidth demands, the elastic optical network (EON) is indispensable. The performance of the EON depends on its network and node architectures. This chapter presents the architecture of the EON and its operation principle. To complete the discussion of network architecture, this chapter focuses on the different node architecture, and compares their performance in terms of scalability and flexibility.

Elastic Optical Network Architecture

Figure 3.1 shows the typical architecture of the EON, which mainly consists of Bandwidth-variable transponders (BVTs) and bandwidth-variable crossconnects (BV-WXCs). These basic components and their working principles are explained in the following subsections.

Bandwidth-Variable Transponder

BVTs [32,52-55] are used to tune the bandwidth by adjusting the transmission bit rate or modulation format. BVTs support high-speed transmission using spectrally efficient modulation formats, e.g., 16-quadrature amplitude modulation (QAM), with 64-QAM used for shorter distance lightpaths. Longer distance lightpaths are supported by using more robust but less efficient modulation formats, e.g., quadrature phase-shift keying (QPSK) or binary phase-shift keying

Architecture of elastic optical network

Figure 3.1: Architecture of elastic optical network.

(BPSK). Therefore, BVTs are able to trade spectral efficiency off against transmission reach.

However, when a high-speed BVT is operated at lower than its maximum rate due to required reach or impairments in the optical path, part of the BVT capacity is wasted. In order to address this issue, sliceable bandwidth-variable transponders (SBVTs) [46,56-59] have been presented that offer improved flexibility; they are seen as promising transponder technology. An SBVT has the capability to allocate its capacity into one or several optical flows that are transmitted to one or several destinations. Therefore, when an SBVT is used to generate a low bit rate channel, its idle capacity can be exploited for transmitting other independent data flows. An SBVT generates multiple optical flows that can be flexibly associated with the traffic coming from the upper layers according to traffic requirements. Therefore, optical flows can be aggregated or can be sliced based on the traffic needs. Figure 3.2 distinguishes BVT and SBVT functionalities.

The SBVT architecture [46, 56] was introduced in order to support slice- ability, multiple bit rates, multiple modulation formats, and adaptive code rates. Figure 3.3 shows the architecture of an SBVT; it mainly consists of a source of N equally spaced subcarriers, a module for electronic processing, an electronic switch, a set of N photonic integrated circuits (PICs), and an optical multiplexer. In this architecture, the N subcarriers are generated by a single multi wavelength source. However, such a source may be replaced by N lasers, one per subcarrier. Each client is processed in the electronic domain (e.g., for filtering) and then is routed by the switching matrix to a specific PIC. The generated carriers are equally spaced according to the spectral requirements and the transmission technique adopted. Generated subcarriers are selected at the multi wavelength source, and they are routed to the appropriate PICs. Each PIC is utilized as a singlecarrier transponder that generates different modulated signals, such as 16-QAM

(a) Functionalities of (a) BYT. and (b) SBVT

Figure 3.2: (a) Functionalities of (a) BYT. and (b) SBVT.

and QPSK, in order to support multiple modulation formats. Finally, subcarriers are aggregated by the optical multiplexer in order to form a super channel. Sometimes, subcarriers may be sliced and directed to specific output ports according to the traffic needs. A detailed description of PIC generation of different modulated signals is given in [56].

Bandwidth-Variable Cross-Connect

The BV-WXC [28, 60, 61] is used to allocate an appropriate-sized cross- connection with the corresponding spectrum bandwidth to support an elastic optical lightpath. Therefore, a BV-WXC needs to configure its switching window in a flexible manner according to the spectral width of the incoming optical signal.

Figure 3.4 shows an implementation example of a BV-WXC, where bandwidth-variable spectrum selective switches (BV-SSSs) in the broadcast-and- select configuration are used to provide add-drop functionality for local signals as well as a groomed signal, and routing functionality for transit signals. Typically, a BV-SSS performs wavelength demultiplexing/multiplexing and optical switching functions using integrated spatial optics. The light from an input fiber is di-

Architecture of SBVT

Figure 3.3: Architecture of SBVT.

vided into its constituent spectral components using a dispersive element. The spatially-separated constituent spectra are focused on a one-dimensional mirror array and redirected to the desired output fiber. Liquid crystal on Silicon (LCoS) or Micro-Electro Mechanical System (MEMS)-based BV-SSSs can be employed as switching elements to realize an optical cross-connect with flexible bandwidth and center frequency. As the LCoS is deployed according to phased array beam steering, which utilizes a large number of pixels, LCoS-based BV-SSSs can easily provide variable optical bandwidth functionality. A detailed description of a BV-WSS employing LCoS technology can be found in [9,62]. Similarly, details of an MEMS-based BV-SSS can be found in [9,63].

 
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