**5. Key functional building blocks for next generation elastic optical network: wavelength conversion, format conversion and multicasting**

The foundation of a photonic network's physical layer is transmission technology in the links and switching technology in the nodes. Different optical multiplexing techniques and modulation formats such as differential quadrature phase shift keying (DQPSK) and quadrature amplitude modulation (QAM) can increase the transmission line rates per fiber to more than 100 Tbit/s [46]. Now research effort are targeted at the Optical Transport Network layer switching where optical processing in the network node allows highly efficient use of capacity, and the abstraction of client data rates from the super-channel data rate. Since some of the networking functions are difficult to carry out electrically, novel processing schemes are required. Alloptical processing techniques remove the need for optical-to-electrical conversion, and electronic processing, resulting in optically transparent networks [47]. **Figure 6** illustrates the key issues relating to the component and subsystem requirements for the main parts of the future all optical network [48]. As discussed earlier, flexibility is the important issue that will drive optical subsystem research and development over the next few years.

One of the challenges being faced by the elastic network is bandwidth assignment and channel numbering for different bit rates. When increasing number of channels, the spectrum gets fragmented as channels are added and removed leaving behind noncontiguous empty slots. When high bandwidth requests arrive there may

## **Figure 6.** *Key issues of future photonics subsystem [27].*

not be sufficient contiguous bandwidth to accommodate them, resulting in blocking [49]. Techniques for spectrum defragmentation involve relocation of existing wavelengths by means of wavelength conversion. Providing such functionality to those channels that require it, and doing it in an efficient manner, is a major challenge. This holds true for other types of functionality such as time multiplexing, format conversion, regeneration, etc. Importantly all-optical modulation format conversion is likely to be used for future all-optical networks in order to add the optical network flexibility [50, 51]. In these networks, systems deployed in different regions could have different bit rates and modulation formats depending on the network size. Therefore, a critical requirement will be the transparent interconnection of these different network islands, which should take place by all-optical means at the network gateways [52].

Transparent optical multicast by multiwavelength conversion has revealed a brand-new way for performing data multicast function directly in the optical domain without passing through any electronics. It provides new visions of optical network designs in terms of optical network switching and forwarding efficiency, transparency, and effectiveness [53]. One-to-many or multichannel wavelength converters are very attractive because they could potentially reduce the number of converters in a routing node without adding more complexity in the switch design. Applications for optical multicast include teleconferencing, video distribution, multiparty gaming and global enterprise virtual private networks (VPNs), etc. [54].
