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Enabling Technology for EONs

Advances in optical transmission techniques and devices have favored the emergence of EONs [32,42-44]. The introduction of advanced modulation formats and wavelength cross-connects (WXCs) enable carrying the growing traffic volume over long-haul distances without optical-electrical-optical (OEO) conversion [45]. The paths with bandwidths determined by the volume of client traffic are allocated through rate-flexible transponders from the transmitter and sent through bandwidth-variable (BV) wavelength cross-connects (WXCs) to the receiver [42].

Spectrally Efficient Superchannel

There are two common schemes used in rate-flexible transponders to achieve a spectrum efficient modulation for super-channel transmitter [46,47]. The first scheme is based on OFDM. A frequency-locked multicarrier generator is utilized to generate equally spaced subcarriers. The generated subcarriers are first separated by a wavelength-division demultiplexer (DMUX), then individually modulated with parallel modulators, and finally coupled to generate a spectrally overlapped superchannel. The second scheme is based on WDM of subchannels which have an almost rectangular spectrum with a bandwidth close to the Nyquist limit for intersymbol-interference-free transmission [48]. The subchannels are aligned with the frequency spacing close to the baud rate, which is the Nyquist limit, while avoiding inter-subchannel spectral overlap. This scheme is referred to as Nyquist-WDM [46,49]. In Nyquist WDM [49], subcarriers are spectrally shaped in order to occupy finer granularity, which corresponds to the baud-rate. These narrow subcarriers are multiplexed at the transmitter with spacing close or equal to the baud-rate, with limited interference and form a super wavelength.

Optical Transponders

Three models of transponder can be found [50], namely mixed-line-rate (MLR) model, multi-flow (MF) model, and bandwidth-variable (BV) model. The MLR model employs a few types of transponders, each with a different bit rate, for example 40, 100 and 400 Gb/s transponders to suit a wide range of traffic demands. The MF model uses a MF transponder with several sub-transceivers, that can then be allocated to different demands, each of which has a fixed bit- rate capacity. The BV model supports all types of traffic demands with a single BV transponder, which assigns the fewest possible spectral resources to support traffic demands with a 400 Gb/s maximum bit-rate. As shown in the study presented in [50], the BV model offers better spectrum efficiency and the lowest port consumption rate. Also it is more suited for an energy reduction purpose owing to the reduced active resource due to the use of sub-transceivers. A next generation sliceable bandwidth-variable optical transponder (SBVT) has been investigated in [51]. The authors provided the design architecture and considered several transmission techniques to build the transponder.

Exercises

  • 1. What is the relation between bit rate and baud rate?
  • 2. What are the rationales behind EONs?
  • 3. How does offer OFDM technology better resource utilization than WDM technology?
  • 4. How are sub and super wavelength channels constructed in EONs?
  • 5. What is the relationship between OFDM technology and distance-adaptive modulation?
  • 6. What are the main technical issues of OFDM?
  • 7. Discuss unique properties of EONs over WDM based optical networks.
  • 8. What are the roles of bandwidth-variable optical transponder to design EONs?
  • 9. Consider the C band (1530-1565 nm). Calculate the total number of spectrum slots that can be used, when the frequency spacing is considered 12.5 GHz.
  • 10. Consider a network mentioned in Fig. 2.6. Estimate the number of required slots for each lightpath request, which are AB, AC AD, AE, AF, BC. BD, BE, BF, CD, EC, FC, ED, FD, and FE, under the following conditions.
Network topology

Figure 2.6: Network topology.

i The routing of each lightpath requests is considered according to the shortest path routing.

ii The bit rate requirement of each lightpath is 100 Gbps.

iii Spectrum slot granularity is 12.5 Gbps.

iv BPSK, QPSK, 8QAM, and 16QAM are used for the transmission reach of > 4500 km, 3500 to 4500 km, 1500 to < 3500 km, and < 1500 km, respectively.

v m = 1, m = 2, m = 3, and in = 4 are considered for BPSK, QPSK, 8QAM, and 16QAM, respectively; m represents the modulation level.

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