Page 722 - Introduction to Information Optics

12. Networking with Optics

Gateway Node: 3R Q Optical Cross Connect
I Optical Amplifier

Fig, 12.22(c). Tomorrow's ultra long-haul backbone network.

combine more than 100 wavelengths carrying life traffic into a single piece of
fiber. EDFA, which is a much more cost-effective solution, has substantially
reduced the use of OEO devices in today's networks. Nevertheless, the cost of
such OEO regenerators is still the largest portion of overall network cost.
Because of the economic factor, it is generally agreed that tomorrow's terres-
trial backbone network should be an ultra-long-distance (over thousands of
kilometers) one between OEO regenerators. Ideally, we should see a network
similar to Fig. 12.22(c) without OEO regenerators from coast to coast.

12.4.1.2. Enabling Technologies for Ultra-Long-Haul Transmission
In this section several key enabling technologies, including forward error
correction, Raman amplification, dynamic gain equalization filters, second-
order dispersion compensation, and soliton transmission are briefly discussed.

12.4.1.2.1. Forward Error Correction (FEC)

We have seen previously that in order to achieve required system BER, a
minimum OSNR has to be guaranteed. FEC is one of the coding technologies
that can improve system BER without additional requirements on OSNR
improvement. Figure 12.23 demonstrates how FEC is implemented. The
encoder introduces extra bits to the input data on the transmitter site. On the
receiver site, the decoder uses the extra bits to restore the input data sequence
when the data is corrupted by the various impairments associated with the
transmission link. As a result, FEC enables the system to achieve the same
BER performance with less channel OSNR compared to a system without