Need + Promise = Challenge!

Life in our increasingly information-dependent society requires that we have access to information at our finger tips when we need it, where we need it, and in whatever format we need it. The information is provided to us through our global mesh of communication networks, whose current implementations -- such as the present-day Internet and asynchronous transfer mode (ATM) networks -- unfortunately, don't have the capacity to support the foreseeable bandwidth demands.

Enter fiber optic technology which can be considered our ``saviour'' for meeting our above need because of its potentially limitless capabilities []:

1. huge bandwidth (nearly 50 terabits per second (Tbps),
2. low signal attenuation  (as low as 0.2 dB/km),
3. low signal distortion,
4. low power requirement,
5. low material usage,
6. small space requirement, and
7. low cost.

Thus, the basic premise of this book's topic -- viz., optical communication networks -- is that, as more and more users start to use our data networks, and as their usage patterns evolve to include more and more bandwidth-intensive networking applications such as data browsing on the world wide web (WWW), java applications, video conferencing, etc., there emerges an acute need for very high-bandwidth transport network facilities, whose capabilities are much beyond those that current high-speed (ATM) networks can provide. Another term that we are increasingly hearing today from anyone who uses the Internet is ``network lag," or ``net lag" for short. That is, the network is taking longer to accomplish a task today, e.g., to access a WWW server and display a picture, relative to how long it took to perform the same task a few days back. There is just not enough bandwidth in our networks today to support the exponential growth in user traffic!

Given that a single-mode fiber's potential bandwidth_optical bandwidth is nearly 50 Tbps, which is nearly four orders of magnitude higher than electronic data rates of a few gigabits per second (Gbps), every effort should be made to tap into this huge opto-electronic bandwidth mismatch.

Realizing that the maximum rate at which an end-user -- which can be a workstation or a gateway that interfaces with lower-speed subnetworks -- can access the network is limited by electronic speed (to a few Gbps), the key in designing optical communication networks in order to exploit the fiber's huge bandwidth is to introduce concurrency among multiple user transmissions into the network architectures and protocols. In an optical communication network, this concurrency may be provided according to either wavelength or frequency [wavelength-division multiplexing (WDM)], time slots [time-division multiplexing (TDM)], or wave shape [spread spectrum, code-division multiplexing (CDM)].

Optical TDM and CDM are somewhat futuristic technologies today. Under (optical) TDM, each end-user should be able to synchronize to within one time slot. The optical TDM bit rate_optical TDM!bit rate is the aggregate rate over all TDM channels in the system, while the optical CDM chip rate_optical CDM!chip rate may be much higher than each user's data rate. As a result, both the TDM bit rate and the CDM chip rate may be much higher than electronic processing speed, i.e., some part of an end user's network interface must operate at a rate higher than electronic speed. Thus, TDM and CDM are relatively less attractive than WDM, since WDM -- unlike TDM or CDM -- has no such requirement.

Specifically, WDM is the current favorite multiplexing technology for optical communication networks since all of the end-user equipment needs to operate only at the bit rate of a WDM channel, which can be chosen arbitrarily, e.g., peak electronic processing speed. Hence, most of this book's material (Chapters 1 through 15) will concentrate on WDM networks; but Chapter 16 will be devoted to optical TDM and CDM.