Nowadays most of us have a fast internet connection, either at home or in the office. Lot of data is exchanged over the web with just a mouse click: mails, files, faxes, web pages, videos, chats, phone calls, and so on. Due to this fact, during the last few years the carriers had to face with a big problem: the increasing demand for high bandwidth services was leading to fiber exhaust. Many service providers neared one hundred per cent of capacity utilization on significant portions of their fiber-optics networks. A bandwidth of 1 Gbps was no longer enough to support all the customers’ requests. Just to give you an idea, with a capacity of 1 Gbps it is possible to transmit one thousand books in just one second.

Basically there were two main ways to solve the fiber exhaust issue. The first, and probably the more obvious, was to increase the available bandwidth by laying more fiber. For some kind of networks that could also be the most economical solution, but in any case it did not allow the carriers to provide new services, or experience the potentially higher bandwidth available with a fiber optic connection. The second way, started at the beginning of the 1980s, was focused on increasing the speed (actually the bit rate) with which data travel on the fiber. That solution was based on the time division multiplexing (TDM) technique. TDM slices time into small intervals, assigns each of them to different data sources, and then multiplexes them on the same fiber before transmission. The following figure gives you an idea of what TDM does: four signals with a 2.5 Gbps bit rate are multiplexed and transmitted on the same fiber which will achieve a 2.5x4=10 Gbps bit rate. The inverse operation will have to be done at the receiving node, where a de-multiplexer function extracts the four original signals with their respective bit rate.

The way data frames are transmitted with TDM technique is similar to the traffic flowing on a highway with just one lane. The following figures show how the traffic rate might be increased by filling the empty space between cars (which corresponds to unused time in TDM) inserting additional vehicles (in this case the traffic rate has been multiplied by four).

TDM has been a winning choice and it is still used today especially in voice transmission applications. Its fortune is due to the fact that most adopted high-speed transmission standards (mainly SONET and SDH) are synchronous; that means the laser signals flowing on the optical network has been synchronized with an external clock, and so it is easy to multiplex and de-multiplex different data sources on a time bases.

Fiber exhaust can be faced with a different technology, known as DWDM (dense wavelength division multiplexing). Instead of increasing the transfer rate by multiplexing several sources as in TDM, DWDM assigns to each source a specific wavelength (also called lambda, or channel) and transmit all the lambdas on the same fiber optic. In the following picture, four different signals, each at a data rate of 10Gbps, are transmitted on the same fiber assigning a different lambda to each of them.

Also in this case we can do an analogy with the traffic running on a highway: in this case we will have a lane for each channel (lambda) and cars move on each lane in a parallel way without modifying their speed.

DWDM gives to the carriers the possibility to dramatically increase the available bandwidth without having to deploy new fiber. To give you an idea of what DWDM is able to do, consider that, in research laboratories, the following configurations have already been successfully tested:

• 320 channels (lambdas) each with a rate of 2.5 Gbps (total: 800 Gbps)
• 160 channels (lambdas) each with a rate of 10 Gbps (total: 1.6 Tbps)
• 128 channels (lambdas) each with a rate of 40 Gbps (total: 5.12 Tbps)

DWDM takes advantage of an important feature of fiber optic: when the light enters a fiber at a given incidence angle, it is almost totally reflected (nearing one hundred per cent of reflection) by the fiber core and propagates inside it until it reaches the other extremity. There are two types of fiber optic: multimode (with a diameter size of about 50-60 micrometers) and singlemode (with a diameter size less than 10 micrometers). In multimode fiber, the light propagates following more than one path (the path is also called as mode), whereas singlemode fiber is so narrow that light can propagate following only one path. Due to this fact, singlemode fiber is more suitable for DWDM applications.

In precedence, we talked about wavelengths. Which are the lambda values used in DWDM systems and how they are chosen? Again, the answer lays in the physical characteristics of fiber optic. When the light propagates inside a fiber, it is attenuated in a way that depends on the wavelength value. In particular, there are some wavelength areas in which the attenuation can be considered as constant, and others in which it has an almost linear dependency. Experimental researches have found three regions (also called as optical windows) in which the attenuation is very low: the first one is centered on the 850 nanometer frequency, the second one is centered on the 1310 nanometer frequency, and the last one is centered on the 1510 nanometer frequency. The most adopted values are 1310 and 1510 nanometer; right now millions kilometers have been covered with such systems. It should be noted that in DWDM systems a fiber can transmit the signal in both directions, not in just one. Moreover, channels are usually spaced with a frequency of 50 or 100 GHz. For instance, the two lambdas 1543.73 nm and 1544.53 nm are spaced at 100 GHz frequency.

Another key factor which contributed to the DWDM success is represented by the recent progress made by optical fiber amplifier (OFA). Data transmitted with DWDM does not require amplification for span lengths less than 80 kilometers. Prior to OFA arrival, DWDM amplification occurred with the ‘3R’ regeneration process: the amplifier received and converted the signal from optical to electric, reshaped, retimed, and amplified it again. That resulted in complex and expensive networks, especially for long distance transmission. OFA, instead, are able to amplify directly the optical signal, regardless of the number and value of the lambdas it contains. The most common type of OFA used in DWDM networks is the erbium doped fiber amplifier (EDFA), where a piece of fiber is doped with erbium ions (Er3+).

In conclusion we can say that DWDM offers several benefits over traditional fiber optic transmission techniques:

• it drastically reduces fiber usage allowing the aggregation of many channels on the same fiber
• it is scalable and protocol independent
• it allows long haul transmission keeping low the regeneration costs