Dense Wavelength Division Multiplexing

Dense wavelength division multiplexing (DWDM) is a technology that puts data from different sources together on an optical fiber, with each signal carried at the same time on its own separate light wavelength. Using DWDM, up to 80 (and theoretically more) separate wavelengths or channels of data can be multiplexed into a light stream transmitted on a single optical fiber. Each channel carries a time division multiplexed (TDM) signal. In a system with each channel carrying 2.5 Gbps (billion bits per second), up to 200 billion bits can be delivered a second by the optical fiber. DWDM is also sometimes called wave division multiplexing (WDM). Since each channel is demultiplexed at the end of the transmission back into the original source, different data formats being transmitted at different data rates can be transmitted together. Specifically, Internet (IP) data, Synchronous Optical Network data (SONET), and asynchronous transfer mode (ATM) data can all be traveling at the same time within the optical fiber.

Multiplexing, by definition, is the process where multiple channels are combined for transmission over a common transmission path. In the early 1990s, fiber could only carry one wavelength, or color, of light at a time. Lasers were used by quickly turning them on and off. By the mid 1990s wave division multiplexing could split the light into two colors. The number of colors rapidly grew and today as many as 160 colors can be carved out by using the most advanced systems, in what is now called dense wave division multiplexing (DWDM). In other words, DWDM combines multiple optical signals so that they can be amplified and transported over a single fiber. An example would be a DWDM network with a mix of SONET signals operating at OC-48 (2.5 Gbps) and OC-192 (10 Gbps) over a DWDM infrastructure can achieve capabilities of over 40Gbps. The reliability of the system is maintained throughout this process. DWDM networks are self-regulated at the bit-rate and format level. They can also accept any combination of interface rates on the same fiber at the same time. This greatly increases the flexibility of the system. The communication industry can become fully integrated, using multiple vendor interfaces with distinct technologies into one physical infrastructure. The fiber itself would remain transparent to the protocol or type of information. If a carrier operates both ATM and SONET networks, it is not required that the ATM signal be multiplexed up to the SONET rate. A DWDM network can merge signals operating at different rates. DWDM networks are self-regulated at the bit-rate and format level. They can also accept any combination of interface rates on the same fiber at the same time. This greatly increases the flexibility of the system. The fiber itself would remain transparent to the protocol or type of information.

A key feature of the DWDM network is that it exists at the Physical Layer. The fiber-optic amplifier section of the DWDM system rids you of the need to change the optical signals into electrical when amplifying. The pump laser is used to energize the erbium with light at a specific wavelength. The erbium acts as a gain medium that amplifies the incoming optical signal. In optical networking, DWDM could be compared to accessing the unused lanes of the highway. With each individual color being a lane, the lanes of the highway are blind to the types of traffic that travel on them. If you increase the capacity for data traffic within the lanes as well as the number of lanes themselves, you have greatly enhanced the capacity of the fiber path. The following description is one possibility of how to increase capacity without increasing the amount of fiber. The various digital signals come from homes, businesses, phone companies, and many other sources. The signals up until this point can be light or electrical pulses. They first need to be brought into what is known as a time division multiplexing box. A time division multiplexer (TDM) provides for the transmission of multiple signals over a common path by using successive time intervals. A preset time interval is used and the discrete data streams are sampled and interleaved by the TDM to provide a single data stream at a much higher data rate than the input data stream. This can then be multiplexed even further using the DWDM system. A classic DWDM system would have a light source on the transmitter side that operates at a specific wavelength