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Bandwidth is the capacity for carrying digital information. In the past few years, bandwidth has been increasing at roughly twice the rate of computing power. Data traffic has been doubling every 100 days (according to the U.S. Department of Commerce) and it is predicted that that Internet traffic will reach 16 million terabytes by 2003. Fiber is the transport medium of choice. It out performs copper, coaxial cable, and high frequency radio in data-carrying ability. As service needs increase, fiber exhaust becomes a problem. In some areas it is not economically feasible to lay new fiber. In these areas, it is important to look at alternatives to provide economical solutions. One of the biggest areas of concentration is how to increase the amount of bandwidth available.

The bandwidth of an optical network is principally determined by how fast the light can travel through the glass as well as how much light the fiber can have capacity for at any given nanosecond. Although higher transmission speeds are trying to be achieved, the speed of light is constant. Furthermore, the fiber increases in efficiency as the diameter decreases. This makes efforts to increase bandwidth focus largely on how to push more data through the same fiber cable at any given time.

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. A DWDM network can merge signals operating at different rates. 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 key feature of the DWDM network is that it exists at the Physical Layer. An all-optical network implies that the service provider will have optical access to traffic at various nodes in the network. Wavelengths can be added or dropped to or from a fiber without the aid of a SONET terminal. Optical cross-connects offers service providers the ability to have admittance to the network at these locations.

The fiber-optic amplifier section of the DWDM system rids you of the need to change the optical signals into electrical when amplifying. An optical fiber is used that has been treated with the element erbium. 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. If a connector were to be used rather than a splice, slight amounts of dirt on the surface could cause the connector to become damaged. Fluoride or Silica based fiber amplifiers could also be used. In the 1530 to 1565 nm range with filters, silica based amplifiers and fluoride based amplifiers work equally well. However, they cost more. Fewer amplifiers are needed due to the multiplexed system, using a broad range of wavelengths in the 1.55 um region. A multiplexed system with 16 wavelengths on a single fiber can decrease the number of amplifiers by that number at each regenerator site.

It becomes clear that service providers can establish infrastructures were the capacity could be increased as needed by combining multiple technologies. 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 from 1530nm to 1565nm at .8nm spacing. A DWDM multiplexer is then used to combine the wavelengths onto the same transport fiber, where the information carried by each is converted into a different color. The collection of colors is then shipped down different lanes along a single piece of fiber. Fiber amplifiers are spaced through the system. They operate at the same wavelength band as the source laser. The amplifier increases the gain of the incoming signal for retransmission at equal power levels for each wavelength channel. A demultiplexer is used at the receiving end to separate the wavelengths. It would then be necessary in this scenario for the signals to be processed in a time division demultiplexer. There are many different types of receivers that can then take the optical signal and convert it into lower speed electrical signals for distribution.

This scheme is just one way utilized in creating more bandwidth. It could even be taken a step further by adding frequency-division multiplexing (FDM). A lens sits in the nodes between the strands of fiber, between the TDM and the DWDM multiplexers. The FDM box converts light beams from the TDM boxes into radio frequencies which are packed closely together by staggering them for about a billionth of a second. The frequencies are then stacked creating multiple lanes of information within one composite radio wave. The FDM box then converts the radio wave into a continuous analog wave of light that travels to the DWDM. Of course, the signals would again have to be demultipluxed at the receiving end.

DWDM systems in the past were predominately deployed in long haul and submarine cable markets. Long haul optics are essential due to cost-cutting advantages and increased efficiency. However, these systems have increased in use for the metropolitan markets. To date North America has led in the deployment of WDM systems. The fastest growth portion occurring in the metropolitan applications. On the other hand, in Europe, the span lengths are greatly reduced which may cause systems to be extensively deployed in the future. In 1998 the worldwide demand for DWDM systems was estimated be at $2.1 billion and was expected to grow to as much as $12.1 billion by 2005 (as forecasted by Electronicast).

The growth into the metropolitan areas is driven largely by the transparency of the network and the opportunity for revenue generating services. Competitive Local Exchange Carriers (CLECs) use lines that are leased from the regional Bell Operating Companies (BOCs). The CLECs are driven to use the new technologies in order to maximize their profits. In turn, the BOCs must utilize the new technologies in order to stay as efficient and therefore competitive with the CLECs.

In conclusion, when the different ways of using multiplexing are combined it improves the amount of bandwidth dramatically. These technologies are extremely important in today's society were we are seeing high demands for data traffic. By having an all-optical network, we are provided with the ability to integrate the many different technologies into one system. The DWDM network enables the service providers to go long distances as well as provide for data lines and other growth in metropolitan area. There are more ways to multiplex than are discussed in this paper and different combinations can be used.


References Used:

All are Internet URL sites

1) http://telecom.tbi.net/mux1.htm

2) http://www.columbia.edu/cu/cie/techlists/patents/4926423.htm

3) http://www.cis.ohio-state.edu/~jain/talks/h_5opt.htm

4) http://www.techguide.com/comm/dwave.shtml

5) ftp://ftp.netlab.ohio-state.edu/pub/jain/courses/cis788-99/h_aipwd6.pdf

6) http://www.iec.org/tutorials/dwdm/topic08.htm

7) http://www.ert.rwth-aachen.de/Projekte/Theo/OFDM/node6.html
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Dwdm. EssayMania.com. Retrieved on 11 Oct, 2010 from