Optical Networking Drivers

Optical Networking Drivers

The performance improvements in speed, cost, and capacity of fiber have been fast and furious. The number of bits per second that fiber can carry doubles every nine months for every dollar spent on the technology. The number of bits per second (bps) per lambda (l), or wavelength, doubles every 12 months. The cost of transmitting a bit of information optically drops by 50% every nine months, and fiber prices, on a per-megabits-per-second basis, are falling by 60% each year. In comparison, the number of transistors on a computer chip doubles only every 18 months. Therefore, over a five-year period, optical technology far outpaces silicon chips and data storage.

Optical equipment has helped to drive down the price of moving a bit of information over long distances to 0.006% of what it was in 1996. If the automotive industry could match that, a BMW could be had for US$2.50 today.

In 2000, network capacity was reported to be doubling every nine months; 2001 statistics suggest that network capacity is doubling every five months. New fiber networks are increasing long-distance transmission capacity incredibly quickly and relatively inexpensively.

What is driving these advancements in optical networking? First, carriers want to boost capacity by orders of magnitude. Dense Wavelength Division Multiplexing (DWDM) enables multiple wavelengths of light to be carried over a single strand of fiber, which allows for the elegant expansion of capacity. We're also now seeing developments in applying Frequency Division Multiplexing (FDM), whereby we can combine more streams of traffic onto the same wavelength, promising a several-fold boost to the carrying capacity of fiber. (DWDM, FDM, and other multiplexing techniques are discussed in Chapter 2, "Telecommunications Technology Fundamentals.") Thus, we can extract more wavelengths, and over each wavelength we can derive more channels, and on each channel we can achieve more bps.

Second, carriers want to slash costs. Advances are being made in eliminating the need for regeneration stations on long-haul networks. Currently, because most optical networks still use electronic repeaters, every 200 miles (320 kilometers) a light signal has to be converted back into an electrical signal in order to be reshaped, resynchronized, and retimed. Then the signal is again converted back into a light pulse. This process alone accounts for about half the cost of optical networking. Provisioning of services needs to occur in minutes. But at this point, a carrier may have to wait six to nine months for an OC-3 (that is, 155Mbps) line to be provisioned. Optical switches will automate the provisioning process while also boosting capacities to handle thousands of wavelengths; that is the promise of end-to-end optical networking.

As described in Chapter 3, "Transmission Media: Characteristics and Applications," today's networks maintain mostly separate electronic connections for voice and data (see Figure 12.1). They achieve reliability in the network by using dual-counter-rotating rings based on the SDH/SONET communications standard (which is discussed in Chapter 5, "The PSTN"). With dual-counter-rotating rings, traffic normally flows in a clockwise direction over the primary fiber. There is also a protect fiber, which is designed to carry traffic in a counterclockwise direction. A SDH/SONET multiplexer aggregates traffic onto the rings. If the primary link is cut, traffic is switched to the protect fiber very quickly in a matter of 50 milliseconds. This ensures a high degree of network survivability and it is a major strength of SDH/SONET.

Figure 12.1. An example of today's optical networks

Optical networks in the near future will channel all traffic over a single fiber connection and will provide redundancy by using the Internet's mesh of interlocking pathways. When a line breaks, traffic can flow down a number of different pathways. Optical switching will become the foundation for building these types of integrated networks (see Figure 12.2).

Figure 12.2. An example of tomorrow's optical networks