How to deal with fiber exhaust –

Enterprise networks face a condition called fiber exhaust when the demand for backbone fiber exceeds the availability of installed fiber strands.

The most obvious and expensive solution to fiber exhaust is to install more fiber. However, the costs associated with running and terminating the new fiber, and the restrictions involved with rights of way, open tray or duct space, and access to buried fiber often make this a less-than-attractive option.

An alternative is to say no to those who want to place new applications on the fiber infrastructure. However, saying no is neither popular nor politically correct.

Another option is to remove the less critical applications from your fiber network to make room for the newer ones. However, telling users that they can no longer use their fiber connections won't win you any popularity contests either.

DWDM to the rescue

Fortunately, a relatively new technology called Dense Wave Division Multiplexing (DWDM) can boost the capacity of a single fiber pair by as much as 32 times.

Prior to DWDM, only one light beam at a time traveled down each single- or multi-mode fiber strand. Since network data travels in two directions, it took a pair of strands to make a single network connection -- one strand for transmission, the other for reception.

DWDM is able to increase the total bandwidth of the fiber because instead of using a single-color laser beam (typically a white beam), it sends multiple-color beams, called lambdas or channels, down the same fiber pair. Multiple frequencies of light offer more bandwidth in a fiber strand in much the same way that different radio frequencies offer many FM stations on a single radio band.

Each lambda can carry its own independent signal, providing the same overall bandwidth per channel (approximately 2.4 Gbps with most of today's fiber) that a single-color laser does. Thus, if you run DWDM with eight lambdas, you increase the capacity of a fiber pair from 2.4 Gbps to 19.2 Gbps. This creates virtual dark fiber, which enterprise networks can use to run multiple higher-layer technologies such as ATM and Gigabit Ethernet simultaneously over the same physical fiber strands.

DWDM can be configured in a simple point-to-point configuration, a hub architecture in which all connections run to a single DWDM device, a chain, or a ring. In a ring configuration, DWDM can bring fault tolerance to point-to-point networks such as ATM and Gigabit Ethernet.

Traffic on DWDM's single ring may run in two directions, one called the east ring and the other called the west ring. Traffic runs in what is called protected mode, in which either the east or west ring is used for traffic, with the DWDM devices deciding the direction on a per-channel basis. Should a break occur, the DWDM devices switch all the connections to the other ring.

Those who like to live a little more dangerously can use the ring topology in unprotected mode and double its capacity. While protected mode typically supports up to 32 channels in DWDM gear, unprotected mode can double this by using both the east and west rings to carry nonredundant traffic. However, a fiber break in a ring running in unprotected mode will disrupt all services running along the broken section of the ring. Some vendors allow network administrators to decide which channels run in protected and which in unprotected mode.

One of DWDM's biggest assets is that it is scalable. You can start with only a few channels and add more as your fiber requirements increase. The basic chassis cards that are needed to make DWDM work will cost of at least $100,000; most of the incremental upgrade cost is in the purchase of additional channel cards, which may run around $10,000 apiece. That is a bit pricey, but that's typical of new enterprise technology.

For more details on DWDM, check out Dense Wavelength Division Multiplexing, an excellent online guide. You can also take a look at the IEC's Web ProForum on DWDM.

Enterprise DWDM products include:

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