Optical/IP Networks

Battle Begins Between FDM Twins

Here’s how FDM works. On one side, an FDM mux takes in multiple optical signals (from Sonet ADMs, a router with a fiber interface, or any other optical networking device) and uses digital signal processors (DSPs) to convert them -- from optical into electrical signals, as well as from digital signals to RF (radio frequency) signals. The FDM mux then performs a series of error correction and equalization techniques before modulating the electrical signals down into one output, reconverting it into a light signal, and sending it back out onto the optical network. At the other side of the optical connection, the process is reversed to unravel the different streams of traffic, in exactly the same way as sub-rate muliplexers work in conventional carrier networks.

Kestrel says its first FDM product will allow it to multiplex an aggregate total of 10 Gbit/s of ‘low-speed’ optical signals into one wavelength. Centerpoint claims its product can stream more than 20 Gbit/s-worth of traffic into one feed.

Either way, it’s a very neat trick.

To understand why, it helps to consider how the signals from optical devices are handled today. Take the example of a service provider using eight 622 Mbit/s OC12 connections to hook up COs (central offices) in a metropolitan area network (MAN). Simply put, it has two choices. First, try to rent eight fibers between the sites -- an expensive proposition even if the lines are available. Second, install an eight-channel DWDM mux to pack all eight OC12 lines onto one fiber connection.

But by installing an FDM multiplexer, the carrier can consolidate all eight OC12 signals and then send them over just one channel, or lambda, on the DWDM multiplexer. The pay-off? Eliminating the need to devote one DWDM channel to each OC12 results in huge cost savings – in this instance by allowing the carrier to use the freed up channels for other traffic.

It’s also worth noting that by front ending a DWDM multiplexer with multiple FDM multiplexers carriers can really supercharge their networks. For example, if Centerpoint is to be believed, adding eight of its FDM multiplexers to an eight channel DWDM mux would allow 160 Gbit/s of data to be sent over a single fiber line. Further, both companies say that their FDM products can be used to bolster capacity with optical technologies other than DWDM – including Sonet.

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