Wave Division Multiplexing

The amount of traffic-data, voice, and multimedia-traveling over the Internet and other networks grows at rates that are hard to quantify, but everyone agrees the increase is beyond anyone 's wildest dreams.

Once all this traffic leaves a local network (or home or small business), it goes into the hands of a carrier or service provider, and typically is sent over a fiber optic cabling infrastructure. Even service providers too small to have their own fiber infrastructure use fiber built and run by someone else.

Fiber optic cabling moves lots of traffic quickly, but even these fat pipes feel the bandwidth pinch . So rather than pay exorbitant amounts to lay new fiber cabling, providers rely on technologies that increase the amount of data a single piece of fiber can handle.

Early Solutions

You can multiplex multiple signals on a single medium by assigning different frequencies to each signal (Frequency Division Multiplexing or FDM), or by assigning different time slots to each signal (Time Division Multiplexing or TDM). Early telephony multiplexing systems used FDM (which is also the standard multiplexing technique used for cable television), but TDM has become the dominant method used by carriers for accommodating multiple data streams on a single cable.

In a TDM-based network, the channels are scanned in sequence, and each channel is given access to the data link at a particular time slot. If a channel has no data to send when its turn comes up, that slot goes unused and the next channel gets its turn .

The TDM network inserts a frame slot at the beginning and end of a group of channels. One pass across all channels results in a frame. Any empty channels get filled with bits to keep frames a consistent size .

Because frames arrive at predictable intervals, TDM is used extensively in the circuit-switched world to send voice traffic. Some examples of TDM are T1 (1.5Mbit/sec) and T3 (45Mbit/sec) circuits, which are used for voice traffic as well as for data.

Another technology that relies heavily on TDM is SONET, a digital fiber optic transport standard used worldwide. It's known internationally as Synchronous Digital Hierarchy (SDH). SONET/SDH is prized because it scales to larger and larger levels of bandwidth. It is an effective Physical-layer technology for such data types as ATM and, more recently, TCP/IP traffic.

The basic building blocks of SONET are groups of DS-0 (64Kbit/sec) circuits that are multiplexed to create a faster Optical Carrier (OC) line rate of 51Mbits/sec. This corresponds to an OC-1 circuit.

From there, OC-1s can be multiplexed to create even faster connections. For example, an OC-3 circuit can carry 155Mbits/sec. An OC-12 circuit (622Mbits/sec) is made up of three OC-3s, and an OC-48 can carry four OC-12s.

The trouble with SONET, and with TDM in general, is that as you multiplex more and more signals on fiber and increase its capacity, you need to add and upgrade the equipment that switches and routes the signals. Add/drop multiplexers and other high-speed transmission equipment are needed to manage the overhead created by the complex system.

Also, as you move to higher and higher speeds, the combination of lower-speed trunks requires SONET add/drop multiplexers to deal with all the data on the new, combined pipe, because none of the multiplexers can deal directly with their own data traffic. All of this makes adopting ever-higher-speed versions of SONET a costly and complicated way to maximize pipes.

TDM devices are required to convert data from light waves to electronic signals and back again-presently there are no commercially available methods for directly switching beams of light. This conversion process adds to the system's complexity, but does not do anything to alleviate bottlenecks.

Another related concern with TDM-based systems is scalability. Currently, the common top speed for TDM is 10Gbits/sec. If you look at the SONET hierarchy, the next logical step for TDM is all the way to 40Gbits/sec (equivalent to OC-768). However, there are doubts as to whether current TDM equipment can handle such a leap.

Making Waves

While TDM-based networks thrive, there is another method for expanding fiber capacity beyond TDM without the bottlenecks and complexity. Wave Division Multiplexing (WDM) divides the light traveling through fiber into wavelengths, also known as lambdas. Lambda is the Greek letter used to represent wavelength. (Because wavelength is inversely proportional to frequency, WDM is logically equivalent to FDM.) Each wavelength can support the high speeds that once required entire optical fibers-even as high as 10Gbits/sec each. In other words, on WDM systems, multiple channels can be transmitted over a single fiber because they are sent at different wavelengths .

WDM wavelengths can each carry independent signals-OC-3 voice on one wavelength, analog video on another wavelength, and OC-12 ATM on yet another one.

Currently, WDM systems can carry as many as two dozen channels, but in the future, capacity should increase to 128 channels or more on a single fiber. The potential bandwidth on WDM systems is mind boggling. For example, a system with 24 channels, each running at OC-48, would have a total capacity of 60Gbits/sec, and a system with 40 such channels would carry 100Gbits/sec.

Today, Dense Wave Division Multiplexing (DWDM) has subsumed WDM. Technically they are similar, but as the name implies, DWDM systems contain many more channels, and therefore much more bandwidth capacity. But most discussions of WDM and DWDM don't strictly define how many channels constitute one or the other, so the terms have become interchangeable (although DWDM is more commonly used).

While DWDM is far from a new technology, major telecommunications vendors and carriers have only recently employed it to boost fiber capacity. So far, DWDM has been used mostly in point-to-point links over great distances. The cost benefits in this scenario are significant because DWDM doesn't require nearly as much equipment as traditional TDM-based systems. Although DWDM has found a strong position in the long distance carrier space, the cost savings are not nearly as compelling for shorter links. Instead, options such as single-fiber SONET are still the best deal for short hauls.

DWDM is starting to attract interest in the medium-haul market, which could include metropolitan area networks. But because of the large amounts of TDM equipment already out there, you'll likely see hybrid deployments of TDM and DWDM technologies.

Because DWDM is economical and a relatively simple method of transporting network data, this technology will probably be used as the basic rails that other types of traffic ride on.

Working Together

In the future, fiber networks will probably have a combination of SONET/SDH that runs over a DWDM infrastructure. For example, data traffic would be multiplexed by SONET add/drop multiplexers before going over a DWDM system. The advantage is that SONET/SDH is easier to manage than raw DWDM. SONET allows for network provisioning and repair, which occurs by means of communications among various points on a SONET network. Also, no current standards define how to manage the different wavelengths in a DWDM system, while SONET/SDH is much more established. It is therefore easier to manage bandwidth using SONET/SDH.

Direct to the Mainline Connecting the ATM and IP traffic directly to an optical-based network such as Dense Wave Division Multiplexing (DWDM) can reduce costs and work around potential WAN traffic bottlenecks.

Today, most wide area traffic, including IP, is converted to ATM cells and then transported over SONET. This usually occurs because IP traffic needs to be multiplexed with ATM traffic or other TDM traffic to make it cost-effective to send. However, this method does include a degree of overhead-the infamous cell tax. (For more on IP over ATM over SONET and other wide area options, see "ATM and Alternatives in the Wide Area Backbone," July 1999.)

Dense Wave Division Multiplexing steps in nicely here by lowering the cost of necessary equipment and by solving the bandwidth bottleneck that could begin plaguing the higher levels of TDM (see Figure). In this scenario, high-speed interfaces (such as OC-48 or OC-192) could let routers and switches connect directly into a DWDM system without going through SONET TDM based on add/drop multiplexers. So you'll see systems in which data and voice traffic are multiplexed by SONET before going over DWDM, and you'll see systems in which IP and ATM data connect directly to DWDM without any interference from SONET.

Down The Road

An infrastructure based on DWDM should let service providers increase capacity within specific places on their networks. Providers can offer different levels of bandwidth to customers by making individual wavelengths available, which should be a more cost-effective solution.

While many businesses, especially very small ones, are doing just fine with a T1 line, many large corporations aren't satisfied with a T3. For the latter group, the option to lease wavelengths might enable new applications that require mass quantities of bandwidth.

Over the next several years , as more and more traffic goes onto existing wide area infrastructures , the need to optimize the fiber already in place will grow. TDM has provided a way to increase fiber capacity, and when combined with the more efficient and cost-effective DWDM, the move toward a system that supports all types of data and voice traffic without running out of bandwidth comes closer to reality.

Resources

For tutorials on SONET, Synchronous Digital Hierarchy (SDH), Dense Wave Division Multiplexing (DWDM), and optical networking, visit www.sonet.com/edu/edu.htm.

A number of white papers on optical networking can be found at www.usa.alcatel.com/telecom/transpt/optical/techpaps/techpaps.htm.

This tutorial, number 133, by Anita Karv, was originally published in the August 1999 issue of Network Magazine.