DWDM for everyone

The cost of spare parts has kept DWDM out of reach for many service providers. But new transmitter card technology is changing that

A serious consideration for service providers as they bring increasing amounts of dense wave division multiplexing equipment into their networks is the cost of "spare parts," specifically transmitter cards, and the flexibility of these units.

Advances in technology now allow providers to use fewer transmitter cards to cover all DWDM channels. With one card doing the work of four or eight channel cards, and eventually scores of channels, this effectively reduces the number of spare cards that a provider must keep on hand. The result is a dramatic reduction in the cost of planning, maintaining and managing such an inventory.

Current generation lasers that transmit data in a DWDM system are set to discrete, narrowband frequencies. As a result, a provider must stock a separate spare transmitter card for each frequency in case one of the active cards fails. For a 16-channel DWDM system transmitting OC-48 traffic, for instance, the provider must keep a spare card for each of those 16 channels. In the event of a card failure, say at lambda 7, the repair technician must locate the spare card that is set to the frequency of lambda 7 and replace the failed card with the spare. Then the technician must order a new spare immediately, so that all channels have a ready spare.

Considering the capital investment necessary for replacement stock for a single central office, the cost quickly adds up when multiplied by the vast number of COs in a typical provider's network. This problem will worsen as wavelength counts increase.

Sparing is currently a bigger issue for interexchange carriers because long-distance applications require more wavelengths, and therefore, most DWDM systems are deployed in this market. A provider using a 16- or 32-channel system, for instance, will have many spare cards to maintain in each locale, and the cost can quickly become prohibitive. In fact, it may inhibit the rollout of DWDM for some providers, denying them and their customers the benefits of increases in bandwidth. The number of spare cards also creates logistical nightmares as carriers must plan, order, track and restock them.

The solution is coming, and coming very soon, in the form of tunable transmitters.

Tuning in

Tunable transmitters allow a provider to stock a spare card with a laser that can be tuned across a range of several frequencies. Therefore, one spare tunable card can fill in for any of four, eight or more pre-set channel cards in case of a failure (Figure 1). The result is a lower capital investment by providers for the parts needed to keep their network operating.

The typical cost of an OC-48 narrowband transmitter card is approximately $12,000 per network element. In a 16-channel DWDM system, that adds up to $384,000 for spares alone: 16 channels at $12,000 per network element, multiplied by two ends.

By using spare transmitter cards that are tunable across a range of four DWDM channels, providers can reduce the number of spares they must purchase by 75% (Figure 2). Using the example above, with 16 channels of OC-48, the total cost would be $96,000 for OC-48 spares inventory, a savings of $288,000. The savings are even greater for OC-192 cards, which cost about $25,000 per network element.

Tunable transmitters bring the same benefits to the companies that manufacture them as they do to the service providers that use them. Equipment suppliers are able to produce fewer card varieties, which facilitates manufacturing forecasts, lowers inventory costs and allows them to react to customer demands more quickly.

Each transmitter card, regardless of the number of wavelengths (lambdas) over which it can be tuned, typically uses a single optical device. In DWDM installations today, a technician must physically remove the failed card and insert a spare plug-in unit that is pre-set to the required frequency.

Tunable transmitters such as those developed by Fujitsu Network Communications enable a single card to be used as a backup for many wavelength channels in a DWDM system. The transmitter can be remotely provisioned using a TL1 software command to assign the specific frequency it must carry. It typically takes less than a minute to provision the card.

If tunable transmitters are offered at a premium price, service providers likely will use their transmitter cards as spares in this way: Once the tunable transmitter card is in place for the failed card, the provider can then order a replacement for the less-expensive, traditional (single-wavelength) card. When the replacement arrives, the provider can remove the tunable spare, insert the single-wavelength card and restock the tunable card for backup purposes.

This approach represents only a temporary solution to providers' sparing needs, as single-wavelength cards will not be around for long. Eventually, all transmitter cards will be tunable to different wavelength frequencies.

How they work

The tunable transmitters being developed today use a thermoelectric cooler as the control mechanism. Using the craft interface, a service provider sends a command to the tunable transmitter to assign a specific wavelength frequency. As the transmitter's laser temperature is changed in response to the command, the laser sweeps across the pre-set range of frequencies until it reaches the required frequency and is locked in place.

Take, for example, a four-channel tunable transmitter card with a frequency range from lambdas 5 through 8 in a 16-channel application. The craftperson types in a command for the laser to set itself to lambda 7. It will start at lambda 5, move past the frequency of lambda 6, and stop at the frequency for lambda 7, being held in place by wavelength-locking circuitry. A four-channel tunable transmitter card can take up to a minute to tune itself to the correct frequency.

The tunable transmitter card must not interfere with active traffic on lambdas 5 and 6 as it makes its sweep to the desired frequency. This is achieved by lowering the power of the tunable card when it is moving across those two frequencies so that the WDM filter effectively eliminates all the remaining power before reaching the multiwavelength signal. Once the transmitter has properly set itself to lambda 7, the power returns to its normal level, and the system can begin to send traffic across lambda 7.

The transmitter cards are tunable to contiguous wavelengths, such as 1 to 4 and 5 to 8 in a four-channel card, and default to the lowest frequency at installation.

The first available tunable transmitter cards will cover a range of four wavelength channels at 100 GHz spacing. These transmitters have been demonstrated at trade shows and became commercially available this spring.

As DWDM channel spacing drops to 50 GHz, the tunable transmitters on those first four-channel cards will begin to support eight wavelength frequencies because the same card can then cover a larger number of wavelengths in its frequency range. Tunable transmitter cards in 8-, 16- and 32-channel versions and beyond should become available later this year.

The obstacles to a high number of tunable wavelength frequencies aren't technological but manufacturing issues. Suppliers must be able to cost-effectively produce a large number of high-wavelength-count cards in a consistent manner so that their service provider customers have a dependable supply of cards to use as spares. Suppliers also must be able to produce these cards so that service providers pay a reasonable premium for the tunable capabilities.

The initial thermally controlled cards will be offered at or near parity prices to the existing single-wavelength units. This allows these tunable units to be used as standard traffic-carrying units or as sparing units.

Suppliers must bring future tunable transmitter cards to market, which will cover an even wider range of frequencies, for less than triple the price of a single-wavelength card for them to even be considered by service providers for spare-only use. For tunable cards that will be used in place of single-wavelength cards, the premium percentage must be lower - in the single digits.

As bandwidth demand continues to skyrocket, service providers will insist on the ability to tune transmitters over the entire C-band of the electromagnetic spectrum. This would enable a single transmitter card to be used for up to 80 different wavelength frequencies. Tunable transmitters should be available with this capability as early as next year.

Being able to handle many channels on a single tunable transmitter card will require an approach other than thermoelectric cooling.

One approach under consideration is the use of laser arrays, which involves producing multiple lasers in one substrate. This will be combined with the initial thermal-control method. Another technique being studied is distributed Bragg reflector, an adaptation of fiber Bragg gratings that uses a reflection for the different wavelengths. This second method offers the promise of reduced costs.

Tunable transmitters will provide welcome relief for the financial impact of maintaining and managing multiple spare cards for a DWDM system. Consequently, service providers will find DWDM much more affordable and will be able to better cost-justify the expansions of their bandwidth capacity that they so desperately need.