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November 1999 Issue
Features

The ABCs of Block Conversion
Add RF Capacity Without Rebuilding
By Mike Whitley

• Optics are great, but. . .
• Why use RF?
• Spanish system reaps the rewards
• Is it right for you?
• Bottom Line

Cable operators are expanding into optical technology at a rapid pace that is likely to continue for some time to come. In short order, virtually all cable traffic aside from the last mile to the home will be in the optical domain.

Expanding reverse traffic capabilities will be key to increasing operator revenue potential. Also, getting signals transmitted from hub sites to headends for processing brings forth several challenges. Limitations in the number of fibers and hub site space, plus finding economical, yet expandable, electronics is important to the return signal path. Block conversion, or frequency stacking, is one way of solving some of these network issues.

Optics are great, but &

Clearly, this optical domain is a positive development for the cable industry, and future development of optical technology, such as advanced lasers and dense wavelength division multiplexing (DWDM) will contribute to cable's growth as a telecommunications player.

Still, many cable engineers and operators are coming to the realization that RF technology will play a continuing role in the optical era because RF is likely to remain the primary connection between the home and the optical node. Multiplexing in the RF domain can be efficient and cost-effective in transmitting return path signals from the home to the headend for processing.

A technique known as block conversion is being deployed at cable systems around the world. Block conversion can be considered the RF equivalent of DWDM. Signals entering a converter are assigned a block, or group of frequencies, to keep them separate from other signals. Each block is then multiplexed onto a broadband RF stream, which ultimately is captured and demultiplexed into individual return blocks. (See Figure 1.)

A DWDM system performs the same function, except that it assigns optical signals to specific frequencies or wavelengths of light.

Why use RF?

So why bother with an RF system when all our training and experience indicates that optical signals are much cleaner and more efficient to work with? Three reasons.

Cost: The first is that the cost of block conversion can be less than DWDM. How much less depends on individual network architectures and the way in which the block converters are deployed. Some early adopters, however, are estimating block conversion at nearly half the cost of DWDM in transporting return path signals.

"We have modeled savings of up to 42 to 45 percent in reverse path costs for the level of segmentation we require," says Oleh Sniezko, vice president of engineering at AT&T Broadband & Internet Services, in his 1999 NCTA technical paper, "Reverse Path for Advanced Services-Architecture and Technology." He continues, "Someone else could have a higher or lower savings depending on the architecture they are using."

Capacity: The second advantage of block conversion is capacity. Current DWDM technology combines eight wavelengths onto a single fiber, with 16-channel and 20-channel systems expected shortly. Block conversion technology available today can multiplex 18 5-42 MHz signals onto a single 870 MHz stream in the United States and can combine 12 5-65 MHz signals onto a single 870 MHz stream in other countries, says Dave Kirkpatrick, network applications engineer at ANTEC. That means every DWDM system carries eight return blocks per fiber, while a block conversion system delivers 18 return blocks per fiber. (See Figure 2.)

Versatility: Third, because these technologies are multiplexing in two different domains, block conversion and DWDM can work together. With current block conversion and DWDM technologies, it's possible to take multiple block-converted bands and input them onto a DWDM transport device. Systems have shown the capability of multiplexing nine 5-42 MHz blocks onto a 16-wavelength DWDM system for a total of 144 return bands onto a single fiber, thereby exponentially increasing the network's overall capacity.

This approach can be most beneficial in systems that already are hitting bandwidth limitations with DWDM in place. In order to keep an all-optical network, operators can either wait for 16-wavelength DWDM to hit their price point, or they can add another eight-wavelength system and install additional fiber. Operators in urban areas know that this can be difficult. By simply installing block conversion, the extra capacity can be gained immediately, without having to install additional fiber.

What about loss of signal robustness in the RF domain? Performance of the block converters always will be limited in the network by the optical transport devices. As long as design rules are followed, the performance will suit all types of return signal modulation schemes: frequency shift keying (FSK), quadrature phase shift keying (QPSK) and quadrature amplitude modulation (QAM). The performance of the block converters individually is determined by three stringent criteria: dynamic range, frequency stability and phase noise.

Spanish system reaps the rewards

In Spain, Cable y Televisio de Catalunya (CTC) is installing block converters for the return path of its Barcelona franchise. The plan is to place converters at the hub level, combining multiple return signals from nodes together. At the hub, the signals are received from the optical nodes, converted to RF in the return path receiver, block-converted into a single RF stream, and then transported optically back to the headend. (See Figure 3.)

Signal robustness is maintained, and the optical carrier handles the actual transmission. CTC will use block conversion to deliver data, telephony and videoconferencing services throughout the Barcelona area.

According to Jaume Salvat, director of technology at CTC, block conversion will allow the company to increase its return path homes-per-fiber ratio from today's 8,000 to more than 90,000.

"We have one hub with block conversion now," he says. "By the end of this year, we will have it installed at seven hubs. In seven years' time, we hope to be serving 1 million homes."

Not only are the block converters less expensive to acquire vs. comparable DWDM devices, but they also cut down on the amount of fiber that CTC has to run from the hub to the headend.

"We would normally run four fibers per hub to the headend," Salvat says. "Now, because of block conversion, we only use one."

Installation of CTC's block conversion is going smoothly, according to Salvat. The devices are practically plug-and-play, with no serious network or enclosure alterations necessary. No special equipment is required-the devices can be aligned using standard return path setup procedures.

Block conversion also gives operators all the segmentation and centralization benefits of DWDM at a reduced cost. By segmenting the return path signals onto their own frequencies, operators can centralize routers, interconnects, servers and other active components at the headend or central office. Not only does this cut equipment costs, but manpower usage also is more efficient because the actives can be serviced and monitored at one central location.

Keeping things centralized at a headend minimizes the physical space required for the hub and reduces real estate and buildings, which would add significantly to the overall cost of installing the network.

Is it right for you?

While block conversion helps make hubs more transparent and simplifies network operation, whether it's right for you depends on your architecture. Plants that can benefit the most are urban systems where the cost of installing new fiber and finding suitable hub sites is high.

"In both rural and metro areas, operators must first look at system construction costs," says Kirkpatrick. "Then, they must factor these costs against the advanced services they plan to deploy. These factors are critical to choosing a technology, whether it is all glass, block conversion or a combination of both."

There are many ways in which block conversion can be deployed. Depending on bandwidth needs, some operators may find it necessary to install block upconverters at the node, with the downconverter at the hub or even headend. A careful evaluation of anticipated bandwidth requirements at each stage of the network will be useful in determining the right approach.

What's important to keep in mind is that block conversion can be another tool used in the transport of return band signals throughout the network. As in CTC's case, the use of block converters can save in both cost and in the number of fibers required.

Mike Whitley is director of product management for outside plant at ANTEC Network Technologies. He may be reached via e-mail at .

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