Fiber-fed regional headends expand bandwidth capacity

Th 05lw38c

The headend in a cable-television plant is the central processing point of the network. All of the external sources for programming are gathered here, processed, and packaged for distribution. The headend is also the place where subscriber information and data communications returning from customers are reconstructed and processed.

In the past, headends have been relatively simple and operators simply duplicated them as often as necessary. As the industry begins offering a range of new digital services, this dynamic is changing. Increasingly, economic and operational imperatives are favoring large regional headends linked to each other and to local distribution networks via fiber optics.

Since the industry's early days, cable operators have been adding channels as fast as the technology and programming sources have become available. In the 1990s, fiber optics has helped accelerate this expansion of programming sources in the traditional broadcast business, while at the same time enabling interactive services to emerge as viable business propositions. The bottom line, in terms of headends, is that they have become considerably more complicated and expensive to build.

Another key trend favoring the regional headend concept is "clustering," a dynamic that has helped fuel the frenzied buying, selling, and trading now underway in the cable industry. Time Warner was one of the first companies to recognize the advantages of grouping its holdings geographically within major market areas. It makes sense to consolidate service organizations and enjoy economies of scale, but more importantly, it allows an operator to capture a larger share of the local advertising market. As interactive services become more prevalent, this concept will become even more important.

These emerging dynamics spurred Cable Labs to examine the regional headend concept several years ago, but it had limited use until the market was ready for them. Today, there are several cable networks that spread over hundreds of square miles. As such scope is being realized, plans are in the works to build and operate regional headends and the fiber-optic transport networks necessary to distribute existing and new services to these expanded service territories.

Narrowcast versus broadcast
Having a single regional headend is a natural fit for broadcasting a large number of channels out to hundreds of thousands (millions in some cases) of subscribers. This broadcast function has been cable's traditional role and today remains its primary business.

If this function were all that was required, then a single fiber could supply all the bandwidth necessary. But this is no longer the case. The bandwidth requirements of spot advertising, video on demand (VOD), and interactive services tailored to individuals or small groups are putting much greater demands on the size of the network pipe.

Broadcast-television stations receive most of their operating revenue from the sale of commercials. The advertiser pays the going rate for a 30- or 60-sec spot based on the size of the market. To the extent they have developed sizable market clusters, cable operators can compete with broadcasters for this market-wide advertising dollar. But unlike broadcasters, they can also segment the market into separate demographic zones and sell the same time slot multiple times. This logic could be applied down to the optical node service area. As cable's advertising business migrates toward this "narrowcasting" model, the demand for bandwidth in the transport network will increase.

Interactive services such as high-speed modems and VOD require data streaming to individuals on

the network. In large metropolitan areas, the amount of data in need of transport may be in the multiple-gigabit range.

If all of this new broadcast and nonbroadcast traffic is to be routed to a regional headend, then the network must be very well thought out.

The first consideration in planning a fiber-optic network to support a regional headend is whether it is to be a private or public network. A public network must interface with outside network providers, their physical routes and connection points, and, if the public-network operator cannot negotiate a "dark" fiber pipe, these other networks' data formats. If it is solely a private network, then there are more options available.

The next step is to decide what remains analog versus what traffic will be digital. Analog fiber-optic equipment has an advantage in broadcast video services, since most of the consumer electronics available today is analog. By leaving the source analog, several layers of complexity and cost can be avoided. The disadvantage of using analog is in the distance that it can be transported. Even with the advent of 1550-nm analog optical amplifiers, the practical maximum distance a good-quality analog video service can be transported is approximately 150 km. This distance may be extended by another 100 km or so if the services are digitized, then modulated, using a QAM or VSB format, onto a radio frequency (RF) carrier. The decision to remain analog is a complex equation that involves the cost per channel, quality of service, and actual network distances.

If analog equipment is not viable, there are a number of digital choices. Digital optical equipment has two major advantages over analog. First, the quality of the video service is set (assuming the network is designed and operating properly) by the quality of the conversion process from analog to digital (sample rate and quantizing error). Second, the signal can be repeated with little or no degradation up to the clock-jitter limitations of the equipment. Both of these advantages allow networks to be scaled to almost any size.

Today, there are several vendors with proprietary digital-video solutions. These companies have tailored their formats to video, which has allowed them to be considerably more cost-effective than telephony-based protocols for this type of application. This advantage has also been a key disadvantage of these solutions, since the propriety protocols were incompatible with telephony standards and had to be used within a private network. Vendors such as ADC Telecommunications and Scientific-Atlanta have since developed products that support Synchronous Optical Network (SONET) compatibility and access to the public network.

SONET and the older digital service protocols are still very inefficient for most of the services delivered by the cable industry, largely because they were designed for a network in which the information flow was symmetrical. Except for telephony and video telephony, all current and future "cable" services are asymmetrical, with the vast majority of the network flow in the downstream direction.

Dense wavelength-division multiplexing (DWDM) will also play an important part in these new network infrastructures. Since the necessary bandwidth required to support VOD, high-speed data, Internet protocol telephony, etc., are enormous, even networks with a large-capacity excess over today's requirements may be stressed in the very near future. In its current stage of development, DWDM remains a fairly expensive proposition for cable applications, especially when used for the distribution portion of the network. It becomes very cost-effective, however, if facing the prospect of building an additional network over the existing one.

Once these regional hubs and fiber-optic transport and distribution networks have been widely deployed, the availability of communications bandwidth will be many multiples of existing levels. This setup will allow us to think outside the current box that today's networks have built around us.

Dealing with satellite outages
Twice every year, and for up to six days each occurrence, the sun blocks out the entire transmission from whatever communications satellite it is directly behind for several minutes at a time. This blocking out occurs twice a year because the earth wobbles on its axis from winter on one end to summer on the other and back again, putting the sun directly above a latitude between the tropics of Cancer and Capricorn twice each year in the spring and fall. Since the earth is also spinning around its axis once every 24 hours, the sun is only directly above any given longitude once each day. The satellites in geostational orbit that send us communications signals, on the other hand, stay in approximately the same spot above the equator and rotate around the earth at the same speed that the earth rotates around its axis. Hence, they are stationary with respect to a given point above the earth (see Fig. 1). When the position of the sun corresponds to the position of a geostationary satellite, an outage can occur.

Because of these orbital mechanics, the actual area where the sun outage occurs is a circle (like a spotlight) with a diameter of 350 to 550 mi (depending on the size of the satellite dish). This circle tracks across the earth from east to west, moving southward every day during the fall and north during the spring (in the Northern Hemisphere). As a result, an outage in a given location will not affect another location 600 mi away (see Fig. 2).

Using a fiber network linking regional headends, it would be relatively easy to import the sun-blocked channels from outside the affected area and return the favor as the sun continues to shift. Such an approach could be applied today if there were a sufficient economic need, but to our knowledge, it has not yet been employed in the cable-TV industry. It could become cost-effective, however, as the availability and cost of fiber bandwidth becomes less expensive.

We present this idea as one small example of the potential implications of a fiber network linking regional headends. Taking the concept another step further, an entire headend could be backed up in case of a local disaster.

Growing need
We firmly believe that today's mega-mergers and clustering activity will continue for the next several years and that the need for regional headends will continue to grow. This, in turn, will increase the need for fiber-optic cable and electronics to link these headends. In our next column, we will look at one of the emerging regional system's networks and plans for the future. Th 05lw38c

Don Gall has been involved with the cable-TV industry for the last 28 years. He was an integral part of the team at Time Warner that developed the first practical applications of analog fiber and hfc networks. He is currently a consultant with Pangrac & Associates (Port Aransas, TX) and can be reached at dongall@pangrac.comTh 05lw39c

Mitch Shapiro has been tracking and analyzing the broadband industry for more than 12 years. He is currently a consultant with Pangrac & Associates, which this summer will publish the first of a series of in-depth reports on clustering, network upgrades, and new service strategies in the cable industry. He can be reached at mshapiro@pangrac.com or via the P&A Web site at http://broadbandfuture.com.

More in Network Design