Fiber keys cable-TV network flexibility
The demand for new services means that hybrid fiber/coaxial-cable networks must be constructed with the future in mind.
Donald T. Gall Time Warner Cable
I have been in the cable-TV industry for the last 27 years. Almost from day one there was talk of high-speed data, video-on-demand, and various telephony services just around the corner. In the greater scheme of things, the time it took to develop broadband services into a reality has been relatively short. Back then these things were not much more than pipe dreams. Advances in electronics, equipment reliability, and above all, fiber-optic technology, finally made these dreams become a reality.
The first community antenna television systems delivered better local television service to small communities. The coaxial bus architecture was small enough to be manageable and relatively reliable. However, during the late 1970s, the cable companies, armed with new satellite programming, expanded into more urban markets by scaling the original bus architecture to fit. Reliability suffered, and the thought of delivering services other than broadcast entertainment dwindled.
Then fiber-optic technology was introduced. By incorporating low-loss analog fiber optics into our architecture we were able to trim the coaxial bus portion back to a reasonable level of complexity. Over the past 10 years, Time Warner has developed a hybrid fiber/coaxial-cable (hfc) architecture that it feels will give the flexibility and reliability needed to deploy any or all of the new data businesses that are emerging in today`s marketplace.
The company uses a ring-ring-star-bus architecture. The bus segment is based on coaxial-cable technology, and the remainder uses fiber optics. The design is structured to be scalable from very small to very large systems (more than one million passings). The star and bus portions of the network have small failure groups, and Time Warner has determined that it is unnecessary and costly to make them redundant. It makes much more sense to harden the electronics and passive devices to ensure reliability. The ring-ring segments are fully redundant, including electronics and path diversity. The following paragraphs describe each part of the network and how they are combined to form a communications system.
It starts with coax
The coaxial-cable bus has a relatively large bandwidth, but has increasingly more through-loss at the higher end of its operational range. Because it is a bus technology, there are also significant losses in dividing the signal to deliver it to each location. To compensate for this loss, amplifiers are added to the system to boost the network signals. Time Warner`s typical design limits the number of cascaded amplifiers to six to ensure reliability, clear broadcast video pictures, and digital signals with extremely low bit-error rates. In an average plant this yields a service area of approximately 500 passings. This becomes the company`s basic network building block. In reality, 500 homes is an overall average because the number of homes passed varies from fewer than 30 to more than 600 per mi depending on the location.
These coaxial-bus serving areas are then grouped together by fiber optics in a star configuration. The maximum optical path budget for each leg of the star is typically 6.0 dB at 1310 nm, with the average being closer to 2.5 dB. This limits the diameter of the star to approximately 10 mi. A system that is close to the norm of 100 homes passed per mi will have about 40 nodes.
In the center of the star is a distribution hub that houses the electronics necessary to support the network. To prepare for future needs, six individ- ual singlemode fibers are deployed to each node. This star group then becomes the next network building block (see Fig. 1).
Star groups are connected to each other by fiber-optic transport rings. Fiber counts are based on supporting broadcast video, video-on-demand, high-speed data services such as Road Runner, and alternate access. The electronics in this ring are a mix of analog 1310 and 1550 nm and digital formats such as Asynchronous Transfer Mode and Synchronous Optical Network. There are three to five distribution hubs in these rings, and the average diameter of each ring is 25 to 30 mi.
One of the hubs is designated a main headend and considered the logical origination point. It contains the interfaces to the rest of the network and outside programming and communications sources. A main headend may be the source for two or more rings.
A description of the logic behind the next building block is as follows: The average coaxial-bus serving area has 500 passings, a distribution hub has 40 areas, each ring has one main headend and three hubs, and if a main headend has two rings, the total is 140,000 passings (500 ¥ 40 ¥ 7). This group then becomes the next network building block (see Fig. 2).
Time Warner has 44 divisions across the United States, and most of them are larger than 200,000 passings. The average division will have two or more main headends, each with its own set of secondary rings. These headends are typically tied together with a primary ring using 1550-nm analog and digital technology. The same criteria are used to select fiber counts but on the larger scale. Using these concepts we can design hfc networks to virtually any size (see Fig. 3).
A look at services
Time Warner is in the forefront of testing and deploying most of the new digital services. In its full-service network test in Orlando, FL, the company learned a great deal about what it takes to deliver video-on-demand and related services. The next step needed in this process is the Peagaus Digital services converter, which will be completing beta tests later this year.
Already under way is the Roadrunner High-speed Data Service, which will be available in 10 divisions (representing several million passings) by the end of the year. This service provides off-peak downstream speeds of up to 10 Mbits/ sec and peak speeds that are many multiples of speeds currently available from most telephone companies. Local telephony and personal communication services have also been tested successfully but will probably not be deployed until its use can be cost-justified.
hfc communications networks are typically very asymmetrical. As in most communications networks, the bandwidth that is available is limited by the technology in the last mile; hence, the coaxial cable is no exception. There is a downstream component that has a very large bandwidth that can deliver complex digital and analog signals. The upstream component also has a wide available bandwidth when compared to twisted-pair technology, but it has only 5% of the capacity of the downstream.
Time Warner is in the process of upgrading the majority of its hfc systems to a downstream bandwidth limit of 750 MHz. This allows the company to carry 80 analog television channels and approximately 1 gigabit of digital information. The format can be configured to be unique to each serving area averaging 500 homes. By making some minor network changes, the serving areas could be divided into increasingly smaller areas, allowing up to eight times the original capacity if future applications require it.
The upstream bandwidth is much smaller. The good news is that the demand for upstream capacity is also smaller. Current upgraded systems have an available bandwidth of 35 MHz for each 500-home serving area. Using a robust digital format such as quadrature phase-shift keying, this translates into approximately 45 Mbits/sec per area. By using a similar technique to subdivide each serving area, this capacity may also be multiplied by eight.
Because the upstream portion of the network has historically not been used, there is much controversy over its overall reliability. Time Warner`s success in implementing new services is directly related to the chosen fiber architecture, as well as good engineering practices used to maintain the physical system.
Time Warner has almost 205,000 mi of coaxial-cable plant in the United States as well as interests in several foreign countries. By year-end, approximately half of this plant will be upgraded. The network infrastructure that is being deployed has the reliability and capacity to handle all of the current and future applications that have been proposed to date.
What if someone dreams up something new? Well, there is always dense wavelength-division multiplexing. u
Donald T. Gall is senior staff engineer at Time Warner Cable, Englewood, CO. He is also a member of Lightwave`s Editorial Advisory Board.
