Maximizing Today’s Network

CWDM offers a cost-effective option to increase capacity. Increasing HFC Network Capacity without Additional Fiber Cable operators are continually pressured to increase the capacity of their HFC networks. Deciding which solution to implement to meet this demand depends on the existing network infrastructure, such as operating bandwidth, the size of existing targeted services groups, and number of available fibers. In addition, decisions made today should not only address current needs, but also facilitate a migration strategy for future capacity enhancements. Making the best short-term decision with an emphasis on meeting both medium- and long-term network goals requires an understanding of the drivers creating the demand for ever-increasing network capacity, the potential solutions, and implementation of the ideal solution for an existing network architecture. Drivers Creating demand for increased network capacity are high definition (HD) television, ever-increasing data download speed, business services, voice over Internet protocol (VoIP) and digital simulcast. Availability of HD programming continues to increase, and reports indicate that more than 40 percent of North American households will have at least one HDTV set by 2009. The rate of HDTV set penetration will depend on the number of programmers who will offer HD channels to capture the advertising dollars of companies wishing to reach this growing audience. Going forward, cable operators will be required to offer both standard definition (SD) and HD channels, placing an even greater demand on network capacity. Download speed for high-speed data service has increased steadily because of competitive pressure and market, but with subscribers’ increasing appetite for even greater speed, cable operators will be pressured to offer higher download speeds, which will require additional quadrature amplitude modulation (QAM) channels and channel bonding. We can easily foresee the need to offer 100 Mbps services to stay competitive. The business data and telecommunication services markets represent a tremendous economic opportunity for cable operators, with average revenue per user (ARPU) exceeding that of the residential market. In addition, the business services market is growing substantially faster than the market for residential video, data, and voice services. The delivery of business services over the HFC network requires dedicated capacity for bidirectional Gigabit Ethernet (GigE) services such as emulated local area network (E-LAN), virtual LAN (VLAN), and Internet protocol virtual private network (IP-VPN); performance sensitive services such as voice over IP (VoIP); and Ethernet private line (E-Line). Residential subscribers are also a segment of the VoIP market, further increasing the demand for greater network capacity. Digital simulcast also drives the demand for increased network capacity by requiring cable operators to carry the same content in both analog and digital form simultaneously. Three solutions A variety of solutions to expand network capacity exist, including advanced video compression, 1,024-QAM and analog harvesting, but the three main solutions to increase capacity that are practical today are bandwidth expansion, node segmentation, and the introduction of switched digital video. Bandwidth expansion. Many cable operators began major network upgrades around 1995. At that time, the bandwidth limit was 750 MHz. Only after 870 MHz equipment became available (1998-1999) did cable operators begin new network upgrade projects at 870 MHz. Many of today’s systems are still predominantly at 750 MHz, and obviously, cable operators at 750 MHz are experiencing the biggest short-term challenge to increase network capacity. Today, widely available and field-proven 1 GHz headend transmitters, fiber-optic nodes and amplifiers enable cable operators to increase forward network capacity with relatively simple drop-in procedures, thereby potentially allowing a typical 40 percent increase over current HDTV channel capacity in a lineup. Node segmentation. Assuming that cable operators have declustered their networks by ensuring that each fiber-optic node has its own headend transmitter, cable operators can further increase network capacity by segmenting to reduce service group size. Service group size may range from 250 homes passed to up to 2,000 homes passed. Segmentable nodes enable cable operators to divide service groups by two or, in some cases, by four to essentially repurpose a 1,000 homes passed node consisting of one optical receiver and one optical transmitter to the equivalent of four 250 homes passed nodes by installing and configuring three additional optical receivers and transmitters. Switched digital video. Also known as switched broadcast, SDV is an emerging technology that enables cable operators to create a personalized broadcast viewing experience for subscribers by dramatically increasing the number of programs that can be provided over HFC networks vs. existing fixed broadcast techniques today. SDV takes full advantage of the HFC broadband transmission spectrum by providing dynamic program management to broadcast video signals. Only the programs actively being viewed within a particular service group are transmitted over the HFC network. Unlike VOD, SDV is a multicast delivery technology. Only a single copy of the program is distributed over the HFC network, allowing all subscribers viewing that particular program to share the same program stream. In a VOD delivery system, program streams are not shared; a unique program stream is transmitted to every user. Bandwidth requirements for multicast stream delivery scale in terms of number of programs viewed as opposed to number of viewers. Traditionally, bandwidth expansion was the only option to increase broadcast capacity. On the other hand, node segmentation was the only option to increase capacity for targeted or narrowcast services, although segmentation also offers a commensurate modest increase in broadcast channel capacity. SDV is a new technology being implemented to offer additional broadcast services through targeted or narrowcast methods. Bandwidth expansion, node segmentation and SDV are not mutually exclusive. In fact, combining SDV with node segmentation and managing the right balance of service group size and SD streams enables cable operators to free up half of their existing bandwidth to support additional broadcast services, such as HDTV channels. Implementation Existing bandwidth and service group size are the key factors in implementing a solution that will meet immediate needs while positioning cable operators to meet the demand for increasing network capacity in the future. Table 1 shows the typical cost per home passed for implementing node segmentation and SDV based on service group size as compared to the fixed cost of bandwidth expansion. Node segmentation is the most economical first step to increase capacity for targeted services. Each time service group size is halved, the capacity for narrowcast services doubles. This recommendation holds true until the size of the service group reaches approximately 250 homes passed. The cost of further segmentation beyond 250 homes passed approaches or even far exceeds the cost of increasing the bandwidth of the HFC network. Implementing SDV alone is cost-justified with any service group size greater than 100 homes passed, but to fully meet the need for additional capacity, SDV may be combined with node segmentation. If this combination is determined to meet capacity needs, it is cost-justified provided that the new service group size is 500 homes passed or higher. Whether node segmentation alone or combined with SDV is the best first step, it requires additional optical paths and implies the need to install new optical fiber or to somehow better use existing optical fiber. The problem in many HFC networks is that the existing fiber infrastructure is rapidly becoming inadequate, if not already so. Because of unexpected growth of a service area combined with the demand for expanded services, the spare fibers installed during an initial system deployment often have already been exhausted and are at capacity. Spare fibers may no longer exist to support the deployment of additional residential and business services. New fiber construction is expensive, time consuming, disruptive and prone to governmental regulations. On average, it costs $10,000/mile or more to install new fiber. The time involved in getting permits and rolling out the fiber makes it impossible to compete with other more fiber-rich networks in the same area that can offer more services, resulting in additional losses in customers and revenue. Therefore, node segmentation is economically advantageous only if segmentation can be implemented without the need for installing new fiber. That’s where coarse wavelength division multiplexing (CWDM) technology comes in. CWDM Why is CWDM the ideal solution for outside HFC networks? Deploying 20 nanometer (nm) spaced CWDM wavelengths over existing optical fiber is both cost-effective and robust. CWDM technology is now economically feasible because it is widely adopted, widely deployed, and does not require the special laser temperature controls and expensive wavelength locking circuitry required with dense WDM (DWDM) applications. These same features give CWDM technology the robustness required in outdoor environments. But how can cable operators implement CWDM and maximize existing fiber infrastructure? (See Figure 1.) Because of the demands for increased capacity in place today, a typical HFC network consists of a single fiber between headend and node, with the use 1,310/1,550 nm WDM to multiplex the forward and return paths on one fiber. CWDM technology can transform this basic two-wavelength architecture to a multi-wavelength system carrying up to—and perhaps more than—10 CWDM wavelengths per fiber, including forward and return wavelengths for both residential and business services, all on a single fiber. But implementing a multi-wavelength CWDM solution over one or, at most, two fibers requires considerable forethought. Specific design parameters—based on underlying, multiple-wavelength, optical nonlinear impairments—dictate the need for a wavelength plan that facilitates future expansion with minimal disruption. The ability to identify all optical nonlinear impairments and to manage their impact on the RF spectrum is the key to making multi-wavelength systems work. Based on these physical limitations, different wavelengths are more appropriate for different services. For example, forward path analog CWDM wavelengths could be in the 1,291 to 1,331 nm range; basic residential, analog return path services could be dedicated to either 1,471 and 1,491 nm or 1,591 and 1,611 nm; while business services could be reserved for the four mid-range CWDM wavelengths of 1,511, 1,531, 1,551, and 1,571 nm. Cable operators should devote the appropriate time and effort to develop their wavelength allocation plans today to maximize capacity for additional services over existing fiber when needed. Plan for growth Subscribers’ insatiable appetite for more will continue: HDTV channels, more channels, faster Internet access, VOD, VoIP. Their demands for increased capacity will drive continued network capacity expansion. How cable operators react today will set the stage for how they position themselves to address the ever-increasing need for network capacity tomorrow. Implementing a future-proof, sustainable network through the strategic implementation of CWDM technology is an economically feasible option that will enable cable operators to maximize the initial capital expenditure for years to come. Jeff Sauter is chief technologist, and Bill Dawson is VP, Access Strategy, both for C-COR. Reach them at [email protected] and [email protected].