In our last column, we talked about the regional headend concept and some of its implications. This month, we discuss a real-life example of a regional headend and the associated fiber network now being deployed in Chicago.
Today, AT&T is in the process of transforming Chicago's patchwork of cable systems into one of the largest broadband telecommunications networks in the nation, perhaps in the world. This massive network is being assembled from TCI's original Chicago-area network, which serves about 35% of the total market, plus a collage of smaller networks the company has acquired from other operators during the past several years.
The largest addition to TCI's core network comes from MediaOne, which accounts for approximately 25% of Chicago's cable subscribers and encompasses about 4000 plant miles. Other pieces have been added from the local holdings of Time Warner, Jones InterCable, and several other operators. In total, the combined network will boast nearly 25,000 network miles that pass 2.9 million homes spread out across an area that extends around Lake Michigan from Wisconsin to Indiana.
One of the challenges AT&T faces is to integrate all the different network segments that have been deployed over the past decade with very little, if any, coordinated effort among the different operators. This integration requires a huge commitment in pre-engineering and ultimately several thousand miles of fiber-optic plant. Such an effort is now underway in Chicago, headed up by Dan Murphy, executive director of engineering, and Roy Boylan, director of advanced technologies. Most of the details on the project were provided by Tom Arvidson, who manages many of the headend/hub projects for the Chicago-area system.
The network being deployed in Chicago will include a primary digital fiber-optic ring that will have two headends and 11 primary hubs, plus a large number of secondary analog rings that will connect the primary locations to 121 secondary hubs. The secondary hubs will use a fiber star configuration to distribute signals to the node serving areas.
The two headends will house all of the satellite receivers, the connections to the local broadcast channels, the master library for pay per view and eventually video-on-demand (VOD), and interconnects to the Internet backbone. The headends will be a mirror image of each other for redundancy and load sharing as necessary. The headends and the 11 primary hubs will be connected with ADC Telecom's DV-6000 digital optoelectronics.
The primary hubs will have access to the digital ring and function as the origination points for all the secondary 1550-nm analog fiber-optic rings. Although they will not have satellite capability, the primary hubs will support the digital video (HITS) sources and house equipment to support narrowcast services such as high-speed data, telephony, digital advertising, and eventually VOD. The physical size of the primary hub buildings will range from 5000 to 10,000 sq ft.
The primary ring, which Arvidson expected to be 90% completed by the end of July, will have a minimum of 24 fibers. The fibers will be configured using both network path and electronic diversity to develop a self-healing ring. The initial electronic deployment will use eight groups of 2.4-Gbit/sec data streams. Each digital stream is multiplexed using dense wavelength-division multiplexing (DWDM) onto two of the 24 fibers in the primary ring. The remaining fibers will be used for future services as needed. The ring is approximately 150 mi (240 km) long. Each 2.4-Gbit/sec data stream is divided into 16 groups of 140 Mbits/sec each. This setup gives the network an initial capacity of 128 digital video or data channels. The ADC equipment can support 8- or 10-bit video, 100Base-T Ethernet, or data formats up to a DS-3 level.
The primary headends and hubs break the Chicago area into service areas that range in size from 120,000 to 300,000 passings each, with the variation mainly due to changes in density across the region. The average density of the Chicago area is approximately 120 homes passed per mile but can vary from under 20 homes passed in the rural suburbs to well over 600 in the urban areas.
Each headend/hub will have from 8 to 12 secondary hubs attached, via 1550-nm analog fiber-optic rings. Each of the more than 120 secondary hubs will serve from 20,000 to 50,000 homes passed, again depending on the density of the homes passed in the area.
Each secondary hub is part of a secondary ring system that has a mix of broadcast analog and narrowcast digital fiber-optic electronics. Broadcast video services are transported as analog because it is the simplest and most cost-effective way to distribute those services.
The secondary rings also support the analog return circuits. These return signals are digitally modulated analog carriers that are collected from the node serving areas via the star segment of the fiber distribution network. The return from each serving area is converted from an optical to electrical RF, then combined into groups of eight. These groups of eight are then optically modulated and combined using DWDM equipment to four groups of eight or 32 serving areas per fiber. The combined total is then transported back to the primary headends/hubs for processing.
In its Chicago systems, most of which had been originally built with 450-MHz equipment and spacing, MediaOne has been deploying a coaxial bus architecture for the node serving area, which the company's engineering group has dubbed FAST (Fiber Asymmetric Serving-area Technology).
In a FAST deployment, existing trunk and bridger locations are grouped into threes, which support a maximum coaxial amplifier cascade of four devices, preserving most of the existing electronic locations and coaxial cable. The fiber serving area, which typically serves about 600, is split; the center location is designated the "A" node and the other two locations become "B" nodes. The fiber feeding the downstream network is split within the serving area to feed all three nodes, each of which serves about 200 homes. The return signals from the B nodes are routed on the existing coaxial trunk cables to the A node, where the three locations are combined and fed to an optical transmitter. The combined signal is then transported back to the hubs.
In addition to the obvious advantages in labor and materials, the FAST architecture solves some of the bottlenecks in managing the upgrade in such a huge system. On average, it yields a node serving area of around 600 homes passed and employs six fibers to link each FAST node area to a secondary hub. Forward optical transmitters are 1310-nm distributed-feedback (DFB) lasers. The return transmitter in the optical A node used a modified Fabry-Perot laser in the past to allow a better dynamic range, but the system is migrating to DFBs, since the cost of DFB lasers has dropped sufficiently enough to be affordable.
Arvidson's comments suggest that MediaOne and TCI are exchanging ideas about hybrid fiber/coaxial-cable architectures, including possible wider deployment of TCI's more aggressive use of DWDM and/or MediaOne's FAST approach. Arvidson adds, however, that this collaborative process is not slowing the pace of upgrade activity, which he says is moving forward at "full warp speed" throughout the Chicago area.
This intense upgrade activity, jokes Arvidson, has turned the Windy City into "contractor heaven." With only about 25% of the area's distribution plant upgraded as of mid-1999 and AT&T hoping to complete the project by the end of 2001, this intense demand for construction labor (and network equipment) is likely to continue for the next few years.
Today's consolidation and upgrade activity in Chicago is the culmination of a process that began earlier in the decade, driven mainly by the imperatives of the advertising insertion business. As fiber-optic advances paved the way for additional narrowcast data services, this clustering trend has intensified in Chicago and virtually all other major markets.
The Chicago upgrade project stands as a prime example of both the opportunities and challenges facing the cable industry. The opportunity is to position cable as the dominant provider of broadband services and targeted advertising in virtually all of the nation's major markets. The challenge is to get all the system swaps and plant upgrades done before competitors can widely deploy competing broadband platforms.
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 firstname.lastname@example.org
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 email@example.com or via the P&A Web site at broadbandfuture.com.