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Broadband Cabling for the First Mile

The drive to provide true broadband connections to all consumers is gaining momentum. A major technology breakthrough is the emerging efm standards being developed by IEEE 802.3.


September 1, 2002  


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The recent decline in the telecommunications marketplace has masked a large opportunity for optimism during the next decade. Recent articles in trade publications have discussed the glut of backbone capacity in the public network. What isn’t mentioned is that the reason there is a glut, is the lack of a broadband pipe from the home or small business to the backbone.

Consumers aren’t able to access the full capacity of the public network. Part of the problem is how broadband is defined. To many, including the government, a 500-kilobit per second DSL connection is broadband.

This definition doesn’t begin to address the requirements future applications will have for the telecommunications network. Most industry experts I have consulted with feel that a 100 Megabits per second (Mbps) connection will be desirable. Some of the bandwidth drivers include high-speed data, IP voice, interactive gaming and most of all, video. As expectations for picture quality become higher, so does the required bandwidth. A standard TV channel needs approximately 2 Mbps of throughput. If DVD quality digital TV is specified this moves to 8 Mbps. For HDTV, 18 Mbps are needed.

The only large-scale service providers possessing high bandwidth, bi-directional capability to the premise are cable television operators. Their Hybrid Fiber Coax (HFC) networks have large downstream capacity for voice, video and data.

The upstream capability is more limited, but adequate for voice and current cable modem data rates and market penetration.

Although technology improvements will improve the bandwidth efficiency of all media, it’s easy to see that upgrades will be needed to transport significant video traffic over current telco networks. The telcos have some limited Fiber to the Home (FTTH) or Fiber to the Curb (FTTC) installations but the vast majority of their last mile network is voice grade twisted pair cabling.

The capital cost to upgrade these networks to FTTH architecture is very high, estimated at over $100 billion for the North American market.

Some municipalities, utilities and competitive carriers are focused on overbuilding current networks with high bandwidth fiber or HFC networks and are successfully competing today.

This article will review the current state of First Mile Cabling and discuss some of the technology driving innovation in this segment.

NETWORK ARCHITECTURE

For CATV providers the HFC model has been adopted almost universally. Most systems have been upgraded to 750 MHz with some at 860 MHz. Optical fiber is run from the head end to the fiber node. Between 500 and 1500 premises are served by each node.

Coaxial distribution cable is used to take the signal from the node into the neighborhood or Multiple Dwelling Unit (MDU). Drop coaxial cable is tapped into the distribution cable and connected to the end user premise. Figure One shows a typical architecture as deployed by Cox Communications Inc. of Atlanta, a full-service provider of advanced video, voice and data services for homes and businesses.

Telco operators have deployed fiber deeper into the network but the majority of cabling in the first mile is voice grade twisted pair. Fiber links the Central Office (CO) with the long haul backbone network.

Fiber is then deployed to a remote terminal with twisted pair connecting to the premise. There have been some large-scale trials for Fiber to the Curb (FTTC), which serve between four and 12 homes from a fiber node, and Fiber to the Home (FTTH), which brings optical fiber all the way to the side of the house. It appears that for new “green field” installations FTTH or FTTC are economically viable.

This has been demonstrated by over builders and municipalities that have successfully deployed this technology.

A major technology breakthrough is the emerging Ethernet in the First Mile (EFM) standards being developed by IEEE 802.3. This is designed to bring up to a Gigabit per second (Gbps) of capacity to each premise. There are three main standards under development. One is designed to use existing copper twisted pair cable.

The capacity for this technology is 10 Mbps for up to 2,500 feet. The Ethernet structure is overlaid on existing DSL technology. While it is designed to work on voice grade copper, no load coils can be present on the pairs. There are two single mode optical fiber proposals designed to deliver 1 Gigabit per second over a 10-kilometer distance.

Optional 100 Mbps versions are also under development. The proposals utilize a switched, Point to Point (PTP) or curb switched network, or a Passive Optical Network (PON). Figure 2 (see p. 22) outlines these systems.

These proposals bring fiber to the home although it would be possible to maintain these throughput rates with a curb switched FTTC type system. Both fiber optic systems have their advantages and should be considered prior to an installation.

HIGH BANDWIDTH CABLING MEDIA

There are many options for providing a high capacity information pipeline to the premise. The cabling systems can be broken out into four main categories: optical fiber, data grade twisted pair, coaxial cable and hybrid cables of fiber, twisted pair and coaxial cables.

Data grade twisted pair cabling is the most widely used media for LAN applications in the workplace. Category 5E and 6 compliant channels are capable of transmitting up to 1 Gbps of throughput up to 100 meters in distance. Some systems utilizing Category 6 cabling to the home have delivered 100 Mbps of information at distances up to 600 feet.

These twisted pair products have been deployed to the premise in combination with a fiber backbone. The cables perform to the same electrical requirements as indoor premise cables but are beefed up for outdoor installation. They are filled with a water blocking compound to prevent ingress of water and are coated with a tough, UV resistant polyethylene jacket.

Armor or double jackets are used for some applications such as direct burial or over lashing. The cables are terminated at the home with the appropriate category components in weatherproof enclosures.

Coaxial cable is the choice for most HFC networks. With the increasing demands of modern networks the cable requirements are becoming more rigorous. New networks require two-way communication, not just broadcast to the premise. Return signals are combined at the node and if proper cabling is not used, excessive noise ingress can create transmission errors.

Semi rigid 75-ohm “hard-line” feeder cables are utilized to go from the fiber node to the service area. Flexible braided drop cables tap into the feeder cable and are attached to the customer premise. Most cabling problems occur with the customer drop and the proper cable must be specified. Cable certified to a minimum of 1 Gigahertz of bandwidth must be used.

The minimum return loss requirement for this application is -20dB. Shield construction is also important. A minimum construction using a single laminated foil and braid should be specified. The braid must be certified to 60 per cent coverage. Preferred for most applications due to improved noise rejection are tri-shield and quad-shield constructions. Outdoor rated jacket materials and corrosion resistant designs will provide longer life and improved reliability for the network. Weather sealed F-connectors should be used at the tap and for the connection to the customer location.

FTTH architectures are becoming more economically feasible for broad use in “green field” new installations. In these cases there is not an installed base to consider when evaluating the payback for the cabling investment. Regulated carriers project that within the next 18-24 months, FTTH will be their architecture of choice for new builds. Overbuilds of aerial plant will also start to utilize FTTH, as needed, for high bandwidth customers. Overbuilding underground plant will be more difficult to justify economically but that hurdle must be overcome in order to provide universal service.

Optical drop cable designs typically utilize central tube constructions. Design criteria are similar to standard outd
oor plant constructions but scaled down for the shorter spans and lower fiber counts utilized. The Insulated Cable Engineers Association (ICEA) is developing a standard for an optical drop cable. Installation loading is rated to a maximum of 300 pounds force as compared to the 600 pounds force specified for standard outside plant cables.

Figure 3 shows a cross-section of an optical drop cable design. All dielectric or armored constructions can be specified. For aerial installations a messenger wire is included.

As seen in Figure 3 in some installations hybrid cables are useful. In the optical fiber and twisted pair composite shown in the diagram the copper pairs were used for network powering. Hybrid cables consisting of coaxial, twisted pair and fiber have been used in various applications.

In some cases, the fiber is left dark and the copper is used in the interim period before lighting up the glass. Installation consumes a large part of the budget for a network and only installing cable once can save significant amounts of money in the long term.

The drive to provide true broadband connections to all consumers is gaining momentum. Significant legislation is being debated which may accelerate the process. To provide the 100 Mbps per second pipeline considered necessary for true broadband, significant investments in network cabling are needed. New architectures and cabling designs will be required.

There will be significant opportunities for telecommunications professionals to help develop and implement this technology. The edge of the public network will be an area of significant growth for the next decade.

Rob Wessels is Vice President of Engineering for CommScope’s Network Division. He has held a variety of engineering management positions at CommScope including CATV, Network and Wireless during the past twelve years. Wessels received his Bachelors of Engineering degree from The Georgia Institute of Technology and an MBA from Georgia State University. He is active in TIA TR-42 and holds seven patents in the field of telecommunications cable design, and also serves on the advisory board for the School of Engineering at the University of North Carolina-Charlotte.


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