Cabling is to a voice/data/video network what arteries and veins are to your circulatory system.
Just as your arteries and veins provide paths for blood flow throughout your body, the cabling in a telecommunications system provides the pathway for voice, data, and video signals. But unlike the unique nature of your arteries and veins, there are a variety of cables you can choose from in designing and constructing a customer-owned outside plant (CO-OSP) system. And depending on what your customer's needs are, selecting the right type of cable could be the most important step in the telecommunications design process.
In the fourth installment of this 12-part series on CO-OSP, we'll review the three most popular cable types used in designing a telecommunications system: coaxial, optical fiber, and twisted-pair.
By definition, a coaxial cable consists of two metallic conductors sharing the same axis. Coax has a metallic center conductor, coaxially positioned within an outer metallic conductor and separated by a dielectric (nonconducting) material. Coax is capable of delivering full motion video, digital and analog data with full duplex transmission, and voice over long and short distances.
Coaxial cable is available in many different jacket and armor configurations. While these configurations are dramatically different in appearance, the cable's impedance and electrical performance will not vary, given the same size cable.
There are three basic coax cable configurations available for aerial applications (Fig. 1 right).
This is the simplest type of coaxial cable. The cable consists of a seamless aluminum tube (outer conductor), foam dielectric, and center conductor, which is usually made of copper clad aluminum or steel. This cable is best used in moderate climates.
This is the same as bare aluminum cable with the exception that it's encased in a high molecular weight polyethylene outer jacket. It's best used in hostile climates and offers protection from salt oxidation as well as ice.
Identical to jacketed cable, this design has an additional supporting strand wire fused to the outer jacket that eliminates the need to separately lash the cable to a separate strand wire. Although this type of cable lowers overall plant construction costs, it inhibits the future possibility of overlashing a second cable onto the existing cable plant.
There are two types of coax configurations available for direct-buried applications.
This is a jacketed cable with a “flooding” compound inserted between the jacket and the aluminum outer conductor. The flooding compound offers extra protection from nicks and tears to the outer jacket caused during the construction process.
This type of cable is similar to a flooded-type cable design, but it has the added protection of a metallic armor jacket and an additional layer of flooding compound underneath the outer polyethylene jacket (Fig. 1). The “armor” provides additional protection from the construction process and helps combat rodent damage and cuts from digging and excavation.
In addition to these mechanical factors, there are six electrical characteristics you must consider when choosing a coaxial cable: capacitance, impedance, velocity of propagation, direct current resistance, radio frequency (RF) attenuation, and structural return loss. The two most pertinent factors that change from one cable size to another are DC resistance and attenuation.
Cable manufacturers' literature usually lists DC resistance in one of three ways: center conductor, outer conductor, and loop resistance. This information, along with cable lengths and amplifiers, is valuable when calculating the power requirements of the network. You should perform power calculations after the plant layout is complete using the cable DC “loop” resistance specification.
Attenuation is dependent upon the size of the cable, the dielectric material used, and the frequency of the system. In general, the higher the frequency, the greater the attenuation. For a given dielectric, the larger the cable OD, the lower the attenuation. Manufacturers list attenuation in dB/100 ft of cable, dB/1,000 ft of cable, or dB per meter. Attenuation is arguably the key factor you must keep in mind when working with coaxial cable. It determines how often the signal has to be amplified in the network.
The subscriber service drop is the last and most important piece of any coax network. Drop cable differs from semirigid or hard-line cable in that it is much smaller, more flexible, and easier to handle. It also has higher attenuation. Drop cable is similar to semirigid in its mechanical makeup with a few exceptions. Its outer conductor isn't a thick seamless aluminum tube, but a thin, flexible, aluminum foil. The foil is wrapped with aluminum braid, which provides an extra layer of shielding. There is also a super shield- or quad shield-type drop cable, which offers an additional foil wrap around the aluminum braid and a second aluminum braid around the second foil wrap.
Optical fiber cabling.
Optical fiber is often used as a backbone between campus buildings to transmit voice, video, data, CATV, and security and fire alarm signals because of its ability to serve several logic protocols and topologies. It's also popular because of its:
Capability to transmit signals over long distances.
Ability to handle high data rates.
Immunity to lightning, electromagnetic interference (EMI), radio frequency interference (RFI), and crosstalk.
Lack of a grounding requirement, as it's an all-dielectric cable.
A backbone that comprises multimode and singlemode fiber is often recommended to satisfy present and future needs.
Optical fiber cable is categorized by its buffering mechanism and function. The two types of buffering mechanisms are known as loose-tube and tight-buffered. Loose-tube cables are constructed so the fibers are decoupled from tensile forces the cable may experience during installation and operation (Fig. 2 on page 48). Loose-tube cables are generally more robust than tight-buffered cables, making them a good fit for outdoor applications. They are also less expensive than indoor cables, specifically at fiber counts above 24. Loose-tube cables are available in armored constructions for use in direct-buried applications, all-dielectric constructions for use in aerial and underground applications, and riser-rated constructions for use in riser applications.
Tight-buffered cables are typically designed with a 900-μm coating applied directly to the fiber strands. They are usually more sensitive to adverse temperatures and outside forces than their loose-tube counterparts, but are desirable because of their increased flexibility, and in the case of low fiber-count cables, for their smaller bend radius and easier handling characteristics.
Tight-buffered cables are typically available in a distribution design and breakout design (Fig. 2). Distribution design cables use one jacket to protect all the tight-buffered fibers. Conversely, breakout design cables use an individual jacket to protect each tight-buffered fiber. Distribution design cables are recommended for typical installations because of their lower cost and smaller diameter.
Twisted-pair copper cabling.
In interbuilding backbone environments, twisted-pair copper cabling can be used between buildings to transmit and receive voice, data, security, and fire alarm signals. The three key factors to consider when designing a twisted-pair copper cable system are:
The resistance of the cable in ohms.
The signaling limits of the telephone and terminating device switch in ohms.
Traditional copper cable selection is based on resistance design of the cable. Use a resistance design worksheet to determine the appropriate size cable required for your particular project. Copper cables are typically available in as many as four sizes: 19, 22, 24, and 26 AWG. GACAN limits usage to no more than two cable gauges.
The second factor to address in loop design is the transmission signaling limits of the switch. These limits are based on the switch's ability to deliver signaling at various loop lengths. Earlier switch platforms could service loops that exhibited resistance values as high as 1,300 ohms. However, as switch technology evolved, the resistance design limits increased to 1,500 ohms, 1,800 ohms, and 2,100 ohms. This allows you to design systems with greater loop lengths.
The third factor impacting loop design is loading. A load is a device designed to counter the effect of capacitance buildup in loop lengths greater than 18,000 ft. The capacitance buildup within a twisted-pair copper cable is the primary reason for the manufacture of low-capacitance cable.
The Insulated Cable Engineers Association, Inc. publishes cable standards to encourage quality and uniformity in the manufacture of telecommunications cables. Even though the standards don't cover all specifications for cable design, they do cover mechanical and electrical requirements.
In next month's installment we'll take a look at cable hardware selection, including splices and splice tray enclosures.
The material for this article was excerpted with permission from BICSI's Customer-Owned Outside Plant Design Manual, Second Edition.