An important concern to users of premises data communications facilities is the type of media needed to connect the interfaces of the data equipment. There are a number of candidates. From the perspective of the user the media choice is influenced by the distance between the data equipment, the desire of immunity to interference and of course cost.

The user is blessed with a number of different media candidates. Topping the list are multi-conductor data, twisted pair, coaxial, and fiber optic cable. Wireless (radio) is also a possibility which is of increasing interest. However, space limitations do not permit any discussion of it.

Data cable is employed when equipment is very close together, within 50 feet. For longer distances, especially beyond several hundred feet, the media alternatives are limited to twisted pair, coaxial and fiber optic cable. All of these media types are discussed below. Undoubtedly, twisted pair is the most widely used. Often we will make comparisons to it.

The reader may find this chapter "overly technical" relative to his/her background and/or needs. If this is the case, my advice is to jump directly to Chapter 4. While you will miss my excellent writing I will keep you interested in reading.


When data equipment is very close together, within 50 feet, it can be connected together using EIA-232C cable. This is usually multi-conductor with 25 conductors. Each conductor is dedicated to a different pin of the EIA-232 interface. In most premises data communications applications only a fraction of the 25 EIA-232C pins are needed. For such applications there is RS-232C data cable available with fewer connections e.g. 12. The data cable comes in two types of packages, ribbon and jacketed. The ribbon version should only be employed when the data equipments being connected are a few feet apart. The jacketed version is used for the longer distances. It is generally shielded to reduce interference and has strain relief to relax inadvertent tension in making a connection.

There is a higher quality version of EIA-232C data cable to connect together equipment separated by several hundred feet. This higher quality cable is referred to as low capacitance, extended distance, EIA-232 cable. Its cost is generally five times the cost of ordinary EIA-232 ribbon cable.

When data cable is purchased it usually comes with the EIA-232 DB25 connectors at the terminations.


Twisted pair cable is exactly what its name represents, two conductors of wire which have been twisted together in order to limit the effects of stray capacitance and cross-talk from adjacent cables. There are usually two twists per foot. It is the least expensive of the connecting media types and the most widely used. There is a simple explanation for this. Generally Unshielded Twisted Pair (UTP) cable is already installed in most buildings or plants, having been put in for other uses. It has its origin in telephony and is familiar to most people as the wire between their phone and the wall jack. Shielded Twisted Pair (STP) exists but it is far more expensive. STP emerged in the early 1980’s when IBM began to use it as an alternative to coaxial cable for their Systems Network Architecture (SNA). It does provide "better performance" than UTP with respect to protection from interference. But, this is only achieved if the shield is properly grounded. If this is not done what you are left with is a more expensive cable which actually yields performance degraded relative to UTP. It suffices to say that many users often do not properly ground the shield.

Connecting communication devices with twisted pair is easy with little in the way of a termination problem. It can support most data rate requirements of premises data communications systems. In fact, recently on short runs, twisted pair has been demonstrated to support transmission rates as high as 10 megabits per second.

Twisted pair conductors come in a number of different "flavors" corresponding to different wire gauges. These are 19AWG, 22AWG, 24AWG and 26AWG. However, 24AWG and 26AWG are most common. The wires are stranded or solid and each wire has a solid or multicolor covering.

As noted in [Ref.1] when choosing the type of twisted pair cabling to employ one needs to consider the following electrical attributes, capacitance (both self and mutual) characteristic impedance, attenuation and velocity of propagation. Such properties of the media are a result of the construction, jacket, insulation, shielding and center conductor. The implications of these parameters are summarized below.

The capacitance measures the electric field energy stored in the dielectric between the conductors of the twisted pair. It is determined by the cable’s dielectric,length and the inter-conductor spacing. Too much capacitance causes signal distortion. There is a rounding of the edges of an originally rectangular shaped data or control signal. This deleterious effect can degrade performance by causing intersymbol interference.

Characteristic impedance measures the resistance to current in the wire. It is a function of frequency as well as the electromagnetic material properties of the cable. Good engineering practice usually dictates a constant characteristic impedance over the entire premises. This minimizes energy loss and makes data communications dependable.

Attenuation is a measure in decibels (dB) of the decrease in signal strength along cable length. Too great an attenuation may reduce the received signal to noise ratio to an unacceptable level. An intolerable received bit error rate may then result.

The velocity of propagation is the speed of an electrical signal on the cable. It is often expressed as a percentage of the speed of light in a vacuum. Its reciprocal is delay. Too great a delay may ultimately impact transactional response time in the higher level applications.

Table 3 summarizes typical physical parameters for twisted pair cable obtain from well known commercial sources. Table 4 displays important parameters of twisted pair cable as a function of frequency.

Typical Physical Parameters for Twisted Pair Wiring Cable, Type 3 Cable and Plenum Grade Cable

Manufacturer Part # Number of Pairs AWG/Strandling Capacitance in pF/Ft. Nominal dc resistance in ohms/1000' Grade
Belden *1154A 4 24/solid 15 25.7 N
Belden *1154A 4 24/solid 15 25.7 P
AT&T *2082
component #1
4 24/solid 11.1 25.7 P
AT&T *2082
component #2
4 24/solid 15.9 25.7 P
AT&T DIW 4/24 4 24/solid 17.5 25.7 N
AT&T 2001 004D 4 24/solid 16 25.7 P
IBM spec Type 3 4 22 or 24/solid n/a 28.6 n/a

Table 3

Physical Parameters of Unshielded Twisted Pair Cable Obtained From Well Known Commercial Sources

Manufacturer Part # Velocity of Prop. Attenuation in dB/100' at Nominal Characteristics impedance in ohms at
1 kHz 256 kHz 1 MHz 1 kHz 256 kHz 1 MHz
Belden *1154A 0.60 n/a 0.27 0.64 n/a 105 105
Belden *1155A 0.60 n/a 0.27 0.64 n/a 105 105
AT&T *2082
Comp. #1
n/a 0.035 n/a 0.48 700 n/a 130
AT&T *2082
Comp. #2
n/a 0.044 n/a 0.66 550 n/a 80
AT&T DIW 4/24 n/a 0.046 n/a 0.64 600 n/a 105
AT&T 2001 004D n/a n/a n/a 0.30 n/a n/a 100
IBM spec Type 3 n/a n/a 0.40 0.80 n/a 90-120 84-113

Table 4

With twisted pair there are four major sources of performance deterioration: noise, distortion, attenuation and crosstalk [Ref. 1]

Excess noise comes from two sources radio frequency interference (radio and television transmitters) and electromagnetic interference (fluorescent lights, arc welders, motors).

Distortion is caused by capacitance and increases with the cable length over which a signal is transmitted.

Excess noise coupled with significant attenuation is a serious problem with twisted pair cable. Long cable runs can realize a low gain antenna and "pick up noise." This coupled with excess attenuation can seriously degrade performance.

An interfering signal in a twisted pair that originates in another pair is termed "crosstalk." The number of cable twists per foot and the dielectric have a significant effect on the amount of crosstalk. The tighter the twists the lower the crosstalk and the more limited its degrading effect.


Coaxial cable consists of two cylindrical conductors, one placed concentric within the other, but separated by an insulator. Figure 8 illustrates the anatomy of coaxial cable. As compared to twisted pair, coaxial cable has a much higher signal bandwidth. It can, as a consequence, support much higher data rates. When it is used in a single ended system, that is, when the outer conductor is grounded, there is an inherent shielding capability which unshielded twisted pair does not have. This shielding capability makes transmission immune to Electro-Magnetic Interference (EMI), Radio Frequency Interference (RFI) and other forms of interference. It provides more reliable communication but is generally more costly than twisted pair. On average it is four times more expensive. Also connecting devices to it (the termination problem) is much more difficult.

Figure 8: Detailed Illustration of Segment of Coaxial Cable

An excellent detailed discussion of the performance characteristics of coaxial cable is provided in [Ref.2]. Particular elements are summarized below.

As illustrated in Figure 8 coaxial cable is a class of cable that is best characterized as having several layers of material surrounding a common axis. A center conductor (solid or stranded) is surrounded by dielectric, or nonconductive, material and then shielded. This shield is often a wire braid or foil jacket, and is covered by an abrasion-resistant jacket. For outdoor applications, the spaces between the dielectric shield, and outer covering can be filled with an inert, waterproof gel for extra protection. This would also be done for buried cable. The dielectric material and outer jacket may be made of plenium or non-plenium material (often Teflon or polyvinyl chloride, respectively).

The cable thickness as well as the composition of the dielectric material, and construction techniques determine the signal properties of a coaxial cable. As with twisted pair, four parameters determine signal quality: characteristic impedance, mutual capacitance, attenuation and velocity of propagation. DC resistance is sometimes mentioned as a fifth parameter. Tables 5 and 6 [Ref.2] provide the electrical and physical characteristics of various types of coaxial cable.

Electrical Characteristics of Various Types of Coaxial Cable Obtained from Well Known Commercial Sources

Manufacturer Part # Cable Designation DC resistance ohms/100' Nominal impedance in ohms Attenuation in dB/100 feet at N MHz
1 5 10 50
Belden 9880 10BASE5, E'net 14.2 50 0.19 0.37 0.52 1.70
Belden 9880 10BASE5, E'net 14.2 50 0.19 0.37 0.52 1.70
Manhattan M4180 Thick Ethernet 14.2 50 N/A N/A N/A N/A
Carol C1154 Thick Ethernet 18.7 52 N/A N/A N/A 1.3
Carol C1152 Thick Ethernet 18.7 52 N/A N/A N/A 1.6
Carol C5015 Thick Ethernet 12.4 50 N/A N/A N/A 1.1
Belden 9907 10BASE2, E/net 95.0 50 0.43 N/A 1.30 2.91
Belden 89907 10BASE2, E/net 95.0 50 0.43 N/A 1.30 2.91
Belden 8259 10BASE2, E/net 108.0 50 0.44 N/A 1.4 3.3
Belden 9201 10BASE2, E/net 101.0 53.5 0.33 N/A 1.2 3.1
Carol C1174 10BASE2, E/net 101.8 53.5 N/A N/A N/A 3.1
Carol C1172 10BASE2, E/net 88.8 50 N/A N/A N/A 3.2
Carol C1163 Arcnet 550.0 93 N/A N/A N/A N/A
Belden 9268 Arcnet 417.0 93 0.25 N/A 0.85 1.9
Manhattan M4276 Arcnet 412.0 93 N/A N/A N/A N/A

Table 5

Physical Characteristics of Various Types of Coaxial Cable Obtained From Well Known Commercial Sources

Manufacturer Part # Cable Designation Weight in lbs./500 ft. AWG Stranding Nominal OD (inch)
Belden 9880 10BASE5, E'net 14.2 50 0.19 0.37
Belden 89880 10BASE5, E'net 14.2 50 0.19 0.37
Manhattan M4180 Thick Ethernet 14.2 50 N/A N/A
Carol C1154 Thick Ethernet 18.7 52 N/A N/A
Carol C1152 Thick Ethernet 18.7 52 N/A N/A
Carol C5015 Thick Ethernet 12.4 50 N/A N/A
Belden 9907 10BASE2, E/net 95.0 50 0.43 N/A
Belden 89907 10BASE2, E/net 95.0 50 0.43 N/A
Belden 8259 10BASE2, E/net 108.0 50 0.44 N/A
Belden 9201 10BASE2, E/net 101.0 53.5 0.33 N/A
Carol C1174 10BASE2, E/net 101.8 53.5 N/A N/A
Carol C1172 10BASE2, E/net 88.8 50 N/A N/A
Carol C1163 Arcnet 550.0 93 N/A N/A
Belden 9268 Arcnet 417.0 93 0.25 N/A
Manhattan M4276 Arcnet 412.0 93 N/A N/A

Table 6

To get a feel for the meaning of these parameters observe Figure 9. It illustrates the combined effect of attenuation and capacitance on the transmitted signal at some distance from the source.

Figure 9: Illustration Showing Mutual Capacitance and Attenuation Significantly Corrupting a Transmitted Signal on Coaxial Cable

Determining the correct coaxial cable type for your application can be confusing. Several dozen types are available. The coaxial cable types most often encountered include RG-58, RG-58A, RG-58A/U, RG-58C/U, 802.3 thick and thin ETHERNET cable, RG-8, RG-8/U, RG-59,RG-62,RG-62A and RG-62A/U, all in polyvinyl chloride and plenum equivalents.

Many cable types, such as RG-58C/U, also meet military specifications, ("mil-specs") and may cost more due to the required certification testing.

Proper reference is made to [Ref. 2] as the original source of the figures and tables of this section.


Fiber optic cable consists of a strand of glass or plastic. A gradient in the refractive index across the diameter of this strand allows the cable to act as a waveguide for light. This provides the essential communication connectivity. Figure 10 [Ref.3] shows the details of the structure of fiber optic cable.

Fiber optic cable is a cylindrical structure made of a dielectric material transparent in the visible and near-infrared region of the spectrum. This structure consists of an inner region called the "core," where light essentially propagates. It is surrounded by a region called "cladding." This outer region has a refractive index smaller than that of the core. It is this constraining condition which allows light to confined in the core region.

The cladding also serves another function. It provides mechanical protection for the core. However, additional outer protection is usually needed for practical handling. Such additional protection typically consists of plastic material.

The basic material for the cladding is silica (SiO2). For increasing the core refractive index, germania (GeO2) is usually added to silica, while fluorine is typically added to decrease the silica refractive index, when needed.

There are many different fiber optic cable types. Fortunately, they can be conveniently grouped into three broad categories, step-index multimode fibers, graded-index multimode fibers and single mode fibers.

Figure 10: General Structure of Fiber Optic Cable

Step-index multimode fibers are characterized by a homogeneous core region with a constant refractive index. As to be expected they are surrounded by a cladding of lower refractive index. Step-index fibers were the first exceptionally low loss fibers to be developed. They have sizes of around 80 um for the core diameter, and 110 to 150 um for the cladding diameter. To date they have not found application in telecommunications outside the laboratory and have not been standardized.

Graded-index multimode fibers exhibit a refractive index profile n(r) (r is the radial coordinate with origin at the core axis). This features a gradual increase from a minimum value n(a) at the core-cladding boundary (generally coincident with the cladding value) to a maximum value of n(0), generally found at the core center. The main advantage of graded-index over step-index fibers is the greatly enhanced bandwidth. This is due to the fact that the speed of the different modes can be almost equalized by suitably shaping the index profile.

Single-mode fibers correspond to the core diameter of the optical fiber taking values comparable to the wavelength of the radiation to be propagated. Reaching this condition allows only the fundamental mode to be guided along the fiber. The most spectacular advantage of single-mode fibers is the increase of bandwidth due to the absence of modal dispersion. The bandwidth limiting factor is essentially constituted by chromatic dispersion which in ordinary step-index profiles has zero value in the region around 1.3 um. As a consequence, with suitably narrow line width sources, the total bandwidth might be two or three orders of magnitude greater than in multimode fibers. Single-mode fibers have other advantages: lower loss, great upgrade capability in view of future ultra-high bandwidth transmission, easy and accurate system design, and compatibility with integrated-optics devices. For these reasons at longer distances than those used in premises data communications single-mode fibers are the only fiber type used. Of course, multi-mode fibers are perfectly acceptable for premises distances.

Essentially, fiber optic cable has a much higher bandwidth than both coaxial cable and twisted pair. Therefore, it has the ability to support much higher data rates. As the demand for bandwidth increases and the cost comes down this will become the media of choice.

The nature of fiber optic cable, the glass or the plastic material from which it is constructed, essentially allows it to have complete immunity to EMI, RFI and other forms of extraneous signals. It is a highly appropriate transmission media for the heavy industrial environment where such forms of interference are a problem. Fiber optic cable is immune to signal leakage and is secure against eaves dropping. It has a much higher reliability than either coaxial or twisted pair. Since fiber optic cable is nonconductive its use eliminates ground loop problems. It also does not need protection from electrical surges or lightning.

Against these advantages it is 25 times more expensive (on average) than twisted pair. Terminating devices to its ends is much more difficult and requires a high degree of skill by the field technician. But, there is some light at the end of this tunnel (No pun intended). Increased demand and utilization in expectation for bandwidth needs is causing cost to come down. Technician handling costs and more convenient termination techniques are likewise arising.

Most fiber optic cable in use today is optimized for operation at either wavelengths of 820 nm or 1300 nm. Table 7 [Ref.4] indicates the attenuation losses of cables at these wavelengths in various situations.

Fiber Optic Cable Attenuation Losses at Premises Communications Distances

Wavelength Worst-case loss/km To desk
(328 feet)
Between closets
(600 feet)
Between buildings
(1,000 feet)
(3,000 feet) (8,200 feet)
820 nm 4 dB 0.4 dB 0.7 dB 1.2 dB 3.7 dB 10 dB
1,300 nm 1 dB 0.1 dB 0.2 dB 0.3 dB 0.9 dB 2.5 dB

Table 7


Please make a selection.