Like any good house, a network must be built on a solid foundation. In the OSI reference model, this foundation is called Layer 1 or the physical layer. The terms used in this chapter describe how network functions are linked to Layer 1 of the OSI reference model. The physical layer is the layer that defines the electrical, mechanical, procedural, and functional specifications for activating, maintaining, and deactivating the physical link between end systems.

In this chapter, you will learn about the network functions that occur at the physical layer of the OSI model. You will learn about different types of networking media that are used at the physical layer, including shielded twisted-pair cable, unshielded twisted-pair cable, coaxial cable, and fiber-optic cable. In addition, you will learn how network devices, cable specifications, network topologies, collisions and collision domains can help determine such things as how much data can travel across the network and how fast.

Shielded twisted-pair cable (STP) combines the techniques of shielding, cancellation, and twisting of wires . Each pair of wires is wrapped in metallic foil. The 4 pairs of wires are wrapped in an overall metallic braid or foil. It is usually 150 Ohm cable. As specified for use in Ethernet network installations, STP reduces electrical noise, both within the cable (pair to pair coupling, or crosstalk) and from outside the cable (electromagnetic interference -- EMI -- and radio frequency interference -- RFI). Shielded twisted-pair cable shares many of the advantages and disadvantages of unshielded twisted-pair cable (UTP). STP affords greater protection from all types of external interference, but is more expensive and difficult to install than UTP.

A new hybrid of UTP with traditional STP is Screened UTP (ScTP), also known as Foil Twisted Pair (FTP) . ScTP is essentially UTP wrapped in a metallic foil shield, or "screen". It is usually 100 or 120 Ohm cable.

The metallic shielding materials in STP and ScTP need to be grounded at both ends. If improperly grounded (or if there are any discontinuities in the entire length of the shielding material, for example due to poor termination or installation), STP and ScTP become susceptible to major noise problems, because they allow the shield to act like an antenna picking up unwanted signals. However, this effect works both ways. Not only does the foil (shield, screen) prevent incoming electromagnetic waves from causing noise on our data wires, but it minimizes the outgoing radiated electromagnetic waves, which could cause noise in other devices. STP and ScTP cable cannot be run as far as other networking media (coaxial cable, optical fiber) without the signal being repeated. More insulation and shielding combine to considerably increase the size, weight, and cost of the cable. And the shielding materials make terminations more difficult and susceptible to poor workmanship. However STP and ScTP still have their role, especially in Europe.

Unshielded twisted-pair cable (UTP) is a four-pair wire medium - composed of pairs of wires - used in a variety of networks. Each of the 8 individual copper wires in the UTP cable is covered by insulating material. In addition, each pair of wires are twisted around each other. This type of cable relies solely on the cancellation effect, produced by the twisted wire pairs, to limit signal degradation caused by EMI and RFI. To further reduce crosstalk between the pairs in UTP cable, the number of twists in the wire pairs varies. Like STP cable, UTP cable must follow precise specifications as to how many twists or braids are permitted per foot of cable.

When used as a networking medium, UTP cable has four pairs of either 22 or 24 gauge copper wire. UTP used as a networking medium has an impedance of 100 ohms. This differentiates it from other types of twisted-pair wiring such as that used for telephone wiring. Because UTP has an external diameter of approximately .43 cm, its small size can be advantageous during installation. Since UTP can be used with most of the major networking architectures, it continues to grow in popularity.

Unshielded twisted-pair cable has many advantages. It is easy to install and is less expensive than other types of networking media. In fact, UTP costs less per meter than any other type of LAN cabling, however its real advantage is its size. Since it has such a small external diameter, UTP does not fill up wiring ducts as rapidly as other types of cable. This can be an extremely important factor to consider, particularly when installing a network in an older building. Also, when UTP cable is installed using an RJ connector, potential sources of network noise are greatly reduced, and a good solid connection is practically guaranteed.

There are disadvantages in using twisted-pair cabling. UTP cable is more prone to electrical noise and interference than other types of networking media, and the distance between signal boosts is shorter for UTP than it is for coaxial and fiber optic cables.

While UTP was once considered slower at transmitting data than other types of cable. However, this is no longer true. In fact, today, UTP is considered the fastest copper-based media.

Coaxial cable consists of a hollow outer cylindrical conductor that surrounds a single inner wire made of two conducting elements. One of these elements - located in the center of the cable - is a copper conductor. Surrounding it is a layer of flexible insulation. Over this insulating material is a woven copper braid or metallic foil that acts as the second wire in the circuit, and as a shield for the inner conductor. This second layer, or shield, can help reduce the amount of outside interference. Covering this shield is the cable jacket.

For LANs, coaxial cable offers several advantages. It can be run, without as many boosts from repeaters, for longer distances between network nodes than either STP or UTP cable. Repeaters regenerate the signals in a network so that they can cover greater distances. Coaxial cable is less expensive than fiber-optic cable, and the technology is well known. It has been used for many years for all types of data communication. Can you think of another type of communication that utilizes coaxial cable?

When working with cable, it is important to consider its size. As the thickness, or diameter, of the cable increases, so does the difficulty in working with it. You must remember that cable must be pulled through existing conduits and troughs that are limited in size. Coaxial cable comes in a variety of sizes. The largest diameter was specified for use as Ethernet backbone cable because it had historically a greater transmission length and noise rejection characteristics. This type of coaxial cable is frequently referred to as thicknet. As its nickname suggests, this type of cable, because of its thickness, can be too rigid to install easily in some situations. The rule of thumb is: "the more difficult the network media is to install, the more expensive it is to install." Coaxial cable is more expensive to install than twisted-pair cable. Thicknet cable is almost never used anymore, except for special purpose installations.

In the past, coaxial cable with an outside diameter of only .35 cm (sometimes referred to as thinnet) was used in Ethernet networks. It was especially useful for cable installations that required the cable to make many twists and turns. Since it was easier to install, it was also cheaper to install. This led some people to refer to it as cheapernet. However, because the outer copper or metallic braid in coaxial cable comprises half the electrical circuit, special care must be taken to ensure that it is properly grounded. This is done by ensuring that there is a solid electrical connection at both ends of the cable. Frequently, installers fail to do this. As a result, poor shield connection is one of the biggest sources of connection problems in the installation of coaxial cable. Connection problems result in electrical noise that interferes with signal transmittal on the networking media. It is for this reason that, despite its small diameter, thinnet is no longer commonly used in Ethernet networks.

Fiber-optic cable is a networking medium capable of conducting modulated light transmissions. Compared to other networking media, it is more expensive; however, it is not susceptible to electromagnetic interference and is capable of higher data rates than any of the other types of networking media discussed here. Fiber-optic cable does not carry electrical impulses, as other forms of networking media that employ copper wire do. Instead, signals that represent bits are converted into beams of light. Even though light is an electromagnetic wave, light in fibers is not considered wireless because the electromagnetic waves are guided in the optical fiber. The term wireless is reserved for radiated, or unguided, electromagnetic waves.

Fiber-optic communication is rooted in a number of inventions made in the 19th century. It was not until the 1960s, when solid-state laser light sources and high-quality impurity-free glasses were introduced, that fiber-optic communication became practical. Its use on a widespread basis was pioneered by telephone companies, who saw its benefits for long-distance communication.

Fiber-optic cable used for networking consists of two fibers encased in separate sheaths. If viewed in cross section, you would see that each optical fiber is surrounded by layers of protective buffer material, usually a plastic such as Kevlar, and an outer jacket. The outer jacket provides protection for the entire cable. Usually made of plastic, it conforms to appropriate fire and building codes. The purpose of the Kevlar is to furnish additional cushioning and protection for the fragile hair-thin glass fibers. Wherever buried fiber-optic cables are required by codes, a stainless steel wire is sometimes included for added strength.

The light-guiding parts of an optical fiber are called the core and the cladding. The core is usually very pure glass with a high index of refraction. When the core glass is surrounded by a cladding layer of glass or plastic with a low index of refraction, light can be trapped in the fiber core. This process is called total internal reflection, and it allows the optical fiber to act like a light pipe, guiding light for tremendous distances, even around bends.

Wireless signals are electromagnetic waves, which can travel through the vacuum of outer space and through media such as air. Therefore, no physical medium is necessary for wireless signals, making them a very versatile way to build a network. Figure represents an electromagnetic wave.

Figure illustrates one of the most important charts in all of science and technology, the Electromagnetic Spectrum chart. You might be amazed that even tough all of the waves - power waves, radio waves, microwaves, Infrared light waves, visible light waves, ultraviolet light waves, x-rays, and gamma rays - look seemingly very different, they share some very important characteristics:

1. All of these waves have an energy pattern similar to that represented in Figure .
2. All of these waves travel at the speed of light, c = 299, 792, 458 meters per second, in vacuum. This speed might more accurately be called the speed of electromagnetic waves.
3. All of these waves obey the equation (frequency) x (wavelength) = c.
4. All of these waves will travel through vacuum, however, they have very different interactions with various materials.

The primary difference between the different electromagnetic waves is their frequency. Low frequency electromagnetic waves have a long wavelength (the distance from one peak to the next on the sine wave), while high frequency electromagnetic waves have a short wavelength.

The interactive calculator in Figure allows you to experiment. Try the interactive calculator by doing the following activities:

1. Enter a frequency and you will notice that the calculator displays the wavelength.
2. Enter a wavelength you will notice that the calculator displays the frequency.

In either case, the calculator display the electromagnetic wave associated with the calculation.

A common application of wireless data communication is for mobile use. Some examples of mobile use includes:

• people in cars or airplanes
• satellites
• remote space probes
• space shuttles and space stations
• anyone/anything/anywhere/anytime that requires network data
• communications, without having to rely on copper or optical fiber tethers

Another common application of wireless data communication is wireless LANs (WLANs), which are built in accordance with the IEEE 802.11 standards. WLANs typically use radio waves (for example, 902 MHz), microwaves (for example, 2.4 GHz), and Infrared waves (for example, 820 nanometers) for communication. Wireless technologies are a crucial part of the future of networking.