Novell's Networking Primer
Now that we understand how information is converted to data and how computers send and receive data over the network, we can discuss the hardware used to transport the data from one computer to another. This hardware can generally be divided into two categories: network transmission media and transmitting and receiving devices. Network transmission media refers to the various types of media used to carry the signal between computers. Transmitting and receiving devices are the devices placed at either end of the network transmission medium to either send or receive the information on the medium.
Network Transmission Media
When data is sent across the network it is converted into electrical signals. These signals are generated as electromagnetic waves (analog signaling) or as a sequence of voltage pulses (digital signaling). To be sent from one location to another, a signal must travel along a physical path. The physical path that is used to carry a signal between a signal transmitter and a signal receiver is called the transmission medium. There are two types of transmission media: guided and unguided.
Guided media are manufactured so that signals will be confined to a narrow path and will behave predictably. The three most commonly used types of guided media are twisted-pair wiring, coaxial cable, and optical fiber cable.
Twisted-pair wiring refers to a type of cable composed of two (or more) copper wires twisted around each other within a plastic sheath. The wires are twisted to reduce crosstalk (electrical interference passing from one wire to the other). There are "shielded" and "unshielded" varieties of twisted-pair cables. Shielded cables have a metal shield encasing the wires that acts as a ground for electromagnetic interference. Unshielded twisted-pair cable is the most common in business networks because it is inexpensive and extremely flexible. The RJ-45 connectors on twisted-pair cables resemble large telephone jacks.
This type of cable is referred to as "coaxial" because it contains one copper wire (or physical data channel) that carries the signal and is surrounded by another concentric physical channel consisting of a wire mesh or foil. The outer channel serves as a ground for electrical interference. Because of this grounding feature, several coaxial cables can be placed within a single conduit or sheath without significant loss of data integrity. Coaxial cable is divided into two different types: thinnet and thicknet.
Thinnet coaxial cable is similar to the cable used by cable television companies. Thinnet is not as flexible as twisted-pair, but it is still used in LAN environments. The connectors on coaxial cable are called BNC twist-on connectors and resemble those found on television cables.
Thicknet is similar to thinnet except that it is larger in diameter. The increase in size translates into an increase in maximum effective distance. The drawback to the increase in size, however, is a loss of flexibility. Because thicknet is much more rigid than thinnet, the deployment possibilities are much more limited and the connectors are much more complex. Thicknet is used primarily as a network backbone with thinnet "branches" to the individual network components.
Optical Fiber Cable
10Base-FL and 100Base-FX optical fiber cable, better known as "fiber optic," are the same types of cable used by most telephone companies for long-distance service. As this usage would imply, optical fiber cable can transmit data over very long distances with little loss in data integrity. In addition, because data is transferred as a pulse of light rather than an electronic pulse, optical fiber is not subject to electromagnetic interference. The light pulses travel through a glass or plastic wire or fiber encased in an insulating sheath.
As with thicknet, optical fiber's increased maximum effective distance comes at a price. Optical fiber is more fragile than wire, difficult to split, and very labor-intensive to install. For these reasons, optical fiber is used primarily to transmit data over extended distances where the hardware required to relay the data signal on less expensive media would exceed the cost of optical fiber installation. It is also used where very large amounts of data need to be transmitted on a regular basis.
Figure 5: Common guided transmission media
Unguided media are natural parts of the Earth's environment that can be used as physical paths to carry electrical signals. The atmosphere and outer space are examples of unguided media that are commonly used to carry signals. These media can carry such electromagnetic signals as microwave, infrared light waves, and radio waves.
Network signals are transmitted through all transmission media as a type of waveform. When transmitted through wire and cable, the signal is an electrical waveform. When transmitted through fiber-optic cable, the signal is a light wave: either visible or infrared light. When transmitted through Earth's atmosphere or outer space, the signal can take the form of waves in the radio spectrum, including VHF and microwaves, or it can be light waves, including infrared or visible light (for example, lasers).
Recent advances in radio hardware technology have produced significant advancements in wireless networking devices: the cellular telephone, wireless modems, and wireless LANs. These devices use technology that in some cases has been around for decades but until recently was too impractical or expensive for widespread consumer use. The next few sections explain technologies unique to unguided media that are especially of concern to networking.
Spread Spectrum Technology
Wireless transmission introduces several challenges not found in wired transmission. First is the fact that when data travels through the air, any device tuned to its frequency can intercept it, such as the way every radio in a city can pick up the same signal broadcast by a radio station. Second, if many devices transmitting on the same frequency are in the same geographical area, the signals can interfere with each other, a phenomenon known as crosstalk.
To prevent wireless transmissions from being intercepted by unauthorized devices and to reduce crosstalk, "spread spectrum" technology is used. A product of the military, spread spectrum technology has only recently become inexpensive and compact enough for use in commercial applications. As its name denotes, spread spectrum technology involves spreading a signal over a bandwidth larger than is needed, according to a special pattern. Only the devices at each end of the transmission know what the pattern is. In this way, several devices transmitting at the same frequency in the same location will not interfere with each other nor can they "listen in" on each other.
Spread spectrum technology can be performed using one of two techniques: Direct Sequence Spread Spectrum (DSSS) and Frequency Hopping Spread Spectrum (FHSS). In DSSS the sending device encodes the digital signal prior to transmission, using another digital signal as the key. The signal's power is then spread across a range of frequencies as it is transmitted. The receiving device has the same key, and upon receiving the transmission uses the key to interpret the signal. Because each connection between devices uses a unique key, the devices "hear" only those signals encoded with that key; all other signals are ignored. Also, by spreading a signal's power over a broader-than-needed spectrum, several signals can be transmitted over the same range of frequencies without interfering with each other.
With FHSS the signal hops from one frequency to another in rapid succession and according to a pattern unique to that transmission. The Federal Communications Commission (FCC) requires that a minimum of 75 frequencies be used per transmission and that the maximum time spent on each frequency be no longer than 400 milliseconds. Because the device at the other end knows to which frequencies the signal will hop and for how long the signal will stay on each frequency, it knows where to find the signal each time it hops. Any other device using FHSS in the same geographical location would be looking for signals that hop frequencies according to a different pattern.
Each method has benefits and drawbacks. DSSS is the faster method of the two: it can achieve data transmission rates in excess of 2 Mbps whereas FHSS data transmission rates do not exceed 2 Mbps. DSSS is also more expensive and consumes more power. FHSS is therefore more cost-effective, but DSSS is best when higher data transfer rates are required.
Because of spread spectrum technology, data transmitted through the air is in many ways more secure than data transmitted over wires. With wired media the frequency at which data is sent remains constant, so a person with a good antenna and some skill could sit in the parking lot of a corporation and intercept unencrypted signals as they travelled over the wires. On the other hand, spread spectrum transmissions cannot be decoded except by the intended device.
Transmitting and Receiving Devices
Once you have selected a transmission medium, you need devices that can propagate signals across the medium and devices that can receive the signals when they reach the other end of the medium. Such devices are designed to propagate a particular type of signal across a particular type of transmission medium. Transmitting and receiving devices used in computer networks include network adapters, repeaters, wiring concentrators, hubs, switches, and infrared, microwave, and other radio-band transmitters and receivers.
A network adapter is the hardware installed in computers that enables them to communicate on a network. Network adapters are manufactured in a variety of forms. The most common form is the printed circuit board, which is designed to be installed directly into a standard expansion slot inside a PC. Many manufacturers of desktop workstation motherboards include network adapters as part of the motherboard. Other network adapters are designed for mobile computing: they are small and lightweight and can be connected to portable (laptop and notebook) computers so that the computer and network adapter can be easily transported from network to network.
Network adapters are manufactured for connection to virtually any type of guided medium, including twisted-pair wire, coaxial cable, and fiber-optic cable. They are also manufactured for connection to devices that transmit and receive visible light, infrared light, and radio microwaves.
The hardware used to make connections between network adapters and different transmission media depends on the type of medium used. Figure 6 illustrates a snap-in RJ-45 connector that is ordinarily used for a 10Mbps Ethernet connection.
Figure 6: An RJ-45 connector links the adapter to the transmission media
Repeaters are used to increase the distance over which a network signal can be propagated.
As a signal travels through a transmission medium, it encounters resistance and gradually becomes weak and distorted. The technical term for this signal weakening is "attenuation." All signals attenuate, and at some point they become too weak and distorted to be received reliably. Repeaters are used to overcome this problem.
A simple, dedicated repeater is a device that receives the network signal and retransmits it at the original transmission strength. Repeaters are placed between transmitting and receiving devices on the transmission medium at a point at which the signal is still strong enough to be retransmitted.
In today's networks, dedicated repeaters are seldom used. Repeaters are "dumb" devices, meaning that they do not have the capability to analyze what they're repeating. They therefore will repeat all signals, including those that should not be repeated, which increases network traffic. Repeating capabilities are now built into other, more complex networking devices that can analyze and filter signals. For example, virtually all modern network adapters, hubs, and switches incorporate repeating capabilities.
Wiring Concentrators, Hubs, and Switches
Wiring concentrators, hubs, and switches provide a common physical connection point for computing devices. (We limit this discussion to devices used for making physical connections. The term "concentrator" can mean something different in a mainframe or minicomputer environment.) Most hubs and all wiring concentrators and switches have built-in signal repeating capability to perform signal repair and retransmission. (These devices also perform other functions.)
In most cases, hubs, wiring concentrators, and switches are proprietary, standalone hardware. There are a number of companies that manufacture such equipment. Occasionally, hub technology consists of hub cards and software that work together in a standard computer.
Figure 7 shows two common hardware-based connection devices: a token-ring switch and an Ethernet 10Base-T concentrator.
Figure 7: Token-ring switch and Ethernet 10Base-T concentrator
Modems provide the means to transmit digital computer data over analog transmission media, such as ordinary, voice-grade telephone lines. The transmitting modem converts the encoded data signal to an audible signal and transmits it. A modem connected at the other end of the line receives the audible signal and converts it back into a digital signal for the receiving computer. Modems are commonly used for inexpensive, intermittent communications between a network and geographically isolated computers.
The word "modem" is derived from "MOdulate and DEModulate"—modems convert digital (computer) signals to analog (audio) signals and vice versa by modulating and demodulating the frequency. However, analog signals consist of a sound wave with three states that can be altered: amplitude, frequency, and phase. Low-speed modems modulate only frequency, but faster modems modulate two or three states at the same time, usually frequency and phase. Faster modems also use full-duplex communication—they utilize both incoming and outgoing telephone lines to transmit data—which further increases their speed.
Microwave transmitters and receivers, especially satellite systems, are commonly used to transmit network signals over great distances. A microwave transmitter uses the atmosphere or outer space as the transmission medium to send the signal to a microwave receiver. The microwave receiver then either relays the signal to another microwave transmitter or translates the signal to some other form, such as digital impulses, and relays it on another suitable medium to its destination. Figure 8 shows a satellite microwave link.
Figure 8: Satellite microwave link
Originally, this technology was used almost exclusively for satellite and long-range communication. Recently, however, there have been developments in cellular technology that allow you complete wireless access to networks, intranets, and the Internet. IEEE 802.11 defines a MAC and physical access control for wireless connection to networks.
Infrared and Laser Transmitters
Infrared and laser transmitters are similar to microwave systems: they use the atmosphere and outer space as transmission media. However, because they transmit light waves rather than radio waves, they require a line-of-sight transmission path.
Infrared and laser transmissions are useful for signaling across short distances where it is impractical to lay cable—for instance, when networks are at sites a few miles apart. Because infrared and laser signals are in the light spectrum, rain, fog, and other environmental factors can cause transmission problems.
Cellular transmissions are radio transmissions and therefore have the advantage of being able to penetrate solid objects. The cellular base station at the center of each cell consists of low-power transmitters, receivers, antennas, and common control computer equipment. The cell tower usually has a triangular array of antennas on top. Unlike conventional radio and television transmitters, whose primary purpose is to cover the largest area possible, cellular transmitters emit signals that do not carry much farther than a few cells. Cellular devices are likewise configured to operate at low power to avoid interfering with other cellular devices in the area.
Wireless LAN Transmitters
Wireless devices interface with LANs at wireless access points (APs). These APs function like hubs and switches in a wired environment, only they propagate signals through radio waves or infrared light instead of wires. An AP consists of a transceiver, usually positioned in a high place such as a tower or near the ceiling, that physically connects to the hard wiring of the LAN. An AP that is connected to the LAN via radio waves is called an extension point (EP). Wireless networking operates under the same principal as cellular phones: each AP or EP covers a cell, and users are handed off from one cell to the next. Therefore, a user with a handheld device can connect to the network in one room and walk to another part of the building or campus and still maintain connectivity.
Other kinds of wireless transmitters reside in wireless devices and interface directly with similar devices, creating an ad-hoc, peer-to-peer network when they are near one another. These transmitters also operate at very low power to avoid unwanted interference.
Currently, technology is being developed to use the human body as a "wet-wire" transmitter. The personal area network (PAN) takes advantage of the conductive powers of living tissue to transmit signals. The PAN device, which can be worn on a belt, in a pocket, or as a watch, transmits extremely low-power signals (less than 1 MHz) through the body. With a handshake, users could exchange business cards or other information with little fear of eavesdropping from remote users. The PAN specification encompasses all seven layers of the OSI model, meaning that it can address application and file transfer as well. (Note: The term PAN is also used to describe ad hoc, peer-to-peer networks.)Return to Primer Index | Next Section