Hotspot | Wireless | Networking

The ISO OSI Model

2008-11-16T21:07:00.000-08:00

As the package delivery example demonstrates, the information itself is only part of the process. When information moves across a network, it’s essential that all of the parties involved the originator, the ultimate recipient, and everything in between agree that they will use the same formatting, timing, and routing rules and specifications. These rules (also called protocols) define the network’s internal “plumbing” and the form of the information that moves through it.

As network communication has become more complex, the community of network designers has accepted the International Organization for Standardization’s (ISO) Open Systems Interconnection (OSI) model to identify the individual elements of a network link. The OSI model applies to just about any kind of data communication system, including the broadband wireless network.

Because everybody in the communication industry uses the OSI model, it encourages hardware and software designers to create systems and services that can exchange information with similar products from other manufacturers. Without the OSI model or something like it, it would not be possible to expect equipment from more than one source to work together.

The OSI model also allows a designer to change just one element of the network without the need to design everything else from scratch. For example, a wireless network uses radio signals instead of cables at the physical layer and adds routing information at the data linklayer, but it keeps the existing protocols and specifications for everything else. A complex network (such as the Internet) can use wired connections for one part of the signal path and wireless connections for another.

The Physical Layer

As the name suggests, the physical layer defines the physical media or hardware that carries signals between the end points of a network connection. The physical layer might be a coaxial cable, a pair of telephone wires, flashing lights, or radio waves. The specifications of a network’s physical layer might include the shape of the shell and the pin numbers in a cable connector, the voltages that define the 0 and 1 (on and off) values, the durations of individual data bits, and the radio frequencies and modulation methods used by a radio transmitter and receiver.

The Data Link Layer

The data link layer handles transmission of data across the link defined by the physical layer. It specifies the format of each data packet that moves across the network, including the destination of each packet, the physical structure of the network, the sequence of packets (to make sure that the packets arrive in the correct order), and the type of flow control (to make sure that the transmitter doesn’t send data faster than the receiver can handle it). Each packet also includes a checksum that the receiver uses to confirm that the data was not corrupted during transmission, as well as the string of bits and bytes that contains the actual data inside the packet. Therefore, it contains the software that creates and interprets the signals that move through the physical layer.

In both wired and wireless Ethernet, every physical device that is connected to the network has a unique 48-bit media access control (MAC) address that identifies it to the network. The header (the first part of the data string inside of a packet) includes the MAC addresses of both the origin and destination of that packet.

The Network Layer

The network layer specifies the route that a signal uses to move from the source to the destination independently of the physical media. At the network level, it doesn’t matter whether the data moves through a cable, radio waves, or if it uses some combination of both because that’s all handled at a lower level.

Within the Internet, the exchange of data between LANs, wide area networks (WANs), and the core Internet trunk circuits occurs at the network layer.

The Transport Layer

Starting at the transport layer, the OSI model is concerned with communication between programs on two different computers rather than the process of moving data from point A to point B. For example, when you view a web page on the Internet, the connection between the browser on your computer (such as Internet Explorer or Firefox) and the webserver that contains that page occurs at the transport layer (but the commands you send to the server occur at the application layer).

The Session Layer

The session layer defines the format that the programs connected through the transport layer use to exchange data. If the programs use passwords or other authentication to assure that the program at the distant end of the connection is allowed to use a local program, that authentication happens in the session layer.

The Presentation Layer

The presentation layer controls the way each computer handles text, audio, video, and other data formats. For example, if a distant computer sends a picture in JPEG format, the software that converts the data string to a picture on a monitor or a printer operates at the presentation layer.

The Application Layer

The application layer handles the commands and data that move through the network. For example, when you send an email message, the content of your message (but not the address or the formatting information) is in the application layer. Most of the words, pictures, sounds, and other forms of information that you send through a network enter the system through the application layer.

Choosing a wireless Service

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Each type of wireless access to the Internet offers a different combination of cost, coverage areas, reliability, ease of use, and security. Your choice will depend on your particular needs and the availability of signals in the locations where you need wireless Internet access.

For example, if you use your computer in just a few places and all of those places are within range of Wi-Fi hot spots, the built-in Wi-Fi adapter (or an inexpensive plug-in adapter) is robably your best choice. It’s likely that Wi-Fi hot spots already exist at your workplace and in the libraries, coffee shops, schools, and conference centers where you regularly spend time, and it’s relatively easy and inexpensive to install one or more access points at home. However, you will probably need a separate account to log in to each Wi-Fi network. Some of these Wi-Fi services are free, but others charge for access by the hour, by the day, or by the month; if you need paid accounts at several locations, the total cost can be more than a single account with a cellular service.

Wi-Fi also allows you to add portable computers to an existing LAN at home or in your school or workplace. And if cost is a primary concern, you will probably choose to use free public Wi-Fi hot spots instead of a cellular or WiMAX service that charges a monthly fee.

On the other hand, if you want constant Internet access wherever you go, the cellular data and WiMAX metropolitan area network services are better choices. Both systems provide coverage throughout large geographic regions, and both allow you to maintain a connection while you’re moving from one place to another. You can use the same account and the same login and password every time you set up a connection. However, it’s important to make sure that there’s a usable cellular or WiMAX signal in all the places where you expect to use them before you commit to a long-term contract. Most wireless data service providers offer a free or low cost trial period that you can use to test the system.

As WiMAX and cellular data services become more common, many laptop computers and add-on network adapters will operate with both types of wireless services. When the computer detects a high-speed Wi-Fi signal, it will automatically try to establish a connection to that network. But when there is no local Wi-Fi signal, or if you haven’t configured your computer to use any of the local signals, it will automatically shift over to your WiMAX or cellular data account and use that service to connect to the Internet.

All three types of wireless Internet services—Wi-Fi, cellular, and WiMAX—offer fast and reliable connections, but each has a different set of strengths and weaknesses. For short-range coverage and for access to local area networks, Wi-Fi is the obvious choice. If you are outside of the service areas of a DSL or cable Internet service, WiMAX is a huge improvement over a slow dial-up service. But when you carry your computer to many locations, a single account with a cellular or WiMAX service will allow you to connect to the Internet without the need to search for a new hot spot and set up a new account.

Wi-Fi Services
The 802.11b, 802.11g, and 802.11n Wi-Fi services all operate in a frequency range at or slightly above 2.4 GHz. The 802.11a signal uses a band close to 5.3 GHz. The specific center frequencies of each Wi-Fi channel are listed in Table below.

Wi-Fi Characteristics Table;

Unless you’re a radio engineer, the important things to know about the different Wi-Fi services are the maximum data transmission rate and the signal range. The differences between the maximum data speeds and the typical speeds are caused by the handshaking and other nondata information that must attach itself to each data packet. Obviously, there’s a tremendous amount of overhead involved in moving information through any kind of Wi-Fi network.

Wireless Bluetooth

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Bluetooth is the other type of wireless networking technology that we ought to describe. Bluetooth uses radio signals to replace the wires and cables that connect a computer or a mobile telephone to peripheral devices, such as a keyboard, a mouse, or a set of speakers. You can also use Bluetooth to transfer data between a computer and a mobile telephone, smartphone, BlackBerry, or other PDA (personal digital assistant).

Bluetooth is an FHSS system that splits the radio signal into tiny pieces. It moves among 79 different frequencies 1,600 times per second in the same unlicensed 2.4 GHz range as 802.11b and 802.11g Wi-Fi services.

Bluetooth is not practical for connecting a computer to the Internet
because it’s slow (the maximum data transfer rate is only about 700Kbps), and it has a very limited signal range (most often about 33 feet, or 10 meters, or less).

In order to prevent interference between Bluetooth and Wi-Fi signals,
many computers that use both technologies (including the widely used Intel
Centrino chip set) coordinate the two services. When either module is active,
it notifies the other, and the active service takes priority. This coordinated
operation is slightly slower than either service operating alone, but the
difference is insignificant.

WiMAX

2008-10-17T04:33:00.000-07:00

Worldwide Interoperability for Microwave Access (WiMAX) is yet another method for distributing broadband wireless data over wide geographic areas. It’s a metropolitan area network service that typically uses one or more base stations that can each provide service to users within a 30-mile radius. The IEEE 802.16 specification contains the technical details of WiMAX networks.

In the United States, the earliest WiMAX services were offered by
Clearwire as a wireless alternative to DSL and cable broadband Internet access in fixed locations (such as homes and businesses), but mobile WiMAX access is not far behind. By early 2008, Clearwire plans to offer access to their wireless networks through an adapter on a PC Card. When those adapters become available, WiMAX, 3G cellular data services, and metropolitan Wi-Fi networks will compete for the same commercial niche: wireless access to the Internet through a service that covers an entire metropolitan area.

Each WiMAX service provider uses one or more licensed operating frequencies somewhere between 2 GHz and 11 GHz. A WiMAX link can transfer data (including handshaking and other overhead) at up to 70Mbps, but most commercial WiMAX services are significantly slower than that. And as more and more users share a single WiMAX tower and base station, some users
report that their signal quality deteriorates.

Unlike the cellular broadband wireless data services that piggyback on existing mobile telephone networks, WiMAX is a separate radio system that is designed to either supplement or replace the existing broadband Internet distribution systems. In practice, WiMAX competes with both 3G wireless services and with Internet service providers that distribute Internet access to fixed locations through telephone lines and cable television utilities. Home and business subscribers to a WiMAX service usually use either a wired LAN or Wi-Fi to distribute the network within their buildings. Below figure shows a typical WiMAX network.

WiMAX provides last mile Internet connections to homes and businesses.

Wireless Data Services

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Because radio signals move through the air, you can set up a network connection from any place within range of the network base station’s transmitter; it’s not necessary to use a telephone line, television cable, or some other dedicated wiring to connect your computer to the network. Just turn on the radio connected to the computer and it will find the network signal. Therefore, a radio (or wireless) network connection is often a lot more convenient than a wired one.

This is not to say that wireless is always the best choice. A wired network is usually more secure than a wireless system because it’s a lot more difficult for unauthorized eavesdroppers and other snoops to monitor data as it moves through the network, and a wired link doesn’t require as many complex negotiations between the sender and receiver on protocols and so forth. In an environment where your computer never moves away from your desk and there are no physical obstacles between the computer and the network access point, it’s often easier to install a data cable between the computer and a modem.

So now we have a bunch of radio transmitters and receivers that all operate on the same frequencies and all use the same kind of modulation. (Modulation is the method a radio uses to add some kind of content, such as voice or digital data, to a radio wave.) The next step is to send some network data through those radios. Several different wireless data systems and services are available to connect computers and other devices to local networks and to the Internet, including Wi-Fi, WiMAX, and a handful of services based on the latest generations of cellular mobile telephone technology.

Benefits of Wireless

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Wireless broadband provides Internet access to mobile devices in addition to allowing network operators to extend their networks beyond the range of their wired connections. For our purposes, two-way radio is the most sensible approach to wireless broadband, but other methods (such as infrared light or visible signaling) are also possible. Connecting your computer to the Internet (or a local network) by radio offers several advantages over connecting the same computer through a wired connection. First, wireless provides convenient access for portable computers; it’s not necessary to find a cable or network data outlet. And second, it allows a user to make a connection from more than one location and to maintain a connection as the user moves from place to place. For network managers, a wireless connection makes it possible to distribute access to a network without the need to string wires or cut holes through walls.

In practice, access without cables means that the owner of a laptop or other portable computer can walk into a classroom, a coffee shop, or a library and connect to the Internet by simply turning on the computer and running a communication program. Depending on the type of wireless network you’re using, you might also be able to maintain the same connection in a moving vehicle.

When you’re installing your own network, it’s often easier to use Wi-Fi links to extend your network and your Internet connection to other rooms because a wired system requires a physical path for the cables between the network router or switch and each computer. Unless you can route those cables through a false ceiling or some other existing channel, this almost always means that you must cut holes in your walls for data connectors and

feed wires inside the walls and under the floors. A radio signal that passes through those same walls is often a lot neater and easier.

INTRODUCTION TO WIRELESS NETWORKS

2008-10-13T01:01:00.000-07:00

Up to a point, it’s quite possible to treat your wireless network as a set of black boxes

that you can turn on and use without knowing much about the way they work. That’s the way most people relate to the technology that surrounds them. You shouldn’t have to worry about the technical specifications just to place a long-distance telephone call or heat your lunch in a microwave oven or connect your laptop computer to a network. In an ideal world (ha!), the wireless link would work as soon as you turn on the power switch.

But wireless networking today is about where broadcast radio was in the late 1920s. The technology was out there for everybody, but the people who understood what was happening behind that Bakelite-Dilecto panel (Figure 2-1) often got better performance than the ones who just expected to turn on the power switch and listen.

In order to make the most effective use of wireless networking technology, it’s still important to understand what’s going on inside the box (or in this case, inside each of the boxes that make up the network). This chapter describes the standards and specifications that control wireless

networks and explains how data moves through the network from one computer to another.

When the network is working properly, you should be able to use it without thinking about all of that internal plumbing—just click a few icons and you’re connected. But when you’re designing and building a new network, or when you want to improve the performance of an existing network, it can be essential to understand how all that data is supposed to move from one

place to another. And when the network does something you aren’t expecting it to do, you will need a basic knowledge of the technology to do any kind of useful troubleshooting.

How Wireless Networks Work

Moving data through a wireless network involves three separate elements: the radio signals, the data format, and the network structure. Each of these elements is independent of the other two, so you must define all three Introduct ion to Wireless Networks 13 when you invent a new network. In terms of the OSI reference model, the radio signal operates at the physical layer, and the data format controls several of the higher layers. The network structure includes the wireless network interface adapters and base stations that send and receive the radio signals. In a wireless network, the network interface adapters in each computer and base station convert digital data to radio signals, which they transmit to other devices on the same network, and they receive and convert incoming radio signals from other network elements back to digital data.

Each of the broadband wireless data services use a different combination of radio signals, data formats, and network structure. We’ll describe each type of wireless data network in more detail later in this chapter, but first, it’s valuable to understand some general principles.

Radio

The basic physical laws that make radio possible are known as Maxwell’s equations, identified by James Clerk Maxwell in 1864. Without going into the math, Maxwell’s equations show that a changing magnetic field will produce an electric field, and a changing electric field will produce a magnetic field. When alternating current (AC) moves through a wire or other physical conductor, some of that energy escapes into the surrounding space as an alternating magnetic field. That magnetic field creates an alternating electric field in space, which in turn creates another magnetic field and so forth until the original current is interrupted.

This form of energy in transition between electricity and magnetic energy is called electromagnetic radiation, or radio waves. Radio is defined as the radiation of electromagnetic energy through space. A device that produces radio waves is called a transmitter, and a complementary device that detects radio waves in the air and converts them to some other form of energy is called a receiver. Both transmitters and receivers use specially shaped devices called antennas to focus the radio signal in a particular direction, or pattern, and to increase the amount of effective radiation (from a transmitter) or sensitivity (in a receiver).

By adjusting the rate at which alternating current flows from each transmitter through the antenna and out into space (the frequency), and by adjusting a receiver to operate only at that frequency, it’s possible to send and receive many different signals, each at a different frequency, that don’t interfere with one another. The overall range of frequencies is known as the radio

spectrum. A smaller segment of the radio spectrum is often called a band.

Radio frequencies and other AC signals are expressed as cycles per second, or hertz (Hz), named for Heinrich Hertz, the first experimenter to send and receive radio waves. One cycle is the distance from the peak of an AC signal to the peak of the next signal. Radio signals generally operate at frequencies in thousands, millions, or billions of hertz (kilohertz or KHz, megahertz or MHz, and gigahertz or GHz, respectively).

The simplest type of radio communication uses a continuous signal that the operator of the transmitter interrupts to divide the signal into accepted patterns of long and short signals (dots and dashes) that correspond to individual letters and other characters. The most widely used set of these patterns was Morse code, named for the inventor of the telegraph, Samuel F.B. Morse, where this code was first used.

In order to transmit speech, music, and other sounds via radio, the transmitter alters, or modulates, the AC signal (the carrier wave) by either mixing an audio signal with the carrier as shown in Figure 2-2 (this is called amplitude modulation, or AM) or by modulating the frequency within a narrow range as shown in Figure 2-3 (this is called frequency modulation, or FM). The AM or FM receiver includes a complementary circuit that separates the carrier from

the modulating signal.

Figure 2-2: In an AM signal, the audio modulates the carrier.

Figure 2-3: In an FM signal, the audio modulates the radio frequency.

Because two or more radio signals using the same frequency can often interfere with one another, government regulators and international agencies, such as the International Telecommunication Union (ITU), have reserved certain frequencies for specific types of modulation, and they issue exclusive licenses to individual users. For example, an FM radio station might be licensed to operate at 92.1 MHz at a certain geographical location. Nobody else is allowed to use that frequency in close enough proximity to interfere with that signal. On the other hand, some radio services don’t require a license. Most unlicensed services are either restricted to very short distances, to specific frequency bands, or both.

Both AM and FM are analog methods because the signal that comes out of the receiver is a replica of the signal that went into the transmitter. When we send computer data through a radio link, it’s digital because the content has been converted from text, computer code, sounds, images or other information into ones and zeroes before it is transmitted, and it is converted back to its original form after it is received. Digital radio can use any of several different modulation methods: The ones and zeroes can be two different audio tones, two different radio frequencies, timed interruptions to the carrier, or some combination of those and other techniques.

Wireless Data Networks

Each type of wireless data network operates on a specific set of radio frequencies. For example, most Wi-Fi networks operate in a special band of radio frequencies around 2.4 GHz that have been reserved in most parts of the world for unlicensed point-to-point spread spectrum radio services. Other Wi-Fi systems use a different unlicensed band around 5 GHz.

Unlicensed Radio Services

Unlicensed means that anybody using equipment that complies with the technical requirements can send and receive radio signals on these frequencies without a radio station license. Unlike most radio services (including other broadband wireless services), which require licenses that grant exclusive use of that frequency to a specific type of service and to one or more specific users, an unlicensed service is a free-for-all where everybody has an equal claim to the same airwaves. In theory, the technology of spread spectrum radio makes it possible for many users to co-exist (up to a point) without significant interference.

Point-to-Point

A point-to-point radio service operates a communication channel that carries information from a transmitter to a single receiver. The opposite of point-topoint is a broadcast service (such as a radio or television station) that sends the same signal to many receivers at the same time.

Spread Spectrum

Spread spectrum is a family of methods for transmitting a single radio signal using a relatively wide segment of the radio spectrum. Wireless Ethernet networks use several different spread spectrum radio transmission systems, which are called frequency-hopping spread spectrum (FHSS), direct-sequence spread spectrum (DSSS), and orthogonal frequency division multiplexing (OFDM). Some older data networks use the slower FHSS system, but the first Wi-Fi networks used DSSS, and more recent systems use OFDM. Table 2-1 lists each of the Wi-Fi standards and the type of spread spectrum modulation they use.

Table 2-1: Wi-Fi Standards and Modulation Type

Wi-Fi Type Frequency Modulation

802.11a 5 GHz OFDM

802.11b 2.4 GHz DSSS

802.11g 2.4 GHz OFDM

Spread spectrum radio offers some important advantages over other types of radio signals that use a single narrow channel. Spread spectrum is extremely efficient, so the radio transmitters can operate with very low power. Because the signals operate on a relatively wide band of frequencies, they are less sensitive to interference from other radio signals and electrical noise, which means they can often get through in environments where a conventional narrow-band signal would be impossible to receive and understand. And because a frequency-hopping spread spectrum signal shifts among more than one channel, it can be extremely difficult for an unauthorized listener to intercept and decode the contents of a signal.

Spread spectrum technology has an interesting history. It was invented by the actress Hedy Lamarr and the American avant-garde composer George Antheil as a “Secret Communication System” for directing radio-controlled torpedoes that would not be vulnerable to enemy jamming. Before she came to Hollywood, Lamarr had been married to an arms merchant in Austria, where she learned about the problems of torpedo guidance at dinner parties with her husband’s customers. Years later, shortly before the United States entered World War II, she came up with the concept of changing radio frequencies to cut through interference. The New York Times reported in 1941 that her “red hot” invention (Figure 2-4) was vital to the national defense, but the government would not reveal any details.

Figure 2-4: Hedy Lamarr and George Antheil received this patent in 1942 for the

invention that became the foundation of spread spectrum radio communication.

She is credited here under her married name, H.K. Markey.

Antheil turned out to be the ideal person to make this idea work. His most famous composition was an extravaganza called Ballet Mechanique, which was scored for sixteen player pianos, two airplane propellers, four xylophones, four bass drums, and a siren. His design used the same kind of mechanism that he had previously used to synchronize the player pianos to change radio frequencies in a spread spectrum transmission. The original slotted paper tape system had 88 different radio channels—one for each of the 88 keys on a piano.

In theory, the same method could be used for voice and data communication as well as guiding torpedoes, but in the days of vacuum tubes, paper tape, and mechanical synchronization, the whole process was too complicated to actually build and use. By 1962, solid-state electronics had replaced the vacuum tubes and piano rolls, and the technology was used aboard US Navy ships for secure communication during the Cuban Missile Crisis. Today, spread spectrum radios are used in the US Air Force Space Command’s Milstar Satellite Communications System, in digital cellular telephones, and in wireless data networks.

Frequency-Hopping Spread Spectrum

Lamarr and Antheil’s original design for spread spectrum radio used a frequency-hopping system (FHSS). As the name suggests, FHSS technology divides a radio signal into small segments and “hops” from one frequency to another many times per second as it transmits those segments. The transmitter and the receiver establish a synchronized hopping pattern that sets the sequence in which they will use different subchannels.

FHSS systems overcome interference from other users by using a narrow carrier signal that changes frequency many times per second. Additional transmitter and receiver pairs can use different hopping patterns on the same set of subchannels at the same time. At any point in time, each transmission is probably using a different subchannel, so there’s no interference between signals. When a conflict does occur, the system resends the same packet until the receiver gets a clean copy and sends a confirmation back to the transmitting station.

For some older 802.11 wireless data services, the unlicensed 2.4 MHz band is split into 75 subchannels, each of them 1 MHz wide. Because each frequency hop adds overhead to the data stream, FHSS transmissions are relatively slow.

Direct-Sequence Spread Spectrum

The direct-sequence spread spectrum (DSSS) technology that controls 802.11b networks uses an 11-chip Barker Sequence to spread the radio signal through a single 22 MHz–wide channel without changing frequencies. Each DSSS link uses just one channel without any hopping between frequencies. As Figure 2-5 shows, a DSSS transmission uses more bandwidth, but less power than a conventional signal. The digital signal on the left is a conventional transmission in which the power is concentrated within a tight bandwidth. The DSSS signal on the right uses the same amount of power, but it spreads that power across a wider band of radio frequencies. Obviously, the 22 MHz DSSS channel is a lot wider than the 1 MHz channels used in FHSS systems.

A DSSS transmitter breaks each bit in the original data stream into a series of redundant bit patterns called chips, and it transmits them to a receiver that reassembles the chips back into a data stream that is identical to the original. Because most interference is likely to occupy a narrower bandwidth than a DSSS signal, and because each bit is divided into several chips, the receiver can usually identify noise and reject it before it decodes the signal.

Figure 2-5: A conventional signal (left) uses a narrow radio frequency bandwidth. A DSSS signal (right) uses a wider bandwidth but a less powerful signal.

Like other networking protocols, a DSSS wireless link exchanges handshaking messages within each data packet to confirm that the receiver can understand each packet. For example, the standard data transmission rate in an 802.11b DSSS WI-Fi network is 11Mbps, but when the signal quality won’t support that speed, the transmitter and receiver use a process called dynamic rate shifting to drop the speed down to 5.5Mbps. The speed might drop because a source of electrical noise near the receiver interferes with the signal or because the transmitter and receiver are too far apart to support full-speed operation. If 5.5Mbps is still too fast for the link to handle, it drops again, down to 2Mbps or even 1Mbps.

Orthogonal Frequency Division Multiplexing

Orthogonal frequency division multiplexing (OFDM) modulation, used in 802.11a Wi-Fi networks, is considerably more complicated than DSSS technology. The physical layer splits the data stream among 52 parallel bit streams that each use a different radio frequency called a subcarrier. Four of these subcarriers carry pilot data that provides reference information about the remaining 48 subcarriers, in order to reduce signal loss due to radio interference or phase shift. Because the data is divided into 48 separate streams that move through separate subcarriers in parallel, the total transmission speed is much greater than the speed of data through a single channel.

The subcarrier frequencies in an OFDM signal overlap with the peak of each subcarrier’s waveform matching the baseline of the overlapping signals as shown in Figure 2-6. This is called orthogonal frequency division. The 802.11a standard specifies a total of eight data channels that are 20 MHz wide. Each of these channels is divided into 52 300 kHz subcarriers.

Figure 2-6: In OFDM, the peaks of overlapping frequencies don’t interfere with one another.

When a Wi-Fi radio receiver detects an 802.11a signal, it assembles the parallel bit streams back into a single high-speed data stream and uses the pilot data to check its accuracy. Under ideal conditions, an 802.11a network can move data at 54Mbps, but like DSSS modulation, the OFDM transmitter and receiver automatically reduce the data speed when the maximum transmission rate is not possible due to interference, weak signals, or other lessthan perfect atmospheric conditions.

The more recent 802.11g specification was designed to combine the best features of both 802.11b (greater signal range) and 802.11a (higher speed). To accomplish this objective, it uses OFDM modulation on the 2.4 GHz frequency band.

Why This Matters

The great science fiction writer Arthur C. Clarke once observed that “Any sufficiently advanced technology is indistinguishable from magic.” For most of us, the technology that controls high-speed spread spectrum radio could just as easily be a form of magic, because we don’t need to understand the things that happen inside a transmitter and a receiver; they’re just about invisible when we connect a computer to the Internet. As mentioned earlier in this chapter, you don’t need to understand these technical details about how a Wi-Fi transmitter splits your data into tiny pieces and reassembles them into data unless you’re a radio circuit designer.

But when you know that there’s a well-defined set of rules and methods that make the connection work (even if you don’t know all the details), you are in control. You know that it’s not magic, and if you think about it, you might also know some of the right questions to ask when the system doesn’t work correctly. If knowledge is power, then knowledge about the technology

you use every day is the power to control that technology rather than just use it.

The ISO OSI Model

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As the package delivery example demonstrates, the information itself is only part of the process. When information moves across a network, it’s essential that all of the parties involved—the originator, the ultimate recipient, and everything in between—agree that they will use the same formatting, timing, and routing rules and specifications. These rules (also called protocols) define the network’s internal “plumbing” and the form of the information that moves through it.

The OSI model also allows a designer to change just one element of the network without the need to design everything else from scratch. For example, a wireless network uses radio signals instead of cables at the physical layer The Physical Layer As the name suggests, the physical layer defines the physical media or hardware that carries signals between the end points of a network connection. The physical layer might be a coaxial cable, a pair of telephone wires, flashing lights, or radio waves. and adds routing information at the data link layer, but it keeps the existing protocols and specifications for everything else. A complex network (such as the Internet) can use wired connections for one part of the signal path and wireless connections for another.

The OSI model is usually portrayed as a stack of seven layers with each layer acting as a foundation for the layer directly above it as shown in figure below ;

== Application Layer ==

== Presentation Layer ==

== Session Layer ==

== Transport Layer ==

== Network Layer ==

== Data Link Layer ==

== Physical Layer ==

The Physical Layer

The specifications of a network’s physical layer might include the shape of the shell and the pin numbers in a cable connector, the voltages that define the 0 and 1 (on and off) values, the durations of individual data bits, and the radio frequencies and modulation methods used by a radio transmitter and receiver.

The Data Link Layer

The Network Layer

Within the Internet, the exchange of data between LANs, wide area networks (WANs), and the core Internet trunk circuits occurs at the network layer.

The Transport Layer

The Session Layer

The Presentation Layer

The Application Layer

INTRODUCTION TO NETWORKING

2008-10-10T05:00:00.000-07:00

Broadband wireless networks are one more step toward the Internet’s ultimate destiny of interconnecting everything in the known universe.

A wireless network combines two kinds of communication technology: data networks that make it possible to share information among two or more computers, and radio (or wireless) communication that uses electromagnetic radiation to move information from one place to another.

The earliest Wi-Fi systems provided a convenient way to connect a laptop computer to an office network and to connect computers to a home network without stringing cables between rooms. Today, Wi-Fi and other broadband services allow millions of users to connect to the Internet when they’re away from their homes or offices, as wireless signals cover entire metropolitan areas.

A variety of products and services use different methods to accomplish essentially the same objective: wirelessly exchanging network data using radio signals. Each service has a somewhat different set of features, and each uses a slightly different technology. The three most widely used systems are Wi-Fi, WiMAX, and 3G cellular service.

The next chapter explains how these three broadband wireless networks work. But before we go into detail about specific wireless data network services, it will be helpful to understand networks in more general terms.

Moving Data Around

To begin, let’s review the general structure of computer data and the methods that networks use to move data from one place to another. This is very basic stuff that might already be familiar to you, but bear with me for a few pages. This really will help you to understand how a wireless network operates.

Bits and Bytes

As you probably know, the processing unit of a computer can recognize only two information states: either a signal is present or not present at the input to the processor. These two conditions are usually described as 1 and 0, on and off, or mark and space. Each instance of a 1 or a 0 is a bit.

The form that each 1 or 0 takes varies in different types of communication channels. It can be a light, a sound, or an electrical charge that is either on or off, a series of long and short sounds or light flashes, two different audio tones, or two different radio frequencies.

Individual bits are not particularly useful, but when you string 8 of them together into a byte, you can have 256 different combinations. That’s enough to assign different sequences to all the letters in the alphabet (both upperand lowercase), the 10 digits from 0 to 9, spaces between words, and other symbols such as punctuation marks and letters used in foreign alphabets.

A modern computer recognizes and processes several 8-bit bytes at the same time. When processing is complete, the computer transmits the same stream of bits at its output. The output might be connected to a printer, a video display, or a data communication channel. Or it might be something else entirely, such as a series of flashing lights.

The inputs and outputs that we’re concerned about here are the ones that form a communication circuit. Like the computer processor, a data channel can recognize only one bit at a time. Either there’s a signal on the line or there isn’t.

However, over short distances, it’s possible to send the data through a cable that carries eight (or some multiple of eight) signals in parallel through eight separate wires. Obviously, a parallel connection can be eight times faster than sending one bit through a single wire, but those eight wires cost eight times as much as a single wire. That added cost is insignificant when the wires are only a foot or two long, but when you’re trying to send the data over a long distance, that additional cost can be prohibitive. And when you’re using existing circuits, such as telephone lines, you don’t have any choice; you must find a way to send all eight bits through the existing pair of wires (or other media). The solution is to transmit one bit at a time with some additional bits and pauses that identify the beginning of each new byte. This is a serial data communication channel, which means that you’re sending bits one after another. At this stage, it doesn’t matter what medium you use to transmit those bits—it could be electrical impulses on a wire, two different audio tones, a series of flashing lights, or even a lot of notes attached to the legs of carrier pigeons—but you must have a method for converting the output of the computer to the signals used by the transmission medium and converting it back again at the other end.

Error Checking

In a perfect transmission circuit, the signal that goes in at one end will be absolutely identical to the one that comes out at the other end. But in the real world, there’s almost always some kind of noise that can interfere with our original pure signal. Noise is defined as anything that is added to the original signal; it could be caused by a lightning strike, interference from another communication channel, or dirt on an electrical contact someplace in the circuit (or in the case of those carrier pigeons, an attack by a marauding hawk). Whatever the source, noise in the channel can interrupt the flow of data. In a modern communication system, those bits are pouring through the circuit extremely quickly—millions of them every second—so a noise hit

for even a fraction of a second can obliterate enough bits to turn your data into digital gibberish.

Therefore, you must include a process called error checking in your data stream. Error checking is accomplished by adding some kind of standard information to each byte. In a simple computer data network, the handshaking information (described in the next section) is called the parity bit, which tells the device receiving each byte whether the sum of the ones and zeroes inside the byte is odd or even. If the receiving device discovers that the parity bit is not what it expected, it instructs the transmitter to send the same byte again. This value is called a checksum. More complex networks, including wireless systems, include additional error checking handshaking data with each string of data.

Handshaking

Of course, the computer that originates a message or a stream of data can’t just jump online and start sending bytes. First it has to warn the device at the other end that it is ready to send data and make sure that the intended recipient is ready to accept data. To accomplish this, a series of handshaking requests and answers must surround the actual data.

The sequence of requests goes something like this:

Origin: “Hey destination! I have some data for you.”

Destination: “Okay, origin, go ahead. I’m ready.”

Origin: “Here comes the data.”

Origin: Data data data data . . . checksum

Origin: “That’s the message. Did you get it?”

Destination: “I got something, but it appears to be damaged.”

Origin: “Here it is again.”

Origin: Data data data data . . . checksum

Origin: “Did you get it that time?”

Destination: “Yup, I got it. I’m ready for more data.”

We can leave the specific contents of the handshaking information to the network designers and engineers, but it’s important to understand that every bit that moves through a computer data network is not part of the original information that arrived at the input computer. In a complex

network, such as a wireless data channel, as much as 40 percent or more of the transmitted data is handshaking and other overhead. It’s all essential, but every one of those bits increases the amount of time that the message needs to move through the network.

Finding the Destination

Communication over a direct physical connection (e.g., a wired connection) between the origin and destination doesn’t need to include any kind of address or routing information as part of the message. You might have to set up the connection first (by placing a telephone call or plugging cables into a switchboard), but after you’re connected, the link remains in place until you instruct the system to disconnect. This kind of connection is great for voice and simple data links, but it’s not efficient for digital data on a complex network that serves many origins and destinations because a single connection ties up the circuit all the time, even when no data is moving through the channel.

The alternative is to send your message to a switching center that will hold it until a link to the destination becomes available. This is known as a store and forward system. If the network has been properly designed for the type of data and the amount of traffic in the system, the waiting time will be insignificant. If the communication network covers a lot of territory, you can forward the message to one or more intermediate switching centers before it reaches its ultimate destination. The great advantage of this approach is that many messages can share the same circuits on an as-available basis.

To make the network even more efficient, you can divide messages that are longer than some arbitrary limit into separate pieces called packets. Packets from more than one message can travel together on the same circuit, reassemble themselves into the original messages at the destination, and combine with packets that contain other messages as they travel between switching centers. Each data packet must also contain another set of information: the address of the packet’s destination, the sequence of the packet relative to other packets in the original transmission, and so forth. Some of this information instructs the switching centers where to forward each packet, and other information tells the destination device how to reassemble the data in the packet back into the original message.

That same pattern is repeated every time you add another layer of activity to a communication system. Each layer can attach additional information to the original message and strip off that information after it has done whatever the added information instructed it to do. By the time a message travels from a laptop computer on a wireless network through a local area network (LAN) and an Internet gateway to a distant computer that is connected to another LAN, a dozen or more information attachments might be added and removed before the recipient reads the original text. A package of data that includes address and control information ahead of the bits that contain the content of the message, followed by an error-checking sequence, is called a frame. Both wired and wireless networks divide the data stream into frames

that contain various forms of handshaking information along with the original data.

It might be helpful to think of these bits, bytes, packets, and frames as the digital version of a letter that you send through a complicated mail delivery system:

1. You write a letter and put it into an envelope. The name and address of the recipient is on the outside of the envelope.

2. You take the letter to the mail room, where a clerk puts your envelope into a bigger Express Mail envelope. The big envelope has the name and address of the office where the recipient works.

3. The mail room clerk takes the big envelope to the post office where another clerk puts it into a mail sack. The post office attaches a tag to the sack, marked with the location of the post office that serves the recipient’s office.

4. The mail sack travels on a truck to the airport, where it is loaded into a shipping container along with other sacks going to the same destination city. The shipping container has a label that tells the freight handlers there’s mail inside.

5. The freight handlers place the container inside an airplane.

6. At this point, your letter is inside your envelope, which is inside the Express Mail envelope, which is inside a mail sack, inside a container, inside an airplane. The airplane flies to another airport near the destination city.

7. At the destination airport, the ground crew unloads the container from the airplane.

8. The freight handlers remove the sack from the shipping container and put it on another truck.

9. The truck takes the sack to a post office near the recipient’s office.

10. At the post office, another mail clerk takes the big envelope out of the sack and gives it to a letter carrier.

11. The letter carrier delivers the big Express Mail envelope to the recipient’s office.

12. The receptionist in the office takes your envelope out of the Express Mail envelope and gives it to the recipient.

13. The recipient opens your envelope and reads the letter.

At each step, the information on the outside of the package tells somebody how to handle it, but that person doesn’t care what’s inside. Neither you nor the person who ultimately reads your letter ever sees the big Express Mail envelope, the mail sack, the truck, the container, or the airplane, but every one of those containers plays an important part in moving your letter

from here to there.

Instead of envelopes, sacks, containers, and airplanes, an electronic message uses strings of data at the beginning or end of each packet to tell the system how and where to handle your message, but the end result is just about the same. In the OSI network model (described in the next section), each mode of transportation is a separate layer.

Fortunately, the network software adds and removes all of the preambles, addresses, checksums, and other information automatically so you and the person receiving your message never see them.

However, each item added to the original data increases the size of the packet, frame, or other package, and therefore increases the amount of time necessary to transmit the data through the network. Because the nominal data transfer speed includes all the overhead information along with the data in your original message, the actual data transfer speed through a wireless network is a lot slower than the nominal speed. In other words, even if your network connects at 11Mbps, your actual file transfer speed might only be about 6 or 7Mbps or even less. That sounds like a huge slowdown, but it really doesn’t matter in a Wi-Fi network that’s connected to the Internet through a 1.5Mbps DSL line or even a 5Mbps cable modem; your wireless

link is still able to handle data transfer more quickly than the DSL or cable modem can provide it. On the other hand, if you’re using Wi-Fi with an ultrafast fiber optic connection to the Internet, or if you want to move very large audio, video, or CAD files around your own local network, you will want to use one of the faster Wi-Fi versions, either 802.11g or (when it becomes available) 802.11n.