2008-10-10
INTRODUCTION TO NETWORKING
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.
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