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When computers are too far apart to be joined by a standard computer cable, a modem can enable communication between them. Remember from Chapter 2, "Basic Network Media," that network cables are limited in length. In a network environment, modems serve as a means of communication between networks and as a way to connect to the world beyond the local network.
Run the c07dem01 video located in the Demos folder on
the CD accompanying this book to view a presentation of how a modem makes
it possible for computers to communicate over a telephone line.
Figure 7.1 Digital signals versus analog waves
A digital signal has a binary form. The signal can have a value of either 0 or 1. An analog signal can be pictured as a smooth curve that can represent an infinite range of values.
Run the c07dem02, c07dem03, and c07dem04 videos located in the Demos folder on the CD accompanying this book for an illustrated overview of modem functions.
As shown in Figure 7.2, the modem at the sending end converts the computer's digital signals into analog waves and transmits the analog waves onto the telephone line. A modem at the receiving end converts the incoming analog signals back into digital signals for the receiving computer.
In other words, a sending modem MOdulates digital signals into analog signals, and a receiving modem DEModulates analog signals back into digital signals.
Figure 7.2 Modems convert digital signals to analog waves, and convert analog waves to digital signals
To use digital lines, you must install a special digital card in the computer.
Run the c07dem05 video located in the Demos folder on the CD accompanying this book to view a presentation of modem cable interfaces.
Modems are available in both internal and external models. An internal modem, as shown in Figure 7.3, is installed in a computer's expansion slot like any other circuit board.
Run the c07dem06 video located in the Demos folder on the CD accompanying this book to view a presentation of internal modems.
Figure 7.3 Internal modem installed in an expansion slot
An external modem, as shown in Figure 7.4, is a small box that is connected to the computer by a serial (RS-232) cable running from the computer's serial port to the modem's computer cable connection. The modem uses a cable with an RJ-11C connector to connect to the wall.
Figure 7.4 External modem connects through the RS-232 cable to the computer serial port
Run the c07dem07 video located in the Demos folder on
the CD accompanying this book to view a presentation of external modems.
In the early 1980s, a company called Hayes Microcomputer Products developed a modem called the Hayes Smartmodem. The Smartmodem became the standard against which other modems were measured, and generated the phrase "Hayes-compatible," just as IBM's personal computer generated the term "IBM-compatible." Because most vendors conformed to the Hayes standards, nearly all LAN modems could communicate with each other.
The early Hayes-compatible modems sent and received data at 300 bits per second (bps). Modem manufacturers currently offer modems with speeds of 56,600 bps or more.
Since the late 1980s, the International Telecommunications Union (ITU) has developed standards for modems. These specifications, known as the V series, include a number that indicates the standard. As a reference point, the V.22bis modem at 2400 bps would take 18 seconds to send a 1000-word letter. The V.34 modem at 9600 bps would take only four seconds to send the same letter, and the V.42bis compression standard in a 14,400 bps modem can send the same letter in only three seconds.
The chart in Table 7.1 presents the compression standards and their parameters since 1984. The compression standard and the bps are not necessarily related. The standard could be used with any speed of modem.
Table 7.1 Modem Compression Standards from 1984 to the Present
|V.32terbo||19,200||1993||Will communicate only with another V.32terbo|
|V.34||28,800||1994||Improved V.FastClass. Backward-compatible with earlier V. modems|
|V.42||57,600||1995||Backward-compatible with earlier V. modems—error-correction standard|
|V.90||56,600||1998||56K modem standard; resolved competition for standard between U.S. Robotic X2 and Rockwell K56 Flex standards.|
"Baud" refers to the speed at which the sound wave that carries a bit of data over the telephone lines oscillates. The term derives from the name of French telegrapher and engineer Jean-Maurice-Emile Baudot. In the early 1980s, the baud rate did equal the transmission speed of modems. At that time, 300 baud equaled 300 bits per second.
Eventually, communications engineers learned to compress and encode data so that each modulation of sound could carry more than one bit of data. This development means that the rate of bps can be greater than the baud rate. For example, a modem that modulates at 28,800 baud can actually send at 115,200 bps. Therefore, the current parameter to look for in modem speed is bps.
Several of the newer modems feature industry standards, such as V.42bis/MNP5
data compression, and have transmission speeds of 57,600 bps; and some
modems go up to 76,800 bps.
Asynchronous Communication (Async)
Asynchronous communication, known as "async," is possibly the most widespread form of connectivity in the world. This is because async was developed in order to make use of common telephone lines.
Figure 7.5 shows an asynchronous environment, in which data is transmitted in a serial stream.
Figure 7.5 Asynchronous serial data stream
Each character—letter, number, or symbol—is turned into a string of bits. Each of these strings is separated from the other strings by a start-of-character bit and a stop bit. Both the sending and receiving devices must agree on the start and stop bit sequence. The receiving computer uses the start and stop bit markers to schedule its timing functions so it is ready to receive the next byte of data.
Communication is not synchronized. There is no clocking device or method to coordinate transmission between the sender and the receiver. The sending computer just sends data, and the receiving computer just receives data. The receiving computer then checks to make sure that the received data matches what was sent. Between 20 and 27 percent of the data traffic in async communication consists of data traffic control and coordination. The actual amount depends on the type of the transmission—for example, whether parity (a form of error checking, discussed in the section that follows) is being used.
Asynchronous transmission over telephone lines can happen at up to 28,800 bps. However, the latest data compression methods can boost the 28,800 bps rate to 115,200 bps over directly connected systems.
Run the c07dem08 and c07dem09 videos located in the Demos folder on the CD accompanying this book for an overview of asynchronous communication.
Error Control Because of the potential for error, async can include a special bit, called a parity bit, which is used in an error-checking and correction scheme called parity checking. In parity checking, the number of bits sent must match exactly the number of bits received.
Run the c07dem10 video located in the Demos folder on the CD accompanying this book to view a presentation of parity bits in asynchronous communication.
The original V.32 modem standard did not provide error control. To help avoid generating errors during data transmission, a company called Microcom developed its own standard for asynchronous data-error control, the Microcom Networking Protocol (MNP). The method worked so well that other companies adopted not only the initial version of the protocol but later versions, called classes, as well. Currently, several modem manufacturers incorporate MNP Classes 2, 3, and 4 standards.
In 1989, the Comité Consultatif Internationale de Télégraphie et Téléphonie (CCITT) published an asynchronous error-control scheme called V.42. This hardware-implemented standard featured two error-control protocols. The primary error-control scheme is link access procedure for modems (LAPM), but the scheme also uses MNP Class 4. The LAPM protocol is used in communications between two modems that are V.42-compliant. If only one, but not both, of the modems is MNP 4compliant, the correct protocol to use would be MNP 4.
Improving Transmission Performance Communication performance depends on two elements:
Run the c07dem11 video located in the Demos folder on the CD accompanying this book to view a presentation of channel speed and throughput.
By removing redundant elements or empty sections, compression improves the time required to send data. Microcom's MNP Class 5 Data Compression Protocol is an example of one current data compression standard. You can improve performance, often doubling the throughput, by using data compression. When both ends of a communication link use the MNP Class 5 protocol, data transmission time can be cut in half.
Run the c07dem12 video located in the Demos folder on the CD accompanying this book to view a presentation of data compression.
The V.42bis standard, because it describes how to implement impressive data compression in hardware, makes even greater performance possible. For example, a 56.6Kbps modem using V.90 can achieve a throughput of 100Kbps.
Although compressing data can improve performance, it is not an exact science. Many factors affect the actual compression ratio of a document or file. A text file, for example, can be compressed more effectively than a complex graphic file. It is even possible to have a compressed file that is actually larger than the original. Remember, compression numbers cited by vendors are usually based on a best-case scenario.
Synchronous communication relies on a timing scheme coordinated between two devices to separate groups of bits and transmit them in blocks known as "frames." Special characters are used to begin the synchronization and check its accuracy periodically.
Run the c07dem13 and c07dem14 videos located in the Demos folder on the CD accompanying this book for an overview of synchronous communication.
Because the bits are sent and received in a timed, controlled (synchronized) process, start and stop bits are not required. Transmission stops at the end of one frame and starts again with a new one. This start-and-stop approach is much more efficient than asynchronous transmission, especially when large packets of data are being transferred. When small packets are sent, this increase in efficiency is less noticeable. Figure 7.6 shows a comparison of asynchronous and synchronous data streams.
Figure 7.6 Asynchronous data stream versus synchronous data stream
If there is an error, the synchronous error-detection and correction scheme implements a retransmission.
Run the c07dem15 video located in the Demos folder on the CD accompanying this book to view a presentation of a synchronous communication error-correction scheme.
Synchronous protocols perform a number of jobs that asynchronous protocols do not. Principally, they:
Asymmetric Digital Subscriber Line (ADSL)
The latest modem technology to become available is asymmetric digital subscriber line (ADSL). This technology converts existing twisted-pair telephone lines into access paths for multimedia and high-speed data communications. These new connections can transmit more than 8 Mbps to the subscriber and up to 1 Mbps from the subscriber.
ADSL is not without drawbacks. The technology requires special hardware, including an ADSL modem on each end of the connection. It also requires broadband cabling, which is currently only available in a few locations, and there is a limit to the connection length.
ADSL is recognized as a physical layer transmission protocol for unshielded
Figure 7.7 Ethernet hubs connected in a series
Figure 7.8 shows how several token-ring hubs can be connected to expand a network.
Figure 7.8 Token-ring hubs connected into one large ring
It is important to be careful when connecting hubs. Crossover cables are wired differently than standard patch cables, and one will not work correctly in place of the other. Check with the manufacturers to determine whether you need a standard patch cable or a crossover cable.
How Repeaters Work
A repeater works at the physical layer of the OSI Reference Model to regenerate the network's signals and resend them out on other segments. Figure 7.9 shows how repeaters regenerate weak signals.
Figure 7.9 Repeaters regenerate weakened signals
The repeater takes a weak signal from one segment, regenerates it, and passes it to the next segment. To pass data through the repeater from one segment to the next, the packets and the Logical Link Control (LLC) protocols must be identical on each segment. A repeater will not enable communication, for example, between an 802.3 LAN (Ethernet) and an 802.5 LAN (Token Ring).
Repeaters do not translate or filter signals. For a repeater to work, both segments that the repeater joins must use the same access method. The two most common access methods are carrier-sense multiple-access with collision detection (CSMA/CD) and token passing (discussed in Chapter 3, "Understanding Network Architecture"). A repeater cannot connect a segment using CSMA/CD to a segment using the token-passing access method. That is, a repeater cannot translate an Ethernet packet into a Token Ring packet.
As shown in Figure 7.10, repeaters can move packets from one kind of physical media to another. They can take an Ethernet packet coming from a thinnet coaxial-cable segment and pass it on to a fiber-optic segment, provided the repeater is capable of accepting the physical connections.
Figure 7.10 Repeaters can connect different types of media
Some multiport repeaters act as multiport hubs and connect different types of media. The same segment limits discussed in Chapter 3 apply to networks that use hubs, but the limits now refer to each segment extending from a hub rather than to the entire network.
Repeaters afford the least expensive way to expand a network. When the need arises to extend the physical network beyond its distance or node limitations, consider using a repeater to link segments if neither segment is generating much traffic or limiting costs is a major consideration.
No Isolation or Filtering Repeaters send every bit of data from one cable segment to another, even if the data consists of malformed packets or packets not destined for use on the network. This means that a problem with one segment can disrupt every other segment. Repeaters do not act as filters to restrict the flow of problem traffic.
Repeaters will also pass a broadcast storm along from one segment to the next, back and forth along the network. A broadcast storm occurs when so many broadcast messages are on the network that the number is approaching the network bandwidth limit. If a device is responding to a packet that is continuously circulating on the network, or a packet is continuously attempting to contact a system that never replies, network performance will be degraded.
Implementing a repeater This section summarizes what you need to consider when deciding whether to implement repeaters in your network.
Use a repeater to:
Repeaters improve performance by dividing the network into segments, thus reducing the number of computers per segment. When using repeaters to expand a network, don't forget about the 5-4-3 rule (introduced in Chapter 3, "Understanding Network Architecture").
Bridges can be used to:
Figure 7.11 A bridge connecting two networks
How Bridges Work
Because bridges work at the data-link layer of the OSI reference model, all information contained in the higher levels of the OSI reference model is unavailable to them. Rather than distinguish between one protocol and another, bridges simply pass all protocols along the network. All protocols pass across bridges,