|[ Team LiB ]|
A network is a collection of ideas, hardware, and software. The software comprises both the programs that make it work and the protocols that let everything work together. The hardware involves the network adapters as well as the wires, hubs, concentrators, routers, and even more exotic fauna. Getting it all to work together requires standardization.
Because of the layered design of most networks, these standards can appear at any level in the hierarchy; and they do. Some cover a single layer; others span them all to create a cohesive system.
Current technology makes the best small computer network a hub-based peer-to-peer design, cabled with twisted-pair wiring and running the software built in to your operating system. The big choice you face is the hardware standard. In the last few years, networks have converged on two basic hardware standards: 10Base-T and 100Base-T. Both are specific implementations of Ethernet.
Just as celebrities are people famous principally for being famous, 10Base-T and 100Base-T are popular because they are popular. They are well known and generally understood. Components for either are widely available and inexpensive. Setting them up is easy and support is widely available.
The distinguishing characteristic of network hardware is the medium used for connecting nodes. Small networks—the kind you might use in your home, office, or small business—most commonly use one of three interconnection types. They may be wired together in the classic style, typically using twisted-pair wires in a star-based topology. They may be linked wirelessly using low-powered radio systems, or their signals may be piggybacked on an already existing wiring system, such as telephone lines or utility power lines.
The elder statesman of networking is Ethernet. It still reigns as king of the wires, and wireless systems have appropriated much of its technology. It shows the extreme foresight of the engineers at Xerox Corporation's Palo Alto research center who developed it in the 1970s for linking the company's early Alto workstations to laser printers. The invention of Ethernet is usually credited to Robert Metcalf, who later went on to found 3Com Corporation, an early major supplier of computer networking hardware and software. During its first years, Ethernet was proprietary to Xerox, a technology without a purpose, in a world in which the personal computer had not yet been invented.
In September, 1980, however, Xerox joined with minicomputer maker Digital Equipment Corporation and semiconductor manufacturer Intel Corporation to publish the first Ethernet specification, which later became known as E.SPEC VER.1. The original specification was followed in November, 1982, by a revision that has become today's widely used standard, E.SPEC VER.2.
This specification is not what most people call Ethernet, however. In January, 1985, the Institute of Electrical and Electronic Engineers published a networking system derived from Ethernet but not identical to it. The result was the IEEE 802.3 specification. Ethernet and IEEE 802.3 share many characteristics—physically, they use the same wiring and connection schemes—but each uses its own packet structure. Consequently, although you can plug host adapters for true Ethernet and IEEE 802.3 together in the same cabling system, the two standards will not be able to talk to one another. No matter. No one uses real Ethernet anymore. They use 802.3 instead and call it Ethernet.
Under the OSI Model, Ethernet provides the Data Link and Physical layers—although Ethernet splits the Data Link layer into two layers of its own: Media Access Control and the Logic Link Control layer (or the MAC-client layer). The Ethernet specifications define the cable, connections, and signals as well as the packet structure and control on the physical medium. Table 12.2 shows the structure of an IEEE 802.3 packet.
Data Link Layer
The basis of Ethernet is a clever scheme for arbitrating access to the central bus of the system. The protocol, formally described as Carrier Sense, Multiple Access with Collision Detection (CSMA/CD) is often described as being like a party line. It's not. It's much more like polite conversation. All the computers in the network patiently listen to everything that's going on across the network backbone. Only when there is a pause in the conversation will a new computer begin to speak. And if two or more computers start to talk at the same time, all become quiet. They will wait for a random interval (and because it is random, each will wait a different interval) and, after the wait, attempt to begin speaking again. One will be lucky and win access to the network. The other, unlucky computers hear the first computer blabbing away and wait for another pause.
Access to the network line is not guaranteed in any period by the Ethernet protocol. The laws of probability guide the system, and they dictate that eventually every device that desires access will get it. Consequently, Ethernet is described as a probabilistic access system. As a practical matter, when few devices (compared to the bandwidth of the system) attempt to use the Ethernet system, delays are minimal because all of them trying to talk at one time is unlikely. As demand approaches the capacity of the system, however, the efficiency of the probability-based protocol plummets. The size limit of an Ethernet system is not set by the number of computers but by the amount of traffic; the more packets computers send, the more contention, and the more frustrated attempts.
The Ethernet packet protocol has many physical embodiments. These can embrace just about any topology, type of cable, or speed. The IEEE 802.3 specification defines several of these, and it assigns a code name and specification to each. Among today's Ethernet implementations, the most basic operates at a raw speed of 10MHz. That is, the clock frequency of the signals on the Ethernet (or IEEE 802.3) wire is 10MHz. Actual throughput is lower because packets cannot occupy the full bandwidth of the Ethernet system. Moreover, every packet contains formatting and address information that steals space that could be used for data.
Originally, Ethernet used coaxial cables, and two versions of the 10MHz IEEE version of Ethernet have proved popular: 10Base-5 (which uses thick coaxial cable, about one-half inch in diameter) and 10Base-2 (which uses a thin coaxial cable, about 2/10th inch in diameter).
Twisted-pair wiring is now used almost universally in Ethernet systems, except in special applications (for example, to link hubs together or to extend the range of connections). Basically the same kind of wire is used for speed that spans two orders of magnitude.
The base level for twisted-pair Ethernet is 10Base-T, which transfers the same signals as the coaxial systems onto two pairs of ordinary copper wires. Each wire pair carries a 10MHz balanced signal—one carrying data to the device and the other carrying it away (in other words, separate receive and transit channels) for full-duplex operation.
The next increment up is 100Base-T, which is likely today's most popular wired networking standard. The 100Base-T system operates at 100MHz, yielding higher performance consistent with transferring multimedia and other data-intensive applications across the network. Its speed has made it the system of choice in most new installations.
During its gestation, 100Base-T wasn't a single system but rather a family of siblings, each designed for different wiring environments. 100Base-TX is the purest implementation and the most enduring—but it's also the most demanding. It requires Class 5 wiring, shielded twisted-pair designed for data applications. In return for the cost of the high-class wiring, it permits full-duplex operation so that any network node can both send and receive data simultaneously. The signals on the cable actually operate at 125MHz but use a five-bit encoding scheme for every four bits. The extra code groups are used primarily for error control. The current 100Base-T standard is formalized in the IEEE 802.3u specification.
To make the transition from 10Base-T to higher speeds easier, 100Base-T4 was designed to work with shielded or unshielded voice-grade wiring, but it only allows for half-duplex operations across four wire pairs. In addition, 100Base-T2 uses sophisticated data coding to squeeze a 100Mbps data rate on ordinary voice-grade cables using one two-wire pair. Currently neither of these transitional formats is in general use.
At the highest-speed end, Gigabit Ethernet moves data at a billion bits per second. The first implementations used fiber optic media, but the most popular format is using the same Category 5 twisted-pair wires as slower versions of Ethernet. Commonly called 1000Base-T and officially sanctioned under the IEEE 802.3ab standard, Gigabit Ethernet's high speed is not a direct upward mapping of 100MHz technology. Category 5 cables won't support a 1GHz speed. Instead, 1000Base-T uses all four pairs in a standard Cat 5 cable, each one operating at 125MHz. To create a 1000Gbps data rate, the 1000Base-T adapter splits incoming data into four streams and then uses a five-level voltage coding scheme to encode two bits in every clock cycle. The data code requires only four levels. The fifth is used for forward error correction. The standard allows both half-duplex (four pairs) and full-duplex (eight pairs using two Cat 5 cables) operation.
The problem with wiring together a network is the wiring, and the best way to eliminate the headaches of network wiring is to eliminate the wires themselves. True wireless networking systems do exactly that, substituting radio signals for the data line. Once an exotic technology, wireless networking is now mainstream with standardized components both readily available and affordable.
Wireless networking has won great favor, both in homes and businesses, because it allows mobility. Cutting the cable means you can go anywhere, which makes wireless perfect for notebook computers (and why many new notebooks come with built-in wireless networking). If you want to tote your computer around your house and work anywhere, wireless is the only way to go. Many airports and businesses also use standard wireless connections, so if you equip your portable computer with wireless for your home, you may be able to use it when you're on the move.
Moreover, wireless networks are easier to install, at least at the hardware end. You slide a PC Card into your notebook computer, plug the hub into a wall outlet, and you're connected. If you're afraid of things technical, that makes wireless your first choice. Of course, the hardware technology does nothing to make software installation easier, but wireless doesn't suffer any handicaps beyond all other network technologies.
On the other hand, wireless has a limited range that varies with the standard you use. Government rules restrict the power that wireless network systems can transmit with. Although equipment-makers quote ranges of 300 meters and more, that distance applies only in the open air—great if you're computing on the football field but misleading should you want to be connected indoors. In practical application, a single wireless network hub might not cover an entire large home.
All wireless networks are also open to security intrusions. Once you put your data on the air, it is open for inspection by anyone capable of receiving the signals. Encryption, if used, will keep your data reasonably secret (although researchers have demonstrated that resourceful snoops can break the code in an hour). More worrisome is that others can tap into your network and take advantage of your Internet connection. When you block them, you sacrifice some of the convenience of wireless.
Wireless networks endured a long teething period during which they suffered a nearly fatal malaise—a lack of standardization. Wireless systems were, at one time, all proprietary. But wireless technology proved so useful that the IEEE standardized it and keeps on adding new standards. Two are currently in wide deployment, and several others are nearing approval. Table 12.3 summarizes the current standards situation.
Through the year 2001, IEEE 802.11b was the most popular wireless standard on the market, popularized under the name Wireless Fidelity (WiFi), given to it by the industry group, the Wireless Ethernet Compatibility Alliance. That group now uses the name WiFi Alliance. WiFi equipment is also sometimes called 2.4MHz wireless networking because of the frequency band in which it operates.
Until October 2001, WiFi was the only official wireless networking standard. Despite its "b" designation, 802.11b was the first official wireless networking standard to be marketed. Its destiny was set from its conception. It was the one IEEE wireless standard with acceptable performance that could readily be built.
The range of 802.11b is usually given as up to 300 meters—nearly 1000 feet—a claim akin to saying that you're up to 150 feet tall. Anything that gets in the way—a wall, a drapery, or a cat—cuts down that range. Moreover, the figure is mostly theoretical. Actual equipment varies widely in its reach. With some gear (such as the bargain models I chose), you'll be lucky to reach the full length of your house.
Moreover, range isn't what you might think. 802.11b is designed to gracefully degrade. What that engineering nonsense means is that the farther away you get, the slower your connection becomes. When you're near the limits of your system, you'll have a 2Mbps connection.
Note that the range is set by the least powerful transmitter and weakest receiver. In most cases, the least powerful transmitter is in the PC Card adapter in your notebook computer. Power in these is often purposely kept low to maintain battery life and to stay within the engineering limits of the card housing. You can often achieve longer distances with an external adapter, such as one that plugs into your computer's USB port.
802.11b is less than ideal for a home wireless network for more reasons than speed. Interference is chief among them. The 2.4GHz band within which 802.11b works is one of the most popular among engineers and designers. In addition to network equipment, it hosts cordless telephones and the Bluetooth interconnection system. Worse, it is close to the frequency used by consumer microwave ovens. Because the band is unlicensed, you can never be sure what you'll be sharing it with. Although 802.11b uses spread-spectrum modulation, which makes it relatively immune to interference, you can still lose network connections when you switch on your cordless phone.
The place to start with 802.11a is with the name. Back-stepping one notch in the alphabet doesn't mean moving down the technological ladder, as it does with software revisions. When the standards-makers at the IEEE put pen to paper for wireless standards, 802.11a did come first, but it was the dream before the reality. The high speed of 802.11a was what they wanted, but they amended their views to accommodate its slower sibling, which they knew they could deliver faster. When manufacturers introduced equipment following the 802.11a standard, they initially promoted it as WiFi-5 because of the similarity with the earlier technology. The "5" designated the higher frequency band used by the newer system. In October, 2002, however, the WiFi Alliance decided to drop the WiFi-5 term—mostly because people wondered whatever happened to WiFi-2, 3, and 4.
From the beginning, 802.11a was meant by its IEEE devisors as the big brother to 802.11b. It is designed to be faster and provide more channels to support a greater number of simultaneous users. The bit-rate of the system is 54Mbps, although in practical application actual throughput tops out at about half that rate. In addition, as the distance between the computers and access points increase, the 802.11a system is designed to degrade in speed to ensure data integrity, slowing down to about 6Mbps at the farthest reaches of its range.
802.11a fits into the UN-II (Unlicensed National Information Infrastructure) band that's centered at about 5.2GHz, a band that's currently little used by other equipment and services. The design of 802.11a provides for eight channels, allowing that number of access points to overlap in coverage without interference problems.
The higher frequency band has a big advantage—bandwidth. There's more room for multiple channels, which means more equipment can operate in a given space without interference.
But the UN-II band occupied by 802.11a isn't without its problems. The biggest is physics. All engineering rules of thumb say the higher the frequency, the shorter the range. The 5.2GHz operating frequency puts 802.11a at a severe disadvantage, at least in theory.
Practically, however, the higher intrinsic speed of 802.11a not only wipes out the disadvantage but also puts the faster standard ahead. Because of the digital technology used by the common wireless networking systems, weaker signals mean lower speed rather than an abrupt loss of signal. The farther away from the access point a user ventures, the lower the speed his connection is likely to support.
Although physics doesn't favor 802.11a's signals, mathematics does. Because 802.11a starts out faster, it maintains a performance edge no matter the distance. For example, push an 802.11b link to 50 feet, and you can't possibly get it to operate faster than 11Mbps. Although that range causes some degradation at 802.11a's higher frequency, under that standard you can expect about 36Mbps, according to one manufacturer of the equipment.
Raw distance is a red herring in figuring how many people can connect to a wireless system. When the reach of a single access point is insufficient, the solution is to add another access point rather than to try to push up the range. Networks with multiple access points can serve any size campus. The eight channels available under 802.11a make adding more access points easier.
There is no such thing as an 802.11b+ standard, at least a standard adopted by some official-sounding organization such as the IEEE. But you'll find a wealth of networking gear available that uses the technology that's masquerading under the name. Meant to fill the gap between slow-but-cheap 802.11b and faster-but-pricier 802.11a, this technology adds a different modulation system to ordinary 802.11b equipment to gain double the speed and about 30 percent more range. Because the technology is, at heart, 802.11b, building dual-mode equipment is a breeze, and most manufacturers do it. As a result, you get two speeds for the price of one (often less, because the 802.11b+ chipset is cheaper than other 802.11b chipsets).
The heart of 802.11b+ is the packet binary convolutionary coding system developed by Texas Instruments (TI) under the trademark PBCC-22. (A convolutionary code encodes a stream of data in which the symbol encoding a given bit depends on previous bits.) The "22" in the name refers to the speed. The only previous version was PBCC-11, the forerunner of PBCC-22 and also developed by TI.
PBCC-22 is a more efficient coding system than that used by ordinary 802.11b. In addition, the TI system uses eight-phase shift keying instead of 802.11b quadrature phase shift keying, but at the same data rate (11Mbps). The result is that the system fits twice the data in the same bandwidth as 802.11b. At the same time, the signal stands out better from background noise, which, according to TI, produces 30 percent more linear range or coverage of 70 percent more area. The same technology could be used to increase the speed of the 802.11b signaling system to 33MHz, although without benefit of the range increase. This aspect of PBCC-22 is not part of the TI chipset now offered for 802.1b+ components and has not received FCC approval.
802.11b+ can be considered a superset of 802.11b. Adapters and hubs meant for the new system also work with all ordinary 802.11b equipment, switching to their high speed when they find a mate operating at the higher speed.
Texas Instruments makes the only chipset to use 802.11b+ technology, so all equipment using the technology uses the TI chipset. TI often prices this chipset below the price of ordinary 802.11b chipsets, so the higher speed of 802.11b+ often is less expensive than the performance of equipment strictly adhering to the official 802.11b standard.
As this is written, 802.11g is a proposal rather than an official standard, although its approval is expected in 2003. In effect a "greatest hits" standard, 802.11g combines the frequency band used by 802.11b with the modulation and speed of 802.11a, putting 54Mb operation in the 2.4GHz Industrial-Scientific-Medical (ISM) band.
Because of the lower frequency of the ISM band, 802.11g automatically gets an edge in range over 802.11a. At any given speed, 802.11g nearly doubles the coverage of 802.11a.
In addition, promoters cite that the ISM band is subject to fewer restrictions throughout the world than the 5GHz band, part of which is parceled out for military use in many countries. Of course, the opposite side of the coin is that the ISM band is more in demand and subject to greater interference. It also does not allow for as many channels as the 5GHz band.
Many of its promoters see 802.11g as a stepping stone to 802.11a. It allows engineers to create dual-speed equipment more easily because the two speeds (11Mbps and 54Mbps) will be able to share the same radio circuits.
Nothing restricts a network or its equipment to using only one standard. To ease people who have already installed WiFi equipment into the higher-performance WiFi-5 standard, many manufacturers offer dual-standard equipment, including hubs that can recognize and use either standard to match the signals it receives. They also offer dual-standard network adapters capable of linking at the highest speed to whatever wireless signals are available. Nothing in the WiFi and WiFi-5 standards requires this interoperability.
Other high-speed standards do, however, guarantee interoperability. The proprietary 802.11b+ system is built around and is compatible with standard WiFi. Equipment made to one standard will operate with equipment made to the other, at the highest speed both devices support.
The various 802.11 standards are promulgated by the IEEE but are promoted by the WiFi Alliance. The organization maintains a Web site at www.weca.net.
One of the original design goals of the 10Base-T wiring scheme was for it to use the same kind of wires as used by ordinary telephones. Naturally, some people thought it would be a great idea to use the same wires as their telephones, but they were unwilling to give up the use of their phones when they wanted to network their computers.
Clever engineers, however, realized that normal voice conversations use only a tiny part of the bandwidth of telephone wiring—and voices use an entirely different part of that bandwidth than do computer networks. By selectively blocking the network signals from telephones and the telephone signals from networking components, they could put a network on the regular telephone wiring in a home or office while still using the same wiring for ordinary telephones.
Of course, there's a big difference between wiring a network and wiring a telephone system. A network runs point to point, from a hub to a network adapter. A telephone system runs every which way, connecting telephones through branches and loops and often leaving stubs of wire trailing off into nowhere. Telephone wiring is like an obstacle course to network signals.
Tut Systems devised a way to make it all work by developing the necessary signal-blocking adapters and reducing the data rate to about a megabit per second, to cope with the poor quality of telephone wiring. Tut Systems called this system HomeRun.
In June, 1998, eleven companies, including Tut (the others being 3Com, AMD, AT&T Wireless, Broadcom, Compaq, Conexant, Hewlett-Packard, IBM, Intel, and Lucent Technologies) formed the Home Phoneline Networking Alliance (HomePNA) to promote telephone-line networking. The alliance chose HomeRun as the foundation for its first standard, HomePNA version 1.0
In September, 2000, the alliance (which had grown to over 150 companies) kicked up the data rate to about 10Mbps with a new HomePNA version—Version 2.0.
On November 12, 2001, HomePNA released market (not technical) specifications for the next generation of telephone-line networking, HomePNA Version 3.0, with a target of 100Mbps speed. The new standard will be backward compatible with HomePNA 1.0 and 2.0 and won't interfere with normal and advanced telephone services, including POTS, ISDN, and xDSL. A new feature, Voice-over-HomePNA, will allow you to send up to eight high-quality telephone conversations through a single pair of ordinary telephone wires within your home. According to the association, the HomePNA Version 3.0 specification will be released by the end of 2002 and was not available as this is being written.
Version 1.0 is a fairly straightforward adaptation of standard Ethernet, with only the data rate slowed to accommodate the vagaries of the unpredictable telephone wiring environment. The system does not adapt in any way to the telephone line. It provides its standard signal and that's that. Low speed is the only assurance of signal integrity. When a packet doesn't get through, the sending device simply sends it again. The whole system is therefore simple, straightforward, and slow.
The design of Version 1.0 allows for modest-sized networks. Its addressing protocol allows for a maximum of 25 nodes networked together. The signals system has a range of about 150 meters (almost 500 feet), although its range will vary with the actual installation—some telephone systems are more complex (and less forgiving) than others.
Although HomePNA Version 1.0 is capable of most routine business chores, such as sharing a printer or DSL connection, it lacks the performance required for the audio and video applications that are becoming popular in home networking systems. Version 2.0 combines several technologies to achieve the necessary speed. It has two designated signaling rates: 2 million baud and 4 million baud (that's 2MHz and 4MHz, respectively). The signal uses quadrature amplitude modulation (the same technology of 28.8Kbps modems), but it can use several coding schemes, from 4 to 256 symbols per baud. As a result, the standard allows for actual data rates from 4Mbps (2 Mbaud carrier with 4 QAM modulation) to 32Mbps (4 Mbaud at 256 QAM modulation). The devices negotiate the actual speed.
Compared to ordinary 10Base-T Ethernet systems, HomePNA Version 2.0 suffers severely from transmission overhead, cutting its top speed well below its potential. To ensure reliability, HomePNA uses its highest speeds only for the data part of its packets (if at all). The system sends the header and tailer of each data frame, 84 bytes of each frame, at the lowest rate (4Mbps) to ensure that control information is error free. As a consequence, in every frame, overhead eats almost one-third of the potential bandwidth at the 32Mbps data rate, about 80 percent at the 16Mbps data rate, and correspondingly less at lower rates.
HomePNA is a peer-to-peer networking system. It does not use a special hub or server. Each computer on the network plugs into a HomePNA adapter (usually through a USB port), which then plugs into a telephone wall jack. If you do not have a wall jack near your computer, you can use extension telephone cables to lengthen the reach of the adapter. No other hardware is required. Once all your computers are plugged in, you set up the network software as you would for any wired network.
The HomePNA specification is maintained by the Home Phoneline Networking Alliance. The group maintains a Web site at www.homepna.org.
The underlying assumption of the HomePlug networking system is that if you want to network computers in your home, you likely already have power wiring installed. If not, you don't likely need a network because you don't have any electricity to run your computers. Those big, fat, juicy electrical wires whisking dozens of amps of house current throughout your house can easily carry an extra milliwatt or two of network signal. In that the power lines inside your walls run to every room of your house, you should be able to plug your network adapter into an outlet and transport the signals everywhere you need them.
You'll want to be careful if you try such a thing. Unlike computer signals, house current can kill you—and the circuitry of your network adapter. House current is entirely different stuff from network signals. That's bad if you stick your fingers into a light socket, but it's good if you want to piggyback network signals on power lines. The big difference between the two means that it's relatively easy for a special adapter (not an ordinary network adapter!) to separate the power from the data.
On the other hand, although those heavy copper wires are adept at conducting high-current power throughout your home, they are a morass for multimegahertz data signals. At the frequencies needed for practical network operation, home wiring is literally a maze, with multiple paths, most of which lead into dead-ends. Some frequencies can squeak through the maze unscathed, whereas others disappear entirely, never to be seen again. Motors, fluorescent lights, even fish-tank pumps add noise and interference to your power lines, not to mention stuff that sneaks in from the outside, such as bursts of lightning. Radio frequency signals like those a network uses bounce through this booby-trapped electrical maze like a hand grenade caught in a pinball machine.
The HomePlug solution is to try, try again. Rather than using a single technology to bust through the maze, HomePlug uses several. It is adaptive. It tests the wiring between network nodes and checks what sort of signals will work best to transport data between them. It then switches to those signals to make the transfer. Just in case, everything that HomePlug carries is doused with a good measure of error correction. The result is a single networking standard built using several technologies that seamlessly and invisibly mate, so you can treat your home wiring as a robust data communications channel.
The basic transmission technique in the HomePlug system is called orthogonal frequency division multiplexing (OFDM), similar to that used in some telephone Digital Subscriber Lines. The OFDM system splits the incoming data stream into several with lower bit rates. Each stream then gets its own carrier to modulate, with the set of carriers carefully chosen so that the spacing between them is inversely proportional to the bit-rate of each carrier (this is the principle of orthogonality). Combining the modulated carriers of different frequencies together results in the frequency division multiplexing. In the resulting signal, it is easy to separate out the data on each carrier because the data on one carrier is easily mathematically isolated from the others.
Each isolated carrier can use a separate modulation scheme. Under HomePlug, differential quarternary phase shift keying (DQPSK) is the preferred method. It differs from conventional QPSK in that the change in the phase of the carrier, not the absolute phase of the carrier, encodes the data. Alternately, the system can shift to differential binary phase shift keying (DBPSK) for lower speed but greater reliability when the connection is poor. DQPSK shifts between four phases, whereas DBPSK shifts between two. The data streams use a convolutionary code as forward error correction. That is, they add extra bits to the digital code, which help the receiver sort out errors.
In the ideal case, with all carriers operating at top speed, the HomePlug system delivers a raw data rate of about 20Mbps. Once all the overhead from data preparation and error correction gets stripped out, it yields a clear communication channel for carrying data packets with an effective bit-rate of about 14Mbps.
If two devices cannot communicate with the full potential bandwidth of the HomePlug system, they can negotiate slower transmissions. To best match line conditions, they can selectively drop out error-plagued carrier frequencies, shift from DQPSK to more reliable DBPSK modulation, or increase the error correction applied to the data.
The HomePlug system uses a special robust mode for setting up communication between channels. Robust mode activates all channels but uses DBPSK modulation and heavy error correction. Using all channels ensures that all devices can hear the signals, and the error correction ensures messages get through, if slowly. Once devices have negotiated the connection starting with robust mode, they can shift to higher-speed transmissions.
The HomePlug system selects its carriers in the range between frequencies of 4.5 and 21MHz, operating with reduced power in the bands in which it might cause interference with amateur radio transmissions. In addition, HomePlug transmits in bursts rather than maintaining a constant signal on the line.
The HomePlug system moves packets between nodes much like the Ethernet system with a Carrier Sense Multiple Access with Collision Avoidance (CSMA/CA) protocol. That is, each device listens to the power line and starts to transmit only when it does not hear another device transmitting. If two devices accidentally start transmitting at the same time (resulting in a collision), they immediately stop, and each waits a random time before trying again. HomePlug modifies the Ethernet strategy to add priority classes, greater fairness, and faster access.
All packets sent from one device to another must be acknowledged, thus ensuring they have been properly received. If a packet is not acknowledged, the transmitting device acts as if its packet had collided with another, waiting a random period before retransmitting. An acknowledgement sent back also signals to other devices that the transmission is complete and that they can attempt to send their messages.
According to the HomePlug alliance, the protocol used by the system results in little enough delay that the system can handle time-critical applications such as streaming media and Voice-over-Internet telephone conversations.
To prevent your neighbors from listening in on your network transmissions or from sneaking into your network and stealing Internet service, the HomePlug system uses a 56-bit encryption algorithm. The code is applied to packets at the Protocol layer. All devices in a given network—that is, the HomePlug adapters for all of your computers—use the same key, so all can communicate. Intruders won't know the key and won't be recognized by the network.
With HomePlug, such intrusions are more than theoretical. Anyone sharing the same utility transformer with you—typically about six households tap into the same utility transformer—will share signals on the power line. Your HomePlug networking signals will consequently course through part of your neighborhood. That's bad if you want to keep secrets, but it could allow you and a neighbor share a single high-speed Internet connection (if your Internet Service Provider allows you to).
HomePlug is a peer-to-peer networking system. It does not use a special hub or server. Each computer on the network plugs into a HomePlug adapter (usually through a USB port), which then plugs into a wall outlet. No other hardware is required. Once all your computers are plugged in, you set up the network software as you would for any wired network.
|[ Team LiB ]|