How Satellite Radio Works

We all have our favorite radio stations that we preset into our car radios, flipping between them as we drive to and from work, on errands and around town. But when you travel too far away from the station, the signal breaks up and fades into static. Most radio signals can only travel about 30 or 40 miles from their source. On long trips, you might have to change radio stations every hour or so as the signals fade in and out. And it's not much fun scanning through static trying to find something -- anything -- to listen to.

Satellite Radio Image Gallery

Photo courtesy XM Satellite Radio Satellite radio broadcasters promise crystal-clear music transmitted from thousands of miles into space. See more pictures of satellite radio.

Now, imagine a radio station that can broadcast its signal from more than 22,000 miles (35,000 kilometers) away and then come through on your car radio with complete clarity. You could drive from Tacoma, Wash., to Washington, D.C., without ever having to change the radio station! Not only would you never hear static interfering with your favorite tunes, but the music would be interrupted by few or no commercials.

XM Satellite Radio and Sirius Satellite Radio both launched such a service at the beginning of the 21st century. Satellite radio, also called digital radio, offers uninterrupted, near CD-quality music beamed to your radio from space.

In February 2007, XM Radio and Sirius Radio announced that they planned to merge into a single satellite radio company. XM and Sirius are both in debt, and a merger could quickly solve that problem. The merger could also lead to lower prices and more programming choices for consumers. Some people are skeptical about the two companies joining, though, fearing a monopoly would only reduce competition, raise prices and affect consumers poorly. XM and Sirius currently must convince the FCC that a merger wouldn't violate anti-trust laws.

Even though XM and Sirius have had financial trouble, satellite radio still has a fairly strong fan base. About 8 million people subscribe to XM Radio, and more than 6 million people tune into Sirius Radio. ­Car manufacturers have been installing satellite radio receivers in some models for a few years now, and several models of portable satellite radio receivers are available from a variety of electronics companies. In this article, you'll learn what separates satellite radio from conventional radio and what you need to pick up satellite radio signals.

The Basics

Satellite radio is an idea over a decade in the making. In 1992, the U.S. Federal Communications Commission (FCC) allocated a spectrum in the "S" band (2.3 GHz) for nationwide broadcasting of satellite-based Digital Audio Radio Service (DARS). Only four companies applied for a license to broadcast over that band. The FCC gave licenses to two of these companies in 1997. CD Radio (now Sirius Satellite Radio) and American Mobile Radio (now XM Satellite Radio) paid more than $80 million each to use space in the S-band for digital satellite transmission.

At this time, there are three space-based radio broadcasters:

• Sirius Satellite Radio
• XM Satellite Radio
• WorldSpace

Satellite radio companies are comparing the significance of their service to the impact that cable TV had on television 30 years ago. Listeners won't be able to pick up local stations using satellite radio services, but they will have access to hundreds of stations offering a variety of music genres. Each company has a different plan for its broadcasting system, but the systems do share similarities. Here are the key components of the three satellite radio systems:

• Satellites
• Ground repeaters
• Radio receivers

Satellite radio works a lot like satellite TV -- you purchase a receiver and pay a monthly subscription fee for a certain number of channels. For the moment, there are slight variances in the three satellite radio companies' systems. In the next three sections, we will profile each of the companies and their current satellite radio services.

The XM/Sirius Merger Both XM and Sirius offer about 100 channels under their current plans. If both the FCC and the Department of Justice approve their proposed merger, though, things might change a little. According to the two companies, the merged company will offer a few options. You could subscribe to 50 channels from either XM or Sirius for $6.99, or you could get a "best-of" package of 100 channels selected from both networks for $14.99. If you're already an XM or Sirius customer, you wouldn’t have to replace your old radio. However, if you wanted to pick and choose your own channels from both networks, you would have to pay a little extra and purchase a new receiver. This all depends on whether the merger is approved, and as of October 2007 there have been no decisions. ­­

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XM Satellite Radio

XM Radio uses two Boeing HS 702 satellites, appropriately dubbed "Rock" and "Roll," placed in parallel geostationary orbit, one at 85 degrees west longitude and the other at 115 degrees west longitude. Geostationary Earth orbit (GEO) is about 22,223 miles (35,764 km) above Earth, and is the type of orbit most commonly used for communications satellites. The first XM satellite, "Rock," was launched on March 18, 2001, with "Roll" following on May 8. XM Radio has a third HS-702 satellite on the ground ready to be launched in case one of the two orbiting satellites fails.

Photo courtesy XM Satellite Radio This graphic illustrates how the XM Radio system works.

XM Radio's ground station transmits a signal to its two GEO satellites, which bounce the signals back down to radio receivers on the ground. The radio receivers are programmed to receive and unscramble the digital data signal, which contains up to 100 channels of digital audio. In addition to the encoded sound, the signal contains information about the broadcast. The song title, artist and genre of music are all displayed on the radio. In urban areas, where buildings can block out the satellite signal, XM's broadcasting system is supplemented by ground transmitters.

Photo courtesy XM Satellite Radio An XM Satellite Radio receiver

Each receiver contains a proprietary chipset. XM began delivering chipsets to its XM radio manufacturing partners in October 2000. The chipset consists of two custom integrated circuits designed by STMicroelectronics. XM has partnered with Pioneer, Alpine, Clarion, Delphi Delco, Sony and Motorola to manufacture XM car radios. Each satellite radio receiver uses a small, car-phone-sized antenna to receive the XM signal. General Motors has invested about $100 million in XM, and Honda has also signed an agreement to use XM radios in its cars. GM began installing XM satellite radio receivers in selected models in early 2001.

Currently, subscribers can receive the XM signal for $12.95 per month. For that price, listeners get up to 100 channels of music, talk and news. They also get access to XM Radio online, a streaming audio service with over 70 channels. Many of the channels have no commercials, with none of the channels having more than seven minutes of ads per hour. XM's content providers include USA Today, BBC, CNN, Sports Illustrated and The Weather Channel. The service bolsters that lineup with its own music channels.

Sirius Satellite Radio

Sirius originally used three SS/L-1300 satellites, instead of GEO satellites, to form an inclined elliptical satellite constellation. Sirius says the elliptical path of its satellite constellation ensures that each satellite spends about 16 hours a day over the continental United States, with at least one satellite over the country at all times. Sirius completed its three-satellite constellation on Nov. 30, 2000. A fourth satellite will remain on the ground, ready to be launched if any of the three active satellites encounters transmission problems. ­In 2006, Sirius purchased a GEO satellite because of its superior signal delivery. The GEO satellite will supplement the elliptical satellites, not replace them. It is currently under construction, and a launch is planned for fall 2008.

The Sirius system is similar to that of XM. Programs are beamed to one of the three Sirius satellites -- the satellites then transmit the signal to the ground, where your radio receiver picks up one of the channels within the signal. Signals are also be beamed to ground repeaters for listeners in urban areas where the satellite signal can be interrupted.

Just like XM Radio, Sirius currently offers a monthly subscription for $12.95 per month. Sirius produces car radios and home entertainment systems, as well as car and home kits for portable use. The Sirius receiver includes two parts: the antenna module and the receiver module. The antenna module picks up signals from the ground repeaters or the satellite, amplifies the signal and filters out any interference. The signal is then passed on to the receiver module. Inside the receiver module is a chipset consisting of eight chips. The chipset converts the signals from 2.3 gigahertz (GHz) to a lower intermediate frequency. Sirius also offers an adapter that allows conventional car radios to receive satellite signals.

WorldSpace

So far, WorldSpace has been the farthest-reaching company in the satellite radio industry. It put two of its three satellites, AfriStar and AsiaStar, in geostationary orbit before either of the other two companies launched one. AfriStar and AsiaStar were launched in October 1998 and March 2000, respectively. AmeriStar, which will offer service to South America and parts of Mexico, has not yet been launched. Each satellite transmits three signal beams, carrying more than 40 channels of programming, to three overlapping coverage areas of about 5.4 million square miles (14 million square km) each. Each of the WorldSpace satellites' three beams can deliver over 50 channels of crystal-clear audio and multimedia programming via the 1,467- to 1,492-megahertz (MHz) segment of the L-Band spectrum, which is allocated for digital audio broadcasting.

The United States is not currently part of WorldSpace's coverage area, although the company has invested in XM Radio and has an agreement with XM to share any technological developments. WorldSpace is going beyond one nation and eyeing world domination of the radio market. That might be overstating the company's intent a bit, but WorldSpace does plan to reach the corners of our world that most radio stations can't. There are millions of people living in WorldSpace's projected listening area who can't pick up a signal from a conventional radio station. WorldSpace says it has a potential audience of about 4.6 billion listeners spanning five continents.

Photo courtesy WorldSpace WorldSpace will be able to broadcast to the majority of the world's population when its AmeriStar satellite is launched.

WorldSpace broadcasters uplink their signal to one of the three satellites through a centralized hub site or an individual feeder link station located within the global uplink beam. The satellite then transmits the signal in one, two or all three beams on each satellite. Receivers on the ground then pick up the signal and provide CD-quality sound through a detachable antenna.

Photo courtesy WorldSpace Two of the WorldSpace satellite radio receivers

WorldSpace satellite receivers are capable of receiving data at a rate of 128 kilobits per second (Kbps). The receivers use the proprietary StarMan chipset, manufactured by STMicroelectronics, to receive digital signals from the satellites.

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do you know SMS Works ?

Just when we're finally used to seeing everybody constantly talking on their cell phones, it suddenly seems like no one is talking at all. Instead, they're typing away on tiny numerical pads, using their cell phones to send quick messages. SMS, or text messaging, has replaced talking on the phone for a new "thumb generation" of texters.

In this article, we'll find out how text messaging works, explore its uses and learn why it sometimes takes a while for your text message to get to its recipient.

SMS stands for short message service. Simply put, it is a method of communication that sends text between cell phones, or from a PC or handheld to a cell phone. The "short" part refers to the maximum size of the text messages: 160 characters (letters, numbers or symbols in the Latin alphabet). For other alphabets, such as Chinese, the maximum SMS size is 70 characters.

SMS Attacks Recently it has been suggested that SMS messages could be used to attack a cell phone system. The basic idea is very simple. If a large number of SMS messages were sent by computers to phones in a small geographical area (like a city), these messages would overwhelm the control channels and make it impossible for the cell phone system to set up calls. Now that cell phone providers know about the possibility of this threat, they can design systems to throttle messages coming from the SMSC onto the network.

But how do SMS messages actually get to your phone? If you have read How Cell Phones Work, you can actually see what is happening.

Even if you are not talking on your cell phone, your phone is constantly sending and receiving information. It is talking to its cell phone tower over a pathway called a control channel. The reason for this chatter is so that the cell phone system knows which cell your phone is in, and so that your phone can change cells as you move around. Every so often, your phone and the tower will exchange a packet of data that lets both of them know that everything is OK.

Your phone also uses the control channel for call setup. When someone tries to call you, the tower sends your phone a message over the control channel that tells your phone to play its ringtone. The tower also gives your phone a pair of voice channel frequencies to use for the call.

The control channel also provides the pathway for SMS messages. When a friend sends you an SMS message, the message flows through the SMSC, then to the tower, and the tower sends the message to your phone as a little packet of data on the control channel. In the same way, when you send a message, your phone sends it to the tower on the control channel and it goes from the tower to the SMSC and from there to its destination.

The actual data format for the message includes things like the length of the message, a time stamp, the destination phone number, the format, etc. For a complete byte-by-byte breakdown of the message format, see this page.

Why 160 Characters? SMS was designed to deliver short bursts of data such as numerical pages. To avoid overloading the system with more than the standard forward-and-response operation, the inventors of SMS agreed on a 160-character maximum message size. But the 160-character limit is not absolute. Length limitations may vary depending on the network, phone model and wireless carrier. Some phones don't allow you to keep typing once the 160-character limit is reached. You must send your message before continuing. However, some services will automatically break any message you send into chunks of 160 characters or less. So, you can type and send a long message, but it will be delivered as several messages.

Advantages of SMS

SMS has several advantages. It is more discreet than a phone conversation, making it the ideal form for communicating when you don't want to be overheard. It is often less time-consuming to send a text message than to make a phone call or send an e-mail. SMS doesn't require you to be at your computer like e-mail and instant messaging (IM) do -- although some phones are equipped for mobile e-mail and IM services. SMS is also a convenient way for deaf and hearing-impaired people to communicate.

SMS is a store-and-forward service, meaning that when you send a text message to a friend, the message does not go directly to your friend's cell phone. The advantage of this method is that your friend's cell phone doesn't have to be active or in range for you to send a message. The message is stored in the SMSC (for days if necessary) until your friend turns his cell phone on or moves into range, at which point the message is delivered. The message will remain stored on your friend's SIM card until he deletes it.

In addition to person-to-person messages, SMS can be used to send a message to a large number of people at a time, either from a list of contacts or to all the users within a particular area. This service is called broadcasting and is used by companies to contact groups of employees or by online services to distribute news and other information to subscribers.

In a 2004 University of Plymouth study on the psychology of SMS users, researchers found that mobile phone users were primarily either "texters" or "talkers" [ref]. Compared to the talkers, the texters sent nearly double the number of SMS messages and made less than half as many voice calls per month. The texters preferred SMS to voice calls for its convenience as well as for the ability to review a message before sending it.

Companies have come up with many uses for the service beyond just your typical person-to-person message. Because SMS doesn't overload the network as much as phone calls, it is frequently used by TV shows to let viewers vote on a poll topic or for a contestant. As a promotional tool, wireless carriers put up giant screens at concerts and other large-scale events to display text messages from people in the audience.

SMS History SMS was created during the late 1980s to work with a digital technology called GSM (global system for mobile communications), which is the basis for most modern cell phones. The Norwegian engineers who invented it wanted a very simple messaging system that worked when users' mobile phones were turned off or out of signal range. Most sources agree that the first SMS message was sent in the UK in 1992. As SMS was born in Europe, it's not surprising that it took a little longer to make its way to the United States. Even today, texting enjoys much greater popularity in Europe, though its stateside use is on the rise. A July 2005 study found that 37 percent of U.S. mobile phone owners had sent or received at least one text message in the previous month [ref].

You can use text messaging subscription services to get medication reminders sent to your phone, along with weather alerts, news headlines or even novels broken into 160-character "chapters." Internet search engines such as Yahoo! and Google have short messaging services that enable users to get information such as driving directions, movie showtimes or local business listings just by texting a query to the search engine's phone number. Social networking services such as Dodgeball use SMS to alert people who live in big cities when their friends or crushes are nearby. The possibilities for integrating SMS into your lifestyle seem endless.

Next, we'll discuss the disadvantages of SMS and look at some alternative communication technologies.

SMS Criticism and Alternatives

SMS in the News Because of the impersonal nature of SMS, it raises certain questions of etiquette -- namely, what kind of information is OK to send in a text instead of delivering it in person? Recently, several people have sought legal action after they were fired or notified of divorce proceedings via SMS. Broadcast text messages have been used to rally political activists in Beijing and to mobilize young people for riots in Belfast. Recently, a contest pitted the efficiency of SMS against Morse code (the Morse coders won).

Despite their popularity, short messaging services have received some criticism. Here are a few of the disadvantages of SMS:

• You have to pay for it. Most wireless plans charge for a certain number of text messages a month. Some only charge for user-originated messages, while others charge for incoming messages as well. If you exceed your message allowance, you may be charged 10 cents per message, and those little charges can add up.

• Speedy message delivery is not guaranteed. During periods of high traffic, it might be minutes or even hours before a message gets through.

• It's strictly for sending text messages. SMS does not support sending pictures, video or music files.

Alternatives to SMS
Alternative messaging services allow for more elaborate types of messages. With EMS (Enhanced Messaging Service), you can send formatted text, sound effects, small pictures and icons. MMS (Multimedia Messaging Service) allows you to send animations, audio and video files in addition to text. If your mobile phone is EMS- or MMS-enabled, you can use these standards just as you would SMS. However, the cost per message will be higher.

Another alternative to using SMS is using an instant messaging program, such as AOL IM, on your cell phone. This can be in the form of software that's pre-installed on your phone, or you can use WAP (Wireless Application Protocol) to access the Internet and sign into your IM account. WAP is a protocol that gives you small, simplified versions of web pages that are easily navigable on your mobile phone or PDA (check out How WAP Works for more information). You can use it to send instant messages or actual e-mails from your phone.

A common complaint about SMS is its inefficient delivery structure -- when the message center is backed up, messages take longer to reach their destination. To make message delivery faster, networks are using more new next-generation technologies such as GPRS (General Packet Radio Service).

To learn more about SMS and other forms of mobile communication, check out the links on the following page.

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How DSL Works

Introduction to How DSL Works

When you connect to the Internet, you might connect through a regular modem, through a local-area network connection in your office, through a cable modem or through a digital subscriber line (DSL) connection. DSL is a very high-speed connection that uses the same wires as a regular telephone line.

Photo courtesy HowStuffWorks Shopper A DSL modem

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Here are some advantages of DSL:

• You can leave your Internet connection open and still use the phone line for voice calls.
• The speed is much higher than a regular modem
• DSL doesn't necessarily require new wiring; it can use the phone line you already have.
• The company that offers DSL will usually provide the modem as part of the installation.

But there are disadvantages:

• A DSL connection works better when you are closer to the provider's central office. The farther away you get from the central office, the weaker the signal becomes.
• The connection is faster for receiving data than it is for sending data over the Internet.
• The service is not available everywhere.

In this article, we explain how a DSL connection manages to squeeze more information through a standard phone line -- and lets you make regular telephone calls even when you're online.

Telephone Lines
If you have read How Telephones Work, then you know that a standard telephone installation in the United States consists of a pair of copper wires that the phone company installs in your home. The copper wires have lots of room for carrying more than your phone conversations -- they are capable of handling a much greater bandwidth, or range of frequencies, than that demanded for voice. DSL exploits this "extra capacity" to carry information on the wire without disturbing the line's ability to carry conversations. The entire plan is based on matching particular frequencies to specific tasks.

To understand DSL, you first need to know a couple of things about a normal telephone line -- the kind that telephone professionals call POTS, for Plain Old Telephone Service. One of the ways that POTS makes the most of the telephone company's wires and equipment is by limiting the frequencies that the switches, telephones and other equipment will carry. Human voices, speaking in normal conversational tones, can be carried in a frequency range of 0 to 3,400 Hertz (cycles per second -- see How Telephones Work for a great demonstration of this). This range of frequencies is tiny. For example, compare this to the range of most stereo speakers, which cover from roughly 20 Hertz to 20,000 Hertz. And the wires themselves have the potential to handle frequencies up to several million Hertz in most cases.

The use of such a small portion of the wire's total bandwidth is historical -- remember that the telephone system has been in place, using a pair of copper wires to each home, for about a century. By limiting the frequencies carried over the lines, the telephone system can pack lots of wires into a very small space without worrying about interference between lines. Modern equipment that sends digital rather than analog data can safely use much more of the telephone line's capacity. DSL does just that.

Asymmetric DSL
Most homes and small business users are connected to an asymmetric DSL (ADSL) line. ADSL divides up the available frequencies in a line on the assumption that most Internet users look at, or download, much more information than they send, or upload. Under this assumption, if the connection speed from the Internet to the user is three to four times faster than the connection from the user back to the Internet, then the user will see the most benefit most of the time.

Photo courtesy Corning DSL signals can't pass through fiber-optic cables.

Precisely how much benefit you see from ADSL will greatly depend on how far you are from the central office of the company providing the ADSL service. ADSL is a distance-sensitive technology: As the connection's length increases, the signal quality decreases and the connection speed goes down. The limit for ADSL service is 18,000 feet (5,460 meters), though for speed and quality of service reasons many ADSL providers place a lower limit on the distances for the service. At the extremes of the distance limits, ADSL customers may see speeds far below the promised maximums, while customers nearer the central office have faster connections and may see extremely high speeds in the future. ADSL technology can provide maximum downstream (Internet to customer) speeds of up to 8 megabits per second (Mbps) at a distance of about 6,000 feet (1,820 meters), and upstream speeds of up to 640 kilobits per second (Kbps). In practice, the best speeds widely offered today are 1.5 Mbps downstream, with upstream speeds varying between 64 and 640 Kbps. Some vast improvements to ADSL are available in some areas through services called ASDL2 and ASDL2+. ASDL2 increases downstream to 12 Mbps and upstream to 1 Mbps, and ASDL2+ is even better -- it improves downstream to as much as 24 Mbps and upstream to 3 Mbps.­

You might wonder -- if distance is a limitation for DSL, why is it not also a limitation for voice telephone calls? The answer lies in small amplifiers called loading coils that the telephone company uses to boost voice signals. Unfortunately, these loading coils are incompatible with ADSL signals, so a voice coil in the loop between your telephone and the telephone company's central office will disqualify you from receiving ADSL. Other factors that might disqualify you from receiving ADSL include:

• Bridge taps - These are extensions, between you and the central office, that extend service to other customers. While you wouldn't notice the­se bridge taps in normal phone service, they may take the total length of the circuit beyond the distance limits of the service provider.
• Fiber-optic cables - ADSL signals can't pass through the conversion from analog to digital and back to analog that occurs if a portion of your telephone circuit comes through fiber-optic cables.
• Distance - Even if you know where your central office is (don't be surprised if you don't -- the telephone companies don't advertise their locations), looking at a map is no indication of the distance a signal must travel between your house and the office.

Next, we'll look at how the signal is split and what equipment DSL uses.

Splitting the Signal

The CAP System
There are two competing and incompatible standards for ADSL. The official ANSI standard for ADSL is a system called discrete multitone, or DMT. According to equipment manufacturers, most of the ADSL equipment installed today uses DMT. An earlier and more easily implemented standard was the carrierless amplitude/phase (CAP) system, which was used on many of the early installations of ADSL.

CAP operates by dividing the signals on the telephone line into three distinct bands: Voice conversations are carried in the 0 to 4 KHz (kilohertz) band, as they are in all POTS circuits. The upstream channel (from the user back to the server) is carried in a band between 25 and 160 KHz. The downstream channel (from the server to the user) begins at 240 KHz and goes up to a point that varies depending on a number of conditions (line length, line noise, number of users in a particular telephone company switch) but has a maximum of about 1.5 MHz (megahertz). This system, with the three channels widely separated, minimizes the possibility of interference between the channels on one line, or between the signals on different lines.

The DMT System
DMT also divides signals into separate channels, but doesn't use two fairly broad channels for upstream and downstream data. Instead, DMT divides the data into 247 separate channels, each 4 KHz wide.

One way to think about it is to imagine that the phone company divides your copper line into 247 different 4-KHz lines and then attaches a modem to each one. You get the equivalent of 247 modems connected to your computer at once. Each channel is monitored and, if the quality is too impaired, the signal is shifted to another channel. This system constantly shifts signals between different channels, searching for the best channels for transmission and reception. In addition, some of the lower channels (those starting at about 8 KHz), are used as bidirectional channels, for upstream and downstream information. Monitoring and sorting out the information on the bidirectional channels, and keeping up with the quality of all 247 channels, makes DMT more complex to implement than CAP, but gives it more flexibility on lines of differing quality.

Filters
CAP and DMT are similar in one way that you can see as a DSL user.

If you have ADSL installed, you were almost certainly given small filters to attach to the outlets that don't provide the signal to your ADSL modem. These filters are low-pass filters -- simple filters that block all signals above a certain frequency. Since all voice conversations take place below 4 KHz, the low-pass (LP) filters are built to block everything above 4 KHz, preventing the data signals from interfering with standard telephone calls.

DSL Equipment

ADSL uses two pieces of equipment, one on the customer end and one at the Internet service provider, telephone company or other provider of DSL services. At the customer's location there is a DSL transceiver, which may also provide other services. The DSL service provider has a DSL Access Multiplexer (DSLAM) to receive customer connections.

The Transceiver
Most residential customers call their DSL transceiver a "DSL modem." The engineers at the telephone company or ISP call it an ATU-R. Regardless of what it's called, it's the point where data from the user's computer or network is connected to the DSL line.

Photo courtesy Allied Telesyn DSL modem

The transceiver can connect to a customer's equipment in several ways, though most residential installation uses USB or 10 base-T Ethernet connections. While most of the ADSL transceivers sold by ISPs and telephone companies are simply transceivers, the devices used by businesses may combine network routers, network switches or other networking equipment in the same platform.

The DSLAM
The DSLAM at the access provider is the equipment that really allows DSL to happen. A DSLAM takes connections from many customers and aggregates them onto a single, high-capacity connection to the Internet. DSLAMs are generally flexible and able to support multiple types of DSL in a single central office, and different varieties of protocol and modulation -- both CAP and DMT, for example -- in the same type of DSL. In addition, the DSLAM may provide additional functions including routing or dynamic IP address assignment for the customers.

The DSLAM provides one of the main differences between user service through ADSL and through cable modems. Because cable-modem users generally share a network loop that runs through a neighborhood, adding users means lowering performance in many instances. ADSL provides a dedicated connection from each user back to the DSLAM, meaning that users won't see a performance decrease as new users are added -- until the total number of users begins to saturate the single, high-speed connection to the Internet. At that point, an upgrade by the service provider can provide additional performance for all the users connected to the DSLAM.

For information on ADSL rates and availability in the United States, go to Broadband Reports. This site can provide information on ADSL service companies in your area, the rates they charge, and customer satisfaction, as well as estimating how far you are from the nearest central office.

ADSL isn't the only type of DSL, and it's not the only way to get high-speed Internet access. Next, we'll look at ADSL alternatives.

Alternatives to ADSL

There are lots of variations in DSL technology -- many of them address DSL's distance limitations in one way or another. Other types of DSL include:

• Very high bit-rate DSL (VDSL) - This is a fast connection, but works only over a short distance. It is capable of handling Internet access, HDTV and on-demand services at rates of 52 Mbps downstream and 12 Mbps upstream.
• Symmetric DSL (SDSL) - This connection, used mainly by small businesses, doesn't allow you to use the phone at the same time, but the speed of receiving and sending data is the same.
• Rate-adaptive DSL (RADSL) - This is a variation of ADSL, but the modem can adjust the speed of the connection depending on the length and quality of the line.
• ISDN DSL (IDSL) - This is a combination of the Integrated Services Digital Network (ISDN) and DSL technology. ISDN was the solution to dial-up Internet -- it allowed voice, text graphics, video and other data to share one telephone line. This made it possible to talk on the phone and use the Internet at the same time. IDSL is faster than ISDN connections but slower than DSL. It can travel a longer distance of 5 to 6 miles, so it is usually a good option for people who can't get DSL in their area.
• Universal DLS (Uni-DSL) - This emerging technology, developed by Texas Instruments, is backwards compatible with all existing versions of DSL. It offers somewhat of a middle ground between ASDL and VDSL -- at longer distances, it can reach the speeds of ASDL, but it can provide greater speeds than VDSL at shorter distances. In some locations, Uni-DSL can provide four times the amount of speed as VDSL.

Alternatives to DSL
With DSL's distance limitation and lower availability, what are some other options? There are two major alternatives to DSL -- cable and wireless.

Cable and DSL are the two big rivals in the world of broadband. Cable isn't limited by distance like DSL -- cable wires reach most neighborhoods, and signal strengths don't weaken over long distances. While DSL allows you to use the telephone and Internet simultaneously, cable lets users watch television and surf the Internet at the same time. Many cable companies are also beginning to bundle services with cable TV, Internet and digital telephone on one bill. Although cable and DSL speeds are about the same, the one disadvantage with cable is bandwidth -- connection speeds can slow down if too many people are using a cable service at the same time.

A new technology, known as WiMax or 802.16, looks to combine the benefits of broadband and wireless. WiMax will provide high-speed wireless Internet over very long distances and will most likely provide access to large areas such as cities. WiMax technology will be available in most American cities in 2008.

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What is the Apple iPhone?

For several years, Apple enthusiasts have been asking each other the question, "Does it exist? Is Apple really making a cell phone?" Rumors came and went, but Apple, a company known for its secrecy about products in development, said nothing. In the last six months, those rumors began to take on a life of their own as industry analysts joined the blogosphere in predicting that Apple would produce a phone sometime in the first half of 2007. In mid-November, news broke that Apple had placed an order with Hon Hai Precision Industry, Co., a longtime Taiwanese manufacturing partner, for 12 million units of a new device believed to be the fabled "iPhone" [Forbes]. That news sent the rumor mill into overdrive, but Apple and Steve Jobs, the company's chief executive officer, again said nothing. Analysts began issuing predictions of cost, features and when the phone would be available, with many of them claiming to have inside information. Given Apple's extraordinary track record with the iPod and its recent innovations in desktop and laptop design, expectations were very high, perhaps impossibly so.

On January 9, at Apple's annual product showcase, Macworld Expo, Jobs finally revealed the already legendary phone, and it's beyond what anyone expected. Jobs briefly tricked his audience during his Macworld Expo keynote address, announcing three new Apple products: a widescreen iPod, a cell phone and an "Internet communicator." Each announcement drew thunderous applause from the crowd, but what Jobs then revealed was that these three products were actually all part of one device -- the Apple iPhone.

Image courtesy Apple/ ©2006 Apple Computer, Inc. All rights reserved.

Touted as a "revolutionary mobile phone," the iPhone can make calls, play music, navigate the Web, edit photos, play movies and text message, among many other capabilities. Although many of the iPhone's functions can be found in other devices, the iPhone appears to be unique in that it seamlessly blends these abilities together, while also throwing a bevy of innovations into the mix.

Jobs demonstrated many of the iPhone's features for the audience. One sequence began with a call from Phil Schiller, Apple's senior vice president of Worldwide Product Marketing. Jobs answered the call on his iPhone, added Schiller to his address book, and when Schiller asked for a photo, Jobs emailed it to him -- all while continuing the call. Later Jobs showed off the phone's ability to integrate multiple applications by using the integrated Google maps application, which knew his location, and typing in a search for Starbucks. Every Starbucks location in San Francisco showed up on screen. Jobs chose one and in a few minutes he was on the phone with Starbucks, ordering 4,000 lattes before abruptly hanging up.

In order to seamlessly integrate Web, phone, media and messaging features, the iPhone employs remarkable, groundbreaking technology. Unlike traditional smartphones that have small, finger-cramping keyboards, the iPhone has only one button for "home." Instead, its 3.5-inch high resolution, color screen, which occupies most of the phone's face, doubles as a "multi-touch" display. The display shows different controls based on what you're doing. If you're typing a text message or e-mail, a keyboard appears at the bottom of the screen, and you can easily type a message and send it to someone from your address book. The multi-touch technology also has an auto-corrective feature that accounts for unintentional taps and corrects misspellings. For music and video, volume and playback controls appear on the screen, and so on for other applications.

The iPhone's multi-touch interface also allows the opportunity for innovative uses of the touch display. When viewing a photo or surfing the Web, simply perform a pinching motion with two fingers, and the photo or Web page zooms in. Spread two fingers apart, and the display zooms out. Scrolling in any application is done by just brushing a finger up or down on the screen.

An intriguing innovation in the phone is what Apple calls visual voicemail. No longer will you have to listen to all of your voicemails if you don't want to. Instead, they will appear in a list, much like an e-mail inbox, and you can simply point to the voicemail you want to play.

Apple has also integrated three sensors into its phone. One is an accelerometer, and it senses when you turn the phone on its side, automatically shifting the display to a landscape mode. This feature is incredibly useful for viewing panoramic pictures, videos or shuffling through your albums, which you view by their cover art. A second sensor detects ambient light and adjusts the screen's brightness accordingly in order to save power. The third sensor deactivates the screen when you bring the phone towards your face, so you won't be dialing with your cheek while talking on the device.

Like many of Apple's products, iPhone syncs easily with a Mac or PC. The phone runs a version of Apple's reliable OSX operating system, and its programs and iPod connector (located on the bottom of the phone) will be familiar to many Mac users. Use the iPod connector or a docking station to connect the phone to your computer, the iPhone will automatically sync your address book, photos, movies, music and bookmarks between the computer and the phone.

Now, all of this may sound great, but there are a few catches. First, the iPhone isn't available until June -- Apple needs to get FCC approval before the iPhone can start using a radio band. Second, Apple has an exclusive contract with Cingular through 2009, so if you want a iPhone, you'll have to be a Cingular customer. And third, it's not cheap. A 4 GB iPhone will set you back $499 with a two year service plan, while an 8 GB iPhone will cost $599 with a two year plan. Still, when compared to other high end smartphones, you're getting a lot. This is essentially a small, powerful computer in the palm of your hand, and of course it has Apple's famous sleek, stylish, minimalist design.

Here's a quick rundown of some of the iPhone's other features:

• 802.11 b/g WiFi and Bluetooth 2.0 wireless capabilities
• Quad-band GSM and Cingular EDGE network
• 3.5-inch high resolution screen with 160 ppi (pixels per inch)
• 2.0 megapixel digital camera
• Battery life: 5 hours talk/video/browsing, 16 hours audio playback
• 11.6 mm thin, 4.8 ounces
• IMAP and POP email support, with integrated Yahoo! e-mail client
• Text messages are displayed like instant message conversations, making keeping track of many messages much easier
• Speaker and standard headphone jack

So will the iPhone change the cell phone industry forever? That depends on who you ask. Keep in mind that very few people have even used the iPhone. But given Jobs' impressive demonstration and Apple's recent track record -- over 100 million iPods and 3 billion songs sold -- it's difficult to doubt that this is an extraordinary and important product. Apple's stock surged 7 percent on the day of Jobs' announcement and appears headed for record prices.

In other news, Cisco has filed suit against Apple for infringing on Cisco's trademarked "iPhone" name. Apparently, Apple and Cisco were in talks to negotiate the licensing of the name "iPhone" for Apple's product, but they had not yet reached an agreement when Jobs announced the product at CES.

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